Eliminating the sources of carbonaceous deposits and optimizing furnace atmosphere conditions can produce a clean final product in bell-furnace hydrogen annealing of steel strip.

Fig 1 Ebner HICON/H2(r) bell-annealer facility

Annealing steel strip, rod and wire using advanced bell type furnaces using 100% hydrogen atmosphere produces more consistent mechanical properties and better surface finish than those obtained by annealing in conventional bell type furnaces using HNX (purified exothermic and nitrogen plus 5-7% hydrogen) atmospheres. These furnaces increase bell-furnace throughput, reduce operating costs and provide superior surface quality of the annealed product [1-4]. In addition, the shorter heating and cooling times shorten floor to floor times, increasing productivity. Advanced bell type furnaces made by Ebner Furnaces Inc. (the HICON/H2(r) bell annealer shown in Fig. 1), LOI Inc., Rad-Con and others use high-convection systems that benefit from the high thermal conductivity and low density of hydrogen to provide a high-quality annealed product. The highest quality annealed steel strip, wire and rod is produced by using the optimum hydrogen flow rate profile and annealing cycle to give the most uniform temperatures within the load, the most effective evaporation and evacuation of the rolling lubricant and significantly reduced carbonaceous deposition.

Occasionally, carbonaceous deposition occurs in hydrogen atmosphere annealing in advanced bell type furnaces. In many, if not the most cases, it is possible to reduce or eliminate the problem of carbonaceous deposition encountered in hydrogen annealing of cold-rolled steel strip. One possible approach that can be used in troubleshooting and hydrogen atmosphere technology optimization for reducing the carbonaceous deposition on the annealed carbon and low alloyed steel strip involves:

  • Examining the lubricant and rolling-mill lubricant systems.
  • Obtaining baseline data on the current annealing practice including furnace atmosphere analysis.
  • Identifying all causes of carbonaceous deposition for the analyzed annealing cycles and determining how they relate to the main general causes of carbon deposition. The main causes are the presence of rolling lubricant and tramp oils on the cold rolled strip, thermocracking (dissociation) before evacuation of evaporating rolling lubricants and tramp oils during heating up stage of the annealing cycle, methanation and carbon back deposition [5-7] and formation of graphite as a result of the decomposition of carbide on the surface of the low carbon steel sheets [8,9]. (See Troubleshooting Hydrogen Annealing of Steel Strip: Part 1, February 2003 IH.)
  • Optimizing annealing cycles for the best surface quality of annealed steel strip and minimum hydrogen consumption per ton of annealed product.


Schematic of Ebner HICON/H2(r) bell-annealer

Examining lubricants and rolling-mill lubricant systems

Constituents that make up lubricants used in cold rolling of low-carbon and low-alloy steel strip, as noted in an early reference [10], as well as recent information from lubricant suppliers, such as Quaker Chemical Corp. (Conshohocken, Pa., USA headquaters) and others, include:

  • Mineral oils or solvents (paraffinic, naphthenic, aromatic)
  • Synthetic fluids (polyesters, dibasic acid esters, polyglicols)
  • Compounded oils (mixtures of mineral or synthetic oils with fatty oils or other additives)
  • EP (extreme-pressure) oils (mineral or synthetic oils containing highly active species, for example, free or only loosely bonded sulfur)
  • Water-base (solutions, emulsions, gels, pastes)
  • Mineral oils, fatty oils, synthetic oils and EP oils can be used in water-oil combinations (emulsions) as lubricant-coolers.

In the U.S., the most common cold-rolling lubricant-cooler is a semisynthetic lubricant containing synthetic oil mixed with mineral oil, vegetable and animal fat (in different proportions) in emulsions (at different concentrations). Mineral and vegetable oils (rape, coconut, olive, etc.) vaporize readily and do not dissociate into heavy fractions that deposit carbon on the steel strip surfaces [10]. In Europe, fully synthetic oils in emulsions at different concentrations are mostly used. Fully synthetic oils have an advantage of staying fluid at low temperatures during winter (i.e., they do not coagulate in cold weather like the animal and vegetable oils). The content of lubricant-coolers depends on the quality of material, degree of deformation (reduction) per pass, number of passes and type of rolling mill (tandem, reversing, cluster Steckel and Sendzimir).

After cold rolling, the rolled steel strip has lubricant and tramp oils on its surface. Tramp oils are a mixture of hydraulic oils, gear oils, morg oils (oils for bearings) and shipping oils of varying concentrations. The higher molecular weight of the morg oils implies a higher evaporation temperature, closer to the cracking temperature, which may not allow sufficient time for evaporation and evacuation of these oils from the surface of the strip during heating up.

A study [11] of the lubricants used for two different rolling mills concluded that a mixture of approximately 30% tramp oils and 70% active rolling oil can cover the surface of the strip steel rolled at a tandem mill. At a reversing mill, the surface of the rolled strip steel can be covered with a mixture of approximately 25% tramp oils and 75% active rolling oil. The tramp oil mixture could contain approximately 80% hydraulic oil, 10% of a mixture of gear oil and morg oil and 10% shipping oil.

Thermogravimetric analysis (TGA) diagrams of rolling oils [11] revealed a dissociation temperature of approximately 470 C (875 F). The rolling lubricant supplier recommended 3 to 3.5% rolling oil in the emulsion. Because of the addition of excess rolling oils and contamination with tramp oils, the content of oil in the emulsion had become too high at 6%. As in most annealing applications, the rolled steel strip coils were not cleaned between rolling and annealing, and sometimes were held for one or two days before being annealed. Long exposure of oil-covered strip to air results in oxidation and transformation of the oil into a gummy film, which during annealing, causes difficult-to-remove heavy adherent carbon residue [12].

Removing lubricants and tramp oils from the steel strip before annealing allows a higher rate of heating up to soaking temperature and a significant increase in furnace productivity. This takes advantage of the full capacity of the advanced bell type furnace and produces a high quality surface of the annealed product, especially when using 100% hydrogen atmosphere. However, it should be noted that even very clean steel strip, after cleaning and waiting for a few hours to be annealed in an environment with high moisture or having some condensed water on the freshly cleaned steel surface, could show some light reddish-color corrosion, or "flush rusting."

Fig 2 Temperature and gas flow rate (a) and furnace gas analysis (b) for a typical annealing cycle

Annealing practice baseline data

Several runs (nine) using different annealing cycles carried out in advanced bell type furnaces were analyzed to find solutions to reduce or eliminate heavy carbonaceous deposition. Figure 2a and 2b show a representative example of nitrogen and hydrogen flow rate patterns and the variation of furnace atmosphere components during a selected annealing cycle (temperature vs. time).

Note that nitrogen is used for purging out the air at the beginning of the cycle before introducing hydrogen in the furnace and for purging hydrogen atmosphere at the end of the annealing cycle before removing the inner bell [13]. Typically, after loading and sealing down, the bell furnace was purged with five volume changes of nitrogen in about one-half hour [13]. When the bell atmosphere reached a safe level of oxygen (0.5% O2), hydrogen was introduced to displace the nitrogen and heating was started.

After a pre-set period in the second half of the soaking stage and during cooling stage of the annealing cycle, the bell was capped (i.e., the outlet or exhaust valve was closed) so the hydrogen atmosphere could not be evacuated. Methane is present during the heating-up stage of the annealing cycle in hydrogen atmospheres, as in nitrogen/hydrogen atmospheres. Figure 2b shows that the methane content in the furnace atmosphere increased to a peak of 6.6% CH4, after closing the outlet (exhaust) valve during the second part of the soaking and beginning of cooling. Other similar cycles reached higher methane peaks (7.85% CH4).

At the end of the soak period, the furnace was removed and the bell allowed to cool below 400 C (750 F), assisted by a water spray. It should be noted that the hydrogen flow rate was zero during the time between closing the outlet (exhaust) valve at the beginning of the second half of soaking and opening the outlet valve at the end of cooling and starting the nitrogen purge. The load was cooling in hydrogen atmosphere with the outlet valve closed. When the load reached 100 C (212 F), hydrogen was purged out with nitrogen, and the bell was removed to unload the work. In six of the nine runs, heavy black carbonaceous deposition was obtained on the surface of the strip.

Fig 3 Temperature and gas flow rate (a) and furnace gas analysis (b) for a modified annealing cycle

Optimizing annealing cycles

Although it may have been possible to modify rolling practices to reduce the amount of tramp oils and to improve strip wiping to remove more lubricant, such modifications are expensive and take time to implement. Efforts were, therefore, concentrated on more appropriate annealing cycles that could be applied immediately. However, it was recommended to minimize the waiting time between the rolling and annealing operations to avoid oxidation of oil and its transformation into adherent residue during annealing. Fresh emulsions are significantly easier to remove during annealing.

Atmosphere flow rates and atmosphere flow rate patterns were developed to obtain consistent results for different kinds and amounts of deposited lubricants on cold rolled strip. The strip was received from rolling mills for annealing without previous cleaning. No changes were recommended to the basic annealing cycles with regard to nitrogen flow rates. Some changes in the temperature profile during heating up for some cycles were tested. Most tests were related to the changes in hydrogen flow rate and flow rate pattern.

Various hydrogen flow rates and flow rate patterns during the second half of soaking and beginning of cooling were tested to maximize the evacuation of methane (CH4) and ethylene (C2H4). During the second half of soaking and beginning of cooling, the exhaust valve was turned on and off to reduce CH4 and C2H4 contents in the furnace atmosphere [5-7] and to avoid excessively high hydrogen consumption. Hydrogen flow rate and timing and frequency of hydrogen flow-rate pulses were optimized to extract and evacuate most of the methane and ethylene during the second half of soaking and beginning of cooling.

The times at which peaks in CO and CH4 vol% variation occured during annealing cycles were recorded. Results from a typical cycle are shown in Fig. 3. The modified hydrogen flow rate pattern shown in Fig. 3a reduced the methane peak considerably as shown in Fig. 3b, although there was, in this case, a higher carbon monoxide peak early in the cycle, probably due to the presence of more emulsion on the coils than in other tests.

These data together with data from other cycles enabled determining optimum cycles for the selected annealing cycles and materials. Figure 4 shows an optimized hydrogen flow rate pattern for an annealing cycle with a soaking temperature of 700 C (1292 F).

Using these optimized cycles, it was possible to consistently reduce carbonaceous deposits and meet the high quality requirements for the carbon and low-alloy steel strip of a particular application. No carbonaceous deposition was seen on the annealed strip during the visual inspection.

Fig 4 Optimised flow pattern

Carbonaceous deposit-reduction recommendations

Note that there are cases when the change in hydrogen flow rate pattern may not be sufficient to reduce or remove the carbonaceous deposition. In these cases, the following general recommendations [14-16] should be considered together with recommended changes in the hydrogen flow rate pattern:

  • Select a lubricant that readily vaporizes and evacuates rather than one that dissociates into heavy fractions, which may deposit carbon on the steel strip surfaces. Consult a lubricant supplier for technical advice.
  • Verify the oil content in the rolling emulsion, and, if necessary, reduce as recommended by the lubricant supplier.
  • Check and replace worn rubber and felt wiper blocks used to clean the strip at the rolling mill.
  • Check for oil in the compressed air used in the air wiping system if one is used to clean the strip at the rolling mill; oil could come from leaks in the compressor. Oily compressed air can be a source of contamination of the residual emulsion and cause carbon deposition on the steel strip. It may also be necessary to increase the clean air flow rate at the rolling mill to remove more lubricant.
  • Eliminate leaks in the rolling mill hydraulic systems and other sources of contamination.
  • For a tandem rolling mill, rinse the rolled strip using hot water at the last stand if possible, and keep the oil enrichment in the rinsing water under 1%.
  • Use a filtration system to separate and remove contaminants (hydraulic oil, gear oil, morg oil, etc.) from the emulsion.
  • If not using a filtration system capable of separating and removing heavy oils, grease and other contaminants from the emulsion, carefully perform a skimming operation or request such service from the lubricant supplier for each rolling mill.
  • If low coiling tension is not possible at the rolling mill, use a recoiling line before annealing. A lower coiling tension of about 30 MPa (306 kg/cm2, or 4,351 psi) may significantly reduce carbonaceous deposition. Low coiling tension facilitates evaporation and evacuation of lubricant trapped between the inner turns of the strip coil [4].
  • If possible, dedicate special bases for annealing very dirty rolled steel strip coils.
  • Reduce the waiting time between rolling and annealing operations to avoid oxidation of oil and its transformation into adherent residue during annealing. Fresh emulsions are easier to remove during annealing.
  • Introduce hydrogen into the retort at the start of heating-up, after nitrogen purging, if the O2 content is less than 0.5%. (Note: Hydrogen hydrogenates lubricant hydrocarbons at approximately 400?C, or 725?F [3]. Hydrogenation reaction products have lower boiling points than lubricant hydrocarbons, and they evaporate before dissociation occurs, reducing or eliminating carbonaceous deposition.)
  • Run annealing cycles using the appropriate heating rate and/or heat step(s) during heating-up, the nitrogen and hydrogen flow rate profile and the exhaust valve open.
  • Clean thoroughly the entire exhaust system, base and retort. Check and clean if necessary the beginning of the exhaust pipe (the portion within the base) after every annealing cycle. Carbon deposition could become hard and difficult to remove.
  • Do not keep the exhaust valve continuously closed during the second half of the soaking and at the beginning of cooling.
  • Optimize the hydrogen flow rate and the time and frequency of turning on and off the supply of hydrogen (to evacuate CH4 and C2H4) during the second half or the last hours of the soaking and at the beginning of cooling to minimize hydrogen consumption and obtain superior quality of the annealed steel strip.
  • Verify the programming of the atmosphere exhaust valve to avoid having it automatically closed when the furnace is down.
  • Use the maximum cooling rate through the critical range for sooting (500 to 300 C, or 932 to 572 F). Install a water spray cooling system and use it for temperatures lower than 482 C (900 F). This reduces carbon deposition and cycle time. Reference [4] recommends that the fast cooling rate should not start until reaching a temperature lower than 600 C (1112 F) in the inner turns of the coil.
  • Install the exhaust piping in conformance with the drawings and recommendations of the furnace manufacturers. A smaller pipe diameter than that required in the original furnace manufacturer's design and too many elbows near the base could contribute to carbonaceous deposition and clogging.


Summary and conclusions

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 + 5-7% 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.

References

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