1. Roller surface cracks
Cracks are divided into normal cracks and abnormal cracks. Normal cracks are also called thermal cracks. They are caused by the gradual rupture of the roller surface due to thermal stress generated by multiple temperature cycles. They are a micro-surface layer phenomenon occurring on the roller surface. Abnormal cracks are caused by thermal shock cracks due to accidents such as slipping, steel sticking, steel jamming, steel piling, and tail swinging during rolling. They form cracks with a certain depth and ductility on the roller surface.
2. Indentation
It mainly refers to the continuous or discontinuous distribution of irregular pits on the roller surface due to the intrusion of foreign matter such as oxidized iron scales due to cleaning reasons during the production process.
3. Oxide film shedding
At the beginning of rolling, after a period of hot rolling, a thin silver-gray or light blue oxide film is formed on the roller surface. It is very important to protect the roller surface and improve the surface quality of the rolled piece. Due to the repeated action of rolling force, the oxide film produces micro cracks around the carbide. When the roller cooling capacity is insufficient, it is difficult to form a dense oxide film on the roller surface. Even if an oxide film is formed, it will deteriorate quickly. In the case of unreasonable rolling, the oxide film is quickly destroyed during the generation process, which will cause the oxide film to fall off, leaving defects such as meteor spots on the roller surface, resulting in increased roller surface roughness and increased wear.
4. Sticking steel, abnormal wear, etc.
Due to abnormal rolling, the roller surface and the workpiece are adhered to each other, resulting in sticking steel; unreasonable process parameters during rolling cause excessive consumption of rollers with the same width and kilometer, resulting in uneven wear on the roller surface.
In production, standardized operations and equipment maintenance should be taken to avoid the above roller surface damage.
Material selection
Forged steel rollers and unlimited chilled cast iron rollers have been used in China. In addition to ordinary chilled cast iron, there are also low nickel-chromium-molybdenum, medium nickel-chromium-molybdenum, and high nickel-chromium-molybdenum cast iron materials. High-grade chilled cast iron materials are high nickel-chromium-molybdenum chilled cast iron. The disadvantages of this type of roller are low hardness and poor wear resistance. Later, ductile iron composite cast iron rollers were used. Relatively speaking, the service life has increased several times and is still in use. In foreign countries, semi-steel and high-hardness special semi-steel materials are generally used, which are very effective in overcoming surface roughness and wear resistance.
In order to improve the surface wear resistance of hot rolling rollers, the materials of hot rolling rollers are constantly improved, and their basic development process is from chilled cast iron to high-chromium cast iron to semi-high-speed steel and high-speed steel.
The chemical composition of high-chromium cast iron rollers is: 2.0%~4.0%C, 10%~30%Cr, 0.15%~1.6%Ni, 0.3%~2.9%Mo. Its essence is a high-alloy white cast iron with high wear resistance. The chromium content is generally 10%~15%. Its carbide is mainly M7C3 type, which is different from the continuous M8C type carbide of white cast iron. It not only has good wear resistance, but also has higher hardness (HV can reach 1800). The matrix is austenite and martensite, so its hardness and toughness are well combined. Actual rolling production shows that high chromium cast iron rolls have good resistance to thermal cracking. The reason is that a dense and tough chromium oxide film is formed on the surface of the roll, which can reduce the number and depth of thermal cracks. Therefore, high chromium cast iron rolls were widely used in the front frame of finishing rolling in the 1980s. High chromium cast iron composite rolls have been widely used as hot strip (steel) continuous rolling mills, rough rolling and finishing front section working rolls, wide medium and thick plates; rough rolling and finishing working rolls and small steel and bar mill finishing rolls.
There are two forms of heat treatment for high chromium cast iron rolls, one is subcritical heat treatment below the critical transformation temperature, and the other is high temperature heat treatment above the critical point A3. The pearlite matrix of the surface material of the high chromium roll is expected to have extremely fine interlamellar spacing, and a large number of dispersed secondary carbides on the matrix, requiring as low residual austenite and residual stress as possible, so the latter form of heat treatment is generally selected, specifically normalizing and tempering.
The application of high-speed steel as a hot rolling roll material began in Japan in 1988. The United States and Europe also conducted research in the early 1990s. my country began to develop and use high-speed steel rolls in the late 1990s. The general composition of high-speed steel is 1%~2%C, 0%~5%Co, 0%~5%Nb, 3%~10%Cr, 2%~7%Mo, 2%~7%V, 1%~5%W. Because it has a large amount of alloying elements such as W and V that can form strong carbides, its final microstructure contains about 10%~15% carbides with extremely high hardness and high temperature stability, so it can maintain high strength and hardness when working at high temperatures. Its working layer has a high hardness of 80~85HS, has good wear resistance and thermal cracking resistance, and there is no thermal cracking on the surface of the roll, and generally no peeling phenomenon.
In recent years, foreign countries have also successfully used semi-high-speed steel rolls in hot-rolled thin plate roughing mills. Their wear resistance is twice that of high-chromium steel rolls, and their bite performance and thermal fatigue resistance are good. Therefore, they have become an ideal choice for hot-rolled thin plate roughing mills and wire rod medium rolling mill rolls. The chemical composition range of semi-high-speed steel is: 1.5%~2.5%C, 0.5%~1.5%Si, 0.4%~1.0%Mn, 1.0%~6.0%Cr, 0.1%~4.0%Mo, 0.1%~3.0%V, 0.1%~4.0%W.
1. Roller surface cracks
Cracks are divided into normal cracks and abnormal cracks. Normal cracks are also called thermal cracks. They are caused by the gradual rupture of the roller surface due to thermal stress generated by multiple temperature cycles. They are a micro-surface layer phenomenon occurring on the roller surface. Abnormal cracks are caused by thermal shock cracks due to accidents such as slipping, steel sticking, steel jamming, steel piling, and tail swinging during rolling. They form cracks with a certain depth and ductility on the roller surface.
2. Indentation
It mainly refers to the continuous or discontinuous distribution of irregular pits on the roller surface due to the intrusion of foreign matter such as oxidized iron scales due to cleaning reasons during the production process.
3. Oxide film shedding
At the beginning of rolling, after a period of hot rolling, a thin silver-gray or light blue oxide film is formed on the roller surface. It is very important to protect the roller surface and improve the surface quality of the rolled piece. Due to the repeated action of rolling force, the oxide film produces micro cracks around the carbide. When the roller cooling capacity is insufficient, it is difficult to form a dense oxide film on the roller surface. Even if an oxide film is formed, it will deteriorate quickly. In the case of unreasonable rolling, the oxide film is quickly destroyed during the generation process, which will cause the oxide film to fall off, leaving defects such as meteor spots on the roller surface, resulting in increased roller surface roughness and increased wear.
4. Sticking steel, abnormal wear, etc.
Due to abnormal rolling, the roller surface and the workpiece are adhered to each other, resulting in sticking steel; unreasonable process parameters during rolling cause excessive consumption of rollers with the same width and kilometer, resulting in uneven wear on the roller surface.
In production, standardized operations and equipment maintenance should be taken to avoid the above roller surface damage.
Material selection
Forged steel rollers and unlimited chilled cast iron rollers have been used in China. In addition to ordinary chilled cast iron, there are also low nickel-chromium-molybdenum, medium nickel-chromium-molybdenum, and high nickel-chromium-molybdenum cast iron materials. High-grade chilled cast iron materials are high nickel-chromium-molybdenum chilled cast iron. The disadvantages of this type of roller are low hardness and poor wear resistance. Later, ductile iron composite cast iron rollers were used. Relatively speaking, the service life has increased several times and is still in use. In foreign countries, semi-steel and high-hardness special semi-steel materials are generally used, which are very effective in overcoming surface roughness and wear resistance.
In order to improve the surface wear resistance of hot rolling rollers, the materials of hot rolling rollers are constantly improved, and their basic development process is from chilled cast iron to high-chromium cast iron to semi-high-speed steel and high-speed steel.
The chemical composition of high-chromium cast iron rollers is: 2.0%~4.0%C, 10%~30%Cr, 0.15%~1.6%Ni, 0.3%~2.9%Mo. Its essence is a high-alloy white cast iron with high wear resistance. The chromium content is generally 10%~15%. Its carbide is mainly M7C3 type, which is different from the continuous M8C type carbide of white cast iron. It not only has good wear resistance, but also has higher hardness (HV can reach 1800). The matrix is austenite and martensite, so its hardness and toughness are well combined. Actual rolling production shows that high chromium cast iron rolls have good resistance to thermal cracking. The reason is that a dense and tough chromium oxide film is formed on the surface of the roll, which can reduce the number and depth of thermal cracks. Therefore, high chromium cast iron rolls were widely used in the front frame of finishing rolling in the 1980s. High chromium cast iron composite rolls have been widely used as hot strip (steel) continuous rolling mills, rough rolling and finishing front section working rolls, wide medium and thick plates; rough rolling and finishing working rolls and small steel and bar mill finishing rolls.
There are two forms of heat treatment for high chromium cast iron rolls, one is subcritical heat treatment below the critical transformation temperature, and the other is high temperature heat treatment above the critical point A3. The pearlite matrix of the surface material of the high chromium roll is expected to have extremely fine interlamellar spacing, and a large number of dispersed secondary carbides on the matrix, requiring as low residual austenite and residual stress as possible, so the latter form of heat treatment is generally selected, specifically normalizing and tempering.
The application of high-speed steel as a hot rolling roll material began in Japan in 1988. The United States and Europe also conducted research in the early 1990s. my country began to develop and use high-speed steel rolls in the late 1990s. The general composition of high-speed steel is 1%~2%C, 0%~5%Co, 0%~5%Nb, 3%~10%Cr, 2%~7%Mo, 2%~7%V, 1%~5%W. Because it has a large amount of alloying elements such as W and V that can form strong carbides, its final microstructure contains about 10%~15% carbides with extremely high hardness and high temperature stability, so it can maintain high strength and hardness when working at high temperatures. Its working layer has a high hardness of 80~85HS, has good wear resistance and thermal cracking resistance, and there is no thermal cracking on the surface of the roll, and generally no peeling phenomenon.
In recent years, foreign countries have also successfully used semi-high-speed steel rolls in hot-rolled thin plate roughing mills. Their wear resistance is twice that of high-chromium steel rolls, and their bite performance and thermal fatigue resistance are good. Therefore, they have become an ideal choice for hot-rolled thin plate roughing mills and wire rod medium rolling mill rolls. The chemical composition range of semi-high-speed steel is: 1.5%~2.5%C, 0.5%~1.5%Si, 0.4%~1.0%Mn, 1.0%~6.0%Cr, 0.1%~4.0%Mo, 0.1%~3.0%V, 0.1%~4.0%W.
