JP2000144230A - Cast piece for thin steel sheet small in defect caused by inclusion and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a cast piece which does not depend on the contents of Mn, Si and Al and small in defects caused by inclusions by controlling the number of oxide inclusions with specified size and the number of alumina cluster inclusions in a of carbon steel contg. specified rations of C, Mn, Si, P, S, Al, Ti, Ca, N, oxygen, and the balance iron with inevitable impurities to not more than the specified value. SOLUTION: This carbon steel has a compsn. contg., by weight, 0.001 to 0.2% C, 0.01 to 0.5% Mn, 0.001 to 0.5% Si, 0.001 to 0.3% P, 0.0005 to 0.05% S, 0.006 to 0.1% Al, 0.005 to 0.06% Ti, 0.0005 to 0.01% Ca, 0.0005 to 0.01% N, 0.0005 to 0.0050% oxygen, and the balance iron with inevitable impurities. In the slab, the number of oxide inclusions of >=53 μm is >=200 pieces/kg, and the number of alumina cluster inclusions is <=20 pieces/kg. C deoxidation is executed in an evacuated atmosphere, furthermore, Ti and Ca are successively added thereto, oxygen in the molten steel is removed, and, thereafter, Al is added to suppress the growth of the crystal grains of the steel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous cast slab of carbon steel for thin steel sheets and a method for producing the same, and more particularly to a slab having few inclusion defects and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, defects in inclusions in cast slabs manufactured by a continuous casting method have been extremely reduced. This is the result of the technical improvement of the deoxidation method at the molten steel stage and the measures against various inclusions in continuous casting were effective (No. 126).
・ 127th Nishiyama Memorial Technical Lecture "High Purity Steel" Japan Iron and Steel Association, 1988).

[0003] However, in cast pieces for thin plates, particularly cast pieces for beverage can materials, more and more inclusions are required to be reduced, and it is required to reduce the size as well as to reduce the number of inclusions. As a technique for reducing the number of inclusions in a slab, there are, for example, JP-A-07-200612 and JP-A-05-331522. As a technique for forming fine inclusions, for example, Japanese Patent Application Laid-Open No. 58-2041
No. 17, JP-A-3-267311.

[0004] As a technique for reducing the number of inclusions in a slab for a beverage can, the above-mentioned Japanese Patent Application Laid-Open No. 07-300012 discloses a technique.
In the secondary refining, it is described that a flux is blown into a molten steel from a gas blowing lance to aggregate and coalesce the flux with the inclusions and float.However, the injected flux remains in the molten steel and forms an inclusion. There was a fear.

[0005] In Japanese Patent Application Laid-Open No. 05-331522, after CaO is charged into a converter to solidify slag, steel is poured into a ladle, and then Al is added to the slag on the ladle. Thus, it is described that the FeO concentration in the slag is 2% or less. However, in order to stably reduce the FeO concentration in the slag to 2% or less, a large amount of Al is required and the cost is increased. Further, even if the FeO concentration in the slag is 2% or less, as long as Al deoxidation is performed, alumina, which is a deoxidation product, is formed to form a cluster. Since the specific gravity is large, it is not expected that the number of alumina clusters greatly decreases due to floating on the molten steel surface.

As a technique for reducing the size of inclusions, Japanese Patent Application Laid-Open No. 58-204117 discloses a technique in which Mn, Si and Ti or Al, or REM or Ca is added in order of decreasing deoxidizing power. However, when Mn is 0.1.
It is specified as 8% by weight or more and cannot be applied to a thin plate having a low Mn. Japanese Patent Application Laid-Open No. Hei 3-267311 discloses a deoxidation method using Ti and Ca.
Since 0.005% by weight or more of Zr is essential,
Increase in cost. Also, when the oxygen concentration of molten steel before adding Ti or Ca is high, even if deoxidation is performed by adding Ti or Ca, the effect of miniaturization of inclusions is not sufficiently exhibited, so that the generated inclusions are It will be big.

For these reasons, it is difficult to stably achieve the reduction in the number of inclusions and the miniaturization of the size of the inclusions in the slab for thin steel sheets with the techniques disclosed in the above publications.

[0008]

DISCLOSURE OF THE INVENTION The present invention provides a slab for a thin steel sheet having few inclusion defect and a production thereof by stably achieving the reduction of the number of inclusions in the slab and the miniaturization of the size of the inclusions. Is to provide a way. That is, the present invention is a method of manufacturing a slab and a slab that satisfies the inclusion condition in the slab for the inclusion property defect not to occur in the thin sheet product, and is particularly limited by the slab for the thin steel sheet. It is an object of the present invention to provide a slab having few inclusion defects and a method for producing the same, which does not depend on the contents of Mn, Si and Al.

[0009]

SUMMARY OF THE INVENTION According to the present invention, before adding a deoxidizing agent to molten steel, the oxygen concentration in the molten steel is reduced by performing C deoxidation in a reduced-pressure atmosphere. Ca
And then deoxidize as a metal or alloy in the order of
By adding l, the number of oxide-based inclusions of 53 μm or more is 200 / kg or less, and among them, the number of alumina cluster inclusions is 20 / kg or less. Means 1 is C: 0.001-0.
2% by weight, Mn: 0.01 to 0.5% by weight, Si: 0.
001 to 0.5% by weight, P: 0.001 to 0.3% by weight, S: 0.0005 to 0.05% by weight, Al: 0.0
06 to 0.1% by weight, Ti: 0.005 to 0.06% by weight, Ca: 0.0005 to 0.01% by weight, N: 0.
0005-0.01% by weight, oxygen: 0.0005-0.
Carbon steel containing 0050% by weight, the balance being iron and unavoidable impurities. Of the oxide-based inclusions in the slab, 53%
The number of inclusions of μm or more is 200 / kg or less, and among them, the number of alumina cluster inclusions is 20 / kg
It is a slab for thin steel sheets with less inclusion defect of less than kg.

[0010] Further, the means 2 may include the means 1 in which Nb: 0.
001 to 0.10% by weight, V: 0.005 to 0.20% by weight, Cr: 0.01 to 0.50% by weight, Mo: 0.0
1 to 0.50% by weight, Cu: 0.01 to 0.50% by weight, Ni: 0.01 to 0.50% by weight, B: 0.000%
One to two or more of 0.0020% by weight is contained.

Means 3 comprises 0.001 to 0.2% by weight of C, 0.01 to 0.5% by weight of Mn, and 0.001% of Si.
-0.5% by weight, P: 0.001-0.3% by weight, S:
0.0005-0.05% by weight, Al: more than 0.006 ~
0.1% by weight, Ti: 0.005 to 0.06% by weight, C
a: 0.0005 to 0.01% by weight, N: 0.0005
To 0.01% by weight, oxygen: 0.0005 to 0.0050
When carbon steel molten steel containing the balance of iron and unavoidable impurities is cast by a continuous casting facility to produce cast slabs, the decarbonized molten steel is subjected to C deoxidation in a reduced-pressure atmosphere. This is a method for producing a slab for a thin steel sheet having few inclusion defects, in which the oxygen concentration in molten steel is set to 300 ppm or less, and then added as a metal or an alloy in the order of Ti and Ca to deoxidize, and then Al is added.

[0012] The means 4 is different from the means 3 in that Nb: 0.00.
1 to 0.10% by weight, V: 0.005 to 0.20% by weight, Cr: 0.01 to 0.50% by weight, Mo: 0.01
0.50% by weight, Cu: 0.01 to 0.50% by weight,
Ni: 0.01 to 0.50% by weight, B: 0.0002 to
One or more of 0.0020% by weight is contained.

Means 5 are: C: 0.001 to 0.2% by weight, Mn: 0.01 to 0.5% by weight, Si: 0.001% by weight.
-0.5% by weight, P: 0.001-0.3% by weight, S:
0.0005-0.05% by weight, Al: more than 0.006 ~
0.1% by weight, Ti: 0.005 to 0.06% by weight, C
a: 0.0005 to 0.01% by weight, N: 0.0005
To 0.01% by weight, oxygen: 0.0005 to 0.0050
When carbon steel molten steel containing the balance of iron and unavoidable impurities is cast by a continuous casting facility to produce cast slabs, the decarbonized molten steel is subjected to C deoxidation in a reduced-pressure atmosphere. The oxygen concentration in the molten steel is set to 300 ppm or less, and then, Mn or Mn, Si or Mn, Si and a small amount of Al are added as a metal or an alloy so that the Al concentration in the molten steel is 0.01% by weight or less. And then Ti
Is added as a metal or alloy to deoxidize it, further added as a Ca metal or alloy to deoxidize it, and then the remaining Al is added to the method for producing a cast piece for a thin steel sheet with few inclusion defects.

The means 6 is different from the means 5 in that Nb: 0.00
1 to 0.10% by weight, V: 0.005 to 0.20% by weight, Cr: 0.01 to 0.50% by weight, Mo: 0.01
0.50% by weight, Cu: 0.01 to 0.50% by weight,
Ni: 0.01 to 0.50% by weight, B: 0.0002 to
One or more of 0.0020% by weight is contained.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION First, the present inventors have studied the inclusion conditions of cast slabs in which inclusion defects are unlikely to occur in products. Here, the inclusion means an oxide-based material that easily affects a product defect. Increasing the number of inclusions in a slab tends to cause inclusion defect in the product. Therefore, as a result of investigating the relationship between the size and number of inclusions in the slab and the occurrence of product defects, as shown in FIG. 1, among the inclusions in the slab, those having a size of 53 μm or more were found to be cast. When the number of alumina clusters is 200 or less per 1 kg of a piece and the number of alumina clusters of 53 μm or more is 20 or less per 1 kg of a slab, the product defect occurrence rate is extremely low and good.

On the other hand, in other cases (more than 200 inclusions of 53 μm or more and more than 20 alumina clusters), the product defect rate is high, that is, the product defect tends to occur easily. found.

Here, the alumina cluster is an aggregate of a plurality of particles, and this aggregate is counted as one. In general, alumina, which is a product after Al deoxidation, has small individual particles, but immediately after generation, the particles aggregate to form a cluster and become large in size. Further, since this cluster contains iron between constituent particles, the specific gravity is large and it is difficult to float. In addition, alumina clusters have a greater effect on product defects than other inclusions. The number 53 μm is the size of the stitch of the filter in the inclusion analysis method.

In order to explain the cast slab of the present invention in detail, the reasons for defining the conditions of the present invention will be described. C is an element used for imparting the strength of steel. However, for thin sheets, it is sometimes desirable to reduce C as much as possible by using a steel sheet for deep drawing. However, when C is 0.001% by weight or less, C deoxidation in the present invention becomes very difficult.
The lower limit was set to 0.001% by weight, and the upper limit was set to 0.2% by weight as the maximum amount of carbon used in the plate material.

Mn is also required for obtaining strength and suppressing embrittlement due to S. The upper limit is set to 0.5% by weight, which is the maximum value when used as a high-tensile material or the like. The lower limit was set to 0.01% by weight to inevitably mix them.
Si is also an element used for obtaining strength and improving high-temperature characteristics, and the upper limit is set to 0.5% by weight. Also,
The lower limit was set to 0.001% by weight because of inevitable mixing.

Since P is a harmful element to steel, it is desirable that P be as small as possible. However, since P is inevitably mixed, the lower limit of 0.001% by weight is practical. However, in some cases, a large amount of P must be added from the viewpoint of improving the strength and corrosion resistance of steel, so the upper limit is set to 0.3% by weight. Above this, the effect of embrittlement by P becomes stronger. Similarly, S often causes harm to the product characteristics, and is desirably as low as possible.
5% by weight is realistic. The upper limit is set to 0.05% by weight in order to prevent cracking during continuous casting.

Al is generally used as a deoxidizing element. Among oxide inclusions in a slab, the number of inclusions having a size of 53 μm or more is 200 / kg or less. In order to satisfy that the number of the alumina cluster inclusions in which two or more alumina particles are united is 20 or less, it is a basic idea that the present invention does not use Al as a deoxidizing element as much as possible.

However, the cast piece for a thin steel sheet, which is an object of the present invention, requires Al because of its material. That is, Al acts as AlN in the steel to suppress the growth of crystal grains of the steel. From this viewpoint, Al is standardized as a necessary component. Therefore, the lower limit is set to more than 0.006% by weight. The upper limit is set to 0.1% by weight so as not to impair the effects of Ti and Ca used in the present invention.

Ti and Ca are important elements of the present invention. Among the oxide-based inclusions in the slab, the number of inclusions of 53 μm or more is 200 / kg or less, and among them,
In order to satisfy that the number of alumina cluster inclusions in which two or more alumina particles are combined is 20 or less, it is necessary to use Ti or Ca instead of using Al as a deoxidizing material, as described later. The inventors have found that there is.

The lower limit of Ti is set to 0.005% by weight in order to obtain a deoxidizing effect, and the upper limit is set to 0.06% by weight, since a large amount of Ti impairs the effect of Ca deoxidizing. The lower limit of Ca was also 0.0005% by weight in order to obtain a sufficient deoxidizing effect. The upper limit was set to 0.01% by weight as a level at which the effect would be saturated even if added excessively.

N is used to combine with Al to form AlN and suppress the growth of crystal grains. From this viewpoint, the upper limit of the amount of addition used was 0.01% by weight. The lower limit was set to 0.0005% by weight in consideration of the inevitable mixing.

Most of the oxygen amount in the slab is included as oxide-based inclusions in the slab. It is desirable to minimize the inclusions of 53 μm or more that are harmful to the product. However, if the number of large inclusions decreases, the oxygen content does not always decrease. That is, even if there are many harmless fine inclusions in the product, the amount of oxygen is high. Therefore, when the oxygen content is below a certain level, the oxygen content cannot always be an index of the number of inclusions, but when the oxygen value is extremely high, the tendency is that the number of large inclusions tends to increase, so the upper limit is set. Was set to 0.0050% by weight. Further, the lower limit is set to 0.0
005% by weight.

The above are the basic components of the steel to which the present invention is directed. In order to improve various properties of the material such as strength, corrosion resistance and hardenability, Nb,
Addition of one or more of V, Cr, Mo, Cu, Ni, and B does not impair the effects of the present invention. That is, the range of the addition amount is Nb: 0.00
1 to 0.10% by weight, V: 0.005 to 0.20% by weight, Cr: 0.01 to 0.50% by weight, Mo: 0.01
0.50% by weight, Cu: 0.01 to 0.50% by weight,
Ni: 0.01 to 0.50% by weight, B: 0.0002 to
0.0020% by weight.

As other elements, REM elements may be contained in molten steel, but 10 pp for each element.
Up to m, even if included, the effect of the present invention is not affected.

In the actual manufacturing process, the added elements are not necessarily included in the 100% molten steel, so it is necessary to add them in consideration of the yield. There is no particular limitation on the method of addition. Any method may be used as long as it can be contained in steel so as to satisfy the above conditions. In addition, among the oxide-based inclusions in the slab, the number of inclusions having a size of 53 μm or more was 200 / k.
g or less, and the number of alumina clusters therein is 2
The value of 0 / kg or less is determined from the condition that the incidence of product defects is reduced as shown in FIG.

Next, a manufacturing method for satisfying such inclusion conditions in the slab was examined. The inventors first focused on deoxidizing elements. Generally, Al is widely used as a deoxidizing element of molten steel. However,
Alumina, which is a product after Al deoxidation, has small individual particles, but immediately after generation, the particles aggregate to form a cluster and increase in size. Further, since this cluster contains iron between constituent particles, the specific gravity is large and it is difficult to float. Therefore, in order to float and remove alumina inclusions generated by Al deoxidation, a very long standing time is required, a large amount of Ar gas is blown into the molten steel, and the gas and the inclusions are united to float. Measures such as promotion were necessary.

Therefore, the present inventors considered not using Al as a deoxidizing material, and considered that Ca was used as a deoxidizing element instead of Al.
We paid attention to. When deoxidized with Ca, the deoxidized product Ca
O is produced, but its size is smaller than other deoxidizing elements. However, the size of this CaO largely depends on the oxygen concentration of molten steel before adding Ca.

The inventors obtained laboratory oxygen concentration before molten Ca addition that reduced the size of CaO inclusions. The composition of steel is 0.04% C-0.0010%
The amount of Ti, Ca, and oxygen was changed with N. In addition, other components are not included. FIG. 2 shows the relationship between the average particle diameter of CaO inclusions immediately after Ca deoxidation and the oxygen concentration of molten steel before Ca deoxidation. When the oxygen concentration of molten steel is 50 ppm or less, the average size of CaO inclusions generated Was found to be very small at 10 μm or less.

Next, the oxygen concentration of molten steel before adding Ca was adjusted to 50 p.
The means for controlling the pressure to less than pm was studied. Considering thermodynamically, in order to make the molten steel oxygen concentration 50 ppm or less, it is preferable to select a deoxidizing element having a higher oxygen affinity than Si. This can be inferred from the fact that when oxygen is added in a relatively large amount of 0.5% by weight and deoxidized, the oxygen concentration of molten steel which thermodynamically balances at a molten steel temperature of 1600 ° C. is about 70 ppm.

The deoxidizing element applicable to this is T
i, Al, Mg, and Ca are included, but Ca is excluded because it is used in the subsequent deoxidation. In addition, Mg is a strong deoxidizing element close to Ca and is therefore excluded. Also, Al was omitted because it is a basic idea of the present invention that it is not used as a deoxidizing element. From the above considerations, Ti was used as a means for controlling the oxygen concentration of molten steel before adding Ca to 50 ppm or less. Ti deoxidation is also characterized in that the concentration required for deoxidation is several hundred ppm, which is very small as compared with the case of Mn or Si.

However, even in the deoxidation of Ti, Ca
As in the case of deoxidation, the oxygen concentration of molten steel before the addition of Ti greatly affects the size of the generated Ti oxide. That is, when the molten steel oxygen concentration is high, the generated Ti oxide is large, which contradicts the intention of the present invention. Therefore, the present inventors have proposed a method of reducing the size of the Ti oxide to Ti
The oxygen concentration of molten steel before addition was determined by laboratory experiments. FIG. 3 shows the relationship between the average particle diameter of Ti oxide immediately after Ti deoxidation and the oxygen concentration of molten steel before Ti deoxidation. When the oxygen concentration of molten steel is 300 ppm or more, the size of the generated Ti oxide sharply increases. It turned out to be bigger. Therefore, Ti
It has been found that the oxygen concentration of molten steel before addition needs to be 300 ppm or less.

Next, the oxygen concentration of molten steel before adding Ti was set to 300
Means for controlling the concentration to less than ppm was studied. According to thermodynamic studies, Mn deoxidation and Si deoxidation can be mentioned in order to reduce the oxygen concentration of molten steel to 300 ppm or less. May be constrained lower. Therefore, it was necessary to consider a deoxidation method that does not depend on the Mn or Si concentration.

The present inventors have focused on C and considered that the oxygen concentration of molten steel is reduced to 300 ppm or less by performing C deoxidation under reduced pressure. Considering the C deoxidation equilibrium, for example, when the C concentration is 0.04% by weight, if the molten steel temperature is 1600 ° C. and the CO partial pressure in the atmosphere is about 0.4, the equilibrium molten steel oxygen concentration becomes about 300 ppm. The conditions required by the invention can be satisfied. In C deoxidation, the deoxidation product is C
Another major feature is that because it is O gas, it remains in the molten steel and does not become inclusions.

Next, the addition of Al was examined. In the present invention, the basic idea is not to use Al as a deoxidizing element. However, the cast piece for a thin steel sheet, which is the subject of the present invention, requires Al because of its material. That is, Al is AlN in steel
Thus, it has the function of suppressing the growth of steel crystal grains, and from this viewpoint, it is standardized as a necessary component. However, since Al has a very high affinity for oxygen, if Al is added while molten steel oxygen is high, a large amount of alumina inclusions will be generated, and the intention of the present invention will not be satisfied.

Therefore, in the present invention, the timing of adding Al is defined as after the addition of Ca. Compared to Al, Ca
Has a higher oxygen affinity, so that the oxygen concentration of molten steel is extremely reduced by the addition of Ca. Even if Al is added thereto, Al hardly combines with oxygen and dissolves in molten steel. That is, in this case, Al does not work as a deoxidizing element. Therefore, it is important to add Al after Ca deoxidation. After performing C deoxidation, Mn or M
Ti may be added after adding n, Si and a small amount of Al. Here, the trace amount is a case in which the concentration in the molten steel is 0.01% by weight or less after the addition, and does not significantly affect the formation of inclusions. It may be acid. Then, the remaining Al may be added after the addition of Ca.

In addition, before the deoxidation, CaO or Al is added to the slag on the molten steel in the ladle to reduce the oxygen potential in the slag, that is, to perform so-called slag reforming. The slag reforming can be expected to further reduce the number of inclusions and make the inclusions finer.

[0041]

EXAMPLE A carbon steel having the components shown in Table 1 was manufactured under the manufacturing conditions shown in Table 3, the number of inclusions in the obtained slab, the steel plate obtained by rolling the slab, and the steel plate as a material. The result of processing was investigated. The survey method was as shown in Table 4. The levels A-1, C-1, D
-1 is for slag reforming, before C deoxidation, 1.5 tons of CaO and 5 tons of Al per 300 tons of molten steel on the slag in the ladle.
00 kg was added. In the case of E-1, Mn, Si, and trace amounts were added after Al was deoxidized by C and before Ti was added.
In this example, Mn and Si were added after C deoxidation and before addition of Ti.

[0042]

[Table 1]

[0043]

[Table 2]

[0044]

[Table 3]

[0045]

[Table 4]

Table 2 shows the results. From the table, when the conditions of the present invention are satisfied, the number of inclusions in the slab is small, good results that no rejection due to surface flaws and internal defects do not occur, and further no defects during processing occur was gotten.

On the other hand, the comparative material not satisfying the present invention resulted in the following problem. That is, in the comparative materials B-2, E-2, and F-2, C deoxidation was not performed before the addition of the deoxidizing alloy elements Ti and Ca. Molten steel oxygen concentration is 300p
pm, and the number of inclusions in the slab increases even if the addition is performed in the order of Ti-Ca-Al.

In Comparative Materials A-2 and D-2, the oxygen concentration of molten steel before adding the deoxidizing alloy element satisfies 300 ppm or less even though C deoxidation was performed before adding the deoxidizing alloy element. Therefore, the number of inclusions in the slab is large.
In the comparative materials C-2, G-2, and H-2, the order of adding the deoxidizing element did not satisfy the present invention, and thus defects occurred during product processing.

Further, since the comparative material I-1 has a low Ti concentration and does not satisfy the present invention, and the J-1 has a high Al content and does not satisfy the present invention, the K-1 has low concentrations of Ti, Ca and oxygen. Since the present invention is not high enough to satisfy the present invention, the number of inclusions in the slab increases. Especially in the case of J-1, 53
Although the number of inclusions of μm or more satisfies the condition, the number of alumina clusters is larger than the range of the present invention.

As a result, when the conditions of the present invention are not satisfied, the number of inclusions in the slab is large, and a coil defect after rolling and a defect during product processing also occur. Here, in the column of the machining defect in Table 3, those marked with a minus sign indicate that the product was not rejected at the coil stage and thus did not become a product and did not undergo machining.

[0051]

As described above, according to the present invention, it is possible to obtain a slab for a thin steel sheet in which the number of harmful inclusions has been greatly reduced, and that the number of coil defects after rolling and defects during processing of products are extremely small. Obtained. Therefore, according to the present invention, it is possible to manufacture a cast piece for a thin steel sheet having few inclusion defect.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between the number of inclusions in a slab and the incidence of product defects.

FIG. 2 is a diagram showing a relationship between molten steel oxygen content and inclusion size before Mg addition.

FIG. 3 is a diagram showing a relationship between molten steel oxygen content and inclusion size before Ti addition.

Continuation of the front page (72) Inventor Hiroaki Iiboshi 1 Nishinosu, Oita-shi, Oita Pref.

Claims (6)

[Claims] 1. C: 0.001 to 0.2% by weight, Mn:
0.01 to 0.5% by weight, Si: 0.001 to 0.5% by weight, P: 0.001 to 0.3% by weight, S: 0.000
5 to 0.05% by weight, Al: more than 0.006 to 0.1% by weight, Ti: 0.005 to 0.06% by weight, Ca: 0.0
005 to 0.01% by weight, N: 0.0005 to 0.01
% By weight, oxygen: 0.0005 to 0.0050% by weight, the balance being iron and inevitable impurities, the number of inclusions of 53 μm or more among oxide inclusions in the slab is 200. A slab for a thin steel sheet having few inclusion defect, characterized in that the number of alumina cluster inclusions is not more than 20 pieces / kg. 2. Nb: 0.001 to 0.10% by weight,
V: 0.005 to 0.20% by weight, Cr: 0.01 to
0.50% by weight, Mo: 0.01 to 0.50% by weight, C
u: 0.01 to 0.50% by weight, Ni: 0.01 to 0.
The slab for a thin steel sheet having a small number of inclusion defects according to claim 1, characterized in that it contains one or more of 50% by weight and B: 0.0002 to 0.0020% by weight. 3. C: 0.001 to 0.2% by weight, Mn:
0.01 to 0.5% by weight, Si: 0.001 to 0.5% by weight, P: 0.001 to 0.3% by weight, S: 0.000
5 to 0.05% by weight, Al: more than 0.006 to 0.1% by weight, Ti: 0.005 to 0.06% by weight, Ca: 0.0
005 to 0.01% by weight, N: 0.0005 to 0.01
Decarbonized molten steel containing, by weight, oxygen: 0.0005 to 0.0050% by weight, and cast carbon steel molten steel containing the balance iron and unavoidable impurities in a continuous casting facility to produce cast slabs Was subjected to C deoxidation in a reduced-pressure atmosphere to reduce the oxygen concentration in the molten steel to 300 ppm or less.
a. A method for producing a slab for a thin steel sheet with few inclusion defects, characterized by adding as a metal or an alloy in the order of a, deoxidizing the metal and then adding Al. 4. Nb: 0.001 to 0.10% by weight,
V: 0.005 to 0.20% by weight, Cr: 0.01 to
0.50% by weight, Mo: 0.01 to 0.50% by weight, C
u: 0.01 to 0.50% by weight, Ni: 0.01 to 0.
The method for producing a slab for a thin steel sheet having a small number of inclusion defects according to claim 3, characterized in that one or two or more of 50 wt% and B: 0.0002 to 0.0020 wt% are contained. 5. C: 0.001 to 0.2% by weight, Mn:
0.01 to 0.5% by weight, Si: 0.001 to 0.5% by weight, P: 0.001 to 0.3% by weight, S: 0.000
5 to 0.05% by weight, Al: more than 0.006 to 0.1% by weight, Ti: 0.005 to 0.06% by weight, Ca: 0.0
005 to 0.01% by weight, N: 0.0005 to 0.01
Decarbonized molten steel containing, by weight, oxygen: 0.0005 to 0.0050% by weight, and cast carbon steel molten steel containing the balance iron and unavoidable impurities in a continuous casting facility to produce cast slabs. Is subjected to C deoxidation in a reduced pressure atmosphere to reduce the oxygen concentration in the molten steel to 300 ppm or less, and then to reduce the Mn or Mn, Si or Mn, Si and the Al concentration in the molten steel to 0.01% by weight or less. Add a trace amount of Al as a metal or alloy to deoxidize, then add Ti as a metal or alloy to deoxidize, then add Ca as a metal or alloy to deoxidize, then add the remaining Al A method for producing a cast slab for a thin steel sheet having less inclusion defect. 6. Nb: 0.001 to 0.10% by weight,
V: 0.005 to 0.20% by weight, Cr: 0.01 to
0.50% by weight, Mo: 0.01 to 0.50% by weight, C
u: 0.01 to 0.50% by weight, Ni: 0.01 to 0.
The method for producing a slab for a thin steel sheet having a small number of inclusion defects according to claim 5, wherein one or more kinds of 50 wt% and B: 0.0002 to 0.0020 wt% are contained.