Knowing where carbonaceous deposits come from and why they occur in hydrogen annealing of steel strip is the first step in reducing or eliminating them.
|Fig 1 EBNER HICON/H2(r) bell annealer facility|
Steel strip, rod and wire products usually are heat treated in a controlled atmosphere during their manufacture to maintain a high-quality surface while improving their formability. Consistency and reliability in surface quality are important, particularly for products that will be painted or hot-dip and electrolytically galvanized before shipping. Continuous annealing furnaces (strand annealers, etc.) working separately or as a part of a continuous process line provide higher surface quality than batch furnaces.
However, advanced bell type furnaces made by furnace manufacturers including Ebner Furnaces Inc. (Ebner HICON/H2 bell type furnaces shown in Fig. 1), LOI Inc., Rad-Con and others can provide surface quality comparable to that of a continuous hydrogen-annealed product, and are more accessible and adaptable to change than continuous annealing furnaces. In addition, these high-integrity batch furnaces are compatible with 100% hydrogen atmosphere. Usually, the hydrogen content in the furnace atmosphere varies between 83 and 92%, never reaching 100%, during soaking and cooling phases of the annealing tests. During the annealing cycle, 100% high-purity hydrogen is introduced into the furnace (the retort of a bell type furnace). However, the furnace atmosphere in the retort never reaches 100% hydrogen because the product to be annealed and the furnace base, devices such as dividers and the retort are not perfectly clean, and lubricant vapor, methane, water vapor and other components are present in different amounts during the annealing cycle.
A measure of the surface quality of cold-rolled products is surface cleanliness, which is characterized by carbonaceous deposits, iron fines and oxides present on the strip . The surface quality of steel strip annealed in hydrogen is significantly better than that of the strip annealed in a nitrogen plus 5 to 7% hydrogen atmosphere or in an exothermic atmosphere due to the improved evaporation and evacuation of lubricants on the product during heat up. However, carbonaceous deposition on steel strip is occasionally encountered in 100% hydrogen annealing. Even a thin film of a carbonaceous deposit on the surface of the annealed steel strip interferes with the subsequent coating process.
|Schematic of EBNER HICON/H2(r) bell annealer|
Sources of carbonaceous deposits
Sources of problems in hydrogen annealing in batch furnaces stem from:
- The existence of rolling lubricant and tramp oils on the cold-rolled strip. Incomplete removal of rolling lubricant from the strip at the rolling mill, not using a steel strip cleaning (degreasing) operation or special cleaning equipment and not removing carbon deposits from the base and retort before annealing increase the seriousness of the carbonaceous deposit problem. Also, a smaller diameter exhaust pipe, partially or completely closed exhaust valve or too many changes in direction (elbows) of the exhaust pipe near the base can cause condensation of lubricant vapor, accumulation of vapor deposition and clogging of the exhaust pipe.
- Thermocracking (dissociation) before evacuating evaporating rolling lubricants and tramp oils during the heat up stage of the annealing cycle, which makes the removal of residuals very difficult. Under some conditions, improving the heat transfer using new technology (high convection-high flow) can increase the transient temperature gradient, causing compressive thermal stresses generated in the inner turns of the coil, which can slow down and delay thermocracking [2-4]. However, there is data demonstrating that improved heat transfer from using 100% hydrogen, whether in high convection systems or not, serves to decrease the transient temperature within coils during heating-up and cooling .
- Methanation (production of methane, or CH4) and carbon back deposition, which are related to the existence of rolling lubricants on cold-rolled steel strip and degree of surface cleanness before annealing. Methanation and carbon back deposition are specific to using a 100% hydrogen atmosphere [2-4]. The carbon back deposition can be reduced or eliminated by optimizing the hydrogen flow rate profile of the cycle in advanced bell type furnaces.
- Formation of graphite due to the decomposition of carbide on the surface of the low-carbon steel sheet [5,6]. The formation of graphite with corresponding limited decarburization of steel strip surface is possible, but negligible in most cases compared with the usual larger amounts of carbon residue from other sources. This carbide decomposition product also can be significantly reduced or eliminated by optimizing the hydrogen flow rate profile during the annealing cycle.
|Fig 2 Temperature and gas flow rate for a typical annealing cycle (top), and furnace gas analysis for a typical annealing cycle (bottom)|
Causes of carbonaceous deposition
The following are causes of carbonaceous deposits during annealing that are related to and complementary to the above-mentioned general main causes:
- Some cleaning systems installed at the rolling mills, such as air-wiping systems (using less than sufficient air flow rate and/or compressor air containing oil), rubber-wiping systems and felt-wiper blocks (which can cause scratches on the rolled strip surface) may not be able to remove rolling emulsion or other rolling lubricants from the steel strip. The varying amount of residual lubricant on the product before annealing plays a role in obtaining a consistently good quality annealed product.
- Filtration systems might not contain a centrifuge system to separate contaminants (hydraulic oil, gear oil, morg oil, etc.) from the active rolling oil of the emulsions.
- The production program might not allow annealing the steel strip coils immediately after rolling; coils sometimes are not annealed before 1 to 2 days or longer.
- Heavy carbon deposits (cold-rolling lubricant, soot, tar, etc.) are present on the furnace base, retort and the exhaust pipe (clogging), so evaporated oils cannot be evacuated from the furnace. A smaller diameter exhaust pipe than that required in the original furnace manufacturer design, a partially or completely closed exhaust valve and many changes in directions of the exhaust pipe (elbows) near the base can cause condensation of lubricant vapor, accumulation of vapor deposition and clogging of the pipe.
- A spheroidize annealing cycle might not have an appropriate heat up rate and/or a heat step at appropriate temperature(s) during heat up, and might not have a sufficiently high atmosphere flow to flush the evaporating oils before reaching the cracking temperature.
- The exhaust valve might be mistakenly programmed to be closed at soaking or during the second part, or the last hours of soaking and/or the first hours of cooling stages of the 100% hydrogen annealing cycle, such as it is in the HNX annealing of steel strip coils in bell type furnaces to reduce the consumption of atmosphere. This can be a major cause of carbon deposition in a 100% hydrogen atmosphere. Figures 2a and 2b show that the maximum concentrations of methane (as a result of methanation) occurs at the end of soaking and beginning of cooling stages of the annealing cycle. Methane cannot be evacuated, so heavy carbon back deposition occurs.
- The exhaust valve might be mistakenly programmed to be closed when the furnace system is down (unpredictably) at any time during the annealing cycle to reduce the consumption of hydrogen. This explains why the same annealing cycle (i.e., the same heat up rate and the same heat steps during heat up) and atmosphere flow-rate profiles sometimes work (producing a clean annealed product) and sometimes do not work (the annealed product being covered with the heavy black carbonaceous deposition).
The use of high thermal conductivity, low-density hydrogen at relatively high flow rates combined with the intense recirculation of the furnace atmosphere in high convection advanced bell type furnaces produces more consistent mechanical properties and better surface finish of the annealed products than those obtained for the materials annealed in conventional bell type furnaces using HNX (purified exothermic and nitrogen + 5 to 7% hydrogen) atmospheres. In addition the faster heating and cooling times shorten floor-to-floor times, thus increasing productivity.
Occasionally, carbonaceous deposition occurs in hydrogen atmosphere annealing in advanced bell type furnaces. In many cases, it is possible to reduce or eliminate the problem of carbonaceous deposition encountered in hydrogen annealing of cold rolled steel strip by optimizing the hydrogen flow rate, the hydrogen flow rate pattern and the timing and frequency of turning on and off hydrogen flow at the end of soaking and beginning of cooling stages of the annealing cycle, and, thus, reap the full benefits of 100% hydrogen atmosphere and advanced bell type furnaces.
In cases where the change in hydrogen flow rate pattern may not be sufficient to reduce or remove the carbonaceous deposition, the general recommendations presented above should be added to the changes in the hydrogen flow rate pattern.
Gasse, W.F. and Thomas, S.J, Dofasco Inc., Experience with High Convection Hydrogen Batch Annealing at Dofasco, 1992 Leroy, V., CRM, and Bouquegneau, D., Sambre, C., Belgium, Possibilities and Limits of the 100% Hydrogen Annealing Process, Techno 27, IISI Comm. on Technology, Brussels, 1995 Mataigne, M., Lamberigts, M., Leroy, V., CRM, Abbaye du Val-Benoit, B-400, Liege, Belgium: Gas-Metal Reactions During 100% H2 Batch-Annealing, Developments in the Annealing of Sheet Steels, Edited by R. Pradhan and I. Gupta, TMS, 1992 Leroy, V., CRM, Abbaye du Val-Benoit, B-400, Liege, Belgium: Surface Reactions of Steel in Batch-Annealing: Relation With Residual Carbon Contamination and Partial Selective Oxidation, Vortrag 11, HICON/H2 92, published by Ebner Furnaces Inc., Wadsworth, Ohio, and Ebner Industrieofenbau, Linz, Austria, 1992 Yano, I. et al., Carbon Diffusion on the Surface of Low Carbon Mild Steel During Its Annealing, Jour. Metal Fin. Soc. of Japan, No. 25, 1974 Leroy, V., Richelmi, J., Graas, H., Belgium: Graphite Formation on the Surface in Annealed Low Carbon Steel Sheet, C.R.M., No. 49, 1976 Mantea, S., and Dulamita, T., Teoria si Practica Tratamentelor Termice, Editura Tehnica, Romania, 1966 Jenkins, I., Controlled Atmospheres for the Heat Treatment of Metals, Chapman and Hall Ltd., London, 1946 Iron-Carbon Phase Diagram, Heat Treating Progress, April/May 2001
SIDEBAR: Carbonaceous chemical reactions in H2 annealing
A comparison between HNX (purified exothermic and nitrogen + 5 to 8% hydrogen) atmospheres and hydrogen atmosphere annealing of strip steel in a bell type furnace is given in Ref. 2-4. In HNX annealing, the atmosphere flow usually is shut off by closing the exhaust valve during the second part of soaking and/or cooling to reduce atmosphere consumption in cubic feet/ton. This is possible in HNX annealing because the concentrations of carbon dioxide (CO2), carbon monoxide (CO) and methane (CH4) in the furnace atmosphere significantly decrease after reaching peak concentrations during heating up. Closing the exhaust valve in 100% hydrogen annealing during the second part or the last hours of soaking and the first hours of cooling may produce heavy carbonaceous deposits. A detailed description of the gas-metal reactions in 100% hydrogen annealing in five different temperature ranges, the effect of degreasing before annealing, the effect of internal thermal stress and other valuable information are given in [2-4].
One reason a carbonaceous deposit (C) is obtained can be explained by the decomposition of CO (a Boudouard, or producer-gas, reaction) present in the furnace atmosphere in the critical temperature range for sooting of 500 to 300 C (932 to 572 F) given by Equation 1.
The rate of decomposition of CO at temperatures lower than 700 C (1292 F) is slow unless it is promoted by a catalytic surface, such as iron, so the surface of the steel strip and possible iron fines serve as catalysts. A hydrogen atmosphere has a very high actual (CO)2/CO2 ratio  in reaction (1); much higher than the equilibrium (CO)2/CO2 ratio in the sooting temperature range. The carbonaceous residue after the evaporation and evacuation of lubricant is labeled CTAR [2-4] to distinguish it from both the carbon deposit in reaction (1) labeled CSOOT and the carbon deposit resulting from the possible limited decarburization of low-carbon steel strip [5,6]. In this discussion, C represents all types of carbon deposit on the steel strip.
Surface-carbon removal is possible by means of a Fischer-Tropsch, or direct methanation, reaction in hydrogen atmosphere and by turning the hydrogen flow on and off at the end of soaking and the beginning of cooling stages of the annealing cycle used in high-convection, advanced bell type furnaces [2-4].
Methanation in a H2 atmosphere
Both HNX and 100% hydrogen atmospheres in the furnace contain hydrocarbon gases, such as CH4 and some ethylene (C2H4), from the evaporation of the cold-rolling lubricant during the heating stage of the annealing cycle. Additional production of CH4 (known as methanation) and very low concentrations of C2H4 is reported in 100% hydrogen atmosphere during the soaking stage and the beginning of cooling stage of the annealing cycle [1-3], and is specific to the 100% hydrogen atmosphere [2-4].
Methanation in the 100% hydrogen furnace atmosphere can be explained [2-4] by the Fischer-Tropsch reaction in Equation 2 coupled with the irreversible reaction in Equation 3.
High convection-high hydrogen flow technology uses methanation to remove carbon deposits and to provide a very clean annealed steel-strip surface [2-4]. Methane is the most stable hydrocarbon. However, it is thermodynamically unstable at temperatures higher than 500 C (932 F). Increasing temperatures favor methane dissociation according to the reversible reaction  in Equation 4.
Carbon deposition (sooting) according to reaction (4) could occur at temperatures lower than Ac1 (727 C, or 1340 F for carbon steels), but higher than 500 C [8,9].
Unlike annealing cycles using HNX atmospheres, where hydrogen is shut off to reduce consumption of hydrogen per ton of annealed steel strip, hydrogen flow cannot be turned off during the second part of soaking stage (or at the end of soaking stage) and/or at the beginning of the cooling stage using a 100% hydrogen atmosphere.
To evacuate the products (CH4 and H2O) of reaction (2) and to avoid carbon back deposition by producing CO and carbon deposition according to reaction (1), (2) and (4), it is recommended to turn the hydrogen flow on and off at the end of soaking stage and beginning of the cooling stage of the annealing cycle [2-4]. An alternative solution to the on/off hydrogen feed is to use a continuous high hydrogen flow rate during the second part of soaking and beginning of cooling stage, which could significantly increase the consumption of hydrogen per ton of annealed product from 3 to 20 m3/metric ton (100 to 640 ft3/ton) of annealed steel strip, and, consequently, increase operating costs.