According to the hardness of the roll surface, mill rolls can be divided into:
(1) Soft roller. The Shore hardness (HS) is about 30 to 40, which is used for roughing mills and piercing machines for blooming mill and large-scale steel rolling mills. The material of mill roll is generally cast steel and forged steel. When the load is not large, the gray pig iron can be used.
(2) Semi-hard roll. The Shore hardness (HS) is about 40 to 60, which is used for roughing mills for large, medium and small sections and steel mills. The material of mill roll is semi-cold hard cast iron, cast steel and forged steel.
(3) Hard surface roller. The Shore hardness (HS) is about 60 to 85, which is used for finishing mills for thin, medium, medium and small section mills and support rolls for four-roll mills. The materials are chilled cast iron and alloy steel.
(4) Extra hard roll. The Shore hardness (HS) is about 85 ~ 100 for cold rolling mills. The material is forged alloy steel containing chromium and nickel.
The mill roll is cast in chrome die or an iron mold, and the soft surface roll is directly poured in the sand mold to obtain gray pig iron; the semi-hard roll is cast in coated iron mold, and the cast iron roll cast in this way is called semi-cold hard cast iron roll; the hard-faced rolls are cast in an iron mold without paint, and the pig iron rolls obtained by this method are called chilled cast iron rolls.
The depth of the chill layer depends on the proportion of silicon and manganese in the pig iron. The hardness of the chill layer depends mainly on the carbon content and the content of an alloying element.