JP5712945B2 - Method for melting low-sulfur steel - Google Patents

Description

  The present invention relates to a method for melting low-sulfur steel used for steel pipes or thick steel plates, and in particular, in processing using an RH vacuum degassing apparatus, the sulfur concentration in steel can be easily reduced with a small amount of solvent. Further, the present invention relates to a method for melting low-sulfur steel that can suppress nozzle clogging during continuous casting to a level that does not cause a problem.

  Since sulfur in steel causes a decrease in weldability, toughness, corrosion resistance, etc. of steel materials, many techniques for reducing the sulfur concentration in steel have been developed. However, since the requirements such as [S] <0.002% by mass have been limited to some products called high-grade steel and high-performance steel, complicated processing due to desulfurization and an increase in manufacturing cost are allowed to some extent. I came. On the other hand, a technology capable of highly efficient refining at a lower cost has been developed.

  As a desulfurization technique using an RH type vacuum degassing apparatus, there are methods disclosed in Patent Document 1 and Patent Document 2, for example. As a method of reducing not only desulfurization but also elements such as oxygen and nitrogen as well as sulfur according to Patent Document 3, the present applicant previously described a desulfurization ability of a flux mainly composed of CaO, La, Ce, Nd, Y, and the like. A method of simultaneously achieving low O, low S, and low N by combining the ability of rare earth elements (hereinafter referred to as “REM”) appropriately.

  By the method presented in Patent Document 3, it is possible to manufacture high-performance, high-performance products by simply reducing O, S and N. In this method, “flux mainly composed of CaO” means a flux in which the pure content of calcium oxide (CaO) in the flux is 75% by mass or more, and also in this specification “CaO-based flux”. It may be noted.

  On the other hand, since there is a case where it is necessary to simply reduce N in general steel or there is a case where the N concentration needs to be slightly reduced in order to satisfy a general N standard component, 4 presented a method for denitrifying molten steel more easily and cheaply and enabling mass production of low nitrogen steel.

  The background of the above technological development is the pursuit of advanced desulfurization technology for steel materials that require high performance, and advanced simultaneous desulfurization S deoxygenation, and to achieve higher performance even now. Development is underway.

  On the other hand, in recent years, not only some high-performance steels but also relatively versatile steel materials are demanding a low sulfur concentration. Of course, desulfurization is possible even with versatile steels if the same treatment as high-performance steel is applied, but these steel types need to be manufactured in large quantities and at a lower cost, while the required desulfurization level is higher than that of high-performance steel. However, since it is often low, it is not always optimal to perform the same treatment as that of high-performance steel.

  In addition, when REM is used as a desulfurizing agent, it is known that nozzle clogging may occur during continuous casting due to the presence of inclusions produced by the reaction of REM with O and S. This is also disclosed in Japanese Patent No. 4, but no specific countermeasure is disclosed in any of Patent Documents 3 and 4.

  Furthermore, Patent Document 5 discloses a method for suppressing clogging of an immersion nozzle that suppresses a decrease in operability and an increase in cost. In this method, a rare earth element is added every one or two or more charges during melting in a secondary refining apparatus. However, if this method is used in large quantities, an increase in cost is inevitable.

JP-A-5-171253 JP 2000-297318 A JP 2009-144221 A JP 2010-132982 A JP 2009-274079 A

  The object of the present invention is a steel melting method that reduces the sulfur concentration easily and inexpensively for a wider range of steel components in degassing using an RH vacuum degassing apparatus in view of the above problems. In addition, an object of the present invention is to provide a method for melting low-sulfur steel that can prevent nozzle clogging during continuous casting.

  REM (rare earth elements such as La, Ce, Nd, Y) and CaO, which have a strong de-O force and a strong affinity for S, and CaO are processed in a RH-type vacuum degassing system for a wide range of steel materials. As a basic desulfurization technology, it is based on the technology to desulfurize the sulfur concentration easily and inexpensively. Furthermore, the molten steel components and REM addition amount are controlled within an appropriate range. The policy is to prevent nozzle clogging during casting by appropriately controlling the flow rate of the molten steel afterwards.

  Based on this policy, first of all, steel components other than iron to be treated according to the present invention were identified for the following reasons. In this specification, “%” in the steel composition and REM concentration (details will be described later) means “% by mass” unless otherwise specified.

  Mn: Mn is a deoxidizing element and is an essential element because it improves various steel properties. Therefore, if the Mn concentration is less than 0.1%, deoxidation becomes unstable, and if it exceeds 2%, the activity of S is reduced, making desulfurization difficult. Therefore, the Mn concentration is 0.1% or more and 2% or less.

  Si: Si is an element indispensable for deoxidation stability like Mn. When the Si concentration is less than 0.001%, deoxidation is unstable, and when it exceeds 1%, the N activity is increased to promote denitrification. In the present invention, since the component system that is not easy to reduce N is used, the Si concentration is set to 1% or less.

  Al: Al is an element that plays an important role in deoxidation because it is the element having the strongest deoxidizing power. In order to obtain this deoxidation effect, the Al concentration needs to be 0.005% or more. On the other hand, if the Al concentration exceeds 1%, the dissolved oxygen concentration becomes high again and it becomes difficult to realize low O, so it is necessary to be 1% or less. For this reason, Al concentration shall be 0.005% or more and 1% or less.

  S: S is an impurity and is an element to be removed, but if it exceeds 0.0035%, the amount of desulfurization agent used increases significantly in terms of material balance, so not only costs but also slag emissions increase. . Therefore, molten steel having an S concentration of 0.0035% or less is treated.

  In addition to the above essential components, other elements such as C, Ti, Ca, B, Nb, W, Mo, V, Mg, Cr, as long as they do not affect desulfurization, deoxidation, and / or denitrification Ni etc. may be contained.

  As already clarified in Patent Document 4, in order to achieve low S, low O is advantageous. Then, it is the highest priority to achieve low O, and there is a possibility that the effect may be improved if elements having strong affinity with S are used together. That is, it is desirable to use an element that has deoxidizing power and can achieve low O, and at the same time has an affinity for S. Since the present invention is premised on vacuum processing, it is desirable that the element has a low vapor pressure.

  Considering the above, REM (rare earth elements such as La, Ce, Nd, and Y) is known as an element that satisfies such conditions. That is, if these REMs are used, it is expected that low S, low O, low N steel can be obtained by a simple method. In the technique disclosed in Patent Document 3, REM is added based on this idea.

  However, since the object of the present invention is only desulfurization and not denitrification, the appropriate REM concentration range is considered to be different from the REM concentration range in the technique disclosed in Patent Document 3. Furthermore, in order to apply the desulfurization method according to the present invention to a steel material having a wider component system, it is not necessary to increase the REM concentration in the steel, or it is necessary that the REM concentration after treatment is sufficiently low. In addition, in order to suppress the nozzle clogging problem, it is necessary to reduce the amount of REM used in order to reduce the amount of REM inclusions that causes the problem, and the generated REM inclusions are suspended in the molten steel. It is also necessary to reduce the amount of turbidity as much as possible.

  Further, in the technique disclosed in Patent Document 4, a flux mainly composed of CaO is unnecessary because it aims at denitrification, but it is necessary to determine again whether it is unnecessary or necessary in the present invention intended for desulfurization. .

  In other words, it is necessary to clarify an appropriate REM concentration range that promotes desulfurization and at the same time does not change the molten steel components and does not cause nozzle clogging, and an appropriate amount of flux mainly composed of CaO including necessity. There is. In addition, it is necessary to clarify conditions regarding the oxygen supply amount after REM addition in the RH treatment and the molten steel ring flow rate after the oxygen supply.

Therefore, we tried to clarify these experimentally. The conditions for this experiment were determined as follows.
First, the addition order of REM and CaO was the order in which REM addition was performed first and then CaO was added. Contrary to this addition order, when REM is added after CaO addition, the sulfur concentration in the molten steel changes depending on the progress of the desulfurization reaction by CaO added in advance. For this reason, the reaction amount of REM with sulfur changes, and as a result, the reaction amount of REM with oxygen in molten steel also changes. Therefore, there is a concern that the effect becomes unstable when REM is added after CaO is added. That is, the effect of REM addition is stably acquired by adding CaO after REM addition.

  Moreover, the addition method of CaO decided to use the method of adding CaO by lump addition, without going through a top blowing lance etc. The method disclosed in Patent Document 3 uses a CaO-based flux addition method in which a CaO-based flux is sprayed over the molten steel over 3 to 10 minutes via an upper blowing lance, but this has a slow denitrification rate. Therefore, it aims at ensuring the denitrification reaction time. On the other hand, the present invention is intended only for desulfurization, which has a higher reaction rate than denitrification, and aims to provide a treatment method that is simpler than conventional desulfurization methods. Naturally, it is preferable that the supply of CaO is performed collectively from the viewpoint of easy processing, and as will be described later, CaO is also a short time from the viewpoint of efficient desulfurization (to achieve a high desulfurization rate in a short time). Are preferably supplied. Therefore, in the present invention, the supply amount of CaO per unit time is limited, and CaO is supplied by batch addition rather than supply through an upper blowing lance that requires carrier gas for supply. Here, “collective addition” in the present invention is a method in which the addition of the entire amount of CaO added from an addition hopper or the like is completed in a short time of about several seconds to within one minute, and the alloy is added by RH treatment or the like. Refers to the general method.

  Further, regarding the atmospheric pressure when adding REM and CaO, the present invention does not aim at denitrification, so that a high vacuum requiring complicated exhaust operation and expensive exhaust cost is not required. For this reason, the RH treatment can be performed at an atmospheric pressure of 1.5 kPa or more which is inexpensive and easily obtained. By performing desulfurization at such an atmospheric pressure, in addition to the reduction of the processing cost, the reflux speed is lowered, so that the effect of adsorption removal of inclusions in the vacuum chamber by the CaO-based flux also occurs.

  The experiment method performed under the conditions set as described above will be described in detail. 15 kg of steel was melted in an MgO crucible and the temperature was adjusted to 1873K. The melting atmosphere was an Ar atmosphere and the atmospheric pressure was 1.5 to 6 kPa. After temperature stabilization, the Mn concentration in the molten steel is 0.5-0.6%, the Si concentration in the molten steel is 0.5-0.6%, the Al concentration in the molten steel is 0.01-0.03%, The medium S concentration was adjusted to 0.0025 to 0.0035%.

  Thereafter, a predetermined amount of metal Ce or metal La was added as REM, and a predetermined amount of CaO reagent was added all at once after 1 minute and 30 seconds had elapsed since the addition of REM. Samples were taken from the molten steel after 2 minutes and after 20 minutes from the addition of the CaO reagent, and the S concentration and REM concentration in the molten steel were quantified by chemical analysis. In the experiment in which CaO was not added, samples were taken from the molten steel after 2 minutes and 20 minutes had passed since REM was added.

  The relationship between the CaO addition amount and the desulfurization rate measured with the REM addition amount of 0.1 kg / ton is shown in a graph in FIG. Desulfurization rate is defined by desulfurization rate (%) = {(sulfur concentration in molten steel before REM addition−sulfur concentration in molten steel after CaO addition) / (sulfur concentration in molten steel before REM addition)} × 100, and CaO addition for 2 minutes The desulfurization rate calculated from the S concentration in the molten steel after is indicated by ◯, and the desulfurization rate calculated from the S concentration in the molten steel after 20 minutes of CaO addition is indicated by ▲.

  Focusing on the result of CaO addition amount 0 kg / ton (y-intercept in the graph of FIG. 1), a desulfurization rate of 31% was obtained after 2 minutes of REM addition (◯) even without adding CaO. This is considered to be desulfurization due to the reaction between REM and S in molten steel to produce sulfide or oxysulfide of REM. However, after 20 minutes from the addition of REM (▲), the sulfurization progressed and the desulfurization rate became 0%. This is considered because REM in molten steel reacted with refractories and the like, and the REM concentration in molten steel decreased.

  From the above results, it is understood that when denitrification is intended, CaO is not required, but when desulfurization is intended, the effect is more stable when CaO is used in combination. Further, when attention is paid to the experimental results after 20 minutes of CaO addition at a CaO addition amount of 1 kg / ton, it can be seen that the use of REM and CaO in combination dramatically improves.

  Moreover, the desulfurization rate increases as the CaO addition amount increases, but if it is less than 5 kg / ton, the change in the desulfurization rate relative to the addition amount is large, and the operation becomes somewhat unstable. Since the desulfurization rate is stable at 5 kg / ton or more, the desulfurization rate can be stabilized at a high level by setting the CaO addition amount to 5 kg / ton or more.

  On the other hand, if the amount of CaO added exceeds 15 kg / ton, the desulfurization rate after 2 minutes of CaO addition is very high, but after 20 minutes of CaO addition, the desulfurization rate decreases slightly and is equivalent to 5-15 kg / ton. The rate has changed.

  This is presumably because if the amount of CaO added exceeds 15 kg / ton, the sulfur concentration decreases to 1 to 2 ppm immediately after the addition of CaO, but because the sulfur concentration is low, resulfurization is easy. Therefore, even if the CaO addition amount is 20 kg / ton, there is no problem other than the increase in the amount of CaO used, but it can be said that a sufficient desulfurization effect can be obtained even at 15 kg / ton or less.

  Next, the influence of the REM addition amount was measured. The measurement was performed by changing the REM addition amount while keeping the CaO addition amount constant at 6 kg / ton. Moreover, the desulfurization rate was calculated using the sulfur concentration in the molten steel after 20 minutes of CaO addition.

  FIG. 2 is a graph showing the relationship between the REM addition amount and the desulfurization rate. Even when the amount of REM added was 0, the desulfurization rate was 58% due to desulfurization using only CaO. When the amount of REM added was increased, the desulfurization rate also increased, but a desulfurization rate of 80% or more was obtained at 0.05 kg / ton or more. Further, it was confirmed that the desulfurization rate was improved up to the REM addition amount of about 0.30 kg / ton.

  By the way, the object of the present invention is not only to simply desulfurize but also to be applicable to a wider range of steel materials. For that purpose, it is necessary that REM does not remain in the steel, and nozzle clogging is likely to occur during casting due to inclusion generation accompanying the addition of REM, so the amount of REM added is reduced in order to reduce the amount of generation. It is necessary.

  Therefore, the REM concentration in the molten steel after 20 minutes of CaO addition was evaluated by an index called residual rate. Residual rate was defined as residual rate (%) = {(REM concentration in molten steel 20 minutes after addition of CaO) / (REM concentration in which added REM amount was converted to molten steel concentration)} × 100. The denominator is the REM concentration in molten steel calculated as a yield of 100%.

  The relationship between the REM addition amount and the residual rate is shown in a graph in FIG. However, since the added REM reacts with O or S in the molten steel and its concentration changes, it is necessary to pay attention to the O concentration and the S concentration. In the present invention, since the S concentration is controlled to be 0.0035% or less for the reasons already described, it is necessary to pay attention to the O concentration. Therefore, examination is made on molten steel with an O concentration of 30 ppm. When the O concentration exceeds 30 ppm, the REM concentration in the inclusions decreases, and the deoxidation effect by REM may become unstable. Therefore, in order to stably obtain the effects of the present invention, it is preferable to set the O concentration before REM addition to 30 ppm or less.

  From the graph of FIG. 3, it can be seen that when the amount of REM added exceeds 0.2 kg / ton, the residual rate rapidly increases. On the other hand, when the amount of REM added is 0.1 kg / ton or less, the added REM is consumed by the reaction with O or CaO in the molten steel, and therefore hardly remains in the molten steel. However, as shown in FIGS. 1 and 2, a sufficiently high desulfurization rate can be achieved even when the REM addition amount is in the range of 0.05 to 0.1 kg / t.

  Since the amount of addition exceeds the amount of reaction consumption with O, the REM remains in the molten steel as a solute, so the present invention cannot be applied to steel materials that do not allow the inclusion of REM. Therefore, it is feared that a large amount of oxygen blowing is required to remove residual REM, which causes a nozzle clogging problem. Therefore, the required amount of REM addition is in the range of 0.05 to 0.1 kg / t when the O concentration in the steel is known to be 30 ppm or less by the rapid analysis method of the O concentration in the steel or the operational experience value. It is preferable that

  However, if it is not certain whether the O concentration in the steel is 30 ppm or less, considering the effect of improving the desulfurization rate shown in FIG. It is possible to expand the types of steel materials to which the REM addition desulfurization process according to the present invention can be applied by increasing the amount of oxygen blown after REM addition and removing residual oxygen as necessary. Even if 0.3 kg / ton of REM is added and the residual rate is 30%, the REM concentration in the molten steel is about 0.010%, so the removal by the top blowing oxygen and the inclusion removal operation by the molten steel recirculation thereafter This is because it is considered that nozzle clogging during continuous casting of molten steel can be prevented.

  The oxygen top blowing for removing the residual REM is performed as follows. After addition of the CaO flux, the reduction of S and O is completed, the purpose of REM refining is achieved, and the presence of REM becomes unnecessary. Moreover, S is separated from the molten steel into the ladle slag together with the CaO flux, and the inclusion alone with the molten steel reflux. Therefore, even if REM in molten steel is removed, the S and O concentrations do not increase rapidly again.

To remove REM from molten steel, a small amount of oxygen may be added to the molten steel using its high deoxidizing power. Specifically, the method of spraying oxygen gas on the surface of the molten steel in the vacuum chamber is the simplest, and the amount of oxygen gas sprayed is 0.1 to 0.6 Nm ³ per ton of molten steel. Since the equilibrium oxygen activity of REM is as very low as 0.0001 to 0.0003 and the REM concentration used in the present invention is also low, REM can be completely removed by applying a very small amount of oxygen.

After this experiment, different amounts of oxygen gas (0.05, 0.1, 0.3, 0.4 and 0.6 Nm ³ ) were sprayed onto the molten steel to confirm this point. As a result, at 0.05 Nm ³ , the REM concentration was 0.0001%, and a part remained. However, at 0.1 Nm ³ or more, the REM concentration was below the analysis limit. If oxygen gas is blown too much, there is a possibility that the oxygen concentration in the steel is likely to increase. Therefore, the oxygen gas blowing amount is 0.6 Nm ³ / t or less, which is actually sufficient. However, in the present invention, REM is added, CaO is added, and then alloy iron is added to adjust the components. It does not prevent the temperature adjustment by blowing up oxygen in the presence of Al. Therefore, when oxygen is blown up for component adjustment or temperature adjustment, the required amount of oxygen is blown up by adding the oxygen gas blowing amount for that purpose together with the above-described oxygen gas for REM removal.

Moreover, when the S concentration in steel before addition is 35 ppm or less and the O concentration in steel is 30 ppm or less and the amount of REM added is suppressed to 0.05 to 0.1 kg / t, residual REM in molten steel Since the concentration is low, it is sufficient that the amount of oxygen gas blown up is 0.1 Nm ³ / t or more and 0.3 Nm ³ / t or less. However, in this case as well, when oxygen is blown up for component adjustment or temperature adjustment, the oxygen gas blowing amount for that purpose is combined with the oxygen gas for removing REM described above to blow up the required oxygen amount. Will do.

  Further, in the present invention, the CaO addition method is limited to batch addition for the above-mentioned reasons, but this effectiveness was examined. An experiment in which the total amount of CaO added is 6 kg / ton, the amount of La added is 0.1 kg / ton, and 6 kg / ton of CaO is added at a time (collective addition). 3 kg / ton is added twice at 1 minute intervals. Experiment, adding 2 kg / ton 3 times at 1 minute intervals, and adding 1.5 kg / ton 4 times at 1 minute intervals, with the first addition being 0 minutes and 5 After 10 minutes, 10 minutes, 20 minutes, and 40 minutes of desulfurization rate were measured. The results are shown graphically in FIG. The time required for one addition is about 20 to 30 seconds.

  As shown in FIG. 4, the desulfurization rate after 40 minutes, which is in a state of almost reaching equilibrium, is equal regardless of the number of additions. However, the desulfurization rate tends to increase as the number of additions decreases in a short time such as 5 minutes or 10 minutes before reaching the equilibrium.

  This is because, in addition to increasing the number of times of addition, the time required to add the whole amount becomes longer, and when added in divided portions, it takes time to mix CaO added previously and CaO newly added. Therefore, it is considered that one of the reasons is that the reaction time is long. Industrially, it is important that the treatment time is shorter, so that it is effective to add all at once in the case of simple desulfurization treatment.

From the above, it can be seen that the desulfurization efficiency of molten steel is increased by adding CaO all at once after REM addition.
Furthermore, the effectiveness with respect to the residual rate of adding CaO all at once was examined. The experimental conditions are the same as the experimental conditions shown in FIG. 4, and the experimental results are shown graphically in FIG. As shown in FIG. 5, the residual rate becomes 0% when held for a long time of 40 minutes, but when the holding time is short, the residual rate increases as the number of times of addition increases. This is considered to be caused by a decrease in the amount of REM in molten steel that reacts with CaO because the amount of addition per one decreases as the number of additions increases. From the above results, it is understood that batch addition is effective not only from the viewpoint of increasing the desulfurization efficiency but also from the viewpoint of REM concentration control in the technique using REM as in the present invention.

  In the experiments shown in FIGS. 4 and 5, the time required for one addition is 20 to 30 seconds, but the addition time in the present invention, which is one addition, that is, batch addition, is within 1 minute. is necessary. When one addition becomes longer than 1 minute, the addition speed with respect to the recirculation speed of the process which prescribes | regulates the molten steel residence time in a vacuum tank will become relatively slow. As a result, it is predicted that the state will be close to the divided addition in the laboratory experiment. Therefore, the time required for the addition needs to be within 1 minute.

  In addition, the denitrification rate in this experiment was as very small as 0-5.3%, and the denitrification promotion effect like the process which blows up a CaO type | system | group flux through an upper blowing lance was not acquired. However, the present invention is difficult to denitrify, such as short processing time, no need for top blowing lance and carrier gas used for top blowing, high vacuum, etc. There are many industrial advantages.

Next, after adding a predetermined amount of REM and CaO to the molten steel in the predetermined composition range according to the present invention, and further spraying 0.1 Nm ³ / t or more of the blown oxygen to the molten steel, the circulating operation of the molten steel For at least 10 minutes. In this molten steel recirculation operation, REM-containing inclusions are generated in the molten steel by addition of REM, even in a small amount, and the inclusions are discharged from the vacuum tank of the RH vacuum degassing apparatus into the molten steel in the ladle. Furthermore, it is a treatment for absorbing the ladle slag present on the molten steel in the ladle. The molten steel reflux time for this purpose requires 10 minutes or more, as will be described later, because the floating separation from the molten steel to the slag is difficult to proceed due to the nature of the REM-containing inclusions. However, since this is a molten steel recirculation operation for absorbing the REM-containing inclusions generated with the addition of REM into the ladle slag, component adjustment and temperature adjustment may be performed during this recirculation operation.

  In the RH type vacuum degassing process for a molten steel amount of 250 tons, the present inventors conducted a test of continuous casting by performing molten steel recirculation after oxygen was blown up after Nd addition and CaO addition. The experimental method will be described in detail.

The hot metal that had been subjected to hot metal desulfurization and hot metal dephosphorization treatment in advance as needed was placed in a 230 ton (t) scale upper bottom blowing converter and decarburized, and the steel was discharged into a ladle.
Various deoxidizers and alloys are added at the time of steel removal, and the molten steel components in the ladle are changed to C: 0.04 to 0.07%, Si: 0.1 to 0.3%, Mn: 0.5 to 0.00. 6%, S: 20-25 ppm, sol. Al: 0.05 to 0.07%.

Furthermore, CaO was added at the time of steel output, and the CaO / Al ₂ O ₃ weight ratio in the slag was adjusted to 1.9 to 2.1, and the total concentration of FeO and MnO in the slag was adjusted to 5% or less.
Nd is used as the REM, and the amount of Nd added is 0.08 kg / ton to 0.30 kg / ton from the hopper, CaO is added 10 to 15 kg / ton, and then the residual REM is removed and the molten steel temperature is adjusted. Then, 0.3 to 1.4 Nm ³ / ton of oxygen was sprayed from the top blowing lance to adjust the molten steel temperature to 1580 to 1590 ° C., and then the molten steel was subjected to 5 to 15 minutes. Thereafter, continuous casting was performed at a casting speed of 0.5 to 3.6 m / min using a continuous casting machine having a mold size of φ191 to φ360 mm and 6 strands, and the clogging state of the immersion nozzle was evaluated.

  Table 1 shows the amount of Nd added, the amount of top blowing oxygen, the recirculation time after oxygen blowing, and the number of continuous casting charges during continuous casting (determination of casting when the nozzle opening reaches the standard opening + 20%) The survey results of the relationship are summarized.

Test Nos. 1 to 8 were performed by blowing molten steel 0.3 to 1.1 Nm ³ / t and then performing molten steel reflux for 5 to 7 minutes, but the number of charges that could be continuously cast was 3 to 3. Stayed at 5 charges.

On the other hand, as for test numbers 9-16, after spraying top blowing oxygen 0.5-1.4Nm < ³ > / t, as a result of performing molten steel recirculation for 10-15 minutes, the number of charge which could be cast continuously is We were able to achieve 7 charges as planned.

  As shown in Table 1, when the molten steel reflux time after blowing up oxygen is less than 10 minutes, the levitation absorption of oxides such as REM to the slag formed on the molten steel is insufficient. Clogging occurs. Therefore, the molten steel reflux time is secured for 10 minutes or more after the oxygen top blowing.

The present invention completed based on the above knowledge is as follows.
(1) Mn: 0.1% or more and 2% or less, Si: 0.001% or more and 1% or less, S: 0.0035% or less, Al: 0.005% or more and 1% or less, molten steel containing other alloy components When the refining treatment is carried out in the RH type vacuum degassing apparatus, the molten steel is one or more selected from the group consisting of La, Ce and Nd at a total amount of 0.05 kg / molten steel ton or more 0.30 kg / After adding molten steel ton or less, add CaO-based flux in a quantity of 5kg / ton or more and 20kg or less of molten steel ton as pure CaO within 1 minute from the inside of the vacuum chamber without going through the top blowing lance. and, further, after the addition of the flux mainly composed of the CaO, oxygen to the molten steel after blowing 0.1 Nm 3 / ^ton or more from the lance onto the vacuum chamber, the refluxing of the solution steel 10 minutes And carrying out on desulfurization method 溶綱.

(2) Mn: 0.1% to 2%, Si: 0.001% to 1%, S: 0.0035% or less, Al: 0.005% to 1%, O: 0.003% Hereinafter, when refining the molten steel containing other alloy components with an RH vacuum degassing apparatus, one or more selected from the group consisting of La, Ce and Nd is added to the molten steel in a total amount of 0.00. After adding 05 kg / molten steel ton or more and 0.10 kg / molten steel ton or less, the CaO-based flux in the amount of 5 kg / ton or more and 15 kg / molten steel ton or less is contained in the pure CaO, without going through the top blow lance was added within 1 minute in a batch, and further, after the addition of the flux mainly composed of the CaO, oxygen to the molten steel after blowing 0.1 Nm 3 / ^ton or more from the lance onto the vacuum chamber, solution steel And performing refluxed for 10 minutes or more, the desulfurization method of 溶綱.

  According to the present invention, in degassing using an RH type vacuum degassing apparatus, the sulfur concentration can be reduced easily and inexpensively for a wider range of steel components, and nozzle clogging is prevented during continuous casting. it can.

It is a graph which shows the relationship between the amount of CaO addition and the desulfurization rate after CaO addition 20 minutes progress after CaO addition 2 minutes progress. It is a graph which shows the relationship between REM addition amount and a desulfurization rate. It is a graph which shows the relationship between REM addition amount and a residual rate. It is a graph which shows the change by the desulfurization process time of the relationship between the frequency | count of CaO addition and a desulfurization rate. It is a graph which shows the change by the desulfurization process time of the relationship between the frequency | count of CaO addition, and a residual rate.

In the RH-type vacuum degassing process, processes such as degassing, component adjustment, and temperature adjustment are arbitrarily selected and performed in an arbitrary order. However, according to the present invention, the molten steel having a predetermined composition range is used. A desulfurization method (hereinafter referred to as “the present method”) in which a predetermined amount of REM and CaO are added, and the top blown oxygen of 0.1 Nm ³ / t or more is sprayed on the molten steel, and then the molten steel is refluxed for 10 minutes or more. May be performed in any order as in these techniques. However, it is desirable to perform this method at the beginning of the RH vacuum degassing process. This is because the effect of the present invention may become unstable if the present invention is carried out in a state where the influence of the treatment carried out before the method is carried out. For example, if this method is carried out immediately after adding an auxiliary material containing sulfur as an impurity to the molten steel, the supply of sulfur from the auxiliary material to the molten steel and the desulfurization from the molten steel according to the present invention proceed simultaneously, and the effect is unstable. It may become. It is desirable to carry out the present method as the first step (that is, prior to degassing and component adjustment) after 2 minutes have passed since the molten steel reflux was started in the RH treatment.

  Since REM which is a strong deoxidizing element is used in the present invention, the effect can be further enhanced by setting the Al concentration in the molten steel before addition of REM to 0.05% or more and 0.15% or less. When the Al concentration is less than 0.05%, several ppm of dissolved oxygen is present in the molten steel, so that part of the added REM reacts with these. On the other hand, if the Al concentration exceeds 0.15%, the effect is saturated.

In addition, the ladle slag is preferably CaO—Al ₂ O ₃ —SiO ₂ based, and the CaO / Al ₂ O ₃ mass concentration ratio is 1.2 or more, and the CaO / SiO ₂ mass concentration ratio is 5 or more. It is desirable to be. The slag is excellent in sulfur absorption capacity, and further, the effects of the present invention can be further stabilized by controlling each mass concentration ratio. In addition, as a method for adjusting the mass concentration ratio, there is a method of adding a medium solvent such as alumina or silica at the time of steel leaving the converter. Furthermore, the effect can be further enhanced by setting the total concentration of FeO and MnO in the slag to 5% or less.

  Furthermore, sol. After adjusting the Al concentration to 0.005% or more, preferably 0.05% or more, and performing the reflux time of RH treatment for 3 to 10 minutes before adding REM, the O concentration in the molten steel can be reduced to 30 ppm or less. Therefore, it is more preferable. During this time, the components may be adjusted by adding an alloy or the like. Further, the O concentration can be reduced to 30 ppm or less by carrying out inert gas stirring / smelting under atmospheric pressure before the RH treatment. In addition, although it is desirable that the O concentration before REM addition is 30 ppm or less, the cleanliness after the treatment according to the present invention can be increased by setting it to 20 ppm or less. This is because, in the present invention, since CaO-based flux is added all at once, a large amount of CaO is temporarily present in the vacuum chamber of the RH vacuum degassing apparatus, and during this time, inclusions in the molten steel are adsorbed. Conceivable.

  REM is preferably added to the molten steel in the vacuum chamber in order to avoid oxidation of REM by the atmosphere. The REM used includes lanthanoid metals such as La, Ce, and Nd, or a mixture, alloy, or misch metal composed of two or more of these. Further, impurities such as Al, Ca and Mg can be allowed up to 5% in total. When the content of impurities exceeds 5%, the deoxidation reaction with elements such as Al and Ca proceeds, and the effect may become unstable.

  Although the conditions for the RH treatment at the time of REM addition are not particularly specified, the pressure in the vacuum chamber may be 4 kPa or more, or a predetermined value of 6.5 kPa or more because it is not the purpose of degassing as in the technique disclosed in Patent Document 4. The effect of can be obtained. However, since it is necessary to maintain a molten steel reflux, it must be 10 kPa or less.

  Subsequently to the addition of REM, a flux mainly composed of CaO (CaO-based flux) is added all at once from a hopper or the like instead of the powder top blowing method via the top blowing lance. The time required for addition is required to be within 1 minute, and more preferably within 30 seconds. The time from the addition of REM to the addition of CaO-based flux is preferably 2 minutes or more and 5 minutes or less. If it is less than 2 minutes, the added REM does not have a uniform concentration in the molten steel, and the effect may become unstable. If the time is longer than 5 minutes, the REM concentration in the molten steel decreases, and a sufficient effect may not be obtained.

  The composition of the CaO-based flux to be added is preferably such that CaO has a purity of 80% by mass or more. When the purity is lower than this, the total amount of flux to be added increases and the slag thickness increases, so that the dip tube of the RH vacuum degassing apparatus may not reach the molten steel, and the RH vacuum degassing process may be difficult. is there. Substances allowed up to 20% by mass in the CaO flux include oxides such as Mg, Al, and Si, and unavoidable metal components, but of course, the lower the sulfur or sulfur compound, the better.

  The shape of the CaO-based flux may be any form such as a powder or a lump of several millimeters to a few centimeters, but is preferably a lump in order to suppress dissipation to the exhaust system.

  The conditions of the RH-type vacuum degassing treatment at the time of adding CaO-based flux are not particularly defined, but since the purpose is not to degas as in the technique disclosed in Patent Document 4, the pressure in the vacuum chamber may be 4 kPa or more, A predetermined effect can be obtained even at 6.5 kPa or more. However, since it is necessary to maintain a molten steel reflux, it must be 10 kPa or less.

  The CaO-based flux may be added from an alloy cutting device (hopper) installed in the vacuum chamber, but if the capacity of the hopper is limited, it is only twice when the amount of CaO-based flux added exceeds 8 kg / ton. It may be added separately. When the amount of CaO-based flux exceeds 8 kg / ton, the amount added per time increases to 4 kg / ton, and the difference between the one time addition and the second time addition shown in FIGS. is there. However, the time until the first addition is preferably within 2 minutes to within 5 minutes after the addition of REM, and the interval between the first addition and the second addition is preferably within 1 minute, and more preferably within 30 seconds. .

After adding the CaO-based flux, 0.1 Nm ³ / t or more of oxygen gas is blown onto the molten steel from an oxygen blowing lance installed on the molten steel in the vacuum chamber of the RH vacuum degassing apparatus. The amount of oxygen gas to be blown is 0.1 to 0.6 Nm ³ per ton of molten steel for REM removal. However, in the present invention, REM is added, CaO is added, and then alloy iron is added to adjust the components. Since it does not prevent the temperature adjustment by blowing up oxygen in the presence of Al, when oxygen is blown up for component adjustment or temperature adjustment, the amount of oxygen gas sprayed for that purpose is the above-mentioned REM removal. The necessary oxygen amount is blown up in combination with the oxygen gas for use.

Moreover, when the S concentration in steel before addition is 35 ppm or less and the O concentration in steel is 30 ppm or less and the amount of REM added is suppressed to 0.05 to 0.1 kg / t, residual REM in molten steel Since the concentration is low, it is sufficient that the amount of oxygen gas blown up for REM removal is 0.1 Nm ³ / t or more and 0.3 Nm ³ / t or less. However, in this case as well, when oxygen is blown up for component adjustment or temperature adjustment, the oxygen gas blowing amount for that purpose is combined with the oxygen gas for removing REM described above to blow up the required oxygen amount. Will do.

In the RH process, processes such as degassing, component adjustment, and temperature adjustment are arbitrarily selected and performed in an arbitrary order. However, a predetermined amount of REM is added to molten steel in the predetermined composition range according to the present invention. And the addition of CaO, and after spraying 0.1 Nm ³ / t or more of top blowing oxygen to the molten steel, the molten steel is refluxed for 10 minutes or more. The order of molten steel component adjustment, REM addition, CaO addition, top blowing oxygen spraying, molten steel reflux operation to this predetermined composition range needs to be managed as a unit, but other degassing, component adjustment, temperature adjustment, etc. This process may be performed before the molten steel component adjustment to the predetermined composition range, or after the CaO addition. Further, the top blowing oxygen spraying and the molten steel reflux operation according to the present invention may be performed in common with other processes such as degassing, component adjustment, and temperature adjustment.

  Therefore, the molten steel component adjustment, REM addition, and CaO addition to the predetermined composition range according to the present invention should be performed in common with other processes such as degassing, component adjustment, and temperature adjustment at the beginning of the RH treatment. Therefore, it can be said that it is advantageous in terms of the operation efficiency of the RH treatment.

1) Molten steel desulfurization treatment Hot metal that has been subjected to hot metal desulfurization and hot metal dephosphorization treatment in advance is placed in a 230-ton (t) scale bottom-bottom converter, decarburized, and steel is discharged into a ladle. did. Various deoxidizers and alloys are added at the time of steel removal, and the molten steel components in the ladle are changed to C: 0.04 to 0.07%, Si: 0.1 to 0.3%, Mn: 0.5 to 0.00. 6%, S: 20-25 ppm, sol. Al: 0.05 to 0.07%. Furthermore, CaO was added at the time of steel output, and the CaO / Al ₂ O ₃ weight ratio in the slag was adjusted to 1.9 to 2.1, and the total concentration of FeO and MnO in the slag was adjusted to 5% or less.

  Thereafter, the ladle was transferred to the RH type vacuum degassing apparatus, and the inside of the vacuum chamber was immediately depressurized to 4 kPa, and the reflux treatment was started. After 3 minutes from the start of treatment, REM was added to the molten steel in the vacuum chamber in the amount shown in Table 2. The added REM is an alloy of Ce and La, the composition of which is 65 mass% for Ce and 35 mass% for La. After 3 minutes from the addition of REM, massive CaO-based flux was added to the molten steel in the vacuum tank in the amount shown in Table 2. Note that the CaO-based flux used in this example is 98% or more of pure CaO. When the CaO flux addition amount was 5 kg / ton or more, it was added in two portions, and when it was less than 5 kg / ton, it was added all at once. Samples are taken from the molten steel in the ladle before the RH treatment and from the molten steel in the ladle immediately after the completion of the RH vacuum degassing treatment, and the sulfur concentration and the REM concentration in the molten steel are quantified. Calculated. The results are shown in Table 2.

  Test with addition of REM and CaO flux, while desulfurization rate when adding only CaO flux of test number 15 is 67%, desulfurization rate when adding only REM of test number 16 is 15% In numbers 1 to 14, a desulfurization rate of 80% or more was obtained, and it can be seen that the desulfurization rate is improved by adding CaO after REM addition.

  However, in Test Nos. 12 and 14, the amount of REM added was as small as 0.03 kg / ton, so the desulfurization rate was slightly low. In Test Nos. 13 and 14, since the addition amount of CaO was as small as 4 kg / ton, the desulfurization rate was slightly low.

  In addition, from the results of test numbers 1 to 6, when the REM addition amount is 0.05 to 0.10 kg / ton, the residual ratio is substantially 0.1% or less, and the REM addition amount is relatively small. At 0.20 to 0.30 kg / ton, the residual rate was slightly high at 1.3 to 1.4%. Therefore, in order to satisfy the desulfurization rate and the residual rate at the same time, the amount of REM added is preferably 0.05 kg / ton or more and 0.1 kg / ton or less. However, if it is assumed that oxygen for removing REM is used, REM is used. It can be said that it was confirmed that the low sulfidation treatment was easily achieved in the range of 0.05 kg / ton to 0.30 kg / ton.

  Furthermore, from the results of test numbers 7 to 10, the desulfurization rate was stable at 84 to 86% when the CaO flux addition amount was 5 to 17 kg / ton. From this, it was confirmed that the addition amount of CaO-based flux was 5 kg / ton or more and 20 kg / ton or less, and it was confirmed that it was almost the same at 5 kg / ton or more and 15 kg / ton or less.

In addition, it was confirmed from Test No. 11 that when both the REM addition amount and the CaO-based flux addition amount are large, the desulfurization rate increases, but the residual rate also increases.
2) Residual REM removal and prevention of nozzle clogging Hot metal that has been subjected to hot metal desulfurization and hot metal dephosphorization as needed is placed in a 250-ton scale top-bottom blowing converter, decarburized, and discharged to the ladle. Made of steel. Various deoxidizers and alloys are added at the time of steel output, and the molten steel components in the ladle are changed to C: 0.04 to 0.07%, Si: 0.001 to 0.003%, Mn: 0.5 to 0.00. 6%, S: 20-25 ppm, sol. Al: 0.200 to 0.220%. Furthermore, CaO was added at the time of steel output, and the CaO / Al ₂ O ₃ weight ratio in the slag was adjusted to 1.9 to 2.1, and the total concentration of FeO and MnO in the slag was adjusted to 5% or less.

Then, the ladle was transferred to the RH type vacuum degassing apparatus, and the inside of the vacuum chamber was immediately depressurized to 4 kPa, and the reflux treatment was started. After 3 minutes from the start of the treatment, 0.1 kg / ton REM was added to the molten steel in the vacuum chamber. The added REM is Nd. After a lapse of 3 minutes from the addition of REM, 12 kg / ton of a massive CaO-based flux is added as a CaO component, and 0.1 to 1.5 Nm ³ / ton of blown oxygen is further blown to remove residual REM and a molten steel temperature of 1580 to After adjusting to 1590 degreeC, the molten steel reflux process was performed for 3 to 15 minutes. Thereafter, continuous casting was performed at a casting speed of 0.5 to 3.6 m / min using a continuous casting machine having a mold size of φ191 to φ360 mm and 6 strands. The results are shown in Table 3.

In Test Nos. 1 to 3, since all the requirements of the present invention were satisfied, continuous casting could be continuously performed for 8 charges as planned.
However, in Test No. 4, the amount of oxygen sprayed after the addition of REM was as small as 0.08 Nm ³ / ton, so the number of charges that could be cast continuously remained at 5 charges. Moreover, in the test number 5, since the molten steel reflux time after blowing up oxygen was as short as 3 minutes, the number of charges that could be continuously cast was only 4 charges.

Claims (2)

In mass%, Mn: 0.1% to 2%, Si: 0.001% to 1%, S: 0.0035% or less, Al: 0.005% to 1%, other alloy components included When refining molten steel with RH vacuum degassing equipment,
After adding one or two or more selected from the group consisting of La, Ce and Nd to the molten steel in a total amount of 0.05 kg / molten steel ton to 0.30 kg / molten steel ton,
A CaO-based flux is added in a quantity of 5 kg / ton to 20 kg / molten steel ton in terms of pure CaO within one minute at a time without going through the top blowing lance from the inside of the vacuum chamber,
After the addition of the flux mainly composed of CaO, oxygen is blown to the molten steel from the top blowing lance in the vacuum chamber at least 0.1 Nm ³ / ton,
A method for desulfurizing a molten steel, wherein the molten steel is refluxed for 10 minutes or more. In mass%, Mn: 0.1% to 2%, Si: 0.001% to 1%, S: 0.0035% or less, Al: 0.005% to 1%, O: 0.003 %, When refining the molten steel containing other alloy components in the RH vacuum degassing equipment,
After adding one or two or more selected from the group consisting of La, Ce and Nd to the molten steel in a total amount of 0.05 kg / molten steel ton or more and 0.10 kg / molten steel ton or less,
A CaO-based flux is added in a quantity of 5 kg / ton to 15 kg / molten steel ton in terms of pure CaO within one minute at a time without going through the top blowing lance from the inside of the vacuum chamber,
After the addition of the flux mainly composed of CaO, oxygen is blown to the molten steel from the top blowing lance in the vacuum chamber at least 0.1 Nm ³ / ton,
A method for desulfurizing a molten steel, wherein the molten steel is refluxed for 10 minutes or more.

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