KR19980053458A - Method of preventing carburizing of cast iron surface by mold powder during continuous casting - Google Patents

Abstract

The present invention relates to a method for preventing slag carburization by mold powder during continuous casting, wherein in continuous casting of stainless steel, the average particle radius (R) of the base material in the mold powder and the average particle radius (r) of carbon are Ultrafine carbon having a ratio (R / r) of 500 or more is used, and at this time, the content of carbon is satisfied by the following formula by weight percent, and carburizing by cast powder carbon while maintaining castability and product cleanliness. It discloses a method of preventing.

[expression]

Where n = 2.5-3.5, d = density of carbon, D = density of substrate

Description

Method of preventing carburizing of cast iron surface by mold powder during continuous casting

The present invention provides a method for reducing the particle size of carbon on the basis of the carbon coated mother of mold powder particles during continuous casting of stainless steel, and also has a suitable carbon content according to the size ratio of the mold powder base particles and the size of the carbon particles. The present invention relates to a method of preventing carburizing in a mold of molten steel by carbon in a mold powder during continuous casting of stainless steel.

In general, in the steelmaking process of stainless steel, molten iron and ferroalloy are melted in an electric furnace to prepare a molten metal, and then, after securing a target composition and temperature of the molten steel in a refining furnace, the molten steel is transferred to a continuous casting process by ladle. The moved molten steel is a continuous casting process. It is fed to a mold, which is water cooled through a tundish, which solidifies to produce slabs (blooms, blooms, billets).

Figure 1 shows schematically the situation in the mold during continuous casting. Since the mold 1 is cooled by the cooling water, the molten steel 5 solidifies from the mold to form the solidification layer 6. On the other hand, 2, 3 and 4 represent the state of the mold powder added during continuous casting. Mold powder injected in the form of powder on the molten steel surface has a composition containing CaO-SiO ₂ -Al ₂ O ₃ -Na ₂ O as a main component (base materill) and some carbon added as a skeleton material. . Table 1 shows the composition of representative mold powders used in the casting of stainless steel.

Mold powder composition (% by weight) CaO F C 39.5 34 8 8 8 2.5

Such mold powder is sintered and melted by the heat of molten steel to form a slag pool on the molten steel. Therefore, in the mold powder in a steady state, as shown in FIG. 1, the liquid slag layer 4 is on the molten steel, and the powder layer (including the sintered layer) 2 is present thereon. The function of the molten slag layer 4 of the mold powder during continuous casting is to dissolve the nonmetallic inclusions emerging from the molten steel during casting, and to prevent the oxidation of the molten steel by the atmosphere and to infiltrate between the mold and the solidification layer, which is the most important role. Continuous casting is possible by forming a slag film and lubricating between cast and mold. On the other hand, the powder layer 2 mainly functions to insulate by insulating molten steel. Meanwhile, 3 in FIG. 1 shows a carbon integrated layer. About 2-3% of the carbon in the mold powder is added in the form of fine particles, and its function is to first control the melting speed of the mold powder by surrounding the substrate particles of the mold powder and preventing direct contact between the particles of the mold powder. Do it. In other words, without carbon, the mold powder particles are quickly melted by the heat of the molten steel to form a large amount of molten slag in contact with the molten steel, and the particles in contact with the atmosphere are sintered with each other to form the gaseous mold mentioned above. Since it does not form a layer of powder, the casting itself is impossible.

The second function of carbon is to keep the molten steel warm by the heat generated by the combustion of carbon. This carbon does not burn to the carbon integrated layer indicated by 3 in FIG. 1 and serves as the melting rate control mentioned above. However, below the carbon integrated layer, the substrate of the mold powder melts to form a mold leg, and the carbon remains as it is and accumulates, and a combustion reaction occurs at the same time. Therefore, the carbon concentration of the carbon integrated layer is very high, 8-10%, which is 4-5 times higher than the concentration of carbon in the raw mold powder.

On the other hand, carburizing problem of molten steel caused by contact between molten steel and carbon integrated layer is very serious. In other words, stainless steel, unlike carbon steel, contains a large amount of chromium. Therefore, when molten steel locally increases carbon by carburization, chromium and carbon react to form chromium carbide (Cr ₂₃ C ₆ , CrC, etc.) to produce hot rolled and cold rolled coil products. Causes fatal defects on the surface. Therefore, carburizing of molten steel must be prevented when casting stainless steel. Carburization of such molten steel occurs mainly in the following cases.

1) When the molten steel surface is excessively fluctuated up and down and the molten steel is in contact with the carbon layer.

2) Too much carbon content in the mold powder or carbon particles is so large that the carbon is properly combusted and extinguished and the rate of extinction slows down, causing the carbon integrated layer to be in contact with the molten steel due to excessive growth of the carbon integrated layer toward the molten steel. If you do.

3) When the consumption of mold powder is too high or the melting speed of the mold powder is too low, the molten steel and the carbon integrated layer easily contact each other.

Therefore, the conventional casting layer carburization prevention techniques include a method of increasing the stability of the molten steel surface for most casting and a method of controlling the melt rate of the mold powder with a new material instead of carbon. First of all, the method of improving the stability of molten steel surface during casting is the most basic requirement to solve all the quality problems due to the mechanical problem of the player. However, this method can prevent carburization due to the problem of (1), but it is impossible to prevent molten steel carburization by (2) and (3). On the other hand, the replacement of new melt rate control materials instead of carbon eliminates the cause of the charcoal in molten steel and thus prevents complete carburization. However, the carbon substitutes studied and used so far have limitations in that they must be very melting point and have similar specific gravity in order to have carbon-like properties. Such materials are mainly nitride systems such as BN, AIN, TIN, Cr ₂ N, and BN is currently commercially available. However, these materials have the decisive disadvantage of being very expensive compared to carbon. In addition, it is difficult to expect the warming effect of the molten steel by the combustion reaction of carbon described above, and in some steels, vanadium (B) is picked up in the molten steel to cause a serious defect in the product. Therefore, carbon pre-mold powder using BN is mostly experimental and is applied to very few steel grades, making it almost impossible to adapt to the casting of stainless steel.

An object of the present invention is to solve the above problems, to provide a method for effectively preventing carburization by mold powder of molten steel during casting of stainless steel.

An object of the present invention is to use ultrafine carbon having a ratio (R / r) of the average particle radius (R) of the base materil in the mold powder (R / r) to 500 or more, wherein It is achieved by the method for preventing carburizing of the surface of the cast steel by the mold powder during continuous casting, characterized by satisfying that the content of carbon (W) in weight percent is satisfied.

[expression]

Where n = 2.5-3.5, d = density of carbon, D = density of substrate

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 schematically shows the mechanism of carburization of slabs by carbon of a mold powder during continuous casting;

FIG. 2 is a diagram schematically illustrating a state in which carbon in a mold powder covers a base material of the mold powder; FIG.

FIG. 3 is a graph showing a calculation result of a mold powder base coating coating of carbon in a mold powder, and shows a graph showing a change in the carbon content (% by weight) in the mold powder to have the same number of coating layers when the size ratio of carbon and substrate is changed. ;

Figure 4 is a graph showing the carburizing state compared with the conventional method as a result of applying the method of the present invention in the type 430 stainless steel casting;

Figure 5 is a graph comparing the defects caused by chromium carbide due to carburization of the hot rolled coil as a result of applying the method of the present invention to the conventional method.

(Explanation of symbols for the main parts of the drawing)

1: mold of continuous casting machine 2: powder of mold powder 3: carbon integrated layer

4: molten mold slag layer 5: molten steel 6: solidification cell

7: carbon powder in the mold powder coating of carbon

8: Mold powder base materil when carbon coated mold powder

9: Mold powder particle and carbon particle size ratio (R / r) and carbon content area of existing mold powder particle

10: Mold powder particle and carbon particle size ratio (R / r) and carbon content area of the invention mold powder

11: Carburizing layer depth of existing mold powder

12: carburizing layer depth of the invention mold powder

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

In the present invention, a technique for preventing carburizing of molten steel has been approached by reducing the size and content of the particles of carbon in the mold powder. In other words, if the absolute amount of carbon decreases, the degree of carburization decreases. It is also because the carbon concentration of the carbon integrated layer will decrease because the continuous velocity of carbon increases if the particle size of carbon decreases. However, since carbon in the mold powder plays a very important function of controlling the melt rate of the mold powder, it is very dangerous to arbitrarily change the existing carbon content and size. In other words, when the size and content of carbon are inadequate and the content is too much, the melt rate of the mold powder is greatly reduced, and the formation of the mold slug pool is reduced, which is the main function of the mold powder between the cast and the mold. The friction between the cast and the mold increases, which greatly increases the possibility of break-out in which the cast, the biggest accident during casting, is torn in the mold. On the contrary, when the carbon content is too small, the formation of mold slag is excessively large, and the mold slag particles into the molten steel are mixed during casting, greatly reducing the cleanness of the cast steel. Therefore, even if changing the content of carbon and the size of the particles should find a condition that does not change the existing melting rate. Therefore, it was conceived that an appropriate carbon content and particle size could be determined based on the field operation results and calculations by setting the coated mother of carbon powder particles. FIG. 2 schematically shows a carbon-covered mother in the present invention, wherein one base of a mold powder having an average radius R is surrounded by one layer of carbon particles 7 having an average radius r. Indicates a state of presence. The weight percentage of carbon for the carbon to cover the substrate in n layers in such a momel can be written as follows when the density of the substrate is D and the density of carbon is d.

[Equation 1]

=

= (Thus R + (i-1) r = R, Rr)

Here k is the number of carbon particles required to cover the n-ply, and when k is substituted into [Formula 1], it is represented by the following [Formula 2].

[Equation 2]

=

Figure 3 shows the substrate, carbon and particle size ratio and the carbon content as a function of the number of carbon coating layers using the above equation. Here, the density of carbon was 1.9 g / cc, and the average density of the mold powder base material was 2.8 g / cc, which is a general value. 8 shows the area of the condition of the mold powder currently in use, and it can be seen from the figure that the carbon covers 2.5-3.5 layers of the mold powder substrate, and the content of carbon is 2-3%.

Conventional powder is a condition that satisfies the thickness of 8-10mm, the appropriate thickness of the mold slag layer during casting, even if the carbon particle size and content is reduced in order to prevent carburizing in the present invention, the carbon coating number is 2.5- Maintaining 3.5 ensures the same melt rate and prevents carburization. On the other hand, the size of the industrially available carbon particles having smaller particles than the existing carbon particles is in the range of 500Rr600, which is 9 areas of FIG. Therefore, when the existing R / r value is the same as the substrate size and the carbon particles are smaller to 500Rr600, as shown in Fig. 1, in order to have the same melting rate (the same number of coatings: 2.5-3.5) It can be seen that the content should also be lowered. Therefore, it can be seen that the content of carbon should be reduced according to R / r within the range of 2-3% to 1-2% of the conventional.

This effect of the present invention is verified through another embodiment.

Hereinafter, embodiments of the present invention will be described.

Example 1

First, in order to investigate the effect of carbon particle size reduction and appropriate carbon content on the slab carburization prevention, the first cast carburizing prevention technology, the conventional mold powder of Comparative Material 1 and carbon particles of Comparative Material 2 as shown in the following [Table]. Application test during casting of type 430 steel by making three mold powders, one of which is reduced in size, one that has the same carbon content, and one which has reduced carbon particles and adjusted carbon content according to the conditions of the present invention. Was done.

As can be seen from Table 2, the present invention exhibited good castability with similar results in conventional mold powder consumption and mold slag thickness, whereas Comparative Material 2, which reduced only carbon particles and did not change carbon content, showed mold slag. The thickness of the layer was reduced by half compared to the conventional and the present invention, and as a result, the nonmetallic inclusion index in the cast steel was also high. The reason for this is that the carbon content becomes excessively large compared to the size of the carbon, which surrounds the base of the mold powder in multiple layers (three or more layers), thereby greatly reducing the melting rate of the powder, thereby reducing the mold slag layer and consequently floating in molten steel. It is interpreted as a result of inability to effectively capture and dissolve inclusions.

[table]

Field application results of the mold powder of the present invention and the conventional comparison mold powder

Comparative material Invention One 2 Exam conditions Carbon particle radius (R. m) 0.048 0.028 0.028 Mold powder base radius (r. m) 15 15 15 R / r 313 536 536 Carbon content in mold powder (%) 2.5 2.5 1.5 Casting result Mold powder consumption (kg / T-s) 0.5 0.47 0.52 Mold slag pool thickness 7-10 4-5 7-10 Cast Interval Rate (ea / cm2) 0.25 0.4 0.24

Example 2

Figure 4 shows the carburizing degree of the comparative material 1 and the present invention material. In the carburizing investigation, analytical specimens were made by grinding 0.5 mm from the cast surface, and the carbon was analyzed using a CS analyzer. As can be seen in the drawing, the comparative material has a surface carburized layer 11 of about 3 mm and a maximum surface carbon concentration of 0.04%, which is superior in preventing slab surface carburization compared to conventional materials.

Example 3

5 is a comparison of the black band index due to the chromium carbide (Cr23C6) due to the surface carburization when the mold powder of the present invention and the conventional mold powder is used. Defective Coils / Coils 100). As a result of using the mold powder of the present invention, the defect is reduced by more than half compared to the conventional powder.

As such, the present invention optimizes the carbon particles and content in the mold powder to prevent carburization without using expensive carbon substitute material, and effectively prevents carburization by molten steel mold powder during casting of stainless steel.

Claims (1)

In continuous casting of stainless steel, ultrafine carbon is used in which the ratio (R / r) of the average particle radius (R) of the base materill in the mold powder to the average particle radius (r) of the carbon is 500 or more. A method of preventing carburizing by molten mold powder during continuous casting, characterized by satisfying the following formula [Formula] by weight of carbon. [expression] W = Where n is 2.5-3.5, d is the density of carbon, and D is the density of the substrate.