INTRODUCTION OF UNSHAPED REFRACTORIES IN THE WEAR LINING OF STEEL LADLES UPORABA NEOBLIKOVANEGA OGNJESTALNEGA MATERIALA ZA OBRABNO OBSTOJNO OBZIDAVO JEKLARSKIH PONOVC F. ETIENNE1, E. ZIAROVSKI2 'Lafarge Monolithics (LRM) -Lafarge Feuerfest GmbH, Kaltenbornweg 6, 50679 Koln Prejem rokopisa - received: 1996-10-04; sprejem za objavo - accepted for publication: 1997-10-21 Steel ladles are essential tools in steelmaking. In the past, research was almost entirely centred on converters but nowadays attention has focused on ladles. The determining factor for the choice of a Iining is the cost per tonne of steel as well as reliability, security, availability and metal purity. The cost per tonne of steel has now reached a constant level vvith shaped products and can hardly be improved. The introduetion of nevv technology and radical changes are the only possible means to make improvements in the economic situation. The application of unshaped products to the ladle may provide a solution. Experience in Japan has shovvn that improvements are possible vvhen techniques utilising monolithics in ladles are adapted. Here vve intend to deseribe the trials carried out by Lafarge Refractaires Monolithiques and the first significant industrial results vvith the use of monolithic ladles in European conditions. Key vvords: steel ladles, vvear lining, monolithic lining, industrial experience Jeklarske ponovce so bistveno orodje za izdelavo jekla. V preteklosti je bila pozornost usmerjena predvsem v konvertorje, sedaj pa se pozornost usmerja v ponovce. Odločujoči dejavnik pri izbiri obzidave so strošek na tono jekla ter zanesljivost, varnost, razpoložljivost in čistost kovine. Strošek na tono jekla je pri oblikovanih proizvodih sedaj dosegel konstantni nivo in ga bo težko izboljšati. Razvoj nove tehnologije in radikalne spremembe so edina možnost za izboljšanje gospodarskega položaja. Uporaba neoblikovanih proizvodov za ponovce bi lahko bila rešitev. Izkušnje iz Japonske so pokazale, da je mogoč napredek z uporabo monolitov v ponovcah. Tu želimo opisati preizkuse, ki so bili izvršeni pri Lafarge Refractaires Monolithiques in prve pomembne industrijske rezultate pri uporabi monolitskih ponovc v evropskih pogojih. Ključne besede: jeklarske ponovce, obrabno obstojna obzidava, monolitska obzidava, industrijske izkušnje 1 EUROPEAN CONDITIONS AND THE LAFARGE CONCEPT a) LININGS IN EVROPE In the steelmaking and secondary steelmaking process, the lining is submitted to different thermomechani-cal stresses. If the evolution of refractories in Europe is taken into account, it is not surprising that a vvide range of ladle linings is found on the market. • Brick linings - Basic linings: The slag line is often magnesia-carbon bricks, the other areas are magnesia or dolomite. The basicity of the lining may be considered a favourable factor for secondary steelmaking. - Aluminosilicate linings: only the sidevvalls and the bottoms are in such bricks, generally bauxite. Alu-mina-spinel is rarely used. • Ravv materials Let us compare the relative costs of the ravv materials for the above compositions: Spinel 100 Tabular alumina 75 Bauxite 15-40 Magnesia 15 - 40 Dolomite 5 - 10 Ladle linings have previously been made from less expensive ravv materials (bauxite, magnesia, dolomite). In Japan, this vvas not the čase and mostly more expen-sive materials vvere used. Therefore, from an economic point of vievv, the starting point for monolithic design differs in Japan and Europe. • Metal quality The steel ladle has become a metallurgical reactor vvhere many operations of secondary steelmaking are carried out. Ladle refractories in contact vvith steel may influence the steel quality, either directly or through sec-ondary effects. For example, refractories may cause al-terations in slag composition, or indirectly affect steel quality as a result of the performance of the refractory involved in the stirring operation. Consequently refractories play an important role m metal quality. Basic linings, in particular dolomite, have been developed for metallurgical reasons and in addition also offered to European steelmakers a cheap solution. In the past steelmakers vvhere also concerned by the pres-ence of alumina in ladle linings. b) THE CONCEPT For technical reasons, basic slag line products vvill not be considered in this paper. Basic castables vvith properties competing vvith basic bricks in this area have not yet been developed to a commercial level. Therefore to date our solutions exclude the slag line and our expe-riences are concentrated on the monolithic sidevvall, pre-fabicated and the casting of the in-situ bottom. Our de-signs are based on the follovving five principles: 1 The various zones of the lining Alumina-spinel products are not considered neces-sary for the whole ladle, since the raw material is rela-tively expensive in Europe. Examples are found vvhere this design proved to be wrong from the economic point of view. It is necessary to have different properties for the different conditions encountered in the ladle, i.e. the best quality/price ratio in each zone of the lining. 2 Deskulling and repairs The cost of high technology alumina castables is greater than those of alumina or basic brick. However the repair/veneering of the initial lining after cleaning, not only allovvs the ladle availability to improve, but also re-duces the refractory cost of the ladle. For subsequent repairs/veneers we propose a self flovving castable (MONROX RANGE) which are specifi-cally designed for this type of repair. Their special char-acteristics give a free flowing product which readily fills the lining profile betvveen the former and residual lining, and has an excellent adhesion to the residual materials. 3 The deveiopment of silica-free products With the exception of the slag line vvhich must re-main basic, the sidewall and the bottom of the ladle are lined vvith alumina materials. Bearing in mind the metal quality, LRM developed silica-free products for monolithic ladles. 4 Appropriate placing Mixing, vibrating and drying together account for 50% of the achievement of the maximum campaign life. These features are specific to unshaped materials. 5 User and supplier as partners The key to progress for the user and the supplier is co-operation. Monolithic ladles are a good replacement for shaped linings, but full co-operation is required to optimise the economic factors, a difficult but not impos-sible task. The follovving solutions may be proposed. 2 DEVELOPMENT OF SILICA-FREE PRODUCTS Monolithic materials have vastly improved over the last fevv years. Highly compact products and inereased refraetoriness have been obtained through the technology of ultrafine particles and defloculation. Corrosion resistance can be improved by the addition of fine particles to protect the matrix taking care to avoid possible reactions betvveen the lining and the liquid steel, vvhich may lead to contamination. These fine particles additions can be classified as: • Anti-wetting-agents These agents impair the penetration of the refractory matrix by oxide slags and therefore limit the corrosion to surface phenomena. They are non-oxide compositions vvhich can only be employed in products vvorking in re-ducing or moderately oxidizing atmospheres. Their use is made possible by the reduction of the oxidation rate because of the great compactness of these castables. The most effective are also those vvhich oxidise the most readily sueh as: piteh, resin, and carbon black used alone or in combination. • Agents \vieh reinforce the dissolution resistance These agents vvork by inereasing the viscosity of the diffusion layer and are generally oxydes, sueh as Cr2Oj, Zr02, MgO and MgO A1203. The use of ultra-lovv cement castables allovvs us to obtain a matrix vvith the characteristics close to those of the aggregate materials. Therefore, means limiting the penetration of the corrosive elements using the tvvo methods indicated and employing alumina ultra-fines (no silica) vvere developed. Anti-wetting castables In castables vvith alumina matrix moderate sintering leaves a very fine capillary strueture vvhich is easily and deeply penetrated by the slag. With the addition of silicon carbide (table 1- castable A), test shovvs a slight improvement of corrosion resistance vvhile the slag stili penetrates deeply. This be-haviour is verified by the analysis of a panel used in a ladle vvall: after 50 heats, by the total residual thickness 185 mm, the thickness of the impregnated layer is 60 mm. The transformed area no longer contains silicon carbide as this has been completely oxidised vvith reaction vvith the slag, mullite and anorthite have been formed. Carbon is the key element to avoid penetration (table 1 - castables B, C, D). Hovvever, carbon addition vveak-ens mechanical strength since it diminishes the sintering of the alumina matrix. A compromise, as in castable D, through the choice of the carbon type and the content of carbon and silicon carbide is required depending on the anticipated degree of oxidation and silicon carbide content and according to the risk of oxidation. The principle has been applied to both corundum and bauxite castables. Alumina-spinel castables In several Japanese papers the use of spinel as addition to tabular alumina castables to reduce impregnation is deseribed. The mechanism may be explained as fol-lovvs: - Spinel magnesia fixes in solid solution iron oxide from slag; - Slag lime reacts vvith the alumina of the matrix to form refractory CA6 Table 1: Castables with antiwetting agents A B C D Ahos 72 67 67 73 SiC 10 10 5 5 C 0.9 0,9 0,9 0,6 Water 4 4,2 4,8 4 Density after 1550°C/5 h* 3,30 3,13 2,98 3,18 Compression Mpa 110°C 60 30 30 30 1200°C/5 h* 65 40 25 30 1600°C/5 h* 160 120 50 100 PLC 1500°C -0,1 +0,1 +0,2 -0,1 Corrosion 1600°C-80 mn (C/S= 2,4) Steel XC 38-Synthetic (Fe304=20) slag (CaF =5) Wear (mm) 8,6 8 6,5 7,5 Impregnation (mm) 7 0 0 0 * reducing atmosphere - Silica concentration in slag increases and conse-quently its viscosity increases also - Reaction vvith alumina induces precipitation of C2AS (gehlenit) These results have been verifted in a tabular alumina castable vvith alumina bond. Addition of 10 to 20% of spinel strongly reduces slag penetration. The introduc-tion of silica in the system promotes the dissolution and in this čase undesirable monticellite is formed. In table 2 the evolution of tabular alumina-spinel products is presented. Recent developments have been made considering the follovving parameters: The nature of the spinel: Laboratory corrosion tests had originally shovvn a deeper penetration at the slag-metal interface in over-stoichiometric than in stoichiometric spinel similar crystal size. Facility of installation: Due to their alumina bond, the castables may need care during placement. Improvements are being made in workability and rheology to achieve a high degree of mobility. This is es-sential for the future. 3 INDUSTRIAL RESULTS Many trials have been undertaken and are either stili in progress in laboratory and industrial stage. Today vve have a good background of industrial experiences throughout Europe, each site presenting a different sce-nario: - Integrated and electric steel plants manufacturing flat products, IFS, long products, steel for roll bear-ings or stainless steel; - Ladle capacities from 50 to 310 tonnes, - Equipement for secondary steel making including RH, DH degassers, desulphurisation lances, ladle furnaces, CAS-OB... Table 2: Alumina-spinel castables E F G Alumina 82 59 62 Electro-fused spinel - 20 - Sintered spinel - - 20 Magnesia - 3 1,4 Lime 1,4 1,4 1,4 Mixing vvater 4,3 4,4 4,9 Apparent density 3,07 2,92 2,93 Compression Mpa 100°C 85 55 50 1200°C/5 hr* 90 90 80 1500°C/ hr* 140 100 90 PLC 1600° C/5 hr +0,3 +0,9 +0,7 Corrosion 1600°C-30mn (C/S=2,4) Steel XC 38-Synthetic (Fe203=20) slag (CaF2=5) Wear(mm) not 8 9 detectable Impregnation completely 12 12 * reducing atmosphere - And the different vvork cultures vvithin the European community. Precast wear linings for Ladle Bottoms The development of high performance castables al-lovvs competitive solutions to be put forvvard in order to obtain a balanced ladle life. This is currently been dem-onstrated by the use of pre-cast blocks in steel ladle bottoms. The pre-cast vvear lining of ladle bottom is a solution vvhich offers a competitive and simple alternative vvith many advantages, including: - quick and easy installation, - controlled off site casting, - pre-drying, under controlled conditions, - design flexibility, and - enhanced performance. The concept uses different castables for various pieces and joints, depending on the characteristics and constraints encountered (impact area, porous plug as vvell as tap-hole number and position). a- SITE 1 - Steel Teeming Ladle-Precast bottom blocks This site is an intregrated steel plant operation vvith 3 No. BOF's producing approximatively 2,3 Mt/year of strip product and supplying high quality lovv and ultra lovv carbon steel to the motor industry. Ladle availability is fundamental to this plant and improved ladle bottom performances vvere required. The standard brick lining gave a typical campaign of 30 heats vvith failure occur-ring in the impact pad. Our objectives vvere to achieve 60 heats and provide a ladle vvith an uniform sidevvalls and bottom life. The proposals for a complete precast bottom vvere based on: - improvement of the impact area with introduction of a precast pad in alumina-spinel quality; - providing an economicaly viable balanced bottom lining exhibiting an even wear. This was achieved by introducing bauxite based products in the none impact area of the bottom; - an intregral joint in a self flovving version of our alu-mina-spinel castables, vvhich are designed for this type of installation. Their special characteristics give a free flovving product vvhich readily fills the lining profile betvveen the precast blocks and has an excel-lent user friendly nature; - elimination of additional drying time usually associ-ated vvith the use of monolithics. The drying sched-ule is the same as that used for the bricks. Ali the objectives vvere achieved in performance. A typical campaign of 60 heats has been attained vvith a stop at mid campaign to change the vvellblock. In table 3 some technical and economic benefits acquired are listed. Table 3: Economical and technical benefits on site 1 Actual situation Solution: prefabicated bottom Material cost 100 105 Manpovver cost 100 28 Client profit directs costs -6,5% on the cost ratio Maintenance standstill 42h 26h Duration of a ladle campaign including installation maintenance and production duration 15 days 15 days Client profit -38% on the maintenance time Indirect costs +4,5% on ladle availability Currentely trials are ongoing to extend the vvellblock life and eliminate the stop at mid campaign and further improve ladle availability. In late 1995, this site decided to convert ali their teeming ladle fleet to precast bottoms. b- SITE 2 - Steel Teeming Ladle-Cast in-situ bottom This 2 x 300t BOF plant is similar to other large inte-grated plants in that there is a desire to diminish the re-fractory costs in ladles. The general feeling is that the cost per tonne of steel has novv reached a constant level using bricks. Hovvever, the introduction of monolithic technology vvas felt to be a solution that could offer sig-nificant improvements. In table 4 the clients final gains after adoption of the monolithic concept are listed. Our alumina-spinel products vvere used, the čast vibration version for the initial installation, and repairs vvith self-flovv product. The performance of these products allovved the customer"s objectives to be achieved. The fleet of ladles on this site is sufficient to allovv planned campaigns vvhen the monolothic concept is adopted the clients final gain is: Table 4: gains on site 2 Actual situation Solution :cast in-situ bottom The campaign 90 heats life vvith one stop to repair the impact area 360 heats vvith 3 small repairs of the impact area Material cost 100 92 Manpovver cost 100 29 Client profit:direct cost -14% on the ratio Maintenance stand-still/length 336 h(4 campaigns) 208 h(4 notations) Duration of a ladle campaign 15 days 15 days Client profit -38% on maintenance time Indirect cost +3,5% on ladle availability The next step at this plant is to improve the ladle side-vvall. c- Monolithic sidevvall and bottom lining: our references Monolithic products either as precast shapes or in-situ casting increase the ladle life. The campaign length is the main parameter vvhich allovvs the decrease of costs per tonne. Numerous trials carried out since 1989 shovv a favourable evolution in maintenance costs. Some benefits are in table 5. Table S: Benefits obtained at different sites SITE A SITE B SITE C SITE D Process 2x3 lOt BOF Cas-OB/CC Slag lx70t BOF 3H/LF/Lances Bloom/ Billet lx80t EAF RH/LF/CCR Bloom/Billet lx85t EAF LF/CCR Tube/Pipe Initial lining 80% A1203 bricks Dolomite bricks MgO-C bricks Dolomite bricks Initial life 80 heats 60 heats 50 heats 70 heats Monolithic concept AI2O3 spinel product AI2O3 spinel and bauxite product AI2O3 spinel products for metal quality AI2O3 spinel products for metal quality Life 400 heats vvith 1 relining at 240 heats 300 heats vvith 4 relinings every 60 heats 120 heats vvith one relining 420 heats vvith 3 relinings Achiefed production costs 10% material and manpovver 20% material and availability 10% material and manpovver 10% material and manpovver 4 CONCLUSION The trend tovvards the use of monolithic refractories in ladle applications has been initiated by changes in steelmaking technology and associated product technol- ogy- Hovvever, products development has to continue to further reduce vvear of the steel ladle refractories and to decrease refractory costs. Lafarge Refractaires Monolithiques have considered the metal quality in the development of new products for secondary steelmaking. The new silica-free bond products behave very satisfactorily in industrial service and confirm the results predicted on the base of laboratory corrosion tests. The improvement ofthe installation tech-niques wi!l have a significant effect on the use of mono-lithic refractories since such techniques lend themselves to automation. When the suitability of the product has been established, the required investment in installation equipment will bring inereased benefits. Recent innova-tions have shown that there is a plače within the ladle environment for monolithic refractories and that they can meet the most competitive challenges. 5 REFERENCES M. M. Bourdet-Jeanne, F. Etienne, P. Lafargue, Evolution des Refractaires des Poches Acier D'Unimetal Normandie: Vers la Poche Mono-lithique; ATS Pariš, decembre 1993 F. Etienne, La Rentabilite des Poches Acier Monolithiques; Commis-sion ATS, juin 1994 P. Atkinson, F. Etienne, Development of Monolithic Refractories in Steel Ladles; Sheffield Congress, september 1994 F. Etienne, M. Jacquemier, P. Meunier, B. Clavaud, Introduction of Un-shaped Products vvithout Silica in Secondary Metallurgy for Metal Cleanliness; Unitecr Kyoto, November 1995