阅读说明：本技术 通过离心铸造法制造的轧制用复合辊及其制造方法 (Composite roll for rolling produced by centrifugal casting method and method for producing same ) 是由 上宫田和则 石川晋也 柳鹤彩华 于 2020-03-25 设计创作，主要内容包括：本发明提供具有与高速铸铁辊相当的优异耐磨损性、耐表面粗糙性,并且具有与高合金麻口铸铁辊相当的抗事故性的通过离心铸造法制造的轧制用复合辊。所述通过离心铸造法制造的轧制用复合辊具有外层和内层,所述外层的化学成分以质量比计为C：1.0～3.0％、Si：0.3～3.0％、Mn：0.1～3.0％、Ni：0.1～6.0％、Cr：0.5～6.0％、Mo：0.5～6.0％、V：3.0～7.0％、Nb：0.1～3.0％、B：0.001～0.1％、N：0.005～0.070％、余量：Fe和不可避免的杂质,该外层的化学组成满足以下的式(1),并且,石墨的晶体析出量被抑制为以面积比计小于0.3％,具有以面积比计为1～15％的MC型碳化物,在所述外层与所述内层的边界处,不具有直径φ4mm以上的铸造缺陷,50×N+V＜9.0 (1)。(The invention provides a composite roll for rolling manufactured by a centrifugal casting method, which has excellent wear resistance and surface roughness resistance equivalent to those of a high-speed cast iron roll and accident resistance equivalent to those of a high-alloy pitted cast iron roll. The composite roll for rolling manufactured by the centrifugal casting method comprises an outer layer and an inner layer, wherein the chemical components of the outer layer are C: 1.0-3.0%, Si: 0.3-3.0%, Mn: 0.1 to 3.0%, Ni: 0.1-6.0%, Cr: 0.5 to 6.0%, Mo: 0.5-6.0%, V: 3.0 to 7.0%, Nb: 0.1-3.0%, B: 0.001-0.1%, N: 0.005-0.070%, and the balance: fe and unavoidable impurities, the chemical composition of the outer layer satisfying the following formula (1), and the amount of crystal precipitation of graphite being suppressed to less than 0.3% by area ratio, having 1 to 15% by area ratio of MC type carbide, having no casting defect having a diameter of 4mm or more at the boundary between the outer layer and the inner layer, and having a value of 50 xN + V < 9.0 (1).)

1. A composite roll for rolling manufactured by a centrifugal casting method, characterized by having an outer layer and an inner layer,

the chemical components of the outer layer are calculated according to mass ratio

C：1.0～3.0％、

Si：0.3～3.0％、

Mn：0.1～3.0％、

Ni：0.1～6.0％、

Cr：0.5～6.0％、

Mo：0.5～6.0％、

V：3.0～7.0％、

Nb：0.1～3.0％、

B：0.001～0.1％、

N：0.005～0.070％、

And the balance: fe and inevitable impurities, and the balance of the Fe and the inevitable impurities,

the outer layer has a chemical composition satisfying the following formula (1), and has an MC type carbide in which the amount of graphite crystal precipitation is suppressed to less than 0.3% by area ratio and 1 to 15% by area ratio,

at the boundary between the outer layer and the inner layer, there is no casting defect having a diameter of 4mm or more,

50×N+V＜9.0 (1)。

2. a composite roll for rolling manufactured by a centrifugal casting method, characterized by comprising an outer layer, an intermediate layer and an inner layer,

the chemical components of the outer layer are calculated according to mass ratio

C：1.0～3.0％、

Si：0.3～3.0％、

Mn：0.1～3.0％、

Ni：0.1～6.0％、

Cr：0.5～6.0％、

Mo：0.5～6.0％、

V：3.0～7.0％、

Nb：0.1～3.0％、

B：0.001～0.1％、

N：0.005～0.070％、

And the balance: fe and inevitable impurities, and the balance of the Fe and the inevitable impurities,

the outer layer has a chemical composition satisfying the following formula (1), and has an MC type carbide in which the amount of graphite crystal precipitation is suppressed to less than 0.3% by area ratio and 1 to 15% by area ratio,

at the boundary of the intermediate layer and the inner layer, there is no casting defect having a diameter of 4mm or more,

50×N+V＜9.0 (1)。

3. the composite roll for rolling produced by centrifugal casting according to claim 1 or 2, wherein in the outer layer, the chemical component further contains in mass ratio

Ti：0.005～0.3％、

W：0.01～6.0％、

Co：0.01～2.0％、

S: 0.3% or less of 1 or more.

4. A method of manufacturing a composite roll for rolling by centrifugal casting according to any one of claims 1 to 3,

the relation between the outer layer casting starting temperature (T1) and the outer layer liquidus temperature (T2) in the centrifugal casting method satisfies the following expression (2),

40℃≤T1-T2≤120℃ (2)。

Technical Field

(cross-reference to related applications)

The present application claims priority based on Japanese application No. 2019-071298 filed on 4/3 of 2019, the contents of which are incorporated herein by reference.

The present invention relates to a composite roll for rolling produced by a centrifugal casting method and having excellent wear resistance, crack resistance, and surface roughness resistance, and a method for producing the same.

Background

In recent years, in the field of hot rolling of steel such as section steel, thin plate, and thick plate, there has been an increasing demand for improving the thickness accuracy and surface quality of steel sheet. The rolls for rolling are also required to have high wear resistance, and the application of high-speed cast iron rolls to the front stage of a hot finishing mill for manufacturing a thin steel sheet is being advanced. However, in the latter stage of the hot finishing mill where the probability of a steel seizure accident is high, when the steel seizure accident occurs, deep cracks may occur on the roll surface, and the cracks may be accelerated during the rolling use to cause burst, so that high-alloy ductile cast iron rolls which have been used conventionally are mainly used.

The high-alloy pitted cast iron roll is characterized by consisting of graphite, carbide and a matrix structure, and has extremely little generation and aggravation of cracks even in a steel clamping accident, namely excellent accident resistance. However, their wear resistance is significantly lower than high speed cast iron rolls, and rolls having both accident resistance and wear resistance are needed.

In order to satisfy the requirements for a roll having both accident resistance and wear resistance, patent document 1 discloses a hot rolling roll outer layer material having excellent seizure resistance, which is characterized by having a composition comprising: contains, in mass%, C: 1.8-3.5%, Si: 0.2-2%, Mn: 0.2-2%, Cr: 4-15%, Mo: 2-10%, V: 3-10%, further comprising P: 0.1-0.6%, B: 0.05-5%, and the balance of Fe and inevitable impurities. Patent document 1 describes that the heat treatment after casting is preferably a quenching treatment in which the casting is quenched by heating at 800 to 1080 ℃ and further a tempering treatment is performed 1 or more times at 300 to 600 ℃. However, the roll described in patent document 1 has a problem that the P content is too high, and therefore, the P content segregates at grain boundaries and becomes brittle. In addition, since minute casting defects are likely to occur at the boundary between the outer layer and the inner layer or between the intermediate layer and the inner layer during casting, the frequency of occurrence of cracks during the manufacturing process is high, and there is a technical problem that minute defects remaining in the product grow and progress during the use of rolling, resulting in a high risk of bursting.

Further, patent document 2 discloses a composite roll for rolling, which is characterized by having a structure in which an outer layer made of a centrifugally cast Fe-based alloy and an intermediate layer and an inner layer made of spheroidal graphite cast iron are welded and integrated with each other; the outer layer has the following composition: the alloy comprises, by mass, 1 to 3% of C, 0.3 to 3% of Si, 0.1 to 3% of Mn, 0.5 to 5% of Ni, 1 to 7% of Cr, 2.2 to 8% of Mo, 4 to 7% of V, 0.005 to 0.15% of N, 0.05 to 0.2% of B, and the balance Fe and inevitable impurities; the intermediate layer contains 0.025 to 0.15 mass% of B, the B content of the intermediate layer is 40 to 80% of the B content of the outer layer, and the total content of carbide-forming elements of the intermediate layer is 40 to 90% of the total content of carbide-forming elements of the outer layer. Patent document 2 describes that after casting, quenching treatment is performed as necessary, and tempering treatment is performed 1 or more times. The tempering temperature is preferably 480-580 ℃. However, the roller described in patent document 2 has a high B content, and therefore has a problem that the frequency of occurrence of cracks during the production process is high and the risk of breakage is high. Further, it was found that there was a problem that the segregation phase of B causes surface roughening during use in rolling. In addition, since minute casting defects are likely to occur at the boundary between the outer layer and the inner layer or the boundary between the intermediate layer and the inner layer at the time of casting, there is a technical problem as follows: the frequency of occurrence of breakage during the manufacturing process is high, and the risk of explosion due to the growth and progress of minute defects remaining in the product during the use of rolling is high.

Further, patent document 3 discloses a composite roll for rolling, which is produced by a centrifugal casting method and has an outer layer, wherein the outer layer is C: 2.2% -3.01%, Si: 1.0-3.0%, Mn: 0.3% -2.0%, Ni: 3.0-7.0%, Cr: 0.5% -2.5%, Mo: 1.0% -3.0%, V: 2.5% -5.0%, Nb: greater than 0 and 0.5% or less, the remainder: fe and inevitable impurities, satisfying the condition (a): Nb%/V% < 0.1, condition (b): 2.1 XC% +1.2 XC% -Cr% +0.5 Mo% + (V% + Nb%/2) less than or equal to 13.0%. Patent document 3 describes that the treatment with gammatization heat at 850 ℃ or higher, quenching and tempering can be performed. However, the roller disclosed in patent document 3 has a problem that the wear resistance is significantly reduced as compared with a high-speed roller, and the surface is rough because graphite is excessively crystallized. In addition, since minute casting defects are likely to occur at the boundary between the outer layer and the inner layer or between the intermediate layer and the inner layer during casting, there is a technical problem as follows: the frequency of occurrence of breakage during the manufacturing process is high, and the risk of breakage is high due to the growth and progress of minute defects remaining in the product during the use of rolling.

Documents of the prior art

Patent document

Patent document 1: japanese patent No. 4483585

Patent document 2: international publication No. 2018/147370

Patent document 3: japanese patent No. 6313844

Disclosure of Invention

Problems to be solved by the invention

However, the rolls described in patent documents 1 to 3 have the following technical problems in that fine casting defects are likely to occur at the boundary between the outer layer and the inner layer or at the boundary between the intermediate layer and the inner layer during casting: the frequency of breakage during manufacture is high and, moreover, the risk of remaining micro-defects in the product growing, aggravating and causing bursting during rolling use is high.

In view of these circumstances, an object of the present invention is to provide a composite roll for rolling produced by a centrifugal casting method, which has excellent wear resistance and surface roughening resistance comparable to those of a high-speed cast iron roll, and has accident resistance comparable to those of a high-alloy pitted cast iron roll, and a method for producing the same.

Means for solving the problems

In order to achieve the above object, according to the present invention, there is provided a composite roll for rolling manufactured by a centrifugal casting method, comprising an outer layer and an inner layer, wherein the outer layer has chemical components in a mass ratio

C：1.0～3.0％、

Si：0.3～3.0％、

Mn：0.1～3.0％、

Ni：0.1～6.0％、

Cr：0.5～6.0％、

Mo：0.5～6.0％、

V：3.0～7.0％、

Nb：0.1～3.0％、

B：0.001～0.1％、

N：0.005～0.070％、

And the balance: fe and inevitable impurities, and the balance of the Fe and the inevitable impurities,

the outer layer has a chemical composition satisfying the following formula (1), and has an MC type carbide in which the amount of graphite crystal precipitation is suppressed to less than 0.3% by area ratio and 1 to 15% by area ratio,

at the boundary between the outer layer and the inner layer, there is no casting defect having a diameter of 4mm or more,

50×N+V＜9.0 (1)。

further, according to the present invention, there is provided a composite roll for rolling manufactured by a centrifugal casting method, comprising an outer layer, an intermediate layer and an inner layer, wherein the outer layer has chemical components in a mass ratio

C：1.0～3.0％、

Si：0.3～3.0％、

Mn：0.1～3.0％、

Ni：0.1～6.0％、

Cr：0.5～6.0％、

Mo：0.5～6.0％、

V：3.0～7.0％、

Nb：0.1～3.0％、

B：0.001～0.1％、

N：0.005～0.070％、

And the balance: fe and inevitable impurities, and the balance of the Fe and the inevitable impurities,

the outer layer has a chemical composition satisfying the following formula (1), and has an MC type carbide in which the amount of graphite crystal precipitation is suppressed to less than 0.3% by area ratio and 1 to 15% by area ratio,

at the boundary of the intermediate layer and the inner layer, there is no casting defect having a diameter of 4mm or more,

50×N+V＜9.0 (1)。

in the outer layer, the chemical component further contains a component in mass ratio

Ti：0.005～0.3％、

W：0.01～6.0％、

Co：0.01～2.0％、

S: 0.3% or less of 1 or more.

In addition, from another aspect, the present invention provides a method for manufacturing a composite roll for rolling by centrifugal casting, wherein the relationship between the outer layer casting start temperature (T1) and the outer layer liquidus temperature (T2) in the centrifugal casting satisfies the following expression (2),

40℃≤T1-T2≤120℃ (2)。

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, it is possible to prevent casting defects from occurring at the boundary between the outer layer and the inner layer or between the intermediate layer and the inner layer during casting, and thus it is possible to prevent the problem of breakage during manufacturing, and it is possible to prevent the problem of cracks caused by the growth of minute defects remaining at the boundary in the product during rolling use. As a result, a composite roll for rolling produced by a centrifugal casting method can be produced which has both excellent wear resistance and surface roughening resistance comparable to a high-speed roll and accident resistance comparable to a high-alloy pitted iron roll. The composite roll for rolling produced by the centrifugal casting method of the present invention is suitable for use in a hot strip mill, particularly a post-stage rolling mill for finish hot rolling in which operational stability is sought.

Detailed Description

Hereinafter, embodiments of the present invention will be described. In the present specification, the symbol "%" represents "% by mass".

The composite roll for rolling of the present invention manufactured by centrifugal casting has an outer layer for rolling. Further, an axial core material composed of an intermediate layer and an inner layer, or an axial core material composed of an inner layer is provided inside the outer layer. As the inner layer material constituting the inner layer, there can be exemplified: high-grade cast iron, nodular cast iron and other materials with high toughness; as the intermediate layer material constituting the intermediate layer, there can be exemplified: nickel-chromium wear-resistant cast iron and graphite steel.

The centrifugally cast outer layer is formed of an Fe-based alloy as follows: the alloy comprises, by mass, 1.5 to 3.0% of C, 0.3 to 3.0% of Si, 0.1 to 3.0% of Mn, 0.1 to 6.0% of Ni, 0.5 to 6.0% of Cr, 0.5 to 6.0% of Mo, 3.0 to 7.0% of V, 0.1 to 3.0% of Nb, 0.001 to 0.1% of B, 0.005 to 0.070% of N, and the balance of Fe and unavoidable impurities.

In addition, the outer layer is composed of (a)MC type carbide, (b) with M₃C、M₂C、M₇C₃Eutectic carbide as a main component, (c) a matrix, (d) other components, and 1-15% of MC type carbide. Further, graphite may be contained in the structure of the outer layer, but the presence of graphite is not essential, and for example, the amount of crystal precipitation of graphite is suppressed to less than 0.3%.

(reasons for defining Components)

The reason why the chemical composition of the outer layer of the present invention is limited will be explained below. In the following, the symbol "%" represents "% by mass" unless otherwise specified.

C：1.0～3.0％

C mainly combines with Fe, Cr, Mo, Nb, V, W, etc. to form various hard carbides. In addition, graphite is sometimes formed in some cases. Further, the solid solution is dissolved in the matrix to form pearlite, bainite, martensite, and the like. The larger the content, the more effective the improvement of wear resistance, but when it exceeds 3.0%, coarse carbides and graphite are formed, which causes deterioration of toughness and surface roughness. If the content is less than 1.0%, the amount of carbide is small, and it is difficult to secure hardness, resulting in deterioration of wear resistance. Therefore, the range is set to 1.0 to 3.0%. A more preferable range is 1.5 to 2.5%.

Si：0.3～3.0％

Si is required to suppress the generation of oxide defects by deoxidation of molten steel. In addition, it has the effect of improving the fluidity of molten steel to prevent casting defects. If the content is less than 0.3%, the effect is insufficient, and the risk of casting defects remaining in the rolling layer of the outer layer increases. Therefore, the content is 0.3% or more. However, if it exceeds 3.0%, the toughness is lowered, which causes the crack resistance to be lowered. Therefore, the range is set to 0.3 to 3.0%. More preferably, the content is in the range of 0.5 to 2.0%.

Mn：0.1～3.0％

Mn is added for deoxidation and desulfurization. In addition, MnS is formed in combination with S. MnS has a lubricating effect, and therefore has an effect of preventing sintering of a material to be rolled. Therefore, it is preferable to contain MnS in a range without side effects. When Mn is less than 0.1%, these effects are insufficient; when the content exceeds 3.0%, the toughness is lowered. Therefore, the range is set to 0.1 to 3.0%. A more preferable range is 0.3 to 1.2%.

Ni：0.1～6.0％

Ni has an effect of improving the hardenability of the matrix, and is an element that effectively strengthens the matrix by preventing pearlite formation and promoting bainitization during cooling, and therefore needs to be contained by 0.1% or more. However, if the content exceeds 6.0%, the retained austenite amount becomes too large, it becomes difficult to secure hardness, and deformation, surface roughness, and the like may occur during use of hot rolling. Therefore, the range is set to 0.1 to 6.0%, and a more preferable range is 0.3 to 5.5%.

Cr：0.5～6.0％

Cr is added for the purpose of improving hardenability, improving hardness, improving temper softening resistance, stabilizing carbide hardness, and the like. However, if it exceeds 6.0%, the amount of eutectic carbide becomes too large, and the surface roughness resistance and toughness deteriorate, so the upper limit is set to 6.0%. On the other hand, if the content is less than 0.5%, the above-mentioned effects cannot be obtained. Therefore, the range is set to 0.5 to 6.0%. More preferably in the range of 1.0 to 5.5%.

Mo：0.5～6.0％

Mo mainly combines with C to form hard carbide, contributes to improvement of wear resistance, and improves hardenability of the matrix, and therefore, it is necessary to contain 0.5% or more as the minimum. On the other hand, if it exceeds 6.0%, coarse carbides are formed, and the surface roughness resistance and toughness are lowered. Therefore, the range is set to 0.5 to 6.0%. More preferably, the content is in the range of 0.7 to 5.5%.

V：3.0～7.0％

V is an important element particularly for improving wear resistance. That is, V is an important element that combines with C to form high-hardness MC carbide that greatly contributes to wear resistance. When the amount is less than 3.0%, the amount of MC carbide is insufficient and the improvement of wear resistance is insufficient; if the ratio exceeds 7.0%, a region in which low-density MC carbide is crystallized alone in the form of primary crystals is formed, and when the material is manufactured by the centrifugal casting method, the density of MC carbide is lower than that of molten steel, and therefore, gravity segregation occurs at the boundary portion between the outer layer and the inner layer or the boundary portion between the intermediate layer and the inner layer, and an aggregated portion of MC carbide is formed. The aggregated portion of the MC carbide causes casting defects at the boundary between the outer layer and the inner layer or at the boundary between the intermediate layer and the inner layer. Therefore, the range is set to 3.0 to 7.0%. More preferably, the content is in the range of 3.5 to 6.5%.

Nb：0.1～3.0％

Nb is hardly dissolved in the matrix, and most of Nb forms high-hardness MC carbide, thereby improving wear resistance. In particular, the MC carbide formed by adding Nb has a smaller difference in density from molten steel than the MC carbide formed by adding V, and therefore has the effect of reducing gravity segregation caused by centrifugal casting. When the content of Nb is less than 0.1%, the effect is insufficient; when the content exceeds 3.0%, the MC carbide becomes coarse, resulting in surface roughening and deterioration of toughness. Therefore, the range is set to 0.1 to 3.0%.

B：0.001～0.1％

B is solid-dissolved in carbide and forms a carboride. The carborides have a lubricating effect and have an effect of preventing sintering of the material to be rolled. When the content of B is less than 0.001%, the effect is insufficient; when the content exceeds 0.1%, segregation occurs at the grain boundary, resulting in surface roughness and deterioration of toughness. Therefore, the range is set to 0.001 to 0.1%.

N：0.005～0.070％

N has the effect of refining carbides, combining with V to form nitrides (VN) or carbonitrides (VCN). When the content is less than 0.005%, the refining effect of the carbide is insufficient; when the content exceeds 0.070%, excessive nitride (VN) or carbonitride (VCN) is formed. They are gravity segregated at the boundary between the outer layer and the inner layer or at the boundary between the intermediate layer and the inner layer to form an aggregate of nitrides (VN) or carbonitrides (VCN). These cause casting defects at the boundary between the outer layer and the inner layer or at the boundary between the intermediate layer and the inner layer, and therefore it is necessary to suppress the casting defects to 0.070% or less. Therefore, the range is set to 0.005 to 0.070%.

As described above, the basic component of the outer layer of the present invention may be appropriately selected from the basic components described above and further contain the following chemical components as other chemical components according to the size of the roll to be applied, the required use characteristics of the roll, and the like.

Ti：0.005～0.3％

The composite roll for rolling produced by the centrifugal casting method of the present invention may contain Ti in addition to the above-described essential elements. Ti is expected to outgas N and O and form TiCN or TiC to become the nuclei of MC carbide. When the Ti content is less than 0.005%, the effect thereof cannot be expected; if the content exceeds 0.3%, the viscosity of the molten steel increases, and there is a high risk of casting defects being induced at the boundary between the outer layer and the inner layer or at the boundary between the intermediate layer and the inner layer. Therefore, when Ti is added, the range is set to 0.005 to 0.3%. More preferably, the content is in the range of 0.01 to 0.2%.

W：0.01～6.0％

The composite roll for rolling produced by the centrifugal casting method of the present invention may contain W in addition to the above-described essential elements. W is solid-dissolved in the matrix like Mo to reinforce the matrix, and is combined with C to form M₂C、M₆Hard eutectic carbide such as C contributes to improvement of wear resistance. In order to strengthen the matrix, the minimum content is 0.01% or more, and when it exceeds 6.0%, coarse eutectic carbides are formed, and the surface roughness resistance and toughness are lowered. Therefore, when W is added, the range is set to 0.01 to 6.0%. The selection of whether or not to add W is made, for example, when the amount of eutectic carbide is increased to improve wear resistance, the effect is further enhanced.

Co：0.01～2.0％

The composite roll for rolling according to the present invention produced by centrifugal casting may contain Co in addition to the above-described essential elements. Most of Co is dissolved in the matrix to strengthen the matrix. Therefore, it has an effect of improving hardness and strength at high temperature. When the content is less than 0.01%, the effect is insufficient; if the content exceeds 2.0%, the effect is saturated, and therefore, from the viewpoint of economy, the content is 2.0% or less. Therefore, when Co is added, the range is set to 0.01 to 2.0%. The choice of whether or not to add Co is effective when, for example, it is required to improve wear resistance and increase the amount of eutectic carbide is difficult.

S: less than 0.3%

In general, S is an element that is inevitably mixed into the raw material to some extent, but as described above, it forms MnS to have a lubricating effect, and thus has an effect of preventing sintering of the rolled material. On the other hand, the content in excess may embrittle the material, so that it is preferably limited to 0.3% or less.

[ inevitable impurities ]

The composition of the outer layer of the composite roll for rolling produced by the centrifugal casting method of the present invention is such that the balance is substantially Fe and inevitable impurities, in addition to the above elements. Among the inevitable impurities, P deteriorates toughness, and therefore is preferably limited to 0.1% or less. Further, as other inevitable elements, elements such as Cu, Sb, Sn, Zr, Al, Te, Ce, and the like may be contained within a range not impairing the characteristics of the outer layer. The total amount of inevitable impurities is preferably 0.6% or less so as not to impair the characteristics of the outer layer.

(relational expression relating to chemical composition)

In addition, regarding the chemical composition (chemical composition) of the outer layer of the composite roll for rolling produced by the centrifugal casting method of the present invention, particularly when V, Nb, Mo, Cr as hard carbide forming elements are added, the content (%) of N and V needs to satisfy the following formula (1),

50×N+V＜9.0 (1)。

n has the effect of refining carbides, but bonds with V, Nb, Mo, Cr, which are hard carbide-forming elements, to form nitrides or carbonitrides. In particular, since V is an element having a lower density than molten steel, when excessive nitride (VN) or carbonitride (VCN) is formed, the nitride (VN) or carbonitride (VCN) moves to the inner surface side of the outer layer molten steel by centrifugal force during centrifugal casting, and thereby forms an aggregated portion of the nitride (VN) or carbonitride (VCN).

In the case of inserting the intermediate layer, the intermediate layer is cast after a certain time has elapsed after the outer layer is poured in the centrifugal casting process, and at this time, the intermediate layer is welded to the outer layer by melting the inner surface of the outer layer. At this time, the inner surface portion of the outer layer melted by the molten steel of the intermediate layer and the molten steel of the intermediate layer become mixed molten steel and solidify, thereby forming the intermediate layer portion. On the other hand, when the nitride (VN) or carbonitride (VCN) aggregates are formed on the inner surface of the outer layer, the nitride (VN) or carbonitride (VCN) has a high melting point and does not melt in the molten steel of the intermediate layer. Therefore, the nitride (VN) or carbonitride (VCN) aggregates formed on the inner surface of the outer layer move to the inner surface of the molten steel of the intermediate layer by centrifugal force after the molten steel of the intermediate layer is poured because the density of the nitrides (VN) or carbonitrides (VCN) aggregates is lower than that of the molten steel of the intermediate layer, and the nitrides (VN) or carbonitrides (VCN) aggregates are formed on the inner surface of the intermediate layer.

In the next step, the inner layer molten steel is poured, and when the outer layer or the outer layer and the intermediate layer are solidified by centrifugal casting, the molten steel is taken out from the centrifugal casting machine, assembled with the upper mold and the lower mold, and then cast by static casting. In this case, when the nitride (VN) or carbonitride (VCN) aggregates are formed on the inner surface of the outer layer or the inner surface of the intermediate layer, the nitride (VN) or carbonitride aggregates remain at the boundary between the outer layer and the inner layer or the boundary between the intermediate layer and the inner layer unless the molten steel is melted by the inner layer at the time of casting the inner layer.

However, since the melting point of nitride (VN) or carbonitride (VCN) is much higher than that of molten steel of the inner layer and the inner surface portion of the outer layer or the intermediate layer needs to be melted only at the minimum thickness (about 10mm at the maximum) required for welding by the pouring temperature of the inner layer, it is difficult to set the value of the pouring temperature of the inner layer to a high temperature at which nitride (VN) or carbonitride (VCN) is melted.

Therefore, when the agglomerates of nitride (VN) or carbonitride (VCN) are formed on the inner surface of the outer layer or the intermediate layer at the time of centrifugal casting, it is extremely difficult to avoid the agglomerates of nitride (VN) or carbonitride (VCN) remaining at the boundary between the outer layer and the inner layer of the roll blank or at the boundary between the intermediate layer and the inner layer. Such a nitride (VN) or carbonitride (VCN) aggregate is a cause of forming a casting defect such as a fusion defect or a void at a boundary between the outer layer and the inner layer or a boundary between the intermediate layer and the inner layer, and as a result, a harmful casting defect remains at a boundary between the outer layer and the intermediate layer or the inner layer.

Therefore, in the present invention, the outer layer of the composite roll for rolling manufactured by the centrifugal casting method satisfies the formula (1), thereby preventing the formation of the nitride (VN) or carbonitride (VCN) gathers on the inner surface side of the outer layer at the time of centrifugal casting. Thus, a good roll can be stably supplied without forming harmful casting defects at the boundary between the outer layer and the inner layer or at the boundary between the intermediate layer and the inner layer.

(casting conditions in centrifugal casting method)

The composite roll for rolling produced by the centrifugal casting method of the present invention is produced by a general centrifugal casting method, but the relationship between the outer layer casting start temperature (T1) and the outer layer liquidus temperature (T2) in the centrifugal casting method needs to satisfy the following formula (2),

40℃≤T1-T2≤120℃ (2)。

since a large amount of alloy elements such as V, Nb, Mo, and Cr are added to the outer layer of the composite roll for rolling produced by the centrifugal casting method according to the present invention, when T1-T2 is less than 40 ℃, the melt fluidity at the time of centrifugal casting cannot be sufficiently ensured, and the integrity of the outer layer cannot be sufficiently ensured. Further, at 120 ℃ or higher, the solidification structure becomes coarse, and there is a problem that the surface is rough when the steel is used in rolling, and the like, and therefore, it is necessary to satisfy the above formula (2).

(amount of graphite crystals precipitated)

In the outer layer of the composite roll for rolling produced by the centrifugal casting method of the present invention, it is necessary to suppress the amount of graphite crystals precipitated to less than 0.3%. Since graphite is an extremely soft microstructure component, a large amount of graphite crystals precipitate on the outer layer of the composite roll for rolling produced by the centrifugal casting method of the present invention, which causes significant deterioration in wear resistance. Further, the difference in abrasion loss between the hard carbide, the high-hardness matrix, and the soft graphite is a cause of surface roughness during rolling. The limit of the amount of graphite crystals precipitated without these adverse effects was 0.3% by area ratio. Therefore, the amount of crystal precipitation of graphite needs to be suppressed to less than 0.3% by area ratio.

When the amount of crystal precipitation of graphite is too large, the amount of crystal precipitation of graphite can be suppressed by decreasing the amount of Si as a graphitization-promoting element or by increasing the amount of Cr, V, or the like as a graphitization-inhibiting element within the scope of the present invention.

(content of MC type carbide)

In addition, the outer layer of the composite roll for rolling manufactured by the centrifugal casting method of the present invention needs to contain 1 to 15% by area of MC type carbide. The composite roll for rolling produced by the centrifugal casting method of the present invention is characterized by having high wear resistance comparable to a high-speed roll, and the high wear resistance is satisfied by appropriate amount of devitrification of the MC type carbide having the highest hardness among the microstructure constituent elements of the roll. Therefore, if the content of the MC type carbide is less than 1%, the wear resistance cannot be maintained. On the other hand, if the amount of MC type carbide exceeds 15%, the MC type carbide crystallized at high temperature during centrifugal casting is largely segregated in the outer layer, and therefore, segregation on the inner surface side causes casting defects at the boundary portion and surface roughness during rolling. Therefore, the amount of MC type carbide is limited to 1 to 15% by area ratio.

The amount of the MC type carbide can be adjusted within the range of the present invention to satisfy the predetermined amount by adjusting the amount of the elements (V, Nb, Ti) forming the MC type carbide. If the amount of the MC type carbide exceeds the upper limit of 15%, the amount of the elements (V, Nb, Ti) to be added to form the MC type carbide may be reduced within the range of the present invention. When the amount of the MC type carbide is less than the lower limit of 1%, the amount of the elements (V, Nb, Ti) to be added to form the MC type carbide may be increased within the range of the present invention.

(Defect at the boundary between the outer layer and the inner layer or between the intermediate layer and the inner layer)

In general, in order to improve the wear resistance of the roll, it is considered effective to increase the content of hard carbide-forming elements, i.e., V, Nb, Mo, Cr, etc., but in the prior art, nitrides (mainly VN) formed during centrifugal casting accumulate at the boundary between the outer layer and the inner layer or at the boundary between the intermediate layer and the inner layer, and there is a problem that casting defects occur at the boundary. Further, when these minute casting defects remain in the product, there is a problem that the defects grow and progress during rolling use, and the risk of occurrence of a fracture problem such as flaking is increased. In view of the above problems, the present inventors have found that the occurrence of casting defects at the boundary between the outer layer and the inner layer or at the boundary between the intermediate layer and the inner layer can be suppressed by a configuration in which the relationship between the outer layer casting start temperature (T1) and the outer layer liquidus temperature (T2) at the time of centrifugal casting satisfies formula (2) and the amount of crystal precipitation of graphite is less than 0.3% by area ratio and 1 to 15% by area ratio of MC type carbide is contained, while the amounts of V and N contained in the outer layer satisfy formula (1).

Specifically, the composite roll for rolling produced by the centrifugal casting method of the present invention has a structure in which casting defects having a diameter of 4mm or more do not exist at the boundary between the outer layer and the inner layer or at the boundary between the intermediate layer and the inner layer. By adopting such a structure, the problem of occurrence of fracture due to growth and progress of casting defects during use of the roll can be suppressed. When the defect size at the boundary between the outer layer and the inner layer or the boundary between the intermediate layer and the inner layer is smaller than 4mm, the problem of cracking due to growth does not occur during the rolling use from the current use result, and thus the defect of more than 4mm is limited to be absent.

(Effect)

As described above, the composite roll for rolling according to the present invention manufactured by centrifugal casting is configured as follows: the chemical composition of the outer layer is set to the predetermined composition, the formulas (1) and (2) are satisfied, and the crystal precipitation amount of graphite is set to be less than 0.3% in terms of area ratio and 1-15% in terms of area ratio of MC type carbide is contained, so that the structure is realized that the casting defect with the diameter phi of more than 4mm is not generated at the boundary between the outer layer and the inner layer or the boundary between the intermediate layer and the inner layer. Thus, the problem of breakage during the manufacturing process, the problem of cracking due to the growth of minute defects remaining at the boundary in the product during the use of rolling, and the accident resistance can be improved. That is, a composite roll for rolling manufactured by a centrifugal casting method, which has excellent wear resistance and surface roughening resistance comparable to a high-speed cast iron roll and also has accident resistance comparable to a high-alloy pitted cast iron roll, is realized.

Although the embodiment of the present invention has been described above, the present invention is not limited to the embodiment shown in the drawings. It is obvious that those skilled in the art can conceive various modifications and variations within the scope of the idea described in the claims, and these naturally fall within the technical scope of the present invention.

Examples

Composite rolls having chemical compositions shown in Table 1, namely, Nos. 1 to 16 (inventive examples) and 17 to 28 (comparative examples) were produced by centrifugal casting to form composite rolls for hot finish rolling mill rolling having an inner layer diameter of 600mm, a roll outer diameter of 800mm, an outer layer thickness of 100mm and a body length of 2400 mm. The melting temperature was set to 1550 deg.C, and the differences T1-T2 between the outer layer casting start temperature (T1) and the outer layer liquidus temperature (T2) were set to the values shown in Table 1 below, respectively. After casting, tempering heat treatment is performed at 400 to 580 ℃. After casting, the steel sheet may be heated to a temperature at which the matrix phase changes to austenite (gamma heat treatment), and then subjected to quenching and tempering heat treatment.

The underlined part in table 1 indicates that the chemical composition of the outer layer does not satisfy the above formula (1) and that the conditions in centrifugal casting do not satisfy the above formula (2). In addition, for the casting defects at the boundaries in table 1, the symbol "none" is indicated within the scope of the present invention, and the symbol "x with" is indicated outside the scope of the present invention. Further, regarding whether or not the surface roughness occurs during rolling, the column indicating the presence or absence of the surface roughness when the surface roughness occurs during rolling is denoted by "x" and the column indicating the absence or absence of the surface roughness when the surface roughness does not occur during rolling is denoted by "o".

[ Table 1]

Then, in order to confirm the presence or absence of a casting defect at the boundary between the outer layer and the inner layer or at the boundary between the intermediate layer and the inner layer in each composite roll, the presence or absence of a casting defect was examined by ultrasonic flaw detection. For ultrasonic testing, the sensitivity was adjusted so that defects having a diameter of 4mm or more could be detected by a standard test piece STB-G for ultrasonic testing (JIS Z2345), and the boundary between the outer layer and the inner layer or the boundary between the intermediate layer and the inner layer in the composite roll was tested by the perpendicular method (using a probe: 5Z 20N).

In addition, for the test piece collected from the outer layer portion of the manufactured roll, the area ratio of graphite and MC type carbide in the structure was measured, and it was examined whether or not graphite was less than 0.3% and whether or not MC type carbide was in the range of 1 to 15%. For the area ratio of graphite, each test piece was mirror-finished and an optical micrograph (× 100) was taken without etching, and the obtained image was measured using image analysis software. The area ratio of the MC type carbide was measured by taking an optical micrograph (x 100) in a state of being colored with an on-village reagent and measuring the obtained image with image analysis software.

As a result, in the rolls of invention examples nos. 1 to 16 in which the chemical composition of the outer layer is within the predetermined range described in the above embodiment and the conditions of the above formulae (1) and (2) are within the range of the present invention, no harmful casting defects were detected at the boundary between the outer layer and the inner layer or the boundary between the intermediate layer and the inner layer.

On the other hand, in the rolls of comparative examples No. 17 to 28 in which the conditions of the above-described formulas (1) and (2) were out of the range of the present invention, harmful casting defects were detected at the boundary between the outer layer and the inner layer or between the intermediate layer and the inner layer.

As is clear from the results of the above examples, in the composite roll for rolling produced by the centrifugal casting method, by setting the chemical composition of the outer layer within the predetermined range and setting the conditions of the above formulas (1) and (2) within the range of the present invention, the amount of crystal precipitation of graphite to be less than 0.3% in terms of area ratio and containing MC type carbide in terms of area ratio of 1 to 15%, the composite roll for rolling produced by the centrifugal casting method having excellent wear resistance and surface roughening resistance comparable to the high-speed cast iron roll and having accident resistance comparable to the high-alloy pitted cast iron roll was realized.

Industrial applicability

The present invention is applicable to a composite roll for rolling produced by a centrifugal casting method and having excellent wear resistance, crack resistance, and surface roughness resistance, and a method for producing the same.