JPH0755325B2 - Compound roll for rolling - Google Patents

Description

The present invention relates to a composite roll for rolling.

In the field of plate rolling mills in recent years, instead of the conventional quadruple rolling mill consisting of composite rolls and working rolls, a six-fold rolling mill in which an intermediate roll is further combined between the reinforcing roll and working roll, or a work roll There is a tendency that the type that shifts is used.

Features of these rolling method is to effect significant load to the roll, whereby, the maximum contact pressure of the work rolls increases from 160 kgf / mm ² of conventional 4-high mill to 240 kgf / mm ^2, the As a result, spalling is more likely to occur.

At the same time, in these new mills, in order to exert a large bending force on the work roll, the roll shaft portion is required to have higher strength.

On the other hand, as the roll of the rolling mill, a composite roll using a centrifugal casting method is usually used, but this has a problem in the mechanical properties of the core material, and the function of the new type mill as described above is not enough. The current situation is that we cannot meet the requirements.

The present invention has been made in view of such a state, wear resistance, surface roughening resistance, seizure resistance, without spoiling the roll characteristics such as crack resistance, spalling resistance is significantly. An object of the present invention is to provide a composite roll for plate rolling having an improved tough shaft.

The roll of the present invention has significantly increased spalling resistance, which is based on the following new experimental findings.

Origin of spalling: (a) Spalling grows in the circumferential direction of the roll, starting from a small notch such as a heat crack generated during use.

(B) Depending on the soundness of the microstructure, the morphology of carbides, etc., the effect is less than that of notches such as heat cracks.

Suppression of spalling growth: (a) The point is how shallow the cracks that are the starting points of spalling are to be stopped.

(B) Cracks are suppressed when a large compressive stress is constantly applied.

The present invention has been made on the basis of the above experimental findings, the gist thereof is a composite roll in which the core material is made of cast steel or forged steel, and the outer layer material is cast by the continuous casting overlay method. After quenching, by tempering, the compression residual stress of the roll outer layer material surface is 20 kgf / mm ² or more, and the tensile strength of the core material is 50 kgf / mm ² or more, and the outer layer material and the core material. The composite roll for rolling is characterized in that the tensile strength of the joint portion is set to be equal to or higher than the strength of the weaker one of the outer layer material and the core material.

Further, in a preferred embodiment, in the rolling composite roll, when a defect is present in the core material, the defect is equivalently converted into a disk perpendicular to the tensile direction, and the radius of the disk is r, Fracture toughness value The above is a composite roll for rolling characterized by the above.

The reasons for limiting the numerical values of the present invention are as follows.

Reasons for limiting residual stress: Thermal cracking due to heating and cooling is significantly affected by the magnitude of compressive residual stress. For example, in the case of high chromium cast iron rolls, when compressive stress of 30 kgf / mm ² or more is applied, Under operating conditions, the crack depth can be made shallower than the maximum shear stress generation depth, and spalling starting from the crack tip can be suppressed. Furthermore, in the case of an outer layer material having a high toughness such as adamite roll, the addition of a compressive residual stress of 20 kgf / mm ² or more has a remarkable effect on the generation of spalling for the same reason. The residual stress is limited based on the above reasons. Reason for limiting the tensile strength of the core material: When a compressive stress of 20 kgf / mm ^{2 is} applied to the surface, a tensile stress of 20 kgf / mm ² or more may occur at the center of the roll. Thermal stress of about 10 kgf / mm ² is added to this. Eventually, a stress of about 30 kgf / mm ² or more will be generated in the center part, and this will be multiplied by the safety factor of a normal roll to obtain about
This is because the strength of 50 kgf / mm ² or more is required for the design.

Reason for limiting the fracture toughness value: If there are no defects such as shrinkage cavities inside, the above strength is sufficient, but if defects are present, the tensile strength is 50 kgf /
In order to obtain strength characteristics equivalent to mm ² , it is necessary that the fracture toughness value is the above value or more.

Reasons for limiting the bonding strength: Even if the inner and outer layers are metallurgically bonded,
When the bonding strength at the boundary is weaker than either of the inner and outer layers, cracks are likely to occur at this portion during heat treatment (quenching). This tendency is particularly noticeable when severe heat treatment is performed to obtain high strength.

In order to prevent cracks during heat treatment, it is necessary to prevent the joint from becoming the weakest point. It is for this reason that the bonding strength is limited to the above conditions.

Next, regarding the production method of the present invention, the roll of the present invention cannot be achieved by a method utilizing a centrifugal casting method, and in order to actually realize it, a core material is prepared in advance, and a molten metal is formed around the core material. The method of casting and joining the two is most preferable. As this method, the continuous casting overlay method is suitable for the roll of the present invention.

Next, the roll of the present invention will be described based on specific examples.

Example Roll size: 400φ × 1500l Roll material: High chromium cast iron Chemical composition: Shown in Table 1.

Hardness: Hs76 Manufacturing method: Continuous cast overlay welding Heat treatment cycle: Results of those rolls to measure residual stress in Zack method, the outer surface -31kgf / mm ^2, the center residual stress of 29kgf / mm ² is measured.

A hot turtle produced by contacting this roll with 800 ° C molten aluminum for 5 minutes and then rapidly cooling it with 20 ° C water. The crack depth is 1.2 mm, while the residual stress was reduced to -4 kgf / mm ² by cutting the roll to a length of 200 mm, and the thermal crack test was carried out to find that the depth was 17.4 mm. It was found that the cracks occurred at 15 times deeper than in the case of compressive residual stress of −31 kgf / mm ² . These rolls have a diameter of 300φ mm,
Combined with forged steel with a length of 50 mm and hardness Hs81, Hertz pressure 200 k
As a result of the spalling test at gf / mm ² , it was proved that the deeper the crack of the high chromium cast iron roller, the shorter the spalling life was.

As described above, it is clear that the composite roll obtained by the roll of the present invention is excellent not only in heat crack resistance but also in spalling property. The core material had a tensile strength of 64 kgf / mm ² .

The present invention has the features described above in detail,
It has the following effects.

(1) When a heat crack occurs in the roll, roll grinding is required to remove it, but as a result of the shallow heat crack,
Roll consumption is reduced.

(2) As a result of eliminating the thermal cracks that develop into spalling, product errors caused by spalling are significantly reduced.

(3) When a strong thermal crack or spalling occurs, the mill is stopped and the roll is replaced each time, but this is not necessary and the rolling efficiency is improved.

Claims (2)

[Claims] 1. A composite roll comprising a core material made of cast steel or forged steel, the outer layer material being cast by a continuous casting and overlaying method, wherein the compressive residual stress on the surface of the outer layer material is 20 kgf by quenching and then tempering. / mm ² or more, the tensile strength of the core material is 50 kgf / mm ² or more, and the tensile strength of the joint between the outer layer material and the core material is the weaker strength of the outer layer material and the core material. A composite roll for rolling, which is characterized as described above. 2. The composite roll for rolling according to claim 1, when a defect is present in the core material, the defect is equivalently converted into a disk perpendicular to the tensile direction, The fracture toughness value is The above is the composite roll for rolling characterized by the above.