JP2002066701A - Continuous casting method of stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method of stainless steel to obtain sound slabs, reducing trap of non-metallic inclusions which often occur at an initial stage of continuous casting of Si-killed stainless molten steel. SOLUTION: In a continuous casting method of stainless steel to continuously cast stainless molten steel into a mold through a tundish nozzle installed at the bottom of a tundish, from a start of casting to a stable casting state, an insoluble gas in molten steel is blown into the molten steel in the volume of 2 to 35 NL/t of steel through the tundish nozzle, depending on feeding rate of molten steel from the tundish to the mold, and then a soluble gas is blown into molten steel during the stable casting state.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to continuous casting of stainless steel, and more particularly to a continuous casting method of stainless steel for producing a slab having excellent surface properties.

[0002]

2. Description of the Related Art Continuous casting of steel is generally performed by a continuous casting machine as partially illustrated in FIG. That is, in FIG. 1, the molten steel transferred from the ladle (not shown) to the tundish is fixed to the tundish bottom brick 1 in close contact with the tundish nozzle 2, the fixing plate 3, and the sliding plate 4. , Collector nozzle 5, immersion nozzle 6
Is supplied to the mold 7 from the molten steel discharge port 6a of the immersion nozzle. The molten steel supplied into the casting mold 7 is solidified while being forcibly cooled from the outside, and solidifies while gradually increasing the thickness of the solidified shell to form a slab. At this time, in order to prevent non-metallic inclusions from adhering to the inside of the tundish nozzle 2 or the immersion nozzle 6 to cause nozzle clogging, and to float and separate inclusions from molten steel, a tundish is usually used. An inert gas such as Ar gas is blown from a gas blowing port 2 a of the nozzle 2.

[0003] Continuous casting of stainless steel is carried out in the same manner. However, in the case of stainless steel, Si deoxidation is generally carried out at the stage of a product and also of a slab, since particularly good surface properties are required. It was a target. This is because, in the case of Al deoxidized steel, there is a concern that the surface properties may be deteriorated due to the alumina cluster. However, when continuously casting such Si deoxidized stainless steel molten steel, cellular defects tended to occur. For such cellular defects in the Si deoxidized stainless steel, for example, Japanese Patent Application Laid-Open No. H11-579
As proposed in Japanese Patent Publication No. 58-58, instead of using Ar gas as an inert gas, a gas soluble in molten steel (N
₂ ) has been put into practical use.

[0004]

However, when the molten stainless steel deoxidized as described above is continuously cast under the blowing of a gas soluble in the molten steel as described above, the slab at a portion corresponding to the initial stage of the casting is cast. Hereinafter, this phenomenon is abbreviated as “initial casting slab”), a phenomenon in which entrainment of nonmetallic inclusions and the like increases. This remarkably deteriorates the surface properties of stainless steel, causes an increase in the number of maintenance steps and a decrease in productivity, and a prompt solution is required. Conventionally, in order to improve the surface properties of the initial casting slab, from the viewpoint of separating non-metallic inclusions in molten steel at the start of casting, a predetermined amount of molten steel is stored in a tundish and then casting is started. Although many attempts have been made, it is not yet a decisive improvement.

Accordingly, the present invention has been made in view of the above-mentioned problems of the prior art, and has been developed in view of the above-mentioned problems encountered in the prior art, in the continuous casting of molten stainless steel, particularly, stainless steel deoxidized stainless steel. It is an object of the present invention to provide a continuous casting method of stainless steel for obtaining a sounder initial casting slab by reducing entrainment of an object.

[0006]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have observed the occurrence of these defects in the initial casting slab in detail, and studied a mechanism of occurrence and a method of preventing the defects. As a result, when the gas soluble in the molten steel is blown from the tundish nozzle, the absolute amount of the gas in the molten steel is reduced as compared with the case where the Ar gas is blown alone. In order to weaken the stirring action of the molten steel in the mold due to the blown gas, the molten steel temperature in the meniscus decreased, forming a trapping site for non-metallic inclusions, which was disadvantageous, and the effect of the latter was rather large. It is.

[0007] The present invention has been completed based on the above-mentioned knowledge and further pursued, in which molten stainless steel is supplied into a mold through a tundish nozzle provided at the bottom of a tundish for continuous casting. In the continuous casting method of stainless steel, from the tundish nozzle, during the period from the start of casting to the steady casting state, an insoluble gas or a mixed gas of an insoluble gas and a soluble gas is blown into molten steel to form a steady casting state. Hereinafter, a continuous casting method of stainless steel, characterized by blowing a soluble gas into molten steel from the tundish nozzle. In the above invention, the flow rate (NL / min) of the gas insoluble in the molten steel blown from the tundish nozzle
Is the amount of molten steel supplied from the tundish to the mold (t / min
) Is preferably 2 to 35 NL / t. In the above inventions, insoluble gas molten steel is Ar gas, it is preferred soluble gas is N ₂ gas into the molten steel. Furthermore, these inventions are even more advantageous when applied to Si-deoxidized stainless steel molten steel.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have found that surface defects found in early casting slabs are caused by non-metallic inclusions being trapped in claw-like trapping sites formed at the beginning of casting. Was confirmed. That is, in the early stage of casting start, the temperature of the molten steel decreases due to heat release in the tundish, heat release in the immersion nozzle, heat of melting of the mold powder, etc., and in addition to these temperature reduction factors, especially in the meniscus part Heat capacity shortage overlaps,
A claw-like trapping site will be formed, which grows from the side wall of the mold toward the center. The situation is schematically shown in FIG. The trapping site s thus formed captures the re-oxide r, slag, deoxidized products and the like formed in the mold powder p, the tundish and the mold. These are manifested as non-metallic inclusion defects in cast slabs and products.

Therefore, in order to reduce nonmetallic inclusion defects, it is important to prevent the formation of trapping sites as described above so that nonmetallic inclusions can float upward. The inventors set out to prevent the formation of trapping sites early in the casting start,
An effective method of supplying a sufficient amount of heat to the meniscus was studied. As a result, it was found that it is extremely effective to increase the usage ratio of a gas insoluble in molten steel such as Ar gas without using the entire amount as a gas soluble in molten steel such as N ₂ in the early stage of the start of casting. .

Here, the flow rate (NL / min) of the gas insoluble in the molten steel blown from the tundish nozzle is
It is preferably from 2 to 35 NL / t depending on the amount of molten steel supplied from the tundish to the mold (t / min). That is, if the flow rate of the gas insoluble in the molten steel is less than 2 NL / t, the lift-up effect of the molten steel by the gas is not sufficient, while if it exceeds 35 NL / t, stirring of the molten steel becomes excessive, and powder entrainment is rather avoided. This is because it is easy to occur frequently. In addition, the blowing amount of the gas soluble in the molten steel is 0 to 3
5 NL / t, 35 NL / t in total with insoluble gas
It is preferable to set the following.

As described above, when the inert gas insoluble in the molten steel is increased at the beginning of casting, the insoluble inert gas supplied into the mold together with the molten steel causes the molten steel in the mold to be lifted by the lift-up effect. The mixture is sufficiently stirred, and the formation of trapping sites can be prevented by supplying heat to the meniscus portion. However, such a treatment is performed only in the early stage of the start of casting, and after the casting operation has been in a steady state, only the gas soluble in the molten steel may be restored. If only the gas soluble in the molten steel in the steady state of casting is used, the original action of the soluble gas with respect to the reduction of bubble defects under the steady state is exhibited, and this effectively contributes to the improvement of the surface properties.

Here, the steady state can be strictly defined as a state in which the amount of heat supplied to the meniscus portion is steady for the purpose of the present invention. A state where the reference pouring speed determined by the degree of superheat ΔT has been reached, or a state where the molten steel weight of the tundish has reached a steady (full) state may be regarded as a steady state.

[0013]

Next, the present invention will be described by way of examples. Si
The SUS304 stainless steel molten steel deoxidized by the above method was cast into a slab by a vertical bending type continuous casting machine (full tundish weight: 30 t, mold size: 200 mm, width: 800 to 1600 mm) as partially shown in FIG. Until the molten steel in the ladle was started to be poured and until the tundish weight reached 4 t, the molten steel was accumulated by the tundish stopper and casting was started. Until the tundish weight becomes 20 t, the throughput is 0.3 to 1.5 t / min,
10 NL / min r gas alone from tundish nozzle
(I.e. 33.3~6.7NL / t) blown in, switched after reaching steady state casting full length 2m, a blowing gas to _{N 2} gas of 10NL / min (5.0~6.7NL / t) .

After completion of the casting, the entire length 2 m of the casting start portion is discarded as scrap, and the subsequent cast initial slab is hot-rolled. The number of initial cast slabs generated / the number of all cast initial slabs) was examined and compared with that of the conventional method. As a result, the surface roughness of the initial cast slab was improved by applying the method of the present invention, whereas the baldness generation rate of the hot rolled steel strip was improved by applying the method of the present invention, whereas the baldness generation rate was 45% in the conventional method. The rate was reduced to 10%, and the surface texture was greatly improved.

[0015]

As described above, according to the present invention,
In the period from the start of casting to the steady state, the gas blown from the tundish nozzle is used as a gas insoluble in molten steel such as Ar, so that a sufficient amount of heat is supplied to the meniscus in the mold. As a result, entrapment of nonmetallic inclusions in the early stage of casting is reduced. Therefore, by applying the present invention to continuous casting of stainless steel,
Swarf flaws and the like in the slab and thus the hot-rolled steel strip are reduced, and the surface properties are greatly improved.

[Brief description of the drawings]

FIG. 1 is a side view illustrating a main part of a continuous casting machine used for applying the present invention.

FIG. 2 is a schematic diagram showing claw-shaped trap sites generated at an early stage of the start of casting.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Tundish bottom brick 2 Tundish nozzle 2a Gas injection nozzle 3 Fixing plate 4 Sliding plate 5 Collector nozzle 6 Immersion nozzle 6a Molten steel discharge port 7 Mold g Solidification shell p Mold powder r Reoxide s Nail trap site

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Tetsuo Mochida 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba F-term in Technical Research Institute, Kawasaki Steel Co., Ltd. 4E004 FA01 HA02 NC02 4K013 AA02 AA09 BA14 CA02 DA03 DA17

Claims (4)

[Claims] 1. A continuous casting method for a stainless steel in which molten stainless steel is supplied into a mold through a tundish nozzle provided at the bottom of a tundish and continuously cast, wherein a steady casting state from the start of casting from the tundish nozzle is provided. In the period up to, the molten steel is blown with insoluble gas or a mixed gas of insoluble gas and soluble gas,
A continuous casting method for stainless steel, wherein a soluble gas is blown into molten steel after a steady casting state. 2. The flow rate (NL / min) of a gas insoluble in molten steel blown from a tundish nozzle is set to 2 to 35 NL / t according to the supply rate (t / min) of molten steel from a tundish to a mold. 2. The method according to claim 1, wherein
3. The continuous casting method according to item 1. 3. The gas insoluble in molten steel is Ar gas,
The gas soluble in molten steel is N ₂ gas.
3. The continuous casting method according to item 1. 4. The continuous casting method according to claim 1, wherein the molten stainless steel is obtained by deoxidizing Si.