This article describes the classification of rolls and the selection of roll materials.
Key words: roll; classification; material
Classification of rolls
Cast iron roll
The carbon content of cast iron rolls is about 2.5%~3.5%. According to the main material, they can be divided into four categories: ordinary cast iron rolls, high-nickel-chromium infinite chilled composite cast iron rolls, high-chromium composite cast iron rolls and alloy nodular cast iron rolls. The common structures in cast iron rolls can be divided into three categories: matrix, cementite, and graphite. The matrix structures are mainly tensite, ferrite, pearlite, bainite, and martensite. The morphology of graphite in cast iron generally has flakes. Both shape and spherical shape:
Ordinary cast iron roll
Ordinary cast iron rolls can be divided into chilled cast iron rolls, medium and low alloy infinite chilled cast iron rolls, medium and low alloy ductile iron rolls, chilled cast iron is a cast iron obtained by using the supercooling degree of molten iron itself and the quenching of the mold surface The surface of the roller body is chilled to form a white layer, which has high hardness and good wear resistance. Chilled cast iron rolls can be divided into three categories according to the manufacturing process and core material: non-ductile iron, nodular composite, and spherical core. The infinite chilled cast iron roll is a material between chilled cast iron and gray cast iron. There is graphite in the matrix structure of the working layer of the roll body, and there is no obvious dividing line between the working layer of the roll body and the core. Rubber rolls It’s this low-end roll. Ordinary cast iron rolls are mainly used in stacked thin plate mills, three-roller Lauter plate mills, wire rod mills, bar mills and section steel mills.
High nickel chromium infinite chilled cast composite iron roll
Infinitely chilled cast iron is a material between chilled cast iron and gray cast iron. Infinitely chilled cast iron rolls have evenly distributed graphite in the matrix structure of the working layer of the roll body. The graphite content is from the surface of the roll body to the inside. As the depth increases, the hardness decreases. Therefore, there is no obvious dividing line between the working layer and the core of the roll body. It is also called unbounded chilled cast iron roll. The material of infinite chilled cast iron roll contains high chromium, nickel, molybdenum The alloying elements are high-nickel-chromium infinite chilled cast iron rolls, which are produced by full flushing (overflow method) or centrifugal composite casting process.
There are a large number of carbides in the matrix structure of the working layer of the high-nickel-chromium infinitely chilled cast iron roll body, so it has high wear resistance. The existence of graphite in the matrix structure makes it have good heat resistance and is widely used It is used as finishing rolls for wide, medium and thick plate mills and strip mills.
High chromium composite cast iron roll
High chromium composite cast iron rolls are made of high chromium white wear-resistant cast iron containing 12-22% chromium as the outer layer material of the roll body, and generally use ductile iron as the roll core and roll neck material, and are produced by centrifugal composite casting process High alloy composite cast iron rolls. Due to the presence of lath-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 and are widely used It is used for rough rolling and finishing front work rolls of hot-rolled strip tandem mills, rough and finish rolling work rolls of wide and medium plate mills, and finish rolling of small section steel and plate mills.
Alloy ductile iron roll
Alloy ductile iron rolls have high tensile strength, can withstand heavy loads, and have good wear resistance due to the uniform distribution of graphite from the roll body to the core. The matrix structure is carbide and pearlite or acicular body. Alloy ductile iron rolls can be roughly divided into two types according to the matrix structure of the roll body: pearlite ductile iron rolls and acicular ductile iron rolls. Alloy ductile iron rolls are generally cast as a whole. However, due to the high alloy content and high casting stress of the acicular cast iron roll, the centrifugal composite casting process can be used to obtain ideal comprehensive performance. Alloy ductile iron rolls have good thermal shock resistance and wear resistance, and are widely used as rolls for large blooming mills, section steel mills, continuous bar mills and large seamless tube mills.
Cast steel roll
Cast steel rolls can be divided into two categories: steel rolls (0.4~1.4% carbon content) and semi-steel rolls (1.4~2.4% carbon content). Steel rolls have low carbon content, so their hardness is low. Composition of ferrite and pearlite, with the increase of carbon content, the amount of ferrite decreases and the amount of pearlite increases. In order to increase the hardness, alloying elements such as Cr, Ni, Mo can be added, or special heat treatment can be carried out. Because the steel roll has high tensile strength and toughness, it is suitable for billet rolling mill or blooming mill with high rolling temperature and heavy rolling load. , can also be used as a support roller. If special heat treatment is carried out to obtain high hardness above HS90, it can also be used as a cold rolling work roll.
The carbon content of semi-steel rolls is between cast steel rolls and cast iron rolls. Its structure is mainly composed of pearlite and contains certain carbides. Therefore, its strength is slightly higher than that of cast steel rolls and lower than that of cast iron rolls. The most important feature of general cast steel rolls is that the internal hardness drop is very small. Therefore, it is especially suitable for the manufacture of deep-groove rolls. Working rolls in the front section of rolling and finishing rolling. If the silicon content is added to the material of the semi-steel roll, it will have high thermal crack resistance and toughness through graphitization treatment. Vertical roll for good rolling effect.
Alloy cast steel roll
According to its chemical composition and heat treatment process, the matrix structure of the outer layer of the roll body can be pearlite and bainite. Pearlite can be flaky or spherical, or sorbite type. Tempered sorbite not only has high strength and toughness, but also has excellent thermal cracking resistance, especially tempered sorbite with phase phase. Excellent wear resistance, so when used in the roughing mill, the rolling effect is good.
Alloy cast steel rolls have high tensile strength and toughness, certain wear resistance and thermal crack resistance, and are mainly used for blanking rolls of blooming mills and rough rolls of section steel mills.
Cast high chromium steel roll
High chromium steel rolls are produced with high chromium steel (C: 0.5~1.5%, Cr: 8~15%) as the outer layer material of the roll body, and ductile iron as the core material, and are produced by centrifugal composite casting process. The outer layer of the roll body has a more chromium-rich matrix structure than the high-chromium composite cast iron material, and has excellent wear resistance. In addition, due to the low carbide content in the matrix structure of the working layer of the high-chromium steel roll body, it has excellent thermal cracking resistance. Based on this, high-chromium steel rolls are currently the development trend of rough roll selection for hot continuous rolling mills.
Centrifugal composite high speed steel roll
High-speed steel rolls not only have high thermal cracking resistance, but also have high wear resistance and red hardness, which can greatly prolong the life of the roll and improve the quality of rolled products.
Semi-steel roll
The carbon content of semi-steel rolls is usually 1.4~2.4%. The mechanical properties are between cast steel and cast iron. When the carbon content of semi-steel is greater than 1.9%, it is called high-carbon semi-steel. Due to the high carbon content, there are eutectic carbides in the matrix structure of high-carbon semi-steel. And pseudo-eutectic carbides are used for billet continuous rolling stand rolls, section steel mill intermediate stands and finish rolling stand rolls. When the semi-steel roll is used as a hot roll, in order to improve the crack resistance, the silicon content is added to the composition to become a graphite cast steel roll, which combines the advantages of both cast steel and cast iron, and is widely used in billet hot rolling mill rolls and section steel mill rolls.
Forged steel series roll
Through smelting, forging and heat treatment, forged steel rolls make the working layer of the roll body have uniform high hardness, high wear resistance and excellent accident resistance, and at the same time ensure that the roll neck and core of the roll body have a certain strength and high toughness. The unique performance of forged steel rolls makes them have irreplaceable advantages in cold rolling and non-ferrous rolling. According to the conditions of use, forged steel rolls can be divided into forged steel hot rolls and forged steel cold rolls. Forged steel hot rolls are mainly used for hot rolling billets and section steel rough rolls, and forged steel cold rolls are widely used for cold rolls. The use of forged steel cold rolling rolls is demanding, 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 resistance Accident capacity, for this reason, the original structure of the roll is very strict, one is the pure metallurgical quality, and the other is the uniformity of the structure. At present, our country mainly uses cold roll materials containing about 2% chromium, and the working layer above HS90 is about 10mm, while the newly developed Cr5 series cold rolling rolls containing 5% chromium, the working layer above HS90 is above 30mm, and the working layer above C5 series is cold rolled. The excellent performance of the work rolls in all aspects will gradually replace the traditionally used C2 series cold rolls and become the development direction of cold rolling work rolls.
Selection of roll material
Selection of Work Roll Materia
Work rolls in the front section of rough rolling (R1): The work rolls in the front section of rough rolling must have strength, toughness, wear resistance and thermal crack resistance. The hardness range is about HS40-55. Generally, rolls made of 60CrNiMo cast steel and other materials are used.
Work rolls in the back section of rough rolling (R2): The work rolls in the back section of rough rolling are required to use materials with good thermal cracking resistance. Generally, semi-steel, high-chromium steel, high-speed steel and other materials are used.
Work rolls in the front section of finish rolling (F1-4): The temperature of the work rolls in the front section of finish rolling is high and the load is heavy. Generally, materials such as cast semi-steel and high-chromium centrifugal composite cast iron are used. High chromium centrifugal composite cast iron has high roll surface wear resistance and thermal crack resistance, and can suppress roll surface spotting defects.
Work roll after finish rolling (F5-7): The work roll after finish rolling is used in the final stage of rolling, which has a very important impact on product quality and surface condition. The performance requirements of the roll are mainly high hardness, wear resistance, indentation resistance, spalling resistance and thermal crack resistance. Generally, infinite chilled cast iron (common type, improved type) and other materials are used.
Material selection of backup rolls and vertical rolls
Work rolls in the front section of rough rolling (R1): The work rolls in the front section of rough rolling must have strength, toughness, wear resistance and thermal crack resistance. The hardness range is about HS40-55. Generally, rolls made of 60CrNiMo cast steel and other materials are used.
Work rolls in the back section of rough rolling (R2): The work rolls in the back section of rough rolling are required to use materials with good thermal cracking resistance. Generally, semi-steel, high-chromium steel, high-speed steel and other materials are used.
Work rolls in the front section of finish rolling (F1-4): The temperature of the work rolls in the front section of finish rolling is high and the load is heavy. Generally, materials such as cast semi-steel and high-chromium centrifugal composite cast iron are used. High chromium centrifugal composite cast iron has high roll surface wear resistance and thermal crack resistance, and can suppress roll surface spotting defects.
Work roll after finish rolling (F5-7): The work roll after finish rolling is used in the final stage of rolling, which has a very important impact on product quality and surface condition. The performance requirements of the roll are mainly high hardness, wear resistance, indentation resistance, spalling resistance and thermal crack resistance. Generally, infinite chilled cast iron (common type, improved type) and other materials are used.
