AU2003273150A1

AU2003273150A1 - Metallurgical treatment method on a metal bath

Info

Publication number: AU2003273150A1
Application number: AU2003273150A
Authority: AU
Inventors: Paul Berg; Emile Lonardi; Jean-Luc Roth
Original assignee: Paul Wurth SA
Current assignee: Paul Wurth SA
Priority date: 2002-05-24
Filing date: 2003-05-21
Publication date: 2003-12-12
Anticipated expiration: 2023-05-21
Also published as: CN1665942B; AU2003273150B2; RU2289630C2; LU90924B1; DE60308860D1; BR0311295B1; RU2004138075A; CN1665942A; EP1507876A2; EP1507876B1; JP4405381B2; WO2003100102A3; CA2485633A1; WO2003100102A2; BR0311295A; JP2005531687A; KR20040106583A; ES2273014T3; DE60308860T2; KR100926321B1

Description

CERTIFICATE IN THE MATTER OF: Australian Designation of International Patent 5 Application Number PCTIEP0O3/50183 in the name of PAUL WURTH S.A. - "Metallurgical treatment method on a metal bath" I, Armand Schmitt 10 of OFFICE ERNEST T. FREYLINGER S.A., B.P. 48, 234, route d'Arlon, L-8001 Strassen (Luxembourg) 15 do hereby certify as follows : 1. That I am fully conversant with the English and French languages; 2. That the annexed document is to the best of knowledge and belief the true and correct translation of International Patent Application Number PCT/EPO3/50183; and 20 3. That the aforesaid translation of this Patent Application was verified by me Declared at Strassen This 9 h day of November 2004 25 P-PWU-480/WO Method for metallurgical treatments on molten metal bath Technical field to which the invention relates 5 The present invention relates in general to a method for metallurgical treatments on molten metal bath. It relates more particularly to such a method that comprises a first treatment involving the presence or the formation of an acidic slag on the surface of a molten metal bath and a second treatment involving the presence or the formation of a basic slag on the surface of this 10 molten metal bath. Prior Art A method of this type is, for example, a method for treating raw steel in a 15 ladle in which the molten steel bath is first heated by an alumino-thermal process before carrying out a desulfurization treatment (that is, a treatment to lower the sulfur content) and/or a dephosphorization treatment (that is, a treatment to lower the phosphorus content). During the alumino-thermal process heating, aluminum is reacted with oxygen, forming an acidicAl 2 0 3 slag 20 on the surface of the molten steel bath. In fact, the desulfurization treatment, respectively the dephosphorization treatment, which requires a basic slag on the surface of the molten steel bath, is inhibited by the presence of an acidic AI20 3 slag on the surface of the molten steel bath. In consequence, the acidic A1 2 0 3 slag must first be skimmed (slagged) before starting the desulfurization 25 and/or dephosphorization treatment. In fact, such intermediate slagging substantially lengthens the total time of the treatment and is not feasible in every metallurgical treatment stand. To increase the efficiency of alumino-thermal process heating of a 30 molten metal bath in a ladle, it is known how to carry out this heating under a bell (c.f. for example US-A-4518422). By injecting an inert gas, a "window" is first formed in a layer of initial slag covering the molten metal bath. The bell is 2 then lowered above this "window" until its bottom edge is immersed in the molten metal bath. The alumino-thermal process reactants, that is aluminum and oxygen, are added under this bell. The molten metal bath is simultaneously agitated by injecting an inert gas. It is preferable for the bell to allow the 5 alumino-thermal process heating under a protected atmosphere and with a minimum of losses to the environment. When the alumino-thermal process heating is terminated, the bell is removed. The slag around the bell is mixed with the A1 2 0 3 slag formed under the bell, yielding a slag of which the A20 3 content (>40%) inhibits a subsequent desulfurization and/or dephosphorization 10 treatment. Another method of the type described in the preamble is a method in which a molten raw cast iron bath or a molten ferroalloy bath must undergo both desiliconizing by oxygen injection (that is, a treatment to lower the silicon 15 content) and desulfurization and/or dephosphorization. Desiliconizing by oxygen injection produces an acidic SiO 2 slag on the surface of the molten metal bath. In fact, the subsequent desulfurization treatment requires the presence of a basic slag on the surface of the molten steel bath and is inhibited by an SiO 2 content above 10%. It follows that the acidic slag formed during 20 desiliconizing must be skimmed (slagged) before beginning the desulfurization treatment. As already explained, this intermediate slagging substantially lengthens the time of the method and is not feasible in every metallurgical treatment stand. 25 Object of the invention The object of the present invention is to optimize a metallurgical method in which a first treatment involves the presence or the formation of an acidic slag on the surface of a molten metal bath and a second treatment involves the 30 presence or the formation of a basic slag on the surface of this molten metal bath.

3 Description of the invention According to the invention, this object is achieved by carrying out the two treatments without intermediate slagging, simultaneously or successively in two 5 separate zones, and by providing a physical separation at the surface of the molten metal bath between an acidic slag zone and a basic slag zone. To save the maximum of time, the two treatments should preferably take place simultaneously. In certain cases, it may nonetheless be advantageous to first terminate or start the first treatment before starting the second treatment or vice 10 versa. In any case, it is always advantageous to save the time necessary for intermediate slagging and the two treatments can be carried out in a single metallurgical treatment stand, which is not necessarily equipped for carrying out a slagging operation (the slagging can be carried out elsewhere). 15 In a preferred execution, one of the two treatments is carried out under a deep bell the bottom edge of which is immersed in the molten metal bath and the other treatment is carried out around this deep bell. This deep bell provides the physical separation between the two slag zones at the surface of the bath, while allowing one of the two treatments under a protected atmosphere, with a 20 minimum of losses to the environment. If it is unnecessary to exploit these additional advantages of a deep bell, it is nonetheless also possible to use a single separating wall to provide a physical separation at the surface of the molten metal bath between an acidic slag zone and a basic slag zone. This separating wall can either cooperate with the edges of a metallurgical 25 receptacle to divide the surface of the molten metal bath into two juxtaposed zones, or form a sort of ring to delimit an "island" in the interior of the surface of the molten metal bath. The first treatment is, for example, a chemical heating which is carried 30 out under a deep bell under a protected atmosphere and which produces an acidic slag under this bell. Chemical heating here means a highly exothermic oxidation of a generally metallic element, such as for example aluminum 4 (alumino-thermal process) or silicon (silico-thermal process). The first treatment can also be a desiliconizing treatment by oxygen injection, particularly as part of a treatment of cast iron or ferroalloys (like 5 ferronickel for example) with high silicon content. This desiliconizing treatment by oxygen injection is also advantageously carried out under a deep bell the bottom edge of which is immersed in the molten metal bath. The second treatment is, for example, a desulfurization and/or 10 dephosphorization treatment involving a basic slag, formed for example by adding lime, sodium carbonate, magnesium carbonate, etc. This treatment can be carried out around the deep bell under which the first treatment is carried out. 15 As part of a desiliconizing treatment by oxygen injection, the desulfurization and/or dephosphorization treatment advantageously comprises the addition of limestone, particularly of castine, to the molten metal bath. This is a cheap and very effective desulfurizing agent, but its decomposition in the molten metal bath gives rise to a highly endothermic reaction, which tends to 20 cool the molten metal bath. In fact, in combination with desiliconizing by oxygen injection, this cooling effect hardly causes any problem, because the desiliconizing reaction, which is highly exothermic, produces an excess of heat in any case. 25 If a deep bell is used to provide the physical separation at the surface of the molten metal bath between an acidic slag zone and a basic slag zone, the method proceeds advantageously as follows: an inert gas is first injected to form a "window" in a layer of initial slag covering the surface of the molten metal bath; this "window" is covered using a deep bell the bottom edge of which is 30 immersed in the molten metal bath; one of the two treatments is carried out under the deep bell and the other around the deep bell, while simultaneously agitating the molten metal bath by injection of an inert gas; and at the end of the 5 two treatments, the agitation is stopped, the deep bell is removed, and the two slags are then immediately skimmed (slagged). The termination of the agitation before removal of the deep bell serves to prevent any excessive mixing of the two slags, which could be detrimental to the result of the method. 5 Other features and characteristics of the method according to the invention will appear from the few examples presented below by way of illustration, with reference also to Figure 1 appended hereto, which shows a schematic illustration of the putting into practice of a method according to the 10 invention. Detailed description of some advantageous embodiments of the invention Figure 1 is used to describe in greater detail, by way of illustration of the 15 present invention, a metallurgical method which comprises a ladle desulfurization treatment of a molten raw steel bath, preceded by a ladle chemical heating of this molten steel bath. Figure 1 shows a metallurgical ladle 10 in a metallurgical treatment stand 20 during the putting into practice of the aforementioned method. In the initial state, this ladle 10 contained a molten bath 12 of raw steel from the converter or from the electric furnace, as well as a layer of residual basic slag covering the molten steel bath. In the metallurgical treatment stand, an inert gas has first been injected to form a window 14 in the layer of residual slag, that is, an area of the 25 surface of the molten steel bath 12 that has been at least partially liberated from the residual slag that covered it. Above this window 14, a deep bell 16 has then been positioned, so that its bottom edge 18 is immersed in the molten metal bath 12 to at least 20 cm (the more the splashing of the molten metal bath 12, the greater the immersion depth of the bottom edge of the bell 16). It should be 30 observed that one possible execution of such a deep bell 16 is, for example, described in patent application WO 98/31841, although the bell used in the present method need not necessarily be a rotating bell.

6 Under the bell 16, the molten steel bath is heated by an alumino-thermal process. For this purpose, aluminum is added and oxygen is blown under the bell 16, as shown schematically by the arrows 18 and 20. Simultaneously, the molten metal bath 12 is agitated by means of an inert gas, which is injected, 5 preferably using a side nozzle 22, into the molten metal bath 12. The aluminum reacts in a highly exothermic reaction with oxygen. This reaction results in the formation of an acidic AI 2 0 3 slag under the bell 16. In Figure 1, this acidic AJ 2 0 3 slag is identified by the numeral 24. 10 In the prior art, the bell 16 was lifted at the end of the chemical heating to skim the residual slag highly contaminated with the AI 2 0 3 slag formed under the bell 16. The desulfurization treatment was then carried out on the molten steel bath liberated of slag. In fact, it is well known that, in order to carry out a desulfurization and/or dephosphorization treatment using a basic slag, the A1 2 0 3 15 content of this slag must be lower than 40%. According to the present invention, the desulfurization and/or dephosphorization treatment is carried out around the bell 16 without an intermediate slagging operation. For this purpose, a nozzle 26 is used to inject 20 an agent for the formation of a basic slag 28 in the molten metal bath 12 around the bell 16. This basic slag 28 formation agent can, for example, be lime, limestone, castine, sodium carbonate, magnesium carbonate, etc. The bell 16 prevents the acidic A1 2 0 3 slag formed under the bell 16 from mixing with the basic slag surrounding the ladle 16, making it possible to carry out the two 25 treatments simultaneously or successively without intermediate slagging. It is preferable first to begin the alumino-thermal process heating and then start the desulfurization and/or dephosphorization treatment when the molten steel bath has reached a sufficient temperature. At the end of the desulfurization and/or dephosphorization treatment, all 30 agitation of the molten metal bath 12 is stopped before lifting the bell 16. The two slags are then skimmed together. It should be noted that the treatment carried out under the bell could, for 7 example, also be a desiliconizing treatment of cast iron or ferroalloys, particularly of ferronickel, by oxygen injection. In this case, the silicon reacts with the oxygen blown under the bell to form an acidic SiO 2 slag under the bell. A desulfurization and/or dephosphorization treatment as described above can 5 then be carried out around the bell. The bell prevents the acidic SiO2 slag formed under the bell 16 from mixing with the basic slag surrounding the ladle 16, making it possible to carry out both treatments simultaneously or successively without intermediate slagging. In fact, for an effective desulfurization and/or dephosphorization treatment, the SiO 2 content of the 10 basic slag must not be above 10%. Example 1 This example relates to a ladle treatment of raw converter steel with the aim of 80% desulfurization of this steel. 15 Initial state: A metallurgical ladle contains 160 t of raw converter steel and 600 kg of residual fining slag. The analytical results are as follows: 0.04% C, 600 ppm O, 0.010% S. The temperature of the molten steel bath is 16000C. 200 kg of deoxidation Al and 600 kg of CaO have been added at the time of pouring. 20 Alumino-thermal process heating: The first treatment is an alumino thermal process heating which takes place, as described in relation to Figure 1, under a deep bell positioned above a zone of the molten steel bath that is previously liberated of its layer of residual slag. The temperature of the molten steel bath is increased by about 90oC by the injection of 530 kg of aluminum 25 and 350 m 3 of oxygen in 7 minutes (at the rate of 50 m 3 /min of 02). The agitation under the belt is caused by argon injection using a side nozzle at an injection rate of 0.2 m 3 /min. Desulfurization: The second treatment is an intense 80% desulfurization which takes place around the bell. The desulfurizing agent used is a powder 30 consisting of 60% CaO and 35% A1 2 0 3 , the remainder being impurities. The addition of A120 3 is intended to adjust the fluidity of the slag obtained. Other slag agents can also be added.

8 The desulfurizing agent is injected with the help of a submerged-head nozzle using argon as carrier gas. Before beginning the injection of the desulfurizing agent, the injection nozzle is used to carry out a prior agitation of the molten steel bath. For this purpose, the injection nozzle is supplied for five 5 minutes with argon at a rate of about 0.5 m 3 /min, the supply of desulfurizing agent being cut off. This preliminary agitation makes it possible in particular to homogenize the temperature of the molten steel bath before its desulfurization. Subsequently, within a time interval of about 12 minutes, 960 kg of the aforementioned desulfurizing agent are injected (solid feed rate about 10 80 kg/min) with a flow rate of about 1 m 3 /min of argon as carrier gas. The treatment is terminated by carrying out an intense agitation with the same nozzle for 5 minutes, with a rate of about 1 m 3 /min of argon, the supply of desuffurizing agent again being cut off. The agitation is then stopped and the bell is lifted. 15 Final state: Steel: 0.04% C, 0.002% S, temperature about: 1600 0 C. Slag: about 1000 kg of A1 2 0 3 formed under the bell plus about 2500 kg of desulfurization slag around the bell. Comment: 20 If it is only required to achieve a moderate desulfurization of the steel, it may be unnecessary to inject a desulfurizing agent into the bath using a nozzle. In fact, the residual slag around the bell may already contain a sufficient quantity of desulfurizing agents to achieve a moderate desulfurization of the steel. It then suffices to agitate the molten steel bath around the bell to make it 25 react with the residual slag floating on its surface and, if necessary, to further add slag agents to adjust the consistency of the slag in particular. Example 2 This example relates to a ladle treatment of raw cast iron with the aim of desiliconizing and desulfurization of the cast iron. 30 Initial state: A metallurgical ladle contains 100 t of raw cast iron of which the analytical results are as follows: 4.5% C, 0.8% Si, 0.10% S. The temperature of the molten cast iron bath is 1350 0 C. The cast iron is covered 9 with a layer of residual basic slag. Desiliconizinq treatment: A desiliconizing treatment by oxygen injection is carried out, as described above, under a deep bell positioned above a zone of the bath previously liberated from its layer of residual slag. 450 m 3 of oxygen 5 is injected under the bell in 10 minutes (at the rate of 45 m 3 /min of 02). The agitation under the bell takes place by argon injection using a side nozzle at a rate of 0.2 m 3 /min, Desulfurization: The desulfurization takes place around the bell. The desulfurizing agent used is a powder consisting of 70% CaCO 3 and 30% 10 Na 2

CO

3 . Other slag agents can also be added. The desulfurizing agent is injected with the help of a submerged nozzle using argon as carrier gas. About 1000 kg of aforementioned desulfurizing agent are injected in a period of about 20 minutes (solid flow rate about 50 kg/ min) with about 1 m3/min of argon as carrier gas. After having stopped all 15 agitation, the bell can be lifted and the two slags skimmed together. Final state: Pre-treated cast iron: 4.3% C, 0.4% Si, 0.02% S, temperature about: 1400CC. Slag: about 860 kg of SiO 2 formed under the bell, plus about 700 kg of 20 desulfurization slag around the bell. Comment on the subect of the cast iron treatment: In a conventional once-through cast iron desulfurization, a mixture of Mg CaC 2 or Mg-CaO is normally used as desulfurizing agent. These are highly effective desulfurizing agents but are also very expensive. They are mainly 25 used because they cause limited cooling of the molten metal bath. In fact, the combination of desulfurization with highly exothermic desiliconizing makes it possible to use a more cooling but less expensive desulfurizing agent such as, for example, limestone (CaCO 3 ) or castine. The decomposition of CaCO 3 or NazCO 3 in the molten steel bath also generates oxygen, which contributes to 30 the desiliconizing of the cast iron (1 kg CaCO 3 or Na 2 CO3 reduces the desiliconizing oxygen demand by about 0.1 m 3 ). Moreover, it is preferable to use a mixture of CaCO 3 + Na 2

CO

3 to obtain a more fluid slag and thereby to 10 limit losses by iron entrainment during slagging. However, the use of Na 2

CO

3 also requires limiting the temperature to 1400 0 C to prevent the loss of Na 2

CO

3 by vaporization. Comments on the subject of a ferroalloy treatment: 5 It is advantageous for a molten ferroalloy bath, particularly a molten ferronickel bath, also to be subjected to combined desiliconizing and desulfurization treatment as presented in example 2 for cast iron. However, in the case of ferronickel, taken by way of example, the aim is generally to achieve a much more intense desiliconizing (to lower the Si content 10 by more than 1%). Siliconizing by a jet of oxygen gas, in the absence of an effective cooling agent, would cause the temperature to rise by 3000C or more. As done in certain cast iron desiliconizing processes, an iron ore or oxide can be used as a cooling agent, obtained as a by-product of steel manufacture. However, with the proposed method, which combines desiliconizing with 15 desulfurization, it is particularly advantageous to use castine (CaCO3) and/or sodium carbonate (Na 2

CO

3 ) as desulfurizing agents, because these products are both strong cooling agents and effective desulfurizing agents, provided they are not diluted by an addition of silica (SiO 2 ). Apart from the quantitative aspect (lowering the Si content by 1 to 2% 20 instead of 0.2 to 0.4% for blast furnace cast irons), the proposed method for cast iron is similarly applicable to ferroalloys, by adjusting the proportions of oxygen and of cooling/desulfurizing agent as required.

Claims

1. A method for metallurgical treatment on molten metal bath comprising: a first treatment involving the presence or the formation of an acidic slag 5 on the surface of said molten metal bath; and a second treatment involving the presence or the formation of a basic slag on the surface of said molten metal bath; characterized in that the two treatments are carried out without intermediate slagging, simultaneously or successively in two separate 10 zones, and by providing a physical separation at the surface of said molten metal bath between an acidic slag zone and a basic slag zone.

2. The method as claimed in claim 1, in which one of the two treatments is carried out under a deep bell the bottom edge of which is immersed in 15 said molten metal bath and the other treatment is carried out around said deep bell.

3. The method as claimed in claim 2, wherein said first treatment is a chemical heating treatment which is carried out under said deep bell. 20

4. The method as claimed in claim 3, wherein said chemical heating treatment is an alumino-thermal or silico-thermal process.

5. The method as claimed in any one of claims 1 to 4, wherein said second 25 treatment is a desulfurization andlor dephosphorization treatment based on a basic slag.

6. The method as claimed in claim 1, wherein said first treatment is a treatment for desiliconizing cast iron or ferroalloys, particularly 30 ferronickel, by oxygen injection.

7. The method as claimed in claim 6, wherein said desiliconizing treatment 12 by oxygen injection is carried out under a deep bell the bottom edge of which is immersed in said molten metal bath and said second treatment is a desulfurization and/or dephosphorization treatment carried out around said deep bell. 5

8. The method as claimed in either one of claims 6 or 7, wherein said second treatment is a desulfurization and/or dephosphorization treatment based on lime. 10

9. The method as claimed in claim 8, wherein said second treatment comprises the addition of limestone, particularly of castine, to said molten metal bath.

10.The method as claimed in any one of claims 1 to 9, wherein: 15 at the start of said method, the surface of said molten metal bath is covered with a layer of residual slag; a window is formed in said layer of residual slag by the injection of an inert gas; 20 said window is covered using a deep bell the bottom edge of which is immersed in said molten metal bath; one of the two treatments is carried out under said deep bell and the other around said deep bell, while simultaneously agitating the molten metal bath by injection of an inert gas; and 25 at the end of the two treatments, said agitation is stopped, said deep bell is removed, and the two slags are then immediately skimmed.