JP2019183276A - Centrifugal casting composite roll for hot rolling and manufacturing method therefor - Google Patents

Abstract

To provide a centrifugal casting composite roll for hot rolling of which abrasion resistance is high in the case close to an initial diameter and heat crack resistance is enhanced in the case close to a waste diameter by cutting.SOLUTION: There is provided a centrifugal casting composite roll for hot rolling manufactured by depositing and integrating an inner layer consisting of an iron-based alloy to an outer layer consisting of an Fe group alloy containing, by mass basis, C:2.6 to 3.6%, Si:0.1 to 3%, Mn:0.3 to 2%, Ni:2.3 to 5.5%, Cr:0.5 to 3.2%, Mo:0.3 to 1.6%, V:0.2 to 3.4%, Nb:0.4 to 3%, and B:0.06% or less, and having 0.07≤V/Nb≤2.7, V equivalent (Veq=V+0.55 Nb) of 2.50%mass or more and the balance Fe with impurities, in which the V equivalent of the outer layer satisfies a condition of Veq1/Veq2=1.1 to 5, where Veq1 is V equivalent of an area from initial diameter to depth of 30 mm in a radial direction, and Veq2 is V equivalent in waste diameter.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a cast composite roll suitable for use in the fifth to seventh stands of a steel sheet hot rolling mill, and particularly for use in the final seventh stand because of its high wear resistance when close to the initial diameter. Although it is suitable, the present invention relates to a centrifugal cast composite roll for hot rolling which is suitable for a sixth stand and a fifth stand, and a method for producing the same.

  A hot strip mill heats a slab with a thickness of several hundred millimeters manufactured by continuous casting or the like, and then sequentially passes between rolls of a plurality of rough rolling mills and a plurality of finish rolling mills to a thickness of several to several tens of millimeters. It is to be rolled. The finish rolling mill is usually one in which 5 to 7 quadruple rolling mill stands are arranged in series, and particularly, a finishing mill composed of seven stands is widely used. In a seven-stand finishing mill, the first to third stands are referred to as front stands, and the fourth to seventh stands are referred to as rear stands.

  Since the roll used in the finishing mill needs to withstand the thermal and mechanical loads caused by rolling, a centrifugal cast composite roll with a composite structure in which an outer layer with excellent wear resistance and an inner layer with excellent toughness are welded and integrated ( Simply called "composite roll"). However, depending on the thermal and mechanical loads caused by rolling, the outer layer surface may be damaged by wear, rough skin, heat cracks, etc., so the composite roll is removed from the rolling mill after a certain period of use, and the damage is removed by grinding (machining). To do. By reworking, the body diameter of the composite roll gradually decreases from the initial diameter to the minimum diameter that can be used for rolling (removal diameter). In the present specification, the roll diameter from the initial diameter to the disposal diameter is referred to as a rolling effective diameter (simply referred to as “effective diameter”), and the effective diameter area from the initial diameter to the disposal diameter is referred to as “use area”.

Conventionally, a centrifugal cast composite roll in which a high alloy grain cast iron outer layer and a cast iron inner layer excellent in toughness are integrated in a metallurgical manner has been used in the latter stage stand of a finish rolling mill. The outer layer of high-alloyed grain cast iron is composed of graphite, carbide and matrix structure, and is particularly excellent in seizure resistance. Therefore, even when encountering a squeeze accident, the occurrence and development of cracks is extremely small, that is, it has excellent accident resistance. There is. In particular, in the latter stage stand, a squeeze accident often occurs due to thin steel plates being folded and rolled. Therefore, a composite roll having a high alloy grain cast iron with good accident resistance as an outer layer is often used. The high alloy Glenn cast iron, and the type containing only M ₃ C type (cementite), M ₃ C system in order to improve the abrasion resistance (cementite) and a relatively small amount of MC carbides consisting of elements such as vanadium There are types to contain. However, since high alloy grain cast iron is inferior in wear resistance as compared with high chromium cast iron and high speed steel, various improvements have been made.

  JP-A-2004-68142 (Patent Document 1) describes C: 2.9 to 3.8%, Si: 0.8 to 2.0%, Mn: 0.2 to 1.5% on the basis of mass as an outer layer material used for the outer layer of the composite roll for hot rolling. , Cr: 1.5-3.5%, Mo: 0.8-3.5%, Ni: 3.0-7.0%, V: 1.0-3.5%, Nb: 0.1-0.8%, B: 0.020-0.2%, REM: 0.002-0.030% And formula (1): [2.5 ≦ C− (0.236 × V + 0.129 × Nb) ≦ 3.2] and formula (2): [0.5 <Cr / C <1.0], and the balance Fe and inevitable A roll outer roll material for hot rolling having a composition comprising impurities and excellent in seizure resistance and wear resistance is disclosed. The composite roll having the outer layer material has both seizure resistance and wear resistance, and is described as being suitable for a roll for a subsequent stand for hot rolling of a steel sheet.

  Among the test materials described in Patent Document 1, (1) C: 3.76%, Cr: 1.67%, V: 2.38%, Nb: 0.92%, Ni: 4.62%, Si: 1.64%, Mn: 0.54%, Mo : Test material L containing 1.30%, B: 0.054%, and REM: 0.011% has a problem that strength and toughness are reduced due to excess of graphite and carbides due to the high C content of 3.76%. (2) C: 3.46%, Cr: 2.27%, V: 2.46%, Nb: 0.83%, Ni: 4.55%, Si: 1.78%, Mn: 0.61%, Mo: 0.66%, B: 0.042%, and The test material T containing REM: 0.019% has a problem that the distribution of MC carbides is biased toward the inner periphery of the outer layer because the V / Nb ratio is too large (2.96). (3) C: 3.35%, Cr : Test material W containing 1.97%, V: 3.27%, Nb: 1.57%, Ni: 4.61%, Si: 1.91%, Mn: 0.5%, Mo: 1.32%, and B: 0.074%, B content Has a problem that the whitening effect becomes strong and the graphite is difficult to crystallize.

Japanese Patent Application Laid-Open No. 2001-279367 (Patent Document 2) has C: 2.7 to 4.0%, Si: 0.5 to 2%, Mn: 0.2 to 2%, Mo: 0.2 to 0.8%, Ni: 2.5 to 6.0% on a mass basis , Cr: 1.0-2.0%, B: 0.01-0.1%, Nb: 0.2-1.0% and V: 0.2-2.0%, 1 or 2 types, the balance being Fe and inevitable impurities A centrifugal casting roll for hot rolling comprising an outer layer made of the above and an inner layer made of high-grade cast iron or spheroidal graphite cast iron is disclosed. In the outer layer of this centrifugal casting roll, an appropriate amount of Nb and V is added to the basic composition of a conventional high alloy glen cast iron material, and an appropriate amount of B and the like are added, so that the formed carbide is M ₃ C type ( (Cementite) is the main component, and MC carbide is less than 3% or less, and fine graphite particles are uniformly distributed in the cast structure. Patent Document 2 provides a roll for hot rolling excellent in wear resistance and crack resistance without containing a large amount of an expensive alloy at low cost by a centrifugal casting method.

  In Patent Document 2, as test materials for outer layers, C: 3.2%, Si: 2.4%, Mn: 0.8%, Mo: 0.6%, Ni: 4.2%, Cr: 2.2%, Nb: 1.5%, V: 3.5% , And B: Test material 11 containing 0.04% is described. However, since the test material 11 has too much V content of 3.5%, the MC carbide tends to be biased toward the inner peripheral side of the outer layer.

  Japanese Patent Application Laid-Open No. 2008-50681 (Patent Document 3) has C: 2.5 to 3.4%, Si: 0.5 to 2.0%, Mn: 0.5 to 1.0%, Ni: 3.0 to 6.0%, Cr: 1.0 to 2.0% on a mass basis , Mo: 0.2 to 0.8%, V: 1.0 to 4.0%, Nb: 0.2 to 1.0%, and the mass% of V and Nb is represented by the formula (1): 1 ≦ (V−1) / Nb ≦ 4, And the outer layer material of the composite roll for rolling by centrifugal casting that satisfies the formula (2): 0.7 ≦ 1.8 (V−1) + Nb ≦ 5.0, and the balance Fe and inevitable impurities. Patent Document 3 describes that the outer layer material is excellent in wear resistance and is free from roughening due to gravity segregation and crystallization of coarse MC carbides.

  Patent Document 3 includes C: 3.1%, Si: 1.8%, Mn: 0.8%, Ni: 4.5%, Cr: 1.1%, Mo: 0.3%, V: 3.0%, Nb: 1.0%, B: 0.03% The test piece for outer layers to contain is described. However, this test piece has a problem that the distribution of MC carbides is biased toward the inner peripheral side of the outer layer because the V / Nb ratio is too large as 3.

  JP-A-2017-185548 (Patent Document 4) is C: 2.6-3.8%, Si: 0.1-3.0%, Mn: 0.3-2.0%, Ni: 2.3-5.5%, Cr: 0.5-2.5% on a mass basis , Mo: 0.2 to 3.0%, V: 0.2 to 3.8%, Nb: 0.4 to 6.8%, the balance is composed of Fe and inevitable impurities, and has a structure containing 0.3 to 10% graphite particles on an area basis It has an outer layer and an axial core portion made of ductile cast iron, and the C content at positions 100 mm away from both end faces of the outer layer in the axial direction is the C content in the center of the axial length of the outer layer. On the other hand, the Nb content at a position that is 0.05 to 0.3% by mass more and 100 mm away from both end faces of the outer layer in the axial direction is 0.5 to 3.0% by mass with respect to the Nb content at the center of the axial length of the outer layer % Centrifugal casting composite rolls for hot rolling are disclosed. This roll has local wear on the outer layer so that the wear of the outer layer center portion relative to the width center portion of the steel sheet to be rolled and the outer layer end portions opposite to the width end portions on both sides of the steel plate progress on average. It has an outer layer that can be reduced and has excellent wear resistance. When the outer layer wears in the roll axis direction on average, when the roll is subjected to rolling again, the loss of the outer layer material is suppressed with little loss until the locally worn portion disappears during roll polishing. Can be reduced.

  However, the outer layer of Example 2 of Patent Document 4 is C: 3.51%, Si: 1.45%, Mn: 0.65%, Ni: 4.51%, Cr: 1.58%, Mo: 0.64%, V: 2.55%, Nb: Although it contains 0.92%, since the V / Nb ratio is too large at 2.77, the distribution of MC carbide tends to be biased toward the inner peripheral side of the outer layer.

  In a plurality of stands in a 7-stand finish rolling mill, composite rolls of the same material are usually used. For example, in the rear stage stand, composite rolls of the same material are used in the fifth to seventh stands. In this case, the speed of the rolled material is the fastest in the final seventh stand that rolls the thinnest rolled material, but the larger the roll diameter, the lower the rotation, so the first stand uses a composite roll with the initial diameter. . In addition, the larger the roll diameter, the longer the rolled material per rotation and the lower the number of roll rotations per rolling amount, so the larger diameter roll is damaged on the outer surface of the roll, such as surface damage, heat cracks, etc. Is advantageous to. On the other hand, the influence on the surface quality of the rolled material is greater in the downstream stand, and in the fifth to seventh stands, the seventh stand is the largest. Therefore, the 7th stand uses a large initial roll, the 6th stand uses a reworked roll, and the 5th stand uses a further refurbished and reduced diameter roll (a roll close to the scrap diameter). Is often used. As described above, the composite roll whose effective diameter is reduced by the refurbishment can be transferred from the seventh stand to the sixth stand and from the sixth stand to the fifth stand.

  The composite roll of the final seventh stand is hardened due to the lowest temperature and is in contact with the longest rolled material, so it is susceptible to wear. As the roll wear progresses, not only the shape of the rolled material deteriorates, but also the rough surface of the roll is transferred to the surface of the rolled material to deteriorate the surface quality of the rolled material. Therefore, in the final seventh stand in which the shape and surface quality of the rolled material are the final quality of the hot rolled material, there is a great demand for wear resistance and rough skin resistance of the roll.

  In contrast, in the composite rolls used for the sixth stand and the fifth stand, since the temperature of the rolled material is high, heat cracks are not easily generated on the outer layer surface, but the temperature of the rolled material is higher than that of the seventh stand. Since the extension is short, it is not worn as much as the seventh stand, and the influence on the surface quality of the rolled material is small.

  Thus, although the factors for the surface damage of the outer layer of the roll in the fifth to seventh stands are different, there has never been a composite roll that can cope with all of these different factors by one. The seventh stand and the sixth stand may use a composite roll of the same outer layer material, and the fifth stand may use a composite roll of an outer layer material different from the seventh stand and the sixth stand. However, since the required characteristics are different, it is impossible to satisfy both wear resistance and heat crack resistance with a composite roll of the same outer layer material.

  Therefore, when the effective diameter is large (close to the initial diameter), it is suitable for use in the final seventh stand because of its high wear resistance, but when the effective diameter becomes small (close to the scrap diameter) by refurbishing. A centrifugal cast composite roll for hot rolling that can exhibit the characteristics of improved heat crack resistance and suitable for the sixth stand and the fifth stand is desired.

JP 2004-68142 A JP 2001-279367 A JP 2008-50681 A JP-A-2017-185548

  Therefore, the object of the present invention is to provide high wear resistance when the effective diameter is large (close to the initial diameter), and heat crack resistance is improved when the effective diameter is reduced by cutting (close to the scrap diameter). It is to provide a centrifugal cast composite roll for hot rolling and a method for producing the same.

  As a result of diligent research in view of the above object, the present inventor found that (a) MC carbide contributing to wear resistance mainly from the initial diameter to the scrap diameter in the outer layer of the hot rolling composite roll used in the hot strip mill. (B) V equivalent Veq1 in the region (surface layer) from the initial diameter to the depth of 30 mm in the radial direction and V equivalent Veq2 in the scrap diameter is Veq1 / Veq2 = 1.1-5 By optimizing the composition of the constituent elements including V and Nb so as to satisfy the conditions, and limiting the V / Nb ratio to a predetermined range, the wear resistance is improved when the effective diameter is large (close to the initial diameter). Although it is large, it was discovered that the heat crack resistance is improved as the effective diameter is reduced (approaching the scrap diameter) by refurbishing, and the present invention has been conceived.

That is, the centrifugal cast composite roll for hot rolling of the present invention is C: 2.6-3.6%, Si: 0.1-3%, Mn: 0.3-2%, Ni: 2.3-5.5%, Cr: 0.5- 3.2%, Mo: 0.3-1.6%, V: 0.2-3.4%, Nb: 0.4-3%, and B: 0.06% or less, 0.07 ≦ V / Nb ≦ 2.7, V equivalent (Veq = V + 0 .55Nb) is 2.50% by mass or more, the inner layer made of iron-based alloy is welded and integrated with the outer layer made of Fe-based alloy having the chemical composition consisting of Fe and impurities as the balance, and the V equivalent of the outer layer is Following formula:
Veq1 / Veq2 = 1.1-5
(Where Veq1 is the V equivalent of the region from the initial diameter to the depth of 30 mm in the radial direction, and Veq2 is the V equivalent of the scrap diameter).

  In the centrifugally cast composite roll for hot rolling of the present invention, the outer layer is further based on mass: W: 0.01 to 3%, Ti: 0.01 to 0.5%, Al: 0.001 to 0.5%, Zr: 0.01 to 0.5%, and Co : It is preferable to contain any 1 or more types among 0.1 to 5%.

  In the centrifugally cast composite roll for hot rolling according to the present invention, it is preferable that the V equivalent of the outer layer satisfies the condition of Veq1 / Veq2 = 1.15-3, and more preferably satisfies the condition of Veq1 / Veq2 = 1.15-2.

  In the centrifugally cast composite roll for hot rolling according to the present invention, the outer layer preferably contains 0.3 to 5% graphite particles and 2 to 20% MC carbide on an area basis.

  The manufacturing method of the centrifugal cast composite roll for hot rolling according to the present invention includes C: 2.6 to 3.6%, Si: 0.1 to 3%, Mn: 0.3 to 2%, Ni: Containing 2.3 to 5.5%, Cr: 0.5 to 3.2%, Mo: 0.3 to 1.6%, V: 0.2 to 3.4%, Nb: 0.4 to 3%, and B: 0.06% or less, 0.07 ≦ V / Nb ≦ 2.7 V-equivalent (Veq = V + 0.55Nb) is 2.50 mass% or more, and the Fe-based alloy molten metal for the outer layer having the chemical composition consisting of Fe and impurities in the balance is used as the austenite precipitation start temperature + (30 to 150) ° C. The outer layer is cast by casting at a temperature of 5 ° C., a centrifugal force in the range of 60 to 200 G at a gravity multiple, and an average lamination speed of 0.5 to 3 mm / s.

  The outer layer molten metal is further selected from the group consisting of W: 0.01 to 3%, Ti: 0.01 to 0.5%, Al: 0.001 to 0.5%, Zr: 0.01 to 0.5%, and Co: 0.1 to 5% on a mass basis. It is preferable to contain at least one kind.

  In the outer layer of the centrifugal cast composite roll for hot rolling according to the present invention, Veq1 in the region (surface layer portion) from the initial diameter to the depth of 30 mm in the radial direction and Veq2 at the scrap diameter are Veq1 / Veq2 = 1.1-5. Since the condition is satisfied, the surface layer portion having a large effective diameter is excellent in wear resistance, and the deep portion having a small effective diameter close to the discarded diameter has improved heat crack resistance. Therefore, the centrifugal cast composite roll for hot rolling according to the present invention can be used for (a) a high wear resistance by being used in a stand on the downstream side of a finish rolling mill of a hot strip mill when the effective diameter is large. ) When the effective diameter is reduced by surface modification, there is an advantage that high heat crack resistance can be utilized by using it for the stand on the upstream side.

  The centrifugal cast composite roll for hot rolling of the present invention is suitable for use as a work roll of a hot strip mill having severe rolling conditions. Of course, it can also be used as a hot rolling roll for wire rods, a hot rolling roll for section steel, etc. Can be used.

It is a schematic sectional drawing which shows the centrifugal cast composite roll for hot rolling of this invention. It is a graph which shows roughly distribution of MC carbide in the use range of an outer layer. It is a disassembled sectional view which shows an example of the casting_mold | template used for manufacture of the centrifugal cast composite roll for hot rolling of this invention. It is sectional drawing which shows an example of the casting_mold | template used for manufacture of the centrifugal cast composite roll for hot rolling of this invention. It is a graph which shows distribution of Veq in the outer layer of Examples 1-3 and Comparative Examples 1-3.

  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] Centrifugal cast composite roll for hot rolling FIG. 1 shows a hot roll composite roll 10 comprising an outer layer 1 formed by centrifugal casting and an inner layer 2 welded and integrated with the outer layer 1. The inner layer 2 has a trunk core portion 21 welded to the outer layer 1 and shaft portions 22 and 23 extending integrally from both ends of the trunk core portion 21.

(A) Outer layer
(i) Composition The composition of the outer layer is represented by the composition of the molten iron-base alloy used to form the outer layer. The composition of the iron-based alloy melt corresponds to the average composition of the entire outer layer. The Fe-based alloy that forms the outer layer is basically in the category of “high alloy glen cast iron”.

(a) Essential elements
(1) C: 2.6-3.6 mass%
C combines with V, Nb, Cr and Mo 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 2.6% 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.

  On the other hand, if C exceeds 3.6% by mass, the graphite becomes excessive and the shape thereof becomes string-like, which decreases the strength of the outer layer. 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 2.7% by mass, more preferably 2.8% by mass. The upper limit of the C content is preferably 3.5% by mass, more preferably 3.4% by mass.

(2) Si: 0.1-3 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 0.1% by mass, the deoxidation effect of the molten metal is insufficient, and the effect of crystallization of graphite is small. On the other hand, if Si exceeds 3% by mass, the alloy matrix becomes brittle and the toughness of the outer layer decreases. The lower limit of the Si content is preferably 0.5% by mass, more preferably 1% by mass. The upper limit of the Si content is preferably 2.8% by mass, more preferably 2.5% by mass.

(3) Mn: 0.3-2 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 0.3% by mass, those effects are insufficient. On the other hand, even if Mn exceeds 2% by mass, further effects cannot be obtained. The lower limit of the Mn content is preferably 0.4% by mass, more preferably 0.5% by mass. The upper limit of the Mn content is preferably 1.5% by mass, more preferably 1% by mass.

(4) Ni: 2.3-5.5% 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. If Ni is less than 2.3% by mass, the effect cannot be obtained sufficiently. On the other hand, when Ni exceeds 5.5% by mass, austenite is excessively stabilized, and transformation to bainite or martensite is difficult. The lower limit of the Ni content is preferably 2.5% by mass, more preferably 3% by mass, and still more preferably 3.5% by mass. The upper limit of the Ni content is preferably 5% by mass, more preferably 4.8% by mass.

(5) Cr: 0.5-3.2% by 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 0.5% by mass, the effect of addition is insufficient. On the other hand, if Cr exceeds 3.2% by mass, not only the crystallization of graphite is inhibited, but coarse eutectic carbides are formed and the toughness of the matrix structure is lowered. The lower limit of the Cr content is preferably 0.7% by mass, more preferably 1% by mass. The upper limit of the Cr content is preferably 2.8% by mass, more preferably 2.5% by mass, and still more preferably 2.3% by mass.

(6) Mo: 0.3-1.6% by mass
Mo combines with C to form hard Mo carbides, increasing the hardness of the outer layer and improving the hardenability of the matrix. If Mo is less than 0.3% by mass, those effects are insufficient. On the other hand, if the Mo content exceeds 1.6% by mass, the toughness of the outer layer is deteriorated and the tendency to whitening becomes strong, which inhibits the crystallization of graphite. The lower limit of the Mo content is preferably 0.4% by mass. The upper limit of the Mo content is preferably 1.3% by mass, more preferably 1% by mass.

(7) V: 0.2-3.4% by mass
V is an element that combines with C to form hard MC carbide. When V is less than 0.2% by mass, the amount of crystallization of MC carbide is insufficient. On the other hand, when V exceeds 3.4% by mass, (a) VC carbide with a low specific gravity is concentrated inside the outer layer due to centrifugal force during centrifugal casting, and the range in which the distribution of MC carbide is maximized is discarded from the initial diameter of the outer layer. Not only does it deviate from the area up to the rejection diameter, but (b) the MC carbides become coarse and the alloy structure becomes rough, and the surface becomes rough during rolling. The lower limit of the V content is preferably 0.5% by mass, more preferably 1.2% by mass, still more preferably 1.5% by mass, and most preferably 1.8% by mass. The upper limit of the V content is preferably 3% by mass, more preferably 2.7% by mass, and most preferably 2.5% by mass.

(8) Nb: 0.4-3 mass%
Nb combines with C to form MC carbide. By combining Nb with V, (a) not only strengthens MC carbide by solid solution in MC carbide and improves the wear resistance of the outer layer, but also increases (a) the specific gravity of MC carbide, MC carbide Prevents segregation on the inner peripheral side of the outer layer. If Nb is less than 0.4% by mass, these effects are insufficient. On the other hand, when Nb exceeds 3% by mass, the specific gravity of MC carbide becomes too large, and the distribution of MC carbide becomes too large in the deleted region from the outer periphery to the initial diameter. The lower limit of the Nb content is preferably 0.6% by mass, more preferably 0.8% by mass. The upper limit of the Nb content is preferably 2.5% by mass, more preferably 2% by mass.

(9) B: 0.06% by mass or less
B has the effect of refining the carbide. A small amount of B contributes to crystallization of graphite. However, if B exceeds 0.06% by mass, the whitening effect becomes strong and the graphite becomes difficult to crystallize. Therefore, the B content is 0.06% by mass or less. In order to obtain the effect of adding B, the lower limit of the B content is preferably 0.001% by mass, more preferably 0.002% by mass. The upper limit of the B content is preferably 0.04% by mass.

(10) V / Nb
MC carbides composed of VC and NbC crystallize in the outer layer formed by centrifugal casting of an iron-based alloy containing V and Nb, but since VC has a specific gravity smaller than the base, it tends to segregate to the inner peripheral side of the outer layer. Since NbC has a specific gravity larger than that of the base, it tends to segregate on the outer peripheral side of the outer layer. By the way, as shown in FIG. 2, the outer layer formed by centrifugal casting is cut and removed on the outer peripheral side to the depth of the initial diameter Di (generally 10 mm), and then the discarded diameter Dd ( For example, since MC is used from the initial diameter Di to a depth of 50 mm, it is preferable that MC carbide is mainly distributed in the use range (effective diameter range) from the initial diameter Di to the disposal diameter Dd. An example of such MC carbide distribution is shown in FIG. The MC carbide distribution I has a peak located substantially at the center of the use area, and the MC carbide distribution II has a peak located near the initial diameter Di, which is a preferable MC carbide distribution. On the other hand, the MC carbide distribution III has a peak located near the inner layer side, but is outside the range of use, so that there is no sufficient MC carbide in the use range and sufficient wear resistance cannot be exhibited.

  To have high wear resistance when the effective diameter is large (close to the initial diameter Di), and to improve heat crack resistance when the effective diameter is reduced by cutting (close to the scrap diameter Dd) As described later, the distribution of V equivalents in the outer layer must satisfy the condition of Veq1 / Veq2 = 1.1-5.

  In order to satisfy the above conditions, it is necessary to satisfy the condition of 0.07 ≦ V / Nb ≦ 2.7. When V / Nb exceeds 2.7, MC carbide with a small specific gravity increases and segregates more on the inner circumference side of the outer layer of centrifugal casting, not only reducing the distribution of MC carbide in the use area, but also Veq1 / Veq2 = The condition of 1.1-5 will not be met. On the other hand, when V / Nb is less than 0.07, MC carbide having a large specific gravity increases, and segregates excessively on the outer peripheral side of the centrifugal casting outer layer. The lower limit of V / Nb is preferably 0.5, more preferably 1, and most preferably 1.2. On the other hand, the upper limit of V / Nb is preferably 2.65, more preferably 2.6.

(11) V equivalent The amount of MC carbide formed in the outer layer can be expressed by V equivalent (Veq = V + 0.55Nb). The larger Veq, the more MC carbides are crystallized. Veq is preferably 2.50% by mass or more. When Veq is less than 2.50% by mass, there is too little MC carbide effective for wear resistance. On the other hand, when Veq increases too much, cementite and graphite other than MC carbides decrease too much. In order to secure the required amount of cementite and graphite, it is sufficient to add C according to the MC carbide. However, if too much C and Veq are added, the amount of MC carbide that solidifies faster than the iron base becomes too large. If there is a large amount of MC carbide in the liquid phase during solidification, segregation of MC carbide tends to occur. For this reason, Veq is preferably 5% by mass or less, more preferably 4% by mass or less, and most preferably 3.5% by mass or less. Further, the lower limit of Veq is preferably 2.55% by mass, more preferably 2.60% by mass, and most preferably 2.65% by mass.

(12) Distribution of V equivalents The distribution of MC carbides in the outer layer use range is evaluated by the ratio Veq1 / Veq2 of Veq1 in the region from the initial diameter Di to the depth of 30 mm in the radial direction and Veq2 at the disposal diameter Dd. be able to. Here, the “region from the initial diameter to the depth of 30 mm in the radial direction” is, as shown in FIG. 2, between the initial diameter Di and the diameter D ₃₀ at the depth of 30 mm in the radial direction from the initial diameter Di. Means an area. Veq1 is the average value of V equivalent Veq measured at the initial diameter position, the position from the initial diameter to the radial inner position of 10 mm, the initial diameter to the radial inner position of 20 mm, and the initial diameter to the radial inner position of 30 mm. is there. The region from Hatsu径Di to a diameter D ₃₀ is referred to as "surface layer portion", the area up waste却径Dd exceed the diameter D ₃₀ is referred to as "deep."

  The outer layer of centrifugal casting of the present invention must satisfy the condition of Veq1 / Veq2 = 1.1-5. By satisfying the condition of Veq1 / Veq2 = 1.1 to 5, the outer layer surface part exhibits sufficient wear resistance due to MC carbide, and the deep part with a small effective diameter close to the discarded diameter Dd has heat crack resistance. Become good. Therefore, (a) the outer layer exhibits sufficient wear resistance when using large diameter rolls used in the stand on the downstream side of the finish rolling mill of the hot strip mill (for example, the seventh stand), and (b) As the roll diameter gradually decreases, heat crack resistance suitable for the upstream stand can be exhibited. The lower limit of Veq1 / Veq2 is preferably 1.15. The upper limit of Veq1 / Veq2 is preferably 3, and more preferably 2.

(b) Arbitrary composition The outer layer of the centrifugal cast composite roll for hot rolling of the present invention may contain the following elements in addition to the essential elements.

(1) W: 0.01-3 mass%
W combines with C to produce a carbide of M ₂ C rigid, it contributes to improve 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, when W exceeds 3% 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 3 mass% or less. The lower limit of the W content is more preferably 0.02% by mass. The upper limit of the W content is more preferably 2.9% by mass.

(2) Zr: 0.01-0.5% by 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, when Zr is added, its 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 lower limit of the Zr content is more preferably 0.02% by mass. The upper limit of the Zr content is more preferably 0.4% by mass.

(3) Co: 0.1-5% by mass
Co is an element effective for strengthening the base organization. Co also facilitates crystallization of graphite. However, when Co exceeds 5% by mass, the toughness of the outer layer decreases. Therefore, when Co is added, its content is preferably 5% by mass or less. On the other hand, if Co is less than 0.1% by mass, the effect of addition is insufficient. The lower limit of the Co content is more preferably 0.2% by mass. The upper limit of the Co content is more preferably 4.9% by mass.

(4) Ti: 0.01 to 0.5 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, when Ti exceeds 0.5 mass%, the viscosity of the molten metal increases and casting defects are likely to occur. Therefore, when adding Ti, the preferable content is 0.5 mass% or less. On the other hand, when Ti is less than 0.01% by mass, the effect of addition is insufficient. The lower limit of the Ti content is more preferably 0.02% by mass. The upper limit of the Ti content is more preferably 0.4% by mass.

(5) Al: 0.001 to 0.5 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 0.5% by mass, the outer layer becomes brittle, leading to deterioration of mechanical properties. Therefore, when adding Al, the preferable content is 0.5 mass% or less. On the other hand, when the Al content is less than 0.001% by mass, the effect of addition is insufficient. The lower limit of the Al content is more preferably 0.01% by mass, and still more preferably 0.02% by mass. The upper limit of the Al content is more preferably 0.4% by mass.

(c) Impurities The balance of the outer layer composition consists of Fe and impurities. P, S, Cu, and the like are impurity elements and are inevitably mixed in, but are known to affect graphite formation. In particular, P and S are known to affect graphite even in trace amounts. Also in the centrifugal cast composite roll of the present invention, it is necessary to control the content of the impurity element in order to obtain graphite particles having a preferable area ratio. Further, since P and S cause deterioration of mechanical properties, the content thereof must be suppressed to a predetermined level. Cu also affects graphite, but its degree of influence is small. Other inevitable impurities include elements such as Ca, Ba, Mg, Sb, Te, and Ce. Specifically, the contents of P and S are each 0.1% by mass or less, Cu is 0.5% by mass or less, Ca and Ba are each 0.05% by mass or less, Mg is 0.07% by mass or less, Sb is 0.05% by mass or less, Te And Ce within the range of 0.03% by mass or less are acceptable because the effects of the present invention are hardly impaired.

(ii) Structure The structure of the outer layer of the centrifugal cast composite roll for hot rolling according to the present invention is composed of matrix, graphite, MC carbide and cementite. The outer layer structure of the centrifugally cast composite roll for hot rolling according to the present invention preferably contains 0.3 to 5% graphite particles and 2 to 20% MC carbide on an area basis. 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 particle area ratio: 0.3 to 5%
The area ratio of graphite particles crystallized in the outer layer structure is 0.3 to 5%. If the area ratio of the graphite particles 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 particles exceed 5% by area, the mechanical properties of the outer layer deteriorate. The lower limit of the area of the graphite particles is preferably 0.5%, more preferably 1%. On the other hand, the upper limit of the area of the graphite particles is preferably 4%, more preferably 3%.

(b) MC carbide area ratio: 2-20%
If the area ratio of MC carbide crystallized in the outer layer structure is less than 2%, 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%. The area ratio of MC carbide is more preferably 2.2% or more, and further preferably 2.5% or more. In order to make the area ratio of graphite particles 0.3 to 5%, MC carbide is more preferably 17% or less, further preferably 15% or less, and most preferably 10% or less.

(B) Inner layer The iron-based alloy forming the inner layer is preferably tough ductile cast iron. The composition of ductile iron is C: 2.3-3.6%, Si: 1.5-3.5%, Mn: 0.2-2%, Ni: 0.3-2%, Cr: 0.05-1%, Mo: 0.05-1% on a mass basis Mg: 0.01 to 0.08%, and V: 0.05 to 1%, preferably consisting of the remainder Fe and impurities. In addition to the above essential elements, Nb: 0.7% or less and W: 0.7% or less may be contained. Ductile cast iron mainly contains ferrite and pearlite at the iron base, and mainly contains graphite and a small amount of cementite. An intermediate layer may be interposed between the outer layer and the inner layer for the purpose of suppressing component mixing or buffering.

[2] Method for Producing Centrifugal Cast Composite Roll for Hot Rolling The method for producing a centrifugal cast composite roll for hot rolling according to the present invention includes C: 2.6 to 3.6%, Si: 0.1-3%, Mn: 0.3-2%, Ni: 2.3-5.5%, Cr: 0.5-3.2%, Mo: 0.3-1.6%, V: 0.2-3.4%, Nb: 0.4-3%, and B: Contains 0.06% or less, 0.07 ≦ V / Nb ≦ 2.7, V equivalent (Veq = V + 0.55Nb) is 2.50% by mass or more, and the balance has a chemical composition composed of Fe-based alloy composed of Fe and impurities The outer layer molten metal is cast at an austenite precipitation start temperature + (30 to 150) ° C, a centrifugal force in the range of 60 to 200 G as a gravity multiple, and an average lamination speed of 0.5 to 3 mm / s. It is characterized by casting. The lower limit of the average lamination speed is preferably 0.6 mm / s, and the upper limit is preferably 2.5 mm / s. Here, the “average lamination rate” of the outer layer is a value obtained by dividing the thickness of the outer layer laminated by casting by the casting time, that is, the rate of increase in thickness of the outer layer per unit time.

  3 (a) and 3 (b) show an example of a stationary casting mold used for casting the inner layer 2 after centrifugal casting of the outer layer 1 with a centrifugal casting cylindrical mold 30. FIG. The stationary casting mold 100 includes a cylindrical mold 30 having an outer layer 1 on the inner surface, and an upper mold 40 and a lower mold 50 provided at the upper and lower ends thereof. The cylindrical mold 30 includes a mold body 31, a sand mold 32 formed inside the mold body 31, and a sand mold 33 formed at the lower end portion of the mold body 31 and the sand mold 32. The upper mold 40 includes a mold body 41 and a sand mold 42 formed on the inside thereof. The lower mold 50 includes a mold body 51 and a sand mold 52 formed on the inside thereof. The lower mold 50 is provided with a bottom plate 53 for holding the inner layer molten metal. The inner surface of the outer layer 1 in the cylindrical mold 30 has a cavity 60a for forming the trunk core portion 21 of the inner layer 2, and the upper die 40 has a cavity 60b for forming the shaft portion 23 of the inner layer 2, The lower mold 50 has a cavity 60 c for forming the shaft portion 22 of the inner layer 2. The centrifugal casting method using the cylindrical mold 30 may be any of horizontal type, inclined type and vertical type.

  On the upper end portion 54 of the lower mold 50 for forming the shaft portion 22, the cylindrical mold 30 obtained by centrifugally casting the outer layer 1 is placed upright, and the upper mold 40 for forming the shaft portion 23 is placed on the cylindrical mold 30. When installed, a stationary casting mold 100 is constructed. In the stationary casting mold 100, the cavity 60a in the outer layer 1 communicates with the cavity 60b of the upper die 40 and the cavity 60c of the lower die 50, thereby forming a cavity 60 that integrally forms the entire inner layer 1.

  After the outer layer 1 formed by the centrifugal casting method is solidified, as the ductile cast iron melt for the inner layer 2 is injected into the cavity 60 from the upper opening 43 of the upper mold 40, the molten metal surface in the cavity 60 becomes the lower mold 50. The inner layer 2 including the shaft portion 22, the trunk core portion 21, and the shaft portion 23 is integrally cast.

  The present invention will be described in more detail with reference to examples, but the present invention is not limited thereto.

Examples 1-3 and Comparative Examples 1-3
A cylindrical mold 30 (with an inner diameter of 800 mm and a length of 2500 mm) having the structure shown in FIG. 3 (a) is installed in a horizontal centrifugal casting machine, and the melts having the compositions shown in Table 1 are prepared at the temperatures shown in Table 2. The outer layer 1 was centrifugally cast at a gravity multiple and an average lamination speed. After the outer layer 1 is solidified, the cylindrical mold 30 with the outer layer 1 (thickness: 90 mm) formed on the inner surface is erected, and a hollow lower mold 50 (inner diameter 600 mm, length 1500 for forming the shaft portion 22) 3) and a hollow upper mold 40 (inner diameter of 600 mm and length of 2000 mm) for forming the shaft portion 23 is erected on the cylindrical mold 30 and FIG. A stationary casting mold 100 shown in (b) was constructed.

  In cavity 60 of stationary casting mold 100, C: 3.0%, Si: 2.6%, Mn: 0.3%, Ni: 1.4%, Cr: 0.1%, Mo: 0.2%, Mg: 0.05%, P : 0.03%, and S: 0.03%, ductile cast iron melt having a chemical composition with the balance being Fe and impurities is poured from the upper opening 43, and a graphitized inoculum containing Si is inoculated in the middle. A composite roll in which the inner layer 2 was integrally welded to the inner surface of the outer layer 1 was produced.

注：(1) 外層の平均組成に相当する。

Note: (1) Corresponds to the average composition of the outer layer.

注：(1) Veq＝V＋0.55Nb（単位：質量％）。
(2) 単位：質量％。

Note: (1) Veq = V + 0.55Nb (unit: mass%).
(2) Unit: mass%.

注：(1) γはオーステナイト析出開始温度。

Note: (1) γ is the austenite precipitation start temperature.

About each obtained centrifugal cast composite roll, distribution of V equivalent in an outer layer was measured with the following method. First, at the end in the longitudinal direction of each outer layer, the position of the initial diameter Di (at a depth of about 10 mm from the black skin), and 10 mm, 20 mm, 30 mm, 40 mm and 50 mm from the initial diameter Di, respectively. Samples for analysis with a thickness of 5 mm were collected at a depth of 5 mm, V and Nb contents were measured by ICP (Inductively Coupled Plasma) emission analysis, and the initial diameter was calculated using the equation Veq = V + 0.55 Nb. V equivalent (Veq ₀ , Veq ₁₀ , Veq ₂₀ , Veq ₃₀ , Veq ₄₀ , Veq ₅₀ ) at depths of 10 mm, 20 mm, 30 mm, 40 mm and 50 mm, respectively, from Di and initial diameter Di It was. The position at a depth of 50 mm corresponds to the position of the disposal diameter Dd, and Veq ₅₀ = Veq2. Veq ₀ , Veq ₁₀ , Veq ₂₀ and Veq ₃₀ were averaged to obtain Veq1, and the ratio of Veq1 / Veq2 was calculated. For each outer layer, the ratio of Veq1 / Veq2 is shown in Table 3, and the distribution of Veq is shown in FIG.

  In the outer layers of Examples 1 to 3, Veq1 / Veq2 was in the range of 1.1 to 5, but in the outer layers of Comparative Examples 1 to 3, Veq1 / Veq2 was not in the range of 1.1 to 5. Therefore, as is clear from FIG. 4, the distribution of Veq1 in the outer layer of Examples 1 to 3 had a peak in the usage range, but in the outer layer of Comparative Examples 1 to 3, the distribution of Veq1 was almost flat in the entire usage range. there were. From this, the composite rolls of Examples 1 to 3 have high wear resistance when close to the initial diameter, and heat crack resistance is improved when close to the scrap diameter by refurbishing, but the rolls of Comparative Examples 1 to 3 It turns out that there is no such function.

  In the longitudinal direction end part of the outer layer of Examples 1 to 3, the position of the initial diameter Di (position of a depth of about 10 mm from the black skin) and 10 mm, 20 mm, 30 mm, and 40 mm from the initial diameter Di, respectively. As a result of microscopic observation of the outer layer structure at a depth of 50 mm and 50 mm, it was confirmed that it had 0.3 to 5% graphite particles and 2 to 20% MC carbide on an area basis.

1 ... Outer layer
2 ... Inner layer
10 ... Centrifugal cast composite roll for hot rolling
21 ... trunk
22, 23 ... Shaft
30 ... Cylindrical mold for centrifugal casting
31, 41, 51 ... Mold body
32, 33, 42, 52 ... sand mold
40 ... Upper mold for stationary casting
50 ... Lower mold for stationary casting
60, 60a, 60b, 60c ... cavity
100 ・ ・ ・ Mold for casting

Claims (6)

C: 2.6 to 3.6% by mass, Si: 0.1 to 3%, Mn: 0.3 to 2%, Ni: 2.3 to 5.5%, Cr: 0.5 to 3.2%, Mo: 0.3 to 1.6%, V: 0.2 to 3.4 %, Nb: 0.4 to 3%, and B: 0.06% or less, 0.07 ≦ V / Nb ≦ 2.7, V equivalent (Veq = V + 0.55Nb) is 2.50% by mass or more, and the balance is Fe and In the centrifugal cast composite roll for hot rolling in which an inner layer made of an iron-based alloy is welded and integrated with an outer layer made of an Fe-based alloy having a chemical composition made of impurities, the V equivalent of the outer layer has the following formula:
Veq1 / Veq2 = 1.1-5
(However, Veq1 is the V equivalent of the region from the initial diameter to the depth of 30 mm in the radial direction, and Veq2 is the V equivalent of the scrap diameter.) Casting composite roll.   The centrifugal cast composite roll for hot rolling according to claim 1, wherein the outer layer is further based on mass: W: 0.01 to 3%, Ti: 0.01 to 0.5%, Al: 0.001 to 0.5%, Zr: 0.01 to 0.5% And Co: a composite roll made of centrifugal cast for rolling, characterized by containing at least one of 0.1 to 5%.   The centrifugal cast composite roll for hot rolling according to any one of claims 1 to 2, wherein a V equivalent of the outer layer satisfies a condition of Veq1 / Veq2 = 1.15-3. roll.   The centrifugal cast composite roll for hot rolling according to any one of claims 1 to 3, wherein the outer layer contains 0.3 to 5% graphite particles and 2 to 20% MC carbide on an area basis. Centrifugal cast composite roll for hot rolling.   A method for producing a centrifugally cast composite roll for hot rolling, wherein C: 2.6 to 3.6%, Si: 0.1 to 3%, Mn: 0.3 to 2%, Ni: Containing 2.3 to 5.5%, Cr: 0.5 to 3.2%, Mo: 0.3 to 1.6%, V: 0.2 to 3.4%, Nb: 0.4 to 3%, and B: 0.06% or less, 0.07 ≦ V / Nb ≦ 2.7 V-equivalent (Veq = V + 0.55Nb) is 2.50 mass% or more, and the Fe-based alloy molten metal for the outer layer having the chemical composition consisting of Fe and impurities in the balance is used as the austenite precipitation start temperature + (30 to 150) ° C. The outer layer is cast by casting at a temperature of 5 ° C, a centrifugal force in the range of 60 to 200 G at a gravity multiple, and an average lamination speed of 0.5 to 3 mm / s.   The centrifugal cast composite roll for hot rolling according to claim 5, wherein the outer layer molten metal is further W: 0.01-3%, Ti: 0.01-0.5%, Al: 0.001-0.5%, Zr: 0.01- A method comprising at least one selected from the group consisting of 0.5% and Co: 0.1 to 5%.

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