In facility producing metal sheet materials, in particular a steel-sheet process line, when making the conveying rolls rotate at a high speed to run steel sheet, slip and walk of the steel sheet, sticking of dust and buildup on the surfaces of the conveying rolls, and other phenomena occur. At the time of processing, if slip of the steel sheet occurs, slip flaws are formed on the surface of the steel sheet, and the surface quality is impaired. If walk occurs, the processing speed falls and the productivity drops.
Further, if dust sticks and buildup occurs on the surface of the conveying roll, the shape of the dust and the foreign matter is transferred to the surface of the steel sheet whereby the surface quality is impaired and the grade of the steel sheet dramatically falls. Not only that, touchup to remove the foreign matter stuck to the surfaces of the conveying rolls becomes necessary and the productivity drops.
Such a problem is particularly remarkable in hearth rolls in continuous annealing furnaces processing sheets in a high speed atmosphere.
Slip and walk of the steel sheet occur due to insufficient frictional force between the steel sheet and rolls. Therefore, to prevent slip and walk, it is necessary to increase the surface roughness of the conveying rolls to raise the coefficient of friction and to secure frictional force to suppress rising of the steel sheet. On the other hand, sticking of dust on the surface of the conveying rolls is the phenomenon of iron powder, sludge, or other foreign matter on the surface of the steel sheet sticking to the surfaces of the conveying rolls, while buildup is the phenomenon of iron, manganese oxide, etc. on the surface of the steel sheet sticking and building up on the surfaces of the conveying rolls.
To prevent these, it is effective to suppress sticking and catching of the iron powder, sludge, and foreign matter on the surface of the steel sheet causing sticking of dust and iron and manganese oxide etc. forming the source of buildup. It is necessary to reduce the surface roughness of the conveying rolls. That is, if adjusting the surface roughness of the conveying rolls to achieve both the prevention of slip and walk of the steel sheet and suppression of sticking of dust and buildup, optimization becomes difficult due to the contradictory nature of the measures.
As measures for suppressing slip and walk of the steel sheet and sticking of dust and buildup on the surfaces of the rolls, the method of forming a flame sprayed coating on the surfaces of the rolls, then dulling it (Japanese Patent Publication (A) No. 7-57904), the method of dulling the surface, then forming a flame sprayed coating (Japanese Patent Publication (A) No. 7-47766 and Japanese Patent Publication (A) No. 9-157826), the method of forming a flame sprayed coating, then polishing it (Japanese Patent Publication (A) No. 10-168527 and Japanese Patent Publication (A) No. 10-168528), and the method of dulling the surface, then forming a flame sprayed coating and further polishing it (Japanese Patent Publication (A) No. 7-22773, Japanese Patent Publication (A) No. 7-1021, Japanese Patent Publication (A) No. 7-39918, and Japanese Patent Publication (A) No. 2005-105338) have been proposed.
However, with each of these methods, achievement of both prevention of slip and walk of the steel sheet and prevention of sticking of dust and buildup on the surfaces of the rolls was difficult.
That is, in the method proposed in Japanese Patent Publication (A) No. 7-57904, lasers are used to drill holes. The side walls of the holes become sharp, so clogging by dust more easily occurs. Further, the lasered parts suffer from microcracks, so the surface roughness becomes larger and sticking of dust and buildup easily occur. Further, in the method proposed in Japanese Patent Publication (A) No. 7-47766 and Japanese Patent Publication (A) No. 9-157826, the surface of the flame sprayed coating becomes a rough surface as flame sprayed and sticking of dust and buildup easily occur. Further, with the method proposed in Japanese Patent Publication (A) No. 10-168527 and Japanese Patent Publication (A) No. 10-168528, the surface roughness Ra becomes smaller and slip and walk of the steel sheet easily occur. Further, the method proposed in Japanese Patent Publication (A) No. 7-22773, Japanese Patent Publication (A) No. 7-1021, and Japanese Patent Publication (A) No. 7-39918 reduces the surface roughness of only projecting parts formed at the surface of flame sprayed coatings. There was the problem that when conveying steel sheet softened at a high temperature, the inclined parts near the projections contacted the steel sheet and easily formed buildup at the rolls.
Further, Japanese Patent Publication (A) No. 2005-105338 proposes shot blasting or brushing to treat the surface of the flame sprayed coating and set the ratio R/R′ between a roughness parameter R (any of Ra, Rq, Rp, Rv, and Rz) measured based on JIS B0633 setting the cutoff value to an initial value and a roughness parameter R′ measured setting the cutoff value to 1/10 of the initial value to 4 or more so as to suppress buildup. However, with this method, the fine relief of the surface of the flame sprayed coating could be reduced, but there was the problem that the shot blasting or brushing ended up causing the formation of a new modified layer on the surface of the flame sprayed coating and due to this sticking of dust and buildup could not be completely prevented.