JPS5829177B2 - Method for reducing vertical cracks in steel ingot corners - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for reducing corner cracking in steel ingots without reducing mold life by combining mold corner improvements and mold coatings.

The defects that occur on the surfaces of various products are considered to be a problem as they are a major impediment to improving yields and expanding energy-saving processes.

For example, there are linear surface flaws that occur on the flange surface of section steel.

This mainly appears at a position 10 to 20 degrees from the outer edge of the flange, and the product value is significantly lowered because some products require extensive maintenance over the entire length.

This flaw has traditionally been treated as a flaw caused by the rolling process due to its shape.

Therefore, attempts were made to reduce the number of defects by considering the rolling method, but no results were obtained, and it was concluded that the defects were not caused by the rolling process.

Therefore, after reviewing the integrated process leading to the finished product stage, it was found that the corner cracking scratch 2 at the first stage of the steel ingot shown in Figure 1 remains until the finished product, and it is necessary to prevent this corner cracking. It turned out that.

The present inventors studied the mechanism by which vertical cracks occur in a portion of a steel ingot corner, and estimated the mechanism by which longitudinal cracks occur in a portion of a steel ingot corner as follows.

That is, during solidification contraction at the beginning of completion of molten steel injection, air gaps 4 tend to form in some corners of the mold 5 due to the tensile stress of the shell 3 from both straight parts, as shown in FIG. Become.

Once an air gap is generated, the amount of heat removed from that part is extremely reduced, and solidification is delayed compared to other surfaces that are in good contact with the mold, and the thickness of the shell 3 becomes thinner.

In this case, if the strength of that part becomes smaller than the tensile and shrinkage stress during solidification, it is thought that corner cracks 2 of the steel ingot will occur starting from the material deteriorated parts (inclusions, surface shape, etc.).

As a measure to prevent the occurrence of corner cracks, there is a corrugated mold, which is generally known to be effective in suppressing the occurrence of cracks.

However, corrugated molds make it difficult to repair the mold,
In addition, it has problems such as significantly shortening the life of the mold, so it is not used practically.

In view of the above circumstances, the present inventors conducted various research experiments on alternative methods that do not have the drawbacks of corrugated molds, and as shown in Figure 3, the use of mold paint is effective in preventing the occurrence of these defects. It was acknowledged that it was effective.

Mold paints can be broadly divided into organic and inorganic types, and an outline of their components is shown in Table 1.

FIG. 3 shows the occurrence of corner cracks in the steel ingot as seen from the number of mold uses when no paint was used and when organic paint and inorganic paint were applied.

Steel ingots coated with organic paints are more prone to corner cracks than those coated with inorganic paints.

Therefore, in order to prevent steel ingot cracking, it is important to use mold paints, and among these, inorganic paints were found to be more effective than organic paints.

Although mold paints have long been known to be effective for removing surface defects such as baldness and double skin, their use for preventing cracks is unknown.

This mold paint does not impair mold life at all, unlike full-circumference corrugated paint.

However, in the present invention, this mold paint is used as a base for preventing vertical cracking in some corners of the steel ingot.

Its effectiveness as a mold paint is to prevent seizure of the mold at the meniscus area where the molten steel and the mold come into contact, and to form a reducing atmosphere, which also acts as a heat insulator to form a shell that is easily deformed, helping to prevent cracking. considered to be a thing.

Further, problems related to these phenomena during the early stage of the mold can be explained by the mold inner surface shape and the steel ingot surface shape after mold removal.

Next, even when mold paints are used, there is a difference in the incidence of cracking between organic and inorganic paints.

The surface shape of steel ingots coated with organic paints is smoother (flat) by reducing the surface roughness of the steel ingots, as the cleaning effect due to gas generation during melting is less effective than that of steel ingots coated with inorganic paints. It has become.

Therefore, it is thought that inorganic paints are more advantageous in preventing corner cracks in steel ingots, since the unevenness on the surface of the steel ingot is larger than when the surface is coated with organic paint, and the above-mentioned stress that occurs during solidification shrinkage is alleviated. .

However, as is clear from Figure 3, if we focus on the relationship between the mold paint and the number of times the mold is used, it is clear that the effect on preventing steel ingot cracking is clearly greater when the paint is applied than when it is not applied. cannot control this cracking.

This is because even if a mold paint is applied, the inner surface of the mold is highly flat in the initial mold, so even if the paint mentioned above is used, it is not possible to impart the optimum roughness to the surface of the steel ingot as described above. This seems to be because the paint coating effect is not fully exhibited.

Therefore, the present invention adopts the minimum necessary corrugated shape only at the position where cracking occurs, in order to prevent cracking at a time when it is difficult to prevent it with mold paint alone.

The feature of the present invention is that a mold paint is applied to the inner surface of the mold, and the entire length of the spherical surface at the corners of the mold or the lower part of the mold where corner cracks are particularly common are coated with a pitch of 20 to 50 mm and a depth as shown in FIG. Form vertical grooves 7 of 5 to 20 mm, or form grooves 7 with a diameter of about 20 mm and an uneven height of about 1 at intervals of about 40 mm.
By forming the recessed hole of 01nrIL, vertical cracking of a portion of the corner of the steel ingot can be prevented, especially at the initial stage of use of the mold, and furthermore, vertical cracking of a portion of this corner can be stably prevented even afterward.

Figure 5 is an explanatory diagram of the synergistic effect of the mold paint and the irregularities on some of the inner corners of the mold.As is clear from this Figure, a sufficient effect of preventing vertical cracking cannot be obtained with only some irregularities on the inner corners of the mold. The combination of this and the mold paint provides an excellent effect of preventing vertical cracking in some corners of the steel ingot even in the early stages of mold use.

In this way, the present invention basically suppresses the occurrence of vertical cracking in some corners of the steel ingot using mold paint, and furthermore, by combining this with unevenness in some of the inner corners of the mold, sufficient corner cracking can be achieved even in the early stages of mold use. Partly, the effect of preventing vertical cracking can be obtained.

Next, examples of the present invention will be shown together with comparative examples.

The present invention was carried out in a 15-ton mold having an aspect ratio of 1.2 and a corner radius of 60 mm.

The steel type is aluminum killed steel.

The results are shown in Table 2. The mold paints shown in Tables 3 and 4 were used.

The thickness is such that it will not fall over. As is clear from the above examples, the examples of the present invention have extremely few corner cracks in steel ingots, and the life of the mold is no different from that of a normal mold.

As explained in detail above, the present invention uses a combination of mold paint and mold corner unevenness to prevent the occurrence of vertical cracks on the steel ingot corner from the early stage of mold use without reducing the mold life. Dogs can minimize the occurrence of scratches on the surface of finished products.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the occurrence of cracks in a part of the steel ingot corner.
Figure 2 is an explanatory diagram of the occurrence of cracks in a part of the corner, Figure 3 is a diagram showing the influence of mold paint and the number of times the mold is used on steel ingot corner cracks, and Figure 4 is a diagram showing the form of unevenness on the inner surface of the mold corner. FIG. 5 is a diagram showing the effects of the present invention in comparison with a comparative example. 1: steel ingot, 2: crack, 3: solidified shell, 4: air gap, 5: mold, 6: unsolidified part, 7: groove.

Claims (1)

[Claims] 1. A method for reducing vertical cracks in a part of a steel ingot corner, which is characterized by using a mold with unevenness formed on the four corners of the inner surface of the mold, and forming an ingot by applying an organic or inorganic paint to the inner periphery of the mold.