Cause of fracture

1、 Brittle fracture, the fracture shape of such rolls is relatively flat, and the surface of the roll body around the fracture is relatively neat;

2、 Ductile fracture, the fracture shape of such rollers is mostly in the shape of "mushroom head", and the roller bodies near the fracture are all crushed.

By comparing the two, it was found that the form of roller breakage in this roller breakage accident was ductile fracture. Both brittle fracture and ductile fracture are caused by roller stress exceeding core strength.

The causes are related to the residual stress of the roller itself, the mechanical stress during rolling, and the thermal stress of the roller, especially when the temperature difference between the surface and the core of the roller body is large. This temperature difference may be caused by poor roll cooling, cooling interruptions, or overheating of the roll surface at the beginning of a new rolling cycle. This large temperature difference between the surface and core of a roller causes large thermal stresses, which can cause roller breakage when the large thermal, mechanical, and residual stresses of the roller exceed the core strength of the roller. For example, when the temperature difference between the surface and core of the roller is 70 ℃, the longitudinal thermal stress of the roller will increase by 100 MPa. The greater the temperature difference, the greater the increased thermal stress. Compared to rollers with brittle fractures, the core material of rollers with ductile fractures is more ductile and less prone to fracture.

There are four types of stresses that cause roller failure:

1、 Residual stress during manufacturing;

2、 Mechanical stress during rolling;

3、 The structural stress of the roller during the rolling process;

4、 Thermal stress caused by temperature difference inside and outside the roller.

If the fracture occurs due to excessive manufacturing residual stress, the roll fracture usually occurs several times before the initial use of the roll, and is the first few pieces of rolled material to be rolled. The broken roller has been rolled four times and the working layer has consumed 14 mm. Therefore, it should not be caused by manufacturing residual stress.

If the fracture is caused by mechanical stress, it requires significant mechanical stress. After rough calculation, if a high chromium cast steel roller with such a large cross-section is pulled apart by mechanical stress, it requires a tensile force of more than 100 MN, which is impossible for the rolling mill in which the roller works. The most stressed part of the roller is the transmission end roll neck. If the mechanical property index of the material is insufficient, the first damage to the transmission end roll neck under normal rolling conditions is. Judging from the actual rolling and roll breakage situation, it is not due to mechanical stress that causes the roll body fracture.

The greatest impact on the structural stress is the residual austenite content in the outer layer of the structure. Under the alternating action of rolling temperature, rolling pressure, and water cooling, residual austenite undergoes a transformation from austenite to martensite or bainite. Due to the small specific volume of austenite, while the large specific volume of martensite, during the process of structural transformation, accompanied by volume expansion, greater compressive stress will be generated in the working layer of the roller, and greater tensile stress will be generated in the core. Once the core stress exceeds the strength of the material, it will inevitably cause the roller to fracture. Considering the impact of residual austenite on the structural stress and the working conditions of the hot strip continuous rolling mill rolls, it is generally possible to control the residual austenite content of the rolls to less than 5% to ensure safe use. The residual austenite content in the outer layer of the fractured roller is less than 1%, so the structural stress can be ignored.

The roller fracture may also be related to thermal stress caused by uneven temperature. During the use of the roller on the machine, due to the close contact with the rolled material, the surface temperature of the roller rises rapidly, while the temperature of the roller core rises slowly. At this time, the temperature difference between the roller surface and the roller core is at the maximum value, and the thermal stress of the roller caused by the temperature difference is also at the maximum value. If the thermal stress of the roll and the residual stress of the roll are superimposed and exceed the strength limit of the roll core, a roll fracture accident may occur.

Fracture prevention methods

Prevention of fracture should be carried out by reducing manufacturing residual stress, mechanical stress, tissue stress, and [1] thermal stress.

Generally, most manufacturing residual stresses are eliminated during heat treatment and gradually dissipate as the storage time of the roller increases. Therefore, storing new rollers for a period of time before use can reduce the risk of roller breakage. The main method of avoiding large mechanical stresses is to avoid supercooled steel. The method of reducing structural stress is to control the residual austenite content in the working layer of the roller body to less than 5% through heat treatment. The way to reduce thermal stress is to cool the rollers well during the rolling process.

Manufacturing residual stress, mechanical stress, structural stress, and thermal stress are the main causes of high chromium steel roller fracture. Good heat treatment, rolling conditions, and cooling can effectively prevent high chromium steel roller fracture.