Flame Cleaning and Furnace Preparation

Sendzimir Oxidation/Reduction Method. The original method for flame cleaning was proposed by Tadeusz Sendzimir in the 1930s. His invention involved heating a steel strip in a direct oxidizing flame to burn off the rolling oils and other contaminants. This step left an oxide on the steel surface, which was then cleaned by heating to a much higher temperature in a reducing furnace with a hydrogen-nitrogen atmosphere. This two-furnace cleaning method was converted into a coating line by cooling the strip and immersing it in molten zinc while it was still under the protective hydrogen-nitrogen atmosphere. An additional benefit was that the strip could be annealed while it was being cleaned for coating.

Oxidizing Furnace. In general, both the oxidizing furnace and the newer, nonoxidizing, direct-fired furnace can perform the same job. Both types of furnace configurations preheat the strip before it enters the radiant-tube furnace, and both flash off the volatile materials and oxidize the carbon compounds that are on the surface of the strip.

The role of the oxidizing furnace is to oxidize all of the carbonaceous products without leaving too thick an oxide layer for the reducing furnace to convert back to pure iron. All oxide that forms in the oxidizer must be reduced in the reducing furnace in order to prepare the strip for coating. Unreduced oxide is brittle and, if left on the strip, will result in poor coating adherence.

Oxidizing furnaces usually preheat the strip to approximately 480 °C (900 °F). This temperature, which is somewhat arbitrary, depends on the uniformity of heating and the reducing capability of the radiant-tube furnace that follows. Care should be exercised to avoid overheating the edges of the strip. Because the oxidizing furnace is not sealed, air can be drawn into the furnace, making the atmosphere within this furnace oxidizing to the steel strip, regardless of burner combustion practices. Burners are usually adjusted to perfect combustion for efficiency. If the combustion ratio of the burners is too rich, then carbon deposits (soot) can form on the strip and completely negate the cleaning purpose of the oxidizer. Carbon cannot be removed with a hydrogen atmosphere, and uncoated spots and poor coating adhesion will result.

Direct-Fired Furnace. The use of a nonoxidizing, high-gradient, direct-fired furnace is preferred for strip preparation. The direct-fired furnace is so named because there is direct flame impingement of the combustion products on the strip. This type of furnace is connected directly to a reducing or radiant-tube furnace. The large volume of combustion products that is produced in the direct-fired furnace acts as either a barrier or a seal to pressurize the rest of the reducing furnace, which minimizes the consequences of small furnace leaks.

Direct-fired furnaces usually have zones, or segments, that can be turned off to adjust for lighter-gage strip or lower annealing-temperature requirements. These zones are usually connected to a common flue near the entry of the furnace. This flue usually contains a damper that controls the furnace pressure to a predetermined value. Pressure that is too high can force combustion products into the reducing zones and contaminate the hydrogen-nitrogen atmosphere. The flue must be large enough and the pressure such that all combustion products will flow out of the flue.

The advantages of this type of furnace are that it preheats, removes volatiles and carbon without oxidizing the strip, and provides a much higher preheat temperature than an oxidizing furnace.

Although the direct-fired furnace is in many ways superior to the original oxidizing furnace, it does have negative aspects. Its major disadvantage is that it is expensive to operate and maintain. It also has a high initial cost because of the sophistication of the burners and burner controls, and because of the refractory requirements to operate at high temperatures. In addition, high operating temperatures and thermal shock can cause problems with the maintenance of the refractories. Controls for the fuel-to-air ratio and the furnace pressure are very critical and cannot be neglected. Because the gas feed on the individual burners can easily become plugged, it needs to be checked and cleaned. The ability of the positive pressure to overcome furnace leaks is not a cure-all for poor maintenance practices. Air can be aspirated into the furnace, even with positive furnace pressure.

Combustion Ratios. Because the furnace must rapidly heat a steel strip without oxidizing it, the correct ratio of air to gas must be ensured. Although the optimal amount will vary with the coating line, it should be in the 4 to 5% range for excess combustibles in the last zone of the direct-fired furnace, where the strip temperature is the highest. This ratio should be set without having the strip or any hydrogen in the furnace. This combustion ratio will increase when the coating line is running because of the addition of the hydrogen atmosphere to the furnace. After exiting from the furnace, the strip should be bright and oxide-free. Minor adjustments are often made using strip appearance only as a guide.

If the amount of excess combustibles are greater than 7 to 8%, then carbon can be deposited on the strip surface, resulting in uncoated spots and poor adhesion. In many mills, it is common practice to set the combustion ratios progressively leaner (closer to perfect combustion and, in some cases, excess air) in the zones closest to the furnace entry, where the strip temperatures are lower. Output will be increased and some of the wasted energy will be recovered by burning the hydrogen furnace atmosphere and the excess fuel from the preheater zones.

Strip Exit Temperature. If strip exit temperatures from the direct-fired furnace are too high, then the strip will be oxidized. Usually, the maximum indicated temperature without oxidation is approximately 730 °C (1350 °F). Higher temperatures almost always produce an oxide, regardless of burner ratios. Lower temperatures do not produce an oxide, but they defeat the powerful cleaning capability and lose the advantage of the rapid preheating. The maximum allowable temperature is also very dependent on furnace design, combustion ratios, and dwell time in the furnace.

Zone Temperature. If the zone temperatures of the direct-fired furnace are too low, then the strip will be oxidized. Usually, these zone temperatures are maintained at a minimum of 1100 °C (2000 °F). Control of the direct-fired furnace is best accomplished by turning zones off, rather than by turning them down. It is best to leave the last zone (the one next to the radiant-tube furnace) on full fire and then add or turn off the other zones in order, as needed. A relationship between strip exit temperature and zone temperature is critical to ensure proper cleaning.

Dwell Time. Even under perfect nonoxidizing conditions within a direct-fired furnace, strip will become oxidized if the residence time, or dwell time, is excessive. For any given coating line, the line speeds (dwell time in the direct-fired furnace) for each gage width and grade are predetermined to achieve the highest production rates consistent with producing the desired physical and mechanical properties. This virtually eliminates the possibility that well times will be used that are long enough to oxidize the strip.

A waste-gas preheater is an unfired section of furnace that is put ahead of the direct-fired furnace in the coating line. The exit gases pass through this furnace section before being exhausted through a damper. The incoming strip can be heated to approximately 200 °C (400 °F), just by contacting the very hot exhaust gases of the direct-fired furnace. Because this section of the furnace can be oxidizing if strip temperatures get too high, it is important not to make this section too large. Too much strip preheat will either oxidize or bake on the oils and carbonaceous material, which will seriously affect the ability of the direct-fired furnace to clean the strip. Direct-fired furnaces with waste-gas preheaters produce a tremendous amount of strip heating and cleaning in a relatively small amount of floor space. In addition to their cleaning capability, they produce a means to pressurize the rest of the furnace to negate the problem of small furnace leaks.

Radiant-Tube Furnace. Gas-phase surface preparation in a radiant-tube furnace allows surface iron oxide to be converted to pure iron, using heat and a hydrogen atmosphere. The radiant-tube furnace is so named because the insides of tubes that are positioned throughout the furnace are heated by external burners. These hot tubes radiate heat to the inside of the furnace without exposing the furnace to either air or combustion products.

Radiant-tube furnaces usually operate with an atmosphere mixture of hydrogen and nitrogen. The steel strip that enters the reducing, or radiant-tube, furnace will be heated under this reducing atmosphere. Hydrogen reacts with surface oxides to produce pure, nascent iron and water vapor (dew point). After the strip is cleaned and annealed to achieve the desired properties, its temperature is adjusted (cooled) to a level close to that of the molten coating bath. This is accomplished by passing the strip through an unheated furnace with cooling tubes that are similar to the radiant tubes, except that only air is blown through them.

A more efficient method of cooling the strip is to use gas-jet coolers, which remove the hot furnace atmosphere and pass it over a heat exchanger to cool, before returning the atmosphere to the furnace, where it contacts and cools the hot strip.

Process Variables. The atmosphere of the radiant-tube furnace must be maintained as reducing to steel, in order to achieve the desired surface preparation. This condition is controlled by the amount of hydrogen, the temperature of the strip, and the amount of water vapor in the furnace. Figure 3 shows the relationship of these variables as a function of temperature versus the water/hydrogen ratio.

Fig. 3 Theoretical equilibrium relationship between iron, iron oxide, hydrogen, and water vapor

Another concern is the infiltration of free oxygen into this atmosphere. Oxygen levels between 50 and 100 ppm have been shown to be detrimental to proper surface preparation for hot-dip coatings.

Wet chemical precleaning and strip heating in a hydrogen-nitrogen atmosphere has certain advantages. The mechanical properties of the final product can be predetermined before coating, and then a low-temperature cycle can be used during coating to preserve these properties. This practice does have higher production costs than the in-line annealing processes. Advantages are that less furnace refractory maintenance is required and that there are many skilled builders of radiant-tube furnaces.

The relatively low-gradient heating that characterizes this method requires a long overall furnace length. Wet chemical precleaning requires higher overall maintenance of equipment and solutions. The cleaning section of the coating line must provide a properly dried and rinsed strip, or else problems can develop with alkaline carryover to ceramic-coated hearth rolls.

The maintenance of a radiant-tube furnace is higher, because tubes are fired harder than they are in a direct-fired furnace. There is greater vulnerability to furnace leaks, because it is difficult to maintain significant positive pressure of the hydrogen-nitrogen atmosphere.

The direct-fired preheating method provides positive and consistent thermal cleaning of cold-rolled strip. It is therefore the preferred combination for most modern coating lines. Direct-fired preheaters provide rapid, high-gradient heating, which requires a much shorter furnace length than radiant-tube furnaces with a wet cleaning capability. This system is less sensitive to furnace leaks, because a high positive furnace pressure is maintained by the controlled discharge of combustion products. Fewer radiant tubes are required, which is a factor that reduces the possibility of furnace leaks. In-line annealing is more economical and produces equivalent mechanical properties when modern stabilized substrates are used.

Refractory maintenance is much greater in a direct-fired section than in a radiant-tube furnace. The direct-fired section lends itself to both horizontal and vertical configurations. The steel strip is heated in the direct-fired section to temperatures that are high enough to effect positive shape correction by stretching before tracking difficulty can occur.

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