12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Hearth rolls in continuous annealing furnace of CGL sometimes cause the trouble that some small protrusions grow up on the surface. Such troubles are called as “build-up” or “pick-up” and it can be a cause of surface defects of steel strips such as scratches, dents and so on. A lot of tentative theory is told about “build-up” as below: (1) Producing High-alloyed steel strips accelerates the growth speed. (2) Temperature and atmosphere affect the build-up behaviour. (3) The composition of “build-up” is limited, most of all of them are consisted of MnO when the furnace atmosphere is reductive. But the details of the build-up mechanisms are not clear because it happens in high-temperature furnace. In commercial CGLs, build-ups seem to grow up very slowly during production campaign for over a month. During the campaign, the furnace temperature and atmosphere varies with time, and the composition of the steel strip is also varied. Sometimes build-ups disappear by time and cannot be collected and analysed at the maintenance term. In this study, the simplified mechanisms of hearth roll build-up are proposed, and several types of basic experiments were tried to simulate the situation of strip and hearth roll in the furnace. Selective oxidation tests by heating in reductive atmosphere were tried to clarify the oxide film formed on the strip surface. Selective oxidation is enhanced when the Mn, Si content of the strip gets higher. But the stable oxide phase is different with Mn and Si composition ratio. Reactivity of such oxides and roll surface (thermal splayed layer) was measured to clarify the build-up nucleation behaviour in the earlier stage. And the sintering behaviour of selective oxides were also investigated to clarify grow-up behaviour in the latter stage. It is suggested that the build-up behaviour can be explained and simulated by combining these basic experimental results.