JP3535026B2 - Slab for thin steel sheet with less inclusion defect and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art In recent years, defects in inclusions in a cast product manufactured by a continuous casting method have become extremely small. This is a result of the technical improvement of the deoxidation method at the molten steel stage and the countermeasures against various inclusions in continuous casting. (Twelfth
6.127th Nishiyama Memorial Technical Lecture "High Purity Steel" Japan Iron and Steel Institute, 1988)

However, in castings for thin plates, particularly in castings for beverage can materials, it is increasingly required to reduce inclusions, and it is required to reduce the number and the size thereof. Techniques for reducing the number of inclusions in a slab are disclosed in, for example, Japanese Patent Application Laid-Open Nos. 07-300162 and 05-331522, and as techniques for reducing the size of inclusions, for example, Japanese Patent Application Laid-Open No. 05- 43
There is a 977 publication.

As a technique for reducing the number of inclusions in a slab for beverage cans, Japanese Patent Laid-Open No. 07-300612 discloses a technique.
In the secondary refining, it is described that the flux is blown into the molten steel from the gas blowing lance to cause the flux to aggregate with the inclusions and float, but the blown flux remains in the molten steel to form inclusions. There was a fear of becoming.

Further, in the above-mentioned Japanese Patent Laid-Open No. 05-331522, after CaO is charged into the converter to solidify the slag, steel is tapped into the ladle, and then Al is added to the slag on the ladle. However, it is described that the FeO concentration in the slag is set to 2% or less, but in order to stably reduce the FeO concentration in the slag to 2% or less, it is necessary to add a large amount of Al, which increases the cost. Further, even if the FeO concentration in the slag is 2% or less, alumina, which is a deoxidation product, is formed into clusters as long as Al deoxidation is performed. Since this has a large specific gravity, a drastic reduction in the number of alumina clusters due to floating on the surface of molten steel cannot be expected.

As a technique for reducing the size of inclusions, Japanese Patent Laid-Open No. 05-43977 discloses that Ti and Mg are added to molten steel, but the Al content is set to 0.006% by weight or less. It is limited and cannot be used for steel applications that require Al content. Further, when the molten steel oxygen concentration before addition of Ti or Mg is high, even if Ti or Mg is added to perform deoxidation, the effect of refining inclusions is not sufficiently exerted. It becomes a big thing.

For these reasons, it is difficult for the techniques of the above-mentioned publications to stably reduce the number of inclusions and miniaturize the size of inclusions in the cast slab for thin steel sheet.

[0008]

DISCLOSURE OF THE INVENTION The present invention provides a cast slab for thin steel sheets with few defects in inclusion properties and its production by stably reducing the number of inclusions in the slab and miniaturizing the size of the inclusions. Is to provide a method. That is, the present invention is a slab and a method of manufacturing the slab that satisfies the inclusion condition in the slab to prevent inclusion property defects in the thin plate product, and is particularly limited by the thin steel plate slab, It is an object of the present invention to provide a cast slab having few inclusion property defects that does not depend on the contents of Mn, Si and Al, and a method for producing the cast slab.

[0009]

According to the present invention, before adding a deoxidizing material to molten steel, C deoxidation is performed in a reduced pressure atmosphere to reduce the oxygen concentration in the molten steel. , M
The order of g is added as a metal or alloy for deoxidation, and then Al is added, whereby the number of oxide inclusions of 53 μm or more is 200 / kg or less, and among them, alumina cluster inclusions are included. The number of cast pieces is 20 pieces / kg or less to prevent the occurrence of inclusion defects during product processing. The means 1 is C: 0.001 to 0.2% by weight, Mn: 0. 01-0.5 wt%, Si: 0.00
1 to 0.5% by weight, P: 0.001 to 0.3% by weight,
S: 0.0005 to 0.05% by weight, Al: 0.006
Super-0.1% by weight, Ti: 0.005-0.06% by weight, Mg: 0.0005-0.01% by weight, N: 0.0
005-0.01% by weight, oxygen: 0.0005-0.0
Carbon steel containing 050% by weight and the balance iron and unavoidable impurities. Of the oxide inclusions in the cast slab, 53μ
The number of inclusions of m or more is 200 pieces / kg or less, and
Among them, the number of alumina cluster inclusions is 20 / kg
The following is a slab for thin steel sheets with few inclusion physical defects.

Further, the means 2 is the same as the means 1 except that 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 wt%, Cu: 0.01 to 0.50 wt%, Ni: 0.01 to 0.50 wt%, B: 0.000
2 to 0.0020% by weight of one kind or two or more kinds are contained.

Means 3 are: C: 0.001 to 0.2% by weight, Mn: 0.01 to 0.5% by weight, Si: 0.001
~ 0.5 wt%, P: 0.001-0.3 wt%, S:
0.0005 to 0.05% by weight, Al: more than 0.006
0.1% by weight, Ti: 0.005-0.06% by weight, M
g: 0.0005 to 0.01% by weight, N: 0.0005
~ 0.01 wt%, oxygen: 0.0005-0.0050
When producing a slab by casting a carbon steel molten steel containing the balance by weight and the balance iron and unavoidable impurities in a continuous casting facility, the decarburized molten steel is subjected to C deoxidation in a reduced pressure atmosphere. This is a method for producing a cast slab for thin steel sheets in which the oxygen concentration in molten steel is 300 ppm or less, Ti, Mg are added in that order as metals or alloys to deoxidize them, and Al is then added.

The means 4 is the same as the means 3 except that Nb: 0.001.
~ 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, Cu: 0.01 to 0.50% by weight, N
i: 0.01 to 0.50% by weight, B: 0.0002 to
One or more of 0.0020% by weight is contained.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors first examined conditions for inclusions in a cast product in which inclusion-like defects are less likely to occur in products. Here, the inclusions are oxide-based inclusions that tend to adversely affect product defects. When the number of inclusions in the slab is large, inclusion property defects are likely to occur in the product. Therefore, as a result of investigating the relationship between the size and the 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 are If the number of alumina clusters per kg of the piece is 200 or less and the number of alumina clusters of 53 μm or more per 20 kg of the cast piece is 20 or less, the product defect occurrence rate is extremely low.

On the other hand, in other cases (when the number of inclusions of 53 μm or more is more than 200 per 1 kg of the cast and the number of alumina clusters is more than 20 per 1 kg of the cast), the product defect occurrence rate is high, that is, It was found that product defects tend to occur.

Here, the alumina clusters are aggregates of a plurality of alumina particles, and this aggregate is counted as one. In general, alumina, which is a product after Al deoxidation, has a small size for each particle, but immediately after generation, the particles agglomerate to form clusters and the size increases.
Further, since this cluster contains iron between the constituent particles, it has a large specific gravity and is hard to float. Further, the alumina cluster has a greater effect on the product defect than other inclusions. The number of 53 μm is the size of the filter mesh in the inclusion analysis method.

In order to describe the cast piece of the present invention in detail, the reasons for defining the conditions of the invention will be described below. C is an element used to have the strength of steel, but in the case of thin plates, it may be desirable to reduce C as much as possible with a deep drawing steel plate or the like. However, if the C content is 0.001% by weight or less, the deoxidation of C in the present invention becomes very difficult.
The lower limit was 0.001% by weight, and the upper limit was 0.2% by weight as the maximum amount of carbon used in the plate material.

Further, Mn is also necessary to obtain strength and suppress brittleness due to S, and the upper limit is set to 0.5% by weight, which is the maximum value when used in a high-tensile material or the like. Further, the lower limit is 0.01% by weight because it is unavoidably mixed.
Si is also an element used to obtain strength and improve high temperature characteristics, and the upper limit was set to 0.5% by weight. Also,
Since it is unavoidably mixed, the lower limit was made 0.001% by weight.

Since P is an element harmful to steel, it is desirable that it is as small as possible, but since it is unavoidably mixed, the lower limit of 0.001% by weight is practical. However, a large amount of P may be required from the viewpoint of improving the strength and corrosion resistance of steel, so the upper limit was made 0.3% by weight. Above this, the effect of embrittlement due to P becomes stronger. Similarly, S is often harmful to the product characteristics, and it is desirable to set it to the lowest possible level, but since it is unavoidably mixed, the lower limit value is 0.000.
5 wt% is realistic. The upper limit was set to 0.05% by weight in order to prevent cracking during continuous casting.

Although Al is generally used as a deoxidizing element, the number of inclusions having a size of 53 μm or more in the cast slab is 200 pieces / 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 united is 20 / kg or less, the basic idea is that Al is not used as a deoxidizing element in the present invention as much as possible.

However, the thin steel sheet slab that is the subject of the present invention requires Al as a material. That is, Al acts as AlN in steel and has a function of suppressing the growth of crystal grains of steel, and from this viewpoint, it is standardized as an essential component. Therefore, the lower limit is set to more than 0.006% by weight. Further, the upper limit is set to 0.1% by weight in order not to impair the effects of Ti and Mg used in the present invention.

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

The lower limit of Ti is set to 0.005% by weight in order to obtain the deoxidizing effect, and the upper limit is defined as 0.06% by weight because addition of a large amount of Ti hinders the effect of Mg deoxidizing. Also for Mg, the lower limit is set to 0.0005% by weight in order to obtain a sufficient deoxidizing effect. The upper limit is set to 0.01% by weight as a level at which the effect is saturated even if added in excess.

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 addition amount used is 0.01% by weight. In addition, considering the amount that is unavoidably mixed, the lower limit is set to 0.0005% by weight.

Most of the oxygen content in the cast slab is contained as oxide inclusions in the cast slab. For inclusions of 53 μm or more, which are harmful to the product, it is desirable that the inclusions are as small as possible, but if the number of large inclusions is reduced, the amount of oxygen does not necessarily decrease. That is, even if there are many fine inclusions that are harmless to the product, the amount of oxygen increases. Therefore, when the oxygen amount is below a certain level, the oxygen amount cannot always be an index of the number of inclusions, but when the oxygen value is very high, the number of large inclusions tends to increase, so the upper limit Was 0.0050% by weight. In addition, the lower limit is 0.0, considering the amount that is unavoidably mixed.
It was 005% by weight.

The above are the basic components of the steel targeted by the present invention. In order to improve various properties of the material such as strength, corrosion resistance, and hardenability, Nb, depending on the use of the steel,
The 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
The amount is 0.0020% by weight.

As other elements, elements such as Ca and REM may be contained in the molten steel.
If it is contained up to 0 ppm, it does not affect the effect of the present invention.

In the actual manufacturing process, 100% of the added elements are not contained in the molten steel, so it is necessary to add them in consideration of the yield. The method of addition is not specified. Any method may be used as long as it can be contained in steel so as to satisfy the above conditions. In addition, the number of oxide inclusions of 53 μm or more among the oxide inclusions in the cast slab is 200 / k.
g or less, and the number of alumina clusters in it is 2
The number of 0 pieces / kg or less is determined based on the condition that the occurrence rate of product defects becomes small as shown in FIG.

Next, a manufacturing method for satisfying such inclusion condition in the cast piece was examined. The inventors first focused on the deoxidizing element. Generally, Al is widely used as a deoxidizing element for molten steel. However,
Each particle of alumina, which is a product after Al deoxidation, is small, but the particles agglomerate immediately after the formation, and the size becomes large in a cluster. Further, since this cluster contains iron between the constituent particles, it has a large specific gravity and is hard to float. Therefore, in order to float and remove the alumina inclusions generated by Al deoxidation, a very long standing time is required, and a large amount of Ar gas is blown into the molten steel to combine the gas and the inclusions for floating. Measures such as promotion were necessary.

Therefore, the inventors of the present invention have considered that Al is not used as a deoxidizing material, and therefore Mg is used as a deoxidizing element instead of Al.
I focused on. When deoxidized with Mg, the deoxidized product Mg
O is generated, but its size is smaller than that of other deoxidizing elements. However, the size of this MgO largely depends on the molten steel oxygen concentration before addition of Mg.

The inventors determined the oxygen concentration of molten steel before the addition of Mg, which reduces the size of MgO inclusions, by a laboratory experiment. Steel composition is 0.04% C-0.0010%
The amount of Ti, Mg and oxygen was changed by N. In addition, other components are not included. FIG. 2 shows the relationship between the average particle size of MgO inclusions immediately after Mg deoxidation and the molten steel oxygen concentration before Mg deoxidation. When the molten steel oxygen concentration is 50 pppm or less, the average size of the MgO inclusions produced is shown. Was 10 μm or less, which was extremely small.

Next, the molten steel oxygen concentration before adding Mg is set to 50 p.
The means for controlling to pm or less was examined. From a thermodynamic study, in order to reduce the molten steel oxygen concentration to 50 ppm or less, it is preferable to select a deoxidizing element having a stronger oxygen affinity than Si. This can be inferred from the fact that when a relatively large amount of Si of 0.5% by weight is added for deoxidation, the molten steel oxygen concentration thermodynamically equilibrated at a molten steel temperature of 1600 ° C. is about 70 ppm.

The deoxidizing element applicable to this is T
Although i, Al, Mg, and Ca are included, Mg is excluded because it is used in the subsequent deoxidation. Further, Ca is an element having a stronger oxygen affinity than Mg, and is therefore excluded. Also, Al
Is excluded since it is a basic idea of the present invention that it is not used as a deoxidizing element. From the above consideration, Ti was used as a means for controlling the molten steel oxygen concentration before adding Mg to 50 ppm or less. Ti deoxidation is also characterized in that the concentration required for deoxidation is several hundred ppm, which is much smaller than in the case of Mn or Si.

However, even in Ti deoxidation, Mg
Similar to deoxidation, the molten steel oxygen concentration before addition of Ti greatly affects the size of the Ti oxide produced. That is, when the molten steel oxygen concentration is high, a large amount of Ti oxide is generated, which is inconsistent with the intention of the present invention. Therefore, the inventors have found that the size of Ti oxide is reduced to Ti.
The molten steel oxygen concentration before addition was determined by a laboratory experiment. FIG. 3 shows the relationship between the average particle size of Ti oxide immediately after Ti deoxidation and the molten steel oxygen concentration before Ti deoxidation. When the molten steel oxygen concentration is 300 ppm or more, the size of the generated Ti oxide is sharp. It turned out to be big. Therefore, Ti
It was found that the molten steel oxygen concentration before addition needs to be 300 ppm or less.

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

The inventors of the present invention focused on C and considered that the deoxidation of C under reduced pressure would reduce the molten steel oxygen concentration to 300 ppm or less. Examining from the C deoxidization equilibrium, for example, when the C concentration is 0.04% by weight and the CO partial pressure in the atmosphere is about 0.4 at the molten steel temperature of 1600 ° C, the equilibrium molten steel oxygen concentration is about 300 ppm. The conditions required by the invention can be satisfied. C deoxidation is carried out when the deoxidation product is C
Since it is O gas, it is also a major feature that it remains in the molten steel and does not become inclusions.

Next, the addition of Al was examined. In the present invention, it is a basic idea that Al is not used as a deoxidizing element, but the thin steel plate cast object of the present invention requires Al as a material. That is, Al is AlN in steel
It has a function of suppressing the growth of crystal grains of steel, and from this viewpoint, it is standardized as a necessary component. However, since Al has a very high affinity with oxygen, if Al is added while the molten steel oxygen is high, a large amount of alumina inclusions will be formed, and the intention of the present invention will not be satisfied.

Therefore, in the present invention, the timing for adding Al is defined as after the addition of Mg. Compared to Al, Mg
In this case, since the oxygen affinity is higher, the molten steel oxygen concentration is extremely lowered by the addition of Mg. Even if Al is added thereto, Al hardly bonds with oxygen and is dissolved in molten steel. That is, in this case, Al does not function as a deoxidizing element. Therefore, it is important to add Al after deoxidizing Mg. It should be noted that it is advantageous for the effect of the present invention to perform so-called slag reforming, in which CaO or Al is added to the slag on the molten steel in the ladle before deoxidation to reduce the oxygen potential in the slag. It is a different method, and it is expected that the number of inclusions will be further reduced and the inclusions will be finer by performing slag modification.

[0038]

EXAMPLES Carbon steels having the components shown in Table 1 were produced under the production conditions shown in Table 3, the number of inclusions in the obtained slab, the steel plate obtained by rolling the slab, and the material The results of processing were investigated. The investigation method was as shown in Table 4. Incidentally, the levels A-1, C-1, D
-1 is slag reforming, and before deoxidation of C, 1.5 t of CaO and 5 t of Al are added per 300 t of molten steel on the slag in the ladle.
00 kg was added.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

[Table 3]

[0042]

[Table 4]

The results are shown in Table 2. From the table, when the conditions of the present invention are satisfied, the number of inclusions in the cast is small, no rejection due to surface defects or internal defects occurs, and good results that no defects occur during processing. was gotten.
On the other hand, the comparative materials that do not satisfy the present invention have the following problematic results. That is, comparative materials B-2 and E
-2 and F-2, since C deoxidation was not performed before adding the deoxidizing alloy elements Ti and Mg, as a result, the molten steel oxygen concentration before adding the deoxidizing alloy element was higher than 300 ppm. , Ti-Mg-Al are added in this order, the number of inclusions in the slab increases. Comparative materials A-2, D-
In No. 2, although the C deoxidation was performed before the addition of the deoxidizing alloy element, the molten steel oxygen concentration before the addition of the deoxidizing alloy element was 3
Since it does not satisfy below 00 ppm, the number of inclusions in the slab is large. In Comparative Materials C-2, G-2, and H-2, the order of addition of the deoxidizing element did not satisfy the present invention, and thus defects were generated during product processing.

Further, the comparative material I-1 has a low Ti concentration which does not satisfy the present invention, and the J-1 has a high Al content which does not satisfy the present invention. Therefore, the K-1, Ti, Mg and oxygen concentrations are high. Since it does not satisfy the present invention at a high level, the number of inclusions in the cast piece is large. Especially in the case of J-1, 53
The number of inclusions of μm or more satisfies the condition, but 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 coil defects after rolling and defects during product processing also occur. Here, in the column of processing defects in Table 2, those marked with a minus mark are those that did not become a product and could not be processed because they failed at the coil stage.

[0045]

As described above, the present invention is a slab for thin steel plates in which the number of harmful inclusions is greatly reduced, and therefore, defects caused by inclusions in the rolled steel plate and the occurrence of product The number of defects is extremely small, a good product can be obtained, and the product yield can be improved. Furthermore, according to the present invention, it becomes possible to reliably manufacture a cast slab for thin steel sheets with few inclusion property defects.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the number of inclusions in a cast and the occurrence rate of product defects.

FIG. 2 is a diagram showing the relationship between the amount of molten steel oxygen before inclusion of Mg and the size of inclusions.

FIG. 3 is a diagram showing the relationship between the amount of molten steel oxygen and the size of inclusions before the addition of Ti.

Continuation of front page (51) Int.Cl. ⁷ identification code FI C22C 38/00 301 C22C 38/00 301Z 38/14 38/14 38/54 38/54 (56) Reference JP-A-5-43977 (JP , A) JP 9-104944 (JP, A) JP 10-324956 (JP, A) JP 11-21613 (JP, A) JP 7-97613 (JP, A) JP 10-158723 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) C21C ⁷ /00-7/10 B22D 11/00 C22C 38/00-38/60

Claims (4)

(57) [Claims] 1. C: 0.001-0.2% by weight, Mn:
0.01-0.5% by weight, Si: 0.001-0.5% by weight, P: 0.001-0.3% 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, Mg: 0.0
005-0.01% by weight, N: 0.0005-0.01
% By weight and oxygen: carbon steel containing 0.0005 to 0.0050% by weight, the balance being iron and unavoidable impurities, and the number of inclusions of 53 μm or more among oxide inclusions in the cast piece is 200. A cast piece for a thin steel sheet having few inclusion physicality defects, wherein the number of alumina cluster inclusions is 20 pieces / kg or less, and the number of alumina cluster inclusions is 20 pieces / kg or less. 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.
50% by weight, B: 0.0002 to 0.0020% by weight of one kind or two or more kinds are contained, and the cast piece for a thin steel sheet with few inclusion physical property defects according to claim 1. 3. C: 0.001-0.2% by weight, Mn:
0.01-0.5% by weight, Si: 0.001-0.5% by weight, P: 0.001-0.3% 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, Mg: 0.0
005-0.01% by weight, N: 0.0005-0.01
Wt%, oxygen: 0.0005 to 0.0050 wt%, carbon steel molten steel consisting of balance iron and unavoidable impurities is cast in a continuous casting facility to produce a slab, and then decarburized molten steel Is deoxidized in a reduced pressure atmosphere to reduce the oxygen concentration in the molten steel to 300 ppm or less, and then Ti, M
A method for producing a cast slab for a thin steel sheet with few inclusion physical defects, which comprises adding g in the order of g as a metal or alloy to deoxidize it, 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.
50% by weight, B: 0.0002 to 0.0020% by weight of one kind or two or more kinds are contained, and the method for producing a cast slab for a thin steel sheet having few inclusion property defects according to claim 3.