EP1699940A1

EP1699940A1 - Steel desulphurating agent and use thereof in the desulphuration of steel

Info

Publication number: EP1699940A1
Application number: EP04816601A
Authority: EP
Inventors: François Sorrentino; Michel Gimenez
Original assignee: Lafarge SA
Current assignee: Lafarge SA
Priority date: 2003-12-24
Filing date: 2004-12-23
Publication date: 2006-09-13
Also published as: ZA200605217B; CA2551270A1; FR2864551A1; US20080302210A1; JP2007517137A; FR2864551B1; UA84723C2; RU2006126687A; US7563303B2; BRPI0418087A; WO2005064022A1; US20070144306A1; KR20060127038A; CN1906313A

Abstract

The invention relates to a steel desulphuration agent, characterized in that it comprises at least 10 % SiO2, at least 10 % C2S, at least 35 % of at least one calcium aluminate and, optionally, a calcium silicoaluminate, in relation to the overall weight of the components.

Description

Desulfurization agent for steels and its use for desulfurization of steel

The invention relates to the field of metallurgy and relates in particular to an agent for the desulfurization of steels, comprising a high content of Si0 ₂ , C2S, and of calcium aluminate, and its use for the desulfurization of steels. The production of steel is carried out schematically by two ways: - the transformation of iron ore into steel by means such as blast furnaces or converters, and - the processing of scrap in an electric furnace. It is known that the presence of impurities, phosphorus, sulfur in the steel obtained after refining of the cast iron is particularly harmful to the mechanical properties. It is known that the presence of a high proportion of sulfur in the steel obtained after purification of the cast iron produced by blast furnaces is particularly harmful because the sulfur reduces the cold ductility, the resilience, and the surface quality of the ingots. . The proportions of sulfur that can be tolerated in the metal must be very low, that is to say less than 0.02% or even 0.005%. One of the major stages in the current production of steel is primary metallurgy, by converter or electric furnace, which results in steel which will then be reprocessed in the ladle to give it specific properties. The most notable progress in improving the properties of steel comes from ladle metallurgy. The apparatuses for purifying cast iron and for preparing steel (blast furnaces, converters) make it possible to reduce the sulfur content of the metal in part but do not however lead to total desulfurization which makes it possible to eliminate the abovementioned drawbacks, hence the need to refine the steel. The general principles of ripening can be summarized as described below. To extract the impurities from the steel, it must be put in intimate contact with a product having more affinity for the impurities, therefore having a lower free enthalpy.

This is a thermodynamic equilibrium problem which can be resolved by the use of high temperatures. To lower the content of elements considered harmful to the steel, the essential refining means are: 1 - exchange through a slag 2- the formation of insoluble compounds 3- the decrease in the solubility of impurities in the steel by lowering their partial pressure by subjecting the steel to a vacuum. The chemical reaction leading to the desulfurization of steel is as follows:

[S] _m + (0 ^" ) L -" (S ^~ ) _L + [0] _m ,

in which [S] _m and [O] _m represent the elements dissolved in the metal, and (O ^"" ) ι_ _and (S ^"" ) represent the elements dissolved in the slag. A usual way to lower the content of elements deemed harmful to steel is the use of a slag based on lime. In such a case, the reaction will be as follows:

[S] _m + (CaO) _L ^ (CaS ₂ ) _L + [O] _m

in which [S] _m and [O] _m represent the elements dissolved in the metal, and (CaO) and (CaS ₂ ) L represent the elements dissolved in the slag. As an indication, in% by weight is indicated in Table 1, the usual mineralogical and / or chemical compositions of the slag from steelworks. Table 1

Among the methods commonly used for desulfurization, none is however completely satisfactory. Thus, the use of sodium carbonate leads to a yield of around 60% maximum of the desulphurization and to the emission of harmful fumes, to obtaining particularly aggressive slag The use of calcium carbide leads to recarburization of the metal, moreover, the product must be protected from humidity to avoid the risks of production of acetylene and explosion. The use of calcium cyanamide leads to nitriding and carburization of the metal, which one seeks to avoid. Magnesium is difficult to process because it is vaporized on contact with steel and can lead to explosions, so it must be coated in tar and placed under a bell. The use of silico-calcium, blown into the mass to be purified leads to a globalization of the inclusions, and requires the use of a basic slag and causes nitrogen to be taken up by the steel. The use of lime is advantageous, however its high melting point, around 2200 ° C, prevents the reaction of lime with the liquid metal. Numerous researches have led to consider that a product having good desulfurization qualities could contain 53 to 55% of CaO, 43 to 45% of AI ₂ O ₃ and 1% of FeO. Many products exist around this composition, such as those described in French patent FR2541310, filed on February 18, 1983, or the products available from Wacker or even the dairy products produced by the manufacture of vanadium. These products are however expensive or of limited availability. There is therefore a need for desulphurizing agents, which make it possible to overcome the drawbacks described above, while being less expensive, more available than the compositions of the state of the art, and in particular which could be obtained from industrial residues, in particular slag from steelworks. The above objects are achieved according to the invention, by an agent for ₍ desulfurization of steels comprising, relative to the total weight of the agent: at least 10% of SiO ₂ , at least 10% of C2S, and - at least 35% of at least one calcium aluminate and optionally a calcium silicoaluminate The composition of the desulfurization agent, comprising a high concentration of C2S allows, in addition to obtaining the advantages described above, to obtain an abundance of the desulphurizing agent, and therefore a powder. The desulphurizing agent is preferably in the form of a powder having a specific surface of between 1000 and 5000 cm ² / g, preferably from 1000 to 2000 cm ² / g. Methods for measuring the specific surface of a powder are known to those skilled in the art. cite by way of example the methods based on the physical adsorption of a gas at low temperature, for example the well-known method known as BET. Preferably, the desulfurization agent comprises, relative to the total weight of the agent, the following mineralogical phases: 10 to 60% of C2S, 0 to 50% of C3A, 0 to 50% of C2AS, - 0 to 70% of C12A7, and 0 to 60% of CA, provided that the composition comprises at least 35% of calcium aluminate or of a mixture of calcium aluminate and calcium silicoaluminate. Most preferably, the desulfurization agent comprises, with respect to the total weight of the agent, the following mineralogical phases: 10 to 30% of C2S, 30 to 60% of CA, and 10 to 40% of C2AS; or - 20 to 50% of C2S, 20 to 70% of C12A7 and 0 to 40% of C3A, preferably 10 to 40% of C3A. Preferably, the desulfurization agent is obtained from a steel slag. This embodiment of the invention is particularly advantageous from the economic point of view, because it makes it possible to develop by-products from the steel industry. The desulfurizing agent according to the invention can be obtained by treating a slag from a molten steelworks in a controlled oxidizing atmosphere so as to modify its mineralogical and chemical composition and to remove the impurities so that it can serve as a sponge instead. the mixture of lime and fluxes usually used for refining. In particular, a process for the preparation of the desulfurization agent may consist in producing a mixture of alumina or of products generating alumina and slag from a steelworks, then bring the mixture to a temperature between 1250 ° C and 1450 ° C, under a partial pressure of oxygen, between 10 ^{~ 1} and 10 ^{~ 6} bar. The alumina, or alumina-generating product, can be added to the slag from the molten steelworks. In general, the amount of alumina to be added to obtain the desulfurizing agent from the slag from the steelworks is 10 to 30% relative to the total weight of the slag, depending on the composition of the slag and / or the composition sought for the desulfurizing agent. The addition of alumina or an alumina-generating compound makes the slag more fusible and more suitable for desulfurization. Preferably the source of alumina is chosen from: bauxite, aluminum residues and red mud. The invention also relates to a process for desulfurization of steel comprising the addition to steel, of the desulfurizing agent as described above and of lime (CaO). Preferably, the desulfurizing agent and the lime are mixed together before being added to the steel. Preferably, the weight ratio of the desulfurizing agent to lime varies from 1 / 0.5 to 1/2, and preferably is 1/1. The desulfurization process for steels preferably takes place at a temperature between 1500 ° C and 1600 ° C, and most preferably at 1550 ° C.

Examples Desulfurization agents according to the invention were prepared from the raw materials of mineralogical composition appearing in Table 2. Table 2

The slag and bauxite were mixed at a temperature between 1250 ° C and 1450 ° C, under a partial pressure of oxygen, between 10 ^"1 and 10 ^{" 6} bar, then mixed with lime in the proportions, expressed as a percentage by weight, described in Table 3.

Table 3

The composition in the mineralogical phase of the desulfurization agents obtained from the compositions described in Table 3 is given in Table 4 below.

Table 4

The capacity of the desulfurization agents has been tested in the laboratory. The desulfurization agents were mixed with molten steel in a 1/1 weight proportion. The concentrations (P / P) of sulfur, of the molten steel and of the desulfurization agent were measured by X-ray fluorescence, before and after treatment of the steel with the desulfurization agent. The results are collated in Table 5.

Table 5

The results illustrated in Table 5 show that the sulfur concentration in the molten metal decreases by a factor of 7 after treatment with the desulfurizing agent. These tests clearly confirm the advantages of using the desulfurization agents according to the invention to reduce the sulfur concentration of the molten metal.

Claims

1. Desulfurization agent for steels, characterized in that it comprises, relative to the total weight of the agent: - at least 10% of Si0 ₂ , - at least 10% of C2S, and - at least 35% of '' at least one calcium aluminate and optionally a calcium silicoaluminate.

2. Desulfurization agent for steels according to claim 1, characterized in that it comprises, relative to the total weight of the agent, the following mineralogical phases: 10 to 60% of C2S, 0 to 50% of C3A, - 0 to 50% of C2AS, 0 to 70% of C12A7, and 0 to 60% of CA.

3. Desulfurization agent for steels according to claim 1 or 2, characterized in that it comprises, relative to the total weight of the agent, the following mineralogical phases: 10 to 30% of C2S, 30 to 60% of CA , and 10 to 40% of C2AS; or 20 to 50% of C2S, 20 to 70% of C12A7 and 0 to 40% of C3A, preferably 10 to 40% of C3A.

4. Desulfurization agent according to any one of claims 1 to 3, characterized in that it is obtained from a steel slag.

5. A method for desulfurization of steel, characterized in that it comprises the addition to molten steel, of the desulfurizing agent according to any one of claims 1 to 4 and of lime (CaO).

6. A method for desulfurizing steel according to claim 5, characterized in that the desulfurizing agent and the lime is mixed together before being added to the steel.

7. A method for desulfurization of steel according to claim 5 or 6, characterized in that the weight ratio of the desulfurizing agent to lime varies from 1 / 0.5 to 1/2, and preferably is 1 / 1.