JPH07103443B2 - High hardness graphite crystallized high chromium material with excellent wear resistance and composite roll for rolling - Google Patents

Description

The present invention relates to a high hardness graphite crystallized high chromium material excellent in wear resistance which is mainly used as a roll material, and an outer layer which is a rolling use layer of the high chromium material. Relates to a composite roll for rolling.

The (prior art and problems) high chromium roll material, M ₇ having a high hardness in the tissue in metallurgy manner
It contains C ₃ type chrome carbide and is characterized by excellent wear resistance. A composite roll in which an outer layer as a rolling use layer is formed of such a high chromium roll material is widely used as a work roll for a hot strip mill.

The high chrome roll material is used by adjusting the hardness of the material to Hs50 to 95 by changing the matrix in the structure from pearlite to martensite or bainite depending on the purpose of the roll.

Generally, the higher the hardness, the better the wear resistance,
The residual stress accompanying the transformation also becomes high, and it is restricted to applications where the thermal stress generated during rolling does not become so high.

For example, in the rolling of a hot strip mill, the temperature of the rolled material is as high as 800 to 1000 ° C, and the work roll used for rolling has a large temperature difference between the roll surface in contact with the rolled material and the central portion, which causes a large difference. Generates thermal stress. When a roll having a high residual stress is used, the residual stress and the thermal stress are combined with each other, and the roll may be broken at the initial stage of rolling. Therefore, the high chrome roll used for hot rolling is
The reality is that the outer layer hardness is suppressed to Hs 50-83.

On the other hand, as a cold rolling roll with small thermal stress, Hs90
Even before and after, there is no fear of breakage and it is applicable.

The present invention has been made in view of the above problems, and can be used in both hot rolling and cold rolling, and a rolling composite roll having a high hardness and high chromium layer excellent in wear resistance as an outer layer, and It is mainly intended to provide a high chromium material suitable as the outer layer material.

(Means for Solving Problems) The high chromium material of the present invention made to achieve the above object has a chemical composition in a weight% of C: 2.8 to 3.6% Ni: 4.5 to 10% Si: 2.5 to 4.0. % Cr: 5-10% Mn: 0.5-1.5% Mo: 3.5-6.0% P: 0.1% or less S: 0.08% or less The balance consists essentially of Fe, and the structure is Cr carbide, Mo carbide,
The composition of the invention is composed of a matrix mainly composed of graphite and bainite generated by the isothermal transformation.
Further, the composite roll of the present invention comprises the high chromium material as an outer layer which is a rolling use layer, and has a constitution of the invention.

(Example) First, the reasons for limiting the components of the high hardness graphite crystallized high chromium material of the present invention will be described, and the application to the outer layer of the rolling roll for composites will be mentioned. In the following, all component units are% by weight.

C: 2.8 to 3.6% C bonds with Cr and Mo to form Cr carbide and Mo carbide, and crystallizes as fine graphite due to the graphitization-forming elements of Si and Ni described later. If it is less than 2.8%, the amount of the above-mentioned carbides decreases, the hardness becomes insufficient, and it becomes difficult to crystallize graphite, so that the residual stress due to the presence of graphite and the seizure resistance cannot be improved. On the other hand, if it exceeds 3.6%, Cr,
If the Mo content is excessive, the amount of carbide will be too large and graphite will hardly crystallize, so that it is not possible to reduce the residual stress due to the presence of graphite, and the material may become brittle and crack during production. .

Si: 2.5-4.0% Si is necessary to crystallize graphite. In the case of the present invention, the white pig iron promoting elements Cr and Mo are 5 to 10% and 3.5 to 5%, respectively.
Since it contains 6.0%, if it is less than 2.5%, crystallization of graphite becomes difficult, while if it exceeds 4.0%, crystallization of graphite becomes excessive and wear resistance deteriorates.

In order to easily crystallize graphite, the amount of Si before casting is kept lower than the above-mentioned range of components, inoculation is performed at the time of casting, and finally means for adjusting the amount within the above range is provided. It is effective to take.

Mn: 0.5-1.5% Mn is positively added with Si for deoxidation of the molten metal. 0.5
If it is less than 1.5%, the deoxidizing effect is insufficient, while if it exceeds 1.5%, the mechanical properties, particularly toughness, are significantly deteriorated.

P: 0.1% or less The less the roll material is, the better.
From the viewpoint of preventing embrittlement, the content is 0.1% or less.

S: 0.08% or less S, like P, makes the roll material brittle, so the smaller the amount, the more desirable, and the content is 0.08% or less.

Ni: 4.5-10% Ni is positively contained to improve the matrix structure and crystallize graphite. If less than 4.5%, almost no crystallization of graphite occurs, and 1
If it exceeds 0%, the amount of graphite becomes excessive, and the retained austenite increases and becomes stable, and it becomes difficult to transform in the subsequent heat treatment, and the surface roughening resistance deteriorates.

Cr: 5-10% Cr combines with C to form high hardness Cr carbide. If it is less than 5%, the amount of carbides is small and the wear resistance is insufficient, while if it exceeds 10%, crystallization of graphite becomes difficult with the above Ni and Si contents.

Mo: 3.5-6.0% Mo enhances the quenching and tempering resistance and forms Mo carbide, which is effective in obtaining a material with high hardness. If it is less than 3.5%, the generation of Mo carbide is small, and it becomes difficult to increase the hardness of the material. On the other hand, if it exceeds 6.0%, the crystallization of graphite is remarkably suppressed with the above Si and Ni contents, and almost no crystallization of graphite occurs.

Although Mo contributes to the formation of high-hardness carbides together with Cr, both are elements having a tendency of white pig iron, and Cr is
This tendency is stronger than Mo. Therefore, in order to achieve high hardness with moderate crystallization of graphite, (Cr% + 1 / 2Mo%) is 8.5 to 1
It is desirable to adjust the composition so that it is 1.5%.

The high hardness graphite crystallized high chromium material of the present invention is composed of the balance Fe and unavoidably mixed impurities in addition to the above alloy components.

The high chromium material is an as-cast state, residual austenite,
Cr carbides, Mo carbides and graphite are crystallized in the matrix formed by pearlite and partly martensite, so that the hardness is increased by the carbides and the seizure resistance is improved by the crystallization of graphite.

In the present invention, after the casting, a heat treatment to be described later is further performed to make the matrix mainly composed of bainite by the isothermal transformation. As a result, a hardness of Hs85 or higher is achieved, and the residual stress is significantly reduced in combination with the presence of graphite.

That is, the bainite transformation occurs in a relatively high temperature state (350 to 500 ° C) as described later, and the structure is self-annealed during the isothermal transformation, and crystallized graphite exists in the structure. The stress is quickly relieved.

The heat treatment, after the casting material is heated and maintained at the austenitizing temperature, it is rapidly cooled to 350 to 500 ° C. at a cooling rate of 300 ° C./hr or more, and held at this temperature for 10 to 20 hr, and all or most of the matrix is bainite transformed. Then, the step of gradually cooling is adopted. The reason why the material is rapidly cooled after being transformed into austenite is that it is necessary to prevent the pearlite transformation during cooling and bring it to the bainite transformation region in the austenite state.

Incidentally, in the conventional heat treatment for increasing the hardness of the high chromium material, since the martensite transformation is usually carried out at a temperature of 300 ° C. or lower after austenitization, the residual stress is not relaxed unlike the high chromium material of the present invention.

The above-described graphite crystallized high chromium material is mainly used as a casting material for the outer layer which is the layer used for rolling of the composite roll for rolling, but as the inner layer (axial core) material, it is tough such as high-grade cast iron and ductile cast iron. A cast iron material having good properties or a cast steel material such as graphite cast steel is appropriately used.

The composite roll referred to here is not limited to one having a solid inner layer, but includes one having a cylindrical shape. The latter composite roll (sometimes referred to as a composite sleeve) is fitted and fixed to a separately prepared roll shaft and used as an assembly roll.

As the method for producing the solid composite roll, after casting the outer layer by a mold centrifugal force casting method, a stationary mold is formed by erecting a centrifugal force casting die having an outer layer inside, and an inner layer is formed inside the die. There is a method of pouring a molten material and integrally welding the outer layer and the inner layer, which is generally applied because it is simple.

After the casting of the composite roll, the above-described heat treatment is performed in order to make the base of the outer layer mainly of bainite, and then the composite roll is machined to obtain a predetermined product roll.

As described above, the high chromium material according to the present invention is extremely excellent in wear resistance because it has a high hardness of Hs85 or more, and also has a low residual stress, which is suitable as a rolling use layer of a hot rolling roll. It is also suitable as a roll material for cold rolling which requires high wear resistance, a roller material, and a material for machine parts. As the machine parts to which the material of the present invention is applied, high temperature, wear resistant members at low temperatures, for example, excavator, slurry pump,
An injection molding member, various molds, etc. can be illustrated.

By the way, in addition to high chromium materials as existing high hardness roll materials, there are high alloy grain materials, but the hardness is Hs85 limit, and if the hardness is higher than this, cracking is not likely to occur during roll manufacturing. Not.

Next, compound roll for rolling (product barrel diameter 680mm, barrel length 1450m
m, total length 3600 mm) will be described.

(1) An outer layer molten metal having the chemical composition shown in Table 1 was cast as an outer layer in a thickness of 80 mm (casting weight 2Ton 400 kg) into a centrifugal force casting mold rotating on a casting machine. At the time of casting, in the examples, Fe-Si was inoculated into the tapped maemoto hot water in order to easily crystallize graphite. The Si content of the original hot water was adjusted to be 0.5% lower than the value in Table 1 in consideration of the increase due to inoculation.

Incidentally, Conventional Example 1 shows a conventional high C-based high chromium material without graphite crystallization, and Conventional Example 2 shows a high alloy grain material.

(2) The outer layer completely solidified 22 to 23 minutes after the start of the outer layer casting. Then, the rotation of the mold is stopped, the mold having the outer layer is erected vertically, and the upper mold and the lower mold for forming the roll shaft portion are continuously provided at both ends of the mold to form a stationary mold, The inner layer material (ductile cast iron) shown in Table 2 was melted inside and completely filled, and then covered with a riser heat insulating material.

(3) After the molten metal was solidified and completely cooled, the roll was taken out of the mold, subjected to the following heat treatment, and machined to obtain a product roll.

Example After holding at 1000 ° C. for 5 hours to austenite, it was drawn out from the heat treatment furnace, rapidly cooled to 450 ° C. at a cooling rate of 400 ° C./Hr by spray water cooling, and further held at this temperature for 15 hours to form a bainite structure for the base. After that, the furnace was cooled.

Conventional Example 1 After holding at 1000 ° C for 5 hours to austenite, draw it out from the heat treatment furnace, rapidly cool to 400 ° C at a cooling rate of 400 ° C / Hr, hold at this temperature for 15 hours, and keep the temperature of the outer surface and the center part. Was measured, and then cooled to room temperature to undergo martensitic transformation.

Conventional Example 1 It is a mixed matrix structure of retained austenite and martensite in the as-cast state, and is for decomposition of retained austenite.
The structure was maintained at a temperature of 300 ° C to 450 ° C to form a bainite or tempered martensite structure.

(4) Table 3 shows the product composition of the rolls thus obtained. The composition of the outer layer is almost the same as the composition of the molten metal, and the description thereof will be omitted.

As a result of ultrasonic flaw detection of the body, it was confirmed that the outer layer and the inner layer were completely welded and integrated in both the example and the conventional example.

In addition, as a result of microscopic observation of the implementation exception layer structure, M ₇ C ₃ type Cr carbide and Mo ₂ C are precipitated in the bainite base,
In addition, it was observed that graphite was finely crystallized.

(5) The hardness and residual stress of the outer layer were investigated. The residual stress was measured by X-ray residual measurement.

The results are shown in Table 4.

It is known from Table 4 that in the examples of the present invention, the outer layer has high hardness and low residual compressive stress.

In Conventional Example 1, from the value of the outer layer residual stress, the residual stress in the central part is estimated to be a tensile stress of 15 to 18 kg / mm ² , and when subjected to hot rolling, the core part was broken. There is a risk of

In this case, in order to reduce the residual stress, strain relief heat treatment may be performed, but it is necessary to hold the heat treatment at 500 to 600 ° C. for more than ten hours, so that the hardness also decreases to Hs80 or less. As a result, as a result of heat treatment for strain relief at 550 ℃ × 15Hr, the residual stress was compressed to 25kg / mm ² , but the hardness was Hs73 ~
It dropped to 75.

In Conventional Example 2, the residual stress is slightly higher than that of the Examples, but the hardness is limited to Hs83, and it is impossible to obtain a hardness higher than this.

(Effects of the Invention) As described above, the high chromium material of the present invention has a Cr: 5-10 content.
%, Mo: Since it is a high-chromium cast iron material having a specific composition having a specific composition and graphite having crystallized 3.5 to 6.0%, it has a high hardness of Hs85 or more without causing an increase in residual stress, Extremely excellent wear resistance. Therefore, it is suitable as a roll material, a roller material, and a material for machine parts that require high wear resistance.

In particular, the one in which the high hardness graphite crystallized high chromium material of the present invention is applied to the outer layer, which is the layer used for rolling of the composite roll, has high residual hardness and high abrasion resistance, but the residual stress is low, so that the breakage resistance is high. It can also be used for hot rolling with excellent heat stress.

Claims (2)

[Claims] 1. The chemical composition is% by weight, C: 2.8-3.6% Ni: 4.5-10% Si: 2.5-4.0% Cr: 5-10% Mn: 0.5-1.5% Mo: 3.5-6.0% P: 0.1% or less S: 0.08% or less The balance consists essentially of Fe, and the structure is Cr carbide, Mo carbide,
A high hardness graphite crystallized high chromium material having excellent wear resistance, which is composed of a matrix mainly composed of graphite and bainite formed by constant temperature transformation. 2. The chemical composition is wt%, C: 2.8-3.6% Ni: 4.5-10% Si: 2.5-4.0% Cr: 5-10% Mn: 0.5-1.5% Mo: 3.5-6.0% P: 0.1% or less S: 0.08% or less The balance consists essentially of Fe, and the structure is Cr carbide, Mo carbide,
A composite roll for rolling, which has an outer layer that is a layer used for rolling and is composed of a matrix mainly composed of graphite and bainite formed by constant temperature transformation.