JP6606977B2 - Method for producing composite roll for hot rolling - Google Patents

Description

  The present invention relates to a method for producing a hot-rolling composite roll in which an outer layer excellent in wear resistance, seizure resistance and accident resistance and a shaft core portion excellent in toughness are integrated via an intermediate layer, In particular, the present invention relates to a method for producing a hot-rolling composite roll suitable for a finish-rolling work roll of a hot strip mill of a thin steel plate.

  A heated slab having a thickness of several hundred mm manufactured by continuous casting or the like is rolled into a steel sheet having a thickness of several to several tens mm by a hot strip mill having a roughing mill and a finish rolling mill. The finish rolling mill is usually a series of 5-7 stand quadruple rolling mills. In the case of a 7-stand finishing mill, the first stand to the third stand are referred to as the front stand, and the fourth stand to the seventh stand are referred to as the rear stand.

  Since the work roll used in such a hot strip mill is in contact with the hot thin plate, damage such as abrasion, rough skin, heat cracks, and the like generated on the surface of the outer layer due to thermal and mechanical rolling loads occurs. Therefore, after grinding and removing these damages, the work roll is subjected to rolling again. Grinding and removing the damage on the surface layer of the outer layer of the roll is called “machining”. The work roll is discarded after being cut from the initial diameter to the minimum diameter (disposal diameter) that can be used for rolling. The diameter from the initial diameter to the scrap diameter is called the effective rolling diameter. At the effective rolling diameter, it is desirable that the outer layer has excellent wear resistance, seizure resistance and accident resistance in order to prevent the hot rolling roll from causing large surface damage such as heat cracks.

  The alteration includes light alteration for removing surface damage due to normal rolling wear and heavy alteration for removing surface damage due to a rolling accident. In particular, in a finishing stand at a later stage, a rolling accident called “squeezing” in which rolled steel sheets that are folded or cut are overlapped and rolled easily occurs. When such an accident occurs, the roll surface is subjected to a strong pressure locally, and the steel sheet is seized on the roll surface. In particular, cracks caused by rolling accidents are often extremely deep. Therefore, the roll for hot rolling is not only less worn by rolling (having excellent wear resistance), but also difficult to seize even in a rolling accident (having excellent seizure resistance), and has little crack development (excellent) Having accident resistance).

  As a work roll for a post-stand stand of a hot strip mill that requires such excellent wear resistance, seizure resistance, and accident resistance, it improves the wear resistance of high alloy grain cast iron with good seizure resistance. Therefore, a composite roll using an alloy to which a hard carbide forming element such as Mo or V is added as an outer layer material has been proposed.

For example, Patent Document 1 (Japanese Patent Laid-Open No. 2005-105296) is based on mass, C: 2.5 to 3.5%, Si: 1.0 to 2.5%, Mn: 0.3 to 1%, Ni: 3 to 5%, Cr: 1.5 -2.5%, Mo: 1.0-4%, V: 1.4-3.0%, Nb: 0.1-0.5%, B: 0.0005-0.2%, with the balance consisting of Fe and inevitable impurities and at least one base An outer layer of a roll for hot rolling excellent in wear resistance and surface roughness resistance having a structure containing 5000 to 100000 pieces / mm ² of fine carbide having a maximum length of 0.1 to 5 μm in the part is disclosed. In Patent Document 1, when the wear resistance of Ni grain roll is improved by MC carbide, secondary carbide is precipitated in the base to prevent rough skin, and quenching at 800 to 950 ° C is performed for that purpose. It is described that it is preferable. However, in the process of cooling from the quenching process, a temperature difference occurs between the roll surface and the inside, and compressive residual stress is added to the roll surface side. This stress is superimposed on the compressive residual stress due to transformation expansion of the outer layer, and the compressive residual stress on the roll surface becomes very high. Thus, when the compressive residual stress is high, cracks are likely to occur.

  Patent Document 2 (Japanese Patent Laid-Open No. 2004-82209) is based on the chemical composition of the outer shell layer as a reference: C: 3.0 to 4.0%, Si: 0.8 to 2.5%, Mn: 0.2 to 1.2%, Ni: 3.0 to 5.0% , Cr: 0.5-2.5%, Mo: 0.1-3.0%, V: 1.0-5.0%, balance Fe and unavoidable impurities, plain cast iron or spheroidal graphite cast iron containing C: 2.5-4.0% Disclosed is a composite roll for hot rolling made by centrifugal casting that satisfies the relationship 0.03 ≦ T / R ≦ 0.5 in which the thickness (T) of the outer shell layer and the radius (R) of the shaft core portion are satisfied. . This composite roll has seizure resistance and wear resistance, and prevents radish cracking during production and chill peeling during use. However, since only the tempering treatment at 430 ° C. is performed as the heat treatment, the hardness of the outer layer of the roll is not sufficient, and therefore the wear resistance is also poor.

Patent Document 3 (Japanese Patent Laid-Open No. 2002-88444) describes an outer layer formed of wear-resistant cast iron, an intermediate layer welded to the inner peripheral surface of the outer layer, and a shaft core portion welded to the inner peripheral surface of the intermediate layer. The chemical composition of the outer layer is, on a weight basis, C: 1.0 to 3.0%, Si: 0.1 to 2.0%, Mn: 0.1 to 2.0%, Ni: 0.1 to 4.5%, Cr: 3.0 to 10.0%, Mo: 0.1 to 9.0%, W: 1.5 to 10.0%, V and / or Nb: total 3.0 to 10.0%, and the balance substantially consisting of Fe, and the chemical composition of the intermediate layer is C: 1.0 to 2.5% by weight , Si: 0.2 to 3.0%, Mn: 0.2 to 1.5%, Ni: 4.0% or less, Cr: 4.0% or less, Mo: 4.0% or less, W and / or V: 12% or less in total, W, V and Nb At least one type: a total roll of 12% or less, and the balance substantially consists of Fe, and the shaft core part discloses a composite roll made of flake graphite cast iron, spheroidal graphite cast iron or graphite steel. However, since the outer layer contains a very large amount of Cr, which is 3.0 to 10.0%, graphite is difficult to crystallize and is inferior in seizure resistance and fracture toughness. Also, fracture toughness is low due to crystallization of Cr carbide (M ₇ C ₃ , M ₂₃ C ₆ etc.). If the fracture toughness is low, cracks generated in a rolling accident are likely to progress.

  Patent Document 4 (Japanese Patent Laid-Open No. 09-170041) is a centrifugal casting roll in which an outer layer containing graphite and a shaft core made of ductile cast iron (spherical graphite cast iron) are welded and integrated through an intermediate layer of graphite steel. The outer layer is C: 2.5 to 4.7%, Si: 0.8 to 3.2%, Mn: 0.1 to 2.0%, Cr: 0.4 to 1.9%, Mo: 0.6 to 5.0%, V: 3.0 to 10.0%, and Nb: In addition to containing 0.6 to 7.0%, the following formulas (1) to (4): 2.0 + 0.15V + 0.10Nb ≦ C (%) (1), 1.1 ≦ Mo / Cr (2), Nb /V≦0.8 (3) and 0.2 ≦ Nb / V (4) are satisfied, and the balance consists of Fe and unavoidable impurities. The shaft core has C: 2.8 to 3.8%, Si: 2.0-3.0%, Mn: 0.3-1.0%, P: 0.10% or less, S: 0.04% or less, Ni: 0.3-2.0%, Cr: 1.5% or less, and Mo: 1.0% or less, the balance Fe and Consisting of inevitable impurities, the intermediate layer is C: 1.0-2.0%, Si: 1.6-2.4%, Mn: 0.2-1.0%, P: 0.05% or less, S: 0.03% or less, Ni 0.1 to 3.5% Cr: 1.5% or less, and Mo: contains 0.1 to 0.8%, discloses a centrifugal cast roll and the balance Fe and unavoidable impurities. However, in the case of the intermediate layer of graphite steel, there is a problem that casting defects such as shrinkage cavities are likely to occur in the outer layer or the intermediate layer because the solidification start temperature of the intermediate layer is higher than that of the outer layer.

  Further, in order to generate graphite having seizure resistance, lubricity and crack propagation suppressing action, and carbide for improving wear resistance in the outer layer of the composite roll for centrifugal casting, 2.5 to 3.5% by mass of carbon and 2.5 It has been found that when cast iron to which Mo of at least mass% is added is used, defects such as shrinkage cavities may occur at the boundary between the outer layer and the ductile cast iron shaft core. It was also found that there is a possibility that the roll may break due to repeated stress due to rolling because a tensile residual stress is applied at the boundary portion. Therefore, it is necessary to reliably prevent defects such as shrinkage nests that occur at the boundary between the outer layer and the shaft core.

JP 2005-105296 A JP 2004-82209 A JP 2002-88444 A JP 09-170041 A

  Accordingly, the object of the present invention is to provide excellent wear resistance and seizure resistance, high fracture toughness value, excellent accident resistance, good outer layer and shaft core welding, and subsequent finishing of the hot strip mill. It is providing the method of manufacturing the composite roll for hot rolling suitable for a work roll.

  As a result of intensive research in view of the above-mentioned purpose, by providing an intermediate layer between the outer layer and the shaft core part, diffusion of Mo, Cr, etc. in the outer layer to the shaft core part is alleviated by the intermediate layer, and Mo is formed in the intermediate layer. Since the concentration distribution of Cr and the like can be achieved, it has been found that defects such as shrinkage can be reliably prevented from occurring at the boundary, and the present invention has been conceived.

That is, the method for producing a composite roll for hot rolling according to the present invention comprises: (a) C: 2.5 to 3.5%, Si: 1.3 to 2.4%, Mn: 0.2 to 1.5%, Ni: 3.5 to 5.0% on a mass basis. , Cr: 0.8-1.5%, Mo: 2.5-5.0%, V: 1.8-4.0%, and Nb: 0.2-1.5%, the balance consists of Fe and inevitable impurities, and the mass ratio of Nb / V is A chemical composition satisfying a condition of 0.1 to 0.7, a Mo / V mass ratio of 0.7 to 2.5, and 2.5 ≦ V + 1.2 Nb ≦ 5.5, and a structure having a graphite phase of 0.3 to 10% on an area basis An outer layer made of cast iron having (b) a shaft core portion made of ductile cast iron, and (c) between the outer layer and the shaft core portion, and a Mo concentration from the boundary with the outer layer to the boundary with the shaft core portion. A method of manufacturing a composite roll for hot rolling having a cast iron intermediate layer that gradually decreases,
(1) A first mold having a cylindrical mold for centrifugal casting and an upper mold and a lower mold for stationary casting separately, or a cavity for centrifugal casting and a cavity for stationary casting are integrally formed. Use the second mold provided,
(2) When using the first mold, (i) casting the molten outer layer having the chemical composition into the rotating cylindrical mold for centrifugal casting, and (ii) the inside of the outer layer during or after solidification (Iii) After the solidification of the intermediate layer, the upper mold and the lower mold are provided on the upper and lower ends of the cylindrical mold to constitute a stationary casting mold, and (iv) Casting molten metal for the shaft core into a cavity constituted by the upper mold, the cylindrical mold and the lower mold,
(3) When using the second mold, (i) casting the outer layer molten metal having the chemical composition into the centrifugal casting cavity of the rotating mold, and (ii) the outer layer during or after solidification. (Iii) After the solidification of the intermediate layer, the melt for the shaft core portion is cast into the stationary casting cavity.

  In the intermediate layer, the Mo content at the boundary with the shaft core part is (a) less than 2.1% by mass, or (b) 2.1% by mass or more and less than 5.0% by mass, and the Mo content− (Cr The content / 3) is preferably less than 1.7% by weight.

  The outer layer may further contain 0.1 to 5.0% by mass of W.

The chemical composition of the outer layer is represented by the following formulas (1) to (3):
Si ≦ 3.2 / [0.283 (C−0.2 V−0.13 Nb) +0.62] ... (1),
(C−0.2 V−0.13 Nb) + (Cr + Mo + 0.5 W) ≦ 9.5 (2), and
1.5 ≦ Mo + 0.5 W ≦ 5.5 (3)
It is preferable to satisfy the following condition.

  The outer layer was further selected from the group consisting of Ti: 0.003-5.0%, Al: 0.01-2.0%, Zr: 0.01-0.5%, B: 0.001-0.5%, and Co: 0.1-10.0% on a mass basis. You may contain at least 1 type.

  Preferably, the outer layer base has a Vickers hardness of 560 or more.

  The circumferential compressive residual stress on the outer layer surface at the center in the roll axis direction is preferably 150 to 500 MPa in terms of the discarded diameter.

The fracture toughness value K _IC of the outer layer is preferably 18.5 MPa · m ^1/2 or more.

  The Si content in the base of the outer layer is preferably 3.2% by mass or less.

  The shaft core portion is preferably made of ductile cast iron having a ferrite area ratio of 35% or less.

  The composite roll for hot rolling produced by the method of the present invention is not only excellent in wear resistance and seizure resistance in the outer layer, but also has high fracture toughness value, so it is excellent in accident resistance. Since there is no defect at the boundary between the outer layer and the shaft core portion, and the outer layer and the shaft core portion are joined firmly to each other, it is suitable for a work roll for the subsequent stage of the hot strip mill.

4 is a graph showing the relationship between the Si content of a matrix composition equivalent alloy and the fracture toughness value K _IC . It is the schematic which shows a rolling abrasion tester. It is the schematic which shows a friction thermal shock tester. It is a graph which shows roughly distribution of Mo, Cr, V, and Nb in an intermediate layer and its neighborhood. It is a graph which shows the method of determining a boundary part from distribution of Cr. It is a partial expanded sectional view which shows the boundary part vicinity of an intermediate | middle layer and an axial center part, Comprising: It is a partial expanded sectional view which shows the definition of content of Mo, Cr, V, and Nb in the said boundary part vicinity. 4 is a graph showing the distribution of Mo, Cr, V and Nb in the intermediate layer of test material No. A-8 and in the vicinity thereof. 5 is a graph showing the distribution of Mo, Cr, V and Nb in the intermediate layer of test material No. A-9 and in the vicinity thereof. 4 is a graph showing the distribution of Mo, Cr, V, and Nb in an intermediate layer of specimen No. A-10 and in the vicinity thereof. 4 is a photomicrograph showing the metal structure of the outer layer of specimen No. A-1. It is a schematic front view which shows the test piece for a fracture toughness value measurement.

  Embodiments of the present invention will be described in detail below, but the present invention is not limited to them, and various modifications may be made without departing from the technical idea of the present invention. Unless otherwise specified, when “%” is simply described, it means “mass%”.

[1] Composition of composite roll for hot rolling
(A) Outer layer
(1) Composition
(i) Essential composition
(a) C: 2.5 to 3.5% by mass
C combines with V, Nb, Cr, Mo and W to form hard carbides, which contributes to improved wear resistance. In addition, the graphitization promoting elements such as Si and Ni crystallize in the structure as graphite, thereby giving seizure resistance to the outer layer and improving toughness of the outer layer. When C is less than 2.5% by mass, not only the crystallization of graphite is insufficient, but also the amount of hard carbides crystallized is too small to provide sufficient wear resistance to the outer layer. Furthermore, when C is less than 2.5% by mass, the temperature difference from austenite crystallization to eutectic carbide crystallization is large, so austenite moves to the outer peripheral side by centrifugal force, and carbon tends to concentrate in the molten metal inside the outer layer. As a result, austenite coarse dendrite is likely to be generated and grown in the carbon-concentrated molten metal. Austenitic dendrite transforms into bainite and / or martensite, resulting in coarse speckled segregation.

  On the other hand, when C exceeds 3.5% by mass, the graphite becomes excessive and the shape thereof becomes string-like, so that the strength of the outer layer is lowered. Moreover, since the amount of carbide crystallization is excessive, the toughness of the outer layer is lowered and the crack resistance is lowered, so that cracks due to rolling become deep and roll loss increases. The lower limit of the C content is preferably 2.55% by mass, more preferably 2.65% by mass. The upper limit of the C content is preferably 3.45% by mass, more preferably 3.4% by mass, and most preferably 3.35% by mass.

(b) Si: 1.3-2.4% by mass
Si has a function of reducing oxide defects by deoxidation of the molten metal and promoting crystallization of graphite, and contributes to seizure resistance and suppression of crack propagation. If Si is less than 1.3% by mass, the deoxidizing action of the molten metal is insufficient, and the action of crystallization of graphite is small. On the other hand, if Si exceeds 2.4% by mass, the alloy matrix becomes brittle and the toughness of the outer layer decreases. The lower limit of the Si content is preferably 1.4% by mass, and more preferably 1.5% by mass. The upper limit of the Si content is preferably 2.3% by mass, more preferably 2.25% by mass, and most preferably 2.2% by mass.

(c) Mn: 0.2 to 1.5 mass%
In addition to the deoxidizing action of the molten metal, Mn has an action of fixing S as an impurity as MnS. If Mn is less than 0.2% by mass, these effects are insufficient. On the other hand, even if Mn exceeds 1.5% by mass, further effects cannot be obtained. The lower limit of the Mn content is preferably 0.3% by mass, more preferably 0.4% by mass, and most preferably 0.5% by mass. The upper limit of the Mn content is preferably 1.4% by mass, more preferably 1.3% by mass, and most preferably 1.2% by mass.

(d) Ni: 3.5-5.0% by mass
Ni has the effect of crystallizing graphite and contributes to seizure resistance. Ni also has the effect of improving the hardenability of the base structure. In the present invention, it is desirable not to perform quenching in order to limit the compressive residual stress on the roll surface. If quenching is not performed, the outer layer needs to be cured by cooling after casting. For this reason, the quenchability which raise | generates a bainitic transformation or a martensitic transformation is not required by causing the pearlite transformation by cooling in a centrifugal casting mold. If Ni is less than 3.5% by mass, the effect cannot be obtained sufficiently. On the other hand, when Ni exceeds 5.0% by mass, austenite is excessively stabilized, and transformation to bainite or martensite is difficult. The lower limit of the Ni content is preferably 3.6% by mass, more preferably 3.8% by mass, and most preferably 3.9% by mass. The upper limit of the Ni content is preferably 4.9% by mass, more preferably 4.8% by mass, and most preferably 4.7% by mass.

(e) Cr: 0.8 to 1.5 mass%
Cr is an element effective for improving the hardenability and maintaining the hardness by making the base a bainite or martensite and maintaining the wear resistance. If Cr is less than 0.8% by mass, the effect of addition is insufficient. On the other hand, if Cr exceeds 1.5% by mass, it not only inhibits crystallization of graphite, but also forms coarse eutectic carbides and lowers the fracture toughness value. The upper limit of the Cr content is preferably 1.45% by mass, more preferably 1.4% by mass, and most preferably 1.35% by mass.

(f) Mo: 2.5-5.0 mass%
Mo combines with C to form hard Mo carbides (M ₆ C, M ₂ C), increasing the hardness of the outer layer and improving the hardenability of the matrix. Mo also produces tough and hard MC carbides together with V and Nb to improve wear resistance. In addition, Mo increases the specific gravity of the residual eutectic melt during the solidification process of the alloy melt, prevents centrifugation of the primary γ phase, and suppresses the appearance of spotted segregation of bainite and / or martensite dendrites. If Mo is less than 2.5% by mass, these effects are insufficient. On the other hand, if Mo exceeds 5.0% by mass, the toughness of the outer layer deteriorates and the tendency to whitening becomes strong, so that crystallization of graphite is inhibited and the fracture toughness value is lowered. The lower limit of the Mo content is preferably 2.6% by mass, more preferably 2.7% by mass. The upper limit of the Mo content is preferably 4.6% by mass, more preferably 4.4% by mass, and most preferably 4.2% by mass.

(g) V: 1.8-4.0% by mass
V is an element that combines with C to form hard MC carbide. This MC carbide has a Vickers hardness Hv of 2500 to 3000, and is particularly hard among the carbides. When V is less than 1.8% by mass, the amount of crystallization of MC carbide is insufficient. On the other hand, when V exceeds 4.0% by mass, MC carbide with a low specific gravity is concentrated inside the outer layer due to the centrifugal force during centrifugal casting, and not only the radial segregation of MC carbide becomes significant, but also MC carbide becomes coarse. The alloy structure becomes rough, and the surface becomes rough during rolling. MC carbide is a carbide mainly composed of V, Nb, or Mo. As will be described later, the amount of crystallization is related not only to V but also to the amount of Nb. Furthermore, the interaction between V and other elements changes the amount of Si solid solution and the amount of coarse carbides formed in the matrix, as will be described later. The lower limit of the content of V is preferably 2.0% by mass, more preferably 2.1% by mass, and most preferably 2.2% by mass. The upper limit of the V content is preferably 3.9% by mass, more preferably 3.8% by mass, and most preferably 3.7% by mass.

(h) Nb: 0.2 to 1.5 mass%
Nb combines with C to form MC carbide. Nb, combined with V and Mo, solidifies in MC carbide and strengthens MC carbide, improving the wear resistance of the outer layer. The NbC-based MC carbide has a smaller difference from the molten metal density than the VC-based MC carbide, thereby reducing the segregation of the MC carbide. Furthermore, Nb increases the specific gravity of the residual eutectic melt during the solidification process of the molten alloy, prevents centrifugation of the primary γ phase, and dendritic bainite and / or martensite transformed from austenite segregates in the form of spots. Suppress. If Nb is less than 0.2% by mass, these effects are insufficient. On the other hand, when Nb exceeds 1.5% by mass, MC carbides aggregate and it becomes difficult to obtain a healthy outer layer. The lower limit of the Nb content is preferably 0.3% by mass, more preferably 0.4% by mass. The upper limit of the Nb content is preferably 1.4% by mass, more preferably 1.3% by mass, and most preferably 1.2% by mass.

(i) Nb / V: 0.1 to 0.7, Mo / V: 0.7 to 2.5, and V + 1.2 Nb: 2.5 to 5.5% by mass
Since V, Nb, and Mo all have the effect of increasing hard MC carbide essential for wear resistance, the total amount of these elements needs to be set to a predetermined level or more. V is an element that decreases the specific gravity of the molten metal, whereas Nb and Mo are elements that increase the specific gravity of the molten metal. Therefore, if the contents of Nb and Mo are not balanced with respect to V, the difference between the specific gravity of the molten metal and the specific gravity of austenite becomes large, and carbon is significantly concentrated due to the movement of austenite to the outer layer side by centrifugal force. As a result, the austenite dendrite tends to segregate.

  Therefore, the mass ratio of Nb / V is 0.1 to 0.7, the mass ratio of Mo / V is 0.7 to 2.5, and V + 1.2 Nb is 2.5 to 5.5 mass%. If Nb / V, Mo / V, and V + 1.2 Nb are within these ranges, carbides based on V will contain appropriate amounts of Nb and Mo, the carbides will become heavy, and the dispersion of the carbides will be uniformed. Therefore, the occurrence of spot-like segregation of bainite and / or martensite dendrite is prevented. In particular, when V + 1.2 Nb exceeds 5.5% by mass, MC carbides that are excessively crystallized and have a small specific gravity are concentrated inside the outer layer during the centrifugal casting process, thereby inhibiting welding with the intermediate layer.

  The lower limit of the mass ratio of Nb / V is preferably 0.12, more preferably 0.14, and most preferably 0.18. The upper limit of the mass ratio of Nb / V is preferably 0.6, more preferably 0.55, and most preferably 0.5.

  The lower limit of the mass ratio of Mo / V is preferably 0.75, more preferably 0.8, and most preferably 0.85. The upper limit of the mass ratio of Mo / V is preferably 2.2, more preferably 1.95, and most preferably 1.75.

  The lower limit of V + 1.2 Nb is preferably 2.6% by mass, more preferably 2.7% by mass, and most preferably 2.8% by mass. The upper limit of V + 1.2 Nb is preferably 5.35% by mass, more preferably 5.2% by mass, and most preferably 5.0% by mass.

(ii) Optional composition The outer layer of the composite roll for hot rolling may contain at least one of the following elements in addition to the above essential composition requirements.

(a) W: 0.1-5.0% by mass
W combines with C to form hard M ₆ C and M ₂ C carbides and contributes to improved wear resistance of the outer layer. It also has the effect of reducing the segregation by increasing the specific gravity by dissolving in MC carbide. However, if W exceeds 5.0% by mass, the specific gravity of the molten metal is increased, so that carbide segregation is likely to occur. Therefore, when adding W, the preferable content is 5.0 mass% or less. On the other hand, when W is less than 0.1% by mass, the effect of addition is insufficient. The upper limit of the W content is preferably 4.5% by mass, more preferably 4.0% by mass, and most preferably 3.0% by mass.

(b) Ti: 0.003 to 5.0 mass%
Ti combines with N and O, which are graphitization inhibiting elements, to form oxides or nitrides. Oxides or nitrides are suspended in the molten metal to become nuclei, and MC carbides are refined and homogenized. However, if Ti exceeds 5.0% by mass, the viscosity of the molten metal increases and casting defects are likely to occur. Therefore, when adding Ti, the preferable content is 5.0 mass% or less. On the other hand, when Ti is less than 0.003 mass%, the effect of addition is insufficient. The lower limit of the Ti content is preferably 0.005% by mass. The upper limit of the Ti content is more preferably 3.0% by mass, and most preferably 1.0% by mass.

(c) Al: 0.01 to 2.0 mass%
Al combines with N and O, which are graphitization-inhibiting elements, to form oxides or nitrides, which are suspended in the molten metal to form nuclei, and MC carbides are crystallized finely and uniformly. However, if Al exceeds 2.0% by mass, the outer layer becomes brittle, leading to deterioration of mechanical properties. Therefore, the preferable content of Al is 2.0% by mass or less. On the other hand, when the Al content is less than 0.01% by mass, the effect of addition is insufficient. The upper limit of the Al content is more preferably 1.5% by mass, and most preferably 1.0% by mass.

(d) Zr: 0.01 to 0.5 mass%
Zr combines with C to form MC carbide, improving the wear resistance of the outer layer. Moreover, since the Zr oxide produced | generated in the molten metal acts as a crystal nucleus, the solidification structure becomes fine. It also increases the specific gravity of MC carbide and prevents segregation. However, when Zr exceeds 0.5% by mass, inclusions are generated, which is not preferable. Therefore, the Zr content is preferably 0.5% by mass or less. On the other hand, when Zr is less than 0.01% by mass, the effect of addition is insufficient. The upper limit of the content of Zr is preferably 0.3% by mass, more preferably 0.2% by mass, and most preferably 0.1% by mass.

(e) B: 0.001 to 0.5 mass%
B has the effect of refining the carbide. A small amount of B contributes to crystallization of graphite. However, if B exceeds 0.5% by mass, the whitening effect becomes strong and the graphite becomes difficult to crystallize. Therefore, the content of B is preferably 0.5% by mass or less. On the other hand, when B is less than 0.001% by mass, the effect of addition is insufficient. The upper limit of the B content is preferably 0.3% by mass, more preferably 0.1% by mass, and most preferably 0.05% by mass.

(f) Co: 0.1-10.0 mass%
Co is an element effective for strengthening the base organization. Co also facilitates crystallization of graphite. However, when Co exceeds 10% by mass, the toughness of the outer layer decreases. Therefore, the content of Co is preferably 10% by mass or less. On the other hand, if Co is less than 0.1% by mass, the effect of addition is insufficient. The upper limit of the Co content is preferably 8.0% by mass, more preferably 6.0% by mass, and most preferably 4.0% by mass.

(g) Mo / Cr: 1.7-5.0
The mass ratio of Mo / Cr is preferably in the range of 1.7 to 5.0. When the mass ratio of Mo / Cr is less than 1.7, the Mo content is not sufficient with respect to the Cr content, and the area ratio of carbide particles mainly composed of Mo decreases. On the other hand, if the mass ratio of Mo / Cr exceeds 5.0, there are too many carbides mainly composed of Mo, and the carbides become coarse, so the fracture toughness is poor. Accordingly, the Mo / Cr mass ratio is preferably 1.7 to 5.0. The lower limit of the mass ratio of Mo / Cr is more preferably 1.8. The upper limit of the Mo / Cr mass ratio is more preferably 4.7, and most preferably 4.5.

(iii) Preferred compositional relationship
(a) Si ≦ 3.2 / [0.283 (C−0.2 V−0.13 Nb) +0.62] ... (1)
In order to improve the accident resistance, the fracture toughness value of the outer layer of the roll needs to have a high fracture toughness of, for example, 18.5 MPa · m ^1/2 or more in the case of a work roll for the latter stage of the hot strip mill. Since the fracture toughness value of the base of the outer layer of the roll cannot be measured, for the alloy corresponding to the base of the outer layer of the roll (excluding the influence of carbide), if the relationship between the amount of Si solid solution and the fracture toughness value is investigated, the roll The relationship between the amount of Si solid solution and the fracture toughness value at the base of the outer layer can be estimated. Therefore, for the purpose of eliminating the influence of the amount of carbides first, the C content is reduced to 1% by mass and the contents of carbide forming elements such as V and Nb are reduced to have various compositions corresponding to the base of the roll outer layer. The alloy samples were prepared, and the fracture toughness value of each sample was measured. Figure 1 shows the relationship between the amount of Si solid solution and the fracture toughness value of the matrix composition equivalent alloy. As shown in Fig. 1, the fracture toughness value of the sample is approximately 22 MPa · m ^1/2 or more when the Si solid solution amount in the matrix composition equivalent alloy is 3.2% or less, but if it exceeds 3.2%, 19 MPa · m Decrease below ^1/2 . From this, it can be estimated that the fracture toughness value of the base of the outer layer of the roll also decreases rapidly when the Si solid solution amount of the base exceeds 3.2%. As a result of earnest research on the alloy composition that limits the amount of Si solid solution in the matrix, Si ≦ 3.2 / [0.283 (C−0.2 V−0.13 Nb) +0 .62] was found to be necessary.

(b) (C−0.2 V−0.13 Nb) + (Cr + Mo + 0.5 W) ≦ 9.5 ... (2)
In the solidification process of cast iron containing V, Nb, Cr, Mo and W, first, granular MC carbides such as V and Nb and austenite crystallize, then Cr, Mo and W are concentrated in the liquid phase. Crystallized as a network-like eutectic carbide such as ₂ C, M ₆ C, M ₇ C ₃ , M ₂₃ C ₆ , and M ₃ C. The fracture toughness value of the outer layer greatly depends on the amount and shape of the carbide, and the fracture toughness value is remarkably lowered particularly when the network-like eutectic carbide is large or coarse. If C is excessive with respect to V and Nb that form MC carbide, and Cr, Mo, and W that are concentrated in the liquid phase during the solidification process, coarse carbide is formed, and the fracture toughness value of the outer layer decreases. To do. Whether C is excessive with respect to V and Nb is determined by the term (C−0.2 V−0.13 Nb), and whether Cr, Mo and W are excessive is determined by the term (Cr + Mo + 0.5 W). The As a result of intensive studies, it has been found that the composition condition for preventing the fracture toughness value from decreasing is that (C−0.2 V−0.13 Nb) + (Cr + Mo + 0.5 W) ≦ 9.5. In order to make the fracture toughness value 18.5 MPa · m ^1/2 or more, the value on the left side needs to be 9.5 or less.

(c) 1.5 ≦ Mo + 0.5 W ≦ 5.5 (3)
Mo and W have the effect of forming MC, M ₂ C or M ₆ C hard carbides. Since the action of Mo is twice that of W, the total content of Mo and W can be expressed as (Mo + 0.5 W). (Mo + 0.5 W) forms M ₂ C and M ₆ C carbides and improves wear resistance, so it should be 1.5% or more, but if too much, network-like eutectic carbides will increase. , 5.5% or less.

(iv) Impurities The balance of the outer layer composition is substantially composed of Fe and inevitable impurities. Of the inevitable impurities, P and S cause deterioration of mechanical properties, so it is preferable to reduce them as much as possible. Specifically, the P content is preferably 0.1% by mass or less, and the S content is preferably 0.1% by mass or less. As other inevitable impurities, elements such as Cu, Sb, Te, and Ce may be 0.7% by mass or less in total.

(2) Structure The structure of the outer layer of the composite roll for hot rolling has base, graphite, MC carbide, cementite, MC carbide, and carbides other than cementite (M ₂ C, M ₆ C, etc.). The structure of the outer layer of the composite roll for hot rolling has a graphite phase of 0.3 to 10 area%. The outer layer structure also preferably has 3 to 20 area% MC carbides. The outer layer base structure is preferably substantially composed of martensite, bainite or pearlite. The outer layer base structure preferably further has a cementite phase of 15 to 45 area%.

(a) Graphite phase area ratio: 0.3 to 10%
The area ratio of the graphite phase (graphite particles) crystallized in the outer layer structure is 0.3 to 10%. When the area ratio of the graphite phase is less than 0.3%, the effect of improving the seizure resistance of the outer layer is insufficient. On the other hand, when the graphite phase exceeds 10 area%, the mechanical properties of the outer layer are lowered. The area ratio of the graphite phase is preferably 0.5 to 8%, more preferably 1 to 7%.

(b) MC carbide area ratio: 3-20%
If the area ratio of MC carbide crystallized in the outer layer structure is less than 3%, the outer layer may not have sufficient wear resistance. In addition, due to the coexistence with graphite, it is difficult to make the MC carbide area ratio more than 20%.

(3) Characteristics
(a) wear resistance of the wear-resistant outer layer, MC, M ₂ C, obtained by hard carbides and a hard base structure, such as M ₆ C. In particular, MC carbides composed of V, Nb, etc. are very hard. When (V + 1.2 Nb) is 2.5% by mass or more, sufficient MC carbides are crystallized. Moreover, a hard base structure is obtained by elements such as Mo and W.

(b) Seizure resistance In order to prevent seizure of the steel sheet during narrowing, it is effective to contain a predetermined amount of carbide and Si and a predetermined amount of graphite. For this purpose, 2.5% by mass or more of C and 1.3% by mass or more of Si are required.

(c) Accident resistance Fracture toughness is an index of resistance to the development of cracks that have occurred. Fracture toughness values depend on carbide morphology, size and amount, and matrix toughness. If the carbide is coarse, cracks tend to progress. It was found that the formation of coarse carbide depends on the amount of C remaining in the molten metal after MC carbide crystallization and the amount of Cr, Mo and W that easily form coarse carbide. As a result, the sum of the term (C−0.2 V−0.13 Nb) indicating the amount of residual C after crystallization of MC carbide and the total amount of Cr, Mo and W (Cr + Mo + 0.5 W) is 9.5% by mass. If it is below, it can be judged that generation | occurrence | production of the coarse carbide | carbonized_material which reduces a fracture toughness value is suppressed.

  It was also found that the fracture toughness of the base was significantly reduced when more than 3.2 mass% Si was dissolved. In order to reduce the amount of Si in the base to 3.2 mass% or less, the condition of Si ≦ 3.2 / [0.283 (C−0.2 V−0.13 Nb) +0.62] may be satisfied.

(d) Compressive residual stress The outer layer of the roll needs a predetermined compressive residual stress to prevent the occurrence of cracks. However, if the compressive residual stress exceeds a predetermined value, the progress of cracks is promoted and accelerated. Since the residual stress is generated by elastic deformation due to the difference in strain between the outer layer and the shaft core portion, the elastic deformation increases correspondingly and the compressive residual stress increases as the outer layer becomes thinner. In the present invention, the value of the circumferential compressive residual stress on the outer layer surface is obtained at the center of the roll axis direction at the waste diameter where the compressive residual stress is maximized. The outer layer compressive residual stress at the scrap diameter at the center of the roll axis is preferably 150 to 500 MPa, more preferably 200 to 400 MPa so as to prevent the occurrence of cracks and not promote the progress of cracks. .

  In order to obtain such compressive residual stress, tempering at 450 to 550 ° C. is performed once or more after casting. The holding at 450 to 550 ° C is preferably 1 hour or longer. Residual austenite is transformed into hard martensite or bainite by this tempering temperature, and compressive residual stress is imparted to the roll surface by this transformation expansion. Such transformation increases the base hardness and improves the wear resistance. If quenching is performed by heating the roll above the austenitizing temperature of the outer layer base (about 770 ° C. or higher), the compressive residual stress on the roll surface exceeds 500 MPa, so that the cracks tend to progress quickly.

(e) Vickers hardness The Vickers hardness of the outer base is preferably 560 or more. When the Vickers hardness of the outer base is less than 560, rolling causes significant wear of the base part and falling off of carbides. A Vickers hardness of 560 or more can be obtained by adding Mo and W so as to satisfy 1.5 ≦ (Mo + 0.5 W).

(B) Shaft Core Part The roll core part is preferably made of ductile cast iron (spheroidal graphite cast iron) having excellent toughness. Furthermore, in order to extend the life of the journal part (shaft core part) in accordance with the extension of the life of the outer layer, it is necessary to improve the wear resistance of the journal part. If the backlash between the bearing and the bearing becomes large due to the wear of the journal part, the centrifugally cast composite roll must be discarded. In order to provide a highly wear-resistant journal part, it is preferable to use ductile cast iron having a ferrite area ratio of 35% or less for the shaft core part in which the journal part having a portion in contact with the bearing is formed. In ductile cast iron, the amount of carbon around it decreases due to spheroidal graphite, and it tends to be a low-hardness ferrite structure. As the ferrite area ratio increases, the hardness of the base decreases, and thus the wear resistance decreases. The ferrite area ratio of the ductile cast iron for the shaft core is preferably 32% or less, and most preferably 29% or less.

  The composition of ductile cast iron is C: 2.3-3.6%, Si: 1.5-3.5%, Mn: 0.2-2.0%, Ni: 0.3-2.0%, Cr: 0.05-1.0%, Mo: 0.05-1.0% by mass Mg: 0.01 to 0.08%, and V: 0.05 to 1.0%, and the balance is preferably composed of Fe and inevitable impurities. In addition to the above essential elements, Nb: 0.7% or less and W: 0.7% or less may be contained. Furthermore, in order to reduce the ferrite area ratio, a total of at least one of Cu, Sn, As, and Sb may be added in an amount of 0.005 to 0.5%. P is usually contained in the ductile cast iron as an impurity element of about 0.005 to 0.05%, but may be added up to 0.5% in order to reduce the ferrite area ratio. Ductile cast iron mainly contains ferrite and pearlite at the iron base, and mainly contains graphite and a small amount of cementite.

(C) Intermediate layer The hot-rolling composite roll obtained by the method of the present invention has an intermediate layer made of cast iron between the outer layer and the shaft core portion, and the Mo concentration in the intermediate layer is from the outer layer side to the shaft core portion. It has a structure that gradually decreases toward the side.

  When the shaft core is cast directly on the inner surface of the cylindrical outer layer by the centrifugal casting method, the shaft core is solidified after the innermost surface of the outer layer is remelted. Therefore, the remelted portion of the outermost inner surface of the outer layer is the shaft core portion. It becomes a boundary part which has a composition mixed with. As a result, the solidification temperature of the shaft core portion is reduced by Mo diffusing from the outer layer to the shaft core portion, the solidification temperature of the boundary portion is lower than the solidification temperature of the shaft core portion, and the shaft core portion is the boundary portion. It has been found that it solidifies faster and shrinkage cavities are more likely to occur.

  As a result of intensive studies, when casting an intermediate layer made of cast iron between the outer layer and the shaft core part when casting the shaft core part integrally with the outer layer, the Mo concentration is changed from the boundary between the outer layer and the shaft core part. It has been found that it is possible to prevent defects such as shrinkage nests. Since the shaft core portion is cast on the inner surface of the intermediate layer, the inner surface side portion of the intermediate layer is re-dissolved, and the diffusion of the component elements between the shaft core portion occurs. Therefore, the inner surface side region of the intermediate layer in which the constituent elements are diffused by re-dissolution is referred to as a “boundary portion with the shaft core portion” or simply as a “boundary portion”. In addition, the diffusion of the component elements also occurs between the outer layer and the intermediate layer, and a boundary region where the concentration of the component element changes is formed, but in order to prevent confusion with the “boundary portion with the shaft core portion” The space between the layers is simply called the “boundary”.

  On the other hand, Cr diffusing from the outer layer to the shaft core has the effect of increasing the solidification temperature of the shaft core, so if the amount of Mo diffusion from the outer layer is large, the amount of Cr diffusion at the boundary should be increased. I also understood. Therefore, if (a) the Mo content at the boundary with the shaft core in the intermediate layer is less than 2.1% by mass, solidification shrinkage at the boundary can be reliably prevented, and (b) the shaft in the intermediate layer When the Mo content at the boundary with the core is 2.1 mass% or more and less than 5.0 mass%, the solidification temperature rises due to Cr by setting [Mo content-(Cr content / 3)] to less than 1.7 mass%. The effect offsets the influence of Mo, and solidification shrinkage at the boundary can be reliably prevented. In the case of (a), the Mo content at the boundary with the shaft core in the intermediate layer is preferably less than 2.0% by mass.

  In order to improve the welding between the outer layer and the shaft core, the average thickness of the intermediate layer is preferably 1 to 70 mm, more preferably 3 to 50 mm, and 5 to 30 mm. Is most preferred. The intermediate layer does not necessarily have a uniform thickness over the entire region of the joint, and a part of the joint may be thin.

(1) Composition of melt for intermediate layer The cast iron melt for forming the intermediate layer contains 0 to 3.0 mass% Mo and 0.8 to 3.0 mass% Cr in order to reliably prevent solidification shrinkage at the boundary. It is preferable to contain. Specifically, the composition of the melt for the intermediate layer is C: 1.6 to 3.8%, Si: 0.2 to 3.5%, Mn: 0.2 to 2.0%, Ni: 0 to 5.0%, Cr: 0.8 to 3.0% on a mass basis , Mo: 0 to 3.0%, V: 0 to 2.0%, Nb: 0 to 2.0%, and W: 0 to 3.0%, and the balance is preferably composed of Fe and inevitable impurities. The upper limit of each content of V and Nb is more preferably 0.5% by mass.

(2) Solidification composition of the intermediate layer Since the intermediate layer and the shaft core are sequentially formed on the inner surface of the outer layer, the outer layer components diffuse to the outer region of the intermediate layer (the side closer to the inner surface of the outer layer). Therefore, the solidified composition of the intermediate layer is not only different from the composition of the molten metal for the intermediate layer, but has a gradient in the roll radial direction.

  By setting the Mo content at the boundary with the shaft core in the intermediate layer to be less than 2.1% by mass, solidification shrinkage at the boundary can be reliably prevented. The Mo content at the boundary is preferably more than 0% by mass and less than 2.0% by mass. Further, when the Mo content in the boundary portion is 2.1% by mass or more and less than 5.0% by mass (preferably in the case of 2.1% by mass or more and less than 3.0% by mass), [Mo content− (Cr content / 3)] is set to 1.7. By setting it to less than mass%, the effect of increasing the solidification temperature by Cr offsets the effect of decreasing the solidification temperature of Mo, and solidification shrinkage at the boundary can be reliably prevented.

  The intermediate layer is formed on the inner surface of the outer layer by centrifugal casting, and the shaft core is formed on the inner surface of the intermediate layer by static casting, so that not only the components of both diffuse at the boundary between the outer layer and the intermediate layer. Both components diffuse at the boundary between the intermediate layer and the shaft core. Therefore, the concentration of the alloy element generally gradually decreases from the outer layer to the shaft core portion through the intermediate layer. In particular, the concentration of these elements is greatly reduced at the boundary portion between the intermediate layer and the shaft core portion having different concentrations of Mo and Cr as carbide forming elements.

  As a result of examining the change in the concentration of Mo, Cr, V and Nb at the boundary between the intermediate layer and the shaft core, as shown schematically in Fig. 4-1, (a) the concentration of Mo, V and Nb is intermediate. Since it gradually decreases from the layer to the shaft core, it is difficult to specify the boundary range. (B) The Cr concentration hardly changes from the outer layer to the intermediate layer, but at the boundary between the intermediate layer and the shaft core. It was found that it dropped rapidly and became constant again at the shaft core. Therefore, it can be said that it is better to use the change in Cr concentration to specify the boundary range. Therefore, as shown in FIG. 4-2, the positions of the inflection points A1 and A2 of the Cr concentration curve are defined as the radially outer position and the inner position of the boundary, respectively. In order to obtain the position of such a boundary portion, it is preferable to analyze the Cr concentration at a pitch of 3 mm or less in the radial direction.

  Figure 4-3 shows an enlarged cross section (cross section perpendicular to the axial direction) of the composite roll near the boundary. As shown in FIG. 4-3, the radial position of the end 20 of the boundary is generally not constant. In the vicinity of the boundary portion having such an end portion 20, the concentrations of Mo, Cr, V, and Nb are measured along the radial straight line L at a constant pitch P, but the measurement points M1, M2, M3,. Is hardly located at the end 20 of the boundary. That is, the outer end A1 of the boundary portion is often not coincident with any of the measurement points M1, M2, M3. Therefore, on the radial straight line L, set a position A3 in the radial direction that is a distance X (= 2 mm) away from the outer end A1, and (a) if any measurement point coincides with the position A3 , Adopting the concentration of Mo and Cr at position A3, (b) If no measurement point coincides with position A3, Mo and Cr at the outermost measurement point (M2 in the example shown) from position A3 The concentration of is adopted. Therefore, the Mo content at the position A3 or the nearest outer measurement point M2 is defined as “Mo content at the boundary with the shaft core in the intermediate layer”. Similarly, the Cr content at the position A3 or the outermost measurement point M2 is defined as “Cr content at the boundary between the intermediate layer and the shaft core”. The V content at the position A3 or the outermost measurement point M2 closest to the position A3 is defined as “the V content at the boundary with the shaft core portion in the intermediate layer”. Furthermore, the Nb content at the position A3 or the outermost measurement point M2 is defined as “the Nb content at the boundary between the intermediate layer and the shaft core”. Enter examples of M1, M2, M3, etc. in Figure 4-2.

  In addition, (a) the requirement that the Cr content at the boundary with the shaft core portion in the intermediate layer is 80% or more of the Cr content in the discarded diameter of the outer layer, and (b) the shaft core portion in the intermediate layer The requirement that the total amount of V and Nb at the boundary of the outer layer is 70% or less of the total amount of V and Nb in the outer diameter of the outer layer is high bonding between the outer layer and the intermediate layer, and between the intermediate layer and the shaft core. It is preferable for obtaining strength (tensile strength of 300 MPa or more).

  Regarding requirement (a), the Cr content at the boundary with the shaft core in the intermediate layer is preferably 82% or more, more preferably 85% or more of the Cr content in the discarded diameter of the outer layer. The upper limit is preferably 300% or less, and more preferably 200% or less.

  Regarding requirement (b), the total amount of V and Nb at the boundary with the shaft core in the intermediate layer is preferably 68% or less, and 65% or less of the total amount of V and Nb in the discarded diameter of the outer layer. Is more preferable.

(D) Roll size The size of the centrifugally cast composite rolling roll of the present invention is not particularly limited, but preferred examples include an outer layer having an outer diameter of 200 to 1300 mm, a roll body length of 500 to 6000 mm, and outer layer rolling. The thickness of the used layer (rolling effective diameter) is 50 to 200 mm.

[2] Method for producing composite roll for rolling made by centrifugal casting The method for producing a composite roll for hot rolling according to the present invention comprises a cylindrical mold for centrifugal casting and an upper mold and a lower mold for stationary casting separately. Or a second mold integrally including a centrifugal casting cavity and a stationary casting cavity.

  When using the first mold, (i) casting the molten outer layer having the chemical composition into the rotating cylindrical mold for centrifugal casting, and (ii) the intermediate layer inside the outer layer during or after solidification (Iii) after solidification of the intermediate layer, the upper mold and the lower mold are provided on the upper and lower ends of the cylindrical mold to constitute a stationary casting mold, and (iv) the upper mold, There is a step of casting the molten metal for the shaft core into a cavity constituted by the cylindrical mold and the lower mold.

  When using the second mold, (i) casting the outer layer molten metal having the chemical composition in the centrifugal casting cavity of the rotating mold, and (ii) intermediate in the outer layer during or after solidification. (Iii) after solidifying the intermediate layer, casting the melt for the shaft core into the stationary casting cavity.

(A) Formation of outer layer
(1) Molten metal The chemical composition of the molten metal for the outer layer is C: 2.5-3.5%, Si: 1.3-2.4%, Mn: 0.2-1.5%, Ni: 3.5-5.0%, Cr: 0.8-1.5% on a mass basis. Mo: 2.5-5.0%, V: 1.8-4.0%, and Nb: 0.2-1.5%, the balance consists of Fe and inevitable impurities, the mass ratio of Nb / V is 0.1-0.7, Mo / The mass ratio of V is 0.7 to 2.5, and V + 1.2 Nb is 2.5 to 5.5%.

(2) Casting temperature The casting temperature of the outer layer molten metal is within the range of Ts + 30 ° C. to Ts + 180 ° C. (where Ts is the austenite crystallization start temperature). By the casting temperature within this range, the time during which the liquid phase remains can be shortened, the centrifugal separation of the γ phase crystallized from the liquid by solidification can be suppressed, and segregation can be suppressed. When the casting temperature is lower than Ts + 30 ° C., solidification of the cast molten metal is too fast, and foreign matters such as fine inclusions solidify before separation by centrifugal force, so foreign matter defects tend to remain. On the other hand, when the casting temperature is higher than Ts + 180 ° C., a spot-like region (segregation region) in which coarse dendrites gather inside the outer layer is generated. The casting temperature is preferably Ts + 30 ° C. to Ts + 100 ° C., more preferably Ts + 80 ° C. to Ts + 100 ° C. The austenite crystallization start temperature Ts is a start temperature of solidification exotherm measured by a differential thermal analyzer. Usually, the outer layer molten metal is cast into a centrifugal casting mold from a ladle through a funnel, a pouring nozzle, etc., or from a tundish through a pouring nozzle, etc. The temperature of the molten metal in the ladle or tundish.

(3) Centrifugal force Centrifugal force when casting an outer layer with a centrifugal casting mold is in the range of 60 to 150 G in terms of multiple of gravity. When casting is performed at a gravitational multiple within this range, the acceleration during solidification is limited to slow the movement speed of the γ phase, thereby suppressing the γ phase centrifugation (suppressing segregation). If the gravity multiple is less than 60 G, the outer layer melt is insufficiently wound (leaning). On the other hand, when the gravity multiple exceeds 150 G, γ-phase centrifugation becomes prominent, and coarse dendrites are generated in the molten metal residue with little γ-phase. As a result, spotted segregation of bainite and / or martensite dendrite is generated inside the outer layer. Gravity multiple (G No.) is the formula: G No. = N x N x D / 1,790,000 [where N is the number of revolutions of the mold (rpm) and D is the inner diameter of the mold (corresponding to the outer circumference of the outer layer) ) (Mm). ].

(4) Centrifugal Casting Mold The centrifugal casting cylindrical mold is preferably made of tough ductile cast iron having a thickness of 120 to 450 mm. If the mold is as thin as less than 120 mm, the cooling capacity of the mold is insufficient, and shrinkage defects are likely to occur in the outer layer. On the other hand, the cooling capacity is saturated even when the thickness of the mold exceeds 450 mm. A more preferable thickness of the mold is 150 to 410 mm. The centrifugal casting mold may be a horizontal mold, an inclined mold, or a vertical mold.

(5) Coating mold In order to prevent the outer layer from sticking to the mold, it is preferable to apply a coating mold mainly composed of silica, alumina, magnesia or zircon to a thickness of 0.5 to 5 mm on the inner surface of the mold. . If the coating mold is thicker than 5 mm, cooling of the molten metal is slow and the remaining time of the liquid phase is long, so that the γ phase is easily centrifuged and segregation is likely to occur. On the other hand, if the coating mold is thinner than 0.5 mm, the effect of preventing seizure of the outer layer is insufficient. A more preferable thickness of the coating mold is 0.5 to 4 mm.

(6) Inoculum In order to adjust the crystallization amount of graphite, an inoculum such as Fe-Si or Ca-Si may be added to the molten metal. In that case, the molten metal composition is determined in consideration of the composition change due to the addition of the inoculum. As an inoculation method, a method of adding the inoculum to the molten metal coming out of the melting furnace, a method of adding the inoculum to the molten metal in a ladle, tundish, funnel, etc., a method of adding the inoculum directly to the molten metal in the mold Etc.

(B) Formation of the intermediate layer The intermediate layer melt is cast during or after solidification of the cast outer layer. After the inner surface of the outer layer is redissolved, the intermediate layer is solidified, so that they are metal-bonded.

(C) Formation of shaft core portion After solidifying the intermediate layer, a cylindrical mold having an outer layer and an intermediate layer is erected, and an upper die and a lower die are provided at the upper and lower ends thereof to constitute a stationary casting mold. The upper mold and the lower mold hollow part communicate with the hollow part of the cylindrical mold (inside the intermediate layer), so that the mold having the upper mold, the outer layer and the intermediate layer and the lower mold form an integral cavity. To do. Ductile cast iron, which is a molten metal for the shaft core, is cast into the cavity. After the inner surface of the intermediate layer is redissolved, the shaft core portion is solidified, so that both are metal-bonded.

  Since the elements of both layers diffuse to each other at the boundary between the outer layer and the intermediate layer, the composition of the solidified intermediate layer is not only different from the melt composition but also has a gradient.

(D) Heat treatment A tempering treatment of 400 to 550 ° C after casting of the shaft core portion is performed to reduce the circumferential compressive residual stress of the outer layer surface at the center diameter in the roll axis direction to 150 to 500 MPa at the disposal diameter of the composite roll. It should be done once or more, but it should not be quenched.

  The present invention will be described in detail by the following examples, but the present invention is not limited thereto.

Example 1 (Sample Nos. A-1 to A-7 and B-1 to B-5)
(1) Manufacture of composite rolls Cylindrical molds for centrifugal casting made of ductile cast iron with a high-speed rotating inner diameter of 400 mm, length of 1500 mm, and thickness of 276 mm for each molten metal having the composition (mass%) shown in Table 1 The inner layer was cast by casting and the outer layer was centrifugally cast. The casting temperature of the outer layer melt was between Ts + 80 ° C. and Ts + 100 ° C. (where Ts is the austenite crystallization start temperature). The gravity multiple at the outer perimeter was 120 G. The average thickness of the outer layer obtained was 96 mm and the scrap diameter was 65 mm from the surface.

  Before solidification of the innermost surface of the outer layer, C: 3.1%, Si: 1.5%, Mn: 0.9%, Ni: 2.8%, Cr: 1.0%, Mo: 0.2%, and V on the inner surface of the outer layer : Melted intermediate layer having a composition of 0.1%, balance Fe and inevitable impurities (P: 0.03% or less, S: 0.02% or less, other impurities) was cast, and the intermediate layer was centrifugally cast. The casting temperature of the melt for the intermediate layer was 1362 ° C. The resulting hollow intermediate layer had an average thickness of 15 mm.

  After the hollow intermediate layer has solidified, stop the rotation of the cylindrical mold for centrifugal casting, and install the upper mold (length 1000 mm) and the lower mold (length 1000 mm) at the upper and lower ends of the cylindrical mold, respectively. A stationary casting mold was constructed. C: 3.2%, Si: 2.6%, Mn: 0.6%, P: 0.03% or less, Ni: 0.6%, Cr: 0.1 in the cavity of the stationary casting mold consisting of the upper mold, cylindrical mold and lower mold %, Mo: 0.1%, V: 0.1%, Mg: 0.07%, and the balance was cast with a molten ductile iron having substantially the composition of Fe and inevitable impurities, and the shaft core portion was statically cast. The casting temperature of the ductile cast iron melt for the shaft core was 1450 ° C.

  After completion of solidification of the shaft core part, the stationary casting mold was disassembled, and the obtained composite roll was taken out and tempered at 500 ° C. for 10 hours. Thus, the composite roll (test material No. A-1 to A-7) within the scope of the present invention, and the composite roll (test material No. B-1 to B-5) outside the scope of the present invention. )

The composition of the outer layer is shown in Table 1-1 and Table 1-2. Nb / V, Mo / V, Mo / Cr, (V + 1.2 Nb), (Mo + 0.5 W) and the right side of the following formula (1) The values and the value on the left side of the following formula (2) are shown in Table 1-3.
Si ≦ 3.2 / [0.283 (C−0.2 V−0.13 Nb) +0.62] ... (1)
(C−0.2 V−0.13 Nb) + (Cr + Mo + 0.5 W) ≦ 9.5 ... (2)

Notes: (1) The balance of the outer layer composition is Fe and inevitable impurities.
(2) Value on the right side of equation (1) above.
(3) The value on the left side of equation (2) above.

(2) Organization measurement
(a) Area ratio of graphite particles and MC carbide in outer layer Outer layer of composite roll of each specimen (No. A-1 to A-7, No. B-1 to B-5) From the optical micrograph of the test piece cut out from a position approximately 100 mm away in the axial direction), the area ratio of graphite particles and MC carbide was determined using image analysis software.

(b) Si content (% by mass) in the outer base
Cut out from the outer layer of the composite roll of each specimen (No. A-1 to A-7, No. B-1 to B-5) (position about 100 mm away from the roll barrel end surface in the roll axis direction) For the test piece, the Si content in the base was measured by an energy dispersive X-ray analyzer (EDX).

(c) Homogeneity of the structure The outer surface of the composite roll of each specimen (No. A-1 to A-7, No. B-1 to B-5) (approx. in the roll axis direction from the end surface of the roll body) Surfaces with depths of 10 mm, 30 mm, and 50 mm from the position 100 mm away were mirror-polished, corroded with an aqueous ammonium persulfate solution for about 1 minute, and then a tissue photograph (magnification: 5 to 10 times) was taken. . About each structure | tissue photograph, the presence or absence of the spot-like segregation with a diameter of 1.5 mm or more of the bainite and / or martensite dendrite was observed, and the homogeneity of structure | tissue was evaluated by the following reference | standard.
○: No spot-like segregation with a diameter of 1.5 mm or more.
×: Spotted segregation with a diameter of 1.5 mm or more.

FIG. 6 is a photograph of the metal structure of the outer layer of specimen No. A-1. This is one that corrodes using Pilacle as the corrosive liquid. In FIG. 6, 21 represents MC carbide, 22 represents graphite, 23 represents M ₆ C carbide, 24 represents matrix, and 25 represents cementite.

(d) Ferrite area ratio (%) of shaft core (journal part)
Image analysis in optical micrographs of test specimens cut from the shaft core (journal part) of the composite roll of each specimen (No. A-1 to A-7, No. B-1 to B-5) The area ratio (%) of ferrite was measured using software.

(3) Measurement of characteristics
(a) Outer layer fracture toughness (K _I C)
Each sample (No. A-1~A-7, No. B-1~B-5) of the fracture toughness value K _I C of the outer layer of the composite roll was measured according to ASTM standard E399. Specifically, as shown in FIG. 7, a distance of about 100 mm in the roll axial direction from the outer layer (the roll barrel end faces of the composite roll in order to measure the fracture toughness value K _I C according to ASTM E399, The test piece 30 (48 mm × 50 mm × 15 mm) cut out from the position) has a central notch 31 extending parallel to the outer surface of the roll and holding holes 32, 32 located on both sides of the notch 31. And have. First, a weak stress F, F was applied in the direction of opening the notch 31 by the member engaged with the holes 32, 32, and a crack 33 was previously made from the bottom of the notch 31 as a starting point. Next, the test piece 30 was re-applied with stresses F and F in the direction of opening the notch 31 to cause the crack 33 to propagate, and the crack opening displacement was measured at the opening end P of the notch 31 until failure. It was determined stress and crack opening displacement fracture from toughness value ^{_{K I C (MPa · m 1/2}} ).

(b) Outer base Vickers hardness (Hv)
Cut out from the outer layer of the composite roll of each specimen (No. A-1 to A-7, No. B-1 to B-5) (position about 100 mm away from the roll barrel end surface in the roll axis direction) The Vickers hardness of the base was measured with a load of 200 g using a micro Vickers hardness tester.

(c) Shore hardness of outer layer (Hs)
The surface of the outer layer located at the initial diameter of the composite roll of each sample material (No. A-1 to A-7, No. B-1 to B-5) was measured for Shore hardness with a Shore hardness meter. .

(d) Compressive residual stress (MPa) at the disposal diameter of the outer layer
At the center in the roll axial direction of the outer layer of the composite roll of each specimen (No. A-1 to A-7, No. B-1 to B-5), the removal diameter of the outer layer was removed by machining. The circumferential diameter compressive residual stress of the outer layer surface was measured with an X-ray diffraction residual stress measuring device at the center of the roll axis at the diameter of the outer layer of each composite roll (depth 50 mm from the surface of the initial diameter of the product).

  Table 2 shows the measurement results of the tissues, and Table 3 shows the measurement results of the characteristics.

Note: (1) Judged by the presence or absence of spotted segregation of dendrites with a diameter of 1.5 mm or more.

Notes: (1) Fracture toughness value (K _I C).
(2) Compressive residual stress at the disposal diameter.

(4) Performance test Using the outer layer of each specimen (No. A-1 to A-7, No. B-1 to B-5), the outer diameter is 60 mm, the inner diameter is 40 mm, and the width is 40 mm. A test roll having a sleeve structure was produced. In order to evaluate the wear resistance, a wear test was performed on each test roll using the rolling wear tester shown in FIG. The rolling wear tester includes a rolling mill 1, test rolls 2 and 3 incorporated in the rolling mill 1, a heating furnace 4 for preheating the rolled material 8, a cooling water tank 5 for cooling the rolled material 8, and a rolling A winder 6 that applies a constant tension to the winder 6 and a controller 7 that adjusts the tension. The rolling wear conditions were as follows. After rolling, the depth of wear generated on the surface of the test roll was measured with a stylus type surface roughness meter. The results are shown in Table 4.
Rolled material: SUS304
Rolling rate: 25%
Rolling speed: 150 m / min Rolling material temperature: 900 ° C
Rolling distance: 300 m / time Roll cooling: Water cooling Number of rolls: Quadruple

In order to evaluate seizure resistance, a seizure test was conducted on each test roll using a frictional thermal shock tester shown in FIG. The frictional thermal shock testing machine rotates the pinion 13 by dropping the weight 12 on the rack 11 and brings the biting material 15 into strong contact with the test material 14. The degree of seizure was evaluated by the seizing area ratio as follows. The results are shown in Table 4. The less seizure, the better the seizure resistance.
○: No seizure (seizure area ratio is less than 40%)
Δ: Slight seizure (seize area ratio is 40% or more and less than 60%).
X: Significant seizure (seize area ratio is 60% or more).

As is clear from Tables 2 to 4, the outer layer of each of the test material Nos. A-1 to A-7 has an area ratio of graphite particles in the range of 0.3 to 10%, and in the range of 3 to 20%. MC carbide has an area ratio of Si, the Si content in the matrix is 3.2% by mass or less, and is excellent in the homogeneity of the structure, and the ferrite area ratio of the shaft core (journal part) is 35% or less Met. Furthermore, all of the outer layers of test materials No. A-1 to A-7 have a fracture toughness value of 18.5 MPa · m ^1/2 or more, a Vickers hardness of 560 or more, and a scrap diameter of 150 to 500 MPa. And had excellent wear resistance, seizure resistance and accident resistance.

On the other hand, the outer layer of specimen No. B-1 had a low fracture toughness value of 17.9 MPa · m ^1/2 and a relatively large wear depth (wear resistance) of 2.61 μm. The outer layer of specimen No. B-2 had a fracture toughness value as low as 17.1 MPa · m ^1/2, and the seizure resistance was insufficient. Since the outer layer of specimen No. B-3 has an MC carbide area ratio of 1.29%, the depth of wear was as large as 3.11 μm. The outer layer of specimen No. B-4 had a base Vickers hardness Hv as low as 532, poor homogeneity of the structure, and the area ratio of the graphite particles was as small as 0.12%, so the seizure resistance was poor. The outer layer of specimen No. B-5 was inferior in seizure resistance due to the poor homogeneity of the structure and the area ratio of the graphite particles being as small as 0.28%.

Example 2 (Sample No. A-8)
Using the same method as in Example 1, the outer layer melt and the intermediate layer melt having the composition (mass%) shown in Table 5 are made of ductile cast iron with an inner diameter of 760 mm, a length of 2700 mm, and a thickness of 320 mm. Cast into a metal mold (coating mold mainly composed of 3 mm thick zircon on the inner surface), and an outer layer having an average thickness of 91 mm and an intermediate layer having an average thickness of 20 mm were formed by centrifugal casting. Thereafter, a ductile cast iron melt for the shaft core part having the composition (mass%) shown in Table 5 was cast by the same method as in Example 1 to form the shaft core part.

  After the solidification of the shaft core part is completed, the stationary casting mold is disassembled, and the resulting composite roll is taken out and subjected to a tempering treatment at 500 ° C. for 10 hours. 8 composite rolls were obtained. The discarded diameter of the outer layer was 65 mm from the surface.

Note: (1) Fe and inevitable impurities.

  The distribution of Mo, Cr, V, and Nb in the vicinity of the intermediate layer was measured on a test piece cut out from a position about 100 mm away from the roll barrel end face of the obtained composite roll in the roll axis direction. The results are shown in Figure 5-1. As is clear from Fig. 5-1, the distance from the position of the scrap diameter to the end of the boundary (A3) was about 28 mm. Table 6 shows the position of the outer diameter of the outer layer and the contents of Mo, Cr, V and Nb at the boundary with the shaft core in the intermediate layer, and the total amount of V and Nb, Mo- (Cr / 3), Mo content at the boundary / Mo content at the disposal diameter position, Cr content at the boundary / Cr content ratio at the disposal diameter position, V content / disposition diameter at the boundary Ratio of V content at the location, ratio of Nb content at the boundary / Nb content at the disposal diameter position, and ratio of V and Nb at the boundary / total amount of V and Nb at the disposal diameter position Indicates.

Example 3 (Sample No. A-9)
Using the same method as in Example 1, the outer layer melt and the intermediate layer melt of the composition (mass%) shown in Table 7 were made into ductile cast iron cylinders with an inner diameter of 795 mm, a length of 2700 mm, and a thickness of 302.5 mm. Cast into a metal mold (coating mainly composed of 3 mm thick zircon on the inner surface), and an outer layer with an average thickness of 85 mm and a hollow intermediate layer with an average thickness of 10 mm were formed by centrifugal casting. .

  After the intermediate layer has solidified, stop the rotation of the cylindrical mold for centrifugal casting, and place the upper mold (length 1000 mm) and the lower mold (length 1000 mm) on the upper and lower ends of the cylindrical mold, respectively. A casting mold was constructed. The ductile iron melt for the shaft core part having the composition (mass%) shown in Table 7 is cast into the cavity of the stationary casting mold composed of the upper mold, the mold having the intermediate layer, and the lower mold, and the axial core part is cast by static casting. did. The casting temperature of the ductile cast iron melt for the shaft core was 1450 ° C.

  After completion of solidification of the shaft core part, the stationary casting mold was disassembled, and the obtained composite roll was taken out and tempered at 500 ° C. for 10 hours. In this way, a composite roll of specimen No. A-9 within the scope of the present invention was obtained. The discarded diameter of the outer layer was 65 mm from the surface.

Note: (1) Fe and inevitable impurities.

  The distribution of Mo, Cr, V, and Nb in the vicinity of the intermediate layer was measured on a test piece cut out from a position about 100 mm away from the roll barrel end face of the obtained composite roll in the roll axis direction. The results are shown in Figure 5-2. As is clear from Fig. 5-2, the distance from the disposal diameter position to the end of the boundary (A3) was about 18 mm. Table 8 shows the position of the diameter of the outer layer discarded and the contents of Mo, Cr, V and Nb at the boundary with the shaft core in the intermediate layer, and the total amount of V and Nb, Mo- (Cr / 3), Mo content at the boundary / Mo content at the disposal diameter position, Cr content at the boundary / Cr content ratio at the disposal diameter position, V content / disposition diameter at the boundary Ratio of V content at the location, ratio of Nb content at the boundary / Nb content at the disposal diameter position, and ratio of V and Nb at the boundary / total amount of V and Nb at the disposal diameter position Indicates.

Example 4 (Sample No. A-10)
Using the same method as in Example 1, a melt for outer layer and a melt for intermediate layer having the composition (mass%) shown in Table 9 is a cylinder for centrifugal casting made of ductile cast iron having an inner diameter of 760 mm, a length of 2700 mm, and a thickness of 320 mm. Cast into a metal mold (coating mold mainly composed of 3 mm thick zircon on the inner surface), and an outer layer having an average thickness of 91 mm and an intermediate layer having an average thickness of 20 mm were formed by centrifugal casting. Thereafter, a ductile cast iron melt for a shaft core part having a composition (mass%) shown in Table 9 was cast by the same method as in Example 1 to form a shaft core part.

  After the solidification of the shaft core part is completed, the stationary casting mold is disassembled, and the resulting composite roll is taken out and subjected to a tempering treatment at 500 ° C. for 10 hours. Ten composite rolls were obtained. The discarded diameter of the outer layer was 65 mm from the surface.

Note: (1) Fe and inevitable impurities.

  The distribution of Mo, Cr, V, and Nb in the vicinity of the intermediate layer was measured on a test piece cut out from a position about 100 mm away from the roll barrel end face of the obtained composite roll in the roll axis direction. The results are shown in Figure 5-3. As is clear from Fig. 5-3, the distance from the position of the disposal diameter to the end of the boundary (A3) was about 23 mm. Table 6 shows the position of the outer diameter of the outer layer and the contents of Mo, Cr, V and Nb at the boundary with the shaft core in the intermediate layer, and the total amount of V and Nb, Mo- (Cr / 3), Mo content at the boundary / Mo content at the disposal diameter position, Cr content at the boundary / Cr content ratio at the disposal diameter position, V content / disposition diameter at the boundary Ratio of V content at the location, ratio of Nb content at the boundary / Nb content at the disposal diameter position, and ratio of V and Nb at the boundary / total amount of V and Nb at the disposal diameter position Indicates.

  In any of the composite rolls of Example 2, Example 3 and Example 4 (Sample Nos. A-8, A-9, and A-10), (a) Mo content in the boundary with the shaft core in the intermediate layer The amount of Mo is less than 2.1% by mass, or (b) the Mo content at the boundary with the shaft core in the intermediate layer is 2.1% by mass or more and less than 5.0% by mass, and Mo content− (Cr content / 3) Is less than 1.7% by mass, and (c) the Cr content at the boundary with the shaft core in the intermediate layer is 80% or more of the Cr content in the discarded diameter of the outer layer, and (d) in the intermediate layer Thus, the total amount of V and Nb at the boundary with the shaft core portion was 70% or less of the total amount of V and Nb in the discarded diameter of the outer layer. As a result of inspection by ultrasonic flaw detection, it was confirmed that there was no shrinkage nest at the boundary between the obtained shaft core and the intermediate layer, and that both were welded soundly.

For the outer layers of Example 2, Example 3 and Example 4, the structure and properties were measured in the same manner as in Example 1. Table 2 shows the measurement results of the tissues, and Table 3 shows the measurement results of the characteristics. As is clear from Table 2 and Table 3, also in Example 2, Example 3 and Example 4, the outer layer has a graphite area ratio in the range of 0.3 to 10%, and the Si content in the matrix is 3.2% by mass or less. In addition, the homogeneity of the structure was excellent, and the ferrite area ratio of the shaft core portion (journal portion) was 35% or less. The outer layers of Example 2, Example 3 and Example 4 also have a fracture toughness value of 18.5 MPa · m ^1/2 or more, a Vickers hardness of 560 or more base, and a compression with a waste diameter in the range of 150 to 500 MPa. Has residual stress.

  For the outer layers of Example 2, Example 3 and Example 4, performance tests were conducted in the same manner as in Example 1. Table 4 shows the results of the performance test. As is clear from Table 4, the outer layers of Examples 2, 3 and 4 also had excellent wear resistance, seizure resistance and accident resistance.

1 ... Rolling mill
2 ... Test roll
3 ... Test roll
4 ... heating furnace
5 ... Cooling water tank
6 ... Winding machine
7 ... Controller
11 ... Rack
12 ... Weight
13 ... Pinion
14 ... Test material
15 ... Bite material
20 ... End of boundary
21 ... MC carbide
22 ... graphite
23 ・ ・ ・ M ₆ C carbide
24 ... Base
25 ... Cementite

Claims (11)

(a) by weight, C: 2.5~3.5%, Si: 1.3~2.4%, Mn: 0.2~1.5%, Ni: 3.5~5.0%, Cr: 0.8~1.5%, Mo: 2.5~ 4.4%, V : 1.8-4.0%, and Nb: 0.2-1.5%, the balance consists of Fe and inevitable impurities, the mass ratio of Nb / V is 0.1-0.7, the mass ratio of Mo / V is 0.7-2.5 And an outer layer made of cast iron having a chemical composition satisfying 2.5 ≦ V + 1.2 Nb ≦ 5.5 and a structure having a graphite phase of 0.3 to 10% on an area basis, and (b) an axial core made of ductile cast iron and parts, be between the axis section (c) and the outer layer, Mo concentration possess a cast iron intermediate layer gradually decreases from the boundary between the outer layer to a boundary between the shaft core portion, said intermediate layer In the above, the Mo content at the boundary with the shaft core part is (a) less than 2.1% by mass, or (b) 2.1% by mass or more and less than 5.0% by mass, and Mo content− (Cr content / 3) the hot composite roll for rolling is lower than 1.7 wt% A method of forming,
(1) A first mold having a cylindrical mold for centrifugal casting and an upper mold and a lower mold for stationary casting separately, or a cavity for centrifugal casting and a cavity for stationary casting are integrally formed. Use the second mold provided,
(2) When using the first mold, (i) casting the molten outer layer having the chemical composition into the rotating cylindrical mold for centrifugal casting, and (ii) the inside of the outer layer during or after solidification (Iii) After the solidification of the intermediate layer, the upper mold and the lower mold are provided on the upper and lower ends of the cylindrical mold to constitute a stationary casting mold, and (iv) Casting molten metal for the shaft core into a cavity constituted by the upper mold, the cylindrical mold and the lower mold,
(3) When using the second mold, (i) casting the outer layer molten metal having the chemical composition into the centrifugal casting cavity of the rotating mold, and (ii) the outer layer during or after solidification. (Iii) A method of casting the molten metal for the shaft core into the stationary casting cavity after the solidification of the intermediate layer. 2. The method for producing a composite roll for hot rolling according to claim 1, wherein the Mo concentration in the chemical composition of the outer layer is 2.5 to 4.2% by mass . 3. The method for producing a composite roll for hot rolling according to claim 1 or 2 , wherein the composition of the melt for the intermediate layer is C: 1.6 to 3.8%, Si: 0.2 to 3.5%, Mn: 0.2 to 2.0 on a mass basis. %, Ni: 0 to 5.0%, Cr: 0.8 to 3.0%, Mo: 0 to 3.0%, V: 0 to 2.0%, Nb: 0 to 2.0%, and W: 0 to 3.0%, the balance being A method comprising Fe and inevitable impurities . The method for producing a composite roll for hot rolling according to any one of claims 1 to 3 , wherein the outer layer further contains 0.1 to 5.0% by mass of W. In the manufacturing method of the composite roll for hot rolling in any one of Claims 1-4 , the chemical composition of the said outer layer is following formula (1)-(3):
Si ≦ 3.2 / [0.283 (C−0.2 V−0.13 Nb) +0.62] ... (1),
(C−0.2 V−0.13 Nb) + (Cr + Mo + 0.5 W) ≦ 9.5 (2), and
1.5 ≦ Mo + 0.5 W ≦ 5.5 (3)
A method characterized by satisfying the following conditions. In the manufacturing method of the composite roll for hot rolling in any one of Claims 1-5 , the said outer layer is further Ti: 0.003-5.0% by mass reference | standard, Al: 0.01-2.0%, Zr: 0.01-0.5%, B: A method comprising at least one selected from the group consisting of 0.001 to 0.5% and Co: 0.1 to 10.0%. The method for producing a composite roll for hot rolling according to any one of claims 1 to 6 , wherein the outer layer base has a Vickers hardness of 560 or more. In the hot method for manufacturing a rolling composite roll according to any one of claims 1 to 7 that circumferential compressive residual stress in the outer layer surface in the roll axial direction center is 150 to 500 MPa in the waste却径Feature method. The method for producing a composite roll for hot rolling according to any one of claims 1 to 8 , wherein the fracture toughness value K _IC of the outer layer is 18.5 MPa · m ^1/2 or more. The method for producing a composite roll for hot rolling according to any one of claims 1 to 9 , wherein the Si content in the base of the outer layer is 3.2 mass% or less. The method for producing a composite roll for hot rolling according to any one of claims 1 to 10 , wherein the shaft core portion is made of ductile cast iron having a ferrite area ratio of 35% or less.