Material selection of rolling mill rolls used in large and medium section steel mills

Description：In view of the complex section of large and medium-sized steel and the different main properties of the rolling mill rolls in each stand of the rolling mill, this paper points out the factors that should be considered in the selection of roll materials for the rough rolling unit, intermediate rolling stand and finishing rolling stand, and puts forward the requirements for each stand. The material and hardness control range that should be selected for the roll.

Keyword：Large and medium section steel mill; rolling mill rolls; material selection

Characteristics of large and medium section steel rolling mills

The cross-section of large and medium-sized steel is usually more complex, so during the rolling process, the deformation of the metal in the roll pass is very uneven. Since the temperature and deformation of each part of the profile are different from the relative speed of the roll surface, the temperature stress and rolling force on each part of the roll are also very different, which makes the wear of each part of the pass uneven, so that the roll fails The reasons become very complicated.

Horizontal large and medium-sized steel rolling mills are all reversible rolling mills. The rolls used in these rolling mills are usually processed with deep passes. In order to ensure the dimensional accuracy of each part of the rolled material, it is required that the rolls should have a similar distance from the surface of the roll body to the bottom of the pass. Hardness value, in order to achieve uniform wear inside the pass pattern. In addition, because the pass is very deep, the effective diameter of the roll is reduced. Therefore, the roll should have sufficient strength and toughness to prevent the roll body from breaking and the roll neck from being damaged.

Rolling mill stand and its Rolling mill roll material selection

According to the different rolling loads and rolling tasks of each stand of the large and medium-sized steel rolling mill, the rolling mill is divided into rough rolling unit, intermediate rolling unit and finishing rolling unit. According to the different use environments of each stand, different materials, Rolls with different performances are adapted to the working conditions of the rack.

3.1 Rough rolling unit

The rolls of the large and medium-sized steel roughing mill stands are mainly for secondary blanking. Usually deep grooves are processed, and the rolling temperature is high and the rolling pressure is high. The rolls are required to have high strength and thermal crack resistance. In order to ensure the surface quality of the steel and the size of the billet, the rolls are also required to have certain wear resistance. At the same time, due to the large section size of the raw material and the complicated pass shape, it is also necessary to consider that the roll has a high biting performance.

Most of the steel mills use forged steel rolls or cast steel rolls made of 60Cmo, 60cr~mMo and other materials. These two types of material rolls have certain strength and toughness, which can meet the strength requirements of the rough rolling stand for the rolls. ·

NiCrMo series alloy cast steel roll, through reasonable composition control and special heat treatment process, the structure of the roll body is controlled as tempered sorbite. Because the Ni alloy element contained in the chemical composition can refine the grain, it has a certain effect on the matrix. Reinforcing effect. Therefore, the wear resistance, thermal crack resistance and toughness of the roll are greatly improved compared with CrMO, CrMn-Mo series cast steel rolls. When the roll pass is deep, the process of pre-cutting the pass and then final heat treatment can be used, so that the roll can obtain a uniform hardness layer on the side wall or bottom of the groove, so that the thermal cracking resistance and wear resistance of the roll are greatly improved. . The chemical composition and properties of alloy cast steel rolls used in rough rolling stands are shown in Table 1.

3.2 Intermediate rack

The main task of the intermediate stand is to further shape the rolled piece and provide the rolled piece with suitable specifications and sizes for the finishing stand. Therefore, the rolls used in the middle stand must have good thermal crack resistance and wear resistance at the same time, and because the pass is deep, the rolls are also required to have certain strength and toughness.

Before the 1980s, the roll material used in domestic steel rolling mills generally used nodular semi-chilled cast iron rolls, and the hardness of the roll body was only 40~45HSD. Due to the production of nodular semi-chilled rolls by cold-type sand-hanging process, the hardness is low and the structure is relatively coarse , wear resistance, comprehensive mechanical properties are not ideal. Later, some manufacturers switched to CrMo, NicrMo nodular infinite chilled series cast iron rolls. The rolls are produced by cold spray coating. The graphite, carbide, and matrix structure of the roll body are refined, and the wear resistance and comprehensive mechanical properties are improved. However, when rolling large-scale steel, because the pass is very deep, the chilling effect in the casting at the bottom of the pass is very weak, the matrix structure is not much different from that of the nodular semi-chilled roll, and its wear resistance is still not enough to produce high-precision products. need. Alloy semi-steel rolls should be used for the rolls used in the middle frame of large section steel with a rolling groove depth greater than 100 mm. The carbon content of the alloy semi-steel roll is between 1.3% and 2.3%, and the mechanical properties are between steel and iron, with wear resistance, high strength and good toughness. By adjusting the chemical composition and selecting different heat treatment processes, the body structure of the alloy semi-steel roll can be controlled to be fine pearlite. The hardness drop of the semi-steel roll is small, less than 2.5HSD in the range of 100mm (for the roll as shown in Figure 1, the process of pre-cutting the pass and then final heat treatment can be used, so that the roll can obtain uniformity on the side wall or bottom of the groove. The hardness layer greatly improves the thermal cracking resistance and wear resistance of the roll. Alloy semi-steel rolls have strict requirements on water cooling conditions (usually the surface temperature of the rolling groove is less than 70°C after being off the machine).

For some rolls with relatively simple passes, grooved rolls produced by infinitely chilled nodular cast iron or semi-chilled nodular cast iron can be used. Casting with grooves avoids excessive hardness drop of the roll body during integral casting, so the wear resistance is improved. .

When the pass type is shallow, pearlitic ductile iron rolls can be used. The pearlitic ductile iron roll structure contains a large amount of fine and evenly distributed graphite. When the small microcracks on the surface of the roll are under the action of alternating rolling stress, they will terminate when they encounter graphite. Therefore, graphite can hinder the expansion of thermal cracks, making The thermal cracking resistance of the roll is greatly improved, and the requirements for the cooling water of the rolling mill are relatively low. The chemical composition and properties of the alloy semi-steel and pearlitic ductile iron rolls used in the middle frame are shown in Table 2.

3.3 Finishing stand

The rolling operation of the large section steel finishing stand is an important operation link to ensure the realization of negative tolerance rolling and improve the dimensional accuracy of the section bar. Therefore, the rolls used in large and medium-sized steel rolling mills must have excellent wear resistance and thermal crack resistance. At the same time, due to the deep pass of the rolls, it is necessary to ensure that the rolls have a certain strength.

In terms of material selection for large-scale steel finishing stands, most of them still use ordinary CrMo infinitely chilled cast iron rolls with a large drop in the hardness of the roll body. Large drop leads to low single groove rolling capacity of such rolls, frequent groove changing operations and roll changing operations, resulting in increased roll consumption, low mill operating rate and affects the quality of rolled products. High-carbon semi-steel rolls should be selected for rolls used in large-scale finishing stands, with a carbon content of 1.9%.

Above, due to the increase of the content of alloying elements, especially Ni, Cr, and Mo elements, the structure of the roll body is controlled as bainite or bainite + martensite, and the hardness decreases slowly from the surface to the inside. The high carbon semi-steel roll body The hardness is generally controlled at 50-60HsD.

For rolling mills with poor cooling conditions, graphite steel rolls can be used. Graphite steel rolls are a new type of roll material developed on the basis of semi-steel rolls, and a certain amount of graphite is formed on the matrix structure of the steel. It has been proved by research that the shape of graphite in the matrix structure of steel has a great influence on its performance, and spherical graphite can effectively prevent the generation and expansion of thermal fatigue cracks. The graphite steel material contains a variety of alloying elements that improve the strength and toughness of the matrix, and cooperates with scientific and advanced smelting and pouring processes to reasonably control the content and corresponding proportion of graphitization elements and carbide-forming elements. (fine pearlite) precipitated part of spherical graphite. Graphite has excellent thermal conductivity and acts as a natural lubricant, and the damping performance of graphite evenly distributed in spherical shape can greatly improve the crack resistance. At the same time, fine-grained and semi-network carbides with extremely high microhardness are added to the fine pearlite matrix structure similar to the semi-steel material. The material will eventually undergo a strictly controlled heat treatment process to make it strong, tough and wear-resistant. Therefore, the graphite steel roll material has excellent thermal crack resistance, accident impact resistance and high wear resistance of semi-steel material. For the deep pass section steel rolling mill, the graphite steel roll is subjected to secondary heat treatment after rough opening, so that the wear resistance at the bottom of the pass is almost the same as that of the roll surface.

According to the specific rolling conditions and rolling products, pearlitic alloy ductile iron rolls can also be used for the finishing stand, and the structure of the roll body is pearlite + carbide + spherical graphite, or pearlite + a small amount of bainite + carbide +Spherical graphite, the hardness is 55~65HSD, with the increase of hardness, the wear resistance is improved. The chemical composition and performance comparison of graphite steel, semi-steel and pearlitic rolls used for finishing stands are shown in Table 3.

To sum up, according to the rolling conditions of large and medium-sized section steel mills and the main properties that the rolls of each stand should have, the following principles should be followed in the selection of roll materials.

Alloy cast steel rolls should be selected for the rough rolling stand, and the hardness should be controlled at 35-45HSD.

Alloy semi-steel rolls should be selected for the intermediate rolling stand, and the hardness is controlled at 40-50HsD; alloy ductile iron rolls, the hardness is controlled at 45-60HSD.

The finishing stand should choose high-carbon semi-steel rolls with a hardness of 45-55HsD; graphite steel rolls with a hardness of 45-55HSD; pearlite ductile iron rolls with a hardness of 55-65HsD.