JP2007146276A - Composite roll for rolling - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a composite roll for rolling which has an outer layer and an inner layer welded while forming a sound interface, and has both of excellent abrasion resistance and thermal-crack resistance, by solving a problem of a conventional roll for rolling. <P>SOLUTION: The composite roll for rolling is made by a centrifugal casting technique, and comprises: the outer layer that has a structure in which granular carbides with circle-equivalent diameters of exceeding 10 μm occupy over 20% to 60% by an area rate, and non-granular carbides occupy 5% or less by the area rate, and has a matrix of which the Vickers hardness is over 550 Hv to 900 Hv; and an inner layer which is metallically bonded to the inner surface of the outer layer and is made from an Fe-based alloy having a chemical composition including, by mass%, 0.5-3.0% C, 0.1-3.0% Si and 0.1-3.0% Mn. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a composite roll for rolling using an outer layer material for hot or cold rolling rolls, which is excellent in wear resistance and heat crack resistance, and particularly as a work roll used in a finishing row of a hot sheet rolling mill. Is preferred.

  Important properties that determine the productivity of rolling are the wear resistance and thermal crack resistance of the roll. When the wear resistance is poor, the roll surface is worn away at an early stage, and the dimensional accuracy of the material to be rolled is impaired. In order to prevent this, the rolls must be replaced frequently, resulting in a decrease in rolling mill productivity due to an increase in the frequency of rolling operation interruptions, 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 number of rolls.

  On the other hand, heat cracks are generated on the roll surface due to contact with a hot material to be rolled. If the heat cracking resistance is poor, the cracks tend to propagate to the deep part of the roll, causing problems such as chipping or peeling of the roll surface, and further the roll itself being broken to stop rolling. In order to prevent this, it is necessary to completely remove heat cracks in addition to removing irregularities due to abrasion during grinding of the roll surface after rolling. Thus, a roll with poor heat cracking resistance tends to increase the amount of roll surface grinding, which causes problems such as an increase in cost required for grinding and a decrease in roll basic unit.

  In recent years, there has been a demand for further improvement in productivity and basic unit of roll in rolling mills, and in particular, work rolls used in hot sheet rolling mills are required to have high wear resistance and high thermal crack resistance. More recently, the rolling roll load tends to be extremely high in a so-called large rolling operation where the rolling reduction of the rolled material in one stand is large, such as the manufacturing process of a fine-grained structure hot-rolled steel sheet. It is desired for the rolls used to dramatically improve wear resistance and heat crack resistance.

  Conventionally, as a rolling roll intended to meet the demand for improved wear resistance, a centrifugal cast high speed roll having an outer layer of a high speed alloy manufactured by centrifugal casting has been used. This high-speed outer layer material contains a large amount of alloy elements such as Cr, Mo, W, and V, and its structure contains hard granular carbide and non-particulate carbide. The high-speed outer layer material is obtained by increasing the hardness by these carbides and by obtaining high hardness at room temperature and high temperature by the alloy elements in the matrix.

  As this type of conventional technology, for example, in Patent Document 1, C: 3.5 to 5.5%, Si: 0.1 to 1.5%, Mn: 0.1 to 1.2%, Cr: Tool steel for hot rolling characterized by comprising 4.0 to 12.0%, 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 material for centrifugal casting rolls satisfying 0.2 ≦ Nb / V in the outer layer material for centrifugal casting rolls made of hot rolling tool steel is described.

  According to Patent Document 1, it is possible to obtain tool steel for hot rolling that has both wear resistance, crack resistance, and seizure resistance, and does not cause damage to the surface of the counterpart material when used as a tool. In addition, an outer layer material for rolling rolls that does not cause segregation or the like even when subjected to centrifugal casting can be obtained.

  Further, Patent Document 2 includes an outer layer material having an area ratio of 5-30% granular carbide and 6% or more non-particulate carbide, and having a base hardness of Vickers hardness (Hv) of 550 or more. And a hot-rolling composite roll characterized by comprising a steel core. In the composite roll for hot rolling, the chemical components of the outer layer are C: 1.0 to 4.0% by weight, Si: 3.0% by weight or less, Mn: 1.5% by weight or less, Cr: 2 to 10 Wt%, Mo: 9 wt% or less, W: 20 wt% or less, V: 2 to 15 wt%, P: 0.08 wt% or less, S: 0.06 wt% or less, B: 500 ppm or more, balance Fe And an impurity element. Furthermore, it is described that any of Ni: 5.0% by weight or less, Co: 5.0% by weight or less, and Nb: 5.0% by weight or less is contained.

  According to Patent Document 2, since the outer layer material contains a predetermined amount or more of non-particulate carbide, even if a crack is generated due to a thermal load, the crack is suppressed from extending to the deep part of the roll. Thereby, it is a composite roll for hot rolling with improved heat crack resistance.

  Patent Document 3 discloses that C: 0.8 to 1.9% on the inner surface of the outer layer of a high-speed cast iron material having a C content of 2.0 to 3.2% (% by weight, hereinafter the same), Si: 3.0% or less, Mn: 2.0% or less, Cr: 6.0% or less, Mo: 5.0% or less, W: 5.0% or less, V: 5.0% or less, the balance substance In particular, an intermediate layer made of Fe is welded and integrated, and C: 0.2-0.8%, Si: 0.2-3.0%, Mn: 0.2-2. 0%, Cr: 1.5% or less, Mo: 1.0% or less, W: 1.0% or less, V: 1.5% or less, provided that Cr + Mo ≧ 0.3%, with the balance being substantially Fe An inner layer of a cast steel material made of the above is welded and integrated, and the high-speed cast iron material constituting the outer layer is C: 2.0 to 3.2%, Si: 0.1 to 2.0%, Mn: 0 0.1-2.0%, Cr: 3-10%, 2 × Mo + W: 5-22%, V: 3-8 %, And the balance is substantially composed of Fe.

  According to Patent Document 3, the welded state with the outer layer, the intermediate layer, and the inner layer is good, the outer layer has a predetermined wear resistance, and the inner layer can have a predetermined toughness. A quality roll for rolling steel can be manufactured.

  Patent Document 4 discloses an outer layer material having an area ratio of 5-30% granular carbide and 5% or less non-particulate carbide, and having a base hardness of Vickers hardness (Hv) of 550 or more. And a wear-resistant composite roll characterized by comprising a steel core. In the wear-resistant composite roll, the chemical components of the outer layer are C 1.0 to 3.5 wt%, Si 3.0 wt% or less, Mn 1.5 wt% or less, Cr 2 to 10 wt%, Mo 9 wt% or less, W 20 wt. % Or less, V2 to 15% by weight, P 0.08% by weight or less, S 0.06% by weight or less, balance Fe and impurity elements. 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.

  According to Patent Document 4, since the outer layer material contains a predetermined amount of granular carbide and non-particulate carbide, and contains hard carbide of VC, the wear resistance is also good, and the occurrence of cracks is suppressed, The surface does not wear off by the crystal unit starting from the crack. Thereby, it is a composite roll for rolling with good skin resistance.

JP-A-9-256108 JP-A-4-141553 Japanese Patent Laid-Open No. 9-209071 JP-A-3-254304

As a means for improving the wear resistance of a rolling roll, it is generally known to increase the hardness of the roll material by containing a large amount of alloy elements that combine with C and C to form a metal carbide. Among the alloy elements, V, in particular, combines with C to form hard granular carbides and improve wear resistance. Nb also forms granular carbides like V and improves wear resistance.

However, when a molten metal containing excessive V or Nb is subjected to centrifugal force casting, segregation due to centrifugal separation occurs. For example, in the case of centrifugal casting, the carbide formed by V is light, so that it floats on the inner surface, and the outer surface of the outer layer material used for rolling does not contain carbide corresponding to the content. 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.

  When C, V, and Nb exceed a certain range, primary granular carbides are crystallized from the molten metal, and various ranges are reported as primary crystal projection phase diagrams. For example, in a CV primary crystal projection phase diagram made of Fe and an alloy, granular carbides may be crystallized in the primary crystal in the region where C is 2.5% or more and V is 9.5% or more by mass. It has been reported. Further, in the C—Nb primary crystal projection phase diagram made of an Fe-based alloy, granular carbides may be crystallized as primary crystals in the mass% region where C is 1.5% or more and Nb is 4.0% or more. It has been reported. Therefore, the conventional outer layer material for rolling rolls cast by centrifugal force restricts the content of C, V and Nb in order to suppress primary crystal grain carbide and prevent segregation.

  In addition, means for preventing segregation due to centrifugation by increasing the specific gravity of the carbide has been proposed. For example, in Patent Document 1 described above, 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. Further, it is described that in order to obtain a uniform outer layer material 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.

  On the other hand, 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 2, as a manufacturing method of a roll with less restrictions on the chemical composition of the outer layer material, a hard material is produced by a so-called continuous overlay casting method in which an outer layer is continuously formed around a core material made of steel using a high-frequency coil. It becomes possible to add a large amount of elements such as V, W, and Mo that form carbides of the outer layer material to the outer layer material, and it is possible to manufacture a roll having improved wear resistance and heat crack resistance as compared with a conventional centrifugal casting roll. It is described that it became possible. 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 2 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.

  Here, heat crack resistance, which is another important characteristic of the rolling roll, will be described. In Patent Document 2, when a large amount of non-granular, particularly mesh-generated carbide is present in the structure of the outer layer of the composite roll for hot rolling, fine cracks are preferentially generated in the carbide portion due to thermal stress. However, it is described that this crack stays in the vicinity of the surface of the roll and does not progress to the deep part. Also, the base of the outer layer is sufficiently hardened to the deep part, and even if fine cracks are generated due to heat load, the cracks do not propagate to the deep part of the roll because they are dispersed along the non-particulate carbide. It is described.

  On the other hand, when the inner layer is formed on the inner surface of the outer layer to which such a large amount of alloy component is added, welding defects such as shrinkage cavities and carbide segregation occur between the outer layer and the inner layer, or a large amount from the outer layer to the inner layer. There was a problem that the toughness of the inner layer deteriorated due to the mixing of alloy components. Patent Document 3 describes that when the composite roll having an outer layer of a high C material and an inner layer of a low C material is produced by centrifugal casting, the above problem can be suppressed by providing an intermediate layer.

  Thus, in order to drastically improve the wear resistance of the conventional high-speed outer layer material casted by centrifugal force and the high-speed type outer layer material formed by continuous overlay casting, a large amount of V and Nb should be added. Although good, as described above, it is actually very difficult to manufacture.

  On the other hand, in Patent Document 4, when a large amount of non-particulate, particularly mesh-generated carbides are present in the structure, cracks are preferentially generated and propagated in the parts of the carbides, and rough skin starts from such cracks. It is described that it occurs. When rough skin occurs, there arises a problem that the wear resistance is impaired by limiting the amount of rolling endurance or finely breaking the roll surface from the crack intersection. Thus, it is very difficult for the conventional high-speed outer layer material to have both wear resistance and heat crack resistance.

  Thus, in order to drastically improve the wear resistance of the conventional high-speed outer layer material casted by centrifugal force and the high-speed type outer layer material formed by continuous overlay casting, a large amount of V and Nb should be added. Although good, as described above, it is actually very difficult to manufacture.

  Therefore, the present invention aims to solve the problems in the conventional outer layer material for rolling rolls, and to provide a composite roll for rolling in which the outer layer and the inner layer are welded soundly while having excellent wear resistance and heat crack resistance. And

  The composite roll for rolling according to the present invention is particularly characterized in that the inner layer of the optimum component is provided on the inner surface of the outer layer to which an extremely large amount of alloy such as C and V is added. That is, in the present invention, the optimum component range of C, Si, and Mn of the inner layer was found.

  That is, the present invention is a composite roll for rolling formed by centrifugal casting, and the outer layer has an area ratio of granular carbide exceeding 10 μm in circle equivalent diameter exceeding 20% and not more than 60%, and non-particulate carbide is area ratio. The composition of which the hardness is 5% or less, the outer layer having a Vickers hardness exceeding Hv550 and Hv900 or less, and the chemical composition metal-bonded to the inner surface of the outer layer in mass%, C: 0.5% It is characterized by having an inner layer made of an Fe-based alloy containing ˜3.0%, Si: 0.1% to 3.0%, and Mn: 0.1% to 3.0%.

  The V content in the granular carbide is 30% or more by mass%.

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, V is contained in excess of 11.0% and 40.0% or less, and is composed of the balance Fe and inevitable impurity elements.

Further, any one of Cr: more than 1.0% and not more than 15.0%, Mo: more than 0.5% and not more than 20.0% and W: more than 1.0% and not more than 40.0% in the outer layer. It contains a seed or two or more kinds.

A part of V of the outer layer is substituted with Nb in a range satisfying the following formulas (1) and (2) by mass%.
11.0% <V% + 0.55 × Nb% ≦ 40.0% (1)
0.2> Nb% / V% (2)

Furthermore, the following expression (3) is satisfied.
0 <C% −0.2 × (V% + 0.55 × Nb%) ≦ 2.0% (3)

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

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

  Next, the structural elements of the outer layer of the composite roll for rolling of the present invention will be described.

[1] Size of structure element granular carbide of outer layer for rolling roll It is preferable that the average equivalent circle diameter of the granular carbide exceeds 10 μm. When the equivalent circular diameter of the granular carbide is 10 μm or less, the cracks generated by the thermal stress are not sufficiently dispersed, and the cracks are deeply propagated into the roll, so that sufficient heat crack resistance cannot be obtained. Also, the roll comes into contact with the high-heat rolled steel sheet during hot rolling, and the base softens up to about 10 μm from the surface, so if the average equivalent circle diameter of the granular carbide is less than 10 μm, the base cannot sufficiently support the granular carbide, The roll has insufficient wear resistance. Note that a granular carbide having an equivalent circle diameter exceeding 80 μm is not desirable because it is difficult to uniformly disperse and rough skin is likely to occur. The average equivalent circle diameter of the granular carbide is more preferably more than 10 μm and not more than 40 μm, and most preferably 15 to 30 μm.

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). The equivalent circle diameter is also called the circle equivalent diameter. In FIG. 1, when the area of the measurement object 15 is A, the circle equivalent diameter is the diameter of a circle C corresponding to the area equal to the area A of the measurement object, and is represented by the equation (4).
Equivalent circle diameter = √ (A × 4 / π) (4)

Content of granular carbide The granular carbide of the present invention contributes to improvement of wear resistance. In addition, MC carbide, which is the main component of granular carbide, is stable at high temperatures and hardly metal-bonds with the material to be rolled, and therefore exhibits excellent effects in improving seizure resistance. When the granular carbide is 20% or more by area ratio, the wear resistance and seizure resistance are insufficient. When the granular carbide exceeds 60% in area ratio, the effect of improving the seizure resistance is saturated and the toughness of the outer roll layer is significantly reduced. Accordingly, the granular carbide is more than 20% and not more than 60% in terms of area ratio. A preferred area ratio is 30% to 50%.

Content of non-particulate carbide The outer layer of the present invention can further improve the thermal crack resistance by controlling the non-particulate carbide to 5% or less in area ratio. If the non-particulate carbide exceeds 5% in terms of area ratio, the non-particulate carbide tends to crystallize in a network form, and cracks are preferentially generated and propagated in this part, so that sufficient heat crack resistance cannot be obtained. Therefore, the area ratio of desirable non-particulate carbide is 3% or less. Further, the less non-particulate carbide is preferable, and the area ratio may be 0%.

Base hardness The base is a portion excluding the above-mentioned granular carbide and non-particulate carbide, and is mainly composed of Fe and an alloy element, and the hardness changes due to transformation by heat treatment, precipitation of very fine carbides in the base, and the like. When the hardness of the base is Vickers hardness of Hv550 or less, 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 is lowered and the thermal crack resistance is deteriorated. Therefore, the range of the hardness of a desirable base is more than Hv550 and Hv900 or less in Vickers hardness. Moreover, the more preferable range of the hardness of a base is more than Hv650 and Hv850 or less.

  Next, the chemical components of the outer layer of the composite roll for rolling according to 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.

[2] Chemical component (% by mass) of outer layer for rolling roll
(1) Essential component (a) C: more than 2.5% and not more than 9.0% C is mainly combined with alloy elements such as V and Nb to improve the wear resistance by forming granular carbides. It is an essential element. C that does not bond to the alloy element mainly dissolves in the matrix or precipitates extremely finely, strengthening the matrix. When C is 2.5% or less, the amount of granular 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 heat crack resistance of the outer layer of the roll 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%.

(B) 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. On the other hand, when Si exceeds 3.5%, the outer roll layer becomes brittle. The Si content is preferably 0.2% to 2.5%, more preferably 0.2% to 1.5%.

(C) Mn: more than 0.1% and 3.5% or less Mn has a function of deoxidizing the molten metal and fixing S, which is an impurity, as MnS. When Mn is 0.1% or less, these effects are insufficient. On the other hand, when Mn exceeds 3.5%, retained austenite tends to be generated, the hardness cannot be stably maintained, and the wear resistance is likely to deteriorate. The Mn content is preferably 0.2% to 2.5%, more preferably 0.2% to 1.5%.

(D) V: more than 11.0% and not more than 40.0% V is an element which mainly combines with C to form granular carbides. In order to include a large amount of granular carbide in the outer roll layer, V of more than 11.0% and 40.0% or less is necessary. When V is 11.0% or less, the granular carbide is insufficient, and sufficient wear resistance cannot be obtained. On the other hand, when V exceeds 40.0%, the granular carbide becomes excessive, and the toughness of the outer roll layer deteriorates. The V content is more preferably 15.0% to 40.0%, and further preferably 18.0% to 40.0%.

(E) Nb
Nb has the same action as V in that it forms MC carbides. From the atomic weight ratio, 0.55 × Nb% and V% are equivalent in mass%. Therefore, part or all of V may be replaced with an amount of Nb that satisfies the following formula (1).
11.0% <V% + 0.55 × Nb% ≦ 40.0% (mass%) (1)

Moreover, it is preferable that an outer layer satisfy | fills following formula (2) further.
0.2> Nb% / V% (mass%) (2)
When the value of [Nb% / V%] is 0.2 or more, it becomes difficult to disperse the particulate carbide uniformly in the outer layer of the present invention. That is, when the granular carbides agglomerate with each other, a portion where the interval between the granular carbides is wide and a narrow portion are mixed, and cracks tend to preferentially propagate to the portion where the interval between the granular carbides is narrow. In order to deteriorate, Nb which substitutes V needs to satisfy Formula (2).

Nb preferably satisfies C and V and the following formula (3).
0 <C% −0.2 × (V% + 0.55 × Nb%) ≦ 2.0% (mass%) (3)
When the value of [C% −0.2 × (V% + 0.55 × Nb%)] is 0 or less, sufficient granular carbides cannot be obtained, and V and Nb become excessive in the base, and sufficient hardness is obtained. Satisfaction and wear resistance are not obtained. On the other hand, when the value of [C% −0.2 × (V% + 0.55 × Nb%)] exceeds 2.0%, non-granular such as M ₂ C, M ₆ C, and M ₇ C ₃ carbides. Carbide crystallizes in a network, and the heat crack resistance of the outer layer of the roll deteriorates.

  In addition, it is preferable that the V content in the granular carbide contained in the outer layer of the present invention is 30% by mass or more because the wear resistance is remarkably improved.

(2) Arbitrary component The outer layer may appropriately contain the following elements depending on the application and usage of the rolling roll.

(A) Cr: more than 1.0% and not more than 15.0% Cr not only improves the hardenability by solid solution in the matrix, but also partly bonds with C and precipitates as ultra-fine carbides. To strengthen. When Cr is 1.0% or less, the effect of strengthening the base cannot be obtained sufficiently. On the other hand, when Cr exceeds 15.0%, non-particulate carbide such as M ₇ C ₃ carbide crystallizes in a network shape, and the heat crack resistance of the outer layer of the roll deteriorates. A more preferable Cr content is 3.0% to 9.0%.

(B) Mo: more than 0.5% and not more than 20.0% Mo not only improves the hardenability by solid solution in the matrix, but also partly bonds with C and precipitates as ultrafine carbides. To strengthen. Part of Mo forms granular carbides. When Mo is 0.5% or less, the effect of strengthening the base cannot be sufficiently obtained. On the other hand, when Mo exceeds 20.0%, non-particulate carbides such as M ₂ C and M ₆ C crystallize in a network shape, and the heat crack resistance of the outer layer of the roll deteriorates. The Mo content is more preferably 2.5% to 20.0%, particularly 2.5% to 10.0%.

(C) W: more than 1.0% and 40.0% or less W dissolves in the base part to improve hardenability, and partly bonds with C to precipitate as ultrafine carbides, strengthening the base To do. Part of W forms granular carbides. If W is 1.0% or less, the effect of strengthening the base cannot be sufficiently obtained. On the other hand, if W exceeds 40.0%, non-particulate carbides such as M ₂ C and M ₆ C crystallize in a network shape, and the heat crack resistance of the outer roll layer deteriorates. The W content is more preferably 5.0% to 40.0%, particularly 5.0% to 20.0%.

  In order to give sufficient abrasion resistance to the outer layer of the roll of the present invention, it is preferable to contain at least one of Cr, Mo and W which are base strengthening elements.

(D) 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 hardening effect of the base is insufficient.

(E) Co: 10.0% or less Co dissolves in the base and has the effect of strengthening the base. If Co is contained, the hardness of the base can be maintained even at a high temperature. When Co exceeds 10.0%, the toughness of the outer roll layer is lowered. Since Co is expensive, it is desirable to determine its content in consideration of economy and use conditions.

(F) Ti: 0.5% or less In addition to acting as a deoxidizer in the molten metal, Ti has an effect of forming a nitride by combining with N to form a nucleus of the granular carbide and making the granular carbide fine. Some of them combine with C to form granular carbides. The addition effect of Ti is sufficient to be 0.5% or less.

(G) Al: 0.5% or less Al acts as a deoxidizer in the molten metal and has an effect of making the granular carbide fine. When Al exceeds 0.5%, the hardenability of the outer layer is deteriorated, and sufficient base hardness cannot be obtained.

[3] Centrifugal Casting To manufacture the outer layer for a rolling roll of the present invention, first, a molten metal adjusted to a chemical composition for crystallizing primary granular carbide is cast into a cylindrical mold and centrifugally cast. In the present invention using centrifugal separation of granular carbides by centrifugal casting, the molten metal composition is different from the composition of the outer roll layer. In order to obtain the outer layer composition described in [2], the molten metal composition is mass%, C: 2.2% to 6.0%, Si: 0.1% to 3.5%, Mn: 0.00. It contains 1% to 3.5% and V: 8.0% to 22.0%, and consists of the balance Fe and inevitable impurity elements. Since Nb substitutes at least a part of V and is centrifuged as a granular carbide by centrifugal casting, an amount of Nb satisfying the following formula (5) can be added to the molten metal.
8.0% ≦ V% + 0.55 × Nb% ≦ 22.0% (mass%) (5)

  Preferably, the molten metal composition is, in mass%, C: 2.5% to 6.0%, Si: 0.2% to 1.5%, Mn: 0.2% to 1.5%, and V: It may contain 10.0% to 22.0%, and Nb may be contained in an amount satisfying 10.0% ≦ V% + 0.55 × Nb% ≦ 22.0% (mass%).

  Since optional elements Cr, Mo, Ni, Co and Al are hardly centrifuged, the content in the molten metal may be the same as the content in the outer layer of the roll. Since some of W and Ti are dissolved in the primary crystal carbides, they are slightly centrifuged.

  FIG. 2 is 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 a layer part in which granular carbide is concentrated on the inner surface side by casting a molten metal adjusted to a chemical composition for crystallizing primary granular carbide into a casting mold for centrifugal casting and centrifugal casting. Then, after the formation of the non-particulate carbide-poor layer portion on the outer peripheral side, the non-particulate carbide-poor layer portion is removed by processing or the like, and the granular carbide makes the concentrated layer a portion the outer layer for the rolling roll. Can be obtained. In FIG. 2, the portion 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. In the composite roll for rolling in the present invention, after casting the outer layer portion, the inner layer of the portion C in FIG. 2 is cast by stationary casting. After completion of the roll, the portion shown in FIG. 2 is removed by cutting or the like, and the composite roll for rolling of the present invention having the portion a shown in FIG. 2 as the outer layer is obtained.

  In the method of the present invention, from the distribution of the granular carbide predicted by the molten metal composition and centrifugal casting conditions, the roll outer layer depth at which the area ratio of the granular carbide is 20% or more is predicted. It is preferable that the cylindrical body is made to have an outer diameter larger than a target outer diameter of the outer layer.

[4] Inner Layer of Composite Roll for Rolling 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. In addition, 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.

Moreover, in the composite roll for rolling of this invention, it is preferable that the tensile strength between an outer layer and an inner layer is 40 kgf / mm < ² > or more. Thereby, it is possible to prevent the outer layer from peeling off due to the stress amplitude generated during rolling. More preferably, the tensile strength between the outer layer and the inner layer is 50 kgf / mm ² or more.

  Next, the reason for limiting the chemical component (mass%) of the inner layer will be described next. In addition, the chemical component of the inner layer of the present invention indicates the chemical component of the inner layer in the final product, not the component of the molten metal.

[5] Chemical composition (mass%) of the inner layer of the composite roll for rolling

C: 0.5% to 3.0%
C contributes to improving the strength, but if it is less than 0.5%, the welding tends to be insufficient, and defects such as a cast hole are likely to be generated at the boundary between the outer layer and 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 0.8% to 2.4%.

Si: 0.1% to 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.

[6] Roll structure The composite roll for rolling of the present invention may be solid or hollow as long as it has the outer layer and the inner layer of the present invention. Moreover, you may fit the sleeve which consists of an outer layer and an inner layer of this invention to a shaft material.

  The composite roll for rolling of the present invention exhibits excellent wear resistance and heat cracking resistance in general for the work roll for rolling, but extremely excellent wear resistance particularly for the work roll used in the finishing row of a hot sheet rolling mill. Also, it exhibits heat cracking resistance and contributes to the improvement of productivity and roll unit intensity in rolling mills.

  The outer layer material of the present invention concentrates the granular carbide 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 centrifugal casting mold and centrifugal casting. Layer was formed. Specimen No. 1 to 5 are examples of the present invention. 8 is a comparative example. Specimen No. 1-5 and no. 8 is formed by such centrifugal casting, and is collected from the portion corresponding to the portion A in FIG. In addition, specimen No. Nos. 6 to 7 are comparative examples, sample Nos. 9 is a conventional example, all of which were formed by centrifugal casting and sampled.

  The collected specimens were quenched at 1000 to 1200 ° C. after casting and subjected to heat treatment to temper three times at 500 to 600 ° C., and then processed into various test piece shapes described below. These test materials No. The chemical components (mass%) of 1 to 9 are shown in Table 1. In addition, the chemical component of the outer layer shown in Table 1 indicates not the component of the molten metal but the chemical component of the outer layer in the final product. Here, equation (1) in Table 1 is a value of V% + 0.55 × Nb%, equation (2) is a value of Nb% / V%, and equation (3) is C% −0.2 × ( V% + 0.55 × Nb%).

  Furthermore, the test material No. The V content in the granular carbides 1 to 9, the Vickers hardness of the base, the area ratio of the granular carbide exceeding the equivalent circle diameter of 10 μm, and the area ratio of the non-particulate carbide were measured. Further, a wear test using a rolling wear tester was performed as an evaluation of wear resistance, and a fracture toughness value KIC and a heat crack test were performed as evaluations of toughness and heat crack resistance.

(1) V content in granular carbides The analysis of V content in granular carbides was conducted by mirror-polishing the test material and slightly corroding the base with a picric acid ethanol solution, and then in any one granular carbide In any aspect of the above, a chemical component having an atomic weight of sodium or more was analyzed using a Delta III manufactured by EDX analyzer KEVEX, and a ratio corresponding to V among the analytical elements was obtained in mass%. Each sample material was analyzed for any five granular carbides, and the average value was obtained.

(2) Vickers hardness of base Vickers hardness of base is obtained by mirror-polishing the specimen and lightly corroding the base with a picric acid ethanol solution, and then using a Vickers hardness tester to load the base from 50 g to It measured in the range of 200g. The average value was calculated | required about arbitrary five places of each test material.

(3) Area ratio of granular carbide The area ratio of granular carbide exceeding the equivalent circle diameter of 10 μm is measured by mirror-polishing the specimen and corroding the base with an aqueous solution of ammonium persulfate, followed by an image analyzer (Nippon Avionics Co., Ltd.) It was measured using SPICCA-II) manufactured by company. The area ratio was measured with 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.

(4) Measurement of the area ratio of the non-granular carbides _(M 2 C, M ₆ C and _M 7 _{C 3)} total area ratio non-particulate carbide _(M 2 C, M ₆ C and _M 7 _{C 3)} of the test The material was mirror-polished and the base was corroded with an aqueous ammonium persulfate solution, and then measured using an image analyzer (SPICCA-II manufactured by Nippon Avionics Co., Ltd.). The area ratio (%) of non-particulate carbide was measured with 20 arbitrary fields of view corresponding to a 0.23 mm × 0.25 mm portion of each test piece, and the measured values were averaged. Note that M ₂ C, M ₆ C, and M ₇ C ₃ carbides having a circle-equivalent diameter of 1 μm or more, which are easy to identify, were measured.

(5) Wear depth and surface roughness Rz
For the evaluation of wear resistance and rough skin resistance, the wear depth (μm) and the ten-point average surface roughness Rz for the roll after rolling using a rolling wear tester schematically shown in FIG. It measured as follows. The surface roughness Rz was measured with a stylus type surface roughness meter.

  The rolling wear tester is an example of the present invention. 1-No. 5, Comparative Example No. 6-No. 8 and Conventional Example No. 9, a rolling mill 1 comprising test rolls 2 and 3 made of small sleeve rolls having an outer diameter of 60 mm, an inner diameter of 40 mm and a width of 40 mm, a heating furnace 4, a cooling water tank 5, and a winder 6, And a tension controller 7. The rolling wear test conditions were as follows.

Rolled material S: SUS304
Rolling rate: 25%
Rolling speed: 150 m / min Rolling temperature: 900 ° C
Rolling distance: 300m
Roll cooling: Water cooling Number of rolls: Quadruple

(6) Fracture toughness value KIC
The fracture toughness value KIC was measured by taking a test piece for measuring the fracture toughness value KIC from each test material 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.

(7) Heat crack test In the heat crack test, first, a cylindrical test piece having a diameter of 30 mm and a length of 30 mm was collected from the test material, and the end face was mirror-polished to obtain a test piece for a heat crack test. Thereafter, the operation of alternately immersing the end face of the test piece in a salt bath at 700 ° C. and water at 30 ° C. was repeated five times, and then the test piece was cut in a direction perpendicular to the surface to generate heat generated on the surface. The depth of the crack was measured. An average value was determined for each of the five test pieces.

Specimen No. V content (mass%) in granular carbides 1-9, base hardness (Hv), granular carbide area ratio (%) exceeding 10 μm in equivalent circle diameter, non-particulate carbide area ratio (%), wear amount ( μm), KIC (kg / mm ^3/2 ) and heat crack depth (mm) are summarized in Table 2.

  In FIG. 1 shows a metallographic structure. Also, in FIG. 9 shows the metallographic structure. In FIG. 4, a white part is a granular carbide and a black part is a base. In FIG. 5, a white part is a carbide | carbonized_material and a black part is a base. Specimen No. 1 shows that the granular carbide is distributed in high concentration. On the other hand, the sample No. 9 shows that the granular carbides are partially unevenly distributed and the non-particulate carbides are distributed in a network.

  Sample No. of the present invention. The wear amount of 1 to 5 is the test material No. It is less than half compared to 9, and the wear resistance is very good. Further, the material of the present invention has a fracture toughness value KIC which is equal to or higher than that of the conventional material, has sufficient toughness, has a heat crack depth shallower than that of the conventional material, and is excellent in heat crack resistance.

  Sample No. of Comparative Example 6 is outside the scope of the present invention, C%, Ni%, the value of the formula (3), the base hardness, and the granular carbide area ratio exceeding 10 μm in the equivalent circle diameter, compared with the test material of the present invention. Although the fracture toughness value KIC is good, the wear resistance is extremely poor.

  Sample No. of Comparative Example 7 is V%, Cr%, the value of the formula (1), the value of the formula (3), the V content in the granular carbide, the area ratio of the granular carbide exceeding 10 μm in the equivalent circle diameter, and the area of the non-particulate carbide The rate is out of the range of the present invention, and the wear resistance and heat crack resistance are inferior compared with the test material of the present invention, and in particular, the fracture toughness value KIC is extremely inferior.

  Sample No. of Comparative Example No. 8, the value of the formula (2) is out of the range of the present invention, and the thermal crack resistance is inferior to the test material of the present invention.

  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. Using a mold having an inner diameter of 500 mm and a length of 1200 mm and a centrifugal casting machine, No. 1 in Table 1. The molten metal prepared so that it might become an outer layer shown in 1 was inject | poured. After a predetermined time has passed, No. 1 in Table 3 is further added. The molten inner layer prepared to have the components shown in 11 to 16 was injected. In this way, a composite roll was manufactured.

  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 granular carbides (the portion shown in FIG. 2) in the body portion of the roll material 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 boundary strength was 48 kg / mm < ² > 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 cast holes in the boundary portion, poor welding occurs, and the boundary strength is very low. Further, the test material No. 15 and no. No. 16 showed a large amount of carbide segregation around the boundary and low boundary strength.

  FIG. 6 is a schematic cross-sectional view showing various forms of the composite roll for rolling of the present invention. FIG. 6A shows a solid roll comprising the outer layer A and the inner layer C of the present invention. FIG. 6B shows a hollow sleeve roll comprising the outer layer A and the inner layer C of the present invention. F is a hollow part. FIG. 6C shows a hollow sleeve roll made of the outer layer A of the present invention and an inner layer C fitted to a metal shaft material ho. The outer layer (a) referred to here is the same as the layer (i) where MC carbide is concentrated, which is obtained by processing and removing the MC carbide-poor layer (b) in FIG.

  Moreover, when the composite roll for rolling of the present invention was manufactured and actually rolled, it was confirmed that extremely excellent wear resistance and heat crack resistance were exhibited.

  The outer layer material for rolling rolls of the present invention exhibits excellent wear resistance and heat cracking resistance in general for the work rolls for rolling, but extremely excellent wear resistance particularly in the work rolls used in the finishing row of hot sheet rolling mills. And contributes to the improvement of productivity and roll basic unit in rolling mills.

It is a figure for demonstrating a circle equivalent diameter. It is the cross-sectional schematic for demonstrating the composite roll for rolling of this invention. It is the schematic of a rolling wear tester. Sample No. of the present invention. 1 is a metallographic photograph taken by an optical microscope. Sample No. of conventional material It is a metallographic photograph by the optical microscope of 9. 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,
15 Object A Area of the object, C circle, L Perimeter of the object,
(B) MC type carbide centrifugal concentrating layer, site excluding Lo, (c) inner layer,
E Shaft material, to hollow part

Claims (8)

It is a composite roll for rolling formed by centrifugal casting, and the outer layer thereof has a circle equivalent diameter of more than 10 μm of granular carbide exceeding 20% in area ratio and less than 60%, and non-particulate carbide is less than 5% in area ratio. The structure, the outer layer having a Vickers hardness of more than Hv550 and not more than Hv900, and the chemical composition metal-bonded to the inner surface of the outer layer in mass%, C: 0.5% to 3.0%, A composite roll for rolling having an inner layer made of an Fe-based alloy containing Si: 0.1% to 3.0% and Mn: 0.1% to 3.0%. The composite roll for rolling according to claim 1, wherein the V content in the granular carbide is 30% or more by mass%. 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% 3. The composite roll for rolling according to claim 1, 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, any one of Cr: more than 1.0% and not more than 15.0%, Mo: more than 0.5% and not more than 20.0% and W: more than 1.0% and not more than 40.0% in the outer layer. The composite roll for rolling according to any one of claims 1 to 3, comprising seeds or two or more kinds. The composite for rolling according to any one of claims 1 to 4, wherein a part of V of the outer layer is replaced with Nb in a range satisfying the following formulas (1) and (2) by mass%. roll.
11.0% <V% + 0.55 × Nb% ≦ 40.0% (1)
0.2> Nb% / V% (2) Furthermore, the following (3) Formula is satisfied, The composite roll for rolling in any one of Claims 1-5 characterized by the above-mentioned.
0 <C% −0.2 × (V% + 0.55 × Nb%) ≦ 2.0% (3) Furthermore, the said outer layer contains any 1 type or 2 types of Ni: 2.0% or less and Co: 10.0% or less in the mass%, The one in any one of Claims 1-6 characterized by the above-mentioned. Composite roll for rolling. Furthermore, the outer layer contains any one or two of Ti: 0.5% or less and Al: 0.5% or less in mass%, according to any one of claims 1 to 7. Composite roll for rolling.