It is well known that scale layer on work roll forms in hot sheet rolling of steel and scale layer on work roll plays an important role for hot rolling process. The formation conditions of scale layer on work roll are slightly known qualitatively and are hardly understood quantitatively. In order to investigate quantitatively the conditions of scale formation, three steels with different Si content are used and the slip rolling is carried out at a constant roll speed changing the scale thickness of steel workpiece and the reduction. The formation conditions of scale layer on work roll are examined quantitatively by observation of work roll surface after slip rolling. The experiments are carried out at constant rolling conditions of a velocity ratio of 20, a rolling speed of 50 m/min and a furnace temperature of 800 °C, changing the rolling reductions of 0.3, 0.5 and 1.0 mm and scale thickness of workpiece. The colza oil is used as base oil. The emulsion concentration is 3.0%. The emulsion temperature is controlled at 40 °C. Scale layer on work roll forms easily with increasing rolling reduction and decreasing scale thickness of workpiece for three steels A, B and C. In order to estimate quantitatively the formation condition of scale layer on work roll, parameter α which is given by a ratio of the rolling reduction to scale thickness of workpiece is proposed. Scale layer on work roll forms when values of parameter α become same for each steels. Values of parameter α become larger in order of steels A, B and C and it can be understood that scale layer on work roll forms easily in order of steels A, B and C. When FeO layer in scale of the steel surface adheres on work roll surface, it is expected that scale layer on work roll forms easily and strongly by transformation from FeO to Fe3O4, considering that the chemical composition of scale layer on work roll is Fe3O4.

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