JP2005089774A - Method for uniforming trace amount of rare-earth element added into molten steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for uniforming a trace amount of REM element added into molten steel by which the yield of the REM can be improved, the treating time can be shortened and the REM concentration can be uniformed as the same degree as before. <P>SOLUTION: Into the molten steel deoxidized with Al or Al-Si, the rare-earth elements of one or more kinds selected from among Ce, La, Pr and Nd are added in a trace amount and this molten steel is stirred for a period of time 0.1-0.9 times of a uniformly mixing time τ. As the uniformly mixing time τ, time is used which is defined as the time in which the concentration of S reaches ±5% of the balancing concentration in the case of using S added into the molten steel as a trace. Since the specific gravity of the REM is large, after deeply dipping into the molten steel with the inertial force of the charging from the upper part, the REM is gradually widened into the whole ladle while carrying in the inertial convection after the stirring-treatment of the molten steel in the ladle is performed and thus, the stirring time is shortened. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for uniformizing a small amount of rare earth elements (hereinafter referred to as REM) added to molten steel in a short time with a high yield.

  In the deoxidized molten steel of Al deoxidation or Al-Si deoxidation, high-alumina clusters with accumulated alumina may be generated, which may cause product defects such as thread. Therefore, there is a technique for preventing the occurrence of product defects by adding REM such as Ce, La, Pr, and Nd to molten steel to prevent the accumulation of alumina to refine the cluster.

  Usually, REM is added to molten steel inside a molten steel pan or the like, and is stirred and homogenized. At this time, it is common knowledge in the industry to ensure a uniform mixing time τ and stir. This uniform mixing time τ is defined as the time when the concentration of the tracer in the molten steel reaches ± 5% of the equilibrium concentration, usually by adding S and possibly Cu or the like as a tracer into the molten steel and stirring. The uniform mixing time τ varies depending on the type and size of the molten steel pan, molten steel temperature, stirring energy, etc., but if these are determined, it is a time that can be measured objectively, and even after the addition of REM, this uniform mixing time τ The molten steel was stirred over time.

  In addition, the proposal for shortening this uniform mixing time (tau) itself was made conventionally, for example, when adding an alloy component from the surface of the molten steel in which stirring by bottom bubbling is performed in Patent Document 1, the gas blowing position is set. A method of shortening the uniform mixing time τ by adding from a plurality of sandwiched positions is disclosed.

  Patent Document 2 discloses a method of shortening the uniform mixing time τ by setting the nozzle position for blowing the stirring gas into the ladle and the diameter of the dip tube so as to satisfy a predetermined mathematical formula. . In any case, however, the molten steel is conventionally stirred for a longer time than the uniform mixing time τ, and there is no patent document that mentions stirring for less than the uniform mixing time τ.

  However, the yield of REM in molten steel when the molten steel to which REM is added is stirred for the uniform mixing time τ or more is only about 30%. This is because the added REM is oxidized and consumed by FeO, MgO, etc. in the slag during stirring, and an excessive amount of REM is required to exhibit the original function of preventing alumina clusters.

Moreover, since the uniform mixing time τ of a large molten steel pan exceeds 60 seconds, a considerably long processing time is required to ensure the uniform mixing time τ after the addition of REM. For this reason, the temperature of molten steel fell in the meantime, the melting loss of the refractory progressed, and various problems, such as the cost of the utility for stirring increased, were caused.
JP-A-7-145421 JP 2000-204421 A

  The present invention solves the above-mentioned conventional problems, can improve the yield of the added REM, can reduce the processing time, and makes the REM concentration in the molten steel uniform as in the conventional case. It is made in order to provide the homogenization method of the trace amount REM added in the molten steel which can be performed.

  As a result of repeated studies to solve the above-mentioned problems, the present inventors have found that the specific gravity of REM or REM alloy (Fe-Si-REM) added in molten steel is 6.7 and 6.0, respectively. It was clarified that it is much heavier than 2.3 of S.7 and S and close to the specific gravity of the molten steel, and therefore the behavior during stirring is different from that of S having a small specific gravity.

  The present invention has been made on the basis of the above-described knowledge, and a trace amount of one or more rare earth elements selected from Ce, La, Pr, and Nd are added to Al deoxidized or Al-Si deoxidized molten steel. And stirring for a time that is 0.1 to 0.9 times the uniform mixing time τ. In addition, it is preferable to use what was defined as time which reached | attained +/- 5% of equilibrium density | concentration by using S added in molten steel as a tracer as uniform mixing time (tau). The molten steel can be stirred by any of CAS (component fine adjustment equipment), RH (vacuum degassing equipment), and bottom bubbling (gas blowing from the furnace bottom).

  According to the method for homogenizing a small amount of REM added to the molten steel of the present invention, the molten steel may be stirred for a short time of 0.1 to 0.9 times the conventionally used uniform mixing time τ. Therefore, the possibility that the added REM reacts with the slag and is oxidized is reduced, and the yield of REM can be increased to about 40 to 60%. For this reason, the amount of REM to be added in order to exert the same effect can be reduced.

  In addition, according to the method for homogenizing a small amount of REM added to the molten steel of the present invention, it is possible to shorten the processing time, and the temperature of the molten steel is lowered, the refractory of the refractory progresses, Conventional drawbacks such as increased utility costs can be eliminated. Moreover, despite the shortening of the stirring time in this way, the REM concentration in the molten steel has reached an equilibrium concentration as in the conventional case, and can be sufficiently uniformized.

  FIG. 1 is a view showing a state in which a molten steel 2 in a molten steel pan 1 is stirred by an RH (vacuum degassing facility) 3. The molten steel 2 is Al deoxidized or Al—Si deoxidized in the previous step. One or more types of REM 4 selected from Ce, La, Pr, and Nd are added in a small amount from the surface of the molten steel 2 inside the RH 3.

  In the present invention, the stirring time after the addition of REM4 is a short time that is 0.1 to 0.9 times the uniform mixing time τ. As described above, the uniform mixing time τ is defined as the time when the concentration of the tracer reaches ± 5% of the equilibrium concentration, with S added in the molten steel as the tracer. In the present invention, the stirring time is set to 0.1 to 0.9 times the uniform mixing time τ. If the stirring time is shorter than this, stirring is insufficient and the REM concentration in the molten steel becomes non-uniform. If the length is longer, the reaction with the slag 5 proceeds to cause the same disadvantage as in the prior art. After the above stirring is completed, the molten steel pan 1 is left still.

  FIG. 2 is a graph in which the horizontal axis represents stirring time / uniform mixing time τ, and the vertical axis represents concentration / equilibrium concentration values. In this case, stirring was continued until the uniform mixing time τ reached 1.5 times. As shown in the graph, S normally used as a tracer reaches an equilibrium concentration when the stirring time reaches the uniform mixing time τ. However, REM approaches the equilibrium concentration much earlier than S.

  FIG. 3 is a graph in the case where stirring is performed for a time equal to 0.1 times the uniform mixing time τ after the addition, and then the mixture is left standing, and the horizontal axis and the vertical axis are the same as those in FIG. From this graph, it can be seen that when the stirring time is short, S does not reach the equilibrium concentration, but REM rapidly reaches the equilibrium concentration.

  As described above, REM or REM alloy (Fe-Si-REM) has a specific gravity of 6.7 and 6.0, respectively, as compared with S, which is usually used as a tracer. This is probably because it is much heavier than S 2.3 and close to the specific gravity of molten steel. That is, it is considered that REM enters deeply into the molten steel due to the inertial force applied upward, and then gradually spreads over the entire pan on the inertial convection even after the stirring treatment of the molten steel. Therefore, even if REM does not stir over the uniform mixing time τ or more in the stirring step, it can be sufficiently uniformed in the molten steel pan thereafter.

  FIG. 4 is a graph in which the horizontal axis represents the stirring time / uniform mixing time τ, and the vertical axis represents the REM yield. As shown in this graph, the REM yield gradually decreases as the stirring time becomes longer, and the REM yield decreases to about 30% when the stirring is performed for the uniform mixing time τ or longer as in the conventional case. However, if the value of the stirring time / uniform mixing time τ is 0.1 to 0.9 as in the present invention, the REM yield can be increased to about 40 to 60%. For this reason, according to this invention, the addition amount of REM can be decreased rather than before.

  FIG. 5 is a graph in which the horizontal axis represents the stirring time / uniform mixing time τ, and the vertical axis represents the time required until the REM concentration becomes uniform / uniform mixing time τ. As shown in this graph, when the stirring time is made equal to the uniform mixing time τ, the time required until the REM concentration in the molten steel becomes uniform is minimized. On the other hand, when the value of the stirring time / homogeneous mixing time τ is 0.1 to 0.9 as in the present invention, the time required until the REM concentration in the molten steel becomes uniform is 3 to 9 times. It becomes. This indicates that the stirring itself may be short, but it is necessary to take a long standing time thereafter. However, since the molten steel can be left for a sufficient time in the post-stirring process, this point is not a problem in practice.

  As described above, the stirring time is preferably short from the viewpoints of REM yield, prevention of temperature drop of molten steel, prevention of refractory melting, cost reduction of stirring utility, etc., and uniform REM concentration in molten steel quickly. The longer one is preferable for achieving the above. Considering these comprehensively, in the present invention, stirring is performed for a time that is 0.1 to 0.9 times the uniform mixing time τ.

  In addition, the stirring method of molten steel can employ | adopt arbitrary methods, such as CAS and BB (bottom bubbling) other than RH shown in this embodiment. Examples of the present invention are shown below.

  First Example: 280 tons of 1600 ° C. ultra-low carbon steel poured into the ladle shown in FIG. 1 was stirred by RH. Its uniform mixing time τ is 70 seconds. After adding 10 kg of Fe-Si-REM having the composition shown in Table 1 to the molten steel, the mixture was stirred for 60 seconds. The value of the stirring time / uniform mixing time τ is 0.86. After 300 seconds from the end of stirring, it was confirmed that REM was uniformly mixed. The REM yield was 50%.

  Second Example: 280 tons of 1600 ° C. ultra-low carbon steel poured into the ladle shown in FIG. 1 was stirred by RH. Its uniform mixing time τ is 70 seconds. After adding 10 kg of the same Fe-Si-REM as in the first example to the molten steel, the mixture was stirred for 10 seconds. The value of the stirring time / uniform mixing time τ is 0.18. After 600 seconds from the end of stirring, it was confirmed that REM was uniformly mixed. The REM yield was 60%.

  First comparative example: 10 kg of Fe-Si-REM was added to the molten steel under the same conditions as those of the first and second examples, and then allowed to stand without stirring. In this case, the value of the stirring time / uniform mixing time τ was 0, and uniform mixing was not performed even after 1200 seconds.

  Second Comparative Example: After adding 10 kg of Fe-Si-REM into the molten steel under the same conditions as in the first and second examples, the mixture was stirred for 150 seconds. In this case, the value of the stirring time / homogeneous mixing time τ was 2.14, and the REM yield decreased to 30% even though the mixture was uniformly mixed.

  Third Comparative Example: After adding 10 kg of shot Al having the composition shown in Table 2 to molten steel having the same conditions as those in the first and second examples, the mixture was stirred for 60 seconds. In this case, the value of stirring time / homogeneous mixing time τ was 0.86 as in the first example, but since the added metal was not REM, uniform mixing was not carried out even after 1200 seconds.

It is sectional drawing which shows embodiment of this invention. It is a graph which shows the relationship between the stirring time at the time of continuing stirring, and a density | concentration. It is a graph which shows the relationship between the stirring time at the time of stopping stirring on the way, and a density | concentration. It is a graph which shows the change of the REM yield by stirring time. It is a graph which shows the change of time until the REM density | concentration by stirring time becomes uniform.

Explanation of symbols

1 Molten steel pan 2 Molten steel 3 RH (vacuum degassing equipment)
4 REM
5 Slag

Claims (3)

  A small amount of one or more rare earth elements selected from Ce, La, Pr, and Nd are added into Al deoxidized or Al-Si deoxidized molten steel, and the uniform mixing time τ is 0.1 to 0.9 times as large. A method for homogenizing a trace amount of rare earth elements added to molten steel, characterized by stirring for a period of time.   2. The method for homogenizing a trace amount of rare earth elements added to molten steel according to claim 1, wherein the uniform mixing time τ is defined as the time when S added to the molten steel reaches the ± 5% of the equilibrium concentration using S as a tracer. .   The method for homogenizing a trace amount of rare earth elements added to molten steel according to claim 1 or 2, wherein the molten steel is stirred by any one of CAS, RH, and bottom bubbling.

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