KR20030054282A - Mold without Surface Crack in Thin Slab Continuous Caster - Google Patents

Abstract

PURPOSE: A mold capable of reducing surface crack in continuous thin slab casting is provided to reduce cracks generated on the surface of slab by performing coating treatment on the roughly processed surface of the mold after roughly processing the surface of the mold. CONSTITUTION: The mold includes at least a copper plate(3) for mold on a portion contacted with molten steel of which non-uniformed surface roughness is processed as much as the minimum casting width of caster to lower heat flux of a part of the copper plate(3) where solidification is started; and a coating layer formed by coating a chromium (Cr) or nickel (Ni) based coating material on the upper layer of the roughness processed to extend life cycle of the copper plate(3), wherein the roughness processing is performed to width of 700 to 1,000 mm at a position corresponding to height of 0 to 300 mm from the upper part of the mold, and surface roughness processed is processed to an average surface roughness of 5 to 19 μm and the maximum surface roughness of 100 μm.

Description

Mold for Surface Crack Reduction in Thin Slab Continuous Casting {Mold without Surface Crack in Thin Slab Continuous Caster}

The present invention relates to a surface crack reduction mold in thin slab continuous casting, and more particularly, to surface crack reduction in thin slab continuous casting that can reduce the cracks generated on the surface by roughening the surface of the mold and then coating the mold. It's about molds.

In general, there have been many attempts to eliminate various deterioration factors caused by the production of flawless cast steel and high value-added steel in recent years. Among them, since the surface crack of the cast steel is directly related to the surface quality of the final product, a lot of efforts have been made to reduce such surface cracks.

Most of the surface cracks generated in the continuous casting process are known to occur in the mold, and it is known that the influence in the vicinity of the hot water surface, in which the solidification layer 2 starts to occur, is also the greatest in the mold. This is because the portion where the surface is first formed in the process of solidifying the molten steel 1 in the liquid state is the hot water surface portion in the mold.

The mechanism by which surface cracks occur can be described as follows.

end. As the molten steel 1 in the liquid state solidifies, the solidified solidification layer 2 begins to grow.

I. The solidification layer 2 in the solid state shrinks due to solidification shrinkage as cooling proceeds.

All. As the coagulation layer 2 is contracted and pulled together, the coagulation layer 2 is stretched at a weak portion, and the thinning or falling from the mold occurs.

la. The solidified layer 2, which is separated from the mold, is weakened while being heated again to become a liquid phase, and a bending phenomenon of the solidified layer 2 occurs due to the iron positive pressure.

hemp. If the banding phenomenon is severe, cracks occur in the surface portion.

bar. If the cracking phenomenon gradually worsens, the molten steel 1 flows out due to the tearing of the solidified layer 2, leading to an operation stoppage.

The mechanism by which the surface cracks occur is shown schematically in FIG. 1. Most surface cracks are generated by such a mechanism.

Therefore, in order to contribute to the improvement of quality and the productivity of high value-added steel by reducing the occurrence of surface cracks, it is necessary to reduce the initial solidification shrinkage amount and control the nonuniformity of the solidification layer 2 to grow a uniform solidification layer 2.

Many studies and experiments have been conducted to reduce surface cracks. In particular, since surface cracks are caused by excessive solidification shrinkage during initial solidification growth near the water surface, a mild cooling method was used as a method for controlling the surface cracks. Slow cooling is a method of reducing surface cracks by reducing the degree of solidification shrinkage by reducing the degree of cooling in the mold.

Representative slow cooling can be classified into two types. The first method is to cool down by lowering the thermal conductivity by increasing the crystalline rate of the molding flux used in casting (Molf Flux). This method is a method of performing a slow cooling by lowering the thermal conductivity of the mold solvent present between the mold and the slab as shown in FIG. However, this method requires the use of different mold solvents for many steel grades, and depends on the casting conditions and widths. Therefore, even though many studies have been conducted until now, the exact composition, etc. has not been determined. to be.

The second method is to change the material of the mold or to modify the surface shape to obtain a slow cooling effect. As a method of changing the material of the mold, most of the molds currently use an alloy of copper (Cu). In other words, a copper (Cu) -based alloy having high thermal conductivity is used to extract a large amount of heat from the cast in the mold to sufficiently increase the thickness of the solidified layer 2 so that no operation accident occurs. As a result of this situation, too much heat is rapidly released to the initial solidification portion, which causes large solidification shrinkage and further surface cracks. Therefore, in order to compensate for this point, there are some test results using molds made of stainless steel or nickel (Ni), which have lower thermal conductivity than copper (Cu).

In addition, a method of processing the mold surface shape is a method of filling a groove or a proper bend in the surface of the mold as shown in Figure 3 to fill a low conductivity (Ni), etc. in the groove (hereinafter Grooved Mold), the surface It is known that it is effective to some extent in crack reduction.

However, the drawbacks of these methods include the disadvantage that processing is difficult and costs are increased by processing a mold surface, and filling the inside of the groove with nickel (Ni) and then coating (coating) again. Due to these problems, methods of changing the material or reshaping the mold are currently rarely available.

In particular, a large amount of researches and patents have been submitted to Nippon Steel Co. and Nippon Steel Co., Ltd. for processing molds and cooling them. (Publication No. 1996-257695, Application No. 1995-146941, Application number 1994-242252), most of the cases are for the existing players, and the processing range also presents a surface roughness of 30㎛ or more, such as the depth of the groove (groove) grooves as shown in the table of Figure 7a.

Also in the processing method, there are many environmentally contaminated elements by using photo etching with nitrate.

In the concept of machining, only the expectation of the slowing effect is expected through surface machining, and it is designed without considering other additional factors, and it is not currently commercialized due to many obstacles in terms of machining cost and maintenance. In particular, funnel-type molds (Funnel Mold or Crown Mold) used in thin slab players have not been applied due to the complexity of the shape and the danger of high-speed casting.

The present invention has been made in order to solve the problems of the prior art as described above, in the process of processing only mold for cooling the mold surface shape in the conventional existing playing process, the molten steel (1) of the continuous casting mold By appropriately designing and processing the contact surface shape, the initial solidification shrinkage is reduced, and by controlling the uncooled and the non-uniform growth of the solidification layer 2, the uniform solidification layer 2 can be grown to suppress the occurrence of surface cracks. In order to provide a surface crack reduction mold in a thin slab continuous casting having a copper plate (3) surface shape which is designed to be easy to process and has good applicationability and is effective in reducing surface cracks. will be.

BRIEF DESCRIPTION OF THE DRAWINGS The figure explaining surface crack generation mechanism (Mechanism),

2 is an explanatory diagram of a slow effect generating mechanism using a mold solvent (Mold Flux),

3 is a view showing a surface crack reduction mold shape patented by each manufacturer, Figure 3a is a latticed mold of NKK (Japanese steel pipe), Figure 3b is a grooved mold of SMI (Sumitomo metal) (Grooved Mold), FIG. 3C is a shape diagram of a grooved mold of NSC (New Japan Iron Works),

4 is a view for explaining the principle of the surface crack generation according to the surface roughness, Figure 4a is a surface crack generating mechanism on a flat surface, Figure 4b is a description of the surface crack generating mechanism on a rough surface Degree,

5 is a view showing the shape and processing range of the mold to which the present invention is applied to a test,

6 is a view showing a copper plate surface processing portion and processing procedures,

Figure 7a is a table showing the crack incidence by the shape of the copper plate experimented on the NSC side,

7B is a table showing the surface specifications of the appropriate surface crack reduction mold presented by the NSC side.

<Description of Symbols for Major Parts of Drawings>

1: liquid molten steel 2: solidification layer

3: copper plate

In order to achieve the above technical problem, the present invention largely in the mold in the thin slab continuous casting, at least

A copper plate 3 of a mold formed by imposing a surface roughness that is not uniform as the minimum casting width of the casting machine to a portion in contact with the molten steel 1 in order to lower the heat transfer amount at the start of solidification;

In order to extend the life of the copper plate (3) it comprises a coating layer formed by coating the chromium (Cr) or nickel (Ni) based on the roughened copper plate (3).

Here, the mold is a crown-shaped or funnel-shaped mold used in a mini mill or thin slab player, and the position of roughness processing is 700 to 1000 mm in width at a height of 0 to 300 mm from the top of the mold, and is processed. The surface roughness is preferably an average surface roughness of 5 to 19 µm and a surface roughness of up to 100 µm. And the average surface roughness is preferably to maintain the surface roughness of 5 ~ 19㎛ by coating a chromium (Cr) or nickel (Ni) -based coating on it to extend the life of the copper plate (3).

Hereinafter, the configuration of the present invention as described above will be described in more detail through examples. In the reference drawings for the present invention, detailed description is omitted by using the same reference numerals for components having substantially the same configuration and function.

Comparing the growth process of the coagulation layer 2 according to the surface roughness, as shown in FIG. 4A, when the surface is smooth, the molten steel 1 is cooled on the surface and shrinkage is initiated during the coagulation process. 2) grows, but with time, the thickness of the solidified layer 2 due to solidification shrinkage becomes uneven. As a result, when leaving the mold, the thickness of the coagulation layer 2 becomes nonuniform, causing surface cracks.

On the other hand, when the surface is rough as shown in FIG. 4B, the heat transfer amount becomes uneven due to the uneven surface at the initial solidification time, but the thickness of the solidification layer 2 becomes uneven, but as time passes, the solidification layer ( 2) The thickness grows uniformly. On the mold exit side, the thickness of the solidification layer 2 becomes more uniform than when the surface is smooth.

Conventionally, in order to solve such a phenomenon, only the method of reducing the amount of heat at the surface was used, but in the present invention, in the method of providing surface roughness, in addition to the method of reducing the amount of heat, non-uniformity of the thickness of the solidification layer 2 due to solidification shrinkage The focus is on lowering the degree. That is, the amount of solidification shrinkage is reduced when the amount of heat transfer is lowered on the smooth surface as shown in FIG. 4A, but the non-uniformity of the thickness of the solidification layer 2 can not be reduced. It is possible to control the nonuniformity of the thickness of the coagulation layer 2 by controlling the amount of heat transfer, taking into account until this occurs.

The shape of the shot blasting mold of the present invention is shown in FIG. 5. Roughness processing surface is formed on the mold surface in contact with molten steel (1), width is the minimum casting width for each casting machine (about 700 ~ 1200mm range), after applying a surface roughness of about 5 ~ 19㎛ chrome on it (Cr) Coating (Coating) was used to improve the life of the copper plate (3). In the case of surface treatment in this way, by lowering the amount of heat during initial solidification, it is possible to reduce the non-uniformity of the thickness of the solidification layer 2 together with the mild cooling effect.

In addition, since the processing through a mechanical method, there is no environmental pollution problem caused by the conventional photo etching method using nitrates, and the cost of processing is small, thereby reducing the production cost.

6 is a view for explaining the machining procedure of the present invention.

First, the surface of the copper plate 3 is given the appropriate roughness as described above, and then coated on the nickel (Ni) or chromium (Cr) series, which is the copper plate 3 coating material, to extend the life of the copper plate 3. At this time, in the groove-shaped mold such as digging a groove, the surface is kept smooth, while in the present invention, in addition to the cooling effect, it is suitable even after coating to reduce the nonuniformity of the thickness of the coagulation layer 2. Surface roughness was maintained.

Hereinafter, the operation of the present invention having the above configuration will be described.

5 shows that the present invention is applied to a mold of a mini mill player. In the drawing, the shape of the copper plate 3 is a crown mold, which is different from the shape used in existing players, and the processing width is in the range of 700 to 900 mm from the center of the copper plate 3, and the height is the upper portion of the copper plate 3. The test was applied five times in the range of 250 ~ 300mm.

The average surface roughness was maintained at 5 to 19 mu m for each test, and each maximum surface roughness was set to 70 mu m or less.

As a result of the mini mill (or thin slab player) operation of the present invention, it shows an excellent effect in the production of heavy carbon steel with a large amount of solidification shrinkage. In addition, since the mold of the present invention uses a slowing effect during initial solidification, the total amount of heat is reduced to a range of 5 to 15%.

In addition, the surface defect rate of the cast steel is different depending on the steel type, but the quality of cast steel is significantly increased from 0.45 to 0.02 in the heavy carbon steel, and the linear crack incidence rate has been improved from 0.6% to 0.0%. .

As described above, although the present invention has been described by way of limited embodiments and drawings, the present invention is not limited thereto, and the technical idea of the present invention and the following by those skilled in the art to which the present invention pertains. Various modifications and variations are possible, of course, within the scope of equivalents of the claims to be described.

Therefore, according to the present invention, the surface shape processing is also easily designed to improve the operation applicability, the effect of reducing the surface cracks, and the initial solidification phenomenon can be controlled, thereby demonstrating an excellent effect on the surface quality of the cast steel. Other side effects, namely, solving problems such as blowout caused by air gap generation due to excessive solidification shrinkage and poor inflow of mold solvent, can contribute to productivity improvement of high value-added steel. That has a very breakthrough effect.

Claims (1)

In molds in thin slab continuous casting, at least A copper plate 3 of a mold formed by imposing a surface roughness that is not uniform as the minimum casting width of the casting machine to a portion in contact with the molten steel 1 in order to lower the heat transfer amount at the start of solidification; It comprises a coating layer formed by coating the chromium (Cr) or nickel (Ni) based on the roughened copper plate (3) to extend the life of the copper plate (3), The position of the roughness processing is 700 ~ 1000mm in width from 0 to 300mm in height from the top of the mold, the surface roughness to be processed is characterized by consisting of a surface roughness of 5 ~ 19㎛ average surface roughness of up to 100㎛ Surface crack reduction mold in thin slab continuous casting.