Steel strip preheating method

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characterized in that an emulsion of liquid fuel and

STEEL STRP PREHEATING METHOD water is utilized in the oven.

For carrying out the above-defined method, conven

The present invention relates to a method of preheat tional burners equipped with liquid fuel injection means ing steel strips, this method being applicable interalia to 5 capable of forming an emulsion with water are used. non-oxidizing ovens utilized on galvanizing production The mechanism for producing the flame of a carbon lines. free liquid fuel by the addition of water consists in shift It is known that in the manufacture of steel sheets the ing the hydrocarbon dissociation reaction equilibrium steel strips are galvanized on high-production lines 10 in the presence of water vapor towards the specific methane combustion characteristics. In fact, the C/H.

(FIG. 1) in which the steel strip is firstly cleaned in a ratio of liquid preheating oven A, this preheating step taking place methane, and hydrocarbon is much higher than that of from room temperature to a temperature of 600 to 700 tent is obtained in the smoke.a By consequently lower water-vapor con

C, whereafter the clean steel strip is delivered to a labo and water emulsion it is possible toforming a liquid fuel ratory B where it is still heated under atmosphere up to 15 vapor content of smoke and to assimilate thethe increase water combus a temperature of the order of 800 to 900' C, the strip being subsequently cooled in another compartment C to tionTheofpresentliquid hydrocarbons to that of gas. inventor found that to obtain the desired about 450° C before being quenched in a molten zinc combustion conditions the preheating oven should op bath D. Finally, the strip is dried and cooled. erate with an addition of 0.3 to 1 kilogram of water per FIG. 2 of the attached drawing illustrates the temper 20 kilogram of liquid fuel. With this range of water and ature diagram of the steel strip during its passage fuel mixture a clean soot-free combustion is obtained, through the successive processing compartments A, B with only 2 to 8% of unburnt products in the smoke. and C. According to another feature characterizing this in The function of the preheating oven A is to remove vention, the temperature in the preheating oven may any rolling-mill oil from the steel surface by heating the 25 vary from 900' to 1,500 C, and the smoke water-vapor strip to a temperature within the range of about 400 C contents resulting from the combustion of the water and in the case of oxidizing ovens and to about 700 C in the fuel emulsion according to the method of this invention case of non-oxidizing ovens as generally employed on ranges from 14 to 19%, these values comparing with modern galvanizing production lines, with due consid those obtained with a current natural gas (18.7%). eration for the fact that they afford high production 30 In the specific case of the application of the method rateS. of this invention to a preheating oven on a galvanizing All hitherto known non-oxidizing ovens are gas production line, the rolling-mill oil film can be removed heated, as a rule with natural gas, and operate with an by providing a steel strip dwell time in excess of about incomplete gas combustion. In fact, only gaseous fuels 5 seconds in the preheating oven, and in the heat-treat are suited for obtaining a sub-stoichiometric combustion 35 ment phase a dwell time of only 20 seconds will be with a variable proportion of the order of 2 to 8% of sufficient, at a temperature above 700 C in an atmo unburnt products without leaving the least trace of soot sphere containing 10% hydrogen, the cycle being com pleted by a cooling phase.

(i.e. carbon) in the smoke. The method of this invention is perfectly suited for This last requirement is extremely important, for if properly soot particles contained in the flame were allowed to 0 nary to apreparing the surface of steel strips prelimi high-quality galvanization. However, this form a deposit on the travelling steel strip, it would be invention should not be construed as being strictly lim definitely impossible to obtain a proper adherence of the ited by this specific application, since it can be imple zinc to the steel surface, whatever the treatment applied mented with other types to the strip between the preheating oven A and the 45 non-oxidizing ovens for oftheovens and furnaces, notably galvanizing bath D. Therefore, the presence of any sheets, and also for preparing the surfaceofofsilicon treatment steel steel sheets soot, even in extremely moderate quantities, must defi in continuous aluminizing ovens or furnaces. nitely be avoided in the case of steel strip preheating What I claim is:

OVeS.

1. In a process for galvanizing steel strips in a galva

For this reason, up to now it has scarcely been possi 50 nizing production line, said process including the step of ble to use liquid fuels for operating steel strip preheating preheating said steel strips to clean said steel strips, the ovens on galvanizing production lines. In fact, the com improvement wherein said preheating step comprises: bustion properties of liquid hydrocarbons are such that introducing said steel strips into a non-oxidizing oven; large amounts of soot are released in the flame when the and fuel combustion approaches the stoichiometric limit, 55 heating said oven, and thereby said steel strips, by notably when the burners are operated with an insuffi burning a liquid emulsion of liquid fuel and water cient air supply. within said oven such that the temperature of the It is the essential object of the present invention to atmosphere within said oven is from 900 C. to provide a method directed to avoid these inconve 1500 C.

niences by permitting in the preheating oven of a steel 60 2. The improvement claimed in claim 1, wherein said strip galvanizing production line the combustion of emulsion includes from 0.3 to 1.0 kilogram of water per liquid hydrocarbons with a considerable air deficiency kilogram of liquid fuel.

and without the presence of soot or carbon in the flame. 3. The improvement claimed in claim , wherein said In view of the foregoing, the present invention is emulsion is burned within said oven by means of injec directed to provide a method of preheating steel strips 65 tion burners. s s st in an oven heated with liquid fuel, this method being