JPH06134559A - Method for preventing crack in cast slab in continuous casting for steel - Google Patents

Abstract

PURPOSE:To disperse the stress caused by heat shrinkage, etc., of a solidified shell and to prevent the occurrence of longitudinal cracking in a cast slab by making the carrier gas flowing speed in a specific range, at the time of blowing particles having higher m.p. than molten flux to the upper part of the molten flux near mold surface together with the carrier gas. CONSTITUTION:The particles 7 having a higher m.p. than the mold flux 2 are blown to above the mold flux near the mold surface 1 together with the carrier gas 11. The particles are mixed into the mold flux in the condition of the flowing speed or more decided in the inequality U>=140(rhod)<-0.5> at the time of using U (cm/sec) for carrier gas flowing speed, where rho (g/cm<3>) is density of the particles and (d) (cm) is particle diameter, and also, the flowing speed or less decided in the inequality U<=1720h<0.5> at the time of using (h) (cm) for thickness of the mold flux above the meniscus of molten steel. By this method, the mold flux and the particles are flowed into gap between the mold 1 and the molten steel 4 and the solidified shell 6 is made to be grown in uneven condition and the deformed strain in the solidified shell is dispersed in plural positions and the crack of the cast slab is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for molten steel.

[0002]

2. Description of the Related Art In continuous casting of steel, molten steel is usually supplied to a finely vibrating internal water-cooled copper mold through a pouring nozzle, and mold flux is continuously supplied to a meniscus of molten steel. The molten steel in the mold is deheated to the mold side through the mold flux layer flowing into the gap between the surface of the mold and the molten steel, begins to solidify, and the solidified shell grows. Thermal stress mainly due to thermal contraction of the solidified shell acts on the solidified shell in the width direction of the slab to cause distortion, and when this distortion is excessive, it causes vertical cracking of the slab.

Longitudinal cracking of a slab not only lowers work efficiency by increasing post-work such as fusing and removing the cracked portion after casting, but in the case of a large crack, the molten steel cracks during casting. There was a breakout that leaked out from the department,
Since the casting has to be stopped, it is extremely important to prevent the occurrence of cracks.

[0004] Medium carbon steel having a carbon concentration of about 0.1 to 0.2% is particularly susceptible to vertical cracking. As the cause of vertical cracking, it is considered that one of the main causes is the non-uniform inflow of mold flux into the gap between the mold and molten steel. By changing the melting point, the melting point of the flux, the solidification temperature, the viscosity, etc. are optimized and the technology has been developed so that the flux can be uniformly introduced. Reference 1) Kunio Koyama, Yutaka Nagano, Taketo Nakano: Steelmaking Research,
No. 324, 1987, p39-45.

[0005]

Conventionally, a mold flux for continuous casting is mainly composed of SiO ₂ and CaO, and Al ₂ O ₃ , MgO, MnO, Na ₂ O and Li are added to the main component.
₂ O, K ₂ O, Fe ₂ O ₃ , B ₂ O ₃ , F, C and the like are added, and powder or granules are used. When molten steel is cast using such a mold flux, when the molten steel has a carbon concentration of about 0.1 to 0.2%, it is said to be related to the prevention of vertical cracks and the fluctuation of the molten metal surface and the defects on the surface of the mold. The current situation is that vertical cracks occur even if the occurrence is eliminated. When vertical cracks occur in a slab, there are problems such as an increase in the number of post-processes such as fusing of the cracked portion, and if severe vertical cracks, suspension of casting due to breakout. The present invention solves the above problems and provides a continuous casting method for stably producing a slab of medium carbon steel or the like.

[0006]

SUMMARY OF THE INVENTION In continuous casting of steel, the present invention is directed to casting molten steel by continuously supplying a casting mold flux from the upper part of an internal water-cooled mold toward a meniscus of molten steel. When the particles having a higher melting point than the flux are blown together with the carrier gas above the mold flux in the vicinity of the mold surface, the carrier gas flow rate is set to U (cm / se).
c), where the particle density is ρ (g / cm ³ ) and the particle size is d (cm), the carrier gas flow velocity U is equal to or higher than the speed determined by the inequality of U ≧ 140 (ρd) to ^0.5 , and When the thickness of the mold flux on the molten steel meniscus is h (cm), the particles are mixed into the mold flux at a speed equal to or lower than the speed determined by the inequality of U ≦ 1720h ^0.5. When the solidified shell grows, the solidified shell grows non-uniformly and the deformation strain of the solidified shell is dispersed in a number of places to prevent cracking of the cast slab. Is.

[0007]

FIG. 1 is a drawing showing a state in the vicinity of a continuously cast molten steel meniscus. Generally in continuous casting of steel, molten steel 4 is injected into an internal water-cooled copper mold 1 that vibrates finely in a vertical direction 8 through a pouring nozzle, and at the same time, mold flux 2 is continuously supplied from the upper part of the mold. The supplied mold flux is melted by the heat of molten steel, and the mold 1 and molten steel 4
Flows into the gap. The molten steel is solidified by heat moving to the mold through the inflowing mold flux layer 5, so that a solidified shell 6 is formed, and this solidified shell is continuously drawn in the drawing direction 9.

Since the mold flux 2 continuously supplied into the mold is heated and melted by the heat of the molten steel, the molten layer, the sintered layer, and the unsintered layer are directed upward from the molten steel meniscus having a high temperature. (That is, the as-added mold flux layer that has not been sintered) exists, and the solid mass 3 called slugrim, which is formed by aggregating or melting and solidifying the mold flux, is usually present in the portion in contact with the mold 1. .

The thickness of the inflow mold flux layer 5 has a great influence on the heat transfer to the mold, and the growth of the solidified shell is suppressed in the places where the flux layer is locally thick. The solidified shell is subjected to stress due to heat shrinkage and phase transformation of the solidified shell and friction between the solidified shell and the mold via the mold flux layer. Since they occur intensively, the amount of strain becomes large when the solidification delay portion is small, and vertical cracking occurs when the strain exceeds a certain critical value.

Steel having a carbon concentration of about 0.1 to 0.2% has a large shrinkage due to the transformation from the δ phase to the γ phase during the cooling process,
Since the shrinkage stress that occurs is larger than that of steels of other component systems, it is considered that it is easily cracked. For this reason, when medium carbon steel is cast using normal mold flux, vertical cracks frequently occur at one or two or three places in the width direction of the slab.

As shown in FIG. 1, when particles 7 having a melting point higher than that of the mold flux are conveyed to a gas 11 and injected from a particle injecting device 10, the mold flux forms a recess 12 due to the collision energy of the gas. Particles mix into the mold flux from the depression, and then the particles flow into the gap between the mold 1 and the molten steel 4 together with the mold flux.

FIG. 2 is a schematic diagram showing a locally delayed part of solidified shell growth when particles having a melting point higher than that of the mold flux are mixed and continuously supplied to the mold flux. If high melting point particles are mixed in the mold flux,
Some dissolve in the mold flux, while others partially dissolve or do not dissolve and remain in the mold flux layer in the solid state.

When the high melting point particles 7'exist in the mold flux layer 5 in a solid state, when the mold flux layer is cooled and solidified, the particles due to cooling and the mold flux layer generally have different heat shrinkage rates. 7'and the mold flux layer 5 have a void formed of a vacuum or a gas, and the presence of this void locally delays the heat transfer from the molten steel 4 to the mold 1, so that the solidified shell 6 grows locally. Many delay and solidification delay portions 14 are formed.

In the case of particles having a higher melting point and lower thermal conductivity than the mold flux, particles 7'having a small thermal conductivity.
Exists in the mold flux layer 5, in addition to the formation of voids around the particles, the particles themselves serve as heat transfer resistance, and the heat transfer from the molten steel 4 to the mold 1 is delayed.
Growth is locally delayed and a large number of delayed solidification portions 10 are formed. Therefore, particles having lower thermal conductivity are more effective in forming a delayed solidification portion.

FIG. 3 is a schematic view showing the distribution of the locally delayed portion of the solidified shell growth in the width direction of the slab and the casting direction. Vertical slab cracks are mainly caused by stress in the width direction of the slab, however, by intentionally forming a large number of solidification-delayed portions in this way, stress concentration, that is, concentration at strained locations, can be prevented. When the strain due to the stress in the width direction of the slab is dispersed in a large number of solidification delay portions, the amount of strain generated in one solidification delay portion becomes smaller than the critical strain amount for cracking, and vertical cracking does not occur.

The size of the particles mixed in the mold flux is preferably smaller than the size of this gap so that the particles can flow into the gap between the mold and the molten steel. Since it is desirable to maximize the heat transfer resistance of heat transfer from the molten steel to the mold, solid particles or hollow particles having a thermal conductivity as small as possible than the mold flux are preferable.

[0017]

EXAMPLE A continuous casting test for producing a slab slab having a thickness of 240 mm and a width of 1.6 m was conducted by using a medium carbon steel having a carbon concentration of about 0.1 to 0.2%. As the mold flux, powdery ones having the main components shown in Table 1 were used.
Under the conditions shown in Table 2, particles such as ZrO ₂ , TiO ₂ and hollow MgO were continuously supplied from the upper part of the internally cooled mold to the molten steel meniscus by using a particle injection device.

Two particle jetting devices were used, each jetting port was reciprocally moved along the two long side faces of the casting mold, and jetting was carried out on the mold flux at a distance of about 30 to 60 mm from the casting mold.
Argon was used as the carrier gas for the particles, and the gas flow rate was 0.1.
The range was 5 to 40 m / sec. The melting point of the mold flux in Table 1 was about 1273-1423K.
Further, the particle supply volume ratio in Table 2 is the volume ratio of the mold flux supply amount and the particle supply amount. For reference, the melting points of the feed particles are shown in Table 3. As the main casting conditions, the casting speed is 1.8 m / min, and the slab after the casting mold is 2
Water spray was adopted for the subsequent cooling, and the experiment was conducted under constant conditions except for the conditions of the feed particles.

As a result of the above experiment, no particles were ejected.
In the case of Case, about 2 to 3 vertical cracks were observed on the surface of the slab in the width direction, and the sum of the length of vertical cracks per 1 m of the slab on both long side surfaces was about 2 to 7 m / m. Met. On the other hand, in the case of ZrO ₂ particles of Case, the sum of the lengths of longitudinal cracks is about 0.05 to 0.3 m / m, and in the case of TiO ₂ of Case, it is 0.1 to 0.5 m / m. However, it was found that vertical cracks occurred extremely rarely with Case, but the length of vertical cracks was short, there were few vertical cracks compared to Case, and the effect of preventing vertical cracks was large.

In the case of Case, although the number of cracks is slightly larger than that of Case, the sum of the lengths of vertical cracks is about 0.1 to 0.6 m / m,
There were significantly fewer vertical cracks than Case. As a result of observing the internal structure of the mold flux layer taken during the casting experiment, some of the added particles melt and mix with the mold flux, but they do not dissolve and maintain the same shape as before addition. It was found that there are a lot of them, and they contribute as heat transfer resistance in the mold flux layer between the mold and the molten steel.

If the flow velocity of the carrier gas at the time of injecting particles into the mold flux is too slow, the particles will not penetrate into the mold flux, and if too fast, the recess of the mold flux will be too deep and molten steel meniscus will be formed. As it is exposed, it induces vibration of the meniscus, resulting in more cracks in the slab. FIG. 4 is a summary of the results of this experiment, and shows a good region where particles satisfactorily penetrate into the mold flux and a defective region where particles do not penetrate and float on the mold flux.

As a result of rearranging the boundary between the good region and the bad region by the regression equation, the carrier gas flow velocity is U (cm / sec), the particle density is ρ (g / cm ³ ) and the particle size is d (cm) At that time, it was found that if the flow velocity U of the carrier gas is equal to or higher than the velocity defined by the inequality of U ≧ 140 (ρd) to ^0.5 , the spray particles penetrate into the mold flux without being deposited on the mold flux.

[0023]

[Table 1] Components of the mold flux used in the experiment (mass%)

[0024]

[Table 2] Experimental conditions

On the other hand, FIG. 5 shows a defective region where the recess of the mold flux becomes too deep and the molten steel meniscus is exposed and a good region where it is not exposed. In this bad area,
Even when the particles were supplied, the meniscus vibrated remarkably, so that the vertical slabs were often cracked. As a result of rearranging the boundary between the bad area and the good area by the regression equation, when the mold flux thickness on the molten steel meniscus is h (cm), the speed equal to or lower than the speed determined by the inequality of U ≦ 1720h ^0.5 is the good area, It was found that there was no exposure of the molten steel meniscus and the effect of particle supply appeared.

[0026]

[Table 3] Melting point of added particles

[0027]

According to the present invention, stress caused by thermal contraction of the solidified shell and friction with the mold is not concentrated in the delayed portion of the solidified shell growth, but the stress is dispersed in many places to prevent vertical cracking. It is possible to prevent the occurrence, and it is possible to stably produce a slab that is easily broken, such as medium carbon steel.

[Brief description of drawings]

FIG. 1 is a diagram showing a state in the vicinity of a molten steel meniscus in continuous casting, FIG. 2 is a diagram showing a locally delayed portion of solidified shell growth, and FIG. 3 is a localized portion of solidified shell growth in a width direction and a casting direction of a slab. FIG. 4 is a diagram showing a distribution of a target delay portion, FIG. 4 is a diagram showing a carrier gas basin in which particles are favorably supplied into the mold flux with respect to an appropriate lower limit value of the carrier gas flow velocity.
FIG. 4 is a diagram showing a good region regarding an upper limit value of an appropriate carrier gas flow rate.

[Explanation of symbols]

1: Mold, 2: Mold flux, 3: Slugrim, 4: Molten steel, 5: Mold flux layer, 6:
Solidification shell, 7, 7 ': Particles, 8: Vibration direction of mold, 9: Direction of withdrawing slab, 10: Particle injection device, 11: Gas, 12: Indentation of molten steel meniscus,
13: molten steel meniscus, 14: delayed solidification portion, 1
5: Width direction of slab.

Claims (1)

[Claims] 1. In continuous casting of steel, when a casting mold flux is continuously supplied from the upper part of an internal water-cooled mold toward a meniscus of the molten steel to cast molten steel, particles having a melting point higher than that of the molding flux are produced. When spraying with the carrier gas above the mold flux near the mold surface, the carrier gas flow rate U (cm / sec), the particle density ρ (g / cm ³ ),
When the particle size is d (cm), the carrier gas flow velocity U is equal to or higher than the speed that can be reserved by the inequality of U ≧ 140 (ρd) to ^0.5 , and the mold flux thickness on the molten steel meniscus is h (cm) At this time, by mixing the particles into the mold flux at a speed equal to or lower than the speed defined by the inequality of U ≦ 1720h ^0.5 , the mold flux and the particles flow into the gap between the mold and the molten steel, and when the solidified shell grows, A method for preventing slab cracking in continuous casting of steel, characterized in that cracking of slab is prevented by uniformly growing and dispersing deformation strain of the solidified shell at a number of locations.