JP2985633B2 - Mold powder for continuous casting - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mold powder for continuous casting of steel, and more particularly to a mold powder for continuous casting which is effective for preventing occurrence of pinhole defects in a slab.

[0002]

2. Description of the Related Art In continuous casting of steel, powder or granules are added to the molten metal in a mold for the purpose of preventing oxidation of molten steel in the mold and maintaining good lubricity between the mold and the slab. So-called mold powder (hereinafter, also simply referred to as powder) is added.

A conventional continuous casting mold powder is C
aO-SiO ₂ -Al ₂ O ₃ as a base material, Na, Li, etc. are added in the form of carbonate or fluoride to this to produce alkali metal oxides such as Na ₂ O, Li ₂ O, The solidification point and viscosity are adjusted by adding fluorite. The powder melt (powder slag) flows between the mold and the slab,
By performing lubrication between the mold and the slab sufficiently, stable casting can be achieved.

However, in continuous cast slabs,
Defects in which air bubbles are trapped inside a slab called a pinhole may occur. This pinhole defect is extended in the subsequent processing step and appears on the surface to form a streak-like flaw, or if the defect remains inside, it expands during annealing, for example, a protrusion called a blister flaw on a steel sheet. This results in a shape defect.

It is said that the cause of the pinhole defect is argon (Ar) gas from a hot water supply nozzle or air sucked from a sliding nozzle.
As a countermeasure, a method of reducing the flow rate of Ar gas or the amount of air suction has been adopted. Qualitatively, pinholes can be reduced by reducing the bulk density of the mold powder to increase the heat retention of the molten steel surface, or by adding a heating material to the mold powder to increase the surface temperature of the molten steel. Was evident.

However, depending on the chemical composition of the mold powder, there is a limit to the reduction in bulk density from the viewpoint of the raw material to be compounded, or the amount of the heating material added is limited. There were many cases where it was not possible.

[0007]

SUMMARY OF THE INVENTION The present invention is directed to a method for preventing pinhole defects in continuous cast slabs, which is a method for preventing the formation of pinhole defects, regardless of the above-mentioned conventional method. The task was to provide

[0008]

SUMMARY OF THE INVENTION The powder of the present invention is completely different from the concept of the conventional pinhole reduction method in that the composition is hard to cause pinhole defects, in other words, the composition and the solidification shell are less likely to trap air bubbles. It is a mold powder having physical properties.

CAMP-ISIJ vol.6 (1993) -211 and 212
In the molten iron side near the solid-liquid interface of molten iron such as Fe-Al-O-based or Fe-S-based solidifying, aluminum (A
l) There is a concentration gradient of oxygen (O), sulfur (S), etc., which causes a difference in the interfacial tension of the molten iron at the interface between the particles (inclusions and bubbles) and the molten iron, and changes the force acting on the bubbles. Has been reported to affect the movement of bubbles. Then, it is presumed that sulfur has an action of bringing the bubbles closer to the solid-liquid interface, that is, an action of easily trapping the bubbles by the solidified shell.

[0010] According to this concept, the sulfur concentration in the molten steel is reduced, and the concentration boundary layer (as described above in the vicinity of the solid-liquid interface) is reduced.
The part where the concentration gradient of Al, S, etc. exists. By reducing the concentration gradient in FIG. 2 described later), trapping of bubbles by the solidified shell can be prevented and generation of pinhole defects can be suppressed. However, there is a limit in reducing the sulfur concentration in molten steel, and it is not a practical solution.

The inventor has taken this phenomenon from another viewpoint. That is, if the sulfur concentration gradient in the concentration boundary layer, which causes the force acting on the bubbles due to the difference in interfacial tension, can be reduced without lowering the sulfur concentration in the entire molten steel, the bubbles can be formed at the solid-liquid interface. It was thought that it was possible to reduce the effect of approaching the above, and to reduce pinhole defects generated in the continuous cast slab.

As a specific method for reducing the sulfur concentration gradient in the concentration boundary layer, a method of increasing the equilibrium distribution coefficient relating to the solid-liquid distribution of sulfur in steel can be considered.

FIG. 2 is a diagram schematically showing the concentration gradient of sulfur in the vicinity of the solid-liquid interface of molten iron (Fe-S system) during solidification. In this figure, D is the sulfur concentration of the solidified shell, B
Is the sulfur concentration of the molten iron (bulk), A and C are the sulfur concentrations of the molten iron in the immediate vicinity of the solid-liquid interface, and D / C or D
/ A is the equilibrium distribution coefficient. Therefore, the equilibrium distribution coefficient is D
Ascending from / C to D / A (indicated by arrows in the figure),
The concentration gradient of sulfur in the concentration boundary layer becomes smaller.

Regarding the above-mentioned equilibrium partition coefficient, “Iron and Steel”, vol. 70 (1984), pp. 1583 to 1589, describes the results of studies on the effect of added elements on the equilibrium partition coefficient of sulfur in iron-based alloys. It has been reported that the equilibrium partition coefficient of sulfur in iron-based alloys increases with increasing concentrations of boron (B) and carbon (C). Since the boron and carbon concentrations in the steel are determined by the specifications of each product, it is practically impossible to change them, but the present inventor added boron in the form of oxide in mold powder to form oxides. In addition, it was found that a small amount of boron was reduced during casting and dissolved near the surface of the molten steel, thereby increasing the equilibrium distribution coefficient of sulfur. Since carbon does not dissolve in the molten powder, it is difficult to transfer the carbon from the mold powder to the vicinity of the molten steel surface.

The present invention has been made based on the above findings, and the gist of the invention resides in the following mold powder for continuous casting.

The B ₂ O ₃ concentration in the powder is 1 mass% or more and 10 ma
ss%, the viscosity at 1573K is less than 1 Pa · s, the melting point is 1173K or more, and the following mass concentration ratio is 0.9.
A mold powder for continuous casting, characterized in that the mass is less than 1.3, the concentration of added carbon is 1.0 mass% or more, and the bulk density is less than 900 kg / m ³ .

Mass concentration ratio = (CaO + CaF ₂ × 0.718) / SiO ₂

[0018]

The reasons for determining the chemical composition and physical properties of the powder of the present invention in the molten state as described above will be described below.

B ₂ O ₃ in the powder has the effect of increasing the boron concentration near the molten steel surface and reducing the sulfur concentration gradient in the concentration boundary layer.
It is necessary that the B ₂ O ₃ concentration be 1 mass% or more. However, if it exceeds 10 mass%, the boron concentration in the steel will increase too much and precipitate at the grain boundaries, causing cracks in the slab or affecting the material properties. Therefore, the B ₂ O ₃ concentration in the powder is set to 1 mass% or more and less than 10 mass%.

The viscosity of the molten powder is 1 Pa at 1573K.
・ Less than s. This is to stabilize operations.
If the viscosity at 1573K is 1 Pa · s or more, the molten powder is less likely to flow between the mold and the slab, causing seizure.

If the melting point of the powder is too low, the ratio of the liquid phase in the powder film between the mold and the slab becomes too large, and the heat removal is too large, causing cracks in the slab. Therefore, the melting point of the powder is set to 1173K or more.

The main components of the powder, CaO, SiO ₂ and
CaF ₂ is closely related to the crystallization of the powder film formed between the mold and the slab. The total amount of CaO added to the amount obtained by converting the amount of CaF _{2 to the} amount of CaO, that is, the amount of CaF ₂ × 0.718
It is the ratio (mass concentration ratio) between the aO amount and the SiO ₂ amount (CaO + CaF ₂
× 0.718) / SiO ₂ is a parameter related to the crystallization of the powder film between the mold and the slab, if this value is less than 0.9, the powder film does not crystallize and becomes a glass layer, between the mold The contact heat transfer resistance is reduced, the cooling rate is increased, and the slab is cracked. On the other hand,
In the case of 1.3 or more, the melting point rises too much and slagging is not performed well, which causes a problem.

If the concentration of the added carbon is less than 1.0 mass%, the heat retention becomes poor or the slagification becomes unstable and casting cannot be performed.

The bulk density is less than 900 kg / m ³ . 900kg / m ³
In the above case, the heat retention is poor, and it is impossible to prevent the generation of pinholes even if B ₂ O ₃ is added. In addition,
The bulk density is a bulk density specified in JIS K 5101.

[0025]

EXAMPLES Using the powders shown in Table 1, a casting test of low carbon steel was performed using a single-point straightening continuous casting machine with a curvature radius of 10 m, and the occurrence of cracks and the number of pinholes on the slab surface were investigated. Slab size (cross section) is 1500
mm × 250mm, casting speed is 1.5m / min. The chemical composition of the molten steel is as shown in Table 2.

The occurrence of cracks is determined by visual observation. The number of pinholes is determined by removing the scale on the surface of the slab by pickling and then applying a dye to one surface (upper surface) of the slab to dye the pinhole. It was determined by counting the number.

As a result of the investigation, when powders Nos. 2 to 5 (powder of the present invention) were used, the powder consumption was 0.
It was appropriate at 35 to 0.39 kg / m ² and could be cast without any operational problems such as breakout. The number of occurrences of pinholes is shown in FIG. 1 (the number on the horizontal axis corresponds to the number in Table 1) as an average value obtained by examining several slabs, but the number of pinholes was extremely small.

On the other hand, No. ₂ containing no B ₂ O ₃
If one powder is used, the powder consumption is 0.35
Casting was possible at ~ 0.39 kg / m ² without breakout, etc., but pinholes occurred on the slab surface. High B ₂ O ₃ concentration
When the No. 6 powder was used, the generation of pinholes was extremely small, but BN was precipitated at the grain boundaries and large cracks were generated in the slab.

When the powder No. 7 was used, the powder consumption was only about 0.05 kg / m ^{2 and} breakout occurred due to poor lubrication.

When the No. 8 and No. 9 powders were used, there were no operational problems such as breakout, and the number of pinholes was extremely small. Caused trouble and could not be used.

The mass concentration ratio exceeds the range defined by the present invention.
When the powder of No. 10 was used, the melting point was too high and the powder film formed between the mold and the slab had almost no molten layer, and the powder powder was involved, causing breakout. When No. 10 and No. 7 powders were used, the number of pinholes could not be investigated because breakout occurred.

When the powder of No. 11 was used, the melting of the powder was too fast and skinning occurred on the surface of the molten steel.
Moreover, many pinholes were generated on the surface of the slab.

When the No. 12 powder was used, the powder consumption was 0.35 to 0.39 kg / m ² , and casting was possible without breakout. However, the same as when the No. 11 powder was used. Many pinholes were generated on one surface.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

When the mold powder of the present invention is used for continuous casting of steel, the occurrence of pinhole defects on the surface of the slab can be prevented without causing cracks or breakouts.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the powder used and the number of pinholes generated.

FIG. 2 is a view schematically showing a concentration gradient of sulfur in the vicinity of a solid-liquid interface of molten iron during solidification.

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) B22D 11/10 370 B22D 11/07

Claims (1)

(57) [Claims] 2. The method according to claim 1, wherein the main components are CaO, SiO ₂ and CaF _2.
B ₂ O ₃ concentration in the powder is 1 mass% or more and 10 ma
ss% and the viscosity at 1573K is 1 Pa ·
s, the melting point is 1173K or more, the following mass concentration ratio is 0.9 or more and less than 1.3, and the added carbon concentration is 1.0 or less.
A mold powder for continuous casting, characterized by having a mass density of not less than mass% and a bulk density of less than 900 kg / m ³ . Mass concentration ratio = (CaO + CaF ₂ × 0.718) / SiO ₂