|Title of Invention||
A PROCESS FOR REFINING STEEL IN INDUCTION FURNACE
|Abstract||The present invention provides a novel process wherein the phosphorous is removed in induction furnace during refining of steel, when the furnace is lined with basic/ neutral refractory material.|
|Full Text||FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
THE PATENTS RULES, 2003
(See section 10, rule 13)
"A PROCESS FOR REFINING STEEL IN INDUCTION FURNACE"
ELECTROTHERM (INDIA)LTD, an Indian company, of 72, Palodia, Via Thaltej, Ahmedabad-382 115, India
The following specification particularly describes the invention and the manner in which it is to be performed.
A PROCESS FOR REFINING STEEL IN INDUCTION FURNACE
Field of invention:
The present invention relates to a process for refining steel, more particularly, the present invention relates to a process for refining steel in induction furnace.
Background of the invention:
Usually, in the steel industry, that presence of phosphorus in steel is detrimental in the quality of steel. Phosphorus is not desirable in steel except for certain grades. Therefore, phosphorous has to be eliminated to below a specified amount. Phosphorus is removed from steel by oxidation process as product of oxidation i.e. in the form of held in
combination with basic constituents in the slag generated in the steel refining process. The extent of removal of phosphorus (i.e. dephosphorization) is governed by the equilibrium condition which is characterized not only by metal and slag composition but also by temperature. The operating parameters which affect dephosphorization of steel include temperature, basicity of slag, ferrous oxide activity or iron content of slag, activity, silica content of the slag, slag volume, the following operating parameters are desirable for dephosphorization of steel:
(a) low temperature,
(b) high basicity of slag,
(c) high ferrous oxide activity or iron content of slag,
(d) low activity or phosphorous content of slag,
(e) low silica content of slag, and
(f) high slag volume or double or multiple de-slagging.
The two most important and popular primary steelmaking processes are (a) oxygen steelmaking process and (b) electric arc furnace process.
(a) Oxygen steelmaking process:
This is a process wherein gaseous oxygen is used as the primary agent for oxidizing dissolved impurities like carbon, silicon, manganese and phosphorous, and to a limited extent the oxidizing iron itself.
The process involves injection of oxygen into the bath of the metallic charge through several directions such as top or bottom or both. Such injection of high velocity oxygen directly into the bath enhances the slag-metal-gas reactions at the above mentioned sites. Exothermic oxidation reactions provide heat to the liquid bath, which ultimately increases the temperature, which further enhances the reaction kinetics.
Slag basicity for phosphorous removal is maintained by adding fluxes to the bath. The fluxes may include lime and dolomite in burnt or unburnt condition. Slag is removed from furnace by tilting the vessel of the furnace. Large furnace opening also facilitates easy slag removal.
In oxygen steelmaking processes, vessels are lined with basic refractory materials such as tarred dolomite bricks, dolomite enriched with magnesite, natural magensite etc.
Injection of high velocity oxygen jets in top blowing and combined blowing processes causes a lot of splashing of metal on refractory walls. Many a times metal is also thrown out of the furnace due to splashing. Normally, oxygen steelmaking converters/ furnaces which are less than 30 ton capacity are not economical for operation.
(b) Electric arc furnace steelmaking process:
Steelmaking process is also practised in an electric arc furnace. In electric arc furnace, the heat is generated by arcing between metal and electrodes. Steel making temperatures are maintained by utilizing the power. Phosphorous from the steel is removed in an arc furnace, with basic lining, using highly basic and oxidizing slag. The furnaces are operated at small specific volumes, i.e. a large empty space is available above the liquid bath. Splashing of liquid metal may take place due to carbon boiling and oxygen lancing.
Large empty space above the bath avoids the splashing of liquid steel out side the furnace.
Acid lined furnaces are used in foundries for remelting of the metallic charge. Acid lined furnaces are useful where only remelting of the metallic charge is required without refining. Removal of phosphorous from steel is difficult in such furnaces as basic slag making is difficult.
Apart from the above, induction furnaces in various steel plants are used for melting metallic mass like sponge iron and scrap. In induction furnaces, refractory forms a shape of a crucible, in which metallic mass to be melted is put. The charge supplied in the induction furnace acts as a secondary winding of a transformer and water-cooled copper coil is used as a primary winding. Alternating current is supplied to water-cooled copper coil to melt the charge.
An induction furnace is charged with graded steel scrap for melting. A thin layer of slag is maintained on the surface to prevent oxidation of bath. The induction furnace melting is used only for remelting without any chemical reactions. Therefore, the induction furnace process is, wherein what goes in must come out, as the furnace is used only for remelting and no chemical reaction takes place.
Steelmaking through induction furnace is considered as a secondary route of steelmaking. The furnace size varies from a few kilograms to 30 tons.
Small sizes of the induction furnaces, upto 5 tons are more popularly used in foundries/ laboratories. Some small capacity furnaces are also used for stainless steel melting, alloy steel melting and non-ferrous metal melting using basic lining. Chemistry adjustment is done only by addition of few alloying elements or by dilution of molten metal.
The furnaces greater than 5 ton sizes are popular in India to make steel using sponge iron. Sponge iron in induction furnace is used as a substitute of steel scrap. Usually such
furnaces are provided with acidic lining. Impurities such as phosphorous, sulphur, carbon etc cannot be removed by lancing oxygen in the bath. The induction furnaces have very -small diameter to height ratio and are operated with full volume filled. Lancing of any oxygen in the furnace splashes out liquid metal from the furnace. Splashing of liquid metal outside the furnace leads to decrease in the metallic yield. Hence, practically, no lancing is done in induction furnace for refining.
Inability to remove phosphorous in induction furnace restricts the furnace from making clean steels, alloy steels and special steels.
Accordingly, the present invention provides a process for refining steel using an induction furnace.
Object of the present invention:
The main objective of the present invention is to provide a process for refining steel in induction furnace.
Description of the present invention:
Accordingly, the present invention provides a process for refining steel comprising steps of: (a) providing an induction furnace having basic or neutral refractory lining; (b) providing slag having basicity of the order of 1.5 to 4; and (c) maintaining iron oxide content (FeO) of slag in the induction furnace which raises oxygen potential of the slag.
In the process of refining steel in the present invention the metallic charge having iron oxide (FeO) is supplied in induction furnace which is lined with basic or neutral refractory material lining. The conventional induction furnaces have acidic refractory lining in which basic slag making is not possible and therefore, in the conventional induction furnaces refining of steel is not possible. In the process of the present invention the induction furnace is lined with basic refractory lining material. The furnace -can also be lined with alumina based neutral lining material which allows basic slag making in the
furnace. In the process of the present invention core type or coreless induction furnace can be used.
In the process of the present invention the metallic charge which is supplied in the induction furnace comprises cast iron / pig iron, steel scrap and sponge iron, which contains iron oxide (FeO). Mill scale can also be used as a source of iron oxide (FeO). The process of the present invention utilizes iron oxide (FeO) as primary oxidizing agent and no other source for oxygen has been used for refining.
As discussed earlier removal of phosphorus from steel requires slag basicity of the order of 1.5 to 4 or more. The sponge iron is a main source of silica content (Si02) in the bath. To maintain the basicity of slag, required amount of calcium oxide (CaO) can be supplied in the bath using lime and dolomite. Accordingly, raw or calcined fluxes can be added in the bath of the induction furnace.
Refining of steel by dephosphorisation demands large oxidation potential of the slag. In the process of the present invention this high oxidation potential of slag is achieved by providing high FeO content in the slag. The FeO content of the slag can be maintained by adding sponge iron into the furnace. FeO content can also be maintained by adding mill scale in the liquid steel. The content of FeO in the slag should optimally be in the range of 15% to 40%.
As stated above, iron oxide (FeO) is used as a primary oxidizing agent in the process of the present invention. Phosphorous from the liquid steel is removed into the form of P2O5 as a product of oxidation. is held in the slag which in turn can be removed from the
induction furnace. In addition to phosphorus, carbon, silicon and manganese are also removed. Before addition of sponge iron in liquid bath, if fluxes are added in required quantity and temperature is maintained more than some amount of
desulphurization can also be achieved.
In the process of the present invention the slag/metal temperature has to be controlled because higher temperature of slag reverts back phosphorous into the liquid metal (steel). The slag/metal temperature can be controlled by either controlling furnace power or by addition of cold charge into the furnace during refining process. In a preferred aspect, the bath temperature is maintained in the range of 1450°C to 1700°C.
In the process of the present invention for maintaining the basicity of slag and for sustaining the oxidation, fluxes and sponge iron are added in the bath of the' induction furnace. Addition of fluxes and sponge iron into the bath lead to generation of large slag volumes. The slag thus generated can be removed from the furnace time to time by tilting the furnace. Removal of slag more than once provides better control not only over melting operation but provide better control on refining of liquid steel as well.
As can be observed, from the above description of the process of the invention that oxygen is not used for oxidation of liquid metal. Therefore, in the process of the present invention the problem of splashing of liquid due to oxygen lancing is completely eliminated which provides increased metal yield.
By the above described process of refining steel of the present invention, phosphorus content of the steel can be reduced up to the level of 0.01 %. In an aspect of the present invention by the process of refining steel the phosphorous may be removed from the level 0.15 % in the initial bath to the level less than 0.015% in the final bath or maximum 90% of initial phosphorous level. Not only phosphorus other impurities such as carbon, silicon and manganese can also be effectively removed from steel by the process of the present invention.
Advantages of the present invention can be clearly identified:
(a) Basic/neutral lining of the induction furnace;
(b) Addition of fluxes to maintain basicity of the slag;
(c) Control over slag/metal temperature by addition of cold charge continuously;
(d) High oxygen potential of slag by adding sponge iron into the furnace;
'(e) Phosphorous can be removed without lancing any oxygen in the bath;
(f) Phosphorous to the level of 0.01 % in steel can be obtained;
(g) Carbon, silicon and manganese can be removed to the great extent.
The present invention is described with reference to the figures and specific embodiments; this description is not meant to be construed in a limiting sense. Various alternate embodiments of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such alternative embodiments form part of the present invention.
Furnace size is 750 kg.
Charge requirement is; steel scrap 300 kg, sponge iron 440 kg, cast iron 100 kg, burnt
lime 35 kg and burnt dolomite 30kg. Lime and dolomite are major sources of calcium
oxide, which helps in maintaining the slag basicity. Line the furnace with basic refractory
or neutral refractory. The input metallic charge contains 52% sponge iron. Melting
procedure is as follows:
Melt 100 kg cast iron and 300 kg steel scrap in induction furnace. By controlling
induction furnace power, maintain bath temperature to 1550°C. Initial liquid bath
chemical composition could be as follows,
Silicon - 1.22 %
Manganese - 0.40 %
Add 35 kg burnt lime and 30 kg burnt dolomite in the furnace and melt. Lime and dolomite are added to maintain the basicity of the slag between 1.5 to 4. Add 440 kg of sponge iron in the furnace and melt. During melting of sponge iron the temperature of the
bath may shoot up beyond 1600° C due to carbon boiling. Iron oxide in the sponge iron reacts with carbon, silicon, manganese and phosphorous to form their respective oxides. The oxides form the slag. By controlling the power again, maintain bath temperature to 1550°C. Higher temperature of the slag and liquid metal bath may revert back phosphorous into the metal from slag.. The bath temperature is not allowed to increase beyond 1600 °C.
Sponge iron, lime and dolomite quantity can also be added into the furnace by parts or at once. If the quantity is added in number of batches, slag should be removed from the furnace before adding next batch of sponge iron. Final chemistry of the bath could be as follows,
Carbon - 0.03 %
Sulphur - 0.036 %
Manganese - 0.06 %
After finishing additions, the slag is removed and temperature of metal bath is increased to the required level for tapping.
1. A process for refining steel in induction furnace comprising steps of: (a) providing an induction furnace having basic or neutral refractory lining; (b) providing slag having basicity of the order of 1.5 to 4; and (c) maintaining iron oxide content (FeO) of slag in the induction furnace for continuous oxidation and removal of phosphorus from slag.
2. A process as claimed in claim 1, wherein the induction furnace is provided with alumina based lining.
3. A process as claimed in any one of the preceding claims, wherein the furnace is provided with magnesia based basic lining material.
4. A process as claimed in any of the preceding claims, wherein sponge iron or mill scale is added to liquid steel.
5. A method according to claim 1 and 2, wherein phosphorous is removed without lancing any oxygen in the bath
6. A process as claimed in any of the preceding claims, wherein the iron oxide (FeO) content of the slag is between 15% to 40%.
7. A process as claimed in any of the preceding claims, wherein temperature of the furnace is maintained at a range of 1450°C to 1700°C.
8. A process as claimed in any of the preceding claims, wherein fluxes, raw or calcined are added to the slag.
9. A process as claimed in any of the preceding claims, wherein the induction furnace is a core type or coreless induction furnace.
10. A process for refining steel in induction furnace substantially as herein described with reference to the foregoing example.
Dated this 1 lin day of June 2008
OF K & S PARTNERS
AGENT FOR THE APPLICANT(S)
|Indian Patent Application Number||1242/MUM/2008|
|PG Journal Number||18/2012|
|Date of Filing||12-Jun-2008|
|Name of Patentee||ELECTROTHERM (INDIA) LTD.|
|Applicant Address||72, PALODIA, VIA THALTEJ, AHMEDABAD|
|PCT International Classification Number||C21C7/00|
|PCT International Application Number||N/A|
|PCT International Filing date|