JP2007144444A - Composite roll for rolling - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a composite roll for rolling, which can solve the problems of traditional rolls for rolling, and has excellent wear resistance, excellent surface roughing resistant, and excellent seizure resistance, and in which an outer layer and an inner layer are perfectly welded. <P>SOLUTION: The composite roll for rolling has the outer layer having such a structure that MC carbide has been distributed on a base material having a Vickers hardness of 550 to 900 Hv so as to have an area ratio of 20 to 60%, and the region which does not contain the MC carbide having a diameter of 15 μm or more in terms of a circle does not exceed 150 μm in the diameter of an inscribed circle; and has the inner layer metallically joined on the inside surface of the outer layer, wherein the tensile strength of the portion between the outer layer and the inner layer is 400 MPa or higher. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to an outer layer for a rolling roll and a composite roll for rolling using the same, and is excellent in wear resistance, rough skin resistance, and seizure resistance, and particularly used in a finishing row of a hot sheet rolling mill. Is suitable.

  Important properties that determine the productivity of rolling are the wear resistance and rough surface resistance of the rolling roll. If the wear resistance is poor, the roll surface is worn early and the dimensional accuracy of the material to be rolled is impaired. In addition, if the surface of the rolling roll is worn unevenly due to contact with the material to be rolled or contact with the backup roll, the rough surface is transferred to the material to be rolled and the surface appearance of the material to be rolled is impaired. . In order to prevent these problems, the rolls must be replaced frequently, resulting in a decrease in rolling mill productivity due to an increase in the frequency of rolling operations, an increase in the cost required for roll surface grinding, and the amount of roll surface grinding. There arises a problem that the roll basic unit is reduced due to an increase in the roll size.

  Another important characteristic of the rolling roll is seizure resistance. If the seizure resistance is poor, the material to be rolled will seize on the roll due to heat generation in the roll bite during rolling, and normal rolling will not be possible. In particular, in the latter stage stand of the finishing row of a hot sheet rolling mill, an accident called so-called “drawing” occurs in which the end of the material to be rolled is rolled by two sheets for some reason. At that time, if the seizure resistance is poor, the material to be rolled is baked on the roll, and further, the material to be rolled is wound around the roll body and the rolling is forced to stop. Further, when the material to be rolled is rolled while being baked on a roll, a rolling load is concentrated on the baked portion, cracks are generated, and rolls such as a spall may be damaged starting from the crack.

Therefore, a high-speed alloy containing a large amount of alloy elements such as Cr, Mo, W, and V has been conventionally used as an outer layer for rolling rolls intended to meet the demands of wear resistance and rough skin resistance. In the structure, M ₇ C ₃ type carbide containing a large amount of Cr (M represents a metal element, hereinafter the same), M ₂ C type carbide and M ₆ C type carbide containing a lot of Mo and W, and a lot of V are contained. It contains metal carbide such as MC type carbide. There are many known examples of this type of outer layer, for example:

  In Patent Document 1, the chemical components are by weight, C: 1.0 to 3.0%, Si: 0.1 to 3.0%, Mn: 0.1 to 2.0%, Cr: 2. 0 to 10.0%, Mo: 0.1 to 10.0%, V: 1.0 to 10.0%, W: 0.1 to 10.0%, and Mo + W ≦ 10.0% A solid or hollow centrifugal cast composite roll is described which consists of an outer layer composed of an alloy component and the balance of Fe and impurities, and an inner layer of cast iron or cast steel.

  In Patent Document 2, C: 3.5 to 5.5%, Si: 0.1 to 1.5%, Mn: 0.1 to 1.2%, Cr: 4.0 to 12.0%, A hot-rolling tool steel characterized by Mo: 2.0 to 8.0%, V: 12.0 to 18.0%, the balance Fe and inevitable impurities is described. The hot rolling tool steel is described to contain Nb: 8.0% or less, and further Ni: 5.5% or less. Moreover, the outer layer for centrifugal casting rolls which satisfies 0.2 <= Nb / V in the outer layer for centrifugal casting rolls which consists of tool steel for hot rolling is described.

  Patent Document 3 includes an outer layer material having an area ratio of 5-30% granular carbide and 5% non-particulate carbide, and having a base hardness of Vickers hardness (Hv) of 550 or more, and steel. A hot-rolling composite roll characterized by comprising a core material made of the product is described. In the composite roll for hot rolling, the chemical component of the outer layer is C1.0 to 3.5 wt%, Si 3.0 wt% or less, Mn 1.5 wt% or less, Cr2 to 10 wt%, Mo 9 wt% or less, W20 wt% or less, V2 to 15 wt%, P 0.08 wt% or less, S 0.06 wt% or less, the balance Fe and impurity elements are described. Furthermore, it is described that any one of B 300 ppm or less, Ni 5.0 wt% or less, Co 5.0 wt% or less, and Nb 5.0 wt% or less is contained.

  In Patent Document 4, by weight ratio, C: 1.8-5%, Si: 2% or less, Mn: 2% or less, Cr: 4-6%, W: 2-8%, Mo: 2-10 %, V: more than 11% and 17% or less, Co 7 to 13%, the balance Fe and inevitable impurities, and containing MC type carbide having an average particle size of 1 to 30 μm and an area ratio of 20 to 40% Bonds are listed.

Further, a glen-based alloy has been conventionally used as an outer layer for a rolling roll that is excellent in seizure resistance and skin roughness resistance. The structure contains a large amount of cementite, that is, M ₃ C carbide and a small amount of graphite, and the seizure resistance is improved by the effect of the carbide and graphite. In recent years, the application of materials having improved wear resistance by adding V or the like to this glenic outer layer and containing MC carbide in the structure has been expanding. The following is mentioned as a well-known example of this kind of outer layer, for example.

  In Patent Document 5, the chemical components are C2.0 to 4.0% by weight, Si 0.5 to 4.0%, Mn 0.1 to 1.5%, Ni 2.0 to 6.0%, Cr1. 0 to 7.0%, V: 2.0 to 8.0%, the balance is composed of Fe and impurity elements, the base structure, 0.5 to 5 area% graphite, and 0.2 to 10 area% MC system A wear-resistant and seizure-resistant hot rolling roll having a metal structure consisting of carbide and 10 to 40 area% cementite is described.

JP-A-8-60289 JP-A-9-256108 JP-A-3-254304 JP-A-7-268568 JP-A-6-335712

In order to improve the wear resistance of the rolling roll, generally, the hardness of the roll material is increased. The high-speed roll material contains high-hardness carbides (MC, M ₂ C, M ₆ C, M ₇ C ₃ ) mainly composed of alloy elements, and has improved wear resistance. Among the alloy elements, V and Nb in particular form MC carbide having a very high hardness with a Vickers hardness of about Hv 2400 to 3200 and contribute to improvement of wear resistance.

However, when a molten metal containing excessive V or Nb is subjected to centrifugal force casting, segregation due to centrifugal separation occurs. In the above-mentioned Patent Document 1, when V exceeds 10.0%, the carbide formed in the case of the centrifugal casting method is light and floats on the inner surface, and the carbide corresponding to the content on the outer surface of the outer layer used for rolling. Is not included. Such a phenomenon is likely to occur when the molten metal cast into the centrifugal casting mold solidifies and crystallizes granular carbides in the primary crystal. The primary crystal granular carbides lighter specific gravity relative to a specific gravity of 6 g / cm ³ degree and the melt remaining liquid (specific gravity of about 7~8g / cm ^3), excess by the crystallizes centrifugal force to separate the inner surface is there.

  In addition, means for preventing segregation due to centrifugation by increasing the specific gravity of the carbide has been proposed. According to the aforementioned Patent Document 2, VC carbide has a specific gravity smaller than that of the mother molten metal, and segregates when centrifugal casting is performed. Nb forms a composite carbide {V, Nb} C with V, and increases the specific gravity as compared with a carbide of V alone. Thus, it is described that segregation due to centrifugation is prevented. In addition, it is described that in order to obtain a uniform outer layer for rolls when manufactured by the centrifugal casting method, 0.2 ≦ Nb / V must be satisfied. On the other hand, if V exceeds 18.0%, the effect of improving the seizure property is saturated and there is a risk of producing problems such as poor dissolution. Nb forms composite carbides {V, Nb} C with V, but it is described that when it exceeds 8.0%, production problems such as poor dissolution occur.

    Further, as a method for producing the high-speed outer layer material, there is a continuous overlay casting method in addition to the centrifugal casting method. In Patent Document 3, as a method for manufacturing a roll with less restrictions on the chemical composition of the outer layer material, a so-called continuous overlay casting method in which an outer layer is continuously formed using a high-frequency coil around a core material made of steel, A large amount of elements such as V, W, and Mo that form carbides can be added to the outer layer material, making it possible to manufacture rolls with improved wear resistance and rough skin resistance compared to conventional centrifugal casting rolls. It is described that it became. However, if the primary crystal granular carbide crystallizes excessively in the molten molten metal, the granular carbide floats and separates on the upper part of the molten molten metal and is not included in the outer cladding layer. In order to prevent this, if the stirring of the high-frequency coil is strengthened, carbide segregation occurs due to the heterogeneous inclusion of carbides that have been levitated and separated into the outer cladding. For example, Patent Document 3 describes that it is difficult to uniformly disperse the granular carbide if it exceeds 30%. In order to prevent these, the content of V and Nb contained in the outer layer formed by continuous overlay casting is limited.

  Thus, in order to drastically improve the wear resistance of the high-speed outer layer formed by centrifugal force casting, a large amount of V and Nb may be added. However, as described above, it is actually very difficult to manufacture. .

  Similarly, in the case of the powder HIP method as in Patent Document 4, when a large amount of MC carbide forming elements such as V and Nb are added, MC carbide is used in the melt nozzle used when gas atomizing the melt in the powder manufacturing process. Crystallizes and closes the nozzle. In order to prevent this, if the nozzle diameter is increased, the particle diameter of the powder becomes coarse, and the size of the MC carbide contained in the powder becomes inhomogeneous. In the HIP sintered alloy using such a powder, the size and distribution of MC carbides are non-uniform, and a difference in wear occurs due to micro segregation between a portion with a large amount of MC carbide and a portion with a small amount of MC carbide, resulting in rough skin.

  Moreover, in order to improve the seizure resistance of a rolling roll, it is generally known that a large amount of carbide is contained in the metal structure and the area ratio of carbide is increased on the roll surface. The conventional high-speed outer layer contains about 20% of carbide in area ratio, but is not sufficient to enhance seizure resistance.

Therefore, if the conventional high-speed outer layer contains a large amount of C or an alloy element in order to increase the amount of carbide in order to improve seizure resistance, rough skin due to a decrease in toughness or chipping is likely to occur. In particular, when alloy elements such as C, Cr, Mo, and W are excessively increased, so-called eutectic carbides such as M ₂ C, M ₆ C, and M ₇ C ₃ carbides increase, and cracks easily propagate. . For this reason, the toughness is reduced, and these carbides are coarsened and are liable to cause rough skin due to chipping.

On the other hand, a glen-based outer layer such as Patent Document 5 contains cementite in a large amount of about 40% in area ratio, and has improved seizure resistance. However, the glen-based outer layer contains M ₃ C carbide (cementite) mainly composed of Fe, and its hardness is relatively low at about Hv 1000 to 1800, so that the wear resistance is inferior to that of the high-speed outer layer. There is.

  Accordingly, the present invention provides a composite roll for rolling in which the problems associated with conventional rolling rolls are solved, and excellent wear resistance, rough skin resistance, and seizure resistance are combined, and the outer layer and inner layer are welded in a sound manner. Objective.

  The present inventor casts a molten metal adjusted to a chemical composition for crystallizing primary MC carbide into a casting mold for centrifugal force casting and centrifugally casting to form a layer in which MC carbide is concentrated on the inner surface side in the mold. As a result, it has been found that a structure in which MC carbides are rich and uniformly dispersed in the structure can be obtained, and the present invention has been achieved.

  That is, the first rolling composite roll of the present invention has a structure in which MC carbide is dispersed in an area ratio of 20 to 60% on a base having a Vickers hardness of Hv550 to 900, and the equivalent circle diameter in the structure is 15 μm. The region containing no MC carbide has an outer layer whose inscribed circle diameter does not exceed 150 μm, and an inner layer metal-bonded to the inner surface of the outer layer, and the tensile strength between the outer layer and the inner layer is 400 MPa or more. It is characterized by.

  The second composite roll for rolling of the present invention is a structure in which MC carbide is dispersed in an area ratio of 20 to 60% on a base having a Vickers hardness of Hv550 to 900, and the equivalent circle diameter in the structure is 15 μm. In the composite roll for rolling having an outer layer in which the MC carbide-free region does not exceed an inscribed circle diameter of 150 μm and an inner layer metal-bonded to the inner surface of the outer layer, a bonding boundary portion between the outer layer and the inner layer is formed. When the tensile test piece including the test piece is subjected to a tensile test, the tensile fracture position exists in the outer layer excluding the joining boundary portion.

  The third rolling composite roll of the present invention is a structure in which MC carbide is dispersed in an area ratio of 20 to 60% on a base having a Vickers hardness of Hv 550 to 900, and the equivalent circle diameter in the structure is 15 μm. It has an outer layer whose average interparticle distance between MC carbides is 10 to 40 μm and an inner layer metal-bonded to the inner surface of the outer layer, and the tensile strength between the outer layer and the inner layer is 400 MPa or more. And

  The fourth rolling composite roll of the present invention has a structure in which MC carbide is dispersed in an area ratio of 20 to 60% on a base having a Vickers hardness of Hv550 to 900, and the equivalent circle diameter in the structure is 15 μm. In the composite roll for rolling which has the outer layer whose average interparticle distance between the MC carbides is 10 to 40 μm, and the inner layer metal-bonded to the inner surface of the outer layer, the tension including the joining boundary portion between the outer layer and the inner layer When the test piece is subjected to a tensile test, the tensile breaking position is in the outer layer excluding the joining boundary portion.

  The inner layer is made of an Fe-based alloy containing, by mass, chemical components, C: 0.5 to 3.0%, Si: 0.1 to 3.0%, and Mn: 0.1 to 3.0%. It is characterized by that.

  An average equivalent circle diameter of MC carbide particles dispersed in the outer layer is 10 to 50 μm.

  A ratio (G / H) of an average inter-particle distance (G) between MC carbides having an equivalent circle diameter of 15 μm or more in the outer layer to an average equivalent circle diameter (H) of MC carbides is 2 or less. And

The structure is characterized in that the total amount of M ₂ C, M ₆ C and M ₇ C ₃ carbides having an equivalent circle diameter of 1 μm or more is dispersed in an amount of 0 to 5%.

  Chemical component of the outer layer is% by mass, C: more than 2.5% and 9.0% or less, Si: more than 0.1% and 3.5% or less, Mn: more than 0.1% and 3.5% Hereinafter, it is characterized by containing V: more than 11.0% and 40.0% or less, and the balance being Fe and inevitable impurity elements.

  Further, in the outer layer by mass%, Cr: more than 1.0% to 15.0% or less, Mo: more than 0.5% to 20.0% or less, and W: more than 1.0% to 40.0% or less Any one or two or more of these are contained.

Furthermore, a part of V in the outer layer is substituted with Nb in a range satisfying the following formula (1) in mass%.
11.0% <V% + 0.55 × Nb% ≦ 40.0% (1)

Further, the outer layer satisfies the following expression (2).
0 <C% −0.2 × (V% + 0.55 × Nb%) ≦ 2.0%. . . (2)

  Further, the component in the outer layer is mass% and contains any one or two of Ni: 2.0% or less and Co: 10.0% or less.

  Further, the component in the outer layer is mass% and contains any one or two of Ti: 0.5% or less and Al: 0.5% or less.

  Further, the outer layer of the rolling composite roll of the present invention is formed by centrifugal casting.

First, the structural elements of the outer layer of the composite roll for rolling of the present invention will be described.
The base of the outer layer is a portion excluding carbides such as MC carbide, and is mainly composed of Fe and alloy elements, and the hardness changes due to transformation by heat treatment, precipitation of ultrafine carbides in the base, and the like. When the hardness of the base is Vickers hardness and less than Hv550, the wear resistance is lowered. From the viewpoint of improving the wear resistance, it is desirable that the hardness of the base is high, but if it exceeds Hv900, the toughness of the base decreases. A more preferable range of the hardness of the base is Hv650-850. A more preferable range is Hv650-750.

  MC carbide has higher hardness than other carbides and contributes to improvement of wear resistance. In addition, MC carbide is stable at high temperature and hardly metal-bonds with the material to be rolled, and therefore exhibits an excellent effect in improving seizure resistance. The MC carbide of the present invention has insufficient wear resistance and seizure resistance when the area ratio is less than 20%, and if MC carbide exceeds 60% by area ratio, the effect of improving seizure resistance is saturated and toughness is reduced. It drops significantly. Therefore, MC carbide is preferably 20 to 60% in terms of area ratio. A more preferable area ratio is 30 to 50%.

In the outer layer of the present invention, the total amount of M ₂ C, M ₆ C, and M ₇ C ₃ carbides having an equivalent circle diameter of 1 μm or more can be dispersed in an area ratio of 0 to 5%. If the sum of these carbides exceeds 5% in terms of area ratio, the carbides become coarse and the rough skin resistance and toughness are impaired. The smaller the number, the better. The area ratio may be 0%. A more preferable area ratio is 0 to 3%, and still more preferably 0 to 1%. The outer layer of the present invention may contain a small amount of various carbides other than MC, M ₂ C, M ₆ C, and M ₇ C ₃ carbides.

  The greatest feature of the outer layer of the present invention is that the region not containing MC carbide having an equivalent circle diameter of 15 μm or more in the structure of the outer layer of the present invention does not exceed 150 μm in inscribed circle diameter. When the MC carbide has an equivalent circle diameter of less than 15 μm, the effect of improving the seizure resistance inherent to the MC carbide cannot be expected. In addition, the inscribed circle diameter of the region not including MC carbide having an equivalent circle diameter of 15 μm or more is set to 150 μm or less. If there is a region larger than the inscribed circle diameter, variation in the distribution of MC carbide cannot be ignored. It became large and it turned out that the improvement of seizure resistance is not seen. It is more preferable that the region not including MC carbide having an equivalent circle diameter of 15 μm or more does not exceed 120 μm in inscribed circle diameter. Furthermore, it is more desirable not to exceed 80 μm.

  Moreover, in the structure of the outer layer of the present invention, the average inter-particle distance between MC carbides having an equivalent circle diameter of 15 μm or more is set to 10 to 40 μm. If the average inter-particle distance is smaller than this, the MC carbides are aggregated too much. In addition, the micro-roughness due to the difference in wear occurs between the part with a large amount of MC carbide and the part with a small amount of MC carbide, thereby impairing the rough skin resistance. It was found that when the average interparticle distance was larger, the dispersion of MC carbide distribution was so large that it could not be ignored, and no improvement in seizure resistance was observed. The average interparticle distance between MC carbides having an equivalent circle diameter of 15 μm or more is more preferably 20 to 30 μm.

  The average equivalent circle diameter of MC carbide dispersed in the outer layer structure is preferably 10 to 50 μm. During rolling, the roll surface is exposed to high heat from the rolled steel sheet. In particular, when the heat load is high, the roll surface is affected by heat by about 10 μm, and the roll base is softened, which causes deterioration of the wear resistance and rough skin resistance of the roll. When the average equivalent circle diameter of MC carbide is less than 10 μm, MC carbide cannot support the above-mentioned softened base, and wear resistance and rough skin resistance deteriorate. On the other hand, when it exceeds 50 μm and becomes coarse, the effect of improving seizure resistance is saturated and toughness is lowered. A more preferable range of the average equivalent circle diameter of the MC carbide is 10 to 40 μm, and more preferably 15 to 30 μm.

  In the outer layer structure of the present invention, the ratio (G / H) of the average interparticle distance (G) between MC carbides having an equivalent circle diameter of 15 μm or more and the average equivalent circle diameter (H) of MC carbides is 2 The following is desirable. The outer layer of the present invention contains a large amount of MC carbide excellent in wear resistance and is excellent in wear resistance and rough skin resistance, but MC carbide tends to aggregate during production. When the MC carbide aggregates, microscopic unevenness due to a difference in wear occurs in a portion where the MC carbide is large and a portion where the MC carbide is small, which is a cause of impairing the rough skin resistance. When this G / H value exceeds 2, MC carbide tends to agglomerate, and micro unevenness due to a difference in wear occurs between a portion with a large amount of MC carbide and a portion with a small amount of MC carbide, thereby impairing rough skin resistance. A more preferable value of G / H is 1.5 or less.

Here, the equivalent circle diameter in the present invention will be described. FIG. 1 shows a conceptual diagram of the equivalent circle diameter. The equivalent circle diameter represents the diameter of a circle having the same area as the object (in this case, a carbide). In FIG. 1, when the area of the measuring object E is A, the circle equivalent diameter D is the diameter of the circle B corresponding to the area equal to the area A of the measuring object, and is represented by the equation (3).
Equivalent circle diameter D = √ (A × 4 / π) (3)

  Thus, the equivalent circle diameter D of the MC carbide of the present invention and the average equivalent circle diameter H of the MC carbide obtained by averaging them were obtained.

  The inscribed circle diameter of the region not including MC carbide having an equivalent circle diameter of 15 μm or more in the present invention will be described. FIG. 2 shows a conceptual diagram of the inscribed circle diameter in a region not including MC carbide having an equivalent circle diameter of 15 μm or more.

In FIG. 2, symbols a, a1, a2, a3, and a4 (white coating portions) are MC carbides having a circle equivalent diameter of 15 μm or more, and symbol b (black coating portion) is an MC carbide having an equivalent circle diameter of less than 15 μm, symbol e Is a portion excluding MC carbides a and b, and is a region where the base and M ₂ C, M ₆ C and M ₇ C ₃ carbides exist. In this case, the inscribed circle diameter of the present invention is drawn innumerably on the surface of the region e. As shown in FIG. 2, the inscribed circle diameter d inscribed in all of the MC carbides a1 to a4 of 15 μm or more excluding the MC carbide b of less than 15 μm from the measurement object is the inscribed circle diameter in the present invention. .

The average interparticle distance between MC carbides having an equivalent circle diameter of 15 μm or more in the present invention will be described. FIG. 3 shows a conceptual diagram of the average interparticle distance. In FIG. 3, the symbol a (white-coated portion) is an MC carbide having an equivalent circle diameter of 15 μm or more, the symbol b (black-coated portion) is an MC carbide having an equivalent circle diameter of less than 15 μm, and the symbol e is excluding MC carbides a and b. This is a region where the base and M ₂ C, M ₆ C and M ₇ C ₃ carbides exist. In this case, in an arbitrary straight line L, the MC carbide b of less than 15 μm is excluded from the measurement object, and the shortest distance between adjacent MC carbides a in the 15 μm or more MC carbide a existing on the straight line L Assuming that the line segment is Ln, the average interparticle distance G is expressed by Expression (4). Note that X is any other straight line, and the average interparticle distance G is obtained in the same manner as the straight line L.
Average interparticle distance G between MC carbides with equivalent circle diameters of 15 μm or more G =
(Σ (L ₁ + L ₂ +... + L _n )) / n (4)

  Next, the reason for limiting the chemical component (% by mass) of the outer layer of the composite roll for rolling of the present invention will be described. In addition, the chemical component of the outer layer of the present invention indicates not the component of the molten metal but the chemical component of the outer layer in the final product.

C: More than 2.5% and not more than 9.0% C is an essential element that contributes to wear resistance by forming MC carbide mainly by combining with alloy elements such as V or Nb. Further, C that does not bond with the alloy element mainly contributes to wear resistance by strengthening the matrix by solid solution or precipitation with the alloy element in the matrix. If C is 2.5% or less, the amount of MC carbide is insufficient and sufficient wear resistance cannot be obtained. On the other hand, if C exceeds 9.0%, the amount of carbide becomes excessive and the thermal crack resistance deteriorates. The C content is more preferably more than 3.5% and not more than 9.0%, still more preferably more than 4.5% and not more than 9.0%.

Si: more than 0.1% and 3.5% or less Si acts as a deoxidizer in the molten metal. When Si is 0.1% or less, the deoxidation effect is insufficient and casting defects are likely to occur. Moreover, when it exceeds 3.5%, it will embrittle. The Si content is more preferably 0.2 to 2.5%, and still more preferably 0.2 to 1.5%.

Mn: More than 0.1% and 3.5% or less Mn is effective when deoxidation of molten metal or S, which is an impurity, is fixed as MnS and exceeds 0.1%. When Mn exceeds 3.5%, retained austenite tends to be generated, and the hardness cannot be stably maintained, and the wear resistance is likely to deteriorate. A more preferable Mn content is 0.2 to 2.5%, and further preferably 0.2 to 1.5%.

V: more than 11.0% and not more than 40.0% V is an important element of the present invention which mainly bonds with C to form MC carbide. One of the features of the present invention is that the outer layer contains a very large amount of MC carbide. When V is 11.0% or less, MC carbide is insufficient and sufficient wear resistance cannot be obtained. On the other hand, if V exceeds 40.0%, MC carbide becomes excessive and toughness deteriorates. The V content is more preferably more than 15.0% and not more than 40.0%, still more preferably more than 18.0% and not more than 40.0%.

Cr: more than 1.0% and 15.0% or less Cr dissolves in the matrix to improve hardenability, and part of it combines with C in the matrix and precipitates as ultrafine carbide to strengthen the matrix. . When Cr is 1.0% or less, the effect of strengthening the base cannot be obtained sufficiently. On the other hand, if it exceeds 15.0%, carbides other than MC carbide such as M ₇ C ₃ carbide particularly increase, coarsen or crystallize in a network, and heat cracking resistance deteriorates. A more preferable Cr content is 3.0 to 9.0%.

Mo: more than 0.5% and not more than 20.0% Mo dissolves in the matrix and enhances hardenability, and partly bonds with C in the matrix and precipitates as ultrafine carbides to strengthen the matrix. Furthermore, a part of Mo forms granular carbide together with V, Nb and the like. When Mo is 0.5% or less, the effect of strengthening the base cannot be sufficiently obtained. On the other hand, if it exceeds 20.0%, carbides other than MC carbides such as M ₂ C and M ₆ C are particularly increased, coarsened or crystallized in a network, and heat cracking resistance is deteriorated. A more preferable Mo content is 2.5 to 20.0%, and further preferably 2.5 to 10.0%.

W: More than 1.0% and 40.0% or less W is solid-solved in the base part to improve hardenability, and partly bonds with C in the base and precipitates as ultrafine carbide to strengthen the base part. To do. Furthermore, a part of W forms granular carbide together with V, Nb and the like. If W is 1.0% or less, the effect of strengthening the base cannot be obtained sufficiently. On the other hand, if it exceeds 40.0%, carbides other than MC carbide such as M ₆ C and M ₂ C are particularly increased, coarsened or crystallized in a network, and heat cracking resistance is deteriorated. A more preferable W content is 5.0 to 40.0%, and further preferably 5.0 to 20.0% or less.

  The outer layer of the present invention preferably contains at least one or more of Cr, Mo, or W, which are base strengthening elements, in order to obtain a base necessary for sufficiently exhibiting wear resistance. .

11.0% <V% + 0.55 × Nb% ≦ 40.0% (1)
Nb has the same action as V in that it forms MC carbides. When V is 1.0% in mass%, Nb is equivalent to V at 0.55 × Nb% in mass% from the atomic weight ratio. Therefore, a part of V can be replaced with Nb within the range of the formula (1).

0 <C% −0.2 × (V% + 0.55 × Nb%) ≦ 2.0% (2)
When the value of C% −0.2 × (V% + 0.55Nb%) is 0 or less, the amount of MC carbide cannot be obtained sufficiently, and V and Nb become excessive in the base, and the hardness of the base is obtained. The wear resistance decreases. When the value of C% −0.2 × (V% + 0.55Nb%) exceeds 2.0%, carbides other than MC carbides such as M ₂ C, M ₆ C, and M ₇ C ₃ carbides In particular, it increases, becomes coarse or crystallizes in a network, and heat cracking resistance deteriorates.

  Moreover, according to the use and usage method of a rolling roll, the following components can be suitably added to the outer layer of the present invention.

Ni: 2.0% or less Ni dissolves in the base and is effective in improving the hardenability of the base. If Ni exceeds 2.0%, the base austenite is stabilized, and the base hardness cannot be sufficiently obtained.

Co: 10.0% or less Co dissolves in the base portion and has the effect of strengthening the base. In addition, the hardness of the base can be maintained even at high temperatures. If Co exceeds 10.0%, the toughness decreases. On the other hand, since Co is expensive, it is desirable to determine the content in consideration of economy and use conditions.

Ti: 0.5% or less Ti acts as a deoxidizer in the molten metal, and also has an effect of combining with N to form a nitride, forming a nucleus of granular carbide, and making the granular carbide fine. Part of it is combined with C to become part of granular carbide. A Ti effect of 0.5% or less is sufficient.

Al: 0.5% or less Al acts as a deoxidizer in the molten metal and has an effect of making the granular carbide fine. If it exceeds 0.5%, the hardenability is deteriorated, and it is difficult to obtain sufficient base hardness.

  Next, the inner layer of the present invention will be described. The outer layer of the present invention is a high alloy component in order to ensure wear resistance. Therefore, when the outer layer and the inner layer are joined, alloy components are mixed from the outer layer to the inner layer, and toughness deteriorates. Furthermore, since the outer layer is a high alloy component, a large amount of carbides of V and Nb exist, and particularly when centrifugal casting is performed, segregation occurs on the inner surface of the outer layer material due to the specific gravity of the carbide, and the welded portion with the inner surface Since an unevenly distributed carbide layer is generated, it is difficult to weld the outer layer material and the inner layer material. Therefore, the soundness and strengthening of the welding between the outer layer and the inner layer were attempted by suppressing the generation of the carbide layer by specifying the inner layer component.

  The composite roll for rolling according to the present invention is characterized in that the tensile strength between the outer layer and the inner layer is 400 MPa or more. Thereby, it is possible to prevent the outer layer from peeling off and roll breaking due to the stress amplitude generated during rolling use. More preferably, the tensile strength between the outer layer and the inner layer is 500 MPa or more.

  In addition, when a tensile test piece including a joint boundary between the outer layer and the inner layer is subjected to a tensile test, if the tensile fracture position exists in the portion excluding the joint boundary, that is, the outer layer, the tensile stress becomes maximum and fatigue occurs. The weak point that can be a starting point of fracture is located at a portion excluding the joining boundary, and can sufficiently withstand even under high load conditions such as a work roll of a hot sheet rolling mill.

  The reason for limiting the component (mass%) of the inner layer will be described below. In addition, the component of the inner layer of this invention shows the chemical component of the inner layer in a final product instead of the component of a molten metal.

C: 0.5 to 3.0%
C contributes to improving the strength, but if it is less than 0.5%, welding tends to be insufficient, or defects such as a cast hole are likely to be generated at the boundary portion of the inner layer. On the other hand, if C exceeds 3.0%, carbides are excessive and the strength tends to be lowered. A more preferable range of C is 1.8 to 2.4%.

Si: 0.1-3.0%
Si acts as a deoxidizer in the molten metal. When Si is 0.1% or less, the deoxidation effect is insufficient and casting defects are likely to occur. On the other hand, if it exceeds 3.0%, the hardenability is lowered and embrittled, so that it is not suitable as an inner layer. A more preferable range of Si is 0.2 to 1.5%, and a more preferable range is 0.2% to 1.0%.

Mn: 0.1 to 3.0%
Mn is effective when deoxidation of molten metal or S, which is an impurity, is fixed as MnS and exceeds 0.1%. When Mn exceeds 2.0%, it tends to become brittle and unsuitable as an inner layer. A more preferable range of Mn is 0.2 to 1.5%, and a more preferable range is 0.2 to 1.0%.

  In addition to the above, alloy elements such as Ni, Cr, Mo, W, V, Nb, Co, Ti, and Al may be mixed in the inner layer depending on the purpose.

  Next, the structure of the composite roll for rolling in the present invention will be described. FIG. 4 shows a schematic view of a cross section of a composite roll for rolling formed by centrifugal casting according to the present invention. The outer layer of the present invention is formed by casting molten metal adjusted to a chemical composition for crystallizing primary granular carbide into a casting mold for centrifugal force casting, and forming a layer enriched with granular carbide on the inner surface side by centrifugal casting. Can be obtained. In FIG. 4, the part a is a layer in which granular carbides are concentrated and concentrated. The part B is the other part and is a layer in which the presence of granular carbides is poor. Part C is the inner layer. The composite roll for rolling in the present invention casts the inner layer of the section C in FIG. 4 after casting the outer layer part. After the roll material is completed, the portion shown in FIG. 4 is removed by cutting or the like to obtain the composite roll for rolling of the present invention having the portion shown in FIG. 4 as the outer layer.

  The composite roll for rolling of the present invention exhibits excellent wear resistance, rough skin resistance, and seizure resistance in general for the work roll for rolling, but is extremely excellent for the work roll used in the finishing row of a hot sheet rolling mill. It exhibits high wear resistance, rough skin resistance and seizure resistance, and contributes to improved productivity and roll unit intensity in rolling mills.

  In the outer layer of the present invention, MC carbide is concentrated on the inner surface side of the mold by casting a molten metal adjusted to a chemical composition for crystallizing primary granular carbide into a mold for centrifugal casting and centrifugal casting. A layer was formed. Specimen No. Examples 1 to 5 are embodiments of the present invention, which are formed by centrifugal casting according to the present invention, and are collected from a portion corresponding to the above-mentioned portion a in FIG. In addition, specimen No. Nos. 6 to 8 are comparative examples, specimen Nos. Reference numerals 9 and 10 are conventional examples. Specimen No. No. 6 is formed by stationary casting. 7-10 were formed by centrifugal casting.

  The collected specimen No. 1-8 and no. No. 10 was subjected to heat treatment after quenching at 1000 to 1200 ° C. and tempering at 500 to 600 ° C. three times. In addition, specimen No. No. 9 was heated at 400 to 500 ° C. after casting and subjected to residual austenite decomposition and strain relief heat treatment. Each specimen was processed into various specimen shapes after these heat treatments. These test materials No. Table 1 shows 1 to 10 chemical components (% by mass). Here, formula (1) in Table 1 is a value of V% + 0.55 × Nb%, and formula (2) is a value of C% −0.2 × (V% + 0.55 × Nb%). .

Further, the area ratio of MC carbide, the total area ratio of M ₂ C, M ₆ C and M ₇ C ₃ carbide, the maximum value of the inscribed circle diameter of the region not including MC carbide having an equivalent circle diameter of 15 μm or more, base The Vickers hardness, the average equivalent circle diameter of MC carbide, and the average interparticle distance of MC carbide having an equivalent circle diameter of 15 μm or more were measured.

  In addition, as an evaluation of wear resistance and rough skin resistance, the wear amount and roughness of a wear test using a rolling wear tester are measured. A seizure resistance evaluation is a measurement of a seizure area ratio by a frictional thermal shock test. The fracture toughness value KIC was measured.

  The area ratio of MC carbide was determined by first polishing the specimen material and then corroding the MC carbide in black by electrolytic corrosion in an aqueous potassium dichromate solution, followed by an image analysis device (SPICCA- manufactured by Nippon Avionics Co., Ltd.). II) and measured.

In addition, the area ratio of M ₂ C, M ₆ C, and M ₇ C ₃ carbides is determined by first polishing the specimen to a mirror surface and then corroding it with Murakami's reagent, so that M ₂ C, M ₆ C, and M ₇ C ₃ carbide corrosion black or after colored black or gray, and using an image analyzer measurement. Note that M ₂ C, M ₆ C, and M ₇ C ₃ carbides having an equivalent circle diameter of 1 μm or more, which are easily discriminated, were measured.

  In these image analyses, the area ratio was measured in a field of view corresponding to 0.23 mm × 0.25 mm of the specimen. This measurement was performed for any 20 visual fields of each test material, and the average value was obtained.

  The average equivalent circle diameter of MC carbide was determined by first mirror-polishing the test material and then corroding MC carbide in black by electrolytic corrosion in an aqueous potassium dichromate solution, followed by an image analyzer (manufactured by Nippon Avionics Co., Ltd.). Measurement was performed using SPICCA-II). The measurement range of the image analysis was such that one visual field corresponds to 0.23 mm × 0.25 mm of the test material, and each test material was measured for 20 arbitrary visual fields, and the average value of the measured values was obtained.

The average interparticle distance between MC carbides having an equivalent circle diameter of 15 μm or more is that the specimen is first mirror-polished and then the base is corroded with a picric alcohol solution. When observed with an optical microscope, the matrix appears dark gray, the MC carbides appear light gray, and the M ₂ C, M ₆ C, and M ₇ C ₃ carbides appear white. In this way, the average interparticle distance of MC carbide having an equivalent circle diameter of 15 μm or more is measured using an optical microscope texture photograph of 200 × magnification of an arbitrary 1.0 mm × 1.5 mm surface of the test material sample. did.

In order to measure the inscribed circle diameter in a region not containing MC carbide having an equivalent circle diameter of 15 μm or more, the specimen is first mirror-polished and then the base is corroded with a picric acid alcohol solution. When observed with an optical microscope, the matrix appears dark gray, the MC carbides appear light gray, and the M ₂ C, M ₆ C, and M ₇ C ₃ carbides appear white. In this way, an optical microscope texture photograph of a magnification of 100 times on an arbitrary 2.0 mm × 3.0 mm surface of the test material sample was prepared, and the maximum value of the inscribed circle diameter of the present invention was measured.

  The Vickers hardness of the base was measured in a load range of 50 g to 200 g using a Vickers hardness tester after mirror-polishing the specimen and slightly corroding the base with a picric acid ethanol solution. The average value was calculated | required about arbitrary five places of each test material.

  FIG. 5 shows a schematic view of a rolling wear tester. In FIG. 5, the rolling wear tester includes a rolling mill 1, a heating furnace 4 for preheating the rolled material S, a cooling water tank 5 for cooling the rolled material S, a winder 6 for the rolled material S, and a tension controller 7. Consists of Test rolls 2 and 3 are incorporated in the rolling mill 1. A test roll was prepared from each of the above-described test materials, and a small sleeve roll having an outer diameter of 60 mm, an inner diameter of 40 mm, and a width of 40 mm was used. A test roll is incorporated in the rolling wear tester, and the test conditions are rolling material: SUS304, rolling reduction: 25%, rolling speed: 150 m / min, rolling temperature: 900 ° C., rolling distance: 300 m / time, roll cooling: water cooling The number of rolls: The test was performed by a quadruple type. After rolling, the depth of wear and the ten-point average roughness (Rz) generated on the surface of the test roll were measured with a stylus type surface roughness meter.

  FIG. 6 shows a schematic diagram of a frictional thermal shock tester. In this test, the weight 9 is dropped on the rack 8 to rotate the pinion 10 and bring the biting material 12 into strong contact with the test material 11. By this test, the test material 11 is indented, and the biting material adheres to and adheres to a part or the entire surface thereof. The seizing area ratio is obtained by dividing the seizing adhesion area by the indentation area in percentage. This test was performed twice for each specimen, and the average of the seizure area ratio was determined.

  The fracture toughness value KIC was measured by taking a test piece for measuring the fracture toughness value KIC from each specimen and performing a test based on ASTM E399. The measurement was performed on two test pieces for each specimen, and the average value was obtained.

Table 2 shows the results of various measurements. That is, the area ratio of MC carbide (%), the total area ratio (%) of M ₂ C, M ₆ C and M ₇ C ₃ carbides of 1 μm or more, and the region not including MC carbides having an equivalent circle diameter of 15 μm or more Maximum value of inscribed circle diameter (μm), Vickers hardness of base (Hv), average inter-particle distance (μm) of MC carbide with equivalent circle diameter of 15 μm or more, wear amount (μm), surface roughness Rz (μm) ), Seizure area ratio (%), and fracture toughness value KIC (kg / mm ^3/2 ).

  In FIG. 1 shows a metallographic structure. In FIG. 7, the white part is MC carbide, and the black part is the base. Specimen No. 1 shows that MC carbide is distributed in high concentration.

  In FIG. 6 shows the metal structure. In FIG. 8, the white part is MC carbide and the black part is the base. Specimen No. 6 shows that MC carbide is partially unevenly distributed.

FIG. 9 shows a specimen No. of a conventional high-speed roll material. 10 metallographic structures are shown. In FIG. 9, white fine-grained portions are MC carbides, white mesh-like portions are M ₂ C, M ₆ C, and M ₇ C ₃ carbides, and black portions are bases. Specimen No. 10 shows that MC carbides are partially unevenly distributed and M ₂ C, M ₆ C, and M ₇ C ₃ carbides are distributed in a network.

  No. of the example of the present invention. The wear amount of 1 to 5 is No. in the conventional example. It is less than half compared to 9, and the wear resistance is very good. In addition, the examples of the present invention have roughness, seizure area ratio, and fracture toughness value KIC that are higher than those of the conventional materials, and have both rough skin resistance, seizure resistance, and sufficient toughness.

  Comparative Example No. 6, the region not including MC carbide having an equivalent circle diameter of 15 μm or more exceeds 150 μm in inscribed circle diameter, and the average inter-particle distance (G) between MC carbides having an equivalent circle diameter of 15 μm or more, and the MC carbide The ratio (G / H) to the average equivalent circle diameter (H) is outside the range of the present invention exceeding 2 and the roughness and seizure area ratio are less than those of the conventional materials, and the rough skin resistance and seizure resistance are Inferior.

  Comparative Example No. 7 is C%, Ni%, the value of formula (2), the area ratio of MC carbide, the base hardness, the average equivalent circle diameter of MC carbide is outside the scope of the present invention, and the equivalent circle diameter is 15 μm or more. The ratio (G / H) between the average inter-particle distance (G) between MC carbides and the average equivalent circle diameter (H) of MC carbides exceeds 2, and the amount of wear and seizure area ratio are conventional materials. The wear resistance and seizure resistance are inferior.

Comparative Example No. 8 is V%, Cr%, the value of formula (1), the value of formula (2), the area ratio of MC carbide, and the total area ratio of M ₂ C, M ₆ C and M ₇ C ₃ carbide of 1 μm or more. The ratio (G / H) between the average inter-particle distance (G) between MC carbides having an equivalent circle diameter of 15 μm or more and the average equivalent circle diameter (H) of the MC carbide is 2 outside the scope of the present invention. The roughness and KIC are lower than those of the conventional materials, and the rough skin resistance and toughness are inferior.

  No. of the conventional example. 9 is V%, Ni%, the value of the formula (1), the area ratio of MC carbide, the average equivalent circle diameter of MC carbide is outside the scope of the present invention, and between MC carbides having an equivalent circle diameter of 15 μm or more. The ratio (G / H) between the average interparticle distance (G) and the average equivalent circle diameter (H) of MC carbide exceeds 2, and the wear resistance is inferior to that of the present invention material.

  Next, the inner layer of the composite roll for rolling of the present invention was examined. Table 3 shows chemical components (% by mass) in the final product of the inner layer. Specimen No. 11 to 13 are inner layer materials of the present invention. Specimen No. 14, no. 15 is a comparative example. Reference numeral 16 denotes spheroidal graphite cast iron which is a conventional inner layer material. Using these materials, the following welding test was performed.

  The outer layer used in the welding test is the material No. 1 of the present invention shown in Table 1. It was set to 1. The molten metal was injected using a mold having an inner diameter of 500 mm and a length of 1200 mm and a centrifugal casting machine. After a predetermined time has passed, No. 1 in Table 3 is further added. The inner layer molten metal prepared so that it might become a component shown to 11-15 was inject | poured. In this way, a composite roll was manufactured.

  In Table 3, the conventional material No. No. 16 in Table 1 using a mold having an inner diameter of 500 mm and a length of 1200 mm and a centrifugal casting machine. The molten metal prepared so that it might become an outer layer shown in 1 was inject | poured. After a predetermined time had elapsed, the inner layer melt of spheroidal graphite cast iron was injected to produce a composite roll.

  The test roll was naturally cooled together with the mold, and disassembled when the roll reached room temperature, and the test roll material was taken out. A portion with a small amount of carbide in the body portion of the roll material (the portion shown in FIG. 4) was removed by lathe processing to produce a test roll having an outer diameter of 350 mm and a body length of 600 mm. After performing an appropriate heat treatment on the test roll, a disc-shaped material (material including the outer layer and inner layer of the roll) was cut out from the center of the roll body by sticking, and various test pieces were collected from the disc-shaped material. And the soundness of the welding part was confirmed. Further, a boundary tensile test between the outer layer and the inner layer was performed.

  As shown in Table 3, the test material No. The boundary of 11-13 was welded normally, and the tensile strength between an outer layer and an inner layer was also 480 MPa or more, and it turned out that it is suitable for an inner layer. On the other hand, the test material No. In No. 14, it is recognized that there are a large amount of casting voids at the joint boundary and defective welding occurs, and the tensile strength between the outer layer and the inner layer is very low. Further, the test material No. 15 and no. No. 16 was found to have a large amount of carbide segregation in the vicinity of the joint boundary and low tensile strength.

  FIG. 10 shows the specimen No. of the present invention. The cross-sectional structure | tissue photograph after the tension test of 11 is shown. In FIG. 10, it can be seen that the test material of the present invention does not have a cast hole or an aggregated carbide layer at the joint boundary between the outer layer and the inner layer, and is broken on the outer layer side.

  FIG. 11 shows a conventional sample No. The cross-sectional structure | tissue photograph after 16 tensile tests is shown. In FIG. 11, it can be seen that the test material of the conventional example has an agglomerated carbide layer at the joint boundary between the outer layer and the inner layer, and is broken at the fragile joint boundary.

  FIG. 12 is a schematic cross-sectional view showing various forms of the composite roll for rolling of the present invention. (A) of FIG. 12 is the solid roll which consists of the outer layer a of this invention, and the inner layer c. F is a hollow part. FIG. 12B shows a hollow sleeve roll comprising the outer layer A and the inner layer C of the present invention. FIG. 12 (c) shows the hollow sleeve roll made of the outer layer (i) and the inner layer (c) of the present invention fitted to a metal shaft member (e). The outer layer (a) referred to here is the same as the layer (i) where MC carbide is concentrated in FIG.

  Moreover, when the composite roll for rolling of this invention was manufactured and actually rolled, it was confirmed that it was excellent in wear resistance, rough skin resistance, and seizure resistance.

  The composite roll for rolling of the present invention exhibits excellent wear resistance, rough skin resistance, and seizure resistance in general for the work roll for rolling, but is extremely excellent for the work roll used in the finishing row of a hot sheet rolling mill. It exhibits high wear resistance, rough skin resistance and seizure resistance, and contributes to improved productivity and roll unit intensity in rolling mills.

It is a figure for demonstrating a circle equivalent diameter. It is a figure for demonstrating an inscribed circle diameter. It is a figure for demonstrating the distance between average particle | grains. It is a figure for demonstrating the composite roll for rolling of this invention. It is the schematic of a rolling wear tester. It is the schematic of a friction thermal shock tester. Sample No. of the present invention. 1 is a metallographic photograph taken by an optical microscope. Sample No. of Comparative Example 6 is a metallographic photograph of the optical microscope of No. 6. Sample No. of conventional example It is a metallographic photograph by 10 optical microscopes. Sample No. of the present invention. The cross-sectional structure | tissue photograph after the tension test of 11 is shown. Sample No. of conventional example The cross-sectional structure | tissue photograph after 16 tensile tests is shown. It is a schematic sectional drawing which shows the example of the various form of the composite roll for rolling which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Rolling wear test machine, 2 Test roll, 3 Test roll, 4 Heating furnace,
5 Cooling water tank, 6 Winder, 7 Tension controller, S Rolled material,
8 racks, 9 weights, 10 pinions, 11 test materials,
12 Biting material, A Area of the object, B circle, D circle equivalent diameter,
E measurement object,
a (a1, a2, a3, a4) MC carbide having an equivalent circle diameter of 15 μm or more,
b MC carbide with an equivalent circle diameter of 15 μm or less, c inscribed circle, d inscribed circle diameter,
e Region excluding MC carbides a and b, L arbitrary straight line,
L1, L2, L3 interparticle distance,
B MC carbide centrifuge concentrating layer, site excluding loi, c inner layer,
E Shaft material, F Hollow part

Claims (15)

A structure in which MC carbide is dispersed in an area ratio of 20 to 60% on a base having a Vickers hardness of Hv 550 to 900, and a region not including MC carbide having an equivalent circle diameter of 15 μm or more in the structure is an inscribed circle diameter. A composite roll for rolling comprising an outer layer not exceeding 150 μm and an inner layer metal-bonded to the inner surface of the outer layer, wherein the tensile strength between the outer layer and the inner layer is 400 MPa or more. A structure in which MC carbide is dispersed in an area ratio of 20 to 60% on a base having a Vickers hardness of Hv 550 to 900, and a region not including MC carbide having an equivalent circle diameter of 15 μm or more in the structure is an inscribed circle diameter. In a composite roll for rolling having an outer layer not exceeding 150 μm and an inner layer metal-bonded to the inner surface of the outer layer, when a tensile test piece including a bonding boundary between the outer layer and the inner layer is subjected to a tensile test, tensile fracture A composite roll for rolling, characterized in that the position is in the outer layer excluding the joining boundary. A structure in which MC carbide is dispersed in an area ratio of 20 to 60% on a base having a Vickers hardness of Hv550 to 900, and an average interparticle distance between MC carbides having an equivalent circle diameter of 15 μm or more in the structure is 10 to 40 μm. A rolling composite roll characterized by having an outer layer as described above and an inner layer metal-bonded to the inner surface of the outer layer, wherein the tensile strength between the outer layer and the inner layer is 400 MPa or more. A structure in which MC carbide is dispersed in an area ratio of 20 to 60% on a base having a Vickers hardness of Hv550 to 900, and an average interparticle distance between MC carbides having an equivalent circle diameter of 15 μm or more in the structure is 10 to 40 μm. In the composite roll for rolling having the outer layer and the inner layer metal-bonded to the inner surface of the outer layer, when a tensile test piece including a bonding boundary between the outer layer and the inner layer is subjected to a tensile test, the tensile fracture position is A composite roll for rolling characterized by existing in an outer layer excluding a joining boundary portion. The inner layer is made of an Fe-based alloy containing, by mass, chemical components, C: 0.5 to 3.0%, Si: 0.1 to 3.0%, and Mn: 0.1 to 3.0%. The composite roll for rolling according to any one of claims 1 to 4, wherein: The composite roll for rolling according to any one of claims 1 to 5, wherein an average equivalent circle diameter of MC carbide dispersed in the outer layer is 10 to 50 µm. A ratio (G / H) of an average inter-particle distance (G) between MC carbides having an equivalent circle diameter of 15 μm or more in the outer layer to an average equivalent circle diameter (H) of MC carbides is 2 or less. The composite roll for rolling according to any one of claims 1 to 6. The rolling according to any one of claims 1 to 7, wherein the total amount of M ₂ C, M ₆ C and M ₇ C ₃ carbides having an equivalent circle diameter of 1 µm or more is a structure in which 0 to 5% is dispersed. Composite roll. Chemical component of the outer layer is% by mass, C: more than 2.5% and 9.0% or less, Si: more than 0.1% and 3.5% or less, Mn: more than 0.1% and 3.5% The composite roll for rolling according to any one of claims 1 to 8, wherein V is contained in excess of 11.0% and 40.0% or less, and consists of the remaining Fe and inevitable impurity elements. Further, in the outer layer by mass%, Cr: more than 1.0% to 15.0% or less, Mo: more than 0.5% to 20.0% or less, and W: more than 1.0% to 40.0% or less Any 1 type or 2 types or more of these are contained, The composite roll for rolling in any one of Claims 1-9 characterized by the above-mentioned. Furthermore, a part of V of the said outer layer is substituted with Nb of the range which satisfies the following (1) formula by mass%, The composite roll for rolling in any one of Claims 1-10 characterized by the above-mentioned.
11.0% <V% + 0.55 × Nb% ≦ 40.0% (1) Furthermore, the said outer layer satisfies the following (2) Formula, The composite roll for rolling in any one of Claims 1-11 characterized by the above-mentioned.
0 <C% −0.2 × (V% + 0.55 × Nb%) ≦ 2.0%. . . (2) Furthermore, the component in the said outer layer is the mass%, and contains any 1 type or 2 types of Ni: 2.0% or less and Co: 10.0% or less, The one of Claims 1-12 characterized by the above-mentioned. A composite roll for rolling as described in 1. Furthermore, the component in the said outer layer is the mass%, and contains any 1 type or 2 types of Ti: 0.5% or less and Al: 0.5% or less, The one of Claims 1-13 characterized by the above-mentioned. A composite roll for rolling as described in 1. The composite roll for rolling according to any one of claims 1 to 14, wherein the outer layer is formed by centrifugal casting.