Influence of the Scaling upon the Heating Steel Slabs in a Pusher-type Furnace Utjecaj ogorine na proces zagrijavanja celičnih potisnog tipa M. Kundak, J. Črnko, Metalurški fakultet Sisak, Croatia In the frame of this paper an existing and a proposed more optimal temperature regime for heating of steel slabs in a pusher-type furnace is analysed. The influence of steel oxidation upon the heating process and its influence upon the optimal temperature regime calculation is also investigated. Key words: pusher-type furnace, scaling, heating of steel. U okviru ovog rada analiziran je postoječi i predloženi optimalniji temperaturni režim zagrijavanja čeličnih slabova u peči potisnog tipa. Takoder je istražen utjecaj oksidacije čelika na proces zagrijavanja i njezin utjecaj pri proračunu optimalnog temperaturnog režima. Ključne besede: peč potisnog tipa, zagrijavanje, oksidacija. Process of slabova u peči 1. Introduction The intensitv of oxidation of the heated steel generally depends on the affinitv of the basic material, i.e. of iron and alloy-ing elements towards oxygen, on gas composition above the heated steel. on the temperature of furnace space and the time of exposure to high tenipcratures. Oxidation doesn't depend solely on the presence of free oxygen in the furnace atmosphere. Aqueous vapour, carbon and sulphuric dioxide also appear as re-aetants. Results achieved regarding the influence of these factors on steel scaling revealed differences, the cause of vvhich has not been explained adequately so far. Quantitive values regarding the extent of the scaling achieved during the investigation stated in references, are given as medium values of several repeated investigations. Sueh data can be useful for determining the influence scale has on steel heating. especially due to the changes in scale composition throughout the deptli of the scale layer, as stated in references. the layer may significantlv influence the coefficient of heat conductiv itv. Hovvever, it should bc taken into consideration that iron o.vidation in the course of steel heating also introduces heat into the process. Yet. exploitation of sueh heat is insignificant. as the oxygene necessarv for iron oxidation is hrought from the furnace surroundings. i.e. vvith the air required for fuel combustion. therefor a eorresponding quantity of nitro-gen is present. vvhich is heated to the temperature of vvaste gas-es. A part of the heat produced by iron oxidation is lost by this heating. Formation of scale diminishes utilization of heat from vvaste gases inside the furnace. as the heat source on the steel surface decreases the possibilitv of heat transfer from the vvaste gas to steel, and due to the lovv coefficient of conductivity, heat con-ductivitv is limited as vvell. The latter indicates the importance of defining the thickness of the scale layer on the surface of the heated steel semiproduct. The thickness of the scale layer in re-lation to the one obtained by calculation from referential data may be checked bv means of plant investigations in aetual * Doc. dr. Mijo KUNDAK. Metaluški fakultet Sisak. Aleja narodnih heroja 44103 Sisak. Croatia process conditions of given steel heating. This paper also pre-sents the research results regarding the influence of scale layer thickness on the time required for the lovv carbon steel heating in a pusher-type furnace of a nominal capacity of 67 t/h. 2. A pusher-tvpe furnace and the research results To studv the influence scaling has 011 the rate of charge heating, in other vvords the temperature regime of the furnace, a temperature profile of the vvall and of the upper surface of the charge lengthways of the pusher-type furnace is determined by an opti-cal pyrometre and presented by a diagram in Fig. 1. The length of the furnace from the slab charging line up to the front vvall is 26.5 m. and the furnace length covered vvith slabs is 24,4 111. The inside vvidth of the furnace is 4.6 m. A schematic presentation of the furnace profile and of its main dimensions is given in refer- Figure 1: Temperature profile of the vvall and of the upper charge surface along a pusher-tv pe furnace Slika I: Temperaturni profil zida i gornje površine ulo.ška po dužini peči potisnog tipa enee F In the course of determining the temperature profile slabs of the St 12 (per DIN) quality, with dimensions of 430x190x3800 mm and mass of 2500 kg vvere heated. The furnace capacitv vvas 31.4 l/It. To be able to estimate the value of the determined temperature regime, due to lovv furnace produc-tivitv'. a numcrical method vvas used to obtain an optimal temperature regime in such conditions. The numcrical method: starts bv dividing the calculation into scveral sections vvithin vvhich variablcs mav bc considcred constant vvith regard to temperature. Whilc the material vvas passing along the seetion length. the chargc vvas considcred motionlcss and its heating vv as calculated as such as in a hearth furnace of the same temperature, the temperature flovv of the flue gas vv as determined on the basis of the seetion heat equilibrium. According to this, a preheating and heating zone vvas divid-cd into five sections, the soaking zone itself consisted one of the sections. The calculation presumes that heating cffects from belovv vvere cqual to those from above in the heating and preheating zone vvhich vv as confirmed during determination of the temperature regime. The calculation starts vvith the required final temperature of the material and is successively carried out for each seetion tip to the beginning of the preheating zone. The i n i tial temperature vvas alvvavs the final temperature of the material of the preccding seetion. The calculation vv as considcred to bc completed vvhen the obtained initial temperature of the material in the first preheating zone seetion vvas app. 20°C. II. hovvever. the initial temperature of the material differed considerablv from the one stated, the calculation vvould bc repeated. Table 1 pre-sents values for some calculated variablcs and the results of the temperature regime calculation for a 31.4 t/h productivitv of the pusher-tvpc furnace. The vvall temperature and the chargc temperature along the furnace during heating vvere obtained bv cal-culating on optimal temperature regime that is presented in a diagram form in Fig. I. Table 1: Some variablcs and the results of the pusher-type furnace temperature regime calculation Nmiiher Lirah of II h k N, c 11, <>„ Al) 0, ol a seclion seclion (niml C W/m k W/mK kVVs/kgK C C C C 1. 4(166 131X1 2b'l 2i) 2 8,88 11.74 0,71 1275 I22S 4 1230 T 4(166 14511 308 20.2 1.113 0.73 D.7I 1260 1270 IS 1303 3. 41 Ki h 141111 27.4 1.01 11.7! 11.71 1171 1200 37 1260 4. 4ll(i(i 135(1 252 2(i.S ().')!) 11.75 11.71 11)114 1050 71) 1110 X 41 Idli 12(10 33.') 11.55 0.84 0.62 713 761) 64 '175 6. 4ll(i(, SIKI 91 51,3 0.17 (13)7 0.51 318 330 15 650 Duc to the effect of the furnace atmosphere an oxide film (scale), consisting of the Fc,();, FeX), and FeO bound more or less tightlv onto the iron base, is formed on the vvhite hot steel surface. On the basis of the accepted steel scale composition: consisting of 5c/t Fc.O„ 10% Fe.A and 85 p/r FeO11,1 and den-sitv ranging from 5200 kg/m\ 5100 kg/m' for Fe304 and 5900 kg/m'151, a correspoding scaling dcnsity of 5785 kg/m1 is determined. The density and the scale composition enable the calculation of the percentage of steel burn off in regard to the heated chargc. Fig. 2 shovvs diagrams of the dependence of the scale lav er thickness in regard to the percentage of steel burn off and slabs thickness heated both sidedly in a pushcr-tvpe furnace. The amount of burn off steel per unit of steel chargc surface may be calculated according to the temperature and duration of detainment al that temperature, as stated in reference 6. 7, 8 vvhen fuels of higher heating values (cokc-oven gas, natural gas Figure 2: Dependence of the scale laver thickness in regard to percentage ol' steel burned off and the slabs thickness during heating from both sides Slika 2: Odvisnost debljine sloja ogorine od postotka čelika koji odgori i debljine dvostrano zagrijavanog uloška Figure 3: Scale heat conductivity and lovv-carbon steel in regard to temperature Slika 3: Toplinska vodljivost /a ogorinu i niskougljični Celik u ovisnosti od temperature and heavy fuel oil) are used the generated flue gases have a similar effect upon the formation of scale. The quantity of the steel burn off per unit of the heated chargc surface is presented in the references quoted in a form of a table and diagram in regard to various air factors during fuel combustion, i.e. from 0.6 to 1.1. Fig. 3 shovvs diagrams regarding the dependence of scale heat conductivity and lovv -carbon steel to temperature1*. From the determined relation it can bc seen that the scale heat conductiv-ity is more than ten times lovver than that of lovv-carbon steel. therefor it acts as an isolating laver on the heated steel chargc surface. Based on this data a specific heat resistance, depending on the thickness of the scale layer formed during 190 mm thick slabs thickness of the scale layer, mm Figure 4: Specific heat resistanee of low-earbon steel covered with a scale layer in regard to scale thickness and temperature for ll)() 111111 thick slahs Slika 4: Specifični toplinski olpor niskougljičnog čelika sa slojem ogorine u ovisnosti od debljine ogorine i temperature za uložak debljine ll>0 mm were heated and their temperature, as tliis is the most common charge of a pusher-type furnace (over 70'/f). was determing and is presented in a form of a diagram in Fig. 4. Hovvever. for the operating conditions investigated, first average temperatures of the charge surfaces in single sections of the furnace were defined from the diagram presented in Figure 1. the calculated optimal temperature regime was determined on the basis of data from Table 1 regarding for the temperature of the charge surface at the end of a single seetion an average temperature of two neighbouring sections was taken. On the basis of these temperatures and the tirne of charge detention in single sections along the furnace the parametres regarding scale, as well as the extra time required for charge heating due to scale, were calculated. Natural gas used as fuel (Hu 37300 kJ/nr'), the air fac-tor was 1.1. data and methods stated in reference 8 were used for the calculation. The results of calculation regarding the scaling of the steel St 12 are presented in Table 2. Table 2: Calculation results in regard to steel scaling during charge heating in a pushcr-tvpe lurnacc Length of a seetion (mm For operating conditions For calculated conditions 0. (T) Burned steel (kg/m;l <7r Lav er (1111111 Al c;> s, rO Burned steel t kg/nri '!, Laver I111111) Al (%> 4066 3 SO - - - - - - - 4066 950 (1.710 0.09 0.12 11 545 - - - 4(l(i6 115(1 1.872 0.26 0.32 5.0 905 0.42(1 0,06 0,07 0.7 4066 121)11 2,650 0.36 0.45 7.0 1125 0.907 0.12 0.16 2.5 4066 1230 3.600 0.49 0.61 9.5 1235 2,430 0,33 0.42 6,3 406(i 123(1 4.372 0.6(1 0.75 11.0 1249 3.3 IS 0.45 0,57 8,5 Measurements of the scale layer thickness were carried out in the working plant. after the charge was puslied out of the furnace and adequately cooled, data 011 thickness of the scale layer formed in different working conditions of the furnace with regard to the delavs in the vvorking of the furnace due to war cir-cumstances in the countrv were obtained. Table 3 shows the re- sults of the measurements of scale laver thickness on slabs vvith dimensions of 430x 190x3800 mm. steel grade St 12. during con-tinuous and discontinuous furnace operation vvhen the slabs vvere kept too long in the furnace''. Table 3: Results of measurements of the scale layer thickness on the surface of steel St 12 during continuous and discontinuous operation of a pusher-type furnace Working Continuous Discontinuous vvorking of the furnace conditions vvorking of the of llie Heating 16 h 12 h furnace furnace Blind firing X h 12 h 60 h Laver thickness 1.(1111111 4.8 mm 7.(1111111 10,0 mm Slabs vvere kept in the furnace for 264 min. The calculated optimal temperature regime not only improved the heat flovv of the slabs in the initial period, but also shortened the period of slab detention at highcr temperatures and reduced scale loss. the grovvth of the furnace floor and the heat energy consumption. 3. Discussion of the results In addition to disadvanlages that scaling causes sueh as are reduetion of charge mass and defects in products, scale also be-haves as an isolating layer on the heating surface of a charge. therefor more energy is required for its heating as the heating ca-pacitv of a pushcr-tvpe furnace decreases. Figure 1 sliovvs data aquired from a pushcr-tvpe furnace during operation regarding the temperature regime and the calculated optimal temperature regime at a productivitv of 31.4 t/h in a diagram form. The influence of the scale laver thickness upon the heating of a charge could be studied and necessarv corrections in the optimal temperature regime calculation could be applied front these data. Generallv. 011 the basis of the scale layer thickness, slabs thickness and heating procedures (one or both sidedlv), a percentage of steel burn offcan be predieted. From the diagram in Fig. 2 it may be scen that for the scale layer of thickness 2 mm. vvhich does not differ vvith slab thicknesses in a corresponding furnace atmospherc. the steel burn off at the charge thickness of 350 mm is 0,45 % and at the charge thickness of 90 mm is 1.75 c/<. As sueh the percentages of steel burn off inerease or decrease in regard to inerease or decrease of charge thickness, the specific heat resistanee also inereases or decreases. From the expounded it may be noted that the oxidation atmospherc of the furnace space contributes a great deal to a formation of scale on the charge surface, and the inerease of specific heat resistanee vvas larger vvhen thinner charges vvere heated. To vvhat degree scale behaves as an isolating layer can be seen from the diagram in Fig. 3. The coefficient of scale heat conductivity, in regard to the temperature level, is more than ten times lovver than that of steel. I11 Fig. 4 specific steel heat resistanee for different scale layer thicknesses of a 190 111111 thick charge heated in a pusher-type furnace is presented in a diagram. As the specific heat resistanee inereases in regard to temperature, it is necessary to correct the calculated heating time of the charge. From the diagram in Fig. 4 it is also possible to define parametres for the charge of various dimensions vvhen the atmospherc remains unehanged inside the furnace. as alreadv evi-dent in the diagram in Fig. 2, inerease or decrease of the specific heat resistanee parameter presented in diagram, Fig. 4 for the percentage of inerease or the decrease of the steel burn off for the same scale layer thickness. Table 2 presents quantities of scale determined by calculation follovving the method referred to in reference 8 during op- eration and a calculated optimal temperature regime of a pusher-tvpe furnace. It has been noted that scaling is greater in the temperature regime achieved during operation than that at the calculated optimal one, which can be explained bv the higher surface temperature of the charge in the first temperature regime. From the data (At) in Table 3 it may be ascertained that deten-tion at certain temperature sections should be altered, i.e. in a manner that the corresponding temperatures of flue gas and of the vvalls inside the furnace are higher in the 5,h and 6"1 seetion in regard to calculated optimal temperature regime, duc to the iso-lating behavior of the scale. However, the temperature regime determined during operation in fact corresponds to the optimal temperature regime of the furnace vvith a heating capacity of 40 t/h. That increase of capacity would not have an essential influence upon the quantity of scale, as formation due to detention at higher temperatures (which is shortened due to capacity increase) has a significantly greater influence. Research carried out in a pusher-type furnace has shown that in the operating conditions. vvith a heat regime obtained by con-tinuous vvork, ;t larger quantity of scale is formed in compare with the quantity of scale determined through calculation (Table 3). The differenee is: (1 - 0,745) 100/0,745 = 34,23 %. The increase of steel burn off may be explained due to ;t greater oxida-tion atmosphere inside the furnace due to the air sucking through the openings on the furnace. This vvas confirmed by additional investigations. Particular attention should be paved to the formation of scale during discontinuous vvork after longer periods of delay in operation brought about by vvar circumstanccs. Five-day operation of the rolling mili train vvith 12 hours (one and a half) shifts vvas follovved by 60 h delay, vvhieh led to a formation of 10 mm thick scale layer on the slabs surface. By normalising of conditions the continuous vvork vvas obtained. The result of abnormal conditions vvas a mass loss of the charge and an in-creased consumption of energy, because the time necessary for the charge to be heated vvas more than tvvice as long as usually. It vvas noticed that scale formed vvhen the charge vvas kept in the furnace for over 60 hours, consisted of many layers of probably different composition and microstructure. This might be vvorth vvhile to investigate in the future due to the influence on heat con-ductivity and due to great loss of the charge vveight. Research carried out lit the harm that scale makes as an iso-lating layer on the slab surface, during its heating. A correction of calculated heating time is necessary in the čase of determina-tion of proper heating conditions (regime) or by aetual heating proces analysis. They can be useful as a ground for further stud-ies of scaling influence on the quantity of air necessary for the start of fuel combustion, or the amount of 02 in vvaste flue gasses at the end of the furnace. 4. Conclusion The scale on the surface of a charge behaves as an isolating laycr, vvhieh enlarged the heating time and inereased energy consumption. This is much apparent in the čase of thinner charges heating in an oxidating atmosphere. The heating is slovverdue to the influence of scale. By the determining of an optimal temperature regime is necessary to consider the influence of scale formed. Inaccuracy by the calculation of a temperature regime, due to the increase in specific heat resistance is higher than in the čase if no scale is present. A čase of a pusher-type furnace vvith 190 mm thick steel charge heated and vvith a correction applied in the course of optimal temperature regime calculation, con-tributing to a better quality of the heated charge is deseribed. In particular. the influence of a thicker scale laver. formed due to larger stoppage of the rolling mili, the span of the heating time and energv consumption is deseribed. During longer stop-pages in a rolling mili multi-lavered scale is noticed on a charge surface. A long time a blind fired furnace probablv has an influence on scale formation and on its heat conductivity, and also on other parametres connected to charge heating. These investigations may also be useful as a ground for further studies of scale influence on quantity of air required at the beginning of operation and for the determination of fuel to air rate for the automat-ic regulation of the burners of the pusher-tvpe furnace. List of simbols cp - specific heat of steel. Ws/(kg K) h - heat transfer coefficient, W/(nr K) k - thermal conductivitv of steel, W/(m K) NBi - Biot number, At - extention of heating time. % A0m - the highest temperature differenee in slabs, C t - coefficient for thick-vvall bodies (•„ - temperature of flue gas, C ()m„ - average temperature of slabs, C 6ms - temperature of slabs surface, C \),„s - medium temperature of slabs surface, C ()„ - temperature of furnace vvalls, C 5. References 1 J. Črnko: The of the Heat F.nergv Consumption upon the Working Intensitv and the Frequency of the Isolation Maintenance of a Pusher-tvpe Furnace. Kovine zlitine tehnologije, 26, 1992. 4. 239-231. M. Kundak. Z. Acs: Analiza toplinskog režima u valjaonicama traka, gredica i bešavnih cijevi, II dio. Metalurgija (Stsak), 16, 1977, 1.9-13. E. 1. Kazancev: Promišlennie peči, IzdateFstvo Metallurgija. Moskva, 1975. 1 W. Lehnert: Wdrmetecluiische Grundlagen fiir Industrieofen, Bcrgakademie Freiberg, 1979. K. Ražnjevič: Termodinamičke tablice. Izdavač "Veselin Masleša", Sarajevo. 1989. " M. Cauševič: Obrada metala valjanjem. Izdavač "Veselin Masleša". Sarajevo, 1983. M. Kundak: Optimalizacija zagrijavanja materijala u kružnoj peči teške pruge VBC-a. Elaborat za Valjaonicu bešavnih cijevi Zeljezare Sisak izraden it Institutu za metalurgija Sisuk. Stsak. 1982. * W. Heiligenstaedt: Wdrmetechnische Rechnungen fiir Industrieofen, Verlag Stahleisen M.B.H.. Diisseldorf, 1966. 9 B. Kocijančič: Analiza utjecaja potrošnje zemnog plina i stvaranje oksidnog sloja za različite varijante rada VTG-a, Interni izvještaj Holdinga Zeljezare Sisak. Sisak. 1993.