JP2001225152A - Continuous casting method for steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method for steel which allows the easy precipitation of crystal phases into mold powder and ensures effective slow cooling by the mold powder. SOLUTION: Continuous casting is executed by using the mold powder and maintaining the steam vapor pressure in the atmosphere near a casting mold at 0.05 to 0.7 atm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for continuously casting steel using mold powder.

[0002]

2. Description of the Related Art In continuous casting of steel, in general, molten steel poured into a tundish from a ladle is injected into a mold through an immersion nozzle, and a solidified shell formed in the mold continues to the bottom of the mold. It is solidified while being cooled by the spray nozzles between the guide rolls provided, and is pulled out as a cast by a pinch roll. At this time, the mold powder is floated on the molten steel surface (hereinafter, referred to as a meniscus portion) in the mold, the mold powder is melted by the heat of the molten steel, and flows between the mold and the solidified shell. It reduces friction and improves lubricity. In addition, in order to prevent the slab from burning on the inner wall of the mold by the mold powder, the mold is moved at a predetermined frequency,
A device that vibrates at a predetermined amplitude is used. In continuous casting of steel using such a mold powder, if an appropriate mold powder is not selected, a vertical crack is generated on the surface of the slab, and there is a possibility that a slab surface defect which is a quality defect is generated. . Therefore, in order to stably produce an unmaintained continuous cast slab for rolling, it is important to select an appropriate mold powder.

[0003] Incidentally, the carbon content is 0.08 to 0.2.
In continuous casting of 0% steel, uneven solidification is apt to occur, so vertical cracks are likely to occur in the slab. To prevent this, heat transfer between the mold and the slab (solidified shell) is slowed down. It has been pointed out that the so-called "slow cooling" is effective (for example, iron and steel: 67 (1981), 1508.). As a means of this slow cooling, attempts have been made to change the material of the mold or to reduce the amount of cooling water in the mold, but in any case, it was not possible to obtain a sufficient vertical crack preventing effect.

In such a situation, in recent years, as a result of applying mold powder having a high crystal precipitation temperature to continuous casting of steel, a result has been obtained that the frequency of occurrence of vertical cracks in the slab is greatly reduced (for example, Iron and steel:, 83 (1997), 115
etc). Here, the crystal precipitation temperature refers to the temperature at which primary crystals precipitate when the mold powder is once melted and then cooled at a constant cooling rate. In general, a mold powder having a high crystal precipitation temperature has a crystal precipitation region in the TTT diagram. Is big. That is, the key is how to precipitate the crystal phase in the molten mold powder. Therefore, in the prior art, Cuspid, one of the crystal phases in the mold powder, is used.
ine as _{(3CaO · 2SiO 2 · CaF 2} ) is likely to precipitate, by devising the added element has been subjected to mold powder design (supra, iron and steel:, 83 (1997), 115, etc.).

[0005] However, in recent years, the emission of fluorine (F) into the environment has become a problem due to the growing interest in environmental issues. Since Cuspidine contains F, the addition of a small amount of F is There is a need for a process that can precipitate the crystal phase of Cuspidine.

On the other hand, according to measurement by a laser flash method or the like, the crystal phase of the mold powder has a higher thermal conductivity than the glass phase, and when such a crystal phase is interposed between the mold and the slab, The slab should be strongly cooled, but in fact, the temperature rise of the cooling water of the mold is low and the slab is slowly cooled. In many cases, the heat transfer behavior between the mold and the slab has not been elucidated in many cases, and the mold powder is actually manufactured and tested by trial and error.

[0007]

DISCLOSURE OF THE INVENTION The present invention has been made in view of such circumstances, and a crystal phase is easily precipitated in a mold powder, and a slow cooling can be effectively performed by the mold powder. It is an object of the present invention to provide a continuous casting method of steel capable of producing a cast piece with extremely few vertical cracks.

[0008]

Means for Solving the Problems In conventional continuous casting, when powder or granule mold powder absorbs moisture, heat is taken away when the mold powder is put into the upper part of the meniscus and melted, and the mold powder is removed. Generally, the atmosphere near the mold is dried because it is difficult to melt. On the other hand, the present inventors have conducted various studies to produce a slab having extremely few vertical surface cracks. As a result, when steam is present in the atmosphere near the mold when the molten mold powder solidifies, the mold powder It was found that the number of starting points of crystal nucleus formation increased and crystals were easily precipitated.

The present invention has been made based on such findings, and is a method for continuously casting steel using mold powder, wherein the partial pressure of water vapor in the atmosphere near the mold is set to 0.05 to 0. The present invention provides a continuous casting method for steel, wherein continuous casting is performed at 0.7 atm.

[0010] In the above continuous casting method for steel,
Furthermore, the surface roughness of the solidified mold powder is 5 μm.
A continuous casting method for steel characterized by performing continuous casting as described above.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described specifically. In the present invention, the starting point of crystal nucleation in the molten mold powder is increased by performing continuous casting with the partial pressure of water vapor in the atmosphere in the vicinity of the mold being 0.05 to 0.7 atm, for example, Cuspidine (3CaO.2Si).
Since a crystal phase such as O ₂ · CaF ₂ ) can be more efficiently precipitated than in the past, slow cooling of the slab can be effectively performed, and vertical cracks generated in the slab can be prevented.

If the partial pressure of water vapor in the atmosphere near the mold is less than 0.05 atm, the effect of increasing the starting point of crystal nucleus formation is not sufficient, and the crystal phase cannot be efficiently precipitated. On the other hand, when the water vapor partial pressure exceeds 0.7 atm,
F in the mold powder reacts with water vapor to generate HF, which reduces F and makes it difficult to precipitate Cuspidine, which is a crystalline phase. In addition, the mold powder absorbs moisture due to water vapor, so that it is difficult to melt when poured into the molten steel surface near the meniscus, and it is difficult for the powder to flow between the mold and the slab, thereby causing operational troubles such as breakout. Therefore, in the present invention, the partial pressure of water vapor near the mold is set to 0.05%.
To 0.7 atm.

Actually, continuous casting of steel was carried out by using a mold powder shown in A of Table 1 to be described later and changing the steam partial pressure of the casting bed near the mold, and the result shown in FIG. 1 was obtained. Was. FIG. 1 is a diagram showing the relationship between the water vapor partial pressure at that time on the horizontal axis and the number of slab cracks on the vertical axis. As shown in FIG. 1, when the steam atmosphere in the vicinity of the mold is set to 0.05 to 0.7 atm, the vertical cracks generated on the slab surface are remarkably suppressed, and effective slow cooling is performed. I understand.

In order to keep the partial pressure of water vapor in the atmosphere near the mold within the above-mentioned range, for example, a surrounding member surrounding the vicinity of the mold of the continuous casting machine is arranged, and an atmosphere in which the partial pressure of water vapor is adjusted is provided inside the surrounding member. Can be carried out. Further, an atmosphere in which the partial pressure of steam is adjusted may be blown from a nozzle provided near the mold toward the meniscus portion of the mold.

Further, in the present invention, in the continuous casting performed as described above, longitudinal cracks generated in the slab are more effectively suppressed, and a steel slab having extremely few surface defects is obtained. It is preferable that the surface roughness of the mold powder is 5 μm or more. If the surface roughness of the mold powder after solidification is less than 5 μm, it is difficult to exhibit such excellent longitudinal crack suppressing effect.

The surface roughness of the solidified mold powder is as follows:
After performing continuous casting, the mold powder solidified together with the slag rim generated in the meniscus is pulled up and collected,
It can be grasped by measuring the difference between the valley (minimum value) and the peak (maximum value) of the irregularities on the solidified mold powder surface, but this method may hinder the operation of continuous casting, The surface roughness of the mold powder may be estimated by the following method.

That is, before casting steel, the mold powder to be used is heated and melted in a dehydrated Ar atmosphere (ie, an atmosphere having a partial pressure of water vapor of 0 atm) using a laser microscope equipped with an image heating furnace in advance. After cooling to room temperature at 10 K / sec, which is estimated to be the cooling rate between the mold and the slab of the actual machine, the surface shape measurement function, which is the function of the laser microscope, is used to measure the valleys and hills Measure the difference r2. The relationship between r2 measured in this way and the surface roughness r3 of the mold powder solidified in an atmosphere having a water vapor partial pressure of 0.2 atm was ascertained based on actually measured values. As a result, 1.25 × r2 ≦ r3 ≦ 1.4 × r2. When the average value of (r3 / r2) is obtained from these actually measured values, it is 1.31, which can be approximated as r3 = 1.3 × r2. Also, as a result of grasping the relationship between the measured value r1 of the surface roughness of the mold powder solidified during continuous casting and the above-mentioned r3 at a steam partial pressure of 0.05 atm or more where the influence of steam appears, 1.75 × r3 ≦ r1 ≦ 2.20 × r3. When the average value of (r1 / r3) is obtained from these actually measured values, it is 2.00, which can be approximated as r1 = 2.0 × r3. Therefore, if the r2 of the mold powder used before casting is measured, the actual surface roughness r1 of the mold powder solidified during continuous casting can be estimated from the above relational expression.

In the image heating furnace of the laser microscope, since the cell in which the mold powder is placed is small, the surface roughness is reduced due to the surface tension when the mold powder solidifies, but the relative relationship is maintained. Therefore, by performing the correction by multiplying the coefficient as described above, the data obtained by measuring the surface roughness after heating and cooling using the image heating furnace can sufficiently reproduce the situation of the actual machine.

Actually, continuous casting was performed using mold powders having various surface roughnesses in an atmosphere having a partial pressure of water vapor of 0.3 atm, and the number of surface vertical cracks generated in the slab was measured. The results shown were obtained. FIG. 2 is a diagram showing the relationship between the surface roughness of the mold powder after solidification at that time on the horizontal axis and the number of slab cracks on the vertical axis. As shown in FIG. 2, it can be seen that by setting the surface roughness of the mold powder after solidification to 5 μm or more, vertical cracks generated in the slab can be more effectively suppressed. Here, the surface roughness of the mold powder after solidification is a value estimated by the above method.

In the continuous casting method of the present invention, for example,
For example, SiO₂, CaO, Al₂O₃, Fe₂O₃, Mg
O, MnO, BaO, B₂O₃And the like as the base material,
In addition, Na₂O, K₂O, Li₂Metal oxides such as O,
NaF, KF, LiF, CaF ₂, MgF₂, Al
F₃, Na₃AlF₆Such as fluoride, carbonic acid of those metals
Commonly used with the addition of
Mold powder can be used.
It is not limited.

[0021]

Next, examples of the present invention will be described. Using mold powders A and B shown in Table 1, steel was continuously cast under the conditions of casting speed and steam partial pressure shown in Table 2, to produce cast pieces having the sizes shown in Table 2. Table 2 shows the number of vertical cracks generated in the obtained slab, the surface roughness r2 when each mold powder was melt-solidified by an image heating furnace in a dry atmosphere, and the case where the melt was solidified at a predetermined steam partial pressure. And the surface roughness r1 measured by collecting and solidifying the mold powder from the actual machine after continuous casting. The values of r1 to r3 shown in Table 2 are the average values of the surface roughness measured at 10 or more locations.

According to Table 2, the partial pressure of water vapor in the atmosphere was set to 0.0
When the surface roughness of the solidified mold powder is controlled to 5 μm or more by controlling the solidified mold powder to 5 to 0.7 atm, it can be seen that the number of vertical cracks in the slab is greatly reduced. Table 2 shows the values of r3 / r2 and r1 / r3, the average values of which are 1.3 and 2.00, and r3 = 1.3.
It was confirmed that the approximate expression of × r2, r1 = 2.0 × r3 was applicable.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

According to the present invention, it is possible to produce a steel slab having extremely few surface vertical cracks, since a crystal phase is easily precipitated in the mold powder, and the mold powder can be slowly cooled effectively. Can be provided. That is, a cast slab that can be maintained without rolling can be stably manufactured, and its industrial effects such as improvement in yield and reduction in manufacturing cost are extremely large. Furthermore, since the crystal phase can be efficiently precipitated in the mold powder, a reduction in the amount of F discharged to the environment can be expected.

[Brief description of the drawings]

FIG. 1 shows a partial pressure of steam near a mold when steel is continuously cast using a mold powder shown in A of Table 1.
The figure which shows the relationship with the number of surface vertical cracks produced in the obtained slab.

FIG. 2 shows the relationship between the surface roughness of mold powder and the number of vertical cracks on the slab surface when steel is continuously cast using mold powders of various surface roughnesses in an atmosphere with a steam partial pressure of 0.3 atm. FIG.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hikaru Hino 02 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi Pref.Graduate School of Engineering, Tohoku University (72) Tetsuya Nagasaka, Aoba 02, Aramaki, Aoba-ku, Sendai, Miyagi F-term in the Graduate School of Engineering, Tohoku University 4E004 MB14 MC02 MC30

Claims (2)

[Claims] 1. A continuous casting method for steel using a mold powder, wherein the continuous casting is performed by setting a partial pressure of water vapor in an atmosphere near a mold to 0.05 to 0.7 atm. Method. 2. The continuous casting method for steel according to claim 1, wherein the continuous casting is performed so that the solidified mold powder has a surface roughness of 5 μm or more.