JPH0293038A - Graphite-crystallized high-chrome cast iron roll material having excellent wear resistance and resistance to roughening of surface and composite roll for rolling - Google Patents

Abstract

PURPOSE:To combinedly provide the title roll material with seizure resistance, crack resistance, wear resistance and resistance to roughening of surface by specifying the contents of C, Si, Mn, Ni, Cr and Mo, forming the balance with Fe and crystallizing out fine graphite in the structure as cast. CONSTITUTION:The chemical compsn. of the graphite-crystallized high-chrome cast iron roll material is constituted of, by weight, 2.0 to 3.6% C, 1.5 to 2.8% Si, 0.5 to 1.5% Mn, 3.0 to 4.3% Ni, 5.0 to 10% Cr, 2.1 to 5.0% Mo and the balance substantial Fe and fine graphite of <=50mum is crystallized out in the structure as cast. Graphite grains of >50mum are easy to form a starting point of the roughening of the surface and is not preferable. The wear resistance and resistance to the roughening of the surface are improved coupled with the suppression of Ni content and much addition of Mo. The above graphite- crystallized high-chrome cast iron roll material is used as a casting material of an outer layer and high grade cast iron or the like are used as an inner layer material.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a graphite-crystallized high-chromium cast iron material that is mainly used as a roll material and has excellent seizure resistance, abrasion resistance, and surface roughness resistance, and a high-chromium cast iron material that is The present invention relates to a rolling composite roll having a chromium cast iron material in the outer layer used for rolling.

(Conventional technology and problems) Does high chromium cast iron roll material have high hardness M in its base structure metallurgically? It is a C3 type chromium chromium carbide and is characterized by excellent wear resistance.・Composite rolls in which the outer layer used for rolling is formed of such high chromium roll material,
Popular as work rolls for hot strip mills, etc.

However, high chromium cast iron roll materials have a problem in that they have low thermal conductivity and are prone to seizure.

Therefore, as disclosed in Japanese Patent Publication Nos. 61-16415 and 61-16336, the present applicant has developed a graphite-crystallized high-chromium cast iron material with significantly improved seizure resistance by crystallizing graphite in a high-chromium structure. was developed.

When special steel such as stainless steel, which is conventionally thought to be susceptible to seizure, was rolled using a rolling roll having this graphite crystallized high chromium cast iron material as a rolling layer, no seizure occurred and good results were obtained. was gotten.

However, in the graphite-crystallized high-chromium cast iron material, the crystallized graphite has a relatively large grain size of 80μ1 or more, so depending on the intended use, the roll surface tends to become rough, resulting in poor wear resistance and roughness resistance. There was a problem.

The present invention has been made in view of these problems, and includes a graphite crystallized high chromium cast iron roll material that has excellent not only seizure resistance but also abrasion resistance and roughness resistance, and a roll material using the roll material as a rolling layer. An object of the present invention is to provide a composite roll for rolling having the following properties.

(Means for Solving the Problems) The graphite-crystallized high chromium cast iron roll material of the present invention, which has been made to achieve the above object, has the following chemical composition in weight percent: C: 2.0 to 3.6% Ni :3.0~4.3%S
i: 1.5-2.8% Cr: 5
．． 0~10%Mn: 0.5~1.5%
Mo: 2.1-5.0% balance substantially F
The composite roll for rolling of the present invention is made of graphite-crystallized high-chromium cast iron having the above-mentioned specific composition and structure. The structure of the invention is that the roll material is applied to the outer layer of the composite roll, which is the layer used for rolling.

(Example) Hereinafter, the reasons for limiting the components of the graphite crystallized high chromium cast iron roll material of the present invention will be described, and the application to the rolling layer of a rolling composite roll will be mentioned.

All component units are weight %.

C: 2.0 to 3.6% C not only combines with Cr to form chromium carbide, but also crystallizes fine graphite with the graphitization forming elements of Si and Ni, which will be described later. However, if C is less than 2.0%, chromium carbide decreases and graphite crystallization also disappears, making it impossible to obtain the desired high-chromium cast iron. On the other hand, if it exceeds 0.386%, depending on the relationship with the Cr content, in the case of the present invention, since Cr is suppressed to 10% or less, supersaturated carbon becomes graphitized due to the high Si and Ni contents, resulting in a graphite product. This is because the amount of ejection becomes excessive and the wear resistance deteriorates.

Si: 1.5-2.8% Si is necessary to crystallize graphite in high chromium materials.
This is because if it is less than 1.5%, this effect will not be achieved, and if it exceeds 2.8%, graphite will be excessively present and the wear resistance will deteriorate.

Regarding Si, it is more effective for graphite crystallization to keep the O3i amount before casting lower than the above component range, perform inoculation at the time of casting, and adjust the components of the final product to within the above range. .

Mn: 0.5-1.5% Mn is actively added together with Si to deoxidize the molten metal. This effect is insufficient when MnO is less than 5%, and 1.5
%, mechanical properties, particularly toughness, will deteriorate significantly.

Ni: 3.0 to 4.3% Ni is actively included for the purpose of improving the matrix structure and crystallizing graphite. However, if Ni is less than 3.0%, no crystallization of graphite is observed, while if it exceeds 4.3%, there will be an excessive amount of graphite as in the case of Si, and at the same time, retained austenite will increase and the austenite will remain stable even with subsequent heat treatment. This is because it poses a problem in terms of roughness resistance when used.

Cr: 5. O = 10% Cr combines with C to form Cr carbide, but 5.0%
If it is less than 10, there will be less carbide and the wear resistance will be inferior.
%, graphite cannot be crystallized even in the above-mentioned ranges of Ni and Si components.

Mo: 2.1-5.0% Mo increases the quenching and tempering resistance and enters into the carbide,
It is effective in increasing carbide hardness and temper softening resistance at the same time, but if the content is less than 2.1%, this effect is small, while if it exceeds 5.0%, there is a noticeable tendency for whitening. This is because almost no graphite is sold anymore.

The graphite-crystallized high-chromium cast iron roll material of the present invention is made of Fe, with the balance of the above components being substantially Fe, but P and S (
In particular, since S) makes the material brittle, it is desirable to have less S).
It is best to keep it at 0.1% or less, S: 0.08% or less. Further, in order to improve the material properties, Nb or V can be contained as necessary in place of a part of Fe. That is, both Nb and V are effective in refining the casting structure, and each can be contained alone or in combination at 1.0% or less. This is because if it exceeds 1.0%, it combines with carbon to form carbides such as VC, NbC, NbzC, etc., which impedes crystallization of graphite and increases costs.

As mentioned above, the graphite-crystallized high chromium cast iron roll material of the present invention is composed of specific alloy components and ranges, so just by performing normal die casting, the entire cast structure remains as cast. It is possible to crystallize graphite with a particle size of approximately 50DII+ or less, and combined with the suppressed Ni content and the addition of a large amount of Mo, it improves not only seizure resistance but also wear resistance and roughness resistance. It can make a big contribution. Note that graphite particles exceeding 50 μm tend to become a starting point for rough skin, and the larger the amount, the less preferable the roll material is.

The graphite crystallized high chromium cast iron roll material described above is mainly used as a casting material for the outer layer, which is the rolling layer of a composite roll for rolling. Cast iron materials or cast steel materials such as graphite cast steel are used as appropriate.

Note that the composite roll referred to herein is not limited to one in which the inner layer is solid, but also includes one in which the inner layer is cylindrical. The latter composite roll (sometimes referred to as a composite sleeve) is used as an assembled roll by being fitted and fixed onto a separately prepared roll shaft. Moreover, the roll material of the present invention can also be used as an outer layer of a hollow composite roller.

As for the manufacturing method of the solid composite roll, the outer layer is cast by the die centrifugal casting method, and then the centrifugal casting die containing the outer layer is erected to form a stationary mold, and the inner layer material is placed inside the centrifugal casting die. There is a method of pouring molten metal and welding the outer layer and the inner layer together, and this method is commonly used because it is simple.

Since the roll material of the present invention has a low Cr content of 10% or less, the roll material applied to the outer layer of the composite roll has the following properties:
Mixing and diffusion of Cr from the outer layer to the inner layer can be reduced, and the toughness of the inner layer can be improved.

The composite roll cast as described above is usually subjected to heat treatment for reducing residual stress and transforming retained austenite into a thermally stable phase.

Various methods can be considered for the above heat treatment depending on the chemical composition, required hardness, rolling conditions, etc., but broadly speaking, A1
A method of repeating tempering at a relatively high temperature below the A point several times to decompose the residual austenite into thermally stable pearlite; This is a method in which Cr is precipitated as secondary carbide, and then the base is made into a mixed phase of secondary carbide and pearlite, or tempered martensite and pearlite, by quenching and tempering heat treatment with controlled cooling rate. By combining such heat treatment methods, the base structure can be selected from pearlite to martensite.

Suitable applications for the roll material of the present invention include hot strip mill (pre-finishing stage, post-finishing stage, roughing stand) work rolls, hot run table rollers, trough pinch rolls, bottom pinch rolls, work rolls for various long steel rolling, and pipe manufacturing work rolls. Roles can be exemplified.

By the way, in addition to the above-mentioned patent publication, BS (British Standard) 4844: Par
t 2 (1972). The differences between these and the roll material of the present invention will be mentioned.

The feature of the roll material of the present invention is that it contains a large amount of chromium carbide and fine graphite of approximately 50 μm or less is crystallized throughout the structure as it is cast.

On the other hand, BS discloses that graphite may crystallize in slow-cooled parts and thick-walled parts, but because the Cr% is too high, graphite crystallization does not occur in the state where it is rapidly cooled by a mold. Therefore, graphite cannot be finely crystallized throughout the structure as in the present invention.

On the other hand, in the case of Japanese Patent Publication No. 16415/1983, which has higher levels of Si and Ni than the present invention, the size of crystallized graphite is 8.
It becomes large, 0 μm or more, and becomes a problem in terms of roughness resistance of the roll.

In the present invention, the reason why Ni is suppressed to a low level and the descendant Mo is used is that Ni stabilizes retained austenite and makes it difficult to be decomposed by heat treatment, so it is difficult to obtain the high hardness required for wear resistance. Therefore, we suppressed Ni to the minimum amount required for crystallization of graphite, made it easy to transform by heat treatment, and increased MO to improve wear resistance by precipitation of MozC carbide and melt into the matrix. It is actively added to improve hardenability.

Next, an example of manufacturing a composite roll having a product body diameter of 760 mmφ, a body length of 2000 mm, and a total length of 4800 mm will be described.

(+) For the outer layer, high chromium cast iron molten metal with the chemical composition shown in Table 1 below was used, and this was placed in a mold rotating on a centrifugal casting machine at a casting temperature of 1350°C with a wall thickness of 85 mm (casting weight 3 '500 kg) was cast.

(Next page) Table 1 (3) After that, the mold was set vertically and ductile cast iron was poured into the upper part as a shaft core material at 1380°C.After the mold was completely filled, the upper end was covered with feeder insulation material. .

(4) After cooling and completely cooling, the roll was taken out from the mold and subjected to the following heat treatment and machining to obtain a final product roll.

・Heat treatment After heating at 1000° CX 511r, it was allowed to cool in the atmosphere, and after that, it was heated at 540° CX 10 hours and then cooled in a furnace.

(5) As a result of performing ultrasonic flaw detection on the body of the roll manufactured in this way, it was confirmed that the outer layer and the shaft core material were completely bonded. Further, Table 2 shows the results of measuring the size of graphite particles.

When the composite rolls shown in Table 2 (6) and above were actually used as hot strip mill finish rolls on an F4 stand, no seizure occurred with any of the rolls. Also,
As for skin roughness resistance, Example 1 had good skin, but Comparative Examples 1 and 2 were related to heat cracks as the graphite particles became larger, causing pit-like depressions and resulting in unsatisfactory skin. became. In terms of abrasion resistance, the number of rolling tons per 1 mm of the outer layer is based on Example 1, Comparative Example 1
was 0.7 times, and Comparative Example 2 was 0.5 times, confirming that the example was superior in terms of wear resistance.

(Effects of the Invention) As explained above, the graphite crystallized high chromium cast iron roll material of the present invention has Si: 1.5 to 2.8% and Ni: 3.0 to 4.
3%, Cr: 5.0-10%, Mo: 2.1-5
．． Since it is made of high chromium cast iron material with a specific composition having 0%
The following fine graphite can be crystallized, and the presence of graphite ensures seizure resistance and crack resistance, as well as good wear resistance and roughness resistance. was completed.

In addition, when the above-mentioned roll material is applied to the outer layer, which is the rolling layer of a composite roll for rolling, not only can the roll material of the present invention have an excellent rolling performance, but also the Cr content of the roll material of the present invention is 10.
% or less, it is possible to reduce mixing and diffusion of Cr into the inner layer, and there is an effect that deterioration of toughness due to Cr incorporation can be prevented.

Claims (2)

[Claims] (1) Chemical composition in weight%: C: 2.0-3.6% Ni: 3.0-4.3% Si: 1
．． 5-2.8% Cr: 5.0-10% Mn: 0.5-1
．． 5% Mo: 2.1 to 5.0% The balance consists essentially of Fe, and is characterized by fine graphite crystallization in the structure as cast.It has excellent wear resistance and roughness resistance. Graphite crystallized high chromium cast iron roll material. (2) Chemical composition in weight%: C: 2.0-3.6% Ni: 3.0-4.3% Si: 1
．． 5-2.8% Cr: 5.0-10% Mn: 0.5-1
．． 5% Mo: 2.1 to 5.0% A composite roll for rolling characterized by having an outer layer, which is a layer used for rolling, in which the balance is substantially Fe, and fine graphite is crystallized in the structure as cast. .