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Vacuum heat treatment of high speed steel rolls

There are many kinds of rolls, which can be divided into 4 categories according to the hardness shown in Table 1. Hard rolls in Table 1 can also be called hot rolls, and their working conditions are equivalent to hot forging dies; extra-hard rolls can also be called cold rolls (including work rolls, flat rolls, and intermediate rolls), and their working conditions are equivalent to cold rolls. Make a mold. Therefore, the performance requirements for hot roll and cold roll materials are basically similar to those of hot work die steel and cold work die steel. The performance requirements for hot roll materials are: high temperature hardness, high temperature strength, thermal fatigue resistance, wear resistance, high hardenability and high thermal conductivity; the performance requirements for cold roll materials are: high hardness, high strength, high resistance Grinding, high toughness, high contact fatigue strength and high hardenability, hot roll and cold roll materials have experienced a development process. The development process of hot roll materials is: chilled cast iron → high Cr cast iron → semi-high-speed steel → high-speed steel → hard The development process of cold roll material is: low alloy tool steel → medium alloy tool steel → high alloy tool steel → high speed steel → cemented carbide.

High Speed Steel for Roll and Its Properties

High-speed steel for rolls includes cast high-speed steel, forged high-speed steel and non-carbide segregation high-speed steel. At present, cast high-speed steel is generally used to make composite rolls. The roll surface is high-speed steel, and the core is nodular cast iron or structure. steel.

1.1 Casting high speed steel

Casting high-speed steel is generally made of composite rolls with ductile iron or medium carbon structural steel. Compared with the high-speed steel for making knives, the composition of cast high-speed steel for making rolls has the following characteristics: ① High C content, the carbon content of general high-speed steel is only about 1%, while the carbon content of cast high-speed steel for rolls is as high as 2%. 2. The W equivalent is low, the W equivalent of general high-speed steel is 18%, and the W equivalent of cast high-speed steel for rolls is lower than 10% when it is the lowest; 3. The Cr content is high, and the Cr content of general high-speed steel is 4%, while the Cr content of high-speed steel is 4%. The Cr content of cast high-speed steel can reach up to 10% or even 20%; ④ The V content is high. Generally, the V content of high-speed steel is below 2%, while the V content of the most widely used cast high-speed steel for rolls is about 5%.

The reason for these differences is to: ① improve the hardenability, because the size of the roll is relatively large, and the maximum diameter exceeds φ 1m; ② improve the oxidation and corrosion resistance to ensure the surface quality of the rolled steel plate; ③ improve the wear resistance; ④ Reduce the vacuum quenching temperature to match the core ductile iron. Table 2 shows the chemical composition of cast high-speed steel for rolls. It can be seen from Table 2 that the chemical composition of cast high-speed steel for rolls varies greatly.

1.2 Forging HSS

Forged high-speed steel includes low-alloy high-speed steel (semi-high-speed steel), low-carbon high-speed steel, ordinary high-speed steel, high-hardness high-speed steel and super-hard high-speed steel. Except for low-alloy high-speed steel (semi-high-speed steel), the main difference between the other four high-speed steels is the difference in carbon content. With the increase of the content, the fracture toughness decreased with the increase of the hardness. However, for high-speed steel with high carbon content, the method of reducing the vacuum quenching temperature can be used to reduce the hardness and improve the fracture toughness. Therefore, these high-speed steels can be used to make rolls.

The main disadvantage of high-speed steel is the existence of carbide segregation, and the larger the size, the more serious the carbide segregation. The size of the rolls is generally relatively large, so the segregation of carbides is relatively serious. When the diameter exceeds φ120 mm, the carbide segregation of forged high-speed steel is comparable to that of cast high-speed steel, so forged high-speed steel is superior to cast high-speed steel only when it is used to make small-sized cold rolls. In order to solve the problem of carbide segregation in large rolls, processes such as spray forming and hot isostatic pressing can only be adopted.

2 Vacuum heat treatment of high speed steel rolls

The key to producing rolls is vacuum heat treatment, especially for large rolls. Compared with various hot and cold molds, the working conditions of the rolls are better, so the performance requirements are lower than those of the molds. However, the size of the roll is much larger than that of the mold, which greatly increases the difficulty of vacuum heat treatment. The main point of vacuum heat treatment of rolls is to prevent cracking on the premise of ensuring performance.

2.1 Vacuum annealing of high-speed steel rolls

Isothermal vacuum annealing must be carried out for the casting and forming of high-speed steel rolls. The purpose is to eliminate the unbalanced structure and internal stress with high hardness that may cause cracking and facilitate machining. The heating temperature and isothermal temperature of vacuum annealing are basically the same as those of general high-speed steel isothermal vacuum annealing. In order to prevent cracking during vacuum annealing, the heating speed should be slow; in order to prevent the generation of new internal stress during the cooling process after isothermal, the cooling speed after isothermal should also be slow. Mechanical processing is performed after vacuum annealing.

2.2 Vacuum quenching of high-speed steel rolls

Vacuum quenching is performed after machining. The determination of the heating temperature of vacuum quenching is basically the same as that of general high-speed steel vacuum quenching. Because the size of the roll is relatively large, the heating temperature of vacuum quenching can be appropriately reduced. Measures must be taken to prevent decarburization during heating. Below 500°C, the heating rate should be slower, and above 500°C should be heated faster. After the furnace temperature reaches the temperature, it can be kept for a period of time and then released, or it can be released immediately when the surface temperature of the roll reaches the vacuum quenching temperature, that is, the so-called differential temperature vacuum quenching. Vacuum quenching and cooling should be sprayed with water. Above 500°C, it should be cooled as quickly as possible, and below 500°C should be cooled slowly. Cool to about 200°C and immediately put into the furnace for vacuum tempering.

2.3 Vacuum tempering of high-speed steel rolls

The vacuum tempering temperature is the same as that of general high-speed steel vacuum tempering, which is also 550 ~ 560 °C. The heating and cooling speed of vacuum tempering should be slow to prevent cracking, and the number of vacuum tempering is 3 to 4 times.

Finally, it should also be pointed out that because the size of the roll is much larger than that of the cutting tool; therefore, the difficulty of vacuum heat treatment of high-speed steel rolls is also much higher than that of high-speed steel tools. The success of vacuum heat treatment is often the key to the production of high-speed steel rolls. Pay attention to.