Abstract
The effects of surrounding radiation—emanating from radiation exchange with neighboring partners in indirect-fired vertical strip annealing furnaces, such as the other radiant tubes, the passing strip, and the enclosing furnace chamber—on the radiant tube lifetime were studied. In-house developed and validated numerical models were used to calculate the thermomechanical behavior, especially creep deformations and the corresponding stresses as lifetime indicating parameters. Different setups of recirculating P-type radiant tubes were investigated, including a reference case of an isolated tube. The investigations could be broadly classified into the study of the effects of different tube arrangements, burner operations (synchronous/asynchronous on/off firing), and changes of strip parameters (width/temperature). Results showed higher creep deformation of the central radiant tube in the setup with three tubes arranged horizontally in a row compared to three tubes stacked in a vertical column, even though the respective characteristic temperature values in a firing cycle were similar. Furthermore, the cases with asynchronous burner firing resulted in lower creep rates than other cases, where the burners were operating in synchronous on/off firing modes. In addition, the change of strip width had a higher impact on radiant tube lifetime compared to locally changing strip temperatures across the furnace. Alternating temperatures, caused by burner operation or process changes, such as change of strip’s speed or cross-section, and local temperature gradients were observed to be the main factors influencing the tube’s service life.
Highlights
In the steel industry, vertical strip galvanizing lines are used for the production of high-quality coated steel strips to be utilized, e.g., in making automotive body parts
The indirect-fired vertical strip annealing furnace in such a continuous strip galvanizing line (CGL) usually comprises of metallic radiant tubes (RT), arranged in several columns with the strip driven between them (Figure 1a)
At a typical cost of 7000 € for a single double-P-type radiant tube, direct tube replacement costs of approx. 260,000 € per annum could be expected for one such CGL alone
Summary
Vertical strip galvanizing lines are used for the production of high-quality coated steel strips to be utilized, e.g., in making automotive body parts. The indirect-fired vertical strip annealing furnace in such a continuous strip galvanizing line (CGL) usually comprises of metallic radiant tubes (RT), arranged in several columns with the strip driven between them (Figure 1a). The furnace, commonly divided into heating and soaking sections, normally operates under a protective gas atmosphere to prevent oxidation of the steel strips. The radiant tubes serve the purpose of heating the steel strips and simultaneously separate the combustion gases from the protective gas atmosphere in the furnace. The potential for cost savings for several such CGLs justifies any effort to understand the factors influencing radiant tube lifetime. To this end, it is important to develop
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