News

Fracture causes

First, brittle fracture, the shape of the fracture of such rolls is relatively flat, the fracture around the surface of the roll body is relatively flush; Second, the toughness of the fracture is mostly "mushroom head" shape, the roll body near the fracture are crushed into pieces.

Second, toughness fracture, the shape of the fracture of such rolls more "mushroom head", the fracture near the roll body are crushed into pieces.

Comparison of the two found that the broken roll accident roll form of toughness fracture. Brittle fracture and toughness fracture is because the roll stress over the core strength.

The reason for this is related to the residual stress of the roll itself, the mechanical stress during rolling and the thermal stress of the roll, especially when the temperature difference between the surface of the roll body and the core is more likely to occur. This temperature difference may be caused by poor roll cooling, cooling interruption or overheating of the roll surface at the beginning of a new rolling cycle. Rolls of this surface and the core of the huge temperature difference between the cause of large thermal stresses, when the larger thermal stresses, mechanical stresses and residual stresses on the rolls over the core strength of the rolls caused by roll breakage. For example, the temperature difference between the roll surface and the core at 70 ℃ when the roll will increase the longitudinal thermal stress of 100MPa, the greater the temperature difference, the greater the increase in thermal stress. Compared with the rolls that produce brittle fracture, the core material of the rolls that produce toughness fracture has better toughness and is less likely to fracture.

Roll failure caused by a total of four kinds of stress:

First, the residual stress in the manufacturing process.

Second, the rolling process of mechanical stress.

Third, the rolling process in the roll of the organization of stress.

Fourth, the thermal stress caused by the temperature difference between inside and outside the roll.

If the manufacturing residual stress is too large to produce fracture, broken rolls usually occur in the initial use of the rolls on the first few times, and for the opening of the first few pieces of rolling material. The broken rolls have been rolled four times, the working layer consumed 14mm, so it should not be due to manufacturing residual stress formation of the fracture.

If the fracture was due to mechanical stress, it would require a large amount of mechanical stress. Roughly calculated, such a large cross-section of high-chromium cast steel rolls if the mechanical stress to pull off, it requires more than 100MN of tension, for the roll work of the mill this is impossible. Roller force is the largest part of the transmission end of the roll neck, if the mechanical properties of the material index is insufficient, the first damage is the transmission end of the roll neck under normal rolling conditions. From the actual rolling and broken roll situation, not due to mechanical stress caused by the roll body fracture.

The greatest impact on the organization of stress is the outer layer of the organization of residual austenite content. Residual austenite in the rolling temperature, rolling pressure and water cooling under the effect of alternating, occurring austenite to martensite or bainite transformation, due to the austenite specific volume is small, while the specific volume of martensite is large, and thus in the organization of the process of transformation is accompanied by the expansion of the volume, which will lead to the roll of the working layer of the greater compressive stresses, the core produces a greater tensile stresses, the core of the stress once more than the strength of the material, will inevitably result in roller Fracture. Taking into account the effect of residual austenite on the organization of stress and the working conditions of the rolls of the hot strip mill, the general control of the residual austenite content of the rolls in less than 5% to ensure safe use. The fracture of the roll of the outer layer of the organization of residual austenite content of less than 1%, so the tissue stress is negligible.

Roll fracture may also be uneven with the temperature caused by thermal stress. Rolls in the process of use on the machine, due to close contact with the rolling material, the surface temperature of the roll rises rapidly, while the core of the roll temperature rises more slowly, the temperature difference between the roll surface and the core of the roll is at its maximum value, the temperature difference caused by the thermal stress of the roll is also at its maximum value. If the thermal stress of the roll and the residual stress of the roll superimposed, and exceeds the strength limit of the core of the roll may occur when the roll fracture accident.

Methods to prevent breakage

Preventing breakage should be carried out from the reduction of manufacturing residual stress, mechanical stress, tissue stress and [1] thermal stress.

Generally most of the manufacturing residual stress will be eliminated in the heat treatment process, and will be gradually eliminated with the prolongation of the storage time of the rolls, so the new rolls stored for a period of time before use, can reduce the risk of breakage. The main method to avoid large mechanical stresses is to avoid overcooling the steel. The way to reduce the organizational stress is to control the residual austenite content of the working layer of the roll body at less than 5% through heat treatment. The way to reduce thermal stresses is to provide good cooling of the rolls during the rolling process.

Manufacturing residual stress, mechanical stress, organizational stress and thermal stress is the main cause of high chrome steel roll fracture, good heat treatment, rolling conditions and cooling can effectively prevent and control high chrome steel roll fracture.