KR20040045966A - Molds for irregular heat flow direction in continuous castings - Google Patents

Abstract

PURPOSE: A mold capable of forming equiaxed crystal solidified structure at casting slab as growing randomized solidification layer by guiding non-uniformity of electric heat direction as reducing solidification contraction amount by initial solidification delay is provided. CONSTITUTION: In a continuous casting mold comprising long side part and short side part, the continuous casting mold having randomized electric heat directional characteristics is characterized in that a surface roughness having a depth of 0.01 to 0.08 mm and a gap between valleys of 0.01 to 0.08 mm is formed at a region corresponding to an initial solidification part on the inner surface of the long side part, wherein the region corresponding to the initial solidification part is a region ranging from an upper part of the long side part to an arbitrary point that is 300 to 350 mm distanced downward from the upper part of the long side part, wherein the surface roughness is formed by shot blast using emery powder, and wherein a coating layer of Ni and Cr is formed in a single layer or double layers on the inner surface of the mold, and the surface roughness has 0.01 to 0.08 mm of a depth from an upper part of the coating layer and 0.01 to 0.08 mm of a gap between the valleys of the coating layer.

Description

Mold for continuous heat flow in non-uniform heat transfer characteristics {Molds for irregular heat flow direction in continuous castings}

The present invention relates to a mold for continuous casting, and more particularly, to form a surface roughness in the area of the initial solidification portion of the inner surface of the mold to reduce the amount of solidification shrinkage, thereby making it possible to generate equiaxed crystal structure in the cast. The present invention relates to a continuous casting mold.

As expectations for high-quality cast steel have risen, various attempts have been made to identify and eliminate the causes of various defects in the cast. Among them, the surface quality of cast steel is caused by surface defects in the later process, and work is being done to minimize defects occurring on the surface. The types of surface defects that are frequently generated in the continuous casting process can be classified into three types as follows.

(1) Scab or Sliver defects on board

In the case of silicic defects, nonmetallic inclusions or mold powders, which are mold solvents, exist under the surface of the cast steel and protrude to the surface during rolling to generate defects, which are mainly related to molten steel flow.

(2) Blow Hole or Pencil Pipe Defects

Pencil pipe defects inject sealing gas into the nozzle to prevent clogging of the immersion nozzle between the tundish and the immersion nozzle.The blown gas flows into the mold together with the molten steel and is not injured. If present, defects occur during rolling.

(3) duty free and horizontal cracks

In the case of the duty free crack, when the molten steel solidifies, the shrinkage occurs and the contact between the cast and the mold decreases, so that the crack is generated by the tensile stress on the cast.

The occurrence of (1) silicic defects and (2) pennastic pipe defects are mostly related to the flow of molten steel, but are more correlated with the length of oscillation hooks caused by the vibration of the mold. The oscillation hook is a portion generated by deforming the solidification layer in the process of vibrating the mold up and down so that the cast is not constrained to the mold. The generating mechanism is shown in FIG. 1. When the oscillation hook grows for a long time, the floating inclusions are attached to the bottom surface (just below the surface) to generate the silicon defect, and when the sealing gas is attached to the surface layer, it becomes a penpipe pipe defect. A schematic diagram of the inclusion adhesion phenomenon according to the depth of the oscillation hook is shown in FIG. 2.

In order to reduce the defects related to inclusions or bubbles, the technique of controlling the molten steel flow pattern in the mold and floating the inclusions is mainly used. That is, a method using electromagnetic (Japanese Patent Laid-Open Publication No. 2002-028761, 1998-249297, 1998-005945, etc.), or a method of promoting floating separation of inclusions by changing the shape of the immersion nozzle. However, such an attempt has not been able to significantly reduce the silicon defect and the Pensil pipe defect to date, due to the large amount of additional costs associated with the device modification or the weak effect.

Surface cracks are caused by excessive solidification shrinkage during initial solidification growth near the surface of the water so that many methods are used to control this. 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. There are three major slow cooling methods.

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 cast as shown in FIG. However, when using this method, it is necessary to use different mold solvents for all kinds of steels, and it depends on the casting condition and width. Therefore, even though many studies have been conducted until now, the exact composition is not determined. It is true.

The second method is to change the material of the mold to achieve a slow cooling effect. Currently, most of the molds use Cu-based alloys. That is, a 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 solidification layer 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. In order to compensate for this, molds made using stainless steel or Ni-based materials having lower thermal conductivity than Cu-based materials have been tested, but are currently being tested.

The third method is to modify the shape of the mold surface to achieve a slow cooling effect.

This method is a method of filling the surface of the mold or giving a proper bend to fill Ni such as low conductivity in the groove (Grooved Mold) is known to have some effect on the surface crack reduction. . However, the problems of such methods include the disadvantage that processing is difficult and costs increase by going through a complicated processing procedure in which the surface of the mold is precisely processed in a uniform shape, and Ni is filled in the groove and then coated again. Therefore, it is currently rarely available.

Looking specifically at other techniques for modifying the shape of the mold surface, Japanese Patent Laid-Open Nos. 6-297103, 8-257695 and 9-206891 have been proposed.

Japanese Patent Application Laid-Open No. 6-297103 is to uniformly form a plurality of irregularities having a diameter of 0.1 to 3 mm and a depth of 5 to 300 µm on a powder solidification layer forming portion of the mold inner surface as shown in FIG. It is claimed that this unevenness causes the powder solidification layer to be uniform, thereby forming a solidification shell of uniform thickness. In other words, the prior art uniformly distributes the unevenness having a diameter of 0.1 ~ 3mm, there is a constant gap between the unevenness and unevenness. If the diameter of the unevenness is large and there is a space (interval) not processed between the unevenness (FIG. 5A), the slow cooling effect is increased (in FIG. 5, 1 is a mold, 3 is powder, 4 is a solidified shell, and 5 is Irregularities).

However, as shown in FIG. 6 (a), the mold flux is thickened at the recessed portion, and the mold flux is thinned at the unprocessed portion, and the amount of heat transfered to the mold flux is thinned, so that the mold flux is solidified. The layer thickness becomes thicker. Therefore, as the distance from the hot water surface is maintained by maintaining the constant heat transfer direction as it is, the interval for maintaining the thickness of the mold flux is increased. Therefore, the solidification layer growth is promoted in the place where the mold flux is thin, thereby producing columnar texture.

Japanese Unexamined Patent Application Publication Nos. Hei 8-257695 and Hei 9-206891 also show an example of the processing form in Fig. 4 (b) and (c) as a technique for processing a groove of 1 mm or more inside the mold.

As described above, the prior art of processing grooves or irregularities on the inside of the mold is focused on preventing surface defects of the cast through the slow cooling effect, isoaxial with uniformity of the solidification layer due to lack of awareness of the heat transfer direction There is no study on the structure of cast iron. Such prior art is not currently commercialized due to many obstacles in terms of processing cost and maintenance in actual use.

The present invention is derived from the research process for solving the problems of the prior art described above, and reducing the amount of shrinkage due to the initial coagulation delay while inducing unevenness in the direction of heat transfer in the cast steel while growing a uniform solidification layer The purpose is to provide a mold capable of forming equiaxed coagulation tissue.

1 is a schematic diagram showing a surface crack generating mechanism;

Figure 2 is a schematic diagram of the inclusions attached under the surface layer according to the depth of the hook

3 is a schematic diagram showing a mechanism for generating a slow cooling effect using a mold solvent.

4 is a conventional mold grooved on the surface of the mold

5 is a conventional mold in which the irregularities are formed on the surface of the mold;

6 is a schematic diagram showing the concept of solidified layer growth.

Figure 6 (a) is a conventional mold for forming irregularities on the surface of the mold

Figure 6 (b) is a mold of the present invention to form a surface roughness on the surface of the mold

7 is a mold of the present invention showing the surface roughness forming area on the surface of the mold

8 is a schematic diagram showing a surface crack generating mechanism;

Figure 8 (a) is a smooth conventional mold

Figure 8 (b) is a mold of the present invention to form a surface roughness

9 is a graph showing the rate of heat transfer according to the distance between bone and bone on the mold surface

10 is a cross-sectional view of the surface processed mold.

Fig. 10 (a) shows a conventional mold having uneven processing on the mold surface

Figure 10 (b) (c) is a mold of the present invention to form a surface roughness on the mold surface

11 is a graph showing the defect occurrence rate of the cast steel according to the mold

12 is a surface coagulation structure photograph of the cast steel according to the mold

Continuous casting mold of the present invention for achieving the above object, in the continuous casting mold consisting of a long side portion and a short side portion, the depth of 0.01 ~ 0.08mm in the area corresponding to the initial solidification portion on the inner side of the long side portion Surface roughness of 0.01 ~ 0.08mmm distance between the valleys is formed, and has a non-uniform heat transfer characteristic.

Hereinafter, the present invention will be described in detail.

In the present invention, by forming a constant surface roughness on the inner side of the long side part to secure the nonuniformity of the heat transfer direction to prevent the growth of the coagulation layer in any direction, the nucleation site is randomly generated, the surface coagulation tissue has a lot of equiaxed crystals To grow. That is, the inventors of the present invention, when the surface roughness is properly formed on the surface of the mold, the heat transfer amount becomes uneven by the uneven surface at the beginning of coagulation, so that the thickness of the coagulation layer becomes uneven, but the thickness of the coagulation layer grows uniformly over time. The present invention was completed by observing the densification of the surface coagulated tissue and the formation of equiaxed crystal tissue.

In the present invention, the surface roughness is formed on the initial solidification portion on the inner side of the long side portion. The initial solidification part is a region where a solidification layer having a constant thickness is formed on the cast steel. When the thickness of the coagulation layer is about 10-20mm, it is called initial coagulation. In this case, the initial coagulation part is an area corresponding to an arbitrary point in the range of 300-350mm from the upper side to the lower side. In any section in the range from 300 to 350 mm from the top to the bottom direction, a solidified layer of a constant thickness is formed on the cast steel.

In the width direction of the long side portion, the surface roughness is imparted to the region corresponding to the casting region. Since the continuous casting mold adjusts the casting area in the long side width direction according to the width of the cast, it is recommended to form the surface roughness in the minimum casting width area, which is the surface roughness in the area of the long side that is interviewed with the thickness of the short side part. It is because it is preferable that is not formed. The minimum casting width is determined within 700 ~ 1500mm around the center of the long side. An example of a mold having a surface roughness formed on a mold long side portion according to the present invention is shown in FIG. 7.

According to the present invention, the surface crack generation suppression mechanism of the slab according to the surface roughness on the surface of the mold is shown in FIG. 8.

In the smooth mold (FIG. 8A), the initial solidification layer grows to a uniform thickness, and starts to generate air gaps due to solidification shrinkage. As the air gap grows and the coagulation delay portion expands as the air gap grows, the thickness of the coagulation layer decreases and cracks in the coagulation layer due to iron static pressure occur.

In contrast, in the mold having the surface roughness (FIG. 8B), the initial solidification layer grows to a non-uniform thickness, but the surface roughness is formed on the mold surface to restrain the shrinking solidification layer, thereby preventing the formation of air gaps. Since no air gap is generated, surface cracks can be suppressed by inducing the growth of a uniform solidification layer.

In the present invention, the formation of the air gap is prevented to induce the growth of the uniform solidification layer, which is possible through the control of the heat transfer amount. That is, Figure 9 shows the heat transfer rate increase according to the distance between the bone and the bone when the short blast treatment to the surface of the long side by forming a short blast treatment with gold steel yarn. 9, the heat transfer increase is great when the distance between the bone and the bone is 0.05mm, but the heat transfer rate is low when the distance between the bone and the bone is less than 0.01mm or more than 0.1mm, which is caused by the air gap. The amount of heat is lowered. In view of these results, the surface roughness increases the amount of heat transfer (prevention of air gap) when the distance between the bone and the bone is 0.01 to 0.1 mm, preferably about 0.01 to 0.08 mm. When the distance between the bone and the bone is 0.01 ~ 0.08mm, the depth of the surface roughness satisfies this degree.

Therefore, the present invention forms a surface roughness of 0.01 ~ 0.08mm depth in the area corresponding to the initial solidification portion on the inner side of the long side, wherein the distance between the bone and the bone in the surface roughness is 0.01 ~ 0.08mm. Such surface roughness is, for example, to form the diamond steel particles by a shot blast method (shot blast). Short blast treatment with emery steel gives a surface roughness of various shapes with no regularity on the surface of the mold, which is shown in Figure 10 (c) of the surface. This is represented conceptually in Figure 10 (b). Fig. 10 (a) is a cross section of a mold for the prior art which uniformly forms a plurality of irregularities having a diameter of 5 to 300 µm with a diameter of 0.1 to 3 mm on the surface of the mold (Japanese Patent Laid-Open No. 6-297103).

In addition, the concept of solidification progress in the case of continuous casting by a mold as shown in FIG. 10 (a) is shown in FIG. 6 (a), and the concept of solidification progress in the case of FIG. b).

That is, Figure 6 (a) is a mold formed uniformly with a large diameter irregularities having a constant interval, so that even if the distance from the hot water is kept constant heat transfer direction, the thickness of the mold flux as the distance away from the hot water The interval which is kept constant becomes large. Therefore, the solidification layer growth is promoted where the mold flux is thin, and the solidification structure has columnar tablets.

On the contrary, in FIG. 6 (b), since the mold having the surface roughness having a short distance between the bones is not regular, the heat transfer direction is not maintained and randomization is delayed at any part due to randomization. The phenomenon can be prevented. In other words, as the cooling occurs randomly, the site of nucleation occurs randomly, so that the surface coagulation tissue shows almost no columnar tablets, and many isomers grow.

According to the present invention, the mold has a depth of 0.01 to 0.08 mm on the coating layer for extending the life of the copper plate, and the distance between the bone and the bone satisfies 0.01 to 0.08 mm. Such coating is performed in a large number of single and multiple layers of Ni and Cr. The surface roughness may be directly on the surface of the copper mold, or may be applied to the coating layer (Ni or Cr). Among them, the method of directly processing the surface of the copper plate is most effective. Copper plate is advantageous in terms of thermal conductivity. For example, the thermal conductivity of a copper plate is 350 W / m degreeC, and Ni is about 70 W / m degreeC. The thermal conductivity of the copper plate is about five times greater than that of Ni. Therefore, when the surface roughness is formed on the copper plate, the heat transfer control effect is about five times the improvement effect compared to processing the surface roughness on the Ni coating layer.

According to the present invention, the surface coagulation structure of the cast steel is almost invisible columnar tablets, a lot of equiaxed crystals grow. As such, if a large number of equiaxed crystals exist, not only the quality of the coil is increased after rolling but also the coil strength is increased, so that the thickness of the coil can be thinned for the same strength. In addition, in the present invention, the length of the oscillation hook is shortened due to the initial coagulation delay, so that the inclusions attached to the surface layer are reduced, so that the quality improvement effect is excellent even in the steel grade where the surface quality is important.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

The inner surface of the Cu mold was short blasted with gold steel particles within 300 mm from the top and 1000 mm in width to form a surface roughness of 0.01 to 0.1 mm in depth and 0.01 to 0.1 mm in distance between the bone and the bone.

The ultra low carbon steel, low carbon steel, medium carbon steel, used carbon steel, and Ca-treated steel were continuously cast according to the bone to bone distance of the mold, and the temperature of the cooling water was measured. Then, continuous casting was carried out in a smooth mold for the same steel grade, and after measuring the coolant temperature, the difference in the coolant temperature was shown in FIG.

As shown in Figure 9, the heat transfer was the highest when the surface of the mold 0.05mm.

Fig. 11 shows the results of investigating the surface defect incidence of the obtained slabs for the molds having a surface roughness of 0.05 mm. In FIG. 11, it can be seen that in the case of using the mold of the present invention, the incidence of defects in post-process quality dropped to about 1/8 of that of the conventional mold. Therefore, when the mold of the present invention is used, it was found that the effect of improving the surface quality of the cast is excellent, and the quality improving effect is very high for the product whose surface quality is important. In addition, a number of operation accidents such as break out occurred in the side using a conventional mold, but did not occur during the application of the mold of the present invention was confirmed that it is very effective in terms of operation stability.

The actual application results for this phenomenon are shown in FIG. 12. As shown in FIG. 12, when the existing mold is applied, dendrite grows in one direction due to deformation of the coagulation layer, and thus cooling of the coagulation layer is uneven. However, it can be seen that in the case of the cast steel obtained according to the present invention, the general dendrite growth direction disappears and most of them are formed as equiaxed crystals.

As described above, according to the present invention, by reducing the non-uniform solidification phenomenon by the initial solidification delay and the non-uniformity (randomize) effect of the heat transfer direction, it is possible to generate enormous expected profits by improving the surface quality of cast steel and stabilizing operation.

Claims (4)

In the continuous casting mold comprising a long side and a short side, Mold for continuous casting having non-uniform heat transfer characteristics, characterized in that the surface roughness of 0.01 ~ 0.08mm in depth and 0.01 ~ 0.08mmm distance between the bone and the bone is formed in the area corresponding to the initial solidification portion in the inner side of the long side . According to claim 1, wherein the area corresponding to the initial solidification portion continuous casting mold having a non-uniform heat transfer direction characteristics, characterized in that from any point in the 300 to 350mm range from the upper side to the lower side. [Claim 2] The mold for continuous casting according to claim 1, wherein the surface roughness is formed by a shoblasting method using gold steel yarn. According to claim 1, wherein the inner surface of the mold is a coating layer of Ni and Cr is formed in a single layer or a double layer, the surface roughness is 0.01 ~ 0.08mm depth and the distance between the bone and the bone on the coating layer is 0.01 ~ 0.08m Continuous casting molds with nonuniform heat transfer characteristics.