JP2007254818A - Continuous cast slab of aluminum-killed steel and producing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To further surely solve such problem caused by alumina cluster as a nozzle-clogging etc., when a continuous cast slab of aluminum-killed steel is produced. <P>SOLUTION: When the steel slab is produced by continuously casting after adding REM into the molten steel deoxidized with Al, the addition of the REM is performed by using Fe-Si-REM alloy containing ≤30 mass% REM and having ≤10mm grain size at ≤25g/min/ton of the molten steel of additional speed. In the steel slab produced in such way, as the number ratio of oxide-base inclusion of 1-10μm grain size interposed in the steel slab, the oxide-base inclusion of 0.5-15 mass% REM oxide concentration is ≥30%, the oxide-base inclusion exceeding 15 mass% REM oxide concentration is <10% and the rest is the oxide-base inclusion of <0.5 mass% REM oxide concentration. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a continuous cast slab of aluminum killed steel used for, for example, rolling for machine structures, forging materials, automobile steel plates, and the like, and a method for producing the same.

  In general, when casting aluminum killed steel used for rolling for machine structures, forging materials, automobile steel plates, etc., first, the hot metal discharged from the blast furnace is decarburized in a converter to be steel output. Next, Al as a deoxidizing element is added to the discharged molten steel, and the added Al is reacted with oxygen in the molten steel for deoxidation. Then, continuous casting is performed after deoxidation to produce a continuous cast slab. In this way, by casting after deoxidizing sufficiently with Al, the molten steel is solidified without generating gas when casting, so high quality slabs without bubbles or defects inside Can be manufactured.

However, in aluminum killed steel, alumina (Al ₂ O ₃ ) produced by reaction of Al, which is a deoxidizing element, with oxygen aggregates to form a huge alumina cluster. This alumina cluster clogs nozzles such as a nozzle that pours molten steel from the ladle into the tundish (so-called pan nozzle) and a nozzle that pours molten steel from the tundish into the mold (so-called immersion nozzle), making casting difficult, , The casting speed is reduced to remove the deposits, and the production efficiency is lowered. Moreover, since a considerable amount of alumina clusters remain in the manufactured slab, the quality deteriorates. Furthermore, if casting is continued with the submerged nozzle clogged, the discharge flow from the submerged nozzle will be biased, and the mold powder placed on the molten steel will be involved, resulting in product defects.

  Therefore, in order to solve the problems caused by the above-described alumina clusters, Patent Document 1 discloses that REM (lanthanoid elements and Sc, Y are collectively referred to as REM) is added to Al deoxidized molten steel to obtain alumina. A method for obtaining a steel material with few clusters is disclosed.

JP 2004-52076 A

When REM is added to molten steel deoxidized with Al, aggregation and coalescence of alumina particles in molten steel can be suppressed, and the amount of alumina clusters formed by aggregation of alumina particles can be reduced. Such an effect is insufficient if the REM oxide concentration of the oxide inclusions present in the steel slab obtained by casting the aluminum killed steel is less than 0.5% by mass, and the clustering of alumina particles does not occur. It cannot be prevented sufficiently. On the other hand, when the REM oxide concentration in the oxide inclusions exceeds 15% by mass, the oxide inclusions are easily aggregated and coalesced to generate coarse clusters. Therefore, in Patent Document 1, the oxide inclusions (average inclusion composition) are mainly composed of Al ₂ O ₃ and REM oxide, and the content of REM oxide is 0.5 to 15% by weight in weight%. The range.

  However, even if the REM oxide concentration of oxide inclusions is regulated as in Patent Document 1, problems such as nozzle clogging may still occur, and various problems caused by alumina clusters will be further solved. Was desired.

  An object of the present invention is to more reliably avoid problems caused by alumina clusters such as nozzle clogging in the production of continuously cast steel pieces of aluminum killed steel.

  In order to solve the above-mentioned problems, the present inventors proceeded with further consideration, collected a sample from a continuous cast steel slab of aluminum killed steel manufactured by adding REM after deoxidizing Al, and included each sample contained in the sample. The REM oxide concentration of the oxide inclusions was measured and the variation was investigated. As a result, the REM oxide concentration of the oxide inclusions present in the steel slab is not constant for each oxide inclusion and varies, and the REM added to the molten steel after Al deoxidation. It has been found that the REM oxide concentration of each oxide inclusion in the steel slab varies widely or, on the contrary, the variation becomes small.

  And in order to solve problems such as nozzle clogging, it is not sufficient to set the REM oxide concentration (average concentration) of the oxide inclusions in the continuous cast steel slab in the range of 0.5 to 15% by mass. Furthermore, in the number ratio of oxide inclusions having a particle diameter of 1 to 10 μm interposed in the steel slab, oxide inclusions having a REM oxide concentration of 0.5 to 15% by mass are 30% or more, REM It is necessary to regulate the distribution so that oxide inclusions exceeding 15% by mass of oxide are less than 10%, and the rest are oxide inclusions having a REM oxide concentration of less than 0.5% by mass. Obtained knowledge. Moreover, in order to obtain such a continuous cast steel piece, the addition of REM performed on molten steel after Al deoxidation is performed by adding Fe-Si containing a particle size of 10 mm or less, mass% and REM of 30% or less. -It was found that it was necessary to use a REM alloy at an addition rate of 25 g / min or less per ton of molten steel.

  Based on such knowledge, according to the present invention, after adding REM to molten steel deoxidized with Al, a method of continuously casting to produce a steel slab, the addition of the REM is performed with a particle size of 10 mm or less, There is provided a method for producing a continuous cast steel slab characterized by using an Fe-Si-REM alloy containing 30% or less of REM by mass% at an addition rate of 25 g / min or less per 1 ton of molten steel.

  Note that the addition of the REM may be performed by an RH degasser.

  Further, according to the present invention, a steel piece continuously cast after adding REM to molten steel deoxidized with Al, in the number ratio of oxide inclusions having a particle diameter of 1 to 10 μm interposed in the steel piece. 30% or more of oxide inclusions having a REM oxide concentration of 0.5 to 15% by mass, less than 10% of oxide inclusions having a REM oxide concentration of more than 15% by mass, and the rest having a REM oxide concentration of 0.1%. An aluminum killed steel continuous cast slab characterized by being less than 5% by mass of oxide inclusions is provided.

  According to the present invention, the addition of REM performed on molten steel after Al deoxidation is performed under predetermined conditions, so that the occurrence of nozzle clogging and the like can be more reliably avoided, and an aluminum killed steel continuous cast steel slab is suitable. Can be manufactured. The continuously cast steel slab thus produced contains oxide inclusions having a REM oxide concentration of 0.5 to 15% by mass in the ratio of the number of oxide inclusions having a particle size of 1 to 10 μm interposed in the steel slab. Less than 10% of oxide inclusions with a REM oxide concentration of more than 15% by mass and less than 10%, and the remainder becomes oxide inclusions with a REM oxide concentration of less than 0.5% by mass, and the remaining amount of alumina clusters is low. Excellent quality.

Hereinafter, preferred embodiments of the present invention will be described.
In the present invention, when REM is added to molten steel deoxidized with Al and then continuously casted to produce a steel slab, REM is added to Fe containing 30% or less of REM at a particle size of 10 mm or less by mass. -Using a Si-REM alloy, the addition rate is 25 g / min or less per ton of molten steel. By adding REM, agglomeration and coalescence of alumina particles in molten steel can be suppressed, and the amount of alumina clusters formed by aggregation of alumina particles can be reduced. The REM (rare earth metal) in the present invention refers to lanthanoid elements and Sc, Y from La having atomic number 57 to Lu having atomic number 71.

  The steel slab manufactured by the present invention is preferably C: 0.0005-1.5%, Si: 0.005-1.2%, Mn: 0.05-3.0%, P: 0.001 to 0.1%, S: 0.0001 to 0.05%, Al: 0.005 to 1.5%, or Nb: 0.005 to 0.1%, Ti: 0 Carbon steel containing 0.001 to 0.25%, the balance being Fe and inevitable impurities, applied to thin plates, thick plates, steel pipes, sections, steel bars, etc. by applying necessary rolling to the steel pieces it can. The reason why this range is preferable is as follows.

  Since C is a basic element that improves the strength of steel most stably, the content is adjusted in the range of 0.0005 to 1.5% depending on the strength of the desired material. In order to ensure strength or hardness, it is desirable to contain 0.0005% or more, but if it exceeds 1.5%, the toughness is impaired, so 1.5% or less is preferable.

  If Si is 0.005 to 1.2%, pre-treatment is required if it is less than 0.005%, which imposes a large cost burden on refining and impairs economic efficiency. This is because defects occur and surface properties and corrosion resistance deteriorate.

  The reason why Mn is set to 0.05 to 3.0% is that if it is less than 0.05%, the refining time becomes long and the economic efficiency is impaired. If it exceeds 3.0%, the workability of the steel material is greatly deteriorated. It is to do.

  If the P content is less than 0.001%, the hot metal pretreatment will take time and cost and the economic efficiency will be impaired. If it exceeds 0.1%, the workability of the steel will be increased. This is because it deteriorates.

  S is set to 0.0001 to 0.05%. If it is less than 0.0001%, the hot metal pretreatment takes time and cost, which impairs the economy. If it exceeds 0.05%, the workability and corrosion resistance of the steel material are reduced. This is because of a significant deterioration.

  The reason why Al is 0.005 to 1.5% is that if it is less than 0.005%, N is trapped as AlN, and solid solution N cannot be reduced. Further, if it exceeds 1.5%, the surface properties and workability deteriorate, so 1.5% or less is preferable.

  The above is the basic component system, but in the present invention, Nb and Ti, for example, may be contained in addition to these according to the respective applications.

  Nb and Ti are elements that improve the strength of the steel by precipitation strengthening. Nb contains 0.005% or more, and Ti contains 0.001% or more. However, Nb is 0%. .1% and Ti exceeding 0.25% may impair toughness, so Nb is in the range of 0.005 to 0.1% and Ti is in the range of 0.001 to 0.25%.

In the present invention, first, molten steel is discharged from a steelmaking furnace such as a converter into a ladle, a predetermined alloy element is added, Al is then added, and Al deoxidation is performed. In this way, REM is added to the molten steel after Al deoxidation, and then steel pieces are manufactured by continuous casting.
Here, the conditions for adding REM to the molten steel are as follows. That is, when REM is added into molten steel, the REM concentration locally increases in the region where REM is added, and as such, excessive oxide inclusions with a high REM oxide concentration are generated. Measures to avoid this include (1) reducing the REM component concentration in the REM alloy added to the molten steel, (2) reducing the size of the REM alloy to be added, and (3) the rate of addition of the REM alloy. For example, slowing down and (4) adding a REM alloy to a region where the stirring force is large can be mentioned. Therefore, in the present invention, the addition is performed at an addition rate of 25 g / min or less per 1 ton of molten steel, using an Fe—Si-REM alloy having a particle size of 10 mm or less, mass%, and 30% or less of REM.

  It is desirable that the REM alloy is added to the refluxing molten steel in the vacuum tank of the RH degassing apparatus having the largest stirring power, and the REM added to the molten steel is rapidly stirred to make the concentration uniform. In addition, after adding REM with a ladle, stirring with Ar gas or the like is also conceivable.

  Then, after adding REM, the molten steel is poured from the ladle into the tundish, and the molten steel is poured into the mold to perform continuous casting. Thus, the molten steel poured into the mold can be cooled and solidified to produce a slab.

  By performing continuous casting under the above conditions, it becomes possible to stably produce a continuous cast steel slab of aluminum killed steel excellent in cleanliness by avoiding nozzle clogging and the like. In the continuously cast steel slab of aluminum killed steel thus manufactured, the REM oxide concentration is 0.5 to 15% by mass in the number ratio of oxide inclusions having a particle size of 1 to 10 μm interposed in the slab. Oxide inclusions of 30% or more, REM oxide concentration exceeding 15% by mass is less than 10%, and the remainder becomes oxide inclusions having a REM oxide concentration of less than 0.5% by mass. The remaining amount of the cluster is low and the quality is excellent.

  The present invention will be described using examples and comparative examples. In the following examples and comparative examples, aluminum killed steel (steel types A and B) having the components shown in Table 1 was used.

  First, REM alloy was added and the situation of nozzle clogging (pan nozzle and immersion nozzle) was investigated. The target steel type was steel type A in Table 1, which is most likely to clog the immersion nozzle. The REM alloy used was an Fe-Si-30% REM alloy having a particle size of 10 mm, and the addition rate of the alloy was not controlled. The REM alloy was added so that the REM concentration was 8 ppm as the molten steel equivalent concentration. The molten steel equivalent concentration is obtained from the previously known molten steel weight (kg) and the amount of added REM (REM concentration (%) in REM alloy × REM alloy amount (kg) / 100). The weight of the molten steel tested this time is 370 tons, and the number of castings is 8 in a row.

  As a result of the test, it was found that clogging of the immersion nozzle in the tundish does not occur, but clogging of the ladle nozzle in the ladle may occur. Therefore, oxide inclusions when both the pan nozzle and the immersion nozzle are not clogged and when the immersion nozzle is not clogged and only the pan nozzle is clogged (1 to 10 μm in the slab) The compositions were compared.

  As a result, as shown in FIG. 1, when the clogging of both the pan nozzle and the immersion nozzle did not occur, the number ratio of oxide inclusions having a REM oxide concentration of 0.5 to 15% by mass was 30% or more. When the REM oxide concentration exceeds 15% by mass, the number ratio of oxide inclusions is less than 10%. It was found that the number ratio of the objects was 10% or more.

  From this result, the number ratio of oxide inclusions having a REM oxide concentration of 0.5 to 15% by mass of oxide inclusions (particle size of 1 to 10 μm) in the slab is 30% or more, REM oxide concentration It was found that if the number ratio of oxide inclusions exceeding 15 mass% is less than 10%, clogging does not occur in both the immersion nozzle and the pan nozzle.

  The difference in the presence or absence of pan nozzle clogging at the same REM addition amount is due to the difference in the REM oxide concentration distribution in the oxide inclusions due to the variation in the REM concentration in the molten steel when the REM alloy was added. It is guessed. This is considered to be due to the fact that when the REM alloy is added, a region where the concentration of the REM component is locally high is formed, and the inclusions with an excessively high REM oxide concentration formed there are the main causes of the clogging of the pan nozzle. As measures to avoid this, as described above, (1) reducing the REM component concentration in the REM alloy added to the molten steel, (2) reducing the size of the REM alloy to be added, ( 3) Decreasing the rate of addition of the REM alloy, and (4) adding the REM alloy to a region where the stirring force is large. In the present invention, an Fe-Si-REM alloy having a particle size of 10 mm or less, mass%, and 30% or less of REM is added at an addition rate of 25 g / min or less per ton of molten steel. An example of the reason for limitation will be described.

  Here, when the REM component concentration of the REM alloy is high, there is a region where a high concentration of REM component exists locally until mixing with the molten steel immediately after melting in the molten steel. In order to avoid the region, it is necessary to reduce the concentration of the REM component in the REM alloy. Further, regarding the particle size of the REM alloy, it is advantageous to have a small particle size in order to immediately dissolve and homogenize the alloy after the addition of the alloy. Furthermore, regarding the addition rate of the REM alloy, even if the particle size of the REM alloy is reduced, if the REM alloy is added all at once (almost simultaneously) without being dispersed, the REM alloy having a larger particle size is added. There is no big difference from the case. Therefore, it is necessary to slow down the addition rate of the REM alloy and add the REM alloy having a small particle size in a dispersed manner.

  Since these three conditions (REM component concentration, particle size, addition rate) are closely related, it was examined to obtain an appropriate range. The amount of REM added was fixed at 8 ppm in terms of molten steel, and the REM component concentration in the REM alloy, the particle size of the REM alloy, and the addition rate of the REM alloy were examined. Table 2 shows the results of variously changing the above three conditions and arranging the occurrence rate of pot clogging.

  As shown in Table 2, the conditions in which the pan nozzle clogging does not occur are as follows: the REM component concentration in the REM alloy is 30% by mass or less, the particle size of the REM alloy is 10 mm or less, and the REM alloy addition rate is 25 g / ton of molten steel. This is the case of min or less. Under these conditions, the oxide inclusions in the slab (particle size 1 to 10 μm) have a REM oxide concentration of 0.5 to 15% by mass, and the number ratio of oxide inclusions is 30% or more. The number ratio of oxide inclusions having an object concentration of more than 15% by mass is less than 10%.

  The REM alloy addition amount is determined from the REM alloy addition rate (g / min / ton), the molten steel amount (ton), and the addition time (min). Actually, the amount of REM added may be controlled by adding time at a constant rate of addition, or a predetermined amount of REM alloy may be cut out in advance from a hopper or the like, and the rate of addition may be controlled to 25 g / min or less per 1 ton of molten steel. Here, the latter method was used for control. The amount of REM alloy added is preferably 3 to 10 ppm in terms of molten steel equivalent concentration of the REM component.

  Next, molten steel of aluminum killed steel (steel types A and B) having the components shown in Table 1 was secondarily refined with an RH degasser. At that time, REM was appropriately added to the molten steel sucked into the vacuum chamber. Table 3 shows the added REM, its addition conditions, casting conditions, and the like. The reason why the components of each steel type A and B vary is that a plurality of aluminum killed steels are used for each of the steel types A and B, and the variations of the components of the plurality of aluminum killed steels used in the examples are as follows. It was shown that it was in the range of 1. The amount of molten steel in the molten steel pan was 370 tons, and the number of castings was 8 continuously.

  In the present invention example, an Fe-Si-REM alloy having a REM concentration of 10% by mass or 30% by mass was added. The comparative example showed the case where the REM density | concentration of 30 mass% Fe-Si-REM alloy was added, the case where 100% density | concentration REM was added, and the case where REM (or REM alloy) was not added. In addition, REM (or REM alloy) was added by putting it into a vacuum chamber from a hopper. The addition time of REM (or REM alloy) is 2 minutes after adding deoxidation Al to molten steel.

  After that, the molten steel returned from the vacuum tank to the ladle was advanced from the ladle to the mold via the tundish, and continuous casting was performed to check whether the nozzle clogging occurred in the pan nozzle or the immersion nozzle. The casting speed for continuous casting was 1.3 m / min to 1.5 m / min, and the casting size was casting thickness 280 (mm) × casting width 1700 mm, 1800 mm, 1900 mm. The amount of inert gas (Ar) blown into the immersion nozzle is 1 Nl / min, 2 to 2.5 Nl / min, 2.5 to 3 Nl / min, 3 Nl / min. In the mold, an electromagnetic brake and electromagnetic stirring were applied to facilitate the removal of inclusions. Table 4 shows whether nozzle clogging occurred during casting in each of the inventive examples and the comparative examples.

  In addition, for oxide inclusions having a particle diameter of 1 to 10 μm interposed in each slab produced, oxide inclusions having a REM oxide concentration of 0.5 to 15% by mass and REM oxide concentration of 15 masses Table 4 shows the respective number ratios of oxide inclusions exceeding 50% and oxide inclusions having a REM oxide concentration of less than 0.5% by mass.

Note that oxide inclusions in the slab may generate new inclusions during solidification in the central segregation zone at the center of the slab thickness. Take a sample from the removed part. In the example, a sample was taken from a ¼ width and ¼ thickness portion of the slab. The collected sample is identified by SEM using inclusions with a circle equivalent diameter of 1 × 1 mm ² and a size of 1 to 10 μm. Thereafter, the inclusion composition is determined by wavelength dispersion type detection using EPMA. Identified. The REM oxide detection limit in EPMA is 0.5% by mass. The ratio of each inclusion composition was calculated | required using the identified inclusion composition.

  As for nozzle clogging, the case of the pan nozzle that pours molten steel from the ladle into the tundish and the case of the immersion nozzle that pours the molten steel from the tundish into the mold are individually described. The SN opening index shown in Table 4 is a value (ratio of opening area) indicating an actual nozzle opening ratio with respect to an ideal immersion nozzle opening state (1) where clogging does not occur. This is an indirect index indicating that the larger this value is, the more clogged the immersion nozzle is. If the SN opening index is 1.1 or less, it can be determined that the submerged nozzle is not clogged. If the SN opening index exceeds 1.1, it is determined that the submerged nozzle has clogged. , Remove the internal deposits by sticking from above the immersion nozzle. In the case of stick pricking, the charge (the number of the molten steel pan when pouring) was specified. In addition, when the molten steel is poured from the molten steel pan to the tundish, the pan nozzle clogging is possible even if the opening rate of the SN attached to the top of the pan nozzle is 100% (fully open), the molten steel supply rate to the tundish is the throughput of continuous casting. Suppose that it was impossible to follow. When a pan nozzle clogging occurs during casting, the pan nozzle is clogged.

  As shown in Tables 3 and 4, in the measurement data 1 to 5 of the examples of the present invention, good results were obtained without clogging of the immersion nozzle and the pan nozzle. In particular, the measurement data 1 to 3 are in the range of 3 to 10 ppm of REM alloy addition, which is a preferred embodiment, and the change in the SN opening index is small and good results are obtained. On the other hand, in the measurement data 6 of the comparative example, since the REM concentration in the REM alloy is 100%, the REM concentration locally increases when the REM alloy is added, and the REM oxide concentration in the oxide inclusions is 15 mass. More than 10%, the pan ratio was over 10% and the pan nozzle was clogged. In the measurement data 7 of the comparative example, the REM alloy addition rate exceeds 25 g / min. In the measurement data 8 of the comparative example, the particle size of the REM alloy to be added exceeds 10 mm. In addition, the REM concentration increased, the number ratio of the REM oxide concentration in the oxide inclusions exceeding 15% by mass was 10% or more, and the pan nozzle was clogged. Moreover, the measurement data 9 of the comparative example is a normal Al—K steel, which was not subjected to REM addition at all, so that the immersion nozzle was clogged in the latter half of the casting, and stick pricking was performed.

  The present invention is useful for continuous casting of aluminum killed steel.

It is a graph which shows distribution of the REM oxide density | concentration of the oxide type inclusion in a slab, and the number ratio of an oxide type inclusion.

Claims (3)

A method for producing a steel slab by continuously casting REM after adding REM to molten steel deoxidized with Al,
The addition of the REM is performed at an addition rate of 25 g / min or less per 1 ton of molten steel using a Fe-Si-REM alloy having a particle size of 10 mm or less, mass%, and 30% or less of REM. Manufacturing method of continuous cast steel slab. The method for producing a continuous cast steel slab according to claim 1, wherein the REM is added by an RH degasser. A steel piece continuously cast after adding REM to molten steel deoxidized with Al,
In the number ratio of oxide inclusions having a particle diameter of 1 to 10 μm interposed in the steel slab, oxide inclusions having a REM oxide concentration of 0.5 to 15% by mass are 30% or more, and REM oxide concentration is 15% by mass. A continuous cast steel slab of aluminum killed steel, characterized in that oxide inclusions exceeding 10% are less than 10%, and the rest are oxide inclusions having a REM oxide concentration of less than 0.5% by mass.

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