JP6809256B2 - Mold powder for continuous casting - Google Patents

Description

The present invention relates to low carbon steel (for example, 0.0200 to 0.0800% C in mass%) and Nb-Ti-SULC (ultra-low carbon steel, for example 0.0005 to 0.0200% C in mass%). When continuously casting slabs for high-grade steel such as high-clean beverage cans and large outer panels for high-clean automobiles, it is possible to suitably prevent surface defects and obtain slabs with good surface quality. It relates to a mold powder for continuous casting.

Continuous casting in low carbon steel (eg 0.0200-0.0800% C in mass%) and Nb-Ti-SULC (eg very low carbon steel, eg 0.0005-0.0200% C in mass%) If so, it is necessary to inject powder for lubrication between the mold and the solidified shell. Further, in order to improve the inflow characteristics of the powder, it is necessary to reduce the viscosity of the powder. On the other hand, in order to prevent surface cracks and surface defects of low carbon steel and Nb-Ti-SULC (ultra-low carbon steel) and improve surface quality, it is important to prevent powder entrainment, and therefore SiO ₂ in the powder. The reactivity is reduced by reducing the activity. Even if powder designed and manufactured under such strict restrictions is used, when the steel type is a steel plate, streaks on the hot-rolled plate and streaks after hot-dip galvanizing the steel plate are used. The problem related to the shape of the defect has not been solved yet, and some of the defects are caused by the peculiar component in the powder component.

The above-mentioned streaky flaws on the hot-rolled plate and streaky flaws after hot-dip galvanizing the steel sheet are caused by oxide-based non-metal inclusions and powder components existing from the slab near the surface of the rolled plate in the heating furnace. It occurs because it is crushed or stretched and becomes a surface defect of the steel sheet during heating, subsequent hot rolling, and cold rolling. To solve this problem, by defining the components of non-metal inclusions and powders that have been problematic so far, and by clarifying the behavior in the mold, the composition that does not form surface defects, the method of controlling by the casting method, etc. It has been taken.

For example, Patent Document 1 and Patent Document 2 describe that defects can be reduced by controlling P ₂ O ₅ in the powder to 0.5% or less, or 0.5-10 with a high SiO ₂ concentration powder. A method of non-Newtonian fluidization as suspended particles in a mold containing% Cr ₂ O ₃ is described.

Japanese Patent Application No. 2016-100309 Japanese Patent No. 5454131

However, even if the techniques of Patent Document 1 and Patent Document 2 are used, it is not possible to completely eliminate the streaky flaws of the hot-rolled plate and the streak flaws after hot-dip galvanizing the steel plate. It cannot be dealt with by simply specifying the powder component, that is, it is suspended in the mold due to the component on the molten steel side or by using a high SiO ₂ concentration powder containing 0.5-10% Cr ₂ O _3. This is because it may be caused by factors that cannot be dealt with only by physical operations such as non-Newtonian fluidization as particles.

Therefore, in view of this background, the present invention defines the simple powder components as described above and the suspended particles in the mold when continuously casting low carbon steel or Nb-Ti-SULC (ultra-low carbon steel). Even if there are factors that cannot be dealt with only by the physical operation (physical phenomenon) of the above, we devise and provide a powder that can prevent surface defects and obtain slabs with good surface quality. Therefore, not only the component fluctuation of the powder itself is small, but also the molten steel component fluctuation is small, and the powder component is not reduced by the molten steel component, and the powder component is set using the concept of thermodynamic equilibrium distribution of molten steel-powder. Design and manufacture. Thereby, the component caused by the surface defect can be transferred from the molten steel to the powder side to make it harmless, and the powder for preventing the surface defect and obtaining a slab having good surface quality can be provided.

As a result of diligent research, the present inventor has found that the streaky flaws on the hot-rolled plate caused by the use of powder and the streaky flaws after hot-dip galvanizing the steel plate are problems of the components of the powder only. It was also found that it is a phenomenon based on a chemical reaction between molten steel and powder that cannot be suppressed by a physical phenomenon, and that it can be suppressed by using the theory of such two-phase thermodynamic chemical equilibrium. In the present invention, in order to reduce the flaws caused by the powder component, especially the peculiar component in the powder, in particular, phosphorus ([P]) in steel and phosphoric acid ((P ₂ O ₅ )) in powder are used. We investigated a powder for continuous casting that enables high-speed casting while preventing streaky surface flaws caused by it. That is, using the concept of thermodynamic chemical reaction equilibrium distribution of molten steel-powder, phosphorus in steel generated by the reoxidation reaction of molten steel by powder during casting in the mold is not caused. ([P]) is detoxified by shifting from molten steel to the powder side. As a result, it is possible to obtain a powder for continuous casting that enables high-speed casting by reliably dealing with the streak-like flaws of the hot-rolled plate and the streak-like flaws after hot-dip galvanizing the steel sheet. This idea can be applied to low Si, high Si, high Mn, high Al, high Ti, and high Nb as the chemical reaction equilibrium distribution of molten steel-powder. I found that. The present invention has been completed based on such findings. In addition, [] of [P] indicates the component in molten steel, the unit is mass%, () of (P ₂ O ₅ ) indicates the oxide component in powder and slag, and the unit is mass%.

The present invention has been completed based on such findings. The gist of the present invention is as follows.

Mold powder for continuous casting used for continuous casting of low carbon steel and ultra-low carbon steel in which C is 0.0005% or more and 0.0800% or less in mass%, and the low carbon steel and ultra-low carbon steel. A mold powder for continuous casting, characterized in that the components of the mold powder supplied into the mold during continuous casting of the slabs of the above components satisfy the following formula (1).
([Si] × [P] (4/5) × (Al2O3) (4/3)) / ((SiO2) × (P2O5) (2/5) × [Al] (8/3)) ≧ 1.64 ( 1)
Here, [Si], [P], and [Al] indicate the mass% of the components in the molten steel, and (Al2O3), (SiO2), and (P2O5) are the portions of the interface between the molten powder and the molten steel in the mold. It is the mass% of the oxide component of the melt of the molded powder sampled by charging the analysis sampler.

By using the powder of the present invention, it is possible to prevent surface cracks and scratches due to powder entrainment, and to reduce scratches caused by peculiar components in the powder component.

It is a graph which shows the relationship between the equation (1) and the streak pattern occurrence rate. It is a graph which shows the streak pattern occurrence rate before use and after use of the powder of this invention.

The inventors have conducted many experiments from the viewpoint of preventing powder entrainment in order to improve the surface quality of low carbon steel (including ultra-low carbon steel), and streaky surface flaws caused by powder are physical such as powder entrainment. I realized that it was not a normal phenomenon. Therefore, in the present invention, as described above, streaky flaws in the hot-dip galvanized sheet are particularly caused by phosphorus ([P]) in steel and phosphoric acid ((P ₂ O ₅ )) in powder. In order to deal with the problem of streaky flaws after hot-dip galvanizing a steel sheet, the idea was to increase the phosphorus content capacity in the powder by a new idea thermodynamically using a chemical reaction. The details will be described below.

When continuous casting is performed on low carbon steel or Nb-Ti-SULC (ultra-low carbon steel), the streaks of the hot-rolled plate that occur due to the use of powder and the streaks after hot-dip galvanizing the steel sheet The points to solve the problem of defects are as follows. As mentioned above, the idea of distributing P in the molten steel-powder phase was applied to the idea of concentrating (incorporating) phosphorus [P] into the powder. That is, it can be expressed by the phosphorus distribution ratio (P ₂ O ₅ ) / [P] equation, and the idea was to consider increasing this P distribution equation.

On the other hand, it is considered that each component in the molten steel and each component in the powder undergo a redox reaction to distribute each component to the molten steel-powder phase. The reaction of [P] is shown by the following reaction formula (1), and the components involved in the redox reaction of P are considered as [Al] and [Si], and the corresponding reaction is the reaction formula (2). , (3), it was thought that it could be shown by the redox reaction of [Al] and [Si] in steel.
(P ₂ O ₅ ) → 2 [P] + 5 [O] (1)
2 [Al] +3 [O] → (Al ₂ O ₃ ) (2)
[Si] +2 [O] → (SiO ₂ ) (3)
At this time, assuming that the [O] concentration related to the above equations (1), (2), and (3) is constant, the equations (1), (2), and (3) can be combined, and the following equation ( 4) can be obtained.
(SiO ₂ ) + 2/5 (P ₂ O ₅ ) + 8/3 [Al] → [Si] + 4/5 [P] + 4/3 (Al ₂ O ₃ ) (4)
This equation (4) is based on the assumption that the [O] concentration involved is constant, but this time, we came up with the idea of applying this reaction equation to the (P ₂ O ₅ ) / [P] equation. .. Applying equation (4), adding the terms [Al] and [Si], the following equation (5) is obtained, and this equation (5) is used as the applied P distribution equation, which is an index of surface defects this time. I was conceived to use it as. This equation (5) is used as an index.
([Si] × [P] ^(4/5) × (Al ₂ O ₃ ) ^(4/3) ) / ((SiO ₂ ) × (P ₂ O ₅ ) ^(2/5) × [Al] ^{(8) / 3)} ) (5)

Here, in the present invention, [Si], [P], and [Al] indicate the mass% of each component in the molten steel, and (Al ₂ O ₃ ), (SiO ₂ ), and (P ₂ O ₅ ) are. , The mass% of the oxide component of the melt of the molded powder sampled by charging the analysis sampler at the interface between the molten powder and the molten steel in the mold.

In this way, using this equation (5), when the slab becomes the coil of the steel plate of the final product, the incidence of surface defects per coil is arranged on the vertical axis. As shown in FIG. 1, 1.64 is obtained. At the boundary, a region in which the incidence of surface defects can be suppressed to 0 was obtained, and the present invention was established.

According to the above equation (4), (SiO ₂ ) activity (concentration), (P ₂ O ₅ ) activity (concentration), and (Al ₂ O ₃ ) activity (concentration) can be lowered, lowered, or raised, respectively. , P can be considered to be effective in incorporating into powder. That is, if only considering the uptake of P, it is desirable to set (SiO ₂ ) = 0, (P ₂ O ₅ ) = 0, (Al ₂ O ₃ ) = large, but then it is normal as a powder. It does not satisfy the viscosity required for lubrication and heat retention, solidification temperature, etc., and cannot function as a powder. Therefore, it is necessary to consider the design of a normal powder that satisfies the normal lubrication, the viscosity required for heat retention, the solidification temperature, etc. within the range of 1.64 or less in the formula (5). .. That is, it is possible to design a powder satisfying the equation (5) = <1.64 while obtaining the lubrication, heat retention, viscosity, and solidification temperature of a normal powder, and the viscosity and solidification temperature are not particularly specified in the present invention. As a normal powder, the viscosity at 1300 ° C. is 0.5 to 20 poise, and the solidification temperature is 1000 to 1300 ° C.

The slab cast according to the present invention is subsequently rolled or the like and used for an automobile thin steel sheet, an automobile member, or a low carbon hot-rolled steel sheet. The components of the slab of the present invention are, for example,% by mass.
C: 0.0005-0.0800%,
Si: 0.01-1.00%,
Mn: 0.10 to 1.00%,
P: 0.0500% or less,
S: 0.0100% or less,
T. O: 0.0040% or less,
N: 0.0005 to 0.0100%
Acid-soluble Al: 0.003 to 0.130%,
Acid-soluble Ti: 0.0050 to 0.1000%,
Nb: 0.0050 to 0.1000%,
Residual Fe and unavoidable impurities, which may contain Cr, Mo, V, W, B, Ca, REMCu, Ni, and Zr as selective elements. The main components of the slab and the steel plate according to the present invention will be described below. Unless otherwise specified, the unit is mass%.

<C: 0.0005 to 0.0800%>
C is the most basic element that controls the workability, hardenability and strength of low carbon steel and IF steel, and effectively contributes to improving good workability while ensuring the required strength. To do. That is, C is an element essential for achieving both the strength of the steel sheet and good workability, and in order to obtain good workability, it is preferably contained at least 0.005%. Preferably, it is 0.002% or more. On the other hand, if C exceeds 0.0800%, a large amount of coarse protophilic cementite structure is generated and the workability is lowered. Therefore, in order to secure the necessary workability, C should be 0.08% or less. Is preferable.

<Si: 0.01 to 1.00%>
Si is one of the main deoxidizing elements, and has the function of increasing the number of austenite nucleation sites during quenching and heating, suppressing the grain growth of austenite, and refining the particle size of the quenching hardened layer. .. Further, Si suppresses the formation of carbides, suppresses the decrease in grain boundary strength due to carbides, and is also effective for the formation of bainite structure, so that the strength is increased without significantly impairing the elongation and the yield strength is low. It is an important element for improving hole expansion and bending workability.

In this embodiment, it is an important element because it is involved in the thermodynamic reaction equilibrium with the SiO ₂ system component in the powder. The SiO ₂ component in the powder is reduced with Al to produce alumina-based inclusions or the Al ₂ O ₃ component in the powder. Since it is necessary to add 0.01% or more of Si in order to reduce the dissolved oxygen concentration in the molten steel and once generate SiO ₂ inclusions, the lower limit of Si can be set to 0.01%. preferable. On the other hand, if the Si concentration is too high, the SiO ₂ concentration in the inclusions becomes high, the powder easily causes an oxidation reaction, and reduction by Al is less likely to occur. This also increases surface flaws. In addition to this, excess Si adversely affects good workability, weldability and ductility, and deteriorates moldability. Therefore, it is preferable to set the upper limit of Si to 1.00%.

<Mn: 0.10 to 1.00%>
Mn is an element useful for deoxidation in the steelmaking stage, and together with C and Si, is an element effective for increasing the strength of steel sheets. In order to obtain this effect, it is preferable to contain 0.10% or more. In order to obtain a steel sheet having higher strength, it is more preferably 0.15% or more. However, if it is contained in excess of 1.00%, the ductility is lowered due to the segregation of Mn and the increase in solid solution strengthening. Further, since workability, weldability and base metal toughness are also deteriorated, it is preferable to set the upper limit of Mn to 1.00%.

<P: 0.0500% or less>
Since P is smaller than the Fe atom and acts as a substitution type solid solution strengthening element, it is an element effective for improving the strength. However, if it exceeds 0.0500%, segregation occurs at the grain boundaries of austenite, the grain boundary strength is lowered, the torsional fatigue strength is lowered, and the workability is deteriorated. Therefore, the upper limit is 0.0500%. Is preferable. However, if there is no need for solid solution strengthening, it is not necessary to add P, so the lower limit of P is assumed to include 0.0000%.

<S: 0.0100% or less>
S segregates as an impurity to form MnS-based coarse stretch inclusions, which deteriorates processability and material properties. Further, in order to reduce such an adverse effect, it is desirable that the concentration is as low as possible, and usually, in addition to reducing at the refining stage, Ca may be added or the like. CaS and CaO-Al ₂ O ₃ inclusions produced by the Ca addition, workability, because an adverse effect on workability of the weld, S is desirably a further lower concentration. However, in the present embodiment, since the influence on the surface defect can be ignored, S can be set to 0.0100% or less. Therefore, in the present embodiment, although the influence on the surface defect can be ignored, S is 0.0070% or less in order to prevent adverse effects on the material such as workability or to obtain the ultimate improvement in the material. Is preferable. The lower limit does not need to be set in particular, but the amount that can be realized when desulfurization is performed by strengthening the secondary refining is 0.0001%.

<T. O: 0.0040% or less>
Oxygen forms oxides as impurities. Mass-based total oxygen content T.I. When O is too high, mainly Al ₂ O ₃ system inclusions increase, the oxygen potential of the system cannot be minimized, ductility and workability become extremely poor, leading to an increase in surface defects. Therefore, in the present embodiment, the mass-based total oxygen amount T.I. The upper limit of O is preferably 0.0040%.

<N: 0.0005 to 0.0100%>
N is an element that is inevitably mixed in the steel because nitrogen in the air is taken in during the molten steel treatment. N forms a nitride with Al or the like to promote fine graining of the base metal structure. However, if this N is contained in an amount of more than 0.0100%, coarse precipitates such as Al are formed, and the processability, material properties, and toughness are deteriorated. Therefore, in the present embodiment, the upper limit of the concentration of N is preferably 0.0100%. The upper limit of the more preferable concentration of N is 0.005%. On the other hand, since the cost is high to make the concentration of N less than 0.0005%, it is preferable to set 0.0005% as the lower limit from the viewpoint of industrial feasibility.

<Acid-soluble Al: 0.003 to 0.130%>
In general, acid-soluble Al tends to be coarse due to clustering of its oxide, and when a large number of coarse Al ₂ O ₃ are present, ductility is extremely deteriorated, surface defects are increased, and various materials are used. It is desirable to suppress it as much as possible because it deteriorates. However, the present inventor is important as an element involved in the formation of surface defects by performing Al deoxidation. Therefore, in the present embodiment, it is not necessary to set a restriction that Al is substantially not added in order to avoid coarse clusters of alumina-based oxides as in the conventional case, and the concentration of the acid-soluble Al is particularly free. It is possible to control to prevent surface defects while increasing the degree. Further, by positively adjusting the acid-soluble Al to 0.003% or more, the effect of suppressing the oxygen potential in the steel can be enjoyed. This effect becomes more remarkable when the acid-soluble Al concentration exceeds 0.01%.

Corrosion resistance is also improved due to the high Al concentration. The acid-soluble Al concentration is preferably 0.130% or less from the viewpoint of increasing the effect and the addition cost. Further, the acid-soluble Al concentration referred to here is a measurement of the concentration of Al dissolved in an acid, and is an analysis method utilizing the fact that dissolved Al is dissolved in acid and Al ₂ O ₃ is not dissolved in acid. is there. Here, as the acid, for example, a mixed acid in which hydrochloric acid 1, nitric acid 1, and water 2 are mixed in a ratio (mass ratio) can be exemplified. By using such an acid, Al soluble in acid and Al ₂ O ₃ insoluble in acid can be separated, and the concentration of acid-soluble Al can be measured.

<Acid-soluble Ti: 0.0050 to 0.1000%>
Ti is one of the major deoxidizing elements, forms carbides, nitrides, and carbonitrides, and by sufficient heating before hot rolling, it increases the number of austenite nucleation sites and austenite grains. It is an element that contributes to miniaturization and high strength to suppress growth and has a function of effectively acting on dynamic recrystallization during hot rolling.
The present inventors have experimentally found that it is necessary to add 0.0050% or more of acid-soluble Ti in order to secure the above function. Therefore, the lower limit of acid-soluble Ti was set to 0.0050%. The lower limit of preferred acid-soluble Ti is 0.010%.
On the other hand, if the acid-soluble Ti is contained in excess of 0.1000%, not only the deoxidizing effect is saturated, but also coarse carbides, nitrides and carbonitrides are produced even if sufficient heating is performed before hot rolling. Is generated, and on the contrary, the material is deteriorated and the effect commensurate with the content cannot be expected. Therefore, the upper limit is set to 0.1000%.
Incidentally, the acid-soluble Ti concentration is a measurement of the concentration of Ti dissolved in an acid, and is analyzed and quantified by utilizing the fact that dissolved Ti is dissolved in an acid and Ti oxide is not dissolved in an acid. Here, as the acid, for example, a mixed acid in which hydrochloric acid 1, nitric acid 1, and water 2 are mixed in a ratio (mass ratio) can be used. Using this acid, Ti soluble in acid and Ti oxide insoluble in acid are separated, and the acid-soluble Ti concentration is measured.

<Nb: 0.0050 to 0.1000%>,
Nb is an element that forms carbides, nitrides, and carbonitrides with C or N to promote fine graining of the base metal structure and contribute to improvement of toughness. The present inventors have experimentally found that it is necessary to add 0.0050% or more of Nb. Therefore, the lower limit of Nb was set to 0.0050%. The preferred lower limit of Nb is 0.010%. It is preferable to add Nb in order to obtain the above-mentioned carbides, nitrides and the like. However, if the amount of Nb added exceeds 0.1000%, the effect of atomizing the base metal structure is saturated and the production cost increases, so the upper limit is set to 0.1000%.

Residual Fe and unavoidable impurities, which may contain Cr, Mo, V, W, B, Ca, and REM as selective elements.

Cr, Mo, V, W, and B are elements that improve the strength of steel and the hardenability of steel and can be added. Since these elements are selective elements, the presence or absence of addition is arbitrary, and only one type may be added or two or more types may be added as needed. However, in order to obtain the above-mentioned carbides, nitrides and the like, they are added as necessary. In order to obtain this addition effect, it is preferable to add 0.01% or more of each selected element. However, the addition of a large amount of these elements deteriorates the strength-ductility balance, so the upper limit is set to 0.10%.

B: 0.0002 to 0.0020%
B is added as needed in order to further strengthen the grain boundaries and improve workability. In order to obtain this effect, it is preferable to add 0.0002% or more. However, even if B is added in an amount exceeding 0.0020%, the effect of addition is saturated, and on the contrary, the cleanliness of the steel is impaired and the ductility is deteriorated. Therefore, the upper limit is set to 0.0020%.

Ca: 0.0005 to 0.0050%
Ca is an important element for fixing S and forming inclusions. Further, Ca is an element that strengthens grain boundaries and improves the workability of steel by controlling the form of desulfurization such as spheroidizing sulfide. To obtain this effect, 0.0005% or more of Ca is added. However, even if a large amount of Ca is added, the effect of addition is saturated, and the cleanliness of the steel is impaired and the ductility is deteriorated. Therefore, the upper limit is set to 0.0050%.

REM: 0.0005-0.0100%
REM reduces SiO ₂ generated by Si deoxidation and Al ₂ O ₃ generated by Al deoxidation sequentially, and easily becomes a precipitation site of MnS-based inclusions, and is hard and fine, and deforms during rolling. Difficult inclusions, ie Ce oxides (eg Ce ₂ O ₃ , CeO ₂ ), cerium oxysulfide (eg Ce ₂ O ₂ S), La oxides (eg La ₂ O ₃ , La O ₂ ), Lantern oxysulfide (for example, La ₂ O ₂ S), Ce oxide-La oxide, and / or inclusions containing cerium oxysulfide-lanthanum oxysulfide as the main phase (within 50% or more as a guide). Since it is an element having a function of forming, it is effective and can be added.
If the REM is less than 0.0005%, SiO ₂ and Al ₂ O ₃ inclusions cannot be reduced, while if it exceeds 0.0100%, a large amount of cerium oxysulfide and lanthanum oxysulfide are produced. Even under the above conditions, the low oxygen potential cannot be achieved, and further, it becomes a coarse inclusion and deteriorates the low temperature toughness. The preferred lower limit of REM is 0.0010%. The upper limit of the preferable REM is 0.0085%.

Cu, Ni, Zr In the present invention, Cu, Ni, Cr, and Zr, which are elements that improve the strength of steel and the hardenability of steel, can be further added. Since these elements are selective elements, the presence or absence of addition is arbitrary, and only one type may be added or two or more types may be added as needed. The reasons for limiting the selected elements will be described below.

Cu: 0.10 to 2.00%
Cu can be contained as necessary in order to contribute to strengthening the precipitation of ferrite and improving fatigue strength, and further to secure the strength of the steel sheet, and in order to obtain this effect, 0.10% or more should be added. Is preferable. However, the inclusion of a large amount of this Cu rather deteriorates the strength-ductility balance. Therefore, the upper limit is 2.00%.

Ni: 0.05 to 1.00%
Since Ni can be solid-solved and strengthened of ferrite, it can be contained as needed in order to further secure the strength of the steel sheet, and 0.05% or more may be added to obtain this effect. preferable. However, the inclusion of a large amount of Ni rather deteriorates the strength-ductility balance. Therefore, the upper limit is 1.00%.

Zr: 0.001 to 0.010%
Zr is an element that has the effect of spheroidizing sulfide and improving the toughness of the base metal. In order to obtain this effect, it is preferable to add 0.001% or more. However, the addition of a large amount of Zr impairs the cleanliness of the steel and deteriorates the ductility, so the upper limit is set to 0.010%.

This molten steel component is an analytical value obtained by spark emission spectroscopic analysis (KANT BACK, QV) or chemical analysis of molten steel sampled in a tundish during casting. For the components of the mold powder, the mold powder is supplied onto the molten steel in the mold during casting, an analytical sampler is charged at the interface between the mold and the molten steel, and the sampled melt of the mold powder is subjected to the glass bead method. , Obtained by analysis by luminescence analysis method.

The present invention can be summarized as follows.
(1) Thermodynamic reaction of steel-powder Using the concept of equilibrium distribution, phosphorus ([P]) in steel generated by the reoxidation reaction of molten steel by powder is taken in (transferred) from molten steel to the powder side. [P] Increase the uptake capacity (2) Decrease the SiO ₂ activity in the powder to reduce the reactivity (prevent the reoxidation reaction), and phosphorus ([P]) in the steel generated by the reoxidation reaction of the molten steel by the powder. Eliminate (reduce)
(3) Low viscosity to improve powder inflow characteristics for lubrication between mold and solidified shell. Detoxification of flaws and defects, ensuring lubricity, and high-speed casting while preventing powder entrainment are possible. It becomes a powder for continuous casting. The most important points are (1), and (2) is based on the conventional concept of powder design in low carbon steel and ultra-low carbon steel, but the reaction is controlled so as not to reduce P. (3) is a concept based on a conventional idea, which is a preferable condition in the present invention.

By using the powder of the present invention, surface cracks and scratches due to powder entrainment can be prevented, and scratches caused by peculiar components in the powder component can be reduced. As shown in FIG. 2, the streak pattern occurrence rate can be reduced. Was able to be reduced from the conventional 10 to 0.
By the versatile indexing using the powder component and the molten steel component this time, the same effect can be obtained not only in a limited steel type but also in various steel types.

The steels of the components shown in Table 1 below were continuously cast using the powders shown in Table 2 and rolled to obtain steel sheet coils. In that case, the presence or absence of flaws and the occurrence rate of streaky patterns are shown. In the case of the powder component within the range of the formula described in the present invention, there is no defect and it is good.

Claims (1)

Mold powder for continuous casting used for continuous casting of low carbon steel and ultra-low carbon steel in which C is 0.0005% or more and 0.0800% or less in mass%, and the low carbon steel and ultra-low carbon steel. A mold powder for continuous casting, characterized in that the components of the mold powder supplied into the mold during continuous casting of the slabs of the above components satisfy the following formula (1).
([Si] × [P] ^(4/5) × (Al ₂ O ₃ ) ^(4/3) ) / ((SiO ₂ ) × (P ₂ O ₅ ) ^(2/5) × [Al] ^{(8) / 3)} ) ≧ 1.64 (1)
Here, [Si], [P], and [Al] indicate the mass% of the components in the molten steel, and (Al ₂ O ₃ ), (SiO ₂ ), and (P ₂ O ₅ ) are the molten powders in the mold. The mass% of the oxide component of the melt of the molded powder sampled by charging an analytical sampler at the interface between the and molten steel.