Roll classification, performance and fracture analysis

Rolls are rolling tools that plastically deform metals. This is one of the key components of the rolling mill, which can use the pressure generated when the rolls rotate to roll steel. The dynamic and static loads, wear and temperature changes during the rolling process are the main factors affecting rolling. Its consumption depends on three factors:

• (1) Reasonably choose rolling mills, vehicles, rolling conditions, and rolls.
• (2) Roll material and manufacturing quality.
• (3) Role use and maintenance system.

Role classification

There are many kinds of rolls. At present, the commonly used rolls include cast steel rolls, cast iron rolls and forged rolls. The profile rolling mill also has a small number of cemented carbide rolls.

Roll performance

(1) Strength and heat-resistant cracking

Generally, roughing rolls are needed mainly for strength and heat crack resistance. When selecting rolls, select the main material for safe loading according to the basic strength requirements of the rolling mill.

(2) Hardness and wear resistance

The finishing roll speed is fast, and the surface quality of the final product is stable. Mainly require hardness and wear resistance, consider the wear resistance used when using rolls. Due to the complex wear mechanism of the roll, it includes mechanical stress, heat effect during rolling, cooling, chemical effect of lubricating medium and so on. At present, there is no unified index for comprehensive evaluation of roll wear resistance. The radial hardness curve is often used to approximate the wear resistance index of a roll, because the hardness is easy to measure and can reflect the wear resistance under certain conditions.

(3) Complete

When rolling thin gauge products, the requirements for roll stiffness, structural uniformity, processing accuracy and surface finish are very strict.

(4) Machinability

Cross-section profile When rolling the cross-section, the cutting performance of the processed layer of the roll body also needs to be considered.

Rolling damage

Residual stresses and thermal stresses occur during roll preparation before manufacturing and use. In addition, it is subject to various periodic stresses, such as bending, twisting, shearing, contact stress and thermal stress. The distribution of these stresses along the roll body is not uniform, and is constantly changing not only due to design factors, but also due to roll wear, temperature and roll shape during use. In addition, abnormalities often occur under rolling conditions. Rolls that have been improperly cooled after use may also be damaged by thermal stress. Therefore, in addition to abrasion of the roll, various local damages and surface damages such as cracks, fractures, peeling, dents, etc. often occur. A good roll needs strength, wear resistance and good matching with other various performance indicators. In this way, it is not only durable under normal rolling conditions, but also less damaged under abnormal rolling conditions. Therefore, in the manufacture of rolls, it is necessary to strictly control the metallurgical quality of the rolls or take external measures to increase the bearing capacity of the rolls. Reasonable roll shape, pass, deformation system and rolling conditions can also reduce the work load of the roll, avoid local peak stress, and extend the life of the roll.

During the use of the roll, due to the close contact with the rolled material, the surface temperature of the roll rises rapidly, and the temperature of the roll core rises slowly. At this time, the temperature difference between the roll surface and the roll core is at a relatively large value, and the thermal stress of the roll caused by the temperature difference is also at a relatively large value. If the thermal stress of the roll is superimposed on the residual stress of the roll and exceeds the strength limit of the core of the roll, a roll breaking accident may occur.

Damage prevention must be implemented from the perspective of reducing manufacturing residual stress, mechanical stress, tissue stress, and thermal stress. Under normal circumstances, most of the manufacturing residual stress is removed during the heat treatment process, and gradually removed as the roll storage time increases. Therefore, the new roll can be used to reduce the risk of damage after being stored for a period of time. The main way to avoid large mechanical stress is to avoid the use of supercooled steel. One way to reduce the structural stress is to control the retained austenite content of the processed layer of the roll body below 5% through heat treatment. One way to reduce thermal stress is to cool the roll well during the rolling process.