JPH07112613B2 - Tundish for continuous casting of steel - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a tundish for continuous casting of steel, and in particular, when pouring molten steel from a tundish into a mold through a dipping nozzle, the dipping nozzle is used. The present invention relates to a tundish for continuous casting of steel having an improved pouring nozzle structure in which an inert gas is blown into a molten steel passing therethrough.

[Prior Art] Conventionally, in continuous casting of steel, the molten steel poured from a ladle is received by a tundish receiving part partitioned by a weir, and transferred to the pouring part through the weir. Molten steel is poured into the mold through an immersion nozzle provided at the bottom of the mold, and the molten steel poured into this mold is cooled in the mold and by a cooling zone below the mold, and a slab obtained by this cooling It is carried out by continuously pulling out.

In the continuous casting process of the steel, non-metallic inclusions present in the molten steel, especially Al ₂ O ₃ etc. are attached to the inner wall when passing through the immersion nozzle, and it is known that there is a problem of nozzle clogging, As a measure for preventing the nozzle clogging, an inert gas is generally blown between the tundish and the mold. Specifically, a method is provided in which an insert nozzle made of a porous material is provided on the inner bottom of the tundish, and a pressurized inert gas is blown from the outer peripheral surface to the inner peripheral surface through the pores of the porous material.

Then, the inert gas blown by the above-mentioned blowing of the inert gas, most of which floats up to the molten steel molten metal surface in the mold and is separated, but a very small portion is captured by the meniscus portion, and the cast slab is cast. It is known to appear as pinholes on the surface.

[Problems to be Solved by the Invention]

However, in recent years, ultra-low carbon steel with improved workability (C ≦ 0.01
wt%) and other needs for cold rolled materials made of soft steel have increased, and when this type of soft steel is continuously cast by the above-mentioned method and the obtained slab is processed into cold rolled materials, the surface of the cold rolled material is swollen and linear. In order to investigate the cause, the swelling linear defect part was investigated and Ar gas was detected.As a result of further investigation, Ar gas blown to prevent nozzle clogging during casting was found.
It was found that the gas bubbles of the gas were caught up to a deep part of the epidermis under the epidermis of several tens of mm or more and appeared as swelling linear defects on the surface of the cold rolled material.

Therefore, in order to solve the above problems, the present inventors have
The state of Ar gas trapped in the slab was thoroughly investigated.
As a result, the size and number of pinholes of the subcutaneous 15 to 30 mm pinhole of the cast piece obtained by casting the molten steel into the mold under the conditions where the mold size and the Ar gas flow rate were constant and the discharge rate. The relationship shown in FIG. 7 was obtained.

As is clear from the results shown in Fig. 7, there is a clear difference in the number of pin holes due to Ar gas trapped in the slab between a pin hole diameter of 1.5 mm or less and 1.6 mm or more. Those with a diameter of 1.5 mm or less markedly increased as the molten steel discharge flow rate increased, but those with a pinhole diameter of 1.6 mm or more hardly increased and the number was almost irrespective of the molten steel discharge flow rate. .

The present inventors, from the above grasped results, the bubble diameter of the inert gas blown to prevent nozzle clogging during casting is 1.6 m
If it is generated and blown so that it becomes m or more, the floating separation of gas bubbles is facilitated regardless of the molten steel discharge flow rate, and it is thought that quality defects such as swelling of the soft steel can be resolved, further intensive research, As a concrete means thereof, the tundish for continuous casting of steel according to the present invention has been developed.

[Means for Solving the Problems]

The present invention has been made under the circumstances described above, and the gist thereof is that the pouring port portion located at the inner bottom is made of a porous material having an average pore diameter of 30 μm or more, and the minimum inner diameter portion of the pouring hole of the porous material. Is a tundish for continuous casting of steel having a pouring nozzle structure having a cross-sectional area of 1.5 times or more of the cross-sectional area of the minimum inner diameter portion of the pouring hole continuing below the porous material.

Next, the present invention will be described in detail.

The inert gas bubbles blown from the porous material into the molten steel are sheared by the molten steel flow flowing along the surface of the porous material in the process in which the inert gas is ejected from the pores on the surface of the porous material into the molten steel and grows as bubbles. Therefore, it is considered that the bubble size (bubble diameter) is determined by the balance between the bubble growth rate and the shear force (molten steel flow rate) due to the molten steel flow.

So, using the water model device outlined in Figure 4,
The average pore diameter of the porous material of the porous material insert nozzle 16 provided below the tundish 15 and the site where the porous material is provided were changed, and changes in the diameter of gas bubbles generated were investigated.

In FIG. 4, 17 is a sliding nozzle, 18 is a dipping nozzle, 19 is a mold, and 20 is a water circulating device.

Further, R ₀ / R ₁ = 1 / 1.35 was used as the similarity rule of the gas bubble diameter between the water model and the actual molten steel. However, R ₀ : gas bubble diameter in water, R ₁ : gas bubble diameter in molten steel.

Therefore, the gas bubble diameter observed in the water model is 1.
In actual equipment, the conditions for obtaining a thickness of 15 mm or more are 1.
The gas bubble diameter is 6 mm or more, which is a condition that can prevent the gas bubbles from being trapped in the slab.

First, using the insert nozzle 16 having the shape shown in FIG. 3, the average pore diameter (23 μm and 40 μm) of the porous material 21 constituting the insert nozzle 16, the gas bubble diameter generated by this insert nozzle, and the discharge flow rate. The relationship between and was investigated by the water model device, and the results shown in FIG. 5 were obtained. From this result, it is understood that in order to obtain the gas bubble diameter of 1.15 mm or more, the average pore diameter of the porous material should be increased and the discharge flow rate should be decreased.

Next, the insert nozzle 16 having the same shape as that shown in FIG.
And the cross-sectional area (S ₁ ) of the smallest inner diameter portion of the porous portion 16a
Is prepared by gradually changing the sectional area (S ₀ ) of the straight rectifying section up to the straight rectifying section 22 of the insert nozzle 16, and the discharge flow rate of 350 l / m
The generation of bubbles under the in condition was investigated and the results shown in FIG. 6 were obtained. From this result, in order to obtain a gas bubble diameter of 1.15 mm or more, at the same time as increasing the average pore diameter of the porous material, a porous portion is provided above the insert nozzle with a small molten steel flow velocity to shear the bubbles. I understand that it is good.

Inferring from the investigation results shown in FIGS. 5 and 6 obtained by the above water model device, in order to obtain a gas bubble diameter of 1.6 mm or more in an actual device, the average pore diameter of the porous material is , 30 μm or more, preferably 40 μm
It may be m or more. In addition, the application site of the porous material can be minimized with a molten steel flow velocity nozzle that shears bubbles,
In addition, the top of the nozzle that can stably obtain a gas bubble diameter of 1.6 mm or more, that is, the pouring port at the bottom of the tundish is good, and the cross-sectional area of the minimum inner diameter of the pouring hole of the porous material (S ₁ )
Is preferably 1.5 times or more of the cross-sectional area (S ₀ ) of the minimum inner diameter portion of the pouring hole continuing below the porous material.

〔Example〕

Hereinafter, embodiments according to the present invention will be described with reference to the drawings.

FIG. 1 is a cross-sectional view of essential parts of a tundish for continuous casting of steel having a pouring nozzle structure confirmed by the above water model device.

Reference numeral 1 is a tundish, in which a refractory 3 is stretched on the inner surface of a steel skin 2. 4 is an insert nozzle,
It has a laminated structure of a porous portion 4a and a non-porous portion 4b,
A step portion 6 is placed on an iron plate 5 provided on the iron skin 2, and a refractory mortar 7 is packed and fixed around the step portion 6. 8
Is a submerged nozzle, in which the upper end of the submerged nozzle 8 and the lower end of the insert nozzle 4 are in close contact with each other, and between the iron plate 5 provided on the iron shell 2 and the fixing iron plate 9 of the submerged nozzle 8. It is fixed by three fixing bolts 10 provided.

Reference numeral 11 is a pipe for feeding an inert gas to the porous portion 4a of the insert nozzle 4.

Next, in the tundish having the above-mentioned structure, the insert nozzle 4 is the third one for comparison with that according to the present invention shown in FIG. 2 (average pore diameter of porous portion 40 μm, S ₁ / S ₀ = 1.65). Using the conventional structure shown in the figure (average pore diameter of the porous part 23 μm, S ₁ / S ₀ = 1), C: 30ppm, Si: tr, Mn: 0.
25wt%, P: 0.010wt%, S: 0.007wt%, Al: 0.030wt plate, Ti: 0.0
Discharge flow rate of killed steel containing 6 wt% ultra-low carbon Ti
Continuous casting was performed under the condition of 2.7 ton / min.

During this continuous casting, there was no nozzle clogging in both cases, but the slab obtained by continuous casting was processed into a cold rolled material, and the result of investigating the state of occurrence of blisters and defects, using the tundish according to the present invention The obtained cold-rolled material had a reduction of blisters of more than 60% compared with the cold-rolled material obtained by using the tundish of the conventional structure.

〔The invention's effect〕

As described above, according to the tundish of the present invention, the bubble diameter of the inert gas blown to prevent nozzle clogging during casting is generated to be 1.6 mm or more and blown in, and the gas bubbles float from the molten steel in the mold. Separation is facilitated, and continuous casting of steel that can significantly reduce quality defects such as swelling of soft steel can be performed.

[Brief description of drawings]

FIG. 1 is a sectional view of a main part of a tundish for continuous casting of steel according to the present invention, FIG. 2 is an insert nozzle according to the present invention, FIG. 3 is an insert nozzle having a conventional structure, and FIG. FIG. 5 is a schematic diagram of the water model device, FIG. 5 is an explanatory diagram of the relationship between the discharge flow rate and the minimum gas bubble diameter, FIG. 6 is an explanatory diagram of the relationship between the insert nozzle shape and the minimum gas bubble diameter, and FIG. It is an explanatory view of the relationship between the discharge flow rate of molten steel and the size and number of pinholes of 15 to 30 mm in the epidermis of a slab obtained by casting at the discharge flow rate 1 ... Tundish, 2 ... Iron skin 3 ... Refractory, 4 ... Insert nozzle 4a ... Porous part, 4b ... Non-porous part 5 ... Iron plate, 6 ... Step part 7 ... Refractory mortar, 8 ... Immersion nozzle 9 ... Fixing iron plate , 10 ...... Fixing bolt 11 ...... Inert gas supply pipe

Claims (1)

[Claims] 1. A pouring port located at the inner bottom has an average pore diameter of 30 μm.
Pouring made of porous material of m or more, and the cross-sectional area of the minimum inner diameter portion of the pouring hole of this porous material is 1.5 times or more of the cross-sectional area of the minimum inner diameter portion of the pouring hole that follows the porous material. A tundish for continuous casting of steel, characterized by having a nozzle structure.