WO2002097139A1

WO2002097139A1 - Process and apparatus for the continuous steel degassing and refering

Info

Publication number: WO2002097139A1
Application number: PCT/IT2002/000353
Authority: WO
Inventors: Domenico Capodilupo
Original assignee: Centro Sviluppo Materiali SpA
Current assignee: Rina Consulting Centro Sviluppo Materiali SpA
Priority date: 2001-05-31
Filing date: 2002-05-31
Publication date: 2002-12-05
Anticipated expiration: 2003-11-30
Also published as: ITRM20010299A0; ITRM20010299A1

Abstract

Process and apparatus for the continuous degassing and/or processing of liquid-phase steel, comprising the steps of: collecting said liquid phase in an open air collecting vessel (8), that comprises one or more baffles (12, 17) partitioning said collecting vessel (8) into distinct zones, inletting the steel from a processing and casting furnace (7) thereof from a first end of the collecting vessel (8); determining a substantial vacuum at one or more sections of said collecting vessel (8), each section housing a respective baffle (12, 17), so as to locally raise the level of the liquid phase and to keep the continuity of the liquid phase; carrying out degassing and/or processing treatments at said one or more sections; and extracting said liquid phase from a second end (19) of the collecting vessel (8) opposed to the first end of the processing and casting furnace (7), thereby implementing a process continuous and free from discrete changes in temperature.

Description

PROCESS AND APPARATUS FOR THE CONTINUOUS STEEL DEGASSING AND REFINING

DESCRIPTION The present invention refers to a process and to an apparatus for the continuous degassing and processing of liquid-phase steel by a vacuum-type treatment, in particular for the removal of gases, like Hydrogen, Oxygen, Nitrogen, and of other elements, like Sulfur.

Many systems operating with the vacuum technique are known, contrived in order not to inlet strong deoxidants (Silica, Aluminum) into the liquid phase of the steel, deoxidant traces being difficult to remove, and carrying out controlled temperature drop coolings that are overly onerous, in particular for large-sized pieces. The known vacuum techniques can make use of inert gas scrubbing, in order to stir the bath and to lift the dissolved gases on surface, and can also be employed for the processing of the chemical composition of steel, in particular the adjusting of the Carbon content, and in this case it is defined as Carbon deoxidation.

All of the viable techniques, though attaining satisfactory results, are not free from several drawbacks, generally related to the plants utilized for these processes. In fact, all known plants are referable to a plant design comprising a vessel in which a certain vacuum degree is generated. This vessel can directly be that employed to collect the spilled steel, and in this case there will be required a ladle-containing autoclave vessel or a gastight cover resting on the ladle edge and containing the vacuum generating apparatuses, or it can be a further vessel acting as collector and receiving the steel from an elevator vessel filled up by the ladle spilling. In the first case, vacuum is generated above the bath level over the entire mass, and in order to attain the desired effect the deeper layers of liquid metal should be lifted to the surface by gaseous or magnetic stirring, with a remarkable expenditure of time and thermal losses entailing an undesirable temperature drop. In the second case, there is required the passage of the liquid phase between two distinct vessels, entailing process interruptions for the emptying and the filling of said vessels, and, in this case as well, temperature drops. Moreover, the plant is quite complex.

An alternative foresees the degassing be carried out not on a vessel-contained liquid phase but on a liquid phase jet, that is transferred from one vessel to another, all in a vacuum environment, i.e. in a larger vessel housing either the receiving vessel or the donor vessel. However, also this arrangement fails to overcome the abovementioned drawbacks, and the plant complexity is increased, rather than reduced. The technical problem underlying the present invention is to provide a process and an apparatus overcoming the drawbacks mentioned with reference to the known art according to the same inventive concept.

This problem is solved by a process for the continuous degassing and/or processing of liquid phase steel comprising the steps of: * collecting said liquid phase in a collecting vessel, that comprises one or more baffles partitioning said collecting vessel into distinct zones, inletting the steel from a first end of the collecting vessel;

* determining a substantial vacuum at one or more sections of said collecting vessel, each section housing a respective baffle, so as to locally raise the level of the liquid phase and to keep the continuity of the liquid phase;

* carrying out degassing and/or processing treatments at said one or more sections; and

* extracting said liquid phase from a second end of the collecting vessel opposed to said first end. The abovementioned collecting vessel can be of the open-air type.

It is understood that the quantity of liquid-phase steel extracted can be sent to an ingot mold, e.g. for the making of steel strips by continuous casting.

The steel degassing and processing treatments can be any one, e.g., it being possible to induce, at said baffles and sections, the required vacuum degree, an optional flow of inert gas or of another gas, etc.

Moreover, the vessel regions comprised between said portions could also host further non-vacuum treatments.

The main advantage of the process according to the present invention lies in providing balanced extracting and inletting that be continuous, thereby keeping constant the different levels in the vacuum sections and in the other regions due to the communicating vessels principle.

Hence, vessel emptying and filling steps are unnecessary, and, there being provided a single continuous liquid phase, the temperature is subjected to a moderate drop from one end to the other one of the open air vessel. According to the same inventive process, an apparatus for the degassing and/or the processing of liquid-phase steel comprises:

* a collecting vessel having a first inlet end; a second extraction end opposed to said first end; one or more baffles partitioning said vessel into distinct zones, said collecting vessel being apt to receive liquid-phase steel in said first end; * one or more bell-shaped structures, located at respective said one or more baffles, having a bottom edge apt to be immersed in the steel bath at a level lower than the apex of the respective baffle; * means for determining a predetermined vacuum degree at each of said one or more bell-shaped structures; and

* means for the degassing and or the processing at each of said one or more bell- shaped structures. The present invention will hereinafter be described according to two preferred embodiments thereof, given by way of a non-limiting example with reference to the attached drawings, wherein:

* Fig. 1 is a schematic and functional view of an apparatus for the degassing and/or the processing produced by an electric furnace of the continuously charged type; and

* Fig. 2 is a schematic and functional view of a second apparatus according to the invention.

With reference to Fig. 1, a siderurgical plant is generally indicated with 1. It comprises an inlet section 2 that is employed for the charging of iron scrap and fluxing agents (CaO, MgO) for steelmaking. This section also comprises a slag outlet (not shown in the Fig.).

Said inlet section 2 feeds an electric arc furnace (EAF) 3 operating at temperatures in the order of 1600°C. This furnace is of the type in which the dephosphorizing, decarburizing and oxidizing steps are carried out. The furnace 3 is formed by a container having a bottom section 4, filled with a liquid-phase steel bath, and a top section 5 that houses the process gases and is crossed by a lance-shaped electrode 6, also employable to inlet gas. Opposed to the inlet section 2, the furnace comprises an outlet section 7 that is connected to a collecting vessel 8 of the open-air type, of which the outlet section 7 can be considered as the inlet end.

From the furnace 3 and with the vessel 8, an apparatus for the degassing and/or the processing of the liquid-phase steel 9 extends to an ingot mold 10 that feeds by casting a structure 11 for making strips and the like. The vessel 8 comprises, at the outlet section 7 of the furnace 3, a first section 12 above which the apparatus 9 comprises a first bell-shaped structure 13 that defines a first section, a liquid-phase steel vacuum treating step being carried out thereat. The first section 12 partitions the vessel 8 into distinct zones: the first zone corresponds to the outlet section 7 of the furnace 3 and the second zone is that comprised between the first baffle 12 and the subsequent baffle. The first bell-shaped structure 13 has a respective first bottom edge 14 apt to be immersed into the steel bath at a level lower than the apex 15 of the respective first baffle 12. It has to be noted that the vessel 8, by virtue of its location, collects the liquid-phase steel directly from the oven 3, keeping unaltered the level of the zone delimited by the first baffle 12.

At the first bell-shaped structure 13, the apparatus 9 comprises first means for determining a predetermined vacuum degree thereat, indicated with 16 and merely sketched.

By virtue of said means 16, there can be determined a substantial vacuum at the corresponding section of the collecting vessel 8, so as to locally raise the level of the liquid phase and to keep the continuity of the liquid phase across the first baffle 12. In the present embodiment, the vacuum degree is in the order of 0.05 Torr, which is capable of sucking in the steel below the bell-shaped structure 13, lifting it of a height such as to enable the liquid phase to thus flow from the section 7 to the section 21 of the vessel 8 at a temperature in the order of 1550°C. With said vacuum degree the expected height is of 1.25 m. The overall geometry prevents the slag, optionally floating above the steel level, from being dragged beyond the first baffle 12, as it is staunched by the first edge 14 of the bell-shaped structure.

Together with the vacuum means 16, the apparatus 9 comprises first means for the degassing and/or the processing at the first bell-shaped structure 13. In the present embodiment, the treatment carried out is of decarburizing, deoxidizing (O₂ removal) and degassing (N₂, H₂ removal). The gases to be removed are collected in the bell-delimited ceiling and are suitably extracted.

It is understood that structurally the means for creating the vacuum and the means for degassing, i.e. for extracting the gases from the bell-shaped structure, may overlap. Optionally, the means for degassing and or processing can also comprise the function of scrubbing gases, like an Argon-type inert gas.

The vessel 8 comprises, downstream of the first baffle 12 according to the flowing sense of the liquid phase, a second baffle 17. Above the baffle 17 the apparatus 9 comprises a second bell-shaped structure 18 that defines a second section, at which a vacuum treatment step of the liquid-phase steel is carried out. The second baffle 17 partitions the vessel 8 into distinct zones: the abovementioned second zone in cooperation with the first baffle 12 and a third zone corresponding to a second extraction end 19. The second bell-shaped structure 18 has a respective first bottom edge 20 apt to be immersed in the steel bath at a level lower than the apex 15 of the respective first second baffle 17. In the zone comprised between the two bell-shaped structures 13, 18 and between the baffles 12, 17, the apparatus 9 comprises a desulfuration and alligation section 21 that is provided with a second electrode 22 apt to heat the steel bath in said section up to a temperature in the order of 1650°C.

In this section a Fe-Si alligation for magnetic steels and a Fe-Mn alligation for machining steels and for deep moulding are carried out.

At the second bell-shaped structure 18, the apparatus 9 comprises second means for determining a predetermined vacuum degree thereat, indicated with 23 and merely sketched.

By virtue of said second means 23, there can be determined a substantial vacuum at the corresponding section of the collecting vessel 8, so as to locally raise the level of the liquid phase and to keep the continuity of the liquid phase across the second baffle 17.

In the present embodiment the vacuum degree is in the order of 0.05 Torr, which is capable of sucking in the steel, flowing at a temperature in the order of 1600 °C, below the respective bell-shaped structure 18.

The overall geometry prevents the slag, optionally floating above the steel level, from being dragged beyond the second baffle 12, as it is staunched by the second edge 20 of the second bell-shaped structure 18. Together with the respective vacuum means 23, the apparatus 9 comprises second means for the degassing and/or the processing at the second bell-shaped structure 18.

In the present embodiment, the treatment carried out is of final degassing (N2, H2 removal) and of final deoxidizing (O2 removal). The gases to be removed are collected in the bell-delimited ceiling and are suitably extracted. In this case as well, it is understood that structurally the means for generating the vacuum and the means for degassing, i.e. for extracting the gases from the bell- shaped structure, may overlap. Optionally, the means for degassing and/or processing can also comprise the function of scrubbing gases, like an Argon-type inert gas. Subsequently to the second bell-shaped structure 18, the vessel 8 and the apparatus 9 comprise a thermal homogeneization section having a storage tank 24 in which the temperature, in the present embodiment, is of about 1580°C.

This tank 24 comprises said extraction end 19 and provides liquid-phase steel to the next user of the plant 1 that, in the present embodiment, comprises a continuous casting machine 25 employing said ingot mold 10. With reference to Fig. 2, a siderurgical plant 1 comprises an electric furnace 1 gravity-discharging liquid-phase steel into the collecting vessel 8 as abovedescribed.

In this embodiment, between the inlet end 7 and the first baffle 12, the vessel comprises a further section, indicated with 26, where in the liquid steel steps of dephosphorizing, decarburizing and oxidizing steps are carried out, employing a further electrode 27.

The remainder of the apparatus 9 is unchanged.

Concerning the operation of the apparatus, evidently the level of the liquid-phase steel depends on the overall content of the collecting vessel 8, that self-levels according to the communicating vessels principle.

The level below the bell-shaped structures depends on the pressure difference set up between the inside and the outside thereof. If the steel extraction is equal to the steel inlet, the levels in the vessel will keep constant. Hence, the abovedescribed apparatus provides: a continual flow, since when the continuous casting downstream of the apparatus 9 requires a higher casting rate, then the storage tank 24 provides the steel holding the level in the entire plant, i.e., recalling it from the preceding sections; lesser drops in the temperature, ranging from 1650 to 1550°C, with discharge at 1580°C; and a substantial simplification, with reduced plant dimensions.

To the abovedescribed process and apparatus a person skilled in the art, in order to satisfy further and contingent needs, could effect several further modifications and variants, all however encompassed by the protective scope of the present invention, as defined by the attached claims.

Claims

1. A process for the continuous steel degassing and/or processing of liquid- phase steel comprising the steps of:

* collecting said liquid phase in a collecting vessel (8), that comprises one or more baffles (12, 17) partitioning said collecting vessel (8) into distinct zones, inletting the steel from a first end (7) of the collecting vessel (8);

* determining a substantial vacuum at one or more sections of said collecting vessel (8), each housing a respective baffle (12, 17), so as to locally raise the level of the liquid phase and to keep the continuity of the liquid phase; * carrying out degassing and/or processing treatments at said one or more sections; and

* extracting said liquid phase from a second end (19) of the collecting vessel (8) opposed to said first end (7).

2. The process according to claim 1, wherein the collecting vessel is of the open- air type.

3. The process according to claim 1, wherein at least one degassing step is carried out.

4. The process according to claim 3, wherein at respective baffles (12, 17) of the collecting vessel at least two distinct degassing steps are carried out.

5. The process according to claim 1, wherein the liquid-phase steel is subjected to a desulfurizing step in said collecting vessel (8).

6. The process according to claim 1, wherein the liquid-phase steel is subjected to an alligating step in said collecting vessel (8).

7. The process according to claim 1, wherein the liquid-phase steel is subjected to a dephosphorizing, decarburizing and oxidizing step in said collecting vessel (8).

8. The process according to claim 1, wherein the degassing and processing treatment comprises: decarburizing; deoxidizing (02 removal); and degassing (N2, H2 removal).

9. The process according to claim 1, wherein a vacuum in the order of 0.05 Torr is established.

10. An apparatus (9) for the degassing and/or the processing of liquid-phase steel comprises:

* a collecting vessel (8) having a first inlet end (7); a second extraction end (19) opposed to said first end (7); one or more baffles (12, 17) partitioning said collecting vessel (8) into distinct zones, said collecting vessel (8) being apt to receive liquid-phase steel in said first inlet end (7);

* one or more bell-shaped structures (13, 18), located at respective said one or more baffles (12, 17), having a bottom edge (14, 20) apt to be immersed in the steel bath at a level lower than the apex (15) of the respective baffle (12, 17); * means for determining a predetermined vacuum degree (16, 23) at each of said one or more bell-shaped structures (13, 18); and * means for the degassing and/or the processing at each of said one or more bell- shaped structures (13, 18).

11. The apparatus (9) according to claim 10, wherein the collecting vessel is of the open-air type.

12. The apparatus (9) according to claim 10, comprising two partitions (12, 17) and two respective bell-shaped structures (13, 18).

13. The apparatus (9) according to claim 10, wherein the overall geometry of baffles (12, 17) and bell-shaped structures (13, 18) prevents the slag, optionally floating above the steel level, from being dragged beyond the respective baffle (12, 17), as it is staunched by the respective bottom edge (14, 20) of the corresponding bell-shaped structure (13, 18).

14. The apparatus (9) according to claim 10, comprising a desulfuration and alligation section (21) that is provided with a respective electrode (22) apt to heat the steel bath.

15. The apparatus (9) according to claim 10, comprising a thermal homogeneization section having a storage tank (24) at the extraction end (19).

16. The apparatus according to claim 10, wherein between the inlet end (7) and a baffle (12), the collecting vessel (8) comprises a dephosphorization, decarburization and oxidation section (26), having a respective electrode (27).