JP2011026659A - Method for controlling of lanthanoid concentration in molten steel, method for simultaneously contolling lanthanoid concentration and non-metallic inclusion composition in molten steel, and method for treating molten steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for treating molten steel which improves and stabilizes the accuracy of a yield of the lanthanoid and a control of the inclusion composition. <P>SOLUTION: In the method for treating the molten steel, in which the lanthanoid of 0.1-1.5 kg/ton and Ca of 0.1-1.0 kg/ton are simultaneously added into the molten steel containing, by mass, ≤0.005% S and ≤0.005% O; this method for controlling the lanthanoid concentration in the molten steel has the peculiarity, in which the mixing ratio of the lanthanoid and the Ca is made to be mass ratio of 0.16-1.0, or the method for simultaneously controlling the lanthanoid concentration and the non-metallic inclusion-formation in the molten steel, has the peculiarity, in which the mixing ratio of the lanthanoid and the Ca is made to be mass% of 0.23-1.0. In these methods, an adding speed of the Ca pure content can be made to be 0.01-0.06 kg/(ton*min). <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention improves the concentration controllability of lanthanoids in molten steel at the same time by stabilizing the yield of lanthanoids in molten steel at a high level in the process of adding lanthanoids such as La, Ce, Nd to the molten steel, and at the same time inclusions The present invention relates to a refining method for improving castability by form control (in the present invention, a concept including “inclusion composition control”, hereinafter also simply referred to as “inclusion control”).

  Lanthanoids (atomic numbers 57 to 71, hereinafter referred to as “LN”) such as La, Ce, Pr, and Nd are generally called rare earth elements (REM). LN has a strong affinity for light elements such as oxygen (O), sulfur (S), and phosphorus (P), and thus reacts with these light elements in steel to reduce the adverse effects of light elements on steel materials. It is well known to improve steel performance.

  For this reason, LN-added steel has been widely known, and many addition methods and additives have been developed. Even in recent years, steel materials having excellent performance by addition of LN and methods for producing the same have been actively developed.

  For example, Patent Document 1 discloses a method for producing high-strength steel excellent in weld crack resistance with addition of LN, and Patent Document 2 has excellent toughness and brittle crack stopping characteristics in a weld heat-affected zone added with LN and Ca. Japanese Patent Application Laid-Open No. H10-260260 discloses a thin steel plate having excellent surface cracking resistance during hot rolling to which REM, Ca, and Mg are added.

  In these prior arts, the generation of coarse sulfide inclusions due to the addition of excess LN is commonly cited as a technical problem. Since this coarse inclusion causes a decrease in the toughness and ductility of the steel material, it is necessary to suppress the generation thereof.

  As a method of suppressing the formation of coarse inclusions due to such excessive addition of LN, a method of controlling the LN concentration in steel to an upper limit value or less is common. The upper limit concentration of the LN concentration varies depending on the use of the steel material and the component of the base material, but is 0.02 to 0.1% by mass. Hereinafter, “mass%” for the component composition of steel, molten iron, etc. is also simply expressed as “%”.

  As described above, when LN is added to a steel material, generation of coarse inclusions is a problem, but there is a further problem of productivity. Sulfide inclusions produced when LN is added cause clogging of the ladle or immersion nozzle during casting of the slab. Therefore, when this inclusion is generated, continuous casting becomes impossible, castability is lowered, and productivity may be deteriorated.

  A number of techniques have been devised for avoiding such deterioration of castability due to the clogging of the immersion nozzle. For example, Patent Document 4 discloses a method for avoiding nozzle clogging by reducing oxide inclusions and Patent Document 5 by reducing sulfide inclusions.

  As described above, LN is an element effective for improving the performance of a product (steel), but if its usage is inappropriate, the product is deteriorated due to the coarse inclusions produced or produced due to a decrease in castability. It is also an element that causes problems such as deterioration of sex. Many techniques for avoiding these problems have been proposed.

JP 2006-342421 A JP 2008-88487 A JP 2007-254828 A Japanese Patent Laid-Open No. 2001-20033 JP 2001-60739 A

  However, when a high LN concentration is required for a product, or when inexpensive production due to an improvement in productivity is required, the conventional technology may not be able to cope with it sufficiently.

  For example, the technique of setting the LN concentration in steel to a certain upper limit value or less cannot be applied to steel that requires an LN concentration exceeding that concentration.

  In the technique shown in Patent Document 5, it is necessary to control O concentration and S concentration in molten steel before addition of LN, and it is also necessary to measure these concentrations before addition of LN. There were limits in terms of cost and productivity.

  Further, in Patent Document 2, although the order of addition of LN, Ca, Zr, and the like has been studied, since the described technique is a technique for controlling the upper limit concentration of LN, details of the order of addition are not shown. Absent. In general, LN and other elements are often added separately to molten steel, and even in the example of Patent Document 2, for example, an example in which REM and Ca are added simultaneously is not shown. Such divided addition of elements leads to an increase in the total processing time and a decrease in molten steel temperature, and it has been difficult to significantly improve product productivity.

  Furthermore, in addition to the above-mentioned inclusions and productivity problems, there is also a problem in the accuracy of molten steel processing. As described above, LN has a strong affinity with light elements such as O and S, so when added to molten steel, it reacts with O and S to react with oxides, sulfides, and sulfates of LN. To form inclusions. As a result, the LN concentration in the molten steel decreases, but this decrease varies depending on the O concentration, S concentration in the molten steel, and the balance between the two. In other words, the yield of LN tends to fluctuate and is unstable, and it is difficult to process molten steel with high accuracy. For this reason, the conventional technology that manages the LN concentration, O concentration, and S concentration in molten steel has not been sufficient in yield stability.

  The destabilization of the yield of LN causes the destabilization of the LN concentration in the molten steel, and as a result, the accuracy of inclusion control is also lowered, which may affect the performance of the steel material.

  In view of the above-described problems of the prior art, an object of the present invention is to provide a molten steel processing method that improves and stabilizes the yield of LN and the accuracy of inclusion control. Another object of the present invention is to improve the productivity by further increasing the upper limit of the LN concentration that has been conventionally allowed.

  In order to solve the above-mentioned problems, the present inventors have investigated and analyzed deoxidation, desulfurization reaction and inclusion composition change at the time of LN addition, and obtained the following findings (a) to (d). Thus, the present invention has been completed.

(A) Appropriate range of S and O concentration of target molten iron S: 0.005% or less Sulfur (hereinafter also simply referred to as “S”) is LN when its concentration exceeds 0.005% in molten iron. The amount of sulfide generated increases regardless of the concentration. S, like LN, also has a strong affinity for Ca, so the amount of CaS produced increases, preventing an increase in Ca concentration, which will be described later. Therefore, the appropriate range of S concentration is set to 0.005% or less.

O (oxygen): 0.005% or less Oxygen (hereinafter, also simply referred to as “O”) increases the amount of oxide produced in molten iron exceeding 0.005%, regardless of the LN concentration. Become more. O, like LN, has a strong affinity with Ca, so the amount of CaO produced increases and prevents an increase in Ca concentration, which will be described later. Therefore, the appropriate range of O concentration is set to 0.005% or less.

(B) Improvement of LN yield instability (lower limit of mixing ratio of LN and Ca)
The factor of destabilizing the yield of LN is due to the reaction between LN and S and O in the molten steel. Therefore, in order to improve the instability of the yield of LN, the reaction with S and O may be suppressed. The following two methods can be considered as this method.

  The first method is a method of sufficiently reducing S and O in molten iron, or adjusting the addition amount of LN or LN concentration according to the S concentration and O concentration in molten iron, and reacts in a material balance. It is a method of suppressing. This method is a method mainly used in the prior art.

  The second method is a method of suppressing the reaction between O, S and LN using a chemical reaction. Specifically, a method of adjusting the temperature of the molten iron, an element different from LN having an affinity for O and S that is stronger than the affinity between LN-O and LN-S (hereinafter referred to as “different element”) Or a method of adding flux. Temperature control of the former molten iron is almost impossible industrially. Among the latter, addition of flux can be expected to be stable, but there is a concern that the reaction rate is slow and the productivity is poor. Therefore, it was decided to study a method of adding another element among the latter.

  From the viewpoint of thermodynamic data, candidates for different elements include alkaline earth metals such as Ba, Mg, and Ca, and numerous elements such as Zr. In the present invention, the cheapest Ca is selected. However, since Ca has a boiling point lower than the molten steel processing temperature, there is a problem that its own yield is unstable.

  Next, from the viewpoint of productivity, it is preferable to add LN and Ca simultaneously rather than separately adding them to the molten iron. If each of LN and Ca is a general element that has a high yield and is stable and the reaction of one element does not affect the reaction of the other element, the two are mixed according to the target concentration. It is easy to add all at once.

  However, since LN and Ca are both elements whose yields are unstable and the reactions of the two influence each other, it is not easy to add them all at once. In particular, the cause of unstable LN yield is the reaction between LN and O, S, whereas the cause of unstable Ca yield is evaporation in addition to the reaction between Ca, O, and S. . Further, if the reaction between LN and O, S proceeds, the behavior of Ca reacting with O, S also changes, and conversely, that is, LN reacting with O, S due to the progress of the reaction between Ca, O, and S. Changes in the behavior also occur.

  Therefore, LN and Ca cannot be simply mixed and added together according to the target concentration in the same manner as general elements.

  Therefore, the inventors experimentally investigated and analyzed the influence of the mixing ratio of LN and Ca. The experiment was performed as follows. An experiment was conducted in which LN and Ca were mixed and added to 15 kg of molten steel in which the concentrations of S and O were adjusted to 0.005% or less, respectively, and the molten steel components and inclusion components were investigated. Molten steel temperature was 1848-1898K. In this experiment, metal La, metal Ce, metal Nd, and misch metal were used as LN, and a CaSi alloy having a pure Ca content of 33% was used as Ca. The addition amount of LN (per 1 ton of molten steel) is 0.1 kg / ton or more and 1.5 kg / ton or less, and the addition amount of Ca (per 1 ton of molten steel) is 0.1 kg / ton or more and 1.0 kg / ton in terms of pure Ca. It was as follows.

  LN and Ca were adjusted to a predetermined ratio and mixed well in advance, then wrapped in iron foil and added to the molten steel at once, then a molten steel sample was taken, and after quenching, the molten steel composition and the inclusion composition were quantified. The composition of inclusions was measured by SEM-EPMA.

(B) -1. About lower limit of Ca / LN mixture ratio (LN yield standard)
An experimental result using metal Nd as LN will be described. By the addition of LN (Nd) and Ca in the above amounts, the Nd concentration in the molten steel was 0.03% to 0.17%, and the Ca concentration in the molten steel was 0.0015% to 0.0045%. An example of the experimental results is shown in FIG.

  FIG. 1 is a graph showing the relationship between the Ca / Nd mixture ratio and the Nd yield. In FIG. 1, Ca / Nd mixture ratio = 0 is an experimental result in which only Nd is added without mixing CaSi. As can be seen from FIG. 1, when the Ca / Nd mixture ratio is less than 0.16, the Nd yield is low and unstable. However, when the Ca / Nd mixture ratio is 0.16 or more, the Nd yield is improved and stabilized. Furthermore, when the Ca / Nd mixture ratio is 0.31 or more, the Nd yield is further stabilized.

  Since the same result as Nd was obtained with Ce, La, and Misch metal, if the S / O concentration in the molten steel is 0.005 or less by setting the Ca / LN mixing ratio to 0.16 or more, It can be seen that the LN yield can be stabilized regardless of the S, O concentration and LN addition amount in the molten steel, and as a result, the control accuracy of the LN concentration is improved. When the Ca / LN mixture ratio is 0.31 or more, the effect of further stabilizing the LN yield is enhanced.

(B) -2. About lower limit of Ca / LN mixing ratio (inclusion composition standard)
The composition analysis of inclusions was performed on the molten steel to which metal Nd was added as the above LN, and the Ca / Nd molar ratio in the inclusions was measured.

  FIG. 2 is an example of the composition analysis result of inclusions, and is a graph showing the relationship between the Ca / Nd mixture ratio and the Ca / Nd molar ratio in inclusions. The composition of inclusions (Ca / Nd molar ratio in inclusions) showed the same tendency as the Nd yield shown in FIG. 1 with respect to the Ca / Nd mixture ratio. However, the absolute value of the Ca / Nd mixing ratio is slightly different. When the Ca / Nd mixing ratio is 0.23 or more, the inclusion is modified to a Ca-based inclusion having a low melting point, and the immersion nozzle is clogged. It is difficult and castability is improved. When the Ca / Nd mixing ratio is 0.31 or more, the effect of improving castability is further stabilized.

  Since the same result was obtained with Ce, La, and Misch metal, the Ca / LN mixture ratio must be 0.23 or more in order to stably perform inclusion control for improving castability. The effect of improving castability is further stabilized by setting the / LN mixing ratio to 0.31 or more.

(B) -3. Reasons for the two lower limits of the mixing ratio of Ca and LN As described above, the Ca / LN mixing ratio is 0.16 based on the LN yield and 0.23 based on the inclusion composition. There are two lower bounds, and this reason is estimated as follows.

  LN and Ca react competitively with O and S in molten steel. For example, the reaction is governed by LN-O deoxidation equilibrium, Ca-O deacidification equilibrium. If Ca is relatively insufficient with respect to LN, the reaction between LN and O (LN-O reaction) becomes dominant, and LN is consumed in the oxidation reaction. This LN consumption varies depending on the LN addition amount and the O concentration in the molten steel.

  On the other hand, if Ca is excessive relative to LN, the reaction between Ca and O (Ca—O reaction) becomes dominant, and LN is not consumed by the oxidation reaction. In addition, inclusions generated as a result of the oxidation reaction also change from the LN system to the Ca system. It is considered that the Ca / LN mixture ratio at the boundary between the shortage and excess of the LN yield is 0.16, and the boundary Ca / LN mixture ratio in the inclusion control is 0.23.

  Further, the reason why the absolute value of the lower limit of the boundary Ca / LN mixture ratio is different between the case where the LN yield is used as a reference and the case where the inclusion control is used as a reference is considered as follows. Since the activity of the oxide component in the inclusion is 1 or less, the activity of LN or Ca in the molten steel needs to be higher than the deoxidation equilibrium in the molten steel. For this reason, it is considered that the boundary Ca / LN mixture ratio for inclusion control is larger than the boundary Ca / LN mixture ratio for LN yield. Moreover, since inclusions and molten steel are stabilized by following the same equilibrium reaction, it is considered that the more stable boundary Ca / LN mixing ratio showed 0.31 which is the same value in both cases.

(C) About the upper limit of the mixing ratio of LN and Ca Next, the upper limit of the mixing ratio of Ca / LN will be described. The lower limit of the Ca / LN mixture ratio for achieving the purpose of stabilizing the yield of LN at a high level and controlling the form of inclusions is as described above. As can be seen from FIG. 1 and FIG. Even if the LN mixing ratio is increased beyond the lower limit, the effect does not change within the experimental range.

  However, when FIG. 1 and FIG. 2 are examined in detail, when the Ca / Nd mixture ratio exceeds 1, the Nd yield and the fluctuation range of the Ca / Nd molar ratio in the inclusion increase, that is, the Nd yield. It can also be seen that the Ca / Nd molar ratio in the inclusion is unstable. This is considered to be caused by the fact that the change in the activity of LN or the increase in the saturated oxygen concentration becomes apparent because Ca has an excessively high concentration relative to LN. In order to avoid this instability, the Ca / LN mixing ratio needs to be 1 or less.

(D) About the addition rate of Ca Next, the influence of the addition rate of Ca is demonstrated. As described above, as a cause of destabilization of the Ca concentration, there is an evaporation reaction of Ca from molten steel. Therefore, it was considered that the Ca concentration in the molten steel was further improved by considering the evaporation of Ca.

  Therefore, the effect on the inclusion composition was investigated by changing the rate of addition of pure Ca to the molten steel (unit: kg / (ton · min)). The experimental conditions were as follows: in addition to the molten steel components with S and O concentrations of 0.005% or less, the Ca / Nd mixing ratio was 0.4, and the Nd addition amount to the molten steel was 0.5 to 1.5 kg / ton. It was. The Nd concentration in the molten steel was 0.04 to 0.14%.

  FIG. 3 is a graph showing the relationship between the Ca / Nd molar ratio in inclusions and the Ca pure addition rate obtained in the experiment. The Ca / LN mixing ratio (in this case, the Ca / Nd mixing ratio) satisfies the values specified by the lower and upper limits described in (b) and (c) above. Therefore, as shown in FIG. 3, the Ca / Nd molar ratio in the average inclusion is stable at a high level over the entire range of the Ca pure addition rate in which this experiment was performed. However, in FIG. 3, it can be seen that the inclusion composition distribution is wide on the side where the Ca pure addition rate is low and the side where it is large, and the accuracy of inclusion control is reduced. And in the range where the distribution of inclusion composition is narrow, it can be seen that the addition rate of pure Ca is 0.01 kg / (ton · min) or more and 0.06 kg / (ton · min) or less.

  It is presumed that the stability of the Ca concentration in molten steel depends on the pure Ca addition rate for the following reasons. When the Ca pure addition rate is too low, the evaporation rate exceeds the addition rate, so the Ca concentration in the molten steel during addition cannot be stably maintained. For this reason, the distribution of inclusion composition becomes wide.

  Conversely, when the Ca pure addition rate is excessive, the addition rate exceeds the evaporation rate, so the Ca concentration in the molten steel during addition of Ca is kept very high, but the addition time is shortened. As a result, a high Ca concentration inclusion resulting from a high Ca concentration in the molten steel during the addition of Ca and a low Ca concentration inclusion resulting from a short addition time and a short reaction time are generated. Therefore, the distribution of inclusion composition is widened.

  The present invention has been made on the basis of the above findings, the gist of which is the following (1) method for controlling the lanthanoid concentration in molten steel, and (2) the lanthanoid concentration in molten steel and the nonmetallic inclusion composition in molten steel. Are the simultaneous control method and the molten steel treatment method shown in (3).

(1) A molten steel containing, by mass%, S: 0.005% or less and O (oxygen): 0.005% or less and a lanthanoid of 0.1 kg / ton to 1.5 kg / ton and 0.1 kg / ton lanthanoid concentration in molten steel characterized in that the mixing ratio of lanthanoid and Ca is 0.16 or more and 1.0 or less in mass ratio in the processing method of molten steel to which Ca of ton and 1.0 kg / ton or less is added simultaneously Control method.

(2) In a molten steel containing, by mass%, S: 0.005% or less and O (oxygen): 0.005% or less, a lanthanoid of 0.1 kg / ton to 1.5 kg / ton and 0.1 kg / ton lanthanoid concentration in molten steel, characterized in that, in a method for treating molten steel in which Ca of ton and 1.0 kg / ton or less is added simultaneously, the mixing ratio of lanthanoid and Ca is 0.23 to 1.0 in terms of mass ratio And control method of non-metallic inclusions in molten steel.

(3) In the process of adding lanthanoid and Ca simultaneously and continuously to the molten steel, the addition rate of pure Ca should be 0.01 kg / (ton · min) or more and 0.06 kg / (ton · min) or less. The method for treating molten steel as described in (1) or (2) above,

  According to the method for controlling the lanthanoid concentration in molten steel of the present invention, the yield of LN can be stabilized at a high level by setting the Ca / LN mixing ratio to 0.16 or more and 1.0 or less. According to the simultaneous control method of the lanthanoid concentration in molten steel and the nonmetallic inclusion type in molten steel according to the present invention, the nonmetallic inclusion composition is stabilized by setting the Ca / LN mixing ratio to 0.23 or more and 1.0 or less. Can be controlled accurately. Furthermore, the controllability of the inclusion composition can be further improved by setting the Ca pure component addition rate to 0.01 kg / (ton · min) or more and 0.06 kg · (ton / min) or less.

  Further, the present invention can cope with an LN concentration of 0.1% or more, and contributes to an increase in the variety of manufactured products. At the same time, productivity can be improved by adding Ca and LN in the same process instead of separate processes.

It is a graph which shows the relationship between Ca / Nd mixture ratio and Nd yield. It is a graph which shows the relationship between Ca / Nd mixing ratio and Ca / Nd molar ratio in inclusions. It is a graph which shows the relationship between Ca / Nd molar ratio in inclusions, and Ca pure content addition rate.

  The embodiment of the present invention will be described by taking as an example a case of using a converter and a continuous casting machine. First, the composition of the molten steel will be described.

(A) Molten steel composition (a) -1. As described above, the method of the present invention is intended for molten iron having an S concentration of 0.005% or less and an O concentration of 0.005% or less. It is preferable that the S concentration before addition of LN is 0.0025% or less and the O concentration is 0.0030% or less. Thereby, Nd addition amount and Ca addition amount can further be reduced.

  In order to further improve the controllability of the inclusion composition, the S concentration is preferably 0.0001% or more and 0.001% or less, and the O concentration is preferably 0.0005% or more and 0.0015% or less.

(A) -2. Preferred ranges of C, Si, Mn, Al and other component compositions Next, preferred ranges of C, Si, Mn, Al and other component compositions will be described.

(A) -2-1. C, Si
C and Si are elements having an action of increasing the activity of P in steel when its concentration is high. If the C concentration is higher than 3.5%, the effect on the activity of P becomes significant, and the production conditions of the P compound may change. Therefore, the C concentration may be 3.5% or less. preferable. For the same reason, the Si concentration is preferably 2.5% or less.

  The C concentration is more preferably 0.0015% or more from the viewpoint of securing the steel material characteristics and securing a stable deoxidation action. The Si concentration is more preferably 0.01% or more from the viewpoint of preliminary deoxidation.

(A) -2-2. Mn
Mn is an element having an action of reducing the activity of P in the steel when its concentration is high. If the Mn concentration is higher than 3%, the accuracy of inclusion control may be lowered due to the influence of MnS generation. Therefore, the Mn concentration is preferably 3% or less. The Mn concentration is more preferably 0.2% or more from the viewpoint of securing the steel material strength.

(A) -2-3. Al
Al has an extremely large influence on the dissolved oxygen concentration because of the equilibrium relationship with the dissolved oxygen in the steel. If the Al concentration exceeds 3%, the equilibrium dissolved oxygen concentration increases rapidly, and oxide inclusions increase, which may deteriorate the cleanliness of the steel. Therefore, the Al concentration is preferably 3% or less. Further, the Al concentration is more preferably 0.0035% or more from the viewpoint of improving the yield of LN and ensuring the stability of the yield of LN. In the present specification, “Al concentration” means “concentration of acid-soluble Al (sol. Al)”.

(A) -2-4. Other Elements In the above molten iron, elements such as Ni, Mo, V, Ti, Cr, and B may be contained in the following concentration ranges in place of a part of iron (Fe). This is because these elements have little influence on the reaction between LN, Ca and O, S in molten iron. That is, Ni in a concentration range of 0.01% to 30%, Mo in a concentration range of 0.01% to 1%, V in a concentration range of 0.001% to 0.1%, 0.005% to 0.00. Ti in a concentration range of 3%, Cr in a concentration range of 0.001% to 35%, B in a concentration range of 0.0001% to 0.003%, and the like.

(B) LN and Ca addition method (b) -1. Secondary refining After removing molten steel from the converter into the ladle, or adding alloy elements such as flux, such as CaO, Si, Mn, and Al to the ladle, the ladle is gas blown into the stirring treatment device or RH ( It is transferred to a secondary refining apparatus such as a vacuum degassing apparatus such as a reflux type vacuum degassing apparatus.

  In secondary refining, components such as desulfurization, degassing, and inclusions are removed as necessary, and component adjustment and temperature adjustment are performed by addition of alloy elements. These treatments are optional.

  However, it is desirable that S and O are reduced to the preferred range described above. As a method for reducing S, a method of reacting molten steel and slag by gas stirring using CaO-based slag, a method of blowing a desulfurizing agent such as CaO flux into the molten steel, and a desulfurization flux on the surface of the molten steel with a vacuum degassing apparatus such as RH. There are methods such as spraying, but any method may be used. As a method for reducing O, there are a method of blowing an inert gas into molten steel, a method of dry distillation with a vacuum degassing apparatus such as RH, etc., but any method may be used.

(B) -2. Slag composition and slag amount The preferred range of the slag composition on the surface of the molten steel in the ladle is as follows. The total concentration of FeO, MnO, and Fe ₂ O _{3 in} the slag is 5% or less, more preferably 1.5% or less. When these are high in concentration, they react with LN and Ca, and the accuracy of inclusion control is reduced. In addition, oxygen continues to be supplied from the slag to the molten steel after the secondary refining treatment. Gets worse. These phenomena are referred to as “reoxidation by slag”. In order to suppress this reoxidation, the total concentration of FeO, MnO, and Fe ₂ O _{3 in} the slag is desirably 5% or less. When these total concentrations are reduced to 1.5% or less, reoxidation is almost completely suppressed.

The ratio of the CaO concentration to the Al ₂ O ₃ concentration in the slag (hereinafter also referred to as “C / A”) is preferably 1.0 or more, and more preferably 1.5 or more. When C / A is less than 1.0, the activity of CaO in the slag becomes low, and thus Ca deoxidation in the molten steel may become unstable. Further, when C / A is 1.5 or more, since CaO is saturated in the slag, the activity of CaO becomes 1, and unstable elements can be eliminated.

  The amount of slag (per 1 ton of molten steel) is preferably 10 kg / ton or more and 25 kg / ton or less. If it is less than 10 kg / ton, the amount of slag is too small, and the stability of deoxidation of molten steel due to the slag-metal reaction is lowered. On the other hand, if the amount exceeds 25 kg / ton, the influence of slag becomes excessive, and there is a possibility that even a small amount of MnO in the slag will have an influence.

(B) -3. About the addition of LN and Ca After performing secondary refining such as removal of impurities from steel, adjustment of molten steel composition, adjustment of molten steel temperature, adjustment of slag temperature, etc., using a gas blowing stirring treatment device or a vacuum degassing treatment device such as RH According to the present invention, LN and Ca are added to the molten steel in the ladle.

  LN and Ca are added in accordance with the target concentration of LN. As can be seen from FIG. 1, according to the method of the present invention, since a stable yield of LN can be obtained, the amount of LN added can be easily determined according to the target LN concentration.

  Next, according to the present invention, the Ca addition amount is determined. The amount of Ca added needs to be set in the range defined in (1) or (2) according to the purpose. The Ca addition amount is set to the median value of the specified range unless otherwise specified. When there are restrictions on the molten steel temperature and processing time, the Ca addition amount is set on the lower limit side of the specified range, and on the upper limit side of the specified range to ensure stable castability.

  LN to be added may be any additive such as metal LN such as metal La, alloy such as La-Ce, La-Al, or the like. Any Ca may be used as an additive such as an alloy such as metal Ca, Ca—Fe, Ca—Si, and Ca—Al. Further, the LN content and the Ca content may be previously alloyed, or may be simply mixed.

Furthermore, in addition to these LN and Ca components, oxides such as CaO and Al ₂ O ₃ may be added to the flux as additives. However, the pure LN content and the pure Ca content (also referred to as “Ca pure content”) in the additive need to satisfy the range defined in (1) or (2). The Ca pure component refers to Ca in a metal Ca mixture such as metal Ca or Ca in an alloy, and Ca in a Ca compound such as CaO or CaF ₂ is not added to the Ca pure component.

  Examples of methods for adding LN and Ca include the following methods. One is a method in which the mixture of LN, Ca and other substances satisfying the above (1) or (2) is added to molten steel as an additive. As another method, there is an injection method in which a mixture of the aforementioned LN, Ca, and other substances satisfying the above (1) or (2) is used as an additive and is directly blown into the molten steel together with a carrier gas using a blowing lance immersed in the molten steel. is there. Further, as a further alternative method, there is a wire method in which a mixture of the above-described LN, Ca, and other substances satisfying the above (1) or (2) is filled into an iron-coated wire and fed into molten steel as an additive. .

  Any of these methods may be used as a method for adding LN and Ca, but an injection method or a wire method is desirable. This is because in the case of batch addition, a slight difference in the dissolution rate of LN and Ca may cause a decrease in LN concentration and inclusion control accuracy. On the other hand, in the injection method and the wire method, since LN and Ca are added over a certain period of time, the influence of a slight difference in dissolution rate can be avoided.

  Furthermore, the wire method is desirable to satisfy the addition rate of pure Ca as described in (3) above. This is because the wire method is superior in controllability of the addition rate as compared with the injection method. However, in the case of the wire method, since the stirring of the molten steel becomes insufficient, an inert gas of 0.2 Nl / (min · ton) or more and 1.0 Nl / (min · ton) is used by using a blowing lance during the addition of the wire. It is desirable to blow into the molten steel at the following flow rate. When the amount of inert gas blown is less than 0.2 Nl / (min · ton), the stirring force cannot be obtained, and when it exceeds 1.0 Nl / (min · ton), the amount of the molten steel such as splash is combined with the Ca evaporation reaction. Scattering becomes intense.

(B) -4. Next, the method for controlling the addition rate of pure Ca as described in (3) will be described. As described above, the definition of pure Ca as a reference for the addition rate is Ca in a metallic Ca mixture such as metallic Ca or Ca in an alloy, and does not include Ca in Ca compounds such as CaO and CaF ₂ .

  The method for adjusting the addition rate of pure Ca is to set the powder supply rate (unit: kg / min, etc.) in the injection method or the wire feed rate (unit: m / min) in the wire method within the prescribed range of (3). There are ways to adjust it to your satisfaction. Also, there is a method of adjusting the addition rate of pure Ca by mixing a third component such as MgO or Fe in the additive, for example, with the powder supply rate or the wire feed rate being constant. In the case of the latter method of mixing the third component, although the total amount of the additive added to the molten steel may increase, no modification of the addition device is required.

It is most desirable to add pure Ca at an addition rate that satisfies the specified range of (3) from the start of addition to the end of addition. However, when the addition time becomes long, the addition is started at a rate faster than the prescribed range of (3), and even if the addition rate is changed to satisfy the prescribed range of (3) from the middle of the addition, it is constant. An effect is obtained.
(B) -5. Summary

  As described above, it is desirable to add LN and Ca to molten steel according to the method of the present invention after performing the necessary treatment in secondary refining, and then continuously cast. This is because if some molten steel treatment is performed after the addition of LN and Ca, the distribution of the inclusion composition may spread.

  However, the method of the present invention can be carried out in the tundish of a continuous casting machine at the time of continuous casting after completion of the secondary refining treatment using RH or the like. The method implemented in the tundish includes a method of spraying the additive containing LN and Ca according to the present invention to the molten steel falling and flowing into the tundish from the ladle, and a method of spraying the molten steel in the falling inflow portion and the tundish. There are a method of continuously adding an additive on the surface, a method of adding a wire as an additive to molten steel in a tundish, and the like.

  When carrying out the method of the present invention in a tundish, that is, when applying the provisions of (1) or (2) above, after determining the addition amount and the Ca / LN mixing ratio, from the start to the end of the casting of the molten steel During this time, the additive may be constantly added constantly.

  Therefore, when applying the method of the present invention in tundish, when the target LN concentration is determined, the addition amount (unit: kg / ton) of LN is uniquely determined, and the addition amount and casting time (unit: min) are determined. Once determined, the addition rate (unit: kg / (ton · min)) is uniquely determined. Therefore, the rule (3) cannot be applied as it is. However, the addition rate (unit: kg / (molten steel in tundish ton · min)) using the molten steel amount (unit: ton) in the tundish when casting is in a steady state satisfies the above-mentioned provision (3). It is desirable to do.

  In order to confirm the effect of the method of the present invention, the following continuous casting test was performed, and the yield and inclusion composition of the added LN were evaluated.

1. Test conditions 1500 kg of molten steel in an Ar atmosphere of 101 kPa was heated to 1873 K, and its C, Si, Mn, P, S, O, Cr, and Al concentrations were adjusted to the concentrations shown in Table 1.

  La, Ce, and Nd are mixed metals (indicated as “LN” in Table 1) by mixing metal La, metal Ce, and metal Nd at a ratio (mass ratio) of La: Ce: Nd = 4: 3: 3. did. Further, this mixed metal and metal Ca are mixed at a mixing ratio shown in Table 1 (mass ratio; hereinafter, “mixing ratio of mixed metal and metal Ca” is also simply referred to as “mixing ratio”), and this is filled into an iron wire. And added to the molten steel as an additive. After adding an additive to molten steel, the sample was extract | collected from molten steel and the molten steel component (LN, Ca) and the inclusion composition were analyzed.

  Test numbers 1 to 10 are examples of the present invention, test numbers 11 to 20 are comparative examples, and test numbers 21 to 22 are reference examples. Test Nos. 1 to 10, which are examples of the present invention, satisfied the definition of (1), and Test Nos. 4 to 10 satisfied the definition of (2). Test Nos. 3 and 7 to 10 satisfied the above-mentioned rule (3) with respect to the addition rate of pure Ca.

  Test numbers 11 to 22, which are comparative examples or reference examples, did not satisfy any of the provisions of (1) and (2). Test numbers 11 to 16 are comparative examples in which the mixing ratio deviated to the lower side of the specified range of (1), and test numbers 17 to 20 were comparative examples deviated to the higher side. In Test Nos. 21 and 22, only Ca was added without adding Ca. Test Nos. 18 to 20 satisfied the above-mentioned rule (3) with respect to the addition rate of pure Ca. In the columns of (1) to (3) in Table 1, “◯” satisfied the provisions of (1) to (3), and “x” did not satisfy the provisions of (1) to (3), respectively. It shows that.

2. Test results Table 1 shows the analysis results of the yield of LN and the composition of inclusions together with the composition of the molten steel. The yield of LN is calculated as a value (A / B) obtained by dividing the LN concentration (A) obtained by analysis of the molten steel component by the value (B) obtained by dividing the mass of LN added to the molten steel by the mass of the molten steel. In Table 1, it is expressed as a percentage. In Table 1, the analysis results of the inclusion composition are shown as “Ca / LN molar ratio in inclusions” and “inclusion distribution width”. In Table 1, “Ca / LN molar ratio in inclusions” is the average value of the observed Ca / LN molar ratio of inclusions, and “inclusion distribution width” is the maximum Ca / LN mole of the observed inclusions. The difference between the ratio and the minimum Ca / LN molar ratio.

  Test Nos. 1 to 3 are cases where only the provision of (1) was satisfied. Compared with Test Nos. 11 to 17 as comparative examples, the LN yield was high and stable, and Ca / The LN molar ratio was also high.

  Test Nos. 4 to 6 are cases in which the provisions of (2) were satisfied, and the Ca / LN molar ratio in the inclusions was higher than in Test Nos. 1 to 3, and inclusion control further advanced. I understand. In addition, the LN concentration in Test No. 4 is 0.22%, and it can be seen that it is possible to manufacture high lanthanoid steel, which has been difficult to manufacture.

  Test Nos. 7 to 10 are cases where all the provisions of (1) to (3) were satisfied, and the inclusion distribution width was smaller than those of Test Nos. 1 to 6. This result shows that in the test numbers 7 to 10, the inclusions are stably controlled to Ca, and the castability during continuous casting is remarkably improved.

  On the other hand, Test Nos. 11 to 16 are comparative examples in which the mixing ratio deviates to the lower side of the specified range of (1). Compared with Test Nos. 1 to 10, the LN yield and Ca / LN in inclusions The molar ratio was low and the inclusion distribution width was large.

  Test Nos. 17 to 20 are comparative examples in which the mixing ratio deviated to the higher side of the specified range of (1), and the LN yield and the Ca / LN molar ratio in the inclusions both fluctuated. It can be seen that the difference is large and lacks stability.

  As a reference, the results of test numbers 21 and 22 are shown when only LN is added. From this result, it can be seen that the test numbers 21 and 22 have a very low LN yield compared to the test numbers 1 to 20. In addition, in both test numbers 21 and 22, the inclusion distribution width is zero. This indicates that only LN-O-S inclusions were generated as inclusions. As described in the prior art, when such inclusions are produced, casting is difficult due to blockage of nozzles and the like.

  From the above, by following the provision (1), the LN yield is improved and the manufacturing cost of the product is reduced. In addition, by following the provision of (2), it is possible to control the composition of inclusions while ensuring the LN yield, and at the same time, the castability is improved, and at the same time, it is possible to manufacture high lanthanide concentration steel that was difficult to manufacture in the past. It becomes. Furthermore, by following the provision of the above (3), the accuracy of inclusion control is improved, and manufacturing stability and product performance stability can be obtained.

  According to the molten steel treatment method of the present invention, by adding lanthanoid and Ca adjusted to a predetermined ratio, it becomes possible to stabilize the yield of lanthanoid at a high level and at the same time accurately control nonmetallic inclusions. can do.

  Thereby, reduction of lanthanoid basic unit and improvement of castability can be achieved, and lanthanoid-added steel can be mass-produced at a lower cost. Furthermore, high lanthanide steels that have been difficult to manufacture can be manufactured.

  Therefore, the method of the present invention can be widely applied in the steelmaking technical field as a molten steel treatment method capable of refining and supplying molten steel with high productivity and good continuous castability.

Claims (3)

  A molten steel containing, by mass%, S: 0.005% or less and O (oxygen): 0.005% or less, a lanthanoid of 0.1 kg / ton to 1.5 kg / ton and 0.1 kg / ton to 1 A method for controlling the concentration of lanthanoid in molten steel, characterized in that the mixing ratio of lanthanoid and Ca is 0.16 or more and 1.0 or less in mass ratio in the treatment method of molten steel to which 0.0 kg / ton or less of Ca is simultaneously added. .   A molten steel containing, by mass%, S: 0.005% or less and O (oxygen): 0.005% or less, a lanthanoid of 0.1 kg / ton to 1.5 kg / ton and 0.1 kg / ton to 1 In the molten steel treatment method of simultaneously adding 0.0 kg / ton or less of Ca, the mixing ratio of lanthanoid and Ca is 0.23 to 1.0 in terms of mass ratio, and the lanthanoid concentration in the molten steel and in the molten steel A method for simultaneous control of non-metallic inclusion forms.   In the process of adding lanthanoid and Ca to molten steel simultaneously and continuously, the addition rate of pure Ca is 0.01 kg / (ton · min) or more and 0.06 kg / (ton · min) or less. The processing method of the molten steel of Claim 1 or Claim 2.

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