Secondary Steelmaking 1 Synthetic slag practice, injection ladle metallurgy, deoxidation

Transcription

1 17 Secondary Steelmaking 1 Synthetic slag practice, injection ladle metallurgy, deoxidation Topics to discuss... Secondary steelmaking Synthetic slag practice Injection ladle metallurgy Deoxidation

2 Secondary Steelmaking Primary steelmaking is aimed at fast scrap melting and rapid refining. capable at a micro level to arrive at broad steel specification but not designed to meet the stringent demands on steel quality, and consistency of composition and temperature that is required for very sophisticated grades of steel To achieve such requirements, liquid steel from primary steelmaking units has to be further refined after tapping. This is known as secondary steelmaking and has become an integral feature of virtually all modern steel plants. The advent of continuous casting process, which requires stringent quality control is one of the main reasons for the growth of secondary steelmaking. 3/22 Intended Operations in Secondary Steelmaking Removal of harmful impurities (S, P, O, N, H, etc.) and non-metallic inclusions (containing O and S) as they are responsible for loss of ductility, impact strength and corrosion resistance properties in steel. Production of special sophisticated grades of steel, like interstitial-free (IF) steels, where carbon is considered as an impurity and has to be removed to very low levels (ultra-low C steels). Provisions for heating and temperature adjustment to compensate the following heat losses: tapping from primary steelmaking furnaces C secondary steelmaking C continuous casting (pouring into tundish) C 4/22

3 Table: Various secondary steelmaking processes and their capabilities LF ladle furnace VAD vacuum arc degasser VD vacuum degassing VOD vacuum-oxygen decarburisation IGP inert gas purging in a ladle through bottom porous plugs; or, by lance immersed from the top, i.e. Overhead Lance Purging (OLP) IM injection metallurgy, where some solid agents are injected into liquid steel in a ladle; or, nowadays also by wire feeding. 5/22 Evolution of Ladle Treatment In steelmaking, ladles are principally employed to transfer molten steel from BOF/EAF to ingot casting or continuous casting. It has been realized that ladles can be used very effectively as a reactor which can perform any of the following functions: To desulphurize molten steel tapped from BOF/EAF To homogenize molten steel to minimize gradients in concentration and temperature and to attain desired teeming temperature. To deoxidize molten steel To improve cleanliness of steel by removing inclusions To add alloying elements To remove dissolved gases The effectiveness of each of the function requires modifying the ladle in terms of molten steel flow, and extra heating facility etc. 6/22

4 Ladle is a cylindrical refractory-lined vessel with D/H aspect ratio ~ , indicating that the bath is deep. Bath agitation would be required to carry-out the functions effectively. At high temperature, bath can be agitated either by an inert gas or by induction. The gas can either be injected through the nozzle or porous plugs. Enough height also needed to accommodate the quantity of slag required for refining and to absorb inclusions. Additional heating may be required to keep the molten steel to the teeming temperature. This can be achieved either by tapping steel at slightly higher temperature or to provide addition heating arrangement in the ladle itself. Provision for injecting the slag forming materials is required either for refining or for inclusion engineering. Ladle furnace The most important of all, selection of refractory to meet the refining requirements and for injection elements and their fixing. Bath Stirring Bath agitation would be required to homogenize bath composition and temperature. At high temperature, bath can be agitated either by an inert gas or by induction. One has to determine the amount of stirring gas and location of the injection of gas in the ladle. Argon is usually bubbled into the molten steel covered with slag either through the top lance or through a porous plug fitted at the bottom. An asymmetrically placed bubble plume gives velocities near the bottom which are greater than for symmetrically placed nozzle. 8/22

5 Synthetic Slag Practice Synthetic slag practice is principally employed to obtain clean steels and to desulphurise molten steel. Synthetic slag practice is adopted to meet the following objectives: i) To cover molten steel for cutting down heat losses. ii) To avoid reoxidation of steel from atmospheric oxygen because the molten steel transfer operations are done under atmospheric condition. iii) To remove inclusions from molten steel. iv) Using slag of desired basicity and sulphide capacity, deoxidized steel can be desulphurised to as low as 0.005% v) Synthetic slag practice is attractive due to low capital cost on equipment. 9/22 Design of synthetic slag The principle component of synthetic slag is lime. Calcium fluoride increases the sulphide capacity of slag and helps fluidizing the slag. Often Al is present to deoxidize the molten steel since transfer of sulphur from molten steel to slag is followed by transfer of oxygen from slag to steel. Therefore deoxidation of steel is must for efficient desulphurization. Typically, slag contains: CaO % Ca F % Al 5 16 % SiO 2 0 5% This slag is pre-fused in solid state. Alternative synthetic slag A pre-melted slag based on CaO and Al 2 O 3 with small amount of CaF 2 can ease the problem of refractory wear and hydrogen pick. Composition of CaO and Al 2 O 3 can be selected so as to melt at C. A synthetic slag consisting of 70% (1:1 CaO and Al 2 O 3 ), 25% CaO and 5% CaF 2 could be used. Special synthetic slag can also be designed for a specific purpose. For removal of oxide inclusions, for example, a neutral slag with CaO/SiO 2 = 1 or 1.2 can be used, when no desulphurization is needed. 10/22

6 Injection Ladle Metallurgy This consists of purging molten steel by argon introduced from bottom through porous bricks or slit plugs, fitted at the ladle bottom. Nowadays, purging by argon through a top lance is also practiced. Injection techniques have the advantages of dispersing the reactants in the steel bath and at the same time provide a large reaction surface area. The type of powders used is governed by the purpose of injection. Purpose Dephosphorization Desulphurization Type of powder CaO + CaF 2 + Fe 2 O 3 + mill scale soda CaO + Al CaO + CaF 2 + Al CaC 2 Mg + (MgO, Al 2 O 3, chloride slag) CaC 2 + CaCO 3, CaO Alloying FeSi, CaCN 2, NiO, MoO 2 FeB, FeTi etc Deoxidation and inclusion shape control Al, Ca Si, Ca, Si, Mn, Al, and Ba 11/22 Argon + desulphurisation powder Lance Fume Cored Wire Injection Powder Injection Powder Injection 12/22

7 Alloying with gas injection Alloying can be done during tapping by simply dropping the material on the surface, or with a carrier gas. The dissolution and homogenization of the alloying additions are enhanced by stirring and small particle size. Stirring intermixes top slag with the bath, which should be minimized to avoid oxidation. 13/22 Heating of steel Synthetic slag practice with argon stirring or injection of solid powder requires higher tap temperatures to compensate for the heat losses during refining. But an increased tap temperature causes increased lining wear and poor phosphorus removal at the BOF. increased power and electrode consumption and furnace time in EAF. Arc heated ladle processes developed to allow lower temperatures tapping at BOF/EAF, and to perform the following bath homogenization by argon stirring inclusion removal and inclusion engineering desulphurization by synthetic slag or by injection metallurgy holding of ladles for long periods if and when need arises for example in sequence casting. ability to make addition of alloying elements. Different types of ladle arc heated furnaces: Induction stirred, gas stirred by porous plugs, and gas stirred using a tuyere. 14/22

8 Deoxidation of Steel Refining of hot metal to steel is done under oxidizing atmosphere and oxygen dissolves in steel during refining. Sulphur removal needs a reducing atmosphere, during which oxides are reduced and oxygen is reintroduced into the bath. Oxygen has negligibly small solubility in steel (0.23% at 1600 C, 0.003% at RT) the excess oxygen is rejected by the solidifying steel, which produces defects like blow holes and non-metallic oxide inclusion in solidified casting these defects have considerable harmful effect on mechanical properties of steel Therefore, deoxidation of steel is necessary remove this excess oxygen 15/22 Sources of oxygen in steel Rust on steel Oxygen blowing Steelmaking slag Atmospheric oxygen dissolved in steel during teeming Oxidizing refractories Steel classification according to the degree of deoxidation i. Killed steel: Oxygen is removed completely. Solidification of such steels does not give gas porosity (blow holes). ii. Semi-killed steel: Incompletely deoxidized steels containing some amount of oxygen which forms CO during solidification. iii. Rimming steel: Partially deoxidized or non-deoxidized low carbon steels evolving sufficient CO during solidification. These steels have good surface finish. 16/22

9 Deoxidation practices Simple deoxidation - carried out by single element such as Si, Al, Mn, etc. Complex deoxidation - carried out by mixture of elements such as Si+Mn, Ca+Si+Al, etc. Vacuum deoxidation - elements are added in vacuum in the form of ferroalloys such as FeSi, FeMn, FeSi + FeMn, etc. Deoxidation can be carried out during tapping, in the ladle, in the ladle runner and even in the mould. 17/22 Simple deoxidation can be represented by a[m] + b[o] = (M a O b ) M = deoxidiser M a O b = deoxidation product K = a Ma O b h M a h O b M a [O] b = 1 K log K M = x T + y K M = deoxidation constant X and Y = constants = K M 1 M a [O] b increase in T increases K and the deoxidation process using these equations, one can calculate the variation of [O] with [M] when [M] is small. for larger [M], interaction parameters need to be considered. All oxides are solids, except for Mn, which can be either solid (MnO) or liquid (FeO.MnO). Complex deoxidation with Mn has the following advantages over simple one: higher degree of deoxidation. due to the formation of liquid deoxidation product, agglomeration of the product into large size can be obtained easily and can be floated easily. 18/22

10 [O] b K M = [M] a [O] in steel depends on K M Deoxidation will be effective if K M is 1600 C, [Si] + 2[O] = (SiO 2 ) ; K M = 2.40 x [Al] + 3[O] = (Al 2 O 3 ) ; K M = 3.20 x [Ca] + [O] = (CaO) ; K M = 9.84 x Ca is the most efficient deoxidizer and Si is not so efficient as compared to calcium. Though calcium and aluminum are very efficient deoxidizers, but they oxidize very fast and moreover, their density is much lower than steel. Also Ca has a boiling point 1485 C which means calcium is gaseous phase at the steelmaking temperature. Suitable injection methods or addition methods are to be developed. Aluminum is also a strong deoxidizing element when compared with silicon. 19/22 Kinetics of Deoxidation Total oxygen in steel equals to dissolved oxygen + oxygen present in deoxidation products (SiO 2, Al 2 O 3, MnO, etc). Even if the dissolved oxygen is low, deoxidation products (also called inclusions) have to be removed, otherwise steel is not clean. Kinetics of inclusion is concerned with deoxidation reaction and separation of deoxidation products as well. The kinetics of deoxidation process consists of the following issues: (a) Dissolution and homogenization of deoxidizer. depends on melting point ferro alloys melt at around 1500 C. Al is expected to melt faster (low m.p.) intensity of agitation will govern the homogenization of deoxidizer in steel melt for faster kinetics of reaction between oxygen and deoxidizer. 20/22

11 (b) Nucleation of solid product becomes easier if interface is present. deoxidation by Al produces solid Al 2 O 3 and as such Al 2 O 3 /steel interface is useful for nucleation. (c) Growth of the deoxidation product depends on the state of the product liquid products coalesce easily as compared with solid products deoxidation with Al, Si etc. produce solid deoxidation product at the steelmaking temp.. deoxidation with FeSi + FeMn produces liquid deoxidation product. Boron, titanium zirconium are also quite effective deoxidizers. manganese and silicon are used in the ratio 7:1 to 4:1 in order to obtain a thin liquid slag. 21/22 (d) Removal of deoxidation product achieved by floatation and absorption into a slag deoxidation products are lighter than steel; hence they move up. the rising velosity depends on physical properties of steel (density, viscosity) and the size of the product (as per Stokes law) degree of stirring in the melt is important design and use of synthetic slag to absorb the deoxidation product Stokes law: v t = g d2 Δρ 18η V t = terminal velocity (m/s) of the inclusion g = acceleration due to gravity (m/s 2 ) Δρ= difference in density of steel and deoxidation product η = viscosity of steel (kg/m.s) 22/22