Steel Making Refractory Essay
Ms Sanghamitra, R&D Refractory Department from TATA STEEL, Jamshedpur, gave a talk on “STEEL MAKING REFRACTORY”, on 12th September 2012 at 11:00 AM at Ceramic Engineering department. Process chart of TSL, INDIA was first addressed. So, it all starts with iron ores and fluxes (limestone) to be mixed and then pelletized in a sinter plant. At the same time, raw coal is processed to form coke in a coke plant and then sintered pellets along with coke are fed to a blast furnace. Hot metal is obtained at the bottom of the blast furnace above which slag floats, both of them are tapped out.
Hot Iron metal is processed in LD1, LD2 and LD3 (Ladles) to lower down the carbon content to form steel, this constitutes the steel making step and the technique is “BASIC OXYGEN STEEL MAKING”. Further, rolling is performed in Merchant mill, New Bar mill, Wire mill, hot strip mill and Cold rolling mill to produce various products- long product and flat product. There are three major steps/processes involved in steel making: – 1) Primary steelmaking which involves desulphurization, decarburization and de-phosphorization 2) Secondary Steelmaking involving on-line purging of oxygen and processing in Ladle furnace and 3) Continuous casting.
All these processes demand specific refractory properties of the containers where various refractories are used. Area of the refractory application is in transfer ladle, basic oxygen furnace/ ladle, steel ladle and tundish. Transfer ladle is required when hot metal is moved from bottom of blast furnace to other processing units like converter. Refractories essential for transfer ladle are used in four areas- backup lining, barrel, intermediate and bottom. Backup lining needs to be highly insulating so a porous alumina (70% alumina) LCC is employed.
Lining which is going to face the hot metal should be acidic to avoid any reaction/corrosion, so Al2O3-SiC-C bricks are used. Intermediate lining is made up of 80% alumina split brick with moderate porosity. 80% alumina bricks are used at the bottom. Bricks at the bottom must be able to bear the load of the column and at the same time not corrode/react with the contained liquid. So, the metal has now been transferred to a converter, where steel production will start. But, what actually happens in a converter? Here lime, dolomite and scraps are added into converter containing hot metal (from blast furnace).
Pure oxygen gas is purged at high pressure which preferentially oxidises carbon forming CO2, although some Iron along with P and Mn is also oxidized forming acidic slag. So, carbon content decreases. Formation of slag occurs in three steps. Step -1 constitutes the formation of acidic slag from oxidized Si, Fe and Mn and dissolution of lime. In Step-2, lime progressively dissolves with enrichment of liquid slag in CaO and decrease of FeO content due to dilution and FeO reduction during decarburization. Step-3 has de-phosphorization only.
Thermal (exothermic reaction, gas blowing, thermal shocks etc. ), mechanical (raw material charging, brick installation, motion of converter content etc. ) and chemical (corrosion, oxidation, reduction etc. ) factors are responsible for the wear out of the refractory material/bricks of converter. MgO-C is generally used at charge pad, tap pad and trunion and as slag lines. To achieve effective corrosion resistance, graphite is used as source of carbon in range of 10-15%. Secondary steel making: Primary steelmaking was addressed till now.
Let us discuss the secondary steelmaking process and its refractory requirement. Various methods in secondary steelmaking process are:- 1) OnLine Purging (OLP), LADLE furnace, CAS-OB (Composition Adjustment by Sealed argon bubbling with Oxygen Blowing), VD (Vacuum degassing of liquid steel), VAD (vacuum arc degassing), VADR, VOD (Vacuum oxygen decarburization), DH, RH (Ruhrstahl Heraeus), RH-OB, RH-MFB and Stream Degasser. What is the ultimate reason of secondary steelmaking? It is performed to increase the quality of the steel.
Certain aspects of refining in an oxygen steelmaking furnace are difficult to carry out. This process also reduces the burden of primary steelmaking. And at last, this technique increases the productivity. Online purging: lime+dolomite+ alloy is fed into a container already having hot metal. Killing agents (Al/SI) are also added. Then, through a lance oxygen gas is purged which oxidizes carbon to CO2, along with oxidation of other metal leading to formation of acidic slag. Liquid steel settle down at bottom over-which slag floats.
Ladle furnace: Here, electrodes are used for purifying the hot metal. The main difference is bubbling of argon gases from the bottom to homogenize the liquid steel. Basic slags are formed, here. Function of ladle furnace: first one is the homogenization of steel with respect to temperature and composition. Flotation and removal of inclusion by metal stirring and absorption into top slags and desulphurisation takes place. Inclusion modification by injection of suitable reactants is also possible here. De-phosphorization by special lags is the important step.
The holding of ladles for sequence casting is essential. Top slags which are formed on top of liquid steel have some advantages. They are: Good heat transfer, protection of the Bath from the open air, Protection of Refractory Lining, Alloying with good reproducibility, Desulphurisation and Absorption of non-metallic inclusions. Typical composition of LF out slag is: Aluminium killed: 57. 31 CaO, 3. 04SiO2, 5. 66MgO, 30. 67Al2O3, 0. 84 Fe, 0. 39 MnO. Silicon killed: 57. 74 CaO, 26. 99SiO2, 4. 94MgO, 11. 51Al2O3, 0. 78Fe, 0. 70 MnO. Al-Si Killed: 58. 8 CaO, 14. 49SiO2, 4. 96MgO, 18. 62Al2O3, 0. 87Fe, 0. 59 MnO. Relining of ladles : Fitting and checking of mechanical parts followed by stainless steel anchor welding then shuttering for monolithic casting is done. Further step, is the heating of monolithic for around 72 hours in the temperature range of 200-3000C. Subsequently, intermediate lining of around 32mm thickness is employed which is made of 80% Alumina. At last working line and bottom lining is made. Bottom lining is of 200-300mm thickness generally made of alumina-mag-carbon brick.
This ends the relining of ladles. However, ladle slide gate preparation and ladle porous plug preparation is required before putting the ladle to service. In modern time, only specific refractory is employed for a particular region of the ladle depending on its interaction with hot metal and other mechanical use. Well block is made of Al2O3 / Al2O3- spinel, seating block: Al2O3 / Al2O3- spinel. Slide gate plate: alumina-zirconia-carbon refractory. Porous slag: Al2O3 / Al2O3- spinel. Collector nozzle: Al2O3-C. Ladle nozzle: MgO-C.
Selection of refractory for ladles is dictated by different properties which include- corrosion resistance, abrasion resistance, spalling resistance and steel cleanliness. Factors affecting the quality of AMC or MgO-C refractory: 1) Structure density: Pressure, Mixing sequence, Granulometry & Ageing, 2) Alumina: Type of Alumina-WTA, BFA, Bauxite, Impurity, Density 3) Magnesia: n Type of MgO: Sintered, Fused like 97FM, 98 DBM, Quantity, Grain size, Density, 4) Graphite : Purity, Flake size– aspect ratio, Quantity 5) Antioxidants :Metal powders, e. g.
Al, Si, Al+ Si, Al-Mg Alloy, SiC, B4C, Quantity 6) Binder: Fixed carbon, Quality, Qty. Examples: Pitch, Resin. MgO-C refractory can be used in side walls, bottom (less than or equal to 3%C) and slag line (less than or equal to 3%C). It has high refractoriness and excellent corrosion resistance to basic slag (increasing with large crystal size 60 to 1000 micron meter. ) on the other hand; Dolomite-carbon refractory is cost effective for side wall and bottom. It has been found that a protective layer of C2S and C3S is formed for Si-Killed, due to slag refractory reaction.
When Al-killed melting point decreases due to CA phase. Continuous casting: The name itself suggests that there is a continuous casting and production of steel. Here a tundish is used. Depending on the shape and size of the steel different types of tundish are used. Products have two major shapes- billets and slab. Backup lining is generally made of 70% alumina LCC, working lining is made up of dry vibratable material (DVM), and alumina-silicate board/paper is selected for insulating layer. Patching materials are, sometime, also used as back-up which is often 86% alumina.
Tundish after serving for long time needs repair or in metallurgical term “relining”. Monolithic refractories are chosen to serve the aforementioned purpose. Functional refractories for tundish are used for striker pad: 80-90% Al2O3 based (low cement castable) LCC, Flow Modifier: 90% Al2O3 based LCC, CNC Nozzle / SEN / SES: Al2O3-C with ZrO2-C in slag line and MBS: Al2O3-C with MgO-C or ZrO2-C in the head. It was mentioned that DVM is used in backup lining of tundish. Its vitality is worth a brief address.
DVM is actually a basic material with high MgO content and varied amount of silica forming olivine (a magnesium iron silicates) and (dead burned magnesite) DBM. Magnesia is preferred in two ranges: 60-65% and 75-80%. Its importance lies in the fact that a very fast tundish reline and turn round time is possible by using this material. It offers low cost and dry tundish lining system. Other features are clean working conditions, low inclusion pick-up, improved steel cleanliness, minimal wastage and excellent deskulling properties.