JPH08132184A - Mold for continuous casting round cast billet and continuous casting method using same - Google Patents

Abstract

PURPOSE: To continuously cast a round cast billet without casting defect by uniformizing a contact between a mold and the cast billet and uniformizing the radiation of heat. CONSTITUTION: A mold is the one for continuously cast the round cast billet of a steel. A variative ratio of the inner diameter of the mold per unit length along a casting direction [=(1/D0 ).(dD/dx)×100(%), wherein, D0 is the inner diameter at the outlet side of the mold and D is the inner diameter of the mold at the distance (x) from the upper end part of cooling surface of the mold] is reduced at 12-16%/m in the interval 1b from the upper end part 1a of the cooling surface of the mold 1 at least at the pouring side of the molten steel 2 to 50-100mm, and continuously changed at 12-16%/m to 0.8-1.4%/m in the interval 1c from the upper end part 1a of the cooling surface of the mold to 250-300mm. Further, the interval 1d from the position of 300mm below the upper end part 1a of the cooling surface of the mold to the lower end part of the cooling surface of the mold is made to be 0.8-1.4%/m.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mold suitable for continuously casting a round billet slab of steel and a method for continuously casting a round billet slab of steel using the mold.

[0002]

2. Description of the Related Art When continuously casting a round billet of steel,
Compared to the case where a rectangular billet having a rectangular cross section is continuously cast, the contact between the inner wall of the mold and the cast piece becomes uneven, and the cooling tends to become uneven. Therefore, there arises a problem that vertical cracks are generated, breakouts are caused due to the vertical cracks, and finally casting cannot be performed.

Therefore, various methods have been proposed to reduce the inner diameter of the mold cross section in accordance with solidification shrinkage, and to adjust the contact between the mold and the slab by devising the mold powder supplied into the mold during continuous casting. It has been.

For example, in Japanese Utility Model Publication No. 59-165748,
A round billet continuous casting mold in which the inner diameter is reduced as it goes downward and the rate of reduction is changed in two stages. It proposes a mold for continuous round billet casting in which the change in the inner diameter is matched with the solidification shrinkage, and the contact between the mold and the slab can be made uniform.

Further, the present applicant has filed Japanese Patent Application No. 6-158952.
No., during continuous casting, it was supplied to the molten steel upper surface in the mold,
We have proposed a new mold powder that can flow between the mold and the slab to make the heat transfer between the mold and the slab uniform.

[0006]

However, when continuous casting is performed using the mold proposed in Japanese Utility Model Laid-Open No. 59-165748, the contact state between the slab and the mold in the entire region from the upper part to the lower part of the mold. Is difficult to keep good. Further, when continuous casting is performed using the mold proposed in Japanese Utility Model Laid-Open No. 59-165749, it should be possible to maintain good contact between the slab and the mold in the entire region from the upper part to the lower part of the mold. However, it is difficult to measure the amount of solidification shrinkage of the slab, and since the amount of shrinkage changes when the composition of the steel changes, it is necessary to change the mold for each steel type, and when the casting speed changes, the casting direction of the mold In practice, such a mold cannot be used because the amount of shrinkage with respect to is changed. In addition, the present applicant filed Japanese Patent Application No. 6-1
When the mold powder proposed in No. 58952 is used, the heat transfer between the mold and the slab can be made uniform, but there is a limit in compensating the contact between the mold and the slab only with this mold powder. .

The present invention has been made in view of the above-mentioned conventional problems, and when continuously casting a round billet slab of steel, uniform contact between the mold and the slab and uniform heat removal. It is an object of the present invention to provide a continuous casting mold capable of continuously casting a round billet slab having no casting defect and a continuous casting method using the mold.

[0008]

In order to achieve the above-mentioned object, a mold for continuous casting of round billet slabs of the present invention comprises:
Rate of change of mold inner diameter per unit length along the casting direction [=
(1 / D ₀ ) · (dD / dx) × 100 (%), where
D ₀ is an inner diameter on the mold outlet side, D is a mold inner diameter at a distance x from the upper end of the mold cooling surface], and is 12 to 16% at least from the upper end of the mold cooling surface on which molten steel is injected to 50 to 100 mm. / M from the upper end of the mold cooling surface to 250 ~
From 12 to 16% / m up to 300 mm up to 0.8 to
It is continuously changed to 1.4% / m, and 0.8 to 1.4% / m is set between the position 300 mm from the upper end of the mold cooling surface and the lower end of the mold cooling surface.

Further, according to the continuous casting method for round billet slabs of the present invention, the viscosity at 1573K is 0.1-1.0.1 on the surface of the molten steel injected into the continuous casting mold of the present invention.
0 Pa · s, melting point is 1273 K or higher, (CaO +
CaF ₂ × 0.718) / SiO ₂ mass concentration ratio is 0.6
.About.1.0, the amount of Na converted to Na ₂ O is 5.0 mass% or less, the F concentration is 7.0 mass% or less, and Mg converted to MgO.
The continuous casting is performed while supplying the continuous casting mold powder in an amount of 4 to 20 mass%.

[0010]

In the mold for continuous casting of round billet slabs of the present invention, the mold inner diameter per unit length along the casting direction is at least 50 to 100 mm from the upper end of the mold cooling surface on which molten steel is injected. The rate of change is set to 12 to 16% / m because this range is effective for uniform contact between the mold and the slab. This is because if the section is up to a position of less than 50 mm, the contraction of the mold becomes smaller than the contraction of the solidified shell, the contact becomes non-uniform, and vertical cracking occurs. Further, if it exceeds 100 mm, the shrinkage of the mold becomes too large, and the restriction such as the seizure between the mold and the slab occurs. That is, if the mold inner diameter change rate is larger than the specified value, restraint occurs, and if it is too small, vertical cracking occurs.

Further, in the continuous casting mold of the round billet slab of the present invention, 250 to 300 m from the upper end of the mold cooling surface.
The change rate of the inner diameter of the mold is continuously changed from 12 to 16% / m to 0.8 to 1.4% / m up to m. This is because the contraction of the mold is smaller than the contraction, the contact becomes non-uniform, and vertical cracking occurs. Also, 300 mm
If it exceeds, the shrinkage of the mold becomes too large, and the restriction of seizure between the mold and the slab occurs. That is, if the mold inner diameter change rate is larger than the specified value, restraint occurs, and if it is too small, vertical cracking occurs.

Further, in the continuous casting mold for round billet slabs according to the present invention, the change rate of the inner diameter of the mold is from 0. 300 mm from the upper end of the mold cooling surface to the lower end of the mold cooling surface.
The reason why it is set to 8 to 1.4% / m is that if it is larger than the specified value of the mold inner diameter change rate, constraint occurs, and if it is too small, vertical cracking occurs.

The inventors of the present invention have clarified the optimum rate of change of the inner diameter of the mold when continuously casting the round billet slab as described above, based on a detailed study. The method of changing the inner diameter of the mold in the present invention is described in Japanese Utility Model Laid-Open No. 59-16748.
It does not change in two steps as proposed in No. 59-16749, nor does it change in accordance with the shrinkage of the slab as proposed in Japanese Utility Model Laid-Open No. 59-16749.

By using the mold for continuous casting of round billet slabs of the present invention, good contact between the slab and the mold is achieved, and high quality round billet slabs can be obtained. However, as for the mold powder that serves as the heat transfer medium between the mold and the slab, by using a mold powder having the following physical properties and components, it is possible to obtain a higher quality round billet cast than using conventional mold powder. You can get a piece.

First, the viscosity is preferably 0.1 to 1.0 Pa.s at 1573 K, and 0.1 Pa.s.
If it is less than this, the inflow between the mold and the slab will be non-uniform, and the heat removal will be non-uniform, causing vertical cracks, restraints, or being caught in molten steel and causing defects. To do. Further, if it is 1.0 Pa · s or more, the flow between the mold and the slab is insufficient and restraint occurs.

Next, if the melting point is less than 1273 K, the liquid phase becomes too much between the mold and the slab and cooling becomes too strong, so that the slab is deformed by thermal stress and vertical cracking occurs. To do.

Further, (CaO + CaF ₂ × 0.718)
/ SiO ₂ mass concentration ratio is less than 0.6, MgO
When the amount of Mg converted to is less than 4 mass%, the solidified powder becomes vitreous and cooling becomes too strong between the mold and the slab, so the slab is deformed by thermal stress and vertical cracks occur. appear. In addition, (CaO + CaF ₂ × 0.71
8) / SiO ₂ mass concentration ratio is 1.0 or more, M
If the amount of Mg converted to gO is 20 mass% or more, the shrinkage of the powder film becomes too large and the contact between the slab and the mold deteriorates, causing vertical cracking.

The Na content converted to Na ₂ O is 5.0 mass.
% Or the F concentration exceeds 7.0 mass%, the melting property of the powder becomes poor, and defects such as bite bite occur.

Therefore, by continuously casting while supplying the mold powder having the physical properties and components defined here to the surface of the molten steel in the mold of the present invention, it is possible to manufacture a round billet slab of even better quality.

[0020]

EXAMPLES A mold for continuous casting of round billet slabs of the present invention will be described below based on an embodiment shown in FIG. 1, and a method for continuous casting using this mold will be described. FIG. 1 is a cross-sectional view showing a state where a round billet slab is continuously cast using one embodiment of a mold for continuously casting a round billet slab according to the present invention.

In FIG. 1, reference numeral 1 denotes a continuous casting mold of the present invention, where D ₀ is the inner diameter of the mold 1 on the outlet side, and D is the inner diameter at a distance x from the upper end of the cooling surface of the mold 1.
The rate of change of the inner diameter of the mold per unit length along the casting direction is assumed to be (1 / D ₀ ) · (dD / dx) × 100 (%).

According to the mold 1 of the present invention, the above-mentioned assumed mold inner diameter change rate is at least the mold 1 on the side where the molten steel 2 is injected.
From the upper end 1a of the cooling surface of 50 to 100 mm (hereinafter, referred to as "first section") 1b is 12 to 16% / m,
Between the upper end 1a of the cooling surface of the mold 1 and 250 to 300 mm (hereinafter, referred to as "second section") 1c is 12 to 16
% / M continuously from 0.8 to 1.4% / m, and from the upper end 1a of the cooling surface of the mold to the lower end of the cooling surface of the mold 300 mm (hereinafter referred to as "third section").
1d is configured to be 0.8 to 1.4% / m.

The mold 1 of the present invention is constructed as described above, and when molten steel 2 is injected into the mold 1 from a tundish (not shown) through a dipping nozzle, the round billet slab 3 and the mold 1 come into contact with each other. The round billet slab 3 becomes good, and the round billet slab 3 of high quality can be obtained by pulling out the round billet slab 3 from below the mold 1. Further, in this continuous casting, by using the mold powder 4 supplied to the surface of the molten steel 2 located in the mold 1 having the physical properties and components as shown in Table 1 below, higher quality round billet casting The piece 3 can be manufactured. This is the continuous casting method for round billet slabs of the present invention.

[0024]

[Table 1]

Next, in order to confirm the effect of the present invention, a hypoperitectic steel having the components shown in Table 2 is cast in a single-point straightening continuous casting machine having a bending radius of 10 m as shown in Table 3 below (cooling surface lower end inner diameter is Two
The experimental result when hot water is supplied to 25 mm and the length is 900 mm, various mold powders shown in Table 4 are supplied to the surface of this molten steel, and continuous casting is performed at a casting speed of 2.0 m / min will be described. The casting conditions are summarized in Table 5.

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

[Table 4]

[0029]

[Table 5]

FIG. 2 shows the fluctuation range of the temperature of the copper plate of the mold, which shows the state of contact between the mold and the slab. The temperature fluctuation range of the mold shown in FIG. 2 is the effective value (integrated average) of the temperature fluctuation range of the thermocouple installed at a position 150 mm from the upper end of the mold cooling surface. The thermocouple was installed 15 mm inside from the surface of the copper plate. As is clear from FIG. 2, with respect to the casting conditions A, C, E, and G, the fluctuation range of the mold copper plate temperature was large and became a problem in operation, but other casting conditions were within the range with no problem.

FIG. 3 shows the rate of occurrence of vertical cracks in the cast slab. In addition,
The occurrence rate of vertical cracks is indicated by the crack length per cast piece length. As is clear from FIG. 3, regarding the casting conditions A, C, E, and G, longitudinal cracking breakout occurred at least once in casting with 50 charges (150 tons per charge). Regarding the casting conditions B, D, F and H,
One or more restraint breakout alarms were issued during one-charge casting, and it was found from the observation of the cast pieces after casting that seizure occurred between the cast pieces and the mold.

On the other hand, with respect to the casting conditions I to R and B which are the methods of the present invention, vertical cracks were hardly generated, and no breakout or restraint breakout alarm was issued. Also, the casting conditions S to Z and B using the mold of the present invention were within the range of no problem, although they were not as high as the casting conditions I to R and B of the above-described method of the present invention.

[0033]

As described above, when the continuous casting mold of the present invention is used, good contact between the slab and the mold is obtained, and a high quality round billet slab with few surface defects is obtained. be able to. Further, in that case, in the case of the method of the present invention using a mold powder having specified physical properties and components,
Furthermore, a high quality round billet slab with few surface defects can be obtained.

[Brief description of drawings]

FIG. 1 is a mold 1 for continuous casting of round billet slabs according to the present invention.
It is sectional drawing of the state which is continuously casting the round billet cast piece using an Example.

FIG. 2 is a diagram showing a result of an experiment for confirming an effect of the present invention and a diagram showing a variation rate of a mold copper plate temperature.

FIG. 3 is a diagram showing a result of an experiment for confirming an effect of the present invention and a diagram showing a vertical crack occurrence rate.

[Explanation of symbols]

 1 Mold 2 Molten Steel 3 Round Billet Slab 4 Mold Powder

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Satoshi Satake, 1850 Minato Minato, Wakayama, Wakayama Sumitomo Metal Industries, Ltd. Wakayama Works

Claims (2)

[Claims] 1. A mold for continuously casting round billet slabs of steel, wherein the mold inner diameter change rate per unit length along the casting direction [= (1 / D ₀ ). (DD / dx) × 100
(%), Where D ₀ is the inner diameter at the mold outlet side, D is the mold inner diameter at the distance x from the upper end of the mold cooling surface] is at least 50 to 100 from the upper end of the mold cooling surface on the side where molten steel is injected.
12 to 16% / m up to mm and 12 to 16% / m up to 250 to 300 mm from the upper end of the mold cooling surface.
To 0.8 to 1.4% / m continuously, and 0.8 to 1.4% / m from the position 300 mm from the upper end of the mold cooling surface to the lower end of the mold cooling surface. A mold for continuous casting of round billet slabs. 2. The surface of the molten steel injected into the continuous casting mold according to claim 1 has a viscosity at 1573 K of 0.1 to 0.15.
1.0Pa · s, melting point is 1273K or higher, (Ca
O + CaF ₂ × 0.718) / SiO ₂ mass concentration ratio is 0.6 to 1.0, and Na amount converted to Na ₂ O is 5.0 mass.
%, An F concentration of 7.0 mass% or less, and an amount of Mg converted to MgO of 4 to 20 mass% for continuous casting while supplying a continuous casting mold powder. .