The carbon content of cast iron roll is about 2.5%~3.5%. According to the main material, it can be divided into four categories: ordinary cast iron roll, high Ni-Cr infinite cold-hardened composite cast iron roll, high chromium composite cast iron roll and alloy nodular cast iron roll. Common structures in cast iron rolls can be divided into three categories: matrix, cementite and graphite. The matrix structures are mainly tensitic, ferrite, pearlite, bainite and martensite. The forms of graphite in cast iron generally include flake and spherical:
Ordinary cast iron rolls can be divided into cold cast iron rolls, medium and low alloy infinite cold cast iron rolls, medium and low alloy nodular cast iron rolls, cold cast iron is a kind of cast iron which is obtained by using the method of the supercooling of the hot iron itself and the freezing of the die surface. The surface of the roll body is chilled to produce a white layer, which has high hardness and good wear resistance. According to the manufacturing process and core material, the cold cast iron roller can be divided into three categories: non-ductile iron, ductile composite, and spherical core. Infinite cold-hardened cast iron roll is a kind of material between cold-hardened cast iron and gray cast iron. There is graphite in the matrix of the working layer of the roll body, and there is no obvious boundary between the working layer of the roll body and the core. The rubber roll is this kind of low-end roll. Ordinary cast iron rolls are mainly used in sheet rolling mill, three-high Lauter medium plate rolling mill, wire rolling mill, bar rolling mill and section rolling mill.
Unlimited-hardened cast iron is a material between cold-hardened cast iron and gray cast iron. There is a uniform distribution of graphite in the matrix of the working layer of unlimited-hardened cast iron roll body. The content of graphite increases with the increase of depth from the surface of the roll body, and the hardness decreases accordingly. It is also called unbounded cold-hardened cast iron roll. When the material of cold-hardened cast iron roll contains higher alloying elements of chromium, nickel and molybdenum, it is high nickel-chromium cold-hardened cast iron roll, which is produced by full washing (overflow method) or centrifugal composite casting process.
High Ni Cr infinite chilled cast iron roll has high abrasion resistance because of the presence of large amount of carbide in the matrix of the working layer of the roll body. The graphite in the matrix makes it have good heat resistance and intensity. It is widely used as the finishing roll of wide, medium and thick plate rolling mill and strip rolling mill.
- High chromium composite cast iron roll
The high chromium composite cast iron roll is made of high chromium white wear-resistant cast iron containing 12-22% chromium as the outer material of the roll body, and generally uses nodular cast iron as the material of the roll core and neck. The high alloy composite cast iron roll is produced by centrifugal composite casting process. Due to the presence of strip-like Cr7C3 eutectic carbides, chrysanthemum-like Mo2C eutectic carbides and granular Cr23C6 secondary carbides in the matrix, high chromium cast iron rolls have excellent wear resistance. It is widely used for roughing and finishing rolling of hot strip rolling mill, roughing and finishing rolling of wide and medium thickness plate rolling mill and finishing rolling of small section steel and plate rolling mill.
4, alloy nodular cast iron roll
Alloy nodular cast iron rolls have high tensile strength, heavy load and good abrasion resistance because of the uniform distribution of graphite from the roll body to the core. The matrix structure of alloy nodular cast iron roll can be roughly divided into two categories according to the matrix structure of the roll body: Pearlite nodular cast iron roll and acicular nodular cast iron roll, alloy nodular cast iron roll is generally cast as a whole, but acicular cast iron roll due to high alloy content, casting stress, can be used centrifugal composite casting process, to obtain the ideal comprehensive performance. Alloy nodular cast iron rolls have good thermal shock resistance and wear resistance, and are widely used in large blooming mill, section mill, bar joint mill and large seamless tube mill.
Cast steel roll can be divided into two types: steel roll (carbon content 0.4~1.4%) and semi-steel roll (carbon content 1.4~2.4) Steel roll has low carbon content, so the hardness is low, its structure is generally composed of ferrite and pearlite, with the increase of carbon content, ferrite reduced, pearlite volume increased. In order to improve the hardness, Cr, Ni, Mo and other alloying elements can be added, or special heat treatment, because the steel roll has high tensile strength and toughness, suitable for high rolling temperature, large rolling load of the billet mill or blooming mill, can also be used as a support roll. In case of special heat treatment, high hardness of HS90 or above is obtained, it can also be used as cold rolling work roller.
The carbon content of semi-steel roll is between cast steel roll and cast iron roll, its structure is mainly composed of pearlite, and contains certain carbide, so its strength is slightly higher than cast steel roll, lower than cast iron roll, and its hardness is higher than ordinary cast steel roll. The main characteristic of semi-steel roll is that the internal hardness drop is very small, so it is especially suitable for the manufacture of deep hole roll. At the same time, it is widely used in roughing and finishing stand of section steel mill and the front work roll of roughing and finishing rolling of hot strip mill. If the content of silicon is increased in the material of semi-steel roll, it has high thermal cracking resistance and strong toughness through graphitization treatment, and can be used as the opening roll of blooming mill, the large section steel rough roll, the opening roll of continuous rolling mill and the standing roll of hot strip rolling mill.
According to its chemical composition and heat treatment process, the outer matrix of the roller body can be made into pearlite and bainite. Pearlite can be flake or spherical, or sorbitite type tempered sorbitite not only has high strength and toughness, but also has excellent thermal cracking resistance, especially with phase tempered sorbitite structure also has excellent wear resistance, so when used in blooming mill, rolling effect is good.
Alloy cast steel roll has high tensile strength and toughness, certain wear resistance and heat cracking resistance, and is mainly used for the billet roll of blooming mill and rough roll of section mill.
2, casting high chromium steel roll
The high chromium steel roller is made of high chromium steel (C: 0.5~1.5%, Cr: 8~15%) as the outer material of the roller body and ductile iron as the core material by centrifugal composite casting process. Because the outer layer of the roller body has more chromium rich matrix structure than the high chromium composite iron material, the wear resistance is excellent. In addition, due to the low content of carbide in the substrate of the working layer of the roll body, the high chromium steel roll has excellent hot cracking resistance. Based on this, the high chromium steel roll is the development trend of coarse roll selection in hot rolling mills at present.
- Centrifugal composite high speed steel roll
The high speed steel roll not only has high thermal cracking resistance, but also its high wear resistance and red hardness can greatly prolong the life of the roll and improve the quality of the rolling material. It can be used for the front work roll of the tropical tandem mill finishing rolling.
The carbon content of semi-steel roll is usually 1.4~2.4%, and its mechanical properties are between cast steel and cast iron. It has the characteristics of small hardness drop and high wear resistance. It is widely used in rough rolling and middle rolling stand of section steel mill, rough rolling and finishing rolling of hot strip mill. Eutectic carbides and pseudo-eutectic carbides exist in the matrix, which are used for the stand roller of billet continuous rolling, the middle stand of section mill and the roll of finishing mill. In order to improve the crack resistance, silicon content in the composition of the semi-steel roll is increased to become the graphite cast steel roll. The advantages of cast steel and cast iron are concentrated and widely used in the roll of billet hot rolling mill and the roll of section steel mill.
Forged steel roll through smelting, forging, heat treatment, so that the working layer of the roll body has uniform high hardness, high wear resistance and excellent accident resistance, and at the same time ensure that the roll neck and the core of the roll body have a certain strength and high toughness. Forged steel roll has this unique property, which makes it in cold rolling and non-ferrous rolling, casting roll has irreplaceable advantages. According to the service conditions, forged steel roll can be divided into hot forged steel roll and cold forged steel roll. The hot forged steel roll is mainly used for hot rolling billet and section steel rough roll, and the cold forged steel roll is widely used for cold roll. The cold roll of forged steel has high application requirements, and the surface of the roll body must have high and uniform hardness to ensure the dimensional accuracy and good surface quality of the cold rolled strip or steel plate. The roll body has a certain depth of hardened layer and has high accident resistance. Therefore, the original structure of the roll is required to be very strict, one is the pure metallurgical quality, the other is the uniformity of the structure. At present, cold roll material containing chromium 2% is mainly used in our country, the work layer above HS90 is about 10mm, and the newly developed Cr5 series cold roll containing chromium 5%, the work layer above HS90 is more than 30mm, C5 series cold rolling work roll embodies the excellent properties in all aspects, will gradually replace the traditional C2 series cold roll, It becomes the development direction of cold work roll.