AU605949B2

AU605949B2 - Method for cleaning molten metal and apparatus therefor

Info

Publication number: AU605949B2
Application number: AU27038/88A
Authority: AU
Inventors: Shuzo Fukuda; Toshio Ishii; Yutaka Okubo
Original assignee: NKK Corp; Nippon Kokan Ltd
Current assignee: JFE Engineering Corp
Priority date: 1987-12-25
Filing date: 1988-12-19
Publication date: 1991-01-24
Anticipated expiration: 2008-12-19
Also published as: EP0322763A3; DE3883190D1; BR8806870A; KR930005065B1; EP0322763A2; AU2703888A; DE3883190T2; CA1337744C; EP0322763B1; KR890010223A

Description

059494 AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION Form

(ORIGINAL)

FOR OFFICE USE Short Title: Int. Cl: Application Number: Lodged: Complete Specification-Lodged: Accepted: Lapsed: Published: Priority: Related Art: This document contains the amendments made under Section 49 and is correct for Sprinting.

TO BE COMPLETED BY APPLICANT Name of Applicant: Address of Applicant: NKK CORPORATION 1-2, 1-CHOME, MARUNOUCHI

CHIYODA-KU

TOKYO

JAPAN

GRIFFITH HACK CO., 601 St. Kilda Road, Melbourne, Victoria 3004, Australia.

Actual Inventor: Address for Service: Complete Specification for the invention entitled: METHOD FOR CLEANING MOLTEN METAL AND APPARATUS THEREFOR The following statement is a full description of this invention including the best method of performing it known to me:i y METHOD FOR CLEANING MOLTEN METAL AND APPARATUS THEREFOR Background of the Invention Field of the Invention 0 The present invention relates to a method for cleaning molten metal by removing inclusions suspended in molten metal and an apparatus therefor.

Description of the Prior Art There have been proposed various methods for decreasing or removing inclusions suspended in molten metal since those inclusions can be a cause of defects in quality of a product. Four methods as those methods are shown below: A first method, wherein inclusions in molten metal are trapped by gas bubbles produced by bubbling inert gas in molten metal at atmospheric pressure from the bottom of a vessel having molten metal therein.

The inclusions are removed from the molten metal after the inclusions have risen to the surface of the molten metal.

A second method, wherein inclusions in molten metal are removed by a filter made.from calcium oxide which is put in a flow of the molten metal.

A third method, wherein inclusions in molten S1A -1- -rl St t 0* 0 0 I 09 1 00* 0 a o o o 0 0 a «a o o no o 0 B 4 (1 0 0 a a a 00 0$ 0 0 0 044 0 0 0 oa a 0 0 0 aoc 6 o metal are removed by throwing a solid such as calcium oxide capable of ad ,orbing the inclusions into the molten metal.

A fourth method, wherein inclusions in molten metal are removed by having the inclusions risen to the surface of the molten metal or gone down by use of differences in densities of the inclusions.

However, in case of having an object of manufacturing quality material, a total amount of oxygen in molten metal needs to be limited to 15 ppm or less.

When molten metal is cleaned by use of said methods, there can occur the following problems: In the first method, a zone of the bubbling only spreads upwardly from a gas blowing-in inlet positioned at the bottom of a vessel. Moreover, there is a problem that it is difficult to bubble the molten metal from the whole vessel. When the bubbles produced by bubbling are large, the molten metal flows, bypassing the bubbles, during the bubbles' rising to the surface of the molten metal. In this case, micro-inclusions in the molten metal are hard to be trapped by the bubbles since the micro-inclusions also move together with the flow of the molten metal, bypassing the bubbles.

In the second method, when a filter capable of removing micro-inclusions is used, the filter often is choked and unable to be used soon after it has begun to be used.

-2- L In the third method, when the effectiveness of removing inclusions in molten metal by use of a solid such as calcium oxide lowers, there occurs a necessity for withdrawing the solid out of the molten metal. In this case, there is a problem that the withdrawal of the solid requires a troublesome work and, moreover, the efficiency of the withdrawal of the solid is low.

In the fourth method, due to small particles of the micro-inclusions, the rising or the going-down of the micro-inclusions takes a lot of time. This leads to a decrease of the efficiency of removing the micro-inclusions.

R t 1 Summary of the Invention It is an object of the present invention to #000 r provide a method for cleaning molten metal, wherein not only inclusions of an ordinary size, but also it t micro-inclusions can be removed from molten metal.

According to the present invention there is provided a method for cleaning molten metal comprising the steps of; 'Oft r;cT S(a) applying a pressure of 1 to 10 atm. to molten metal in a pressurized vessel; injecting bubbles of a gas soluble in the 001100 molten metal into the molten metal so ,that a portion of the bubbles entrap inclusions of large particle size suspended in the molten metal and rise to the surface of the molten metal and the remainder of the bubbles dissolve into the molten metal; 3

I

Y-i reducing the pressure applied to the molten metal so that fine gas bubbles form and entrap inclusions small particle size and rise to the surface of the molten metal; and removing the inclusions floating on the surface of the molten metal.

According to the present invention there is provided an apparatus for cleaning molten metal comprising; a a first vessel having an inlet for molten metal at the top end and an outlet for molten metal at the bottom end, wherein the molten metal is pressurized by its own static weight; a second vessel having an inlet for pressurized molten metal at the bottom end and an outlet for molten metal at the top end, wherein the pressure of the -pressurized molten metal reduces as the pressurized molten metal rises up the second vessel; a communicating tube connecting a bottom portion of the first vessel and a bottom portion of the second vessel so that pressurized molten metal can flow from the first vessel into the second vessel; and -4- Ii _Il:_C a bubbling device for bubbling a gas soluble in molten metal into the bottom of the first vessel and into the communicating tube.

The above objects and other objects and advantages of the present invention will now become apparent from a *0 9*4 o 9a 9.

*ar4

S

1.

4a 1/2 r the detailed description to follow, taken in connection with the appanded drawings.

o 0 oo o0 0 a 0 0 0 00 0 0 a o 0* 0 0 «0 Brief Description of the Drawings Fig.1 is a cross sectional view showing schematically an apparatrus for cleaning molten metal by means of a batch process of the present invention; Fig.2 is a cross sectional view illustrating another apparatus for cleaning molten metal by means of a batch process of the present invention; Fig.3 is a cross sectional view showing schematically an apparatus for continuously cleaning molten metal with the use of a U-shaped vessel of the present invention; Fig.4 is a graphic representation of a change in time of a total amount of oxygen in molten metal during processing the molten metal in example-1 and control-1; Fig.5 is a cross sectional view showing schematically an outline of an apparatus used for a method of Preferred Embodiment-2; Fig.6 is a graphic representation of the change of the flutter of the surface of the molten metal, which was produced by applying static magnetic field to the molten metal, relative to magnetic flux density of static magnetic field in the abscissa; Fig.7 is a cross sectional view illustrating an apparatus used for a method in Preferred Embodiment 3.

r' I 0r 0 0 0 0000 00 0 0 4 4 0, 1r 0a O #0 00 0 0s 0 0o 0 *0 0 0: Fig.8 is a graphic representation of a change in time of a total amount of oxygen in molten metal during processing the molten metal in Example-4 of the present invention; and Fig.9 is a cross sectional view illustrating an apparatus for continuously cleaning molten metal which is used in Description of the Preferred Embodiment Preferred Embodiment-1 The gas soluble in molten metal is bubbled in said molten metal under increased pressure. Inclusions of large particle size in the molten metal are trapped by the bubbles produced by a first bubbling, go upwardly to the surface of the molten -metal and are removed.

Since the molten metal under increased pressure is bubbled, a large amount of gas is uniformly dissolved in the molten metal by stirring the molten metal by means of bubbling. Thereafter, the gas dissolved in the molten metal is converted to fine gas bubbles under the pressure rapidly reduced. The fine gas bubbles are produced from the whole area of the molten metal.

Micro-inclusions are trapped by the fine gas bubbles, rise to the surface of the molten metal and are removed.

Nitrogen gas and hydrogen gas are preferred as the gas soluble in molten metal. Hydrogen gas is more desirable if the later removal of the gas having -6remained in the molten metal is taken into consideration. A pressure of from 1 to 10 atm is preferred to the atmospheric pressure for applying a pressure on the molten metal. When the molten metal is pressurized at a pressure of less than 1 atm, it is less effective to pressurize the molten metal. When the molten metal is pressurized at a pressure of more than 10 atm, the apparatus becomes too expensive.

The pressure on the molten metal is desired to be reduced in several stages from a pressure applied to *the molten metal because fine bubbles are produced at every stage of the pressure reduction. For example, the pressure on the molten metal is reduced in such a manner as: 10 atm-+ 7 atm 4 atm 1 atm. The method of the present invention as mentioned above can be carried out by means of a batch process wherein a pressure vessel is used or a continuous process wherein a U-shaped vessel is used.

Fig.1 is a cross sectional view showing schematically an apparatus for cleaning molten metal by means of a batch process. A method for cleaning molten metal by means of the batch process will now be explained with specific reference to Fig.1. Firstly, molten metal is poured into pressure vessel Thereafter, pressure vessel 10 is closed with cover 14 and molten metal 12 is pressurized by controlling pressure valve 16 mounted on a duct connected to cover -7- 0 00 0 9 0 0 04 44 00 4 9 000 0o 0 9 0 o o 0 0 U9 0 0 0U 4 *Y 4 14. Gas 20 soluble in the molten metal is blown in the molten metal under pressure from bubbling device 18 positioned at the bottom of vessel 10. After a bubbling has finished, a pressure applied to molten metal 12 is rapidly reduced by controlling pressure valve 16. Finally, the inclusions which have risen to the surface of the molten metal are removed.

Fig.2 is a cross sectional view illustrating another apparatus for cleaning molten metal by means of a batch process. With the use of this apparatus, a pressure applied to molten metal 12 can be rapidly reduced by transferring the molten metal being kept under pressure to vessel 24 opened for the air through communicating tube 22.

Fig.3 is a cross sectional view showing schematically a method for continuously cleaning molten metal with the use of a U-shaped vessel. Molten metal is continuously charged into a vessel through inlet 32 positioned at the top end of first vessel 30. Molten metal 12 goes down inside first vessel 30 and is gradually pressurized by its static pressure. Molten metal 12 which has reached the bottom of vessel 30 is sufficiently pressurized by its static pressure. There is output port 34 at the bottom of vessel 30. The molten metal flows to communicating tube 36 through outflow port 34. the molten metal is bubbled by bubbling device 44 positioned at the bottom of first vessel -8i,

V*

99 Q 99a 09J Vr V J I 4 and by bubbling device 46 positioned at the bottom of communicating tube 36 connected to the first vessel.

The molten metal, which has passed through communicating tube 36, enters second vessel 40 through inflow port 38 positioned at the bottom of second vessel40. There is output port 42 for discharging molten metal at the top end of second vessel 40. The molten metal rises toward outlet 42 for discharging molten metal inside second vessel 40. With the rise of the molten metal, a pressure on the molten metal is rapidly reduced, and gas dissolved in the molten metal appears in the molten metal as fine gas bubbles. The fine gas bubbles rise to the surface of the molten metal, trapping inclusions in the molten metal. The molten metal, which- has gone out of outlet 42 for discharging molten metal of second vessel 40, enters receptor 48. The inclusions floating on the surface of the molten metal inside receptor 48 are continuously removed.

Preferred Embodiment-2 A method of Preferred Embodiment-1 is highly Nositvs it' effective in removing inclusions in molten metal. 44 was estod that, in order to further decrease the adviscab inclusions in the molten metal, it is/g o d- to decrease d;nfIvrb#ce the iutter of the molten metal surface which is produced when the gas bubbles produced by bubbling and -9- 1

I

the innumertable bubbles produced by a pressure reduction rise to the surface of the molten metal and to take measures so that the inclusions which had risen to the surface of the molten metal could not mix again with the molten metal. Therefore, in a method of Preferred Embodiment-2, static magnetic field is applied to the surface of the molten metal when the inclusions trapped by the bubbles produced by bubbling and by Sa innumerable bubbles produced by the pressure reduction o "rise to the surface of the molten metal.

o° Fig.5 is a cross sectional view showing schematically an outline of an apparatus used for a method of Preferred Embodiment-2. When static magnetic S.e field is applied to molten metal in a direction at right angles to a flow of the molten metal, a braking force against the flow of the molten metal takes place.

i bsfl'urbqnsce A flutter- of the molten metal surface just corresponds f i.

to an up-and-down flow of the molten metal bath. A o force suppressing the up-and-down flow of the molten metal is produced by applying the static magnetic field Sto the surface of the molten metal with the use of o~ltirbnce electromagiet 50. Thereby, the width of the flutter is decreased, and this can prevent the occurrence of waves on .the surface of the molten metal. Force F d0s4rba1ce suppressing the fltter- of the molten metal-surface is represented by the following formula: F x B2 x V S: electric conductivity of molten metal bath B: magnetic induction V: velocity of up-and-down movement of molten metal surface The magnetic induction of the static magnetic field is preferred to be of from 1000 to 5000 gausses. When said magnetic induction is less than 1000 gausses, a force suppressing the flutter of the molten metal surface is small. When the magnetic induction exceeds 5000 gausses, there is no change in the digsvrbance.

effectiveness of the force suppressing the fjutt.e r of the molten metal surface.

Preferred Embodiment-3 The present inventor conducted various tests for the purpose of increasing a processing capability of the apparatus and understood that, when time for leaving the molten metal as it was shortened, micro-inclusions in the molten metal were not removed as expected. In consequence, it was thought to further increase the efficiency in the removal of ordinary inclusions by the bubbling carried out before the pressure reduction process. As a result of the tests conducted later, it -11was understtod that the efficiency in the removal of inclusions by bubbling was increased by applying a low frequency electromagnetic force to molten metal during bubbling and actively stirring the molten metal. It is thought that the reason why the efficiency in the removal of the inclusions is increased is that the inclusions in the molten metal increasingly strike against one another by stirring and the inclusions having grown comparatively larger rise to the surface of the molten metal during bubbling. In addition, since the amount of bubbling gas dissolved in the molten metal was expected to be increased by stirring, the amount of fine gas bubbles was increased by the pressure reduction and the efficiency in the removal of the micro-inclusions was expected to be increased.

I

On the other hand, the temperature of the molten metal was decreased by the bubbling. In this connection, the amount of the bubbling gas dissolved in the molten metal was decreased. Fluidity of the molten metal also was decreased. In consequence, the effectiveness of the stirring by the bubbling decreased.

In order to overcome those difficulties, the molten metal could be actively heated by Joul's heat produced by induced current generated by applying an electromagnetic force to the molten metal during bubbling.

The reason why it was only during bubbling when the -12electromagnetic force was applied to the molten metal was consideration so that the inclusions having once risen to the surface of the molten metal after the pressure reduction could not be mixed again with the molten metal by an electromagnetic stirring.

Example-i The cleaning of molten metal in Preferred Embodiment-1 was carried out by means of a batch process with the use of a pressure vessel of 2 m in inside diameter and 3 m in height as shown in Fig.l.

Firstly, 50 tons of molten steel were poured into pressure vessel 10. Subsequently, pressure vessel was closed with cover 14 and sealed. Atmospheric gas inside pressure vessel 10 was substituted for argon gas. Thereafter, a mixed gas consisting of 70% Ar gas ii and 12 gas was bubbled in the molten steel for minutes at a rate of 200 S /min from bubbling device 18 positioned at the bottom of pressure vessel 10. A atm by means of pressure valve 16. After the bubbling had finished, the gas pressure inside pressure vessel was reduced to the atmospheric pressure by means of 4 pressure valve 16. The molten steel was left as it was for 20 minutes until the gas bubbles produced by the pressure reduction rose to the surface of the molten steel. Finally, the molten steel was transferred to -13b 1 9* Do 9* *r o 9i I I the next process.

The cleaning of molten steel was carried out as control-1 by use of a prior art gas bubbling method.

That is, argon gas was blown in 50 tons of the molten steel under the atmospheric pressure at a rate of 400 S /minlor 40 minutes.

Fig.4 is a graphic representation of a change in time of a total amount of oxygen in the molten steel during processing the molten steel in Example-1 and Control-1. The change of the total amount of oxygen in the molten steel in Example-1 is shown with a solid line and that in Control-1 with a dashed line.

The total amount of oxygen before processing the molten steel was 80 ppm, but the amount of oxygen was decreased to 15 ppm in Example-1 while the amount of oxygen was decreased only to 30 ppm in Control-1. It was understood that the case of Example-1 of the present invention was suprior to the case of Control-1 in the effectiveness of cleaning the molten steel. A total amount of bubbling gas was 16000S in Control-1. A total amount of the bubbling gas was 4000£ 2800& of Ar gas and 1200 1 of H 2 gas in Example-1. In .Example-1 of the present invention, the amount of gas used for the bubbling could be decreased and this could lead to a decrease of running cost.

-14- Example-2 A method of the present invention in Preferred Embodiment-1 was carried out by means of a continuous process with the use of a U-shaped vessel as shown in Fig.3. The dimensions of each portion of the vessel were as follows: Height of a first vessel: 4 m Inside diameter of the first vessel: 1 m S° Length of a communicating tube: 2 m Inside diameter of the communicating oaaa tube: 30 cm Bubbling device zones of 44 and 46 2 m Diameter of a second vessel: 10 cm Molten steel was continuously charged into the vessel from inlet 32 positioned at the top end of first vessel 30 at a rate of 250 t/hr. A mixed gas consisting of 60% Ar gas and 40% H 2 gas was bubbled in the molten steel from bubbling devices 44 and I 46 at a rate of 2009 /min. In consequence, the total |amount of oxygen, which was 80 ppm before the molten steel was processed, was decreased to 12 ppm in the molten steel at the bottom of receptor 48. It was understunderstood that the effectiveness in oxidation of the molten steel became higher.

Examp e-3 The cleaning of molten metal in Preferred Embodiment-2 was carried out by means of a batch process with the use of a pressure vessel of 2 m in inside diameter and 3 m in height as shown in Molten metal was poured into pressure vessel 10 so that the level of the molten steel could rise to 2 m in height in pressure vessel 10. The atmosphere inside pressure vessel 10 was substituted for argon gas.

Thereafter, a mixed gas consisting of 70% Ar gas and H2 gas was blown in the molten steel from bubbling ^o v device 18 positioned at the bottom of pressure vessel S at a rate of 300S /min to bubble the molten steel.

o o A gas presure inside pressure vessel 10 was increased to atm during bubbling. Subsequently, static magnetic field was applied to the surface of the molten steel bath by use of electromagnet 50. Thereby, a force suppressing a flutter of the molten steel surface was produced. Thereafter, the bubbling was stopped. The gas pressure inside pressure vessel 10 was rapidly reduced by means of pressure valve 16, and.fine gas bubbles were produced from the whole area of the molten steel. In Example-3, static magnetic field was applied to the surface of the molten steel even when the gas pressure was reduced. Fig.6 is a graphic representation of the change of the flutter of the molten steel surface which was produced by applying staticmagnetic field -16to the molten steel, relative to magnetic flux density of static magnetic field in the abscissa. In this schematic representation when the magnetic flux was over 1000 gausses, there appeared an effect of suppressing a flutter on the surface of the molten.steel and, when the magnetic flux exceeded 5000 gausses, there was no change in the effectiveness of the force suppressing the flutter of the molten steel.

too Example-4 a t I The cleaning of molten metal in Preferred get# Embodiment was carried out by means of a batch process o 0 with the use of a pressure, vessel of 2 m in inside diameter and 3 m in height as shown in Fig.7.

Firstly, 50 tons of molten steel were poured into pressure vessel 10. Subsequently, pressure vessel was closed with cover 14 and sealed. -Atmospheric gas inside pressure vessel 10 was substituted for argon gas.

-I Thereafter, mixed gas consisting of 70% Ar gas and

H

2 gas was blown in the molten steel from bubbling device 18 positioned at the bottom of pressure vessel at a rate of 200 /min and the molten steel was bubbled for 20 minutes. A gas pressure inside pressure vessel 10 was adjusted to 3 atm by means of pressure valve 16. An electromagnetic force was applied to the molten steel with the use of electromagnetic coils 52 arranged around pressure vessel 10 during bubbling,and -17the molten steel was subjected to electromagnetic stirring. Subsequently, the gas bubbling and the electromagnetic stirring were stopped, and the gas pressure inside pressure vessel 10 was reduced to the atmospheric pressure by means of pressure valve 16.

The molten steel was left as it was until the gas bubbles produced by the pressure reduction rose to the surface of the molten steel.

Fig.8 is a graphic representation of a change in time of a total amount of oxygen in the molten steel during processing the molten steel in Example-4. In Example-4, the total amount of oxygen in the molten I t steel was already fairly decreased by an electromagnetic stirring during bubbling a mixed gas. It is thought that inclusions in the molten steel increasingly stroke against one another, that the inclusions having had grown comparatively l.arger rose to the surface of the molten steel, and that the total amount of oxygen in the molten steel was decreased. Moreover, the total amount of oxygen after the completion of all the processes also was decreased to 10 ppm, which were less than in Example-1. The decrease of the total amount of oxygen can be explained by the following two reasons: Firstly, comparatively small inclusions in the molten steel grew larger by striking against one another being stirred during bubbling, and were easily trapped by gas bubbles; and, subsequently, the amount of gas -18a t a, ta f I oor o c oe a, a a a a *aa a, e a D a aa a a at a *1 a&* a a a a, ao a a ia a: dissolved in the molten steel became large and this increased the amount of fine gas bubbles produced during a pressure reduction. Accordingly, it is necessary to take a little bit more time to leave the molten steel as it is after the pressure reduction so as to make the total amount of oxygen in the molten steel processed in Example-i equal to the total amount of oxygen in the molten steel processed in Example-4.

Preferred Embodiment of the apparatus for cleaning molten metal of the present invention will now be explained with specific reference to Fig.9. The apparatus for cleaning molten metal was composed of first vessel 30, communicating tube 36, second vessel and vacuum storage vessel 56. First vessel was of 1 m in inside diameter and of 5 m in height. An opening at the top end of said first vessel was inlet 32 for charging molten metal. Outlet 34 for discharging molten metal was arranged at the bottom of first vessel 30. The molten metal flows through outlet 34 to communicating tube 36. Communicating tube 36 was of 50 cm in inside diameter and of 6 m in length. First bubbling device 44, was positioned at the bottom of first vessel 30 and second bubbling device 46 at the bottom of communicating tube 36 connected to first vessel 30 so that the molten metal could be -19- 4 ~-_111I1 II__ oo 00 f0 a a 0 0 0 o 000 00 091 9 9 0 00 0 00 0 00 ~O 00 0 0S 0 S1 0 O 0o 0 00 00 0 0000*0 o 0 bubbled from said bubbling devices 44 and 46. Gas storage chamber 54 was arranged at the position located a little bit beyond bubbling device 46 positioned at the bottom of communicating tube 36. Measures were taken by discharging a part of the bubbled gas to storage chamber 54 so that the gas bubbles rising to the surface of the molten metal inside second vessel which would be described later could not grow too large. The gas inside gas storage chamber 54 was discharged by means of pressure valve 55. The molten metal, which had passed through communicating tube 36, entered second vessel 40 through inflow port 38 positioned at the bottom of second vessel 40. Second vessel 40 was of 30 cm in inside diameter and of 5 m in height. The inside diameter of second vessel 40 was made small so that the pressure on the molten metal could be rapidly reduced by having the molten metal more rapidly flowed inside second vessel 40. Outlet 42 for discharging molten metal was positioned at the top end of second vessel 40. Vacuum storage vessel 56 of 2 m in inside diameter connected to outlet 42 for discharging molten metal was arranged, and the molten metal stored in vacuum storage vessel 56 was degassed.

Vacuum storage vessel 56 was arranged for the purpose of removing the gas bubbles produced by the bubbling and the pressure reduction, removing the inclusions rising to the surface of the molten metal, being trapped 7 i I' ti 4 41 So 04 44 4d 4 4 44 by the gas bubbles and discharging the gas dissolved in the molten metal even under atmospheric pressure.

Gas in vacuum storage vessel 56 was exhausted by means of vacuum pump 57. Tube 58 of 30 cm in inside diameter for bringing out the molten metal having been cleaned to the next process was connected to the bottom of vacuum storage vessel 56. In said apparatus,molten metal is continuoitsly charged into a vessel through inlet 32 and the molten metal taken out through outlet 42 to vacuum storage vessel 56 can be the molten metal very well cleaned by bubbling the gas soluble in the molten metal from bubbling devices 44 and 46. That is, molten metal 12 discharged through inlet 32 is gradually pressurized by its static pressure, going down inside first vessel 30. A large amount of the gas bubbled from bubbling devices-44 and 46 dissolves in the molten metal Simultaneously, inclusions of ordinary size are trapped by bubbling gas and flow through communicating tube 36. A part of the bubbling gas enters gas storage chamber 54 and is taken out of gas storage chamber 54 utwardly by means of pressure valve 55. The molten metal having passed through communicating tube 36 enters second vessel 40 through inflow port 38 positioned at the bottom of second vessel 40. The pressure on the molten metal is rapidly reduced when the molten metal goes upwardly inside second vessel 40. Then, the gas dissolved in the molten metal appears as fine gas 4 4e 4 i 3 1 i ~1 -21-

L

id, r 1 I r 1 I

C

I

A

4?ii'ie a at' St St S Stn 44.n tt0 O a 0,44+ o St ttttdt4 Ott o o *40* ,tt a St 4 0 St *0 atB *L g, 4 a 4* a *0 St St S C 44 41 i St 4 4 44 4-i bubbles. Those fine gas bubbles rise to the surface of the molten steel, trapping inclusions in the molten metal. The molten metal enters vacuum storage chamber 56 through outlet 42. In vacuum storage vessel 56,the inclusions are trapped by the gas bubbles and the fine gas bubbles produced by the bubbling and the pressure reduciton respectively and rise to the surface of the molten metal. To remove the gas soluble in the molten metal, the molten metal is degassed inside vacuum storage vessel 56. The molten metal cleaned by removing the soluble gas is taken out through passage 42.

The present inventor conducted a test of processing molten steel containing a total amount of ppm of oxygen by use of the present apparatus. The molten metal was continuously charged into a vessel through .inlet 32 at a rate of 250 t/hr. The molten metal was bubbled by a mixed gas consisting of 60% Ar gas and 40% H 2 gas blown in from bubbling devices 44 and 46 at a rate of 200 S /min for 20 minutes. The molten metal containing a toal amount of 12 ppm of oxygen was taken .out in vacuum storage device at a rate of 250 t/hr through passage 42.

-22-

Claims

1. A method for cleaning molten metal comprising the steps of; applying a pressure of 1 to 10 atm. to molten metal in a pressurized vessel; injecting bubbles of a gas soluble in the molten metal into the molten metal so that a portion of the bubbles entrap inclusions of large particle size suspended in the molten metal and rise to *I :the surface of the molten metal and the remainder of the bubbles dissolve into the molten metal; reducing the pressure applied to the molten metal so that fine gas bubbles form and entrap inclusions small particle size and rise to the surface of the molten metal; and t ,4 removing the inclusions floating on the Ssurface of the molten metal.

2. The method of claim 1 wherein the gas soluble in molten metal is nitrogen gas.

33. The method of claim 1 wherein the aas soluble in molten metal is hydrogen gas. 4. The method of any one of claims 1 to 3, wherein the step of reducing the pressure is carried out in a plurality of divided stages. 23 4 I The method of any one of the preceding claims, further comprising the step of applying a static magnetic field to the surface of the molten metal to suppress diPsturbance of the molten metal when the inclusions of large particle size trapped by the gas bubbles produced by the bubbling rise to the surface. 6. The method of any one of the preceding claims, further comprising the step of applying a static magnetic field to the surface of the molten metal to suppress disturbance of the molten metal when the inclusions of small particle size trapped by fine gas bubbles rise to the surface. 7. The method of claim 5, wherein the static magnetic field has a magnetic flux of 1000 to 5000 8. The method of claim 6, wherein the static magnetic field has a magnetic flux of 1000 to 5000 gausses. S' 9. The method of any one of the preceding claims, further comprising the step of stirring the iolten metal by a low frequency electromagnetic force while injecting gas bubbles into the molten metal. The method of any one of claims 1 to 8, further ,4 comprising the step of stirring the molten metal by a high frequency electromagnetic force while injecting gas bubbles into the metal. ii. The method of any one of the preceding claims, further comprising the step of degassing the molten metal in vacuum while reducing the pressure applied to the molten metal. 24 I 1 12. The method of any one of the preceding claims, wherein the pressure applied to the molten metal is between 3 and 10 atm. 13. An apparatus for cleaning molten r tetal comprising; a first vessel having an inlet for molten metal at the top end and an outlet for molten metal at the bottom end, wherein the molten metal is pressurized by its own static weight; a second vessel having an inlet for pressurized molten metal at the bottom end and an outlet for molten metal at the top end, wherein the pressure of the 1 pressurized molten metal reduces as the pressurized molten metal rises up the second vessel; L a communicating tube connecting a bottom portion of the first vessel and a bottom portion of the second vessel so that pressurized molten metal can flow from the first vessel into the second vessel; 4 and a bubbling device for bubbling a gas soluble in molten metal into the bottom of the first vessel and into the communicating tube. 14. The apparatus of claim 13, further comprising a vacuum storage chamber adjacent the outlet port of the second vessel. z? I Kii 7- O'c The apparatus f claims 13 or 14, further comprising a gas storage vessel for supplying at least a part of the gas to the bubbling device. 16. The apparatus of any one of claims 13 to further comprising electromagnetic coils for stirring the molten metal in the first vessel. 17. A method of cleaning molten metal substantially as hereinbefore described with reference to the accompanying drawings. 18. An apparatus for cleaning molten metal substantially as hereinbefore described with reference O. to the accompanying drawings. I a i Dated this 23rd day of October, 1990 o 2 NKK CORPORATION By its Patent Attorneys: GRIFFITH HACK CO. Fellows Institute of Patent |1 Attorneys of Australia. 26 Iz v\