Title of Invention

A METHOD FOR MANUFACTURING STAINLESS STEEL

Abstract

The present invention relates to a method for manufacturing stainless steels, in particular high-grade chrome and chrome-nickel steels, in a smelting device comprising at least two vessels for the supply of a steel foundry, comprising the step of a) melting in the first vessel a charge, consisting essentially of raw materials containing solid and/or liquid metallic iron, in particular scrap, and in some cases carbon-containing alloy carriers, b) decarburizing the melt after reaching a temperature of 1460°C, to a carbon content of < 0.3% by blowing with oxygen or oxygen mixtures, c) heating the melt to a tapping temperature of 1620 to 1720°C and d) subsequently bring the carbon content of less than 0.1 % with d) the change in the first vessel being decarburized with oxygen blowing at the same time as a second charge is undergoing the melting process in the second vessel.

Full Text

The invention relates to a process for the production of stainless steels, in particular stainless chromium steel and stainless nickel chromium steel, in a melting installation which comprises at least two vessels for supplying a steel casting plant. In the production of chromium and stainless nickel chromium steels, it is customary to use a conventional type of electric furnace which I designed as a direct-current or an alternating-current furnace, in which scrap metal and/or other metal starting materials containing iron, e.g. pig iron or DRI (direct Reduced Iron) are melted down together with an adequate quantity of alloy carriers. The starting material thus melted down is tapped into a ladle at a temperature of 1670 to 1700°C. This ladle is subsequently discharged into a converter, and the melt which contains about 2.5% carbon and about 1% silicon, is refined, in a first step by means of oxygen and, in the case of reduced carbon contents, with oxygen/hydrogen mixtures, and subsequently with oxygen/argon mixtures. In this regard, depending on the different process techniques applied, the decarburization is carried out to a final carbon content of less than 0.1%, it subsequently being necessary for the concomitant chromium losses in the slag to be recovered by_ a reduction with ferrosilicon or secondary aluminium. In addition, in a technique involving a 3-step process, it is known for the metal of the converter to be tapped at carbon, contents of about 0.2 - 0.3 % and to be reduced to the final carbon contents in a separate vacuum oxidation process. A factor common to the process known to date is that, as a result of a single or a repeated transfer of the melt, considerable temperature losses arise and these must be compensated for by a high tapping temperature and, thus, a high energy input in the primary melting vessel, which, in these instances, is the electric arc furnace. In addition to the higher energy input, this also involves an increased wear of the electrodes and of the refractory lining of the electric furnace. For installing the converter, which is required for the second process step, relatively large overall heights are required for the surrounding housing, so as to accommodate the arc-blowing lance and the waste gas system. Accordingly, it is the object of the invention to provide a process for the production of stainless steels, in which the number of process steps is reduced, in which the energy consumed in the individual process steps is reduced, and in which it is possible for the operating units to be designed to have a small overall height. Accordingly the present invention provides a method for manufacturing stainless steels, in particular high-grade chrome and chrome-nickel steels, in a smelting device comprising at least two vessels for the supply of a steel foundry, comprising the steps of a) melting in the first vessel a charge, consisting essentially of raw materials containing solid or liquid metallic iron, or solid and liquid metallic iron in particular scrap, and in some cases carbon-containing alloy carriers, b) decarburizing the melt after reaching a temperature of 1460°C, to a carbon content of Advantageous further developments of this process are set out in the subsequent claims. The process according to the invention is carried out in a melting installation which comprises at least two vessels. The two vessels are operated in parallel, it being possible to use, in each vessel, either electrodes for melting down the charge or arc-blowing lances for blowing and/or injecting oxygen and oxygen mixtures. The vessels thus serve, in the first instance, as a melting unit and subsequently, as a refining unit. The advantage hereof is that, without any temperature losses resulting from a transfer, it is possible to process the melt and to bring it to the desired temperature. In a time-staggered manner, scrap metal, ferronickel, nickel, errochromium and other raw materials containing metal iron are melted down, preferably by electrical energy, in each vessel. In so doing, a starting metal is produced and this is composed, for the greater part, of iron, and has a chromium and nickel content which is close to the final analysis of the steel grade to be produced, i.e. essentially austenite, ferrites and martensites. When using ferrochromium having a high carbon and/or silicon content, an advantageous development provides for oxygen to be injected by means of a lance, thereby reducing the silicon content. Once a melt temperature of 1500 to 1600° C is reached, the electrode arm is pivoted out of the particular vessel, an oxygen lance is pivoted into the very same vessel, and said oxygen lance is used, together with nozzles disposed in the bottom and/or in the side wall of the vessel, for oxygen, as well as mixtures comprising oxygen, argon, hydrocarbon and steam, to oxidize the melt. At an average blowing time of 2 0 to 4 0 min., an oxygen input rate of 0.1 to 2.0 NM X min. for the oxygen lance, and using the above-mentioned injection nozzles, the melt is decarburized to a final carbon content of 0.10 to 0.015 %. It is possible for the quantity of heat released as a result of the blowing process to be used for the addition of cooling agents in the form of Ni, FeNi, ferrochromium, scrap metal, as well as other iron-containing metal starting materials, suoh r as pig iron cakes, DRI or alloying agents, in order to adjust the target analysis and the target temperature. After the blowing operation, the slag is reduced by means of reducing agents, such as, for example, ferrosilicon, aluminium or secondary aluminium, with the simultaneous addition of slag-forming agents, such as lime and fluorspar, for recovering oxidized chromium, and the steel and/or slag is/are tapped, the vessel is again filled with scrap metal and alloy carriers, which are melted down by means of the electrode which is, once again, pivoted into the vessel. The melting process and the subsequent blowing process take place successively in the respective vessel and synchronously in the two vessels, such that, after, in each case, 80 to 120 min., it is possible for a melt to be made available from one vessel, i.e. in the synchronous operation of the two vessels, melt is available for the downstream continuous-casting plant every 4 0 to 6 0 min. This isochronous use of two vessels has not only this advantage, i.e. the continuous supply to a continuous-casting plant, but it is also advantageous for energy-related reasons. Once melting has taken place in the first vessel, it is, for example, possible for the, still hot, electrode withdrawn from the vessel to be introduced into the second vessel for commencing the melting operation in said second vessel. This reduces the outlay in respect of energy and electrode consumption. Since the blowing process, which is carried out until the carbon contents are at their very lowest, of course results in. a considerable stress of the refractory material in the hearth of the vessel, it is also possible, in a variation, for the blowing process to be discontinued at a carbon content of 0.2 -0.4 %. In that case, the metal and the slag would also be jointly discharged into a ladle, the slag would be removed by decanting and by means of scrapers, the ladle, together with the metal melt, would be transferred into a vacuum vessel, which operates according to the state of the art and has the means for blowing oxygen, and refining would be carried out to the final carbon content. The present process would not involve the complex installation of a converter for the blowing process in any of the above-mentioned cases, thus resulting in a considerable reduction in the investment costs in respect of the process. In a similar manner-, there is no longer a transfer of the starting metal, which is melted down by electrical energy and is associated with energy losses, in particular a transfer from a transportation ladle into the converter, for reducing the carbon content by means of oxygen. In order to oxidize the silicon to a specific degree, it is suggested for oxygen to be added during the melting of the charge. With a view to obviating special structural measures,. / a door lance is used in this regard. An example of the invention is illustrated in the attached drawing, in which: Figure 1 is an outline of the individual steps. When viewing the furnace vessel 1 in the process phase 1, a small quantity of liquid phase is present in the furnace vessel at the point of time A, and on it is disposed newly introduced charge. The furnace vessel is sealed by a cover through which electrodes penetrate into the top vessel of the furnace. During phase B, the charge is melted down by means of electrical energy. During this melting, the liquid bath level in the bottom vessel of the furnace rises. In phase C, the charge, which essentially comprises scrap metal, ferronickel, nickel, ferrochromium and other metal materials containing iron, is virtually completely melted down into liquid starting metal. When using alloying agents which contain large quantities of carbon and/or silicon, it is possible for oxygen to be injected by means of a lance, thereby reducing the silicon content. In phase D, the starting material is completely melted, the melt having a temperature of 1500 to 1600° C. For a change¬over to a further process phase in vessel 1, the electrode arrangement is withdrawn from the vessel and an oxygen lance is pivoted into said vessel. In the present diagram, the process phase 2 in the furnace vessel 2 is illustrated, since this process phase in vessel 2 takes place isochronously with (and in the reverse sequence to) the process phase 1 of the furnace vessel 1. " In step A of the process phase 2, the melt is oxidized, by means of a lance and bottom nozzles and/or side nozzles, oxygen or an oxygen mixture being used. The bottom or side nozzles are concentric nozzles. They are formed by an outer pipe, an annular gap and a central pipe. It is possible for oxygen or an oxygen mixture, comprising 0^ + N2, O2 + Ar or O2 + air, to be injected via the central pipe. N2 or Ar or hydrocarbons or steam or a mixture of these gases is injected in parallel via the annular gap. Once the final carbon content and the tapping temperature of 162 0 to 168 0° C are reached, the melted slag is reduced by a reducing agent, such as ferrosilicon or aluminium, in order to recover oxidized chromium, and the metal and slag are subsequently jointly tapped. Alternatively, during step C, it is possible for the melt to be decarburized to a final carbon content of, for example, 0.05. %, or to be transferred, after separation of the slag, to-a vacuum plant while at a carbon content of 0.2 to 0.4 %, where it is brought to the desired final carbon content. The finished steel is subsequently tapped. In step D, a fresh charge of scrap metal and alloy carriers which, in part, contain carbon, is filled into the furnace, it being possible for a liquid phase to be present in the furnace vessel. Subsequently, step A again commences, as already described above. WE CLAIM: 1. A method for manufacturing stainless steels, in particular high-grade chrome and chrome-nickel steels, in a smelting device comprising at least two vessels for the supply of a steel foundry, comprising the steps of a) melting in the first vessel a charge, consisting . raw materials containing solid or liquid metallic iron, or solid and liquid metallic iron in particular scrap, and in some cases carbon-containing alloy carriers, b) decarburizing the melt after reaching a temperature of 1460°C, to a carbon content of 2. The method as claimed in claim 1, wherein after reaching the final carbon content and the tapping temperature, the molten slag is reduced with a reducing agent preferably ferrosilicon, silicon or aluminium and subsequently tapped together with the metal. 3. The method as claimed in claim 1, wherein the blowing of oxygen or oxygen mixtures is carried out in the form of top-blowing combined with bottom-blowing and/or side blowing. 10. A method for manufacturing stainless steels, substantially as herein described with reference to the accompanying drawings.

Documents:

174-mas-1997 abstract duplicate.pdf

174-mas-1997 abstract.jpg

174-mas-1997 abstract.pdf

174-mas-1997 assignment.pdf

174-mas-1997 claims duplicate.pdf

174-mas-1997 claims.pdf

174-mas-1997 correspondence others.pdf

174-mas-1997 correspondence po.pdf

174-mas-1997 description (complete) duplicate.pdf

174-mas-1997 description (complete).pdf

174-mas-1997 drawing.pdf

174-mas-1997 form-13.pdf

174-mas-1997 form-2.pdf

174-mas-1997 form-26.pdf

174-mas-1997 form-4.pdf

174-mas-1997 form-6.pdf

174-mas-1997 others.pdf

174-mas-1997 petition.pdf