JP3389864B2 - 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, which is used by adding it in a mold in continuous casting of steel.

[0002]

2. Description of the Related Art In continuous casting of steel, a mold powder is used on the upper surface of molten steel to prevent oxidation of molten steel, maintain heat, absorb floating inclusions, and flow between a mold and a slab to control lubrication and heat removal. Is going.

Among the steels, in the hypoperitectic steel region having a carbon content of 0.08 to 0.18%, which is called medium carbon steel, the volume shrinkage is large due to the δ → γ transformation during solidification. Since uneven solidification is likely to occur, vertical cracks are likely to occur in the slab. The occurrence of this vertical crack has a correlation with the heat flux in the mold, and it is possible to suppress the occurrence by lowering the heat flux. Therefore, it is desirable that the powder used has the property of being capable of being slowly cooled.

Regarding the mold powder for medium carbon steel,
Shinagawa Technical Report No. 32 (Ichikawa et al., 1989), by increasing the basicity (CaO / SiO2) of the powder to raise the freezing point and accelerate crystallization during solidification, the mold and slab (solidified shell) It is described that, among the molten slag flowing in between, voids are generated in the solidified layer fixed to the mold side, the apparent thermal conductivity of the solidified layer is reduced, and cracks in the cast piece can be prevented.

Further, in Japanese Laid-Open Patent Publication No. 3-193248, from the viewpoint of reducing the thermal conductivity in the solidified layer fixed to the mold, the solidification point is high and the molten slag is crystallized without vitrification, as in the above technique. It is said that it is desirable to add at least one oxide of an element selected from the IIIA group and the IVA group as one component of the mold powder. And it is characterized by containing 0.01 to 15% of one or more of them.

[0006]

The mold powder in the prior art described above is intended to perform slow cooling in the mold by increasing the thermal resistance in the solidified layer in contact with the mold. Certain results can be obtained. However, in order to further improve the quality of the slab, further slow cooling is desired.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a mold powder for continuous casting which can cool the inside of the mold slowly and can significantly improve the quality of the slab. And

[0008]

Means for Solving the Problems The powder in the above-mentioned prior art is characterized by increasing the thermal resistance in the solidified layer by thickening the solidified layer in contact with the mold or by providing pores therein. This is intended for slow cooling in the mold.
However, as a result of various studies by the present inventors, the largest factor that influences the heat flux in the mold is the gap between the mold and the solidification layer (hereinafter referred to as the air gap), which increases the thermal resistance of this interface. By doing so, it was found that the inside of the mold can be significantly cooled.

Since the interfacial heat resistance is closely related to the heating crystallization temperature which is the basic physical property of the powder, the interfacial heat resistance is increased by designing the powder so that the temperature has an optimum value. The inside of the mold can be cooled slowly, and the quality of the slab can be significantly improved.

The present invention has been made on the basis of such findings, and after vitrification by melting and quenching, 1
Provided is a mold powder for continuous casting, which has a heating crystallization temperature in the range of 400 to 490 ° C., which is a temperature for crystallization by heat treatment at a heating rate of C / min .

Further, in such a mold powder, when it is melted by heating and then cooled at 10 ° C./min, crystals of the crystal are formed.
The cooling crystallization temperature, which is the temperature at which generation begins, is 1100 to 12
It is preferably in the range of 50 ° C. The premelt rate is preferably 75% or more.

[0012]

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. It is empirically known that it is effective to use a powder that easily crystallizes in order to perform slow cooling in the mold. As a result of examining the correlation between the crystallization and the heat flux, it was found that the largest factor controlling the heat flux was the interfacial heat resistance between the mold and the powder film layer.
This interfacial heat resistance is determined by the unevenness of the surface of the powder film on the mold side. As a result of further various studies, it was found that crystal formation from a supercooled liquid at a low temperature of 500 ° C. or less greatly affects the unevenness. The formation of crystals from this low temperature supercooled liquid can be determined by the heating crystallization temperature. Heating crystallization temperature is by melt quenching
After vitrification, it is bonded by heat treatment at 1 ° C / min.
It is the temperature for crystallization, and it can be determined from the exothermic peak position by melting and cooling the powder to prepare glass and heating it with a differential thermal analyzer.

When the heating and crystallization temperature is 400 to 490 ° C., the surface irregularities are large and the effect of slow cooling is easily exhibited, but when it exceeds 490 ° C., it is difficult to form crystals in an actual machine and the irregularities are small. Further, if the temperature is lower than 400 ° C., unevenness in size of irregularities occurs, and uniform cooling cannot be expected. A more preferable heating crystallization temperature range is 400 to 460.
C., and in this case, the unevenness is large and the nonuniformity of the size is small, and the inside of the mold is uniformly cooled slowly, so that a good cast piece can be obtained.

Further, it is usually used as a crystallization temperature,
After the powder is heated and melted, the cooling and crystallization temperature used as a temperature at which crystals are generated in the cooling process is 10 ° C.
The temperature at which crystals start to form when cooled at
Therefore, the cooling crystallization temperature is preferably 1100 ° C. or higher because it has a correlation with the uniformity of the thickness of the solidified layer and becomes more uniform as the temperature increases. If the temperature exceeds 1250 ° C, the powder lubrication becomes insufficient, so the crystallization temperature range is 11
It is preferably from 0 to 1250 ° C.

The premelt rate of the powder is preferably 75% or more. When the premelt rate is 75% or more,
This is because the unevenness at the interface becomes uniform and the inside of the mold can be uniformly cooled.

In order to obtain the above characteristics, the ratio of CaO / SiO _{2 in} the mold powder is 1.5 or more, and the amount of CaO + SiO ₂ + F in the powder is 75%.
The above is preferable.

[0018]

EXAMPLES (First Example) Five types of powders including conventional powders were subjected to actual machine tests to evaluate the occurrence of vertical cracks in slabs. Here, the occurrence of vertical cracking was evaluated by an index with vertical cracking when using conventional powder as 100. Further, the heat flux in the mold was determined by a thermocouple embedded in the meniscus portion in the mold. The test was conducted at a casting speed of 1.8 m / min and a mold size of 220 mm × 2000 mm.

Table 1 shows the molten steel composition, and Table 2 shows the base material composition of each powder and the actual machine test result. As the mold powder, a base material shown in Table 2 to which carbon was added so that the weight of the base material was 3 wt% was used as the mold powder.

[0020]

[Table 1]

[0021]

[Table 2]

As shown in Table 2, in the powders of Examples 1 to 4 of the present invention, the heat flux in the meniscus portion was reduced and the vertical cracking index was low as compared with the conventional products.

(Second Embodiment) The casting speed is 2.4 m / mi.
The same test as in Example 1 was performed except that n was used. Table 3 shows the values of the vertical cracking index at that time. As shown in this table, in the conventional products, the vertical cracking index was significantly increased.
In No. 4, the vertical cracking index increased only slightly, showing good results in high speed casting.

[0024]

[Table 3]

[0025]

As described above, according to the present invention,
By increasing the interfacial thermal resistance by optimizing the heating crystallization temperature of the powder, the inside of the mold can be slowly cooled, and the quality of the cast piece can be significantly improved.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-10-216907 (JP, A) JP-A-8-141713 (JP, A) JP-A-10-58104 (JP, A) JP-A-10- 258343 (JP, A) JP 8-332554 (JP, A) JP 10-34301 (JP, A) JP 3-193248 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B22D 11/108 C21C 7/076

Claims (3)

(57) [Claims] 1. A temperature of 1 ° C. after vitrification by melt quenching
Mold powder for continuous casting, characterized in that the heat crystallization temperature, which is the temperature for crystallization by the heat treatment at / min, is in the range of 400 to 490 ° C. 2. After melting by heating, it is cooled at 10 ° C./min.
When the cooling crystallization temperature, which is the temperature at which crystals start to form, is 11
The mold powder for continuous casting according to claim 1, wherein the temperature is in the range of 0 to 1250 ° C. 3. The mold powder for continuous casting according to claim 1, wherein the premelt rate is 75% or more.