JPH03243253A - Manufacturing method for high-purity steel - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention utilizes electromagnetic force in a tundish in a continuous steel casting method to effectively agglomerate and float inclusions to achieve high purity. The present invention relates to a method for manufacturing slabs.

[Conventional technology]

As is well known, in the continuous casting method, a tundish is installed directly above the mold, and slabs are manufactured by continuously supplying molten steel to the mold. Furthermore, the steel is cleaned by increasing the residence time in the tundish and floating and separating inclusions.

However, because the flow falling from the pot is fast, it is not possible to suppress the injected flow only by the weir installed in a normal tundish, and there is a locally fast flow (direct flow) that passes through the bottom of the tundish and reaches the mold. do.

Due to the existence of such a perpendicular flow and the fineness of the inclusions, the actual situation is that a sufficient cleaning effect is not obtained.

Various proposals have been made to eliminate this orthogonal flow and clean the steel. For example, as shown in JP-A-61-193752, a weir with a large flow resistance is installed in the tundish to dam the falling flow of the pot S9, slowing the flow in the downstream area and promoting the floating of inclusions. how to,
In addition, as shown in Japanese Patent Application Laid-Open No. 61-255750, there is a pouring method that uses electromagnetic force to suppress the falling flow from the pot, eliminates bare hot water near the nozzle, and actively prevents secondary oxidation of molten steel. .

[Problem to be solved by the invention]

However, these methods lack operational stability because they focus on calming the flow, and although the mainstream can be calmed down, there are still fast flows in the periphery. However, a sufficient cleaning effect was not obtained.

In other words, as mentioned above, in the method of installing a weir in the tandish, in order to obtain a slow uniform flow, the diameter of the hole in the weir is made small and the flow resistance becomes extremely large, creating a flow that causes local stagnation. .

However, as a result, insufficient preheating of the Tanditshu Weir directly leads to operational troubles, and also tends to cause pore clogging and drifting, making operations unstable.

Furthermore, at the end of the operation, because the flow resistance at the base is too large, the flow rate decreases sharply as the molten steel head decreases, making it necessary to take measures such as extending casting time or stopping casting midway, resulting in lower yields. A decline is inevitable.

In addition, in the latter method, electromagnetic force is applied locally to the mainstream of the falling flow to calm it down, but the flow still unevenly distributed in the depth and width directions within the tundish (so-called orthogonal flow). Therefore, it is impossible to obtain a sufficient floating effect of the inclusions.

Furthermore, even if weirs and electromagnetic forces can be used under the best conditions,
In the conventional method, since the inclusions are too small, the buoyancy of the inclusions is low, and a sufficient cleaning effect cannot be expected.

The present invention aims to fundamentally solve the problems of the conventional methods and provides a method for producing steel that can produce slabs with excellent cleanliness.

[Means to solve the problem]

In order to solve the above-mentioned problems, the present invention, when continuously casting steel, applies electromagnetic force to the molten steel flow in the tundish and the entire vertical cross section of the tundish, sandwiching the part where the molten steel is poured into the tundish. A highly clean steel characterized in that the steel is separated into a strong stirring zone and a downstream slow zone of the tanditshu containing the parts, and inclusions in the steel are removed through a stirring agglomeration zone in the strong stirring zone and a flotation separation zone in the slow zone. This is the manufacturing method.

[For production]

The present invention will be described in detail below along with its effects.

FIG. 1 is a front view of a tundish equipped with a stirring device and an electromagnetic force generating device according to the present invention, and FIG. 2 is a plan view thereof.

In the figure, a stirring agglomeration zone 6 is provided between the pouring areas in the tundish 1 to promote the agglomeration of inclusions by strong stirring of the molten steel, and a flotation separation zone 7 is provided downstream of the aggregation zone 6 to remove the inclusions from the molten steel. Perform flotation separation. Between the two regions, an electromagnetic force is applied to the flow of the molten steel and the entire vertical cross section of the tundish 1 across the pouring area, and an effective inclusion is created by separating the two regions. Agglomeration and flotation separation are possible. In the figure, 2 is a long nozzle for pouring hot water, and 5 is an immersion nozzle.

In the agitation aggregation zone 6, the falling energy from the pot through the long nozzle 2 is utilized, bubbles are further blown into the falling stream by an inert gas bubble blowing device 4 to increase the stirring power, and flux is blown to agglomerate inclusions. promote,
Inclusions can be thoroughly agitated using various conventional methods, such as blowing air bubbles from the bottom, mechanically stirring with a stirring device made of refractory material, or stirring using electromagnetic force. This enables the aggregation of inclusions to be further promoted, and the effect of flotation and separation of inclusions is further enhanced.

Note that adding a flux having a high ability to absorb inclusions, such as CaO-based blacks, to the surface of the molten steel in the tundish 1 is an effective means for separating and removing inclusions from the molten steel.

In addition, in order to effectively perform such cohesive flotation, it is necessary to use a gas blowing method that can easily obtain strong stirring.
f 7 minutes (500 watts converted to energy) or more,
Although the electromagnetic force varies depending on the strength of the stirring force, it is preferable that the electromagnetic force be 1000 Gauss or more and the residence time in the flotation separation area 7 be 120 seconds or more.

The present inventors conducted various types of agitation in order to confirm the effect of flotation and separation of inclusions in the flotation and separation zone 7 described above.

Changes in the number of inclusions remaining in the slab under air-dead conditions were investigated. As a result, it was confirmed that by applying electromagnetic force, there were no large inclusions of 300 μm or more, and that a slow uniform flow was realized in the flotation separation area. It was also confirmed that the number of remaining fine inclusions is proportional to the stirring force, and that the number of remaining inclusions changes depending on whether or not gas is blown into the tundish (whether or not air is cut off).

The reason for this is that the molten steel flows as a uniform flow with no interference due to strong agitation such as gas agitation. As a result, the inclusions are actively floated and separated by buoyancy, making it possible to clean the molten steel.

In the present invention, as described above, an electric current of <1000 Gauss or more
for more than 120 seconds. As a result, the fine inclusions in the molten steel agglomerated in the stirring zone 6, so the fine inclusions in the molten steel were reduced to 50 μm or less on the exit side of the stirring zone, and
The number of inclusions increased. Larger inclusions easily receive buoyancy, and as a result of flotation and separation in the uniform flow of the slow flotation separation zone 7 in the downstream region, the number of inclusions was drastically reduced, and in particular, there were no inclusions larger than 300 μm.

[Example] Explaining based on FIGS. 1 and 2 described above, in this example, the electromagnetic force generating device 3 is installed so as to generate magnetic flux all over the tundish 1 in the flow direction and vertical direction. In addition, an inert gas blowing device 4 is installed at the bottom of the tundish l as an inclusion aggregation technique.

The operating conditions of this example are that low carbon aluminum killed steel is continuously cast at a slab size of 980 mm width and 250 mm thickness, and a casting speed of 1.0 m/min. The electromagnetic force generating device 3 has the ability to generate a magnetic flux density of 5000 Gauss,
In this example, a maximum magnetic flux density of around 5000 Gauss was generated and applied so as to cover the entire height of the tundish in the thickness direction. The test was conducted by changing the flow rate of the gas blown from the inert gas blowing device 4 in the range of 5O-25ON f 7 minutes.

FIG. 3 shows the cleanliness of the slab manufactured by this example in comparison with that of the conventional method. As shown in the figure,
It can be seen that the cleanliness of the slab is improved simply by applying electromagnetic force. Also, the cleanliness of the slab improves in proportion to the amount of blown gas up to 7 minutes at 20ON ffi, but at 25ON
At f 7 minutes, the scene was greatly disturbed and the improvement effect was slightly worse, which is considered to be due to an increase in secondary oxidation in Tanditsh 1.

〔Effect of the invention〕

As described in detail above, by carrying out the present invention, the flow of molten steel in the tundish is suppressed over the entire area, and inclusions that have grown large in the agglomeration zone are floated away. As a result, it becomes possible to manufacture slabs with excellent slab cleanliness.

[Brief explanation of drawings]

FIG. 1 is a front view of a tundish equipped with a stirring device and an electromagnetic force applying device according to the present invention, FIG. 2 is a plan view of FIG. 1, and FIG. 3 is a graph showing the effects of the invention. 1... Tanditshu, 2... Long nozzle, 3.
... Electromagnetic force generator, 4... Inert gas bubble blowing device, 5... Immersion nozzle, 6... Stirring aggregation area, 7.
・・Floating separation area

Claims (1)

[Claims] When continuously casting steel, electromagnetic force is applied to the molten steel flow in the tundish and the entire vertical cross section of the tundish, sandwiching the area where the molten metal is poured into the tundish. and the downstream slow basin,
A method for producing highly clean steel, characterized in that inclusions in steel are removed through a stirring agglomeration zone in a strong stirring zone and a flotation separation zone in a slow stirring zone.