Clarification of hydrogen trap formation mechanism in Al-killed and IF steels for vitreous enamelling. Evaluation of analytical techniques for determination of fish-scale susceptibility

A deep investigation has been carried out on the fish-scaling phenomenon in enamelled thin plate steel products. Fish-scaling is one of the most insidious surface defects in completely enamelled steel sheets as it can occur unexpectedly in finished products also a long time after manufacturing. Fish-scaling consists of a large number of splinters which break away from the enamel outer surface. Fish-scaling is caused by an excess of hydrogen dissolved into the steel during the enamelling process, especially during enamel firing at a temperature of 800 - 850 °C. The hydrogen solubility in steel steeply decreases during subsequent cooling, and, in the absence of any binding agent, it will progressively migrate at the steel-enamel interface in quantities that can cause fish-scaling even after a lapse of time. The damage will occur in those interface areas where a high hydrogen pressure in oversaturated solution into steel builds up. The most wanted characteristics for enamelling steel plates are excellent drawability, excellent enamel adhesion and high resistance to fish-scale defect. The European industrial production of enamelling grade steels was characterized by the well known Al-killed type. Today new enamelling interstitial free (IF) steels produced by Continuous Annealing Process Line are appearing on the market. The fish-scaling behaviour characterization of these new IF steels, together with the development of a new test method to detect fish-scaling susceptibility are the main project goals. To reduce the probability of fish-scaling it is necessary to reduce the adsorbed hydrogen into steel and provide an adequate system for fixing the remaining hydrogen in the steel (trapping). Therefore any control criterion must evaluate not only the quantity of hydrogen that can be stably trapped, but also what may remain free and dangerous for the surface enamel layer. The steel fish-scaling characteristics depend on the number and nature of the hydrogen binding agents inside the metal, as said in other words, traps. Traps are responsible for hydrogen capturing after the enamelling process. Hydrogen trapping agents are microvoids, metallic inclusions, carbides and non-metallic particles. Other traps are grain boundaries, dislocations, interface matrix-precipitates or second phases and various kinds of lattice defects. These sites can act as traps for hydrogen atoms because of deepening of the associated potential well. Traps are normally classified as "strong" or "irreversible", when the probability for hydrogen to be released is extremely low, or "weak" or "reversible" when the probability of hydrogen release is higher. Up to now there were two different modalities for evaluating the fish-scale susceptibility in steel for enamelling: an indirect modality and a direct one. To identify the trapping characteristics of thin plate steels, permeation experiments have to be carried out. A permeation experiment consists of hydrogen crossing of a completely outgassed thin steel plate: one side is hydrogenated and on the opposite side a hydrogen-sensitive atoms flux measuring equipment is placed. The experiment result is a graph with time on the abscissa and hydrogen atoms flux on the ordinate. Three different commercial fish-scaling detecting instruments are available: Strohlein, based on an indirect measurement, evaluates the volume variation in a capillary due to the pressure of the hydrogen that cross the sample. The susceptibility to the defect is currently controlled by the first-passage permeation time after Strohlein or ATEL. These methods give an indication of the susceptibility of the tested material to the defect, but they are far from being accurate. Hyperm, based on an indirect measurement, evaluates the current variation due to the permeated hydrogen. Enamelling test (Smalto Test), based on a direct measurement, evaluates the behaviour of a specimen enamelled in humidity condition critical for the defect formatio