GB1598764A

GB1598764A - Method and apparatus for continuous casting of steel

Info

Publication number: GB1598764A
Application number: GB685478A
Authority: GB
Original assignee: Sumitomo Metal Industries Ltd
Current assignee: Nippon Steel Corp
Priority date: 1977-03-18
Filing date: 1978-02-21
Publication date: 1981-09-23
Also published as: DE2811546A1

Description

(54) METHOD AND APPARATUS FOR CONTINUOUS CASTING OF STEEL (71) We, SUMITOMO METAL IN DUSTRIES, LTD., a Japanese body corporate of 15, 5-chome, Kitahama, Higashi-ku, Osaka, Japan, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is performed, to be particularly described in and by the following statement: The present invention relates to a method and apparatus for continuous casting of steel and in particular to a method and apparatus for automatically and reliably opening a slidable nozzle provided at the base of a tundish, and promoting floatation of internal inclusions in the molten steel.

In pouring molten steel from a ladle into a tundish of a heretofore used continuous casting apparatus, when the molten steel is poured into the tundish slag floating on the surface of the molten steel in the tundish was frequently pulled into the molten steel therein by the stream of the molten steel poured from the ladle, allowed to remain in the molten steel which was then cast into, for example, a slab and caused a slag spot on the surface of the slab.

Accordingly, it was conventionally necessary to provide a certain length of pipe of, for example, stainless steel on the nozzle so as to prevent the molten steel poured into the tundish at the beginning of the operation from flowing immediately out of the nozzle into the mold and to cause the molten steel stay within the tundish until the pipe has been melted. In this conventional method using the pipe of stainless, however, the time required for the molten steel to begin to flow into the mold depended on the time required for the pipe to be melted and was unfixed. Therefore, the depth of the molten steel held in the tundish was variable in the range, for example, from 150 mm to 300 mm.

On the other hand, our investigation on the relationship between the depth of the molten steel in the tundish and the number of the slag spots formed on the surface of the slab reveals that the number of the slag spots on the surface of the resultant slab is large when the depth of the molten steel in the tundish is 300 mm of less while the number is sharply reduced when the depth of the molten steel is 400 mm or more.

A tundish used in continuous casting has a slidable nozzle assembly attached to the bottom thereof. The known slidable nozzle assembly comprises an upper nozzle corresponding to the bottom opening of the tundish, a lower nozzle corresponding to the opening of the upper nozzle, a slider slidable between said upper and lower nozzles, and a frame supporting said upper and lower nozzles.

In continuous casting operation using such a nozzle assembly, certain preliminary measures were heretofore adopted before beginning pouring of the molten steel in order to prevent the poured molten steel from solidifying onto the nozzle walls such as filling the upper nozzle from the lower end to the upper end with such materials as silica sand, graphite powder and/or chromium powder, or mounting a cover of steel on the top of the opening of the upper nozzle. However, these measures had serious disadvantages such that they required much time and labor, their rate of success was low, and adjustment of opening timing of the sliding nozzle assembly was difficult.

Accordingly, an object of the present invention is to provide a method and an apparatus for continuous casting of steel capable of preventing internal inclusions from mixing into the molten steel being poured from the tundish into the mold by causing slag and other internal inclusions to float upward in the molten steel which is poured from the ladle into tundish.

According to one aspect of this invention there is provided a method of continuous casting steel comprising the steps of: receiving molten steel from a ladle into a tundish which has a depth of 400 mm or greater and maintaining the depth of the molten steel in the tundish to at least 400 mm; using a slidable nozzle assembly which includes an upper nozzle having permeable refractory material mounted on the inner wall of the opening thereof, an upper nozzle fixing plate supporting said upper nozzle at the base thereof and having permeable refractory material mounted on the inner wall of the opening thereof, a lower nozzle having the lower end thereof inserted into a mold, a slider inserted horizontaly and slidable between the said upper nozzle fixing plate and said lower nozzle and having permeable refractory material embedded therein at a suitable position thereof, a slider driving unit for moving said slider in a predetermined minimum time related to the solidification time of the molten steel between the closing position and the opening position thereof; supplying an inert gas to said permeable refractory materials of said upper nozzle, fixing plate, and slider while the slider is closed and open so as to blow said gas into the molten steel in the tundish; and activating said slider driving unit to open said slider and pour the molten steel from the tundish into the mold.

According to another aspect of this invention there is provided an apparatus for continous casting of steel comprising a tundish having a depth of 400 mm or greater, and a sliding nozzle assembly, including: an upper nozzle having permeable refractory material mounted on the inner wall of the opening thereof; an upper nozzle fixing plate supporting said upper nozzle at the base thereof and having permeable refractory material mounted on the inner wall of the opening thereof; a lower nozzle having the lower end thereof inserted into the mold; a slider inserted horizontally and slidable between said lower nozzle and having permeable refractory material embedded therein at a suitable position thereof; a slider driving unit for moving said slider in a predetermined minimum time related to the solidification time of the molten material between the closing position and the opening position thereof; and means for supplying inert gas to said permeable refractory materials of said upper nozzle, fixing plate, and slider.

The present invention will be better understood from the following description taken in connection with the accompanying drawings in which: Figure 1 is a schematic sectional view of an embodiment of the continuous casting apparatus according to the present invention Figure 2 is a graph showing the relationship between the depth of the molten steel in the tundish and the number of slag spots in a unit area of the surface of the produced slab; Figure 3 is an enlarged sectional view of a slidable nozzle assembly shown in Figure 1 showing the state in which the nozzle is closed; and Figure 4 is a similar view to Figure 3 showing the state in which the nozzle is opened.

Referring to the drawings, the continuous casting method and apparatus according to the present invention will now be described in detail. In Figure 1 showing a schematic section of the continuous casting apparatus according to the present invention, molten steel in a ladle 1 is poured into a tundish 2, directed from an opening 21 provided at the bottom of the tundish 2 through a slidable nozzle assembly 3, and poured into a mold 4.

In practice of the method according to the present invention, first of all the tundish 2 relatively larger in depth than heretofore used tundish is prepared. The depth H of the molten steel in the tundish 2 is maintained to be always 400 mm or greater and consequently the tundish has a depth of at least 400 mm. Then, an inert gas such for example as argon gas is blown into the molten steel in the tundish 2 from the slidable nozzle assembly 3 as will be described hereinbelow in greater detail.

Namely, in the method according to the present invention, the depth H of the molten steel in the tundish 2 is maintained to 400 mm or greater to thereby prevent slag from being pulled into the molten steel by the flow of the molten steel poured from the ladle, and inert gas bubbles such as argon gas is blown into the molten steel so as to cause the internal inclusions pulled into the molten steel to float upward to the top thereof, so that the number of the slag spots on the surface of the continuous-cast slab is considerably reduced by the multiplication effect of the large depth of the molten steel in the tundish and the inert gas blown in bubbles into the molten steel.

Our experimental continous casting operation using AISI TYPE 304 steel and maintaining the depth of the molten steel in the tundish to 100 mm, 200 mm, 300 mm, 400 mm, 500 mm or 600 mm for each run using or not using argon gas blowing showed a significant relationship as shown in Figure 2 between the number of slag spots caused by internal inclusions of the size 50 U or larger in a unit area (10 m2) on the surface of the resultant slab and the depth of the molten steel in the tundish. In Figure 2, curve A indicates the relationship between said two factors of the runs without argon gas blowing and curve B indicates the runs with argon gas blowing under 3 atmospheres at the rate of 10 Nl/hr from the sliding nozzle assembly 3.

As seen in Figure 2, there is a sharp change in each of the curves A and B with a turning point at the depth 400 mm of the molten steel in the tundish. As seen from the graph of Figure 2, the number of internal inclusions in the molten steel can be considerably reduced by casting while maintaining the depth of the molten steel in the tundish to 400 mm or larger, and the number of the slag spots in the produced slab can be further sharply reduced by blowing an inert gas in bubbles into the molten steel in the tundish.

An embodiment of the slidable nozzle assembly according to the present invention will now be described in greater detail with reference to Figures 3 and 4. The slidable nozzle assembly 3 is mounted in the opening 21 of the bottom of the tundish 2. The slidable nozzle assembly 3 comprises essentially an upper nozzle 31, an upper nozzle fixing plate 32, a slider 33, a lower nozzle 34, inert gas supply pipes 35, and a slider driving unit 36 see Figure 1).

Set bricks 22 are mounted in the opening 21 of the tundish 2, and the upper nozzle 31 is mounted along the inner periphery of the wall of said set bricks 22. The upper nozzle 31 is supported at its base by the upper nozzle fixing plate 32, and the slider 33 is inserted horizontally slidably between the fixing plate 32 and the lower nozzle 34 which is fixed to the bottom of the tundish 2 by a frame 37 (see Figure 1) with its lower end inserted into the mold 4 as shown in Figure 1.

The slider driving unit 36 is, as shown in Figure 1, fixed to the bottom of the tundish 2 with a heat insulating structure therebetween and is connected to an end of the slider 33. While any fluid cylinder or an electric motor can be utilized as the slider driving unit 36, a hydraulic cylinder is the most suitable for this purpose for the reason to be described hereinunder.

Permeable refractory materials 312 and 322 such for example as porous bricks are bonded respectively to the inner peripheral walls of the openings 311 and 321 of the upper nozzle 31 and the fixing plate 32 respectively. A similar permeable refractory material 332 is embedded in a portion adjacent an opening 331 of the slider 33.

The permeable refractory material 332 is determined in size and position so that it blocks the opening 321 of the upper nozzle fixing plate 32 when the slider 33 is in the nozzle closing position as shown in Figure 3.

The inert gas supply pipes 35 are connected to the permeable refractory materials 312, 322 and 323 respectively so as to supply argon gas for example at the pressure of three atmospheres and at the flow rate of 10 Nl/hr to these refractory materials.

When the slider 33 is in the opening position as shown in Figure 4, the openings of all the members of the sliding nozzle assembly 3 are in communication with one another so as to form a passage for the molten steel.

The operation of the slidable nozzle assembly according to the present invention will now be described in fuller detail.

First, molten steel is poured from the ladle 1 into the tundish 2 when the slider 33 is in the closing position as shown in Figure 3. At this time an inert gas is supplied to the pipes 35. The gas is introduced into the refractory materials 312, 322 and 332 through round channels 313, 323 and 333 formed respectively therein then through innumerable small pores therein and is emitted forcibly into the molten steel in the tundish from the entire inner peripheral surfaces of the openings 311 and 321 and from the upper and lower surfaces of the refractory material 332. The gas emitted into the molten steel in the tundish violently floats upward in bubbles.

Heretofore, the molten steel in the neighborhood of the tundish opening 21 and the nozzle openings 311 and 321 tended to be deprived of heat and to solidify. However, such tendency is prevented by blowing the inert gas into the molten steel according to the present invention because a thin solidification shell which might have been formed in the neighborhood of an opening is readily destroyed by the violent upward movement of the gas bubbles and fresh molten steel at high temperature is added thereto from neighborhood to prevent further solidification.

The upward movement of the gas bubbles provides two effects, namely one is to prevent solidification of the molten steel in the tundish and the other is to cause internal inclusions to float unward to the surface of the molten steel.

To open the slidable nozzle assembly 3, the slider driving unit 36 is actuated to move the slider 33, for example, to the left in the drawings to the position shown in Figure 4.

Thus the passage for the molten steel in the tundish is unblocked to permit the molten steel to flow into the mold 4. At this time, supply of the inert gas to the pipe 35 connected to the refractory material of the slider 33 may or may not be continued, but supply of the inert gas to the pipes 35 connected to the refractory materials 312 and 322 respectively must be continued.

Since the violent upward movement of gas bubbles in the molten steel is thus continued, there is little possibility that internal inclusions pass with the molten steel into the mold.

A further feature of the present invention is that the period of time required for the slider 33 to move from the closing position (Figure 3) to the opening position (Figure 4) is predetermined to 1.5 seconds or shorter.

The slider driving unit to attain this purpose is preferably a hydraulic cylinder.

The period of time for the slider 33 to move from the closing position to the opening position is determined by the following formula: Ls Vc 1.5 seconds wherein Ls is the length (mm) of the stroke of the piston rod and Vc is the velocity (mm/sec) of the piston rod. Namely, according to our measurement, the opening success ratio (percentage of obtaining the full opening without any plugging) of the nozzle opening was 100% when Ls = 60 mm and Vc = 40 mm/sec and remained unchanged when Vc was increased. However, the ratio decreased as the value Vc was reduced.

This fact clearly shows that the ratio of Ls to Vc is preferably 1.5 (sec) or smaller.

On the other hand, our measurement of the degree of solidification of the molten steel in the tundish with time showed that the solidification proceeded very slowly for the first 1.5 seconds but after 1.5 seconds the solidification developed rapidly in a sharp upward curve.

Accordingly, in the continuous casting method using the slidable nozzle assembly the inert gas is continuously blown into the molten steel in the tundish through the permeable refractory bricks while the nozzle opening is closed and the slider is moved from the closing position to the opening position within a period of time of 1.5 seconds or shorter.

As will be clearly seen from the foregoing, the present invention provides various significant advantages such that internal inclusions present in the molten steel in the tundish are forced to float upward to the surface of the molten steel, the opening success ratio of the sliding nozzle assembly is considerably increased, the various materials which had to heretofore be filled in the nozzle opening before the beginning of the pouring of molten steel are made unnecessary, the timing of quick opening of the nozzle is predetermined, various associated operations such as replacement of damaged set bricks are made easy, and thus the molten steel pouring operation is made in general very effecient.

WHAT WE CLAIM IS: .1. A method of continuous casting steel comprising the steps of: receiving molten steel from a ladle into a tundish which has a depth of 400 mm or greater and maintaining the depth of the molten steel in the tundish to at least 400 mm; using a slidable nozzle assembly which includes an upper nozzle having permeable refractory material mounted on the inner wall of the opening thereof, an upper nozzle fixing plate supporting said upper nozzle at the base thereof and having permeable refractory material mounted on the inner wall of the opening thereof, a lower nozzle having the lower end thereof inserted into a mold, a slider inserted horizontally and slidable between the said upper nozzle fixing plate and said lower nozzle and having permeable refractory material embedded therein at a suitable position thereof, a slider driving unit for moving said slider in a predetermined minimum time related to the solidification time of the molten steel between the closing position and the opening position thereof; supplying an inert gas to said permeable refractory materials of said upper nozzle, fixing plate, and slider while the slider is closed and open so as to blow said gas into the molten steel in the tundish; and activating said slider driving unit to open said slider and pour the molten steel from the tundish into the mold.

2. A method according. to Claim 1, in which argon gas at the pressure of three atmospheres is supplied at the rate of 10 Nl/hr.

3. An apparatus for continuous casting of steel comprising a tundish having a depth of 400 mm or greater and a sliding nozzle assembly, said slidable nozzle assembly including: an upper nozzle having permeable refractory material mounted on the inner wall of the opening thereof; an upper nozzle fixing plate supporting said upper nozzle at the base thereof and having permeable refractory material mounted on the inner wall of the opening thereof; a lower nozzle having the lower end thereof inserted into the mold; a slider inserted horizontally and slidable between said upper nozzle fixing plate and said lower nozzle and having permeable refractory material embedded therein at a suitable position thereof; a slider driving unit for moving said slider in a predetermined minimum time related to the solidification time of the molten material between the closing position and the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

Since the violent upward movement of gas bubbles in the molten steel is thus continued, there is little possibility that internal inclusions pass with the molten steel into the mold.

A further feature of the present invention is that the period of time required for the slider 33 to move from the closing position (Figure 3) to the opening position (Figure 4) is predetermined to 1.5 seconds or shorter.

The slider driving unit to attain this purpose is preferably a hydraulic cylinder.

The period of time for the slider 33 to move from the closing position to the opening position is determined by the following formula: Ls Vc 1.5 seconds wherein Ls is the length (mm) of the stroke of the piston rod and Vc is the velocity (mm/sec) of the piston rod. Namely, according to our measurement, the opening success ratio (percentage of obtaining the full opening without any plugging) of the nozzle opening was 100% when Ls = 60 mm and Vc = 40 mm/sec and remained unchanged when Vc was increased. However, the ratio decreased as the value Vc was reduced.

This fact clearly shows that the ratio of Ls to Vc is preferably 1.5 (sec) or smaller.

On the other hand, our measurement of the degree of solidification of the molten steel in the tundish with time showed that the solidification proceeded very slowly for the first 1.5 seconds but after 1.5 seconds the solidification developed rapidly in a sharp upward curve.

Accordingly, in the continuous casting method using the slidable nozzle assembly the inert gas is continuously blown into the molten steel in the tundish through the permeable refractory bricks while the nozzle opening is closed and the slider is moved from the closing position to the opening position within a period of time of 1.5 seconds or shorter.

As will be clearly seen from the foregoing, the present invention provides various significant advantages such that internal inclusions present in the molten steel in the tundish are forced to float upward to the surface of the molten steel, the opening success ratio of the sliding nozzle assembly is considerably increased, the various materials which had to heretofore be filled in the nozzle opening before the beginning of the pouring of molten steel are made unnecessary, the timing of quick opening of the nozzle is predetermined, various associated operations such as replacement of damaged set bricks are made easy, and thus the molten steel pouring operation is made in general very effecient.

WHAT WE CLAIM IS: .1. A method of continuous casting steel comprising the steps of: receiving molten steel from a ladle into a tundish which has a depth of 400 mm or greater and maintaining the depth of the molten steel in the tundish to at least 400 mm; using a slidable nozzle assembly which includes an upper nozzle having permeable refractory material mounted on the inner wall of the opening thereof, an upper nozzle fixing plate supporting said upper nozzle at the base thereof and having permeable refractory material mounted on the inner wall of the opening thereof, a lower nozzle having the lower end thereof inserted into a mold, a slider inserted horizontally and slidable between the said upper nozzle fixing plate and said lower nozzle and having permeable refractory material embedded therein at a suitable position thereof, a slider driving unit for moving said slider in a predetermined minimum time related to the solidification time of the molten steel between the closing position and the opening position thereof; supplying an inert gas to said permeable refractory materials of said upper nozzle, fixing plate, and slider while the slider is closed and open so as to blow said gas into the molten steel in the tundish; and activating said slider driving unit to open said slider and pour the molten steel from the tundish into the mold.
2. A method according. to Claim 1, in which argon gas at the pressure of three atmospheres is supplied at the rate of 10 Nl/hr.
3. An apparatus for continuous casting of steel comprising a tundish having a depth of 400 mm or greater and a sliding nozzle assembly, said slidable nozzle assembly including: an upper nozzle having permeable refractory material mounted on the inner wall of the opening thereof; an upper nozzle fixing plate supporting said upper nozzle at the base thereof and having permeable refractory material mounted on the inner wall of the opening thereof; a lower nozzle having the lower end thereof inserted into the mold; a slider inserted horizontally and slidable between said upper nozzle fixing plate and said lower nozzle and having permeable refractory material embedded therein at a suitable position thereof; a slider driving unit for moving said slider in a predetermined minimum time related to the solidification time of the molten material between the closing position and the

opening position thereof; and means for supplying inert gas to said permeable refractory materials of said upper nozzle, fixing plate, and slider.
4. An apparatus according to Claim 3, in which said slider driving unit of said slidable nozzle assembly is a hydrauulic cylinder.
5. An apparatus according to Claim 4, in which the length Ls of the stroke (mm) and the velocity Vc (mm/sec) of the piston rod of said hydraulic cylinder are predetermined to satisfy the relationship expressed by: Ls Vc 1.5 (sec)
6. An apparatus according to Claim 5, in which the values of Ls and Vc are predetermined to 60 (mm) and 40 (mm/sec), respectively.
7. A method of continuous casting substantially as herein described with reference to, and as illustrated in, the accompanying drawings.
8. An apparatus for continuous casting substantially as herein described with reference to, and as illustrated in, Figures 1, 3 and 4 of the accompanying drawings.