JP2012172205A - Desulfurizing method for molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a desulfurizing method which can obtain desulfurizing effect equal to desulfurization using a slag with high Al and high CaO concentration, even when Al concentration in molten steel is low and CaO concentration in the slag is low, and also can obtain a desulfurizing rate higher than that of the conventional method when Al concentration and CaO concentration are equal to those of the convention method.SOLUTION: In the desulfurizing method, one or more of rare earth metal such as La, Ce, and Nd is added to the molten steel, then gas stirring is performed, in the refining treatment of stirring a molten steel in a ladle and a slug on a surface of the molten steel with an inert gas. The amount of the rare earth metal added is 0.2 kg/ton or more and 0.9 kg/ton or less. The amount of Al added before adding the REM is adjusted such that the ratio [Al]/[REM] of Al concentration [Al] (mass%) in the molten steel after desulfurizing treatment to a total concentration of the rare earth metals [REM] (mass%) in the molten steel is 1.2 or more and 20 or less.

Description

  The present invention provides a desulfurization efficiency at the time when the addition amount of slag and Al is reduced compared with the conventional method when desulfurization treatment is performed by blowing an inert gas into a ladle containing molten steel and slag in molten steel treatment in steel refining. The present invention relates to a desulfurization method for high-efficiency, low-emission molten steel that stabilizes at a high level.

S in steel affects many properties such as corrosion resistance and weldability of steel. Steel used in shipbuilding and steel pipes is required to further reduce the S concentration in steel, and in recent years, a desulfurization technique for reducing the S concentration in steel to 10 ppm or less has been established, and further increases its refining efficiency. Many techniques and methods for increasing production efficiency have been developed. For example, Patent Document 1 discloses a desulfurization agent that contains iron oxide, CaO, and Al ₂ O ₃ and exhibits high desulfurization power, and Patent Document 2 stores molten steel decarburized and refined in a converter in a ladle. A technique for desulfurization is disclosed.

  Furthermore, since it is only necessary to enable desulfurization using an RH vacuum degassing apparatus in order to increase efficiency, for example, Patent Documents 3 and 4 disclose techniques for desulfurization using an RH vacuum degassing apparatus. Furthermore, according to Patent Document 5, the inventors of the RH vacuum degassing apparatus use La, Ce, and Nd for the molten steel before the flux is sprayed when the flux mainly composed of CaO is sprayed on the surface of the molten steel for desulfurization treatment. By adding one or two or more selected from the group consisting of the above and appropriately combining the desulfurization ability of CaO-based flux with the ability of rare earth elements (hereinafter referred to as “REM”), not only desulfurization but also O and N Disclosed a technique for reducing elements such as S at the same time as S.

  The conventional desulfurization technology is based on the premise that slag mainly composed of CaO is used for molten steel that has been sufficiently deoxidized with Al, and the concentration of Al in the molten steel is increased and the concentration of CaO in the slag is increased. This was accompanied by an increase in the amount of slag due to the incorporation of a large amount of CaO.

JP 2004-204307 A JP 2002-339014 A JP-A-5-171253 JP 2000-297318 A JP 2009-144221 A

  Due to the recent increase in performance requirements for steel materials, there is an increasing need to reduce the CaO blending amount in order to reduce the Al concentration in the steel, suppress the increase in the H concentration in the steel, and reduce the slag discharge amount. . However, the conventional desulfurization method premised on sufficient Al deoxidation and a large amount of CaO is becoming unable to meet such recent demands. Therefore, a desulfurization method that does not depend on strong deoxidation with Al and high CaO concentration slag is required.

  By the way, as a desulfurization method, as mentioned above, by reacting CaO in slag and S in molten steel, S in molten steel is absorbed in slag as CaS (hereinafter, this method is referred to as “slag desulfurization method”). ) Is common. As a desulfurization method different from the slag desulfurization method, Ca or Mg, which is highly reactive with S, is added to the molten steel to produce CaS and MgS in the molten steel, and these compounds are desulfurized by floating and separating from the molten steel (hereinafter referred to as “sulfur desulfurization”). This method is called “metal addition desulfurization method”. Of course, a method using a combination of a slag desulfurization method and a metal addition desulfurization method, a method of blowing CaO flux into molten steel, a method of mixing CaO flux and desulfurized metal and blowing or blowing the molten steel into the molten steel, and the like are also used. In principle, these are roughly classified into a slag desulfurization method and a metal addition desulfurization method.

  Both the slag desulfurization method and the metal addition desulfurization method have advantages and disadvantages, but the disadvantage of the slag desulfurization method is that the reaction rate is slow, so it is necessary to increase the Al concentration in the molten steel and the CaO concentration in the slag to compensate for the rate. Is that there is. On the other hand, in the metal addition desulfurization method, since the desulfurization metal such as Mg itself has a deoxidizing power, a deoxidizer such as Al is not essential. However, when the Mg concentration decreases, the MgS dissociates and the S concentration in the molten steel increases. The resulfurization phenomenon which increases again tends to occur. Particularly, when Ca or Mg having a high vapor pressure is used, the concentration decreasing rate is fast, so that the resulfurization tends to be remarkable. In order to avoid this, it is necessary to add a large amount of desulfurization metal.

As described above, in the conventional slag desulfurization method, even when the metal addition desulfurization method is used in combination, it is not easy to desulfurize molten steel having a low Al concentration by using a low CaO concentration slag.
The present invention has been made in view of such problems of the prior art, and even when the Al concentration in molten steel is low or when the CaO concentration in slag is low, a high Al concentration and high CaO concentration slag is used. It is to provide a desulfurization method for molten steel that can obtain a desulfurization power equivalent to conventional desulfurization and that can obtain a higher desulfurization rate than conventional when the Al concentration in the molten steel and the CaO concentration in the slag are equal to the conventional one. .

  If the Al concentration in the molten steel is not increased, a deoxidizing agent that replaces Al is necessary. If the CaO concentration in the slag is not increased, an S scavenger that replaces CaO is necessary. As described above, Ca and Mg are well known as typical elements that function both as a deoxidizer and as an S scavenger. However, although Ca and Mg are used as hot metal desulfurization agents, since their boiling points are lower than the molten steel processing temperature, the concentrations of Ca and Mg in the molten steel cannot be sufficiently increased. Therefore, it is not used much for desulfurization of molten steel, and it cannot be used in reduced pressure refining such as VOD in which evaporation of Ca and Mg is accelerated.

  In addition to Ca and Mg, REMs such as lanthanoids such as La, Ce, and Nd are known. Since REM has a low vapor pressure unlike Ca and Mg, there is an advantage that a high concentration can be easily obtained even under molten steel temperature or reduced pressure. However, when it is used as a desulfurizing agent, there are the following two problems.

  The first problem is inclusions. The REM added to the molten steel reacts with O and S in the molten steel to generate non-metallic inclusions (hereinafter referred to as “inclusions”) such as oxides, sulfides, and oxysulfides. In the case of Ca or Mg, the desulfurization of the molten steel proceeds as the inclusion floats and separates from the molten steel, but the inclusion does not rise rapidly because the specific gravity of the inclusion in the REM is close to that of the molten steel. Therefore, desulfurization does not progress so much by the addition of REM, and many REM inclusions remain in the steel.

  The second issue is the stability of the effect. REM may react with slag, refractories or oxygen in the atmosphere due to its high reactivity, and its concentration may gradually decrease. As the REM concentration decreases, reactions such as deoxidation and desulfurization proceed in the reverse direction, and O and S are supplied from the REM inclusions to the molten steel. The decrease in the REM concentration that causes the reverse reaction occurs due to various factors, and therefore, the reproducibility is poor, and therefore, prediction based on control and experience is difficult. Therefore, when desulfurization is performed using REM, the state after the desulfurization treatment may not be continued until casting, and may be resulfurized.

  In addition, the first problem and the second problem are contradictory, such that if there is no inclusion remaining, re-sulfurization will occur, and if re-sulfurization will not occur, inclusion inclusion will occur. It is difficult to improve. For this reason, the desulfurization method which adds REM to molten steel was not easy.

  When REM is used in accordance with the technical idea of the metal addition desulfurization method, a problem occurs. Therefore, if it is possible to supplement only the decrease in reaction rate, which is a disadvantage of the slag desulfurization method, without causing desulfurization by the addition of REM, the disadvantages of the slag desulfurization method and the metal addition desulfurization method can be offset. Considered. In addition to the fact that the validity of this consideration is unknown, if the amount of REM added is too small, the effect cannot be obtained, and if the amount of REM added is too large, it is expected to cause the same problem as the metal desulfurization addition method of REM. The

Therefore, the validity and appropriate conditions of the consideration that the low Al concentration and the low CaO concentration can be achieved by appropriately combining the slag desulfurization method and the metal addition desulfurization method using REM were experimentally examined.
The experiment was performed as follows. 10 kg of molten steel adjusted in advance to an S concentration of 0.002 to 0.003% (in this specification, “%” relating to the concentration or chemical composition means “mass%”) is maintained at 1873 K, and metal Al is added. Thus, the Al concentration in the molten steel was adjusted to 0.08% or 0.008%. Thereafter, 200 g of flux adjusted to CaO: Al ₂ O ₃ : SiO ₂ = 54: 36: 10 or 40:50:10 was added to form slag on the surface of the molten steel. After confirming slag formation by visual observation, a predetermined amount of REM was added. The added REM was a mixture of metal La, metal Ce, and metal Nd in a mass ratio of 40:40:20. The molten steel was held after REM addition, and samples were taken from the molten steel at intervals of 2.5 minutes during holding to quantify the molten steel composition. The desulfurization rate was calculated according to the formula (1) from the S concentration [S] _{i in} the molten steel before addition of the flux and the S concentration [S] _{f in} the molten steel after being held for 15 minutes.
Desulfurization rate (%) = ([S] _i − [S] _f ) / [S] _i × 100 (1)
The desulfurization rate became substantially constant after 5 minutes from the addition of REM, but here, the desulfurization rate will be described using the analysis result of a sample collected by holding for 20 minutes after the addition of REM.

FIG. 1 is a graph showing the relationship between the amount of REM added when the Al concentration in molten steel is 0.08%, the CaO / Al ₂ O ₃ concentration ratio in the slag (hereinafter C / A), and the desulfurization rate. The Al concentration in molten steel of 0.08% and C / A = 1.5 correspond to general slag desulfurization conditions using a general high Al concentration and high CaO slag. From the experimental results, the desulfurization rate tended to increase by the addition of REM regardless of C / A in the slag. In the graph of FIG. 1, when REM is not added, the desulfurization rate decreases when C / A is reduced from 1.5 to 0.8. However, the addition of REM improves the desulfurization rate, and the amount of REM added is 0.2 kg. It was found that a desulfurization rate equivalent to that of a high Al concentration high CaO slag can be ensured even when C / A is 0.8 by setting it to / ton or more.

  On the other hand, when the amount of REM added is larger than 0.9 kg / ton, a tendency for the desulfurization rate to slightly decrease is recognized. This phenomenon is considered to be caused by slag desulfurization inhibition due to the formation of sulfide or oxysulfide inclusions of REM generated in molten steel with an increase in the amount of REM added and sulfurization accompanying a decrease in REM concentration. Therefore, a high desulfurization rate can be stably secured by setting the REM addition amount to 0.9 kg / ton or less.

  FIG. 2 is a graph showing the measurement results obtained by simultaneously changing the Al concentration in the molten steel and the C / A in the slag. The desulfurization rate increases with an increase in the REM addition amount, and when the addition amount exceeds 0.9 kg / ton, the desulfurization rate tends to decrease slightly as in FIG. However, by setting the REM addition amount to 0.2 kg / ton or more and 0.9 kg / ton or less, C / A = 0.8 slag with reduced CaO is used, and the Al concentration in the molten steel is 0.008%. It was confirmed that a desulfurization rate equivalent to general high Al concentration and high CaO slag desulfurization can be obtained even at a low Al concentration.

  The present invention can improve the desulfurization rate by adjusting the amount of REM added. On the other hand, the principle of the present invention is not to directly desulfurize by REM, but to accelerate the reaction between CaO-based slag and Al and S in the molten steel. Strictly speaking, the Al concentration in the molten steel and the REM addition amount, Precisely, the desulfurization rate slightly changes depending on the relationship between the Al concentration in the molten steel and the REM concentration in the molten steel.

  Therefore, the desulfurization rate was measured by changing the Al concentration in the molten steel by controlling the Al addition amount under the conditions of C / A = 0.8 and REM addition amount 0.7 kg / ton. The experimental results were arranged using the ratio of the Al concentration in the molten steel and the REM concentration (hereinafter, R) in the molten steel 20 minutes after the completion of the desulfurization reaction. The results are shown graphically in FIG.

  From FIG. 3, the amount of REM added is 0.7 kg / ton, which satisfies the appropriate range of the present invention, so that the desulfurization rate can be secured at 80% or more even when C / A = 0.8. It can be seen that a particularly high desulfurization rate is obtained in the case of 2 or more and 20 or less, and that a higher desulfurization rate can be obtained by adjusting the Al concentration in addition to the REM addition amount.

Under conditions where R is less than 1.2, since REM is relatively large with respect to Al, Al ₂ O _{3 in the} slag reacts with REM, so the reaction between Al and slag in the molten steel becomes unstable, so further desulfurization There is no increase in rate. On the other hand, when R exceeds 20 and REM is relatively less than Al, the effect of REM is smaller than the appropriate R. When R is 1.2 or more and 20 or less, the reaction between Al ₂ O ₃ and REM in the slag is suppressed, and at the same time, REM can exert an appropriate reaction promoting effect on Al. The desulfurization rate can be further improved.

  As described above, in the present invention, desulfurization is possible even when the Al concentration in molten steel and the CaO concentration in slag are lowered by using an appropriate REM addition amount and a ratio of the REM concentration to the Al concentration. Since a highly evaporable substance from molten steel is not used, the present invention can also be used in vacuum gas refining such as inert gas blowing refining under atmospheric pressure or VOD. The desulfurization reaction mechanism according to the present invention is as follows.

The reason why the desulfurization rate does not increase when the amount of REM added is small is that the deoxidation effect by REM is insufficient.
When the amount of REM added is appropriate, the REM reacts with oxygen in the molten steel, so that the oxygen activity at the slag metal interface decreases rapidly, and along with this, desulfurization with slag also proceeds rapidly. This can be understood from the fact that the desulfurization reaction is described as CaO + S = CaS + O. After this desulfurization progress, the REM concentration in the molten steel gradually decreases, but the desulfurization rate is maintained without causing sulfurization for the following reason. _CaO-Al in the reaction with 2 _{O 3} slag, but slag Metal surface oxygen activity is defined by the deoxidation equilibrium with _Al 2 _{O 3} activity of the molten steel in the Al concentration in the slag, the slag _Al 2 O _{3 Since the} activity is low, the interfacial oxygen activity is sufficiently lower than the oxygen activity in the molten steel. However, since the reaction between Al ₂ O _{3 in the} slag and Al in the molten steel does not reach an equilibrium, the oxygen activity is usually higher than the interface equilibrium oxygen activity. Since the oxygen activity is higher than the equilibrium oxygen activity, the S concentration is also higher than the equilibrium value, and therefore the desulfurization rate is lower than the equilibrium value. In other words, if the interfacial oxygen activity can be reduced to the equilibrium oxygen activity in a short time, it becomes possible to ensure a high equilibrium desulfurization rate, and the slag-metal reaction equilibrium desulfurization value originally brought by Al, Even if REM is lowered, the desulfurization rate does not change. That is, REM is used as an agent for accelerating the reaction in the slag-metal reaction in which REM does not exist.

  However, if the amount of REM added becomes excessive, the oxygen activity becomes lower than the original equilibrium value of soot obtained by Al, so that the desulfurization value by slag desulfurization temporarily increases, but the desulfurization rate increases with the decrease in REM. Decrease to desulfurization rate. Therefore, REM for temporarily showing a high desulfurization rate becomes meaningless. Moreover, since the oxygen activity lower than the original equilibrium value promotes the formation of inclusions such as REM oxysulfides and sulfides, the desulfurization rate is consequently reduced.

In such refining at a production facility, in addition to desulfurization according to the present invention, the temperature of molten steel is often adjusted. Then, the relationship between this molten steel temperature adjustment process and this invention is demonstrated.
The temperature adjustment of the molten steel is performed by the oxidation heat of Al or Si generated by adding Al or Si or both to the molten steel and blowing or blowing an oxidizing gas such as oxygen gas on the molten steel. The slag composition changes depending on Al ₂ O ₃ and SiO ₂ generated by this treatment. This slag composition change lowers the desulfurization ability of slag, so that sulfation occurs.

In the present invention, since the desulfurization rate is increased to the equilibrium desulfurization value by REM, if the slag composition changes, resulfurization occurs more easily than general slag desulfurization. Therefore, when the present invention and molten steel temperature adjustment are performed, an oxidizing gas is blown into or blown into the molten steel, and after the molten steel temperature increasing treatment is performed to react with Si or Al in the molten steel, La, Ce, Nd are subsequently applied to the molten steel. It is effective to add one or more rare earth metals such as. In addition, it is effective to sufficiently absorb Al ₂ O ₃ or the like generated by the molten steel temperature increase process performed in advance and mix the slag with the slag, and therefore, it is preferable to perform the mixing with the stirring gas for 5 minutes or more. On the other hand, even if stirring is performed for more than 15 minutes, the effect is saturated, so it is preferable to set the stirring time to 15 minutes or less.

  It is possible to desulfurize molten steel without using different conditions from conventional desulfurization conditions, that is, without using a large amount of Al or CaO. Thereby, manufacture of the steel material which has a new property different from the existing steel, and reduction of an environmental load can be aimed at.

FIG. 1 is a graph showing the relationship between the REM addition amount measured when the Al concentration in molten steel is 0.08%, the CaO / Al ₂ O ₃ concentration ratio in the slag (hereinafter C / A), and the desulfurization rate. FIG. 2 is a graph showing the results of measurement by simultaneously changing the Al concentration in molten steel and C / A in slag. FIG. 3 is a graph showing the relationship between the desulfurization rate and the ratio between the Al concentration in molten steel and the REM concentration in molten steel after desulfurization. (C / A = 0.8, REM addition amount 0.7kg / ton)

  A mode for carrying out the present invention will be described. In the following description, the case of carrying out using a converter and an inert gas blowing refining apparatus under atmospheric pressure is taken as an example. It should be noted that the present invention can also be implemented by reduced pressure refining, and the form is the same as that of atmospheric pressure refining, but will be described as appropriate when conditions specific to reduced pressure refining exist.

Take the molten steel decarburized in the converter into the ladle, and transfer the ladle to an inert gas blowing refining device under atmospheric pressure. A medium solvent such as quick lime and silica sand is added to the ladle at the time of steeling to form slag. In addition, a deoxidizer such as Al or Si may be added as necessary, but the slag can be adjusted by adjusting the amount of solvent added in consideration of the amount of Al ₂ O ₃ or SiO ₂ produced by deoxidation. The composition and slag amount can be adjusted. In addition, you may perform vacuum degas refining, such as RH, before atmospheric pressure inert gas blowing refining.

  As described so far, the present invention is a technique for controlling the slag-metal interface reaction and its equilibrium and the reaction rate until equilibrium by appropriately using REM and Al for slag with low CaO concentration. Therefore, a more stable effect can be obtained by further controlling the slag main component and the stirring conditions that affect the interface reaction. Details will be described below.

The present invention is a technique intended for CaO—Al ₂ O ₃ —SiO ₂ -based slag, and since the effect becomes remarkable when the CaO concentration in the slag is low, Al ₂ O ₃ , SiO _{2 in the} slag. The total concentration of CaO is 75% or more, and the weight ratio of CaO to Al ₂ O ₃ in the slag CaO / Al ₂ O ₃ is 0.6 or more and 1.0 or less than the conventional desulfurization treatment conditions. Even with a severe slag composition, a high effect can be obtained. However, the slag composition preferably satisfies the following conditions.

The slag composition preferably has a total concentration of CaO, Al ₂ O ₃ and SiO ₂ of 75% or more and C / A of 0.6 or more and 1.5 or less. When C / A is less than 0.6 or greater than 1.5, the liquid phase rate of the slag is lowered and the treatment time may be increased. In addition, since this invention shows a high predominance with respect to the existing technology when C / A is 1 or less, it is preferable to use it by C / A <= 1. A more preferable slag composition is that the SiO ₂ concentration in the slag is 10% or less, the MgO concentration is 10% or less, and the total concentration of FeO and MnO is 5% or less. When the SiO ₂ concentration in the slag is higher than 10%, the SiO ₂ activity is increased, and the effect may be lowered by reacting with Al in the molten steel or added REM. Moreover, when MgO density | concentration exceeds 10% and the fluidity | liquidity of slag falls, the slag metal interface reaction rate may be reduced. If the total concentration of FeO and MnO exceeds 5%, it may react with Al or REM in the same manner as SiO ₂ to reduce the effect.

  The slag amount is preferably 15 kg / ton or more and 25 kg / ton or less. When the amount of slag is less than 15 kg / ton, the amount of S absorbed in the slag decreases, so when the pre-treatment S concentration is 20 ppm or more, the amount of slag is insufficient in terms of material balance. On the other hand, if the amount of slag exceeds 25 kg / ton, slag stirring may not be sufficiently performed. However, when the present invention is carried out using a vacuum refining apparatus such as VOD, the slag amount is allowed up to 40 kg / ton. This is because, in vacuum refining, gas agitation becomes stronger than in atmospheric refining due to gas expansion, so the amount of slag can be increased as compared with atmospheric refining treatment.

When the molten steel temperature rise process is required, it is carried out before adding REM. While stirring the molten steel with an inert gas, after adding Al or Si or both to the molten steel, oxygen gas is sprayed onto the molten steel or slag through an upper blowing lance. The inert gas flow rate at this time is preferably 8 Nl / min or more and 12 Nl / min or less per ton of molten steel. If it is less than 8 Nl / min, stirring of the molten steel becomes insufficient and mixing takes a long time. If the amount exceeds 12 Nl / min, the slag may be entrained in the molten steel. Although the concentration of Al ₂ O ₃ and SiO ₂ concentration in the slag by the process increases, the slag composition along with this increase, it is particularly preferable to be added and quicklime such that C / A does not change. Moreover, the addition time of quicklime etc. is preferable before oxidizing gas supply. When quick lime is added after supplying the oxidizing gas, it takes a stirring time to make the slag composition uniform before adding REM. Therefore, the processing time can be shortened by adding it before supplying the oxidizing gas.

  After the adjustment of the molten steel temperature is completed, REM is added to the molten steel while stirring the molten steel with an inert gas. In the present invention, since it is necessary to adjust the Al concentration and the REM concentration, the control accuracy of [Al] / [REM] can be further increased by measuring the Al concentration before adding the REM. As a method for measuring the Al concentration, a method in which a molten steel sample is collected and subjected to an emission analysis is known. Alternatively, a method may be used in which the solid electrolyte oxygen sensor is immersed in molten steel, the oxygen activity in the molten steel is measured, and the Al concentration is calculated from the obtained oxygen activity. In the present invention, since deoxidation is important, it is desirable to perform measurement with a solid electrolyte oxygen sensor before adding REM.

  In the present invention, as described above, a high desulfurization rate can be obtained at a low Al concentration without increasing the Al concentration in the molten steel. Here, the low Al concentration means less than 0.01%. As described above with reference to FIG. 2 when the Al concentration is 0.008%, the effect can be obtained even when the Al concentration is less than 0.01%. Therefore, in the present invention, by performing REM addition with an Al concentration of less than 0.01%, it is possible to omit the Al addition amount reduction or the Al concentration reduction process during the production of low Al steel. Further, the Al concentration is desirably 0.003% or more, and more desirably 0.005% or more. If the Al concentration is less than 0.003%, it is difficult to sufficiently reduce FeO and MnO in the slag, and if it is less than 0.005%, the processing time required for reducing FeO and MnO becomes long.

  The REM to be added may be a single metal of rare earth metal such as metal La or metal Ce, or a mixture or alloy thereof. However, since the present invention performs control using these total addition amounts, the total number of lanthanoids (La, Ce, Pr, Nd) from atomic number 57 to atomic number 60 that are relatively close in atomic weight is 90% or more. It is preferable.

  The REM to be added is preferably in the form of a lump having a size of 5 mm to 15 cm. This is because the molten steel is reliably charged and settled into the molten steel. When the powder is finer than 5 mm, it may be dissipated into the exhaust system or oxidized quickly on the slag surface. If it is larger than 15 cm, it takes time to dissolve the REM itself, so that it is necessary to lengthen the processing time.

  The amount of REM to be added is 0.2 kg / ton or more and 0.9 kg / ton or less as described in claim 1, but preferably 0.3 kg / ton or more and 0.7 kg / ton or less. If it is less than 0.3 kg / ton, it may be affected by oxidation loss at the time of REM addition, and if it exceeds 0.7 kg / ton, the REM oxide concentration in the slag may increase and the slag fluidity may decrease. is there. More preferably, it is 0.5 kg / ton or more and 0.7 kg / ton or less for the same reason.

  After REM addition, the molten steel is stirred with an inert gas. As already described, the stirring time is 5 minutes or more and 15 minutes or less, more preferably the stirring time is 7 minutes or more. If it is 7 minutes or more, the desulfurization rate becomes more stable.

  Moreover, it is preferable that the flow rate of the inert gas blown into the molten steel is 10 Nl / min or more and 20 Nl / min or less per 1 ton of molten steel as a stirring condition affecting the reaction rate between slag-metals. If it is less than 10 Nl / min, stirring of molten steel and slag becomes insufficient, and the added REM may not be rapidly supplied to the slag metal interface. Moreover, if it exceeds 20 Nl / min, the reaction rate of REM and slag may be excessively accelerated, and the REM concentration in molten steel may be reduced in a short time. In this case, the effect of REM may be reduced. is there.

  In general melting of REM-added steel, the purpose is to ensure solid solution REM concentration or to leave REM inclusions in the molten steel, so the molten steel stirring time after REM addition can be shortened to 3-5 minutes. After addition of REM, casting is performed without any other treatment. On the other hand, since the present invention aims at desulfurization, the stirring force and time required for the desulfurization reaction between slag metals are required after REM addition. Therefore, it is preferable to perform stirring under the above conditions.

  In addition, it is preferable not to add another alloy to molten steel during the inert gas blowing performed after REM addition. This is to avoid desulfurization inhibition due to S or O contained in a trace amount in the added alloy. More preferably, a necessary alloy element is added before REM addition.

  Although the Al concentration in molten steel at the time of REM addition can be measured by a solid electrolyte sensor, it can be determined based on operational results. Although it is not necessary to know the REM concentration immediately after REM addition, the yield when the REM to be added is mainly lanthanoid is 45 to 55%. Since the REM concentration reduction rate and the Al concentration reduction rate after desulfurization treatment, that is, from the addition of REM to the end of stirring of the inert gas, show stable reproducibility, the REM addition amount, the Al addition amount, and the inert gas blowing stirring time From the relationship, each concentration after the desulfurization treatment can be easily estimated. From this relationship, the Al concentration before REM addition may be determined so as to satisfy claim 1, and the Al addition amount may be determined and added accordingly.

  Further, the REM concentration in the molten steel after the desulfurization treatment is preferably 0.002% or more, and more preferably 0.002% or more and 0.02% or less. If it is less than 0.002%, there is a possibility that REM may be lost even if a slight contact between the atmosphere and the molten steel, so 0.002% or more is preferable in refining under atmospheric pressure where there is a possibility of contact with the atmosphere. On the other hand, if the content exceeds 0.02%, the inclusions may be REM-Al-O, REM-Si-O, or Al-Si-REM-O, and the inclusions may change. However, the upper limit of the REM concentration is not particularly specified when there are no restrictions on the product component standard for the REM concentration and the REM inclusion inclusion residue.

  In addition, since O and S fall by REM addition, nitrogen absorption during the inert gas stirring after REM addition becomes easy to occur. For this reason, it is preferable to use an apparatus capable of adjusting the internal atmosphere by blocking the atmosphere from the atmosphere.

  The above is an example using a converter and an inert gas blowing refining apparatus under atmospheric pressure, but the same applies to the case of using a converter and a partial vacuum refining apparatus or an electric furnace and a VOD. That is, the present invention can be implemented in a vacuum refining apparatus in which a slag-metal reaction proceeds, such as a VOD or a straight barrel type vacuum refining apparatus. In addition, the ladle may be transferred to an atmospheric pressure inert gas blowing refining apparatus after the VOD treatment to carry out the present invention.

  After the desulfurization treatment, the ladle may be quickly transferred to a continuous casting apparatus or the like for casting, or vacuum degassing refining may be performed with RH and then cast. Since RH has a very low reaction rate between ladle slag and molten steel, sulfurization is unlikely to occur, and therefore RH treatment can be performed after the present invention is implemented. Further, during or after the RH treatment, a Ca treatment for the purpose of inclusion form control or the like may be performed using a ladle or a tundish.

  250 ton of molten steel was decarburized in a converter and put out into a ladle. Quick lime and Al, Si, and Mn were added to the ladle at the time of steeling. Thereafter, the ladle was transferred to an inert gas blowing and refining apparatus under atmospheric pressure, and Ar gas blowing treatment was started immediately. The Ar gas used was a blow lance immersed in molten steel, the blow depth was 70 to 85% (average 80%) of the molten steel depth from the molten steel surface, and the gas flow rate was 3800 Nl / min.

  When it was necessary to adjust the molten steel temperature after the treatment started, Al was added to the molten steel, and CaO was added to the slag, and then oxygen gas was sprayed through the top blowing lance. The amount of oxygen gas is 50 to 150 Nl / min.

After blowing over oxygen as necessary, REM was added and Ar gas was blown and stirred. Table 1 shows the amount of REM added, stirring time, presence / absence of temperature increase treatment, and C / A.
Test No. 1 is a comparative example in which a small amount of REM is added, Test No. 2 is an example of the present invention that satisfies claim 1, Test Nos. 3 to 8 are examples of the present invention that satisfy claims 1 and 2, and Test Nos. 9 to 12 Is a comparative example in which REM was not added.

  Focusing on the desulfurization rate, it can be seen that the present invention example has a higher desulfurization rate than the comparative example, and according to the present invention, a high desulfurization rate can be obtained even at low Al concentration and low C / A. It can also be seen that a higher desulfurization rate can be obtained by satisfying the conditions of claim 2.

  Further, when attention is paid to test number 8 which is an example of the present invention having an S concentration before treatment as low as 6 ppm and test number 10 which is a comparative example where the S concentration before treatment is as low as 7 ppm, the test number 10 where REM is not added is completely desulfurized. On the other hand, in Test No. 8, since the S concentration before treatment is low, the desulfurization rate itself is low, but it is understood that desulfurization in an extremely low sulfur region is possible. Thereby, it turns out that manufacture of an ultra-low-sulfur steel is also possible by this invention.

Claims (2)

  A desulfurization method in which at least one kind of rare earth metal is added to molten steel in the refining treatment in which the molten steel in the ladle and the slag on the surface of the molten steel are stirred with an inert gas, and the total mass of rare earth metals to be added. Is 0.2 kg or more and 0.9 kg or less per mass (ton) of the molten steel, and the concentration [Al] (mass%) of Al in the molten steel after the molten steel desulfurization treatment and the total concentration of rare earth elements in the molten steel A desulfurization method for molten steel, wherein the amount of Al added before REM addition is adjusted so that the ratio [Al] / [REM] to [REM] (% by mass) is 1.2 or more and 20 or less.   After carrying out a molten steel temperature rise treatment in which oxidizing gas is blown or blown into molten steel to react with one or both of Si and Al in the molten steel, one or more of the rare earth metals are subsequently added to the molten steel, and gas stirring is further performed. The molten steel desulfurization method according to claim 1, wherein the desulfurization is performed for 5 minutes to 15 minutes.

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