JPS60221104A - Composite roll for rolling - Google Patents

Abstract

PURPOSE:To obtain a composite roll for rolling having improved resistance to spalling and a strong and tough shaft part by specifying the residual compressive stress and tensile strength on the surface of the outside layer material of the composite roll and joining the outside layer and core material in such a way that the tensile strength of the joint part thereof is higher than the lower tensile strength of both materials. CONSTITUTION:The residual compressive stress on the surface of the outside layer material of the composite roll of which the core material consists of a cast steel and forged steel is specified to at least >=20kg/m<2>. The tensile strength of the core material is made >=50kg/cm<2> or for the core material having internally a defect, the tensile strength thereof is made 2divided by piX50X(piXr)<1/2>kgmm.<-2> where the defect is coverted equivalently to a disk perpendicular and the disk radius is designated as (r). The tensile strength in the joint part between the outside layer and the core material is made higher than at least the lower tensile strength of the outside layer and the core material.

Description

[Detailed description of the invention] The present invention relates to a composite roll for rolling.

In recent years, in the field of plate rolling mills, instead of the conventional 4-fold rolling mill consisting of a composite roll and an operating roll, a 6-fold rolling mill that further combines an intermediate roll between a reinforcing roll and an operating roll, or a work roll. There is a tendency to use a type that shifts the

A feature of these rolling methods is that a large load is applied to the rolls, which increases the maximum contact pressure of the work rolls from 160% of the conventional 4-high mill to 240%, resulting in spalling. is becoming more likely to occur.

At the same time, in order to apply a large pending force to the work rolls of these new mills, the roll shafts are also required to have higher strength.

On the other hand, the rolls of the above-mentioned rolling mills are usually composite rolls made using the centrifugal casting method, but this has problems with the mechanical properties of the core material, and is not sufficient for the functions of the new type of mills mentioned above. The current situation is that it is not possible to satisfy the requirements.

The present invention was made in view of this situation, and has been developed to improve spalling resistance without impairing roll properties such as abrasion resistance, roughening resistance, seizure resistance, and rack resistance. A composite roll for plate rolling having a shaft portion with significantly improved toughness is provided.

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

Causes of spalling: (a) Spalling grows circumferentially from small notches such as heat cracks that occur during use.

(b) Although it depends on the soundness of the microstructure, the form of carbides, etc., the influence is small compared to notch effects such as heat cracks.

Suppression of spalling growth: (a) The key is to keep the cracks that form the starting point of spalling as shallow as possible.

(b) If a large compressive stress is constantly applied, cracks are suppressed.

The present invention was made based on the above-mentioned experimental findings, and the gist thereof is to provide a composite roll whose core material is made of cast steel and forged steel, wherein the compressor residual stress on the surface of the roll outer layer material is at least 205 or more, and the tensile strength of the core material is 50 or more, or for core materials with internal defects, the defects are equivalently converted into a disk perpendicular to the tensile direction, and the The composite roll for rolling is characterized in that the tensile strength of the outer layer and the core material is at least greater than the strength of the weaker one of the outer layer and the core material.

The reasons for limiting each numerical value in the present invention are as follows.

Reason for limiting residual stress: Thermal cracks caused by heating and cooling are significantly affected by the magnitude of compressive residual stress, but for example, in the case of high chromium cast iron rolls, if a compressive stress of 30~ or more is applied, it will cause cracking under actual operating conditions. At the bottom, the crack depth can be kept shallower than the depth at which the maximum shear stress occurs, and spalling starting from the crack tip can be suppressed. Furthermore, in the case of an outer layer material having high toughness such as an Adamite roll, by applying a compressive residual stress of 20 or more, a remarkable effect on the generation of spalling can be obtained for the same reason. The residual stress is limited based on the above reasons.

Reason for limiting the tensile strength of the core material: If a compressive stress of 20% or more is applied to the surface, a tensile stress of 20% or more may occur in the center of the roll, and during use, this will increase to approximately 10% or more. % thermal stress is added. This is because a stress of about 30 or more will be generated in the center, and by multiplying this by the safety factor of a normal roll, a strength of about 50 or more is required in the design.

Reason for limiting the fracture toughness value: If there are no internal defects such as tensile cavities, the above strength is sufficient, but if there are internal defects, the tensile strength will be 50%
In order to obtain strength characteristics equivalent to , it is necessary that the fracture toughness value is greater than or equal to the above value.

Reason for limited bonding strength: Even if the inner and outer layers are metallurgically bonded, if the bonding strength at the boundary is weaker than either of the inner and outer layers, cracks may occur in this area during heat treatment (quenching). is likely to occur. 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. This is the reason why the bonding strength is limited to the above conditions.

Next, regarding the manufacturing method of the present invention, it is completely impossible to manufacture the roll of the present invention using a method that utilizes the centrifugal casting 3ii method. The most preferred method is to join the two by casting metal. Examples of this method include (a) cast-in method and (b) continuous cast overlay method, and any of these methods can be adopted, but among them, the continuous cast overlay method is suitable for the roll of the present invention. Are suitable.

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

Example Roll dimensions: 400φX1500Q Roll material: High chromium cast iron Chemical components: Shown in Table 1.

Hardness: Hs76 Manufacturing method: Continuous casting overlay & heat treatment cycle: As a result of measuring the residual stress of this roll using the Sachs method, the residual stress was measured at -31 to 29 at the outer surface and at the center.

The depth of thermal cracks produced by contacting this roll with molten aluminum at 800°C for 5 minutes and then quenching it with 20°C water was 1.2 mm. When the stress was lowered to -4'\lI and the thermal cracking test described above was carried out, a crack with a depth of 17.4 m occurred, which was about 1
It turns out you can go five times deeper. These rolls have a diameter of 300
φ1111. In combination with a forged steel material having a length of 50++ ui and a hardness @ s81, and a spalling test at a Hertzian pressure of 200-, it was demonstrated that the deeper the cracks in the high chromium cast iron roller, the shorter the spalling life.

As described above, it is clear that the composite roll made of the roll of the present invention is extremely superior not only in heat cracking resistance but also in spalling properties. The core material had a tensile strength of 64 or higher.

The present invention has the features detailed above,
It has the following effects.

(1> When thermal cracks occur in the roll, roll grinding is required to remove them, but as the thermal cracks become shallower, roll wear is reduced.

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

(3) When strong thermal cracking and spalling occurs, the mill is stopped and the rolls are changed each time, but this is no longer necessary and rolling efficiency is improved.

Patent applicant: Hitachi Metals, Ltd.

Claims (1)

[Claims] It is a composite roll whose core material is cast steel and forged steel, and the compressive residual stress on the surface of the roll outer layer material is at least 20 or more, and the core material has a tensile strength of 50 or more, or there are internal defects. For the underlying core material, if the defect is equivalently converted into a disk perpendicular to the tensile direction, and the direction of the disk is r, the fracture toughness value is 2÷πx s o x i, - or more. A composite roll for rolling, characterized in that the tensile strength of the joint between the outer layer and the core material is at least greater than the strength of the weaker one of the outer layer and the core material.