EP0328851A1

EP0328851A1 - An apparatus for decarbonizing steels directly in the ladle

Info

Publication number: EP0328851A1
Application number: EP88830057A
Authority: EP
Inventors: Giorgio Enrico Falck
Original assignee: ACCIAIERIE E FERRIERE LOMBARDE FALCK SpA
Current assignee: ACCIAIERIE E FERRIERE LOMBARDE FALCK SpA
Priority date: 1988-02-16
Filing date: 1988-02-16
Publication date: 1989-08-23

Abstract

An apparatus (1) for processing and hard vacuum decarbonizing special steels in the ladle comprises a vessel (2) having a lid (6) secured thereon in a tight sealed manner, a ladle (3) containing a pool (4) of molten steel to be treated and being introduced into the vessel (2) under a vacuum condition, and a retractable supersonic oxygen injector (13) fitting in a tight sealed manner through the lid (6) of the vessel (2) to blow oxygen onto the pool (4) of steel.

Description

This invention relates to an apparatus for decarbonizing steels directly in the ladle. More particularly, the invention is concerned with an apparatus for processing and hard vacuum decarbonizing steels, specifically high alloyed and stainless steels, in the ladle, of a type which comprises a vessel adapted to receive a ladle containing a pool of molten steel having its free surface to a set level, and a lid secured on said vessel in a removable tight manner.
As is known, in the production of steels, and especially so-called special steels, it is a widespread practice to provide a vacuum or degassing step for the molten steel processed in conventional furnaces and tapped into a ladle, in order to yield an end product which is free of gaseous inclusions as far as possible and to prevent formation of objectionable non-metallic solid inclusions.
In particular, the vacuum degassing step in the ladle has shown to be quite effective to lower the hydrogen content of steels, and accordingly, remove the risk of flaking in a rapid and economical way.
To carry out such a vacuum treatment step, apparatus are usually employed which consist basically of a vessel provided with a tight sealing lid, into which a ladle containing a steel pool to be treated is placed; thereafter, specially provided pumps are operated to form a vacuum within the vessel on the order of 20 kPa or lower. In this condition, the molten steel will boil and release its dissolved gases, which are then taken away by said pumps. In order to promote agitation of the steel pool, and thus ensure continued stirring within the ladle, conventional apparatus have been equipped with devices operative to introduce a noble gas, usually argon, into the pool through a porous plug provided at the ladle bottom.
Such conventional apparatus for vacuum treating in the ladle are often further equipped with devices for additivating the molten steel with components as required to produce a desired alloy, and with carbon electrodes which are mounted slidingly through the vessel lid to heat the steel pool by means of a three-phase arc and maintain the pool at a set temperature.
Also known are apparatus, quite similar to those just discussed, wherein a suitably equipped ladle has a lid incorporating a means of creating a selected vacuum directly inside the ladle itself.
Where hard decarbonization of steel becomes necessary, such as in the production of high alloyed and stainless steels having as low a carbon content as 0.02%, it has been common practice to use vacuum processing apparatus like the ones noted above but additionally equipped with oxygen injectors dipped into the steel pool to blow oxygen into it. Such apparatus can lower the carbon content to extremely low levels; in fact, the reduced pressure which prevails in the medium where the steel to be treated is placed enhances the decarbonizing effect of oxygen, while both the oxygen and the carbon oxide resulting from blowing oxygen into the pool are removed from the steel pool at the same time.
While representing a significant iprovement over other prior approaches which provide for the hard decarbonizing step to be carried out at separate, and hence, more expensive stations, the above-noted apparatus still have the well-recognized drawback that the injectors are liable to undergo rapid and serious deterioration due to their being immersed in high temperature molten steel. Accordingly, they must be replaced at frequent intervals, which involves unacceptable discontinuance of the production process.
The problem which underlies this invention is to provide an apparatus as indicated for processing and hard vacuum decarbonizing steels in the ladle, which has such constructional and operational features as to overcome the drawbacks affecting the state of the art.
This problem is solved according to the invention by an apparatus of the type specified hereinabove being characterized in that it comprises a supersonic oxygen injector mounted through the lid and extending toward the ladle in spaced apart relationship with said free surface to blow oxygen through the steel pool.
The features and advantages of an apparatus according to the invention may be more clearly understood from the following detailed description of a preferred embodiment thereof, to be taken by way of example and not of limitation in conjunction with the accompanying drawings.
In the drawings:

Figure 1 is a side elevation view showing, in section and diagramatic form, an apparatus according to this invention;
Figure 2 is an enlarged scale detail view of the apparatus shown in Figure 1;
Figure 3 is a side elevation view showing, in section and diagramatic form, the apparatus of Figure 1 in an inoperative condition thereof;
Figure 4 is an enlarged scale view of a detail of Figure 3; and
Figure 5 is a side elevation view showing, in section and diagramatic form, a second embodiment of the apparatus according to the invention.

With reference to Figures 1 to 4, the numeral 1 designates generally an apparatus for processing and hard vacuum decarbonizing special steels, specifically high alloyed and stainless steels, in the ladle.
The apparatus 1 comprises a vessel 2 having a substantially cylindrical shape and an open top, inside which vessel a ladle 3 is supported which contains a pool 4 of a molten steel to be treated having a free surface 4a at a set level within the ladle 3.
Indicated at 5 is a porous plug mounted at the bottom 3a of the ladle 3, a means, not shown, being provided to blow a noble gas, preferably argon, through the molten steel pool 4 via the porous plug 5.
The apparatus 1 further comprises a lid 6 which is secured on the vessel 2 in a removable tight manner; in particular, the lid 6 is movable vertically toward and away from said vessel 2 by the action of a drive known per se and omitted therefore from the drawing.
The lid 6 has a suction inlet 7 open inwardly thereof and adapted for connection to conventional suction pumps to create within the vessel 2 as sealed by means of the lid 6 a set vacuum preferably on the order of 20 kPa or less.
The numeral 8 designates three identical electrodes which are connected to an electric circuit and consist of graphite bars laid vertically and inserted slidingly through the lid 6 at a central location thereon overlying the ladle 3. Sliding seals are provided which fit around each electrode 8 at the lid 6.
The electrodes 8 are mounted inside respective vacuum tight telescoping tubes 9 carried in a position overlying the lid 6; further, the electrodes 8 can be driven vertically toward and away from the steel pool 4 by a motor means known per se, and accordingly omitted from the drawing, to heat the pool by means of a three-phase arc, thereby maintaining the steel temperature at a set level.
A shield 10 is suspended in the lid 6 above the ladle 3 and is formed with three ports 11 to admit the three electrodes 8 therethrough, and with a port 12 which is offset from the ports 11.
In accordance with this invention, the apparatus 1 includes a supersonic oxygen injector 13 mounted through the lid 6 in sealed relationship with it and by the side of the electrodes 8, which extends through the port 12 toward the ladle 3 to blow oxygen, at a pressure of up to about 588.393 kPa, onto the pool 4 of molten steel.
The injector 13 has a body 14 of tubular shape and is provided with a nozzle 15 at a bottom end thereof.
Interspaces 30 are formed in the body 14 through which a cooling fluid is circulated.
The injector 13 is set at an inclination angle of approximately 45° from the vertical and is mounted slidingly through the lid 6, a vacuum seal means 16 being arranged to fit around the injector 13, and a motive means 17 being provided to move the injector 13 from an inoperative position where it is fully retracted into the lid 6 (cf. Figures 3 and 4) and an operative position where it projects toward the ladle 3 (cf. Figures 1 and 2) with the nozzle 15 at a set distance from the free surface 4a of the steel pool 4.
More specifically, the body 14 of the injector 13 fits slidingly within a cylindrical tube 18 which is secured on the lid 6 with a bottom end thereof and in communication with the interior of the vessel 2 through an orifice 19 formed in the lid itself.
Indicated at 20 is a sleeve fitting slidingly over the tube 18 and associated with the body 14 at a top end 20a thereof which is sealed around a top end 14a of the injector body 14.
An annular seal 21 fits slidingly over the tube 18 and is associated with a bottom end 20b of the sleeve 20 by means of a ring-like flexible joint 21b fast with the seal 21 and mounted to the end 20b which is adapted to compensate for any slight misalignment between the tube 18 and the sleeve 20.
The tube 18, sleeve 20, and seal 21 with the joint 21a constitute, therefore, said vacuum seal means 16.
The sleeve 20 is mounted on a carriage 22 running along guides 23 provided on the lid 6 at an inclination angle of 45° and lying parallel to the tube 18.
Said motive means 17 comprises a drive chain 24 associated with the carriage 22 and trained around two sprocket wheels 25 and 26, one (25) of which is driving wheel.
The numeral 27 designates an oxygen supply conduit to the injector 13, which is connected to the top end 14a of the injector body 14, and at 28 and 29 there are indicated delivery and return conduits for the cooling fluid to/from the injector which are connected to the interspaces 30 at the top end 14a.
The apparatus 1 further comprises a gate 31 formed in the lid 6 at a location close to the suction port 7 for a TV camera adapted to monitor the level of the free surface 4a of the molten steel pool 4 and the distance of the nozzle 15 of the injector 13 therefrom. To this aim, the shield 10 would be additionally apertured as at 33 in alignment to the gate 31.
Advantageously, on the lid 6 there is mounted pivotally a screen 34 intended for shutting off, by the action of a motive means 35 known per se, the orifice 19 and protecting the injector 13 in the inoperative position thereof fully retracted into the lid 6.
The apparatus 1 also includes devices, not shown, which are mounted on the lid 6 for the purpose of adding components to the steel contained in the ladle 3.
The operation of the apparatus 1 will be described herein below with particular emphasis on the operation of the supersonic oxygen injector 13.
In order to perform a hard decarbonization of the molten steel pool 4 contained in the ladle 3, with the ladle positioned under a vacuum condition inside the vessel 2 as closed by the lid 6, the electrodes 8 are raised above the shield 10. The injector 13 is then brought to the extended operative position with the nozzle 15 at a set distance above the free surface 4a of the pool 4, which is maintained under continued agitation by blowing in argon through the porous plug 5 in the bottom 3a of the ladle 3.
Simultaneously therewith, the injector 13 is operated to blow a jet of oxygen under pressure onto the pool 4. The jet of oxygen is advantageously directed at an angle of about 45° toward a central area of the steel pool surface.
The pressure of the oxygen jet, together with its inclination angle and the distance of the nozzle 15 off the free surface 4a, is determined to force the jet deeply through the pool 4 so as to achieve optimum combustion of the carbon contained therein. In particular, the distance of the nozzle 15 from the free surface 4a is controlled by moving the carriage 22 at a slow rate along the guides 23 by means of the drive 17.
In operation, the injector would be constantly cooled by the coolant fluid circulated through the interspaces 30.
On completion of the decarbonizing step, the injector 13 is raised by means of the drive 17 to its inoperative position retracted into the lid 6, with the nozzle 15 positioned at the orifice 19 (cf. Figure 4).
Advantageously, the injector 13 would be kept in that inoperative position both with the lid 6 taken off the vessel 2 (cf. Figure 3) and, with the vessel closed by the lid 6, where the steel to be treated requires no hard decarbonizing, and where the electrodes 8 are to be lowered to heat the pool 4.
The inventive apparatus for processing and hard vacuum decarbonizing steels in the ladle has shown to be particularly useful to produce a wide range of special steels having a constant high qualitative level, while considerably shortening the times involved in their processing.
The provision of a supersonic oxygen injector, which is retractable and subjected to constant cooling while in operation, affords significant savings in the operating costs for this apparatus. In fact, the rate of deterioration of the supersonic injector outside the molten steel pool being treated is quite low, and the injector can be reused for a number of times.
Furthermore, the ability to retract and protect the injector into the inoperative position whenever its use is not required affords flexible conditions of operation for the inventive apparatus, which can thus be used contingent on requirements to either produce steels requiring hard decarbonizing or steels which need no such decarbonization.
This apparatus, moreover, lends itself to full automation, especially as relates to the position of the injector nozzle with respect to the free surface of the molten steel pool to be treated, whose level is often liable to change.
Where the vacuum treatment and hard decarbonization of a steel pool are carried out by applying a lid to a ladle and creating a selected vacuum inside the ladle itself, a second embodiment of this invention is advantageously provided as indicated at 101 in Figure 5.
In the apparatus 101, those components which are structurally or functionally equivalent to the ones previously described in connection with the apparatus 1 of the first embodiment are denoted by the same reference symbols and no further discussed.
The apparatus 101 comprises a ladle 102 containing a pool 4 of molten steel having a free surface 4a at a set level.
Indicated at 103 is a lid secured removably and tightly on the ladle 102 in a manner known per se, a suction means being provided which is connected to a port 104 open inwardly of the lid 103 and can form within the ladle a set vacuum.
The apparatus 101 further comprises a supersonic oxygen injector 13 mounted in a sealed fashion through the lid 103 and extending into the ladle 102 in spaced apart relationship with said free surface 4a to blow oxygen through the molten steel pool 4.
In quite a similar manner to that previously described, the injector 13 of the apparatus 101 fits slidingly through the lid 103, seals 16 being provided which fit around the injector 13 and a motive means 17 for moving said injector 13 from an inoperative position where it is fully retracted into the lid 103 to an operative position where it is extended toward the pool 4 of molten steel.

Claims

1. An apparatus for processing and hard vacuum decarbonizing steels, specifically high alloyed and stainless steels, in the ladle, of a type which comprises a vessel (2) adapted to receive a ladle (3) containing a pool (4) of steel having a free surface (4a) to a set level, and a lid (6) secured on said vessel (2) in a removable tight manner, characterized in that it comprises a supersonic oxygen injector (13) mounted through the lid (6) and extending toward the ladle (3) in spaced apart relationship with said free surface (4a) to blow oxygen through the steel pool (4).

2. An apparatus according to Claim 1, characterized in that said injector (13) fits slidingly through said lid (6), seal means (16) being provided around the injector (13) and a motive means (17) being arranged to move the injector from an inoperative position of full retraction into the lid (6) to an operative position extended toward the ladle (3).

3. An apparatus according to Claim 1, characterized in that said injector (13) comprises a tubular injector body (14) formed with interspaces (30) wherethrough a cooling fluid is circulated.

4. An apparatus according to Claim 2, characterized in that said injector (13) is set at an angle of approximately 45° from the vertical.

5. An apparatus according to Claim 2, characterized in that said injector body (14) fits slidingly in a cylindrical tube secured on said lid (6) and communicating with said vessel (2), a sleeve (20) associated with said injector body (14) fitting slidingly over the cylindrical tube (18), an annular seal (21) being mounted between said sleeve (20) and said tube (18), the sleeve (20), cylindrical tube (18), and seal (21) constituting said seal means (16).

6. An apparatus according to Claim 5, characterized in that said sleeve (20) is mounted on a carriage (22) arranged to run along guides (23) formed on said lid (6) by the action of said motive means (17).

7. An apparatus according to Claim 6, characterized in that said motive means (17) include a drive chain (24) associated with said carriage (22) and trained around two sprocket wheels (25,26) one (25) of which is a driving wheel.

8. An apparatus according to Claim 3, characterized in that it comprises a screen (34) mounted pivotally on said lid (6) to protect the injector (13) in said inoperative position thereof of full retraction into the lid (6).

9. An apparatus for processing and hard vacuum decarbonizing steels, specifically high alloyed and stainless steels, in the ladle, of a type which comprises a ladle (102) containing a pool (4) of steel having a free surface (4a) to a set level, and a lid (103) secured on the ladle (102) in a removable tight manner, characterized in that it comprises a supersonic oxygen injector (13) mounted in a tight sealed manner through the lid (103) and extending into the ladle (102) in spaced apart relationship with said free surface (4a) to blow oxygen onto the pool (4) of steel.