The metallographic structure of ductile iron is mainly composed of spheroidal graphite, cementite and metal matrix. Because the graphite is spherical and evenly distributed, the properties of the matrix structure are fully developed. It is this structure that makes the nodular cast iron roll have high strength, excellent thermal crack resistance good wear resistance, and can withstand large rolling load. Therefore, it is widely used in steel, plate, strip and steel mills.
The properties of nodular cast iron roll mainly depend on its structure, which in turn is affected by chemical composition, cooling rate, spheroidization inoculation and heat treatment. Reasonable composition design, effective nodularization and inoculation treatment, and strict control of casting process and heat treatment process are the keys to ensure excellent performance of nodular cast iron rolls.
Main types of Pearlitic ductile cast iron rolls
There are many kinds of Pearlitic ductile cast iron rolls, and the common ones are mainly divided into the following three categories:
Indefinite chilled cast iron roll
The roll body of this kind of roll is formed by metal mold casting, which can adopt static integral casting or composite casting process. Due to the slow solidification rate, the content of cementite decreases gradually from the surface to the core, while the content of graphite ball increases. Therefore, the roll has good wear resistance, while the core has high strength and toughness
With the increase of alloy-element content, the matrix structure of the roll body will change from pearlite to fine pearlite, bainite or martensite, and the hardness range is generally between HS55 and 75. Typically, these rolls are stress-relieved annealed and are mainly used in medium or pre-finishing stands of sheet, section, wire and tube mills. After normalizing and tempering treatment, the strength of the roll neck can reach 400 ~ 500MPa, making it suitable for large billet stand and blooming mill stand.
Pearlitic ductile cast iron rolls
This roll contains high alloying elements (mainly Ni, Mo), and its metallographic structure is composed of spherical graphite, cementite and bainite. Compared with Pearlitic ductile cast iron rolls of the same hardness, the wear resistance of needle nodular cast iron rolls is stronger, and with the increase of hardness, its wear resistance will be further improved.
After high temperature heat treatment, the hardness of these rolls in the radial direction of the roll body changes little, and the thermal crack resistance is enhanced. Needle nodular cast iron rolls can be manufactured by static monolithic casting, composite casting or centrifugal casting processes. Among them, the composite acicular nodular cast iron roll produced by centrifugal casting has a minimal decrease in the hardness of its working layer, which further improves the service life.
This roll is mainly used in hot rolling billet, shape steel mills, pipe, and wire bar mills, which can maintain stable performance under high load and high-temperature environments.
Ductile iron roll occupies an important position in the steel rolling industry because of its unique metallographic structure and excellent performance. Through reasonable composition ratio, scientific casting process and precise heat treatment control, it can ensure the best performance of the roll in actual use, improve the rolling efficiency and extend the service life.