JPH04141553A - Composite roll for hot rolling - Google Patents

Abstract

PURPOSE:To obtain a composite roll for hot rolling having superior wear resistance and improved in heat crack resistance by constituting this composite roll of an outer layer material containing granular carbide and nongranular carbide in specific area ratio and having a matrix with specific hardness and of a core material made of steel. CONSTITUTION:A composite roll for hot rolling is constituted of an outer layer material, which has a structure containing, by area ratio, 5-30% of high hardness granular carbide and >=6% of nongranular carbide and in which the hardness of the matrix is regulated to >=550 Vickers hardness (Hv), and of a core material made of steel. It is preferable that the above outer layer has a chemical composition consisting of, by weight, 1.0-4.0% C, <=3.0% Si, <=1.5% Mn, 2-10% Cr, <=9% Mo, <=20% W, 2-15% V, <=0.08% P, <=0.06% S, >=500ppm B, and the balance Fe with impurity elements and further containing, if necessary, any of <=5.0% Ni, <=5.0% Co, and <=5.0% Nb.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a composite roll for hot rolling, and particularly to a composite roll for hot rolling with improved heat crack resistance.

[Prior Art and Problems to be Solved by the Invention] Conventionally, cast iron rolls manufactured by centrifugal casting have been widely used as work rolls for hot rolling.

In recent years, there has been an increasing demand for such hot rolling rolls to have improved wear resistance, heat crack resistance, and the like.

However, in conventional centrifugal casting rolls, there are restrictions on the chemical composition of the outer layer material in order to suppress gravity segregation of the outer layer material. Therefore, there is a problem that it is not possible to obtain a chemical composition with sufficiently high wear resistance and heat crack resistance.

Therefore, the present inventors developed a so-called continuous overlay casting method, in which a high-frequency coil is used to continuously form an outer layer around a core material made of steel, as a new method for manufacturing rolls with fewer restrictions on the chemical composition of the outer layer material. (Japanese Unexamined Patent Publication No. 6
No. 1-60256, Japanese Patent Application No. 63-502702). The development of this manufacturing method has made it possible to add large amounts of elements such as V, W, and Mo that form hard carbides to the outer layer material, resulting in improved wear resistance and heat crack resistance compared to conventional centrifugal casting rolls. This made it possible to manufacture rolls with improved properties.

However, since hot rolling involves an increase in thermal load, even such rolls do not necessarily have sufficient heat crack resistance.

Therefore, the present inventors studied the improvement of heat crack resistance and the roll structure, and found that the heat cracks of the above-mentioned composite roll for hot rolling are closely related to the microstructure of the roll material, and that it is necessary to improve and control this. It has been found that heat crack resistance can be improved.

Therefore, an object of the present invention is to provide a composite roll for hot rolling that has good wear resistance and improved heat crack resistance.

[Means to solve the problem]

As a result of intensive research in view of the above objectives, the present inventors have found that if there are many non-granular, especially mesh-like carbides in the structure of the outer layer of a composite roll for hot rolling, thermal stress causes these carbides to disintegrate. Although fine cracks preferentially occur, it was discovered that these cracks remain near the surface of the roll and do not extend deep into the roll, leading to the development of the present invention.

That is, the composite roll for hot rolling of the present invention has a structure containing 5 to 30% of granular carbide and 6% or more of non-granular carbide in terms of area, and the hardness of the base is Vickers hardness (Hv ) is characterized by consisting of an outer layer material of 550 or more and a steel core material.

The present invention will be explained in detail below.

In the present invention, granular carbide refers to carbide represented by MC, MaC*, etc. The granular carbide generally has high hardness. The content of the granular carbide in the outer layer material is 5 to 30% in terms of area ratio. When the content of granular carbide is less than 5%, there is no sufficient effect of improving wear resistance, and when it exceeds 30%, it becomes difficult to uniformly disperse the carbide.

Furthermore, in the present invention, non-granular carbide refers to LsCakA
It is a non-granular structure formed by the growth of eutectic carbide represented by tCs, MIC, LC, etc. The content of the non-granular carbide in the outer layer material is 6% or more in terms of area ratio. When the content of non-granular carbide is less than 6%, cracks caused by thermal stress are insufficiently dispersed and propagate deep into the roll, resulting in a decrease in heat crack resistance.

The hardness of the base is Vickers hardness ()Iv), which is 55
It is 0 or more. If the Vickers hardness of the base is less than 550, wear resistance will decrease.

In order to have a structure containing granular carbide and non-granular carbide at the above-mentioned area ratio and to develop the above-mentioned Vickers hardness, specifically, it is preferable to use a material having the following composition as the outer layer. That is, the chemical components of the outer layer are C1.0 to 4.0% by weight, S
i 3.0% by weight or less, Mn 1.5% by weight or less, Cr
2 to 10% by weight, Mo 9% by weight or less, W 20% by weight or less, 72 to 15% by weight, P 0.08% by weight or less, 30.
0.6% by weight or less, 8500ppm or more, and the balance consists of Fe and impurity elements.

The reasons for limiting the chemical components of the outer layer of the hot rolling composite roll are as follows.

C is necessary for forming carbides to improve wear resistance.

When the amount is less than 1.0% by weight, the amount of crystallized carbide is small and the wear resistance is not sufficient.

When the amount of C exceeds 4.0% by weight, the amount of carbide becomes excessive,
The material becomes brittle.

Si is an element necessary as a deoxidizing agent. It is also necessary to maintain the fluidity of the molten metal. The amount is 3゜0% by weight
Exceeding this is disadvantageous as it tends to become brittle.

Mn has a deoxidizing effect as well as an effect of fixing S, which is an impurity, as MnS. If the amount exceeds 1.5% by weight, retained austenite tends to occur and sufficient hardness cannot be stably maintained.

If Cr is less than 2% by weight, hardenability is poor, and non-granular carbide effective for heat crack resistance is not sufficient. If it exceeds 10% by weight, the amount of chromium-based carbide becomes excessive, which is disadvantageous. That is, Cr-based carbide, for example, M21C@ is MC
, M4C, , M, and C have lower hardness and lower wear resistance.

Mo is necessary to obtain hardenability and high temperature hardness,
If it exceeds 9% by weight, M and C-based carbides increase too much in the balance of C, V, and Mo, which is unfavorable in terms of toughness and rough skin. Therefore, the upper limit of the Mo content is 9% by weight.

W is necessary to maintain high-temperature hardness, and if it exceeds 20% by weight, M and C carbides increase, which is undesirable in terms of toughness, so the upper limit is set at 20% by weight.

(2) is an essential element for forming MC-based carbides that are effective in improving wear resistance. Therefore, if it is less than 2% by weight, there is no sufficient effect, and if it is more than 15% by weight, the oxidation of the molten metal becomes severe and it becomes difficult to dissolve it in the atmosphere.

In addition to the above elements, the iron-based alloy consists essentially of iron excluding impurities. The main impurities are P and S, but P
is preferably 0.08% by weight or less to prevent embrittlement, and similarly S is preferably 0.06% by weight or less.

Furthermore, the outer layer material contains a trace amount of B. B promotes the formation of non-granular carbides in the matrix and has the effect of increasing the hardness of the steel deep into the steel. Therefore, microcracks are dispersed along the non-granular carbides, and the hardness is improved deep into the roll, so that cracks are prevented from propagating deep into the roll. In order to obtain such an effect, the concentration of B needs to be 500 ppm or more. When the concentration of B is less than 500 ppm, non-granular carbides are not sufficiently generated in the matrix, so that heat cracks tend to develop deep into the roll.

In addition to the above-mentioned components, Ni and Co5Nb may be added to the outer layer of the composite roll for hot rolling of the present invention, if necessary.

Ni has the effect of improving hardenability. For this reason,
It is particularly preferable to add it to large rolls where the quenching speed cannot be increased. However, if the content exceeds 5% by weight, the austenite becomes too stable and there is too much residual austenite after heat treatment, making it impossible to obtain sufficient hardness.

Co has the effect of improving the toughness and hot hardness of the material. Therefore, by adding Co, the wear resistance can be improved. The above-mentioned improvement effect is almost saturated at a content of 5% by weight, so the upper limit is 5% by weight.

Like V, Nb forms granular carbides. Furthermore, it develops the effect of making MC carbide, which is a granular carbide, finer. This improves wear resistance. However, if the content exceeds 5% by weight, oxidation of the molten metal becomes severe, making it difficult to melt and cast in the atmosphere.

Although a steel material such as cast steel or forged steel is used for the core material of the composite roll for hot rolling of the present invention, the composition thereof may be one commonly used and is not particularly limited.

In order to manufacture a composite roll for hot rolling consisting of an outer layer having the above composition and a steel core material, the outer layer is formed around the core material by the usual continuous overlay casting method, and then processed. However, in order to make the B content 500 ppm or more as in the present invention, the flux used to coat the surface of the core material before overlaying and the surface of the molten metal for the outer layer must contain B or a B compound. It is preferable to use

Examples of the above flux include StL, NazO and/or Ni20, B, 0. Other materials such as oxides and halides of alkali metals and alkaline earth metals may be used.

When such a flux is used, B present in the flux diffuses into the molten metal for the outer layer, so that a large amount of B is contained in the outer layer, which is preferable.

[Effect]

The composite roll for hot rolling of the present invention has improved heat crack resistance. The reason for this effect is that the base of the outer layer is sufficiently hardened to the depths, and even if fine cracks occur due to heat load, they are dispersed along the non-granular carbide, so cracks do not occur. This seems to be because the roll does not progress to the deep part.

In addition, the hot rolling composite roll of the present invention has good wear resistance, which is due to the fact that the amount of granular carbide such as VC is prevented from decreasing in the base of the outer layer, and the hardness of the base is above a certain level. It is thought that this is because of this.

〔Example〕

The present invention will be explained in further detail by the following examples.

Example 1 and Comparative Example 1 Materials having the compositions shown in Table 1 were cast and heat treated to produce test pieces (diameter 30 mm, length 30 mm) for the test described below.

For comparison, a test piece with a B content of less than 500 ppm was prepared in the same manner as in Example 1 (Comparative Example 1).

After repeating the operation of alternately immersing one end of each of these test pieces in a salt bath at 700°C and water at 30°C five times, the roll was cut lengthwise and the heat cracks that had formed on the surface were removed. The depth was measured.

The results are shown in Table 1 along with the area percentages of granular carbides and non-granular carbides.

Note that the depth of the heat crack is a value expressed as the average of five measurements.

Table 1 shows that the depth of heat cracks in the roll of Example 1 is approximately half that of the roll of Comparative Example 1, indicating that even if heat cracks occur in the roll of the present invention, they do not extend deep. Ta.

〔Effect of the invention〕

In the composite roll for hot rolling of the present invention, the outer layer material contains a predetermined amount or more of non-granular carbide. Therefore, even if cracks occur due to thermal load, they are prevented from propagating deep into the roll.

This results in a composite roll for hot rolling with improved heat crack resistance. Also, since it contains VC-based hard carbide, it has good wear resistance.

For this reason, the life of the hot rolling roll is extended, and furthermore, breakage of the roll is extremely unlikely to occur.

Claims (5)

[Claims] (1) An outer layer material consisting of a structure containing 5 to 30% of granular carbide and 6% or more of non-granular carbide in terms of area ratio, and a base hardness of 550 or more Vickers hardness (Hv), and a steel A composite roll for hot rolling characterized by comprising a core material. (2) In the hot rolling composite roll according to claim 1, the chemical components of the outer layer are C1.0 to 4.0% by weight, Si
3.0% by weight or less, Mn 1.5% by weight or less, Cr2-1
0% by weight, Mo 9% by weight or less, W 20% by weight or less, V2
A composite roll for hot rolling, characterized in that it consists of ~15% by weight, P0.08% by weight or less, S0.06% by weight or less, B500ppm or more, and the balance is Fe and impurity elements. (3) The composite roll for hot rolling according to claim 1 or 2, wherein the outer layer contains 5.0% by weight or less of Ni. (4) The composite roll for hot rolling according to any one of claims 1 to 3, wherein the outer layer contains 5.0% by weight or less of Co. (5) The composite roll for hot rolling according to any one of claims 1 to 4, wherein the outer layer contains 5.0% by weight or less of Nb.