In facilities for production of metal sheet materials, in particular steel-sheet-making process lines, when making a conveying roll rotate at a high speed to feed steel sheet, the phenomena of slipping and snake motion of the steel sheet, deposition and buildup of dirt on the surface of the conveying roll etc. occur.
In particular, in a continuous annealing furnace, the hearth rolls convey steel sheet in a high temperature state, so buildup easily occurs on the hearth roll surfaces. When such buildup occurs, the shapes of the buildup are transferred to the steel sheet surface whereby the surface quality is impaired and the grade of the steel sheet deteriorates. Not only this, at the time of periodic repair, it is necessary to clean the hearth rolls to remove foreign matter deposited on their surfaces, so this becomes one cause of a drop in productivity.
The buildup on the hearth roll surfaces is the phenomenon of the iron, manganese oxide, etc. on the steel sheet surface sticking to and building up on the hearth roll surfaces. To prevent this, it is effective to suppress the reaction of the sources of buildup, that is, iron, manganese oxide, etc. with the hearth roll surfaces or facilitate removal of reaction products.
As a measure for suppressing buildup on a hearth roll surface, a coating obtained by providing a flame sprayed alloy layer comprised of only a heat resistant alloy on a hearth roll, flame spraying particles of a carbide or a mixture of a carbide and oxide on the sprayed alloy layer, and depositing metal oxides comprised of Cr2O3 and Al2O3 on the outermost layer of the surface to give a chemically converted layer has been proposed (for example, see Japanese Patent Publication (B2) No. 8-19535).
However, the Cr2O3 and Al2O3 provided at the outermost layer of the surface easily react with manganese oxide, so the coating has the problem of easy generation of buildup due to manganese oxide.
Further, a coating containing, by wt %, chrome carbides: 10 to 25% and Ni: 5 to 15% and having a balance of one or more of a carbide and boride of tungsten and unavoidable impurities has been proposed (for example, see Japanese Patent Publication (A) No. 3-86306).
However, with this coating, WC oxidizes at a high temperature and the coating peels off, so there is the problem that use for a long time in an annealing furnace is not possible.
Furthermore, a coating comprising a cermet material of a particle structure comprised of, by wt %, 50 to 90% of chrome carbides and a balance of unavoidable impurities and a nickel-chrome alloy, at least 70% of the carbide particles enclosed by the alloy, and having an average particle size of 5 to 100 μm has been proposed (for example, see Japanese Patent Publication (A) No. 6-116703).
Further, a cermet coating containing at least one of CrB2, ZrB2, WB, TiB2, and other borides in 1 to 60 vol %, containing at least one of Cr3C2, TaC, WC, ZrC, TiC, NbC, and other carbides in 5 to 50 vol %, and having a balance of substantially metal is provided (for example, see Japanese Patent Publication (A) No. 7-11420).
However, these coatings contain chrome carbides resistant to buildup at a high temperature, but if used for a long time in a continuous annealing furnace, the metal ingredients and chrome carbides in the coatings react resulting in the coatings become brittle and the coatings peeling off.
That is, in the past, various coatings have been proposed, but none of these coatings have been able to completely prevent buildup on hearth roll surfaces.