ELONGATED PRODUCT USED FOR DESULPHURIZATION OF A STEEL BATH

Abstract: A longitudinal product comprises at least one portion, in the following termed reactive composition, which is reactively active when being submerged in a bath of molten steel for the treatment of such melts, particularly for desulphurization and for modification of inclusions. The said reactive composition contains in weight-%: 35-75 % (CaO + CaCO3) and 30-70 % CaSi.

Description: ELONGATED PRODUCT USED FOR DESULPHURIZATION OF A STEEL BATH TECHNICAL FIELD The invention concerns an elongated product comprising at least one portion, in the following termed reactive composition, that is reactively active when being submerged in a bath of molten steel for the treatment of such melts, particularly for desulphurization and for modification of inclusions. BACKGROUND OF THE INVENTION One of those impurities which have a negative impact on the mechanical properties of steels is sulphur. Sulphur has an unlimited solubility in steel at high temperatures, but the solubility is greatly reduced when the steel solidifies. This leads to precipitation of iron sulphides which have a melting point far below that of the steel. The molten iron sulphide will be deposited in grain boundaries, which can cause cracking during working. Conventionally a basic (CaO-, MgO-rich) top slag is used for desulphurization. Also injection of powder in the steel melt in connection with after-treatment of the melt in a ladle is known in the art. Further it has been suggested to add desulphurization agents in the form of a powder or a grain shaped material enclosed in a tube- or hose-shaped enclosure. Metallurgists use wire as a term for a product of that kind, a term which will be used also in the following of this text. It is a condition of an efficient desulphurization that the oxygen activity is low in the steel bath. This is achieved through an initial deoxidation step. Aluminium is a very strong deoxidation agent, but the Al 2 O 3 that is formed through the reaction between added aluminium and oxygen that is dissolved in the melt has a high melting point and forms solid clusters which cause problems in connection with continuous casting. As examples of background art, the following references may be mentioned: (1) S. K. Saxena, Modification and control of non. metallic inclusions in aluminium- killed steel by injection of CaO-containing slag powder , Scaninject, June 9-10, 1977 (2) H-J Langhammer, Influence of injection of powdered materials on oxide and sulphide cleanlininess of steel grades, Scaninject, June 9-10, 1977 (3) K. Wada et al., Investigation of desidfurization and deoxidation in injection metallurgy, Scaninject, June 12-13, 1980 (4) J.M. Diximier et al., Recent developments of calcium alloys cored wire in the steel production, Scaninject, June 12-13, 1980 (5) M. Douchy, Sammansatt produkt med rόrforigt hόljefόr behandling av smάlta metallbad, Swedish translation of European Patent No. 0 281 485 (6) J. Blanpain, Gefullter Metalldraht, DE 29 22 349 Al. DISCLOSURE OF THE INVENTION It is the purpose of the invention to provide a new reactive mixture in the form of an elongated product, preferably in the form of a wire and in the following referred to as wire, which allows a combined action of desulphurization and modification of inclusions, which is not possible by means of known products of today, particularly for treating steel melts which need to be desulphurized, e.g. melts which contain at least 150 ppm sulphur. This can be achieved therein that the reactive portion of the wire consists of a material composition which contains in weight-% 35-75 % (CaO + CaCO 3 ) and 30-70 % CaSi. Preferably the reactive composition contains at least 15 % CaO when the reactive composition also contains CaCO 3 which is decomposed to CaO + CO 2 in the melt. At the desulphurization CaO reacts with dissolved sulphur, oxygen being dissolved in the melt, whether CaO is added in the form of lime (CaO) or is obtained through the decomposition of CaC0 3 . This oxygen is taken up by Ca (from CaSi) and by Si and by possibly existing Al. Preferably the reactive composition of the product contains at least 2 weight-% Al. A conceivable content is 2-10 % Al or 2-5 % Al. The optimal content of the agents included in the reactive composition depends on the composition of the melt that shall be treated, in the first place the content of oxygen and sulphur. For the treatment of melts which already have a low oxygen potential due to a foregoing deoxidation by means of aluminium and/or silicon or possibly any other deoxidation agent, aluminium in the reactive composition of the product can be excluded. According to a conceivable application the composition contains 15-45 % CaO. More precisely the content of CaO may amount to 35-45 %, 15-25 % or 30-40 % within the scope of this application. According to another application the content of CaO amounts to 40-60 %. Preferably the reactive composition contains 5-30 % CaC0 3 or, more precisely depending on the application, 20-30 % CaC0 3 or 5-15 % CaCO 3 . In the case the reactive composition contains 20-30 % CaCO 3 , the content of CaO may, for example, amount to 15-25 %. In the case the reactive composition does not contain CaCO 3 , the content of CaO may, for example, amount to 35-45 %. In a preferred case the composition contains 15-45 % CaO, 40-60 % CaSi and 2-5 Al. In another case the reactive composition contains 30-40 % CaO, 40-60 % CaSi, 3-8 % Al and 5-15 % CaCO 3 . The purpose of the aluminium in the wire that shall be injected in the steel melt is to provide a very low oxygen activity in the injection region and thence locally improve the preconditions for the desulphurization at the same time as conditions are created of a good control of the Al-yield and thence also of the Al-content in the melt, which can allow a comparatively low consumption of aluminium and improved desulphurization, because compensation for reoxidation is facilitated. A wire which contains aluminium in the reactive composition is particularly advantageous for the treatment of steel melts having a high oxygen activity, e.g. 20 ppm or more, and above all for treating steels which have a high oxygen activity in combination with a high sulphur content. The injection of said reactive agents in the form of a wire gives a desired good inclusion control, because the hard aluminium oxide clusters which are formed at the aluminium deoxidation are modified by means of calcium in CaSi to form calcium aluminates having a low melting point, e.g. (CaO) 12 (Al 2 O 3 ) 7 . Inclusions of this type are liquid down to 1455°C and they therefore do not give rise to those problems of clogging of casting pipes and of moulds which solid clusters may cause. Besides they are readily separated to be included in the top slag which covers the surface of the ladle in which the injection is carried out. Preferably a certain amount of CaO is replaced by CaCO 3 , wherein an improved desulphurization can be obtained through calcination and decomposition of CaCO 3 to form CaO and C0 2 (g), which leads to the generation of very fine CaO grains having a large reaction surface, in combination with an increased stirring of the bath as C0 2 (g) rises towards the surface. As far as the content of the various ingredients in the reactive composition is concerned, the following may further be mentioned. CaSi shall exist in an amount of at least 30 %, preferably at least 40 % in order to provide desired deoxidation through its content of Si, at the same time as the hard aluminium oxide clusters that are formed in connection with the oxidation of aluminium which also shall take place in the melt, are modified to calcium aluminates of the type (CaO) i 2 (Al 2 O 3 ) 7 inclusions due to the CaSi calcium. At the same time, for the achievement of a desired desulphurization, the reactive composition should contain at least 15 % CaO. The upper limit of CaSi, which is 70 %, preferably max. 60 %, and of CaO + CaCO 3 , respectively of CaO and CaC0 3 separately, as is mentioned in the foregoing, is limited because the ratio CaO/Al 2 O 3 (whether CaO is added in the form of lime (CaO) or is obtained through decomposition of CaCO 3 ) in the melt shall be such that calcium aluminates of the said type are formed. If the content of CaO that is added, or is formed, would be too high, there will be formed other, not desirable inclusions having higher melting temperature. In order to create a desirably low oxygen activity in the injection region, the mixture of reactive agents, i.e. the reactive composition, contains at least 2 % Al. More than 10 % is excess of aluminium and would give bad yield. Preferably the mixture of reactive agents contains max. 8 % Al, but also max. 5 % Al can be sufficient for many applications. The effect that is achieved by the replacement of a certain amount of CaO by CaC0 3 has been mentioned above. In order to achieve the desired, increased kinetics through the formation of bubbles and by increasing the reaction surface through the almost explosive formation of many small CaO particles, the content of CaCO in the mixture of reactive agents should amount to at least 5 %, and according to an embodiment to at least 20 %. The injection of CaCO 3 , however, causes a local reduction of the temperature in the melt. Further the injection of CaC0 3 leads to a certain increase of the carbon content. Therefore the content of CaC0 3 should be limited to max. 30 %. According to a conceivable embodiment the composition contains 5-15 %. The elongated product - the wire - may physically - i.e. from a design point of view - be shaped in many different ways within the scope of the invention. It shall have a sufficient bendability in order to be able to be coiled and uncoiled on and from a drum but at the same time have sufficient stiffness in order to be able to be fed down through a slag cover on the surface of a metal melt in a treatment ladle and be submerged in the melt down to as close to the bottom as possible, e.g. to a depth of at least about 2 m, which are requirements that the wire shall satisfy. Typically the wire has a substantially circular cross section with a diameter of 10-25 mm. Wires having outer diameters of 13 mm and 16 mm have been found to function very well. Further the wire shall be designed in such a way - and this particularly concerns the tube- or hose-shaped enclosure of the wire - that the mixture of reactive agents, i.e. the reactive composition, does not react with the melt until at a desired depth beneath the surface of the melt, i.e. as near the bottom as possible. The envelope of the wire consists of a tube or a hose of metal, suitably a metal which will be molten and be included in the metal melt, preferably without reacting chemically with elements in the melt. Normally the envelope consists of a tube or a hose of steel having a sufficient thickness in order to give the wire sufficient strength, however, not thicker than the wire can be coiled and uncoiled on and from a drum. BRIEF DESCRIPTION OF DRAWINGS In the drawings, Fig. 1-Fig. 7 shows the wire of the invention in cross section according to some conceivable embodiments. Fig. 1 shows a wire consisting of a seamless steel tube la, which contains a powder or grain shaped product 2 consisting of said reactive composition. Fig. 2 shows a wire consisting of a butt-welded steel tube lb containing said reactive composition 2. Fig. 3 shows a lap-seam welded or glued steel pipe lc containing said reactive composition 2. Fig. 4 shows a lockseamed steel pipe Id containing said reactive composition 2. Fig. 5 shows a butt welded pipe la, which contains an aluminium wire 3, which is responsible for the intended aluminium content in the composition and is embedded in a reactive powder or grain shaped product 2', which contains the other ingredients in the reactive composition. Fig. 6 shows a butt-welded pipe la, which on the inside is covered by a sheet of aluminium or which contains a strip 3' of aluminium. The reactive mixture 2' in the same mode as according to Fig. 5 contains the other ingredients in the reactive mixture. Fig. 7 shows a lockseamed steel pipe Id' which is provided with a longitudinal indentation 4 for close-packing the reactive mixture 2, the indentation being performed after the tube Id' has been filled and sealed through the lockseam. In all the drawings the thickness of the steel pipe has been strongly exaggerated. EMBODIMENTS AND EXAMPLES According to a first preferred embodiment the mixture of reactive agents contains in weight-% 38-42 % CaO, 3-4 % Al and 55-60 % CaSi. An example of a composition according to this first embodiment is a composition consisting of 39.3 % CaO, 3.6 % Al and 57.1 % CaSi. According to a second embodiment the reactive composition in the wire according to the invention contains in weight-%: 18-24 % CaO, 2.5-4 % Al, 45-55 % CaSi and 22-28 % CaCO 3 . An example of such preferred embodiment is a composition which consists of 21.3 % CaO, 3.2 % Al, 50.2 % CaSi and 25.4 % CaCO 3 . According to a third, preferred embodiment the reactive composition in the wire of the invention contains in weight-% 33-38 % CaO, 3-8 % Al, 45-55 % CaSi och 7-13 % CaCO 3 . An example of such preferred embodiment is a composition which consists of 35 % CaO, 5 % Al, 50 % CaSi and 10 % CaCO 3 . In all the cases CaO, CaSi and CaC0 3 exist as powder or grains or as an agglomerate in the tubular envelope, while the aluminium either may exist as a powder or as larger particles in the mixture and/or as a wire, sheet, strip, or the like inside the envelope. In principle it is also conceivable that the aluminium exists as part of or on the inside of the envelope. Besides the said components CaO, Al, CaSi and CaCO 3 , the wire, besides iron in the envelope, which are not included in the said percentages, may contain other components, provided that such components do not impair the intended desulphurization and modification of inclusions. Such conceivable, further components may include, but are not restricted to, e.g. boron and misch metal.

Claims: CLAIMS 1. Elongated product comprising at least one portion, in the following termed reactive composition, that is reactively active when being submerged in a bath of molten steel for the treatment of such melts, particularly for desulphurization and for modification of inclusions, characterized in that said reactive composition contains in weight-%: 35-75 % (CaO + CaCO 3 ) and 30-70 % CaSi. 2. Product according to claim 1, characterized in that the reactive composition contains 40-60 % CaSi. 3. Product according to claim 1 or 2, characterized in that the reactive composition also contains at least 2 % Al. 4. Product according to claim 3, characterized in that the reactive composition contains 2-10 % Al. 5. Product according to claim 4, characterized in that the reactive composition contains 2-5 % Al. 6. Product according to any of claim 1-5, characterized in that the reactive composition contains at least 15 % CaO. 7. Product according to claim 6, characterized in that the reactive composition contains 15-45 % CaO. 8. Product according to claim 7, characterized in that the reactive composition contains 35-45 % CaO. 9. Product according to claim 7, characterized in that the reactive composition contains 15-25% CaO. 10. Product according to claim 7, characterized in that the reactive composition contains 30-40 % CaO. 11. Product according to claim 6, characterized in that the reactive composition contains 40-60 % CaO. 12. Product according to any of claims 1-11, c h a r a c t e r i z e d in that the reactive composition contains 5-30 %b CaCO 3 . 13. Product according to claim 12, characterized in that the reactive composition contains 20-30 CaCO 3 . 14. Product according to claim 12, characterized in that the reactive composition contains 5-15 CaCO 3 . 15. Product according to claim 1, characterized in that the reactive composition contains 15-45 % CaO, 40-60 % CaSi and 2-5 % Al. 16. Product according to claim 1, characterized in that the reactive composition contains 30-40 % CaO, 40-60 % CaSi, 3-8 % Al and 5-15 % CaC0 3 . 17. Product according to any of the preceding claims, characterized in that the reactive composition (2) is enclosed in the form of a particle shaped material or as an agglomerate in a tube-shaped metal envelope (la, lb, lc, Id, Id'), preferably a tube- shaped envelope of steel. 18. Product according to any of claims 1-16, characterized in that the reactive composition is encapsulated in a tube-shaped metal envelope, preferably a tube-shaped envelope of steel, and that said aluminium exists as a strand (3') in the envelope (la), while the other ingredients (2') of the composition exist as particles or as an agglomerate in the envelope. 19. Product according to any of claims 17 and 18, characterized in that the reactive composition is close-packed in the envelope (Id') through at least one longitudinal indentation (4) of the envelope.

Inventors: SVAERD DANIEL, SVAERD, DANIEL
Applicants: PARTEK NORDKALK AKTIEBOLAG, SVAERD DANIEL, SVAERD, DANIEL
Application year: 1999
Main classification: C21C7
IPC codes: C21C1, C21C7