GB2253469A - Agitating molten steel - Google Patents

Abstract

Metal-slag stirring in an electric arc furnace containing molten steel is produced by gas injection through the vessel bottom. Injection may be made through gas permeable ceramic elements which are not penetrated by the metal, even when the gas flow rate is cut off. The gas injection can also be done with tuyere type injection elements to avoid gas flow rate suspension during the complete process. A furnace charging door stopper is used during stirring. Gas stirring of the molten bath produces a higher metal-slag and slag-atmosphere interphase area. The stirring gas may be natural gas. so that the process conditions, oxidizing or reducing, are reinforced according to the slag type in the bath. <IMAGE>

Description

PROCESS TO IMPROVE THE ELECTRIC ARC FURNACE STEELMAKING PROCESS BY NATURAL GAS INJECTION THROUGH THE VESSEL BOTTOM PROCESS DESCRIPTION.

The present invention is related with a procedure to improve the electric arc furnace steelmaking process. Particularly this invention consists in a method to produce liquid bath (metal-slag) stirring in the electric arc furnace, which in combination with adequated slags and stirring gas, the steelmaking processes are improved.

The electric arc furnace is a good melting reactor, but it presents certain metallurgical restrictions, mainly regarding the refining of low impurity level steels. These limitations are due to the almost complete absence of agitation of the molten bath. It is known that if the molten bath is stirred, the heat and mass transfer rates are increased approaching faster the eyui equilibrium con ditions in the slag-metal-atmosphere system. In that regard, some plants are sino electromagnetic stirring, however, the power agitation with this process is limited.

It is therefore the object of the present invention to provide for a process to produce liquid bath agitation by gases injection through the electric arc furnace bottom, with the posibility to operate with periods without gases injection and with periods with high gas flow rate to produce a vigorous liquid bath stirring and without the risk of liquid ejections through the furnace charging door. In processes with stages of no-gases injection, gas permeable ceramic elements should be installed. When the above not be a requirement tuyere type elements can be installed. A characteristic of the agitation produced is the increment in the thermal bath homogenization giving as result a reduction in the energy consumption and in the process time. The above is more significative during the melting period due to the higher contact between scrap and the previously liquid metal melted.During the refining periods, when all metal is melted, the higher thermal homogenization produced by stirring makes unnecessary overheat he liquid bath surface avoiding refractory furnace exposition to high temperatures.

Another characteristic of the present invention is the use of natural gas as stirring gas in substitution of inert gases. This practice has additionally the property to reinforce the process conditions, oxidizing or reducing, according to the slag type in the molten bath: Oxidizing Conditions: The bottom natural gas injection is made in combination with an oxidizing slag. The vigorous stirring increases the metal-slag and the metal-atmosphere interphase area, resulting in greater elimination from the molten metal of the most oxidable elements (C, Mn, Si, P, Al) than the obtained in the conventional practices.

Reducing conditions: The bottom natural gas injection is made in combination with an appropiate reducing slag. The agitation produced increases the metalslag contact, resulting in a greater and faster oxygen and sulphur transfer from the metal to slag. The natural gas is burned in the surface bath, consuming oxygen from the atmosphere and increasing the reducing conditions efficiency. The sulphur elimination from the molten metal, by this practice, is greater than the obtained by conventional practices.

The natural gas permanence inside the liquid bath is small enough that the cracking of Methane in carbon and hydrogen is practically worthless. However, it is possible that a small amount of cracking occurs near the bath surface.

This case has two benefits: It produces a reducing atmosphere (hydrogen base) and carbon deposition produces an additional stirring known as molten metal boiling because of carbon monoxide production.

One of the main metallurgical consequences of natural gas injection is a greater thermodinamicequilibrium(C-O)approach which is manifiested by a reduction in the dissolved oxygen content in the molten bath as compared to the conventional practices. A result of the above is an increment in the metallic yield and a reduction in deoxidants and ferroalloys consumption.

The above and other objects, features and advantages of the invention will become more readily apparent from the following description, reference being made to the drawings whose sole figures are schematic drawings according the process of the present invention.

Fig. 1 Schematically illustrates a cross section of an electric arc furnace with gas permeable ceramic elements and a charging door stopper device in one arrangement.

Figs. ? y 3- Schematically illustrate other embodiments of the charging door stopper device.

Refering to Fig. 1, the gases injection is done through gas permeable ceramic elements or tuyere type elements (1). Said elements are instaled as plugs in the electric arc furnace bottom and the arrangement on the furnace bottom depends on the number of them to be installed, on the furnace physical configuration and on the desired flow pattern.

The gases injection is carried out in combination with a charging door stopper device which avoids liquid bath ejections, through said charging door, during the gas injection stages that require high agitation levels as deep desulphurization.

The furnace charging door stopper device consists of a refractory lid (2), of the same dimensions of the charging door oriffice. The said refractory lid is mounted on one end of a ram with a counterweight (3), forming a battering ram as a charging door stopper device. In another arrangement, Fig. 2, the refrac tory lid (2), is mounted on a hinge mechanism (4), which is mounted on the furnace exterior wall, and it operates in a vertical plane or is mounted to operate laterally, Fig. 3, forming a door as a charging door stopper device.

The said hinge mechanism (4), could be operated by hand, pneumatically or hydraulically.

The practice for molten metal vigorous stirring in the electric arc furnace consists of the following steps: 1. Blockading of the furnace charging door.

2. Bottom gas injection.

3. Gas flow suspension and relieving off the blockade.

The practice to produce thermal homogenization does not require vigorous stirring and it can be carried out without the blockade of the furnace charging door.

The stirring gas could be argon, nitrogen or natural gas, the latter being, the only one that reinforces the process conditions, oxidant or reducing, according to the slag in the bath.

The inventors of this invention have carried out studies in the laboratory and in a pilot plant, with a view to finding the natural gas cracking conditions when that gas is bubbled in a molten iron bath. As a result, it has been found that under steel refining conditions, the natural gas cracking is worthless at least when the bath depth is between 0 to 2 meters. When the natural gas flow is increased, the amount of gas cracked is decreased because of the smaller residence time of such gas in the molten bath.

The following examples of the present invention are to be applied in the elec tric arc furnace practices: Example 1. Practice in the melting period.

The melting practice does not require gas flow rate suspention at any moment and it is used to provnke a greater contact between the previously formed liquid metal and the solid charge by gases injection, preferably natural gas, during the complete melting period.

In the case of furnaces that are used mainly as melting reactors, carryingout the refining operations in the transfer ladle, the injection elements can be tuyere type.

During melting, the higher rate achieved through high sitirring power permits a lower usage of oxygen from lance or burners, resultig in a lower alloy oxidation. A reduction from 0.6 to 3 Nm3/ton is possible to obtain.

Example 2. Practice in the oxidizing period.

The dephosphorization process is carried-out according to the following steps: Melting and buildig up an oxidizing slag by adding fluxes, mineral (FeO) and/or oxygen by lance. The slag basicity index (CaO/S102) must be higher than 3. Then, molten metal stirring by the bottom gas injection process, and finally the slagging off process.

The decarburizing process starts by creating oxidizing conditions in the melted bath, then molten metal stirring by the bottom gas injection process, which ends when the carbon content in the metal is about 0.1%. However, it is possible to obtain extra low levels of carbon (0.002% C) only -with the bottom gas injection at the end of the oxidizing period, with natural gas being the stirring gas.

Example 3. Practice in the reducing period.

The desulfurization process is started after removing the previous oxidizing slag, then building up an appropriate synthetic reducing slag having a basicity index higher than 3, and then molten metal stirring by the bottom gas injection process. The high metal-slag contact that is produced by gas stirring permits the effective sulfur elimination from the molten metal, and gives rise to a greater steel clenness, less refractory wear, and shorter operation times than the conventional process. The natural gas, when it is used as a stirring gas, is burned out in the bath surface, consuming oxygen from the atmosphere that increases the efficiency of the reducing conditions.

It is possible to obtain extra low sulfur levels (0.002 S) when the stirring gas is natural gas.

Example 4. Adjustment period.

This practice starts with the addition of alloys, after the molten bath deoxi dation process and sampling, then molten metal stirring by the bottom gas injection process, then sampling and chemical analysis if necessary alloy adjustment, followed by molten metal stirring by the bottom gas injection process.

The stirring of the molten metal by bottom gas injection produces a -greater and faster thermal and chemical homogenization of the molten metal than the conventional process, and permits a greater quality control and reduction of the operation time. When the stirring gas is natural gas, it protects the additions against atmospheric oxidation by consuming oxygen from the atmosphere, burning it out in the bath surface.

Although the present invention has been described, it is to be understood that modifications and variations may be resorted to, without departing from the spirit of the invention. Such modifications and variations are considered to be within the scope of the present invention as defined by the appended claims.

Claims (8)

1. A process to improve the electric arc furnace steelmaking practices by natural gas injection through the vessel bottom, the improvement which comprises the steps of: providing a plurality of gas injection elements which are installed on the furnace bottom in an electric arc furnace; stirring the molten metal in the furnace by blowing a gas through the injection elements and into the interior of the molten metal in the furnace; and blocking the furnace charging door opening by a stopper that includes a refractory lid of the same dimensions as the charging door opening in such way that the internal furnace wall maintains its continuity in the door zone to avoid liquid ejection through the furnace charging door opening as the molten metal is being stirred.

2. A process as defined in claim 1, wherein the stirring gas is selected from the group consisting of argon, nitrogen, and natural gas.

3. A process as defined in claim 1, including the operations of charging solid charge and melting said solid charge simultaneously with gas injection through the injection elements in order to increase thermal homogenization between metal that is being melted and the solid charge.

4. A process as defined in claim 1, including the step of forming a synthetic oxidizing slag having a basicity index (CaO/SiO2) higher than 3 prior to stirring the molten metal to eliminate phosphorus therefrom.

5. A process as defined in claim 1, including the step of forming a synthetic reducing slag having a basicity index higher than 3 prior of stirring the molten metal to eliminate sulfur therefrom.

6. A process as defined in claim 1 including the step of creating an oxidizing condition in the molten metal prior to stirring the molten metal to reduce the carbon content thereof.

7. A process as defined in claim 1, including the step of adding alloying materials to the molten metal prior to stirring to reduce the thermal and chemical homogeneization time of the molten metal in the electric arc furnace.

8. A steelmaking process substantially as herein described with reference to the accompanying drawings.