Centrifugally cast composite roll for hot rolling and method for manufacturing same

Abstract

This centrifugally cast composite roll for hot rolling has an outer layer and an inner layer, wherein: the outer layer comprises chemical components of 2.0-3.5 mass% of C, 0.9-3.0 mass% of Si, 0.1-2.0 mass% of Mn, 3.5-5.5 mass% of Ni, 0.5-2.0 mass% of Cr, 2.5-5.0 mass% of Mo, 0.1-3.0 mass% of W, 2.0-5.0 mass% of V, 0.1-3.0 mass% of Nb, 0.01-0.05 mass% of P, and 0.005-0.1 mass% of N, with the balance consisting of Fe and inevitable impurities; the chemical composition of the outer layer satisfies expression (1); and an M3C eutectic carbide and an M2C eutectic carbide coexist at an amount ratio satisfying expression (2). (1): -0.55<Z-(0.05×CE)<-0.30 (2): 0.05<Y / X<1.0

Description

Centrifugal cast composite roll for hot rolling and its manufacturing method

[0001] The present invention relates to a centrifugally cast composite roll having excellent wear resistance, crack resistance and surface roughening resistance, which is used in a hot strip mill in a hot rolling process, and a method for manufacturing the same.

[0002]

[0003] Composite rolls for use in hot strip mills for hot rolling are required to have excellent wear resistance, surface roughening resistance, crack resistance, and accident resistance in the outer layer that comes into contact with the steel sheet during rolling. In recent years, there has been an increasing demand for improved thickness accuracy and surface quality of hot-rolled steel sheets, and rolling rolls with particularly high wear resistance are required. High-speed cast iron rolls (hereinafter referred to as "HSS rolls") are widely used in the front stages of hot finishing mills for producing thin steel sheets. However, in the rear stages of hot finishing mills, due to the thin plate thickness, so-called squeezing accidents, in which the rolled material overlaps and gets caught between the upper and lower rolls as it moves between stands, are likely to occur. Therefore, high-alloy grain cast iron rolls have traditionally been mainly used.

[0003] High alloy grain cast iron rolls are made of graphite, carbides and matrix structure, and even when subjected to a squeezing accident, the occurrence and progression of cracks is extremely small, meaning that they have excellent accident resistance. However, their wear resistance is significantly inferior to that of high speed steel rolls, so there is a demand for rolls that combine both accident resistance and wear resistance.

[0004] In order to meet the demand for rolls with excellent wear resistance and accident resistance, Patent Document 1 discloses a hot rolling roll outer layer material with excellent seizure resistance, characterized by having a composition containing, by mass%, 1.8 to 3.5% C, 0.2 to 2% Si, 0.2 to 2% Mn, 4 to 15% Cr, 2 to 10% Mo, 3 to 10% V, 0.1 to 0.6% P, 0.05 to 5% B, the balance being Fe and unavoidable impurities. Patent Document 1 also describes that, after casting, the heat treatment is preferably a quenching treatment in which the material is heated to 800°C to 1080°C and quenched, and further a tempering treatment at 300 to 600°C, which is carried out one or more times. However, the roll described in Patent Document 1 has a high content of P and B, which segregate at grain boundaries to form compounds with low melting points, which causes problems such as surface roughness being easily generated during rolling. In addition, if a crack occurs during rolling, the crack will grow faster than in a high-alloy grain cast iron roll, increasing the risk of explosion.

[0005] Patent Document 2 discloses a centrifugally cast composite roll for rolling having an outer layer, the outer layer containing, in mass %, 2.2% to 3.01% C, 1.0% to 3.0% Si, 0.3% to 2.0% Mn, 3.0% to 7.0% Ni, 0.5% to 2.5% Cr, 1.0% to 3.0% Mo, 2.5% to 5.0% V, more than 0% to 0.5% Nb, the balance being Fe and unavoidable impurities, and satisfying the following conditions (a): Nb% / V%<0.1, and (b): 2.1×C%+1.2×Si%−Cr%+0.5×Mo%+(V%+Nb% / 2)≦13.0%. Patent Document 2 also discloses that a gamma-annealing heat treatment at 850°C or higher, quenching, and tempering may be performed. However, the roll described in Patent Document 2 has a problem that the wear resistance is significantly inferior to that of a high-speed steel roll because the eutectic carbide is cementite only, and that the surface is roughened due to excessive crystallization of graphite. Furthermore, when a squeezing accident or the like occurs during rolling, a problem occurs in that a deep crack is more likely to occur in the surface of the outer layer of the roll than in the case of a high-alloy grain cast iron roll. Furthermore, a problem occurs in that the outer layer residual stress value of the roll surface is likely to be excessive, resulting in a fast crack growth rate and a high risk of explosion.

[0006] Patent Document 3 discloses a hot rolling roll having an outer layer made by centrifugation, which is a composite roll for rolling, and which has an outer layer material characterized in that the outer layer contains, in mass %, C: 3.0% to 4.5%, Si: more than 0% but not more than 2.0%, Mn: more than 0% but not more than 1.5%, Ni: 3.0% to 5.0%, Cr: 1.4% to 4.0%, Mo: 0.1% to 3.0%, V: more than 0% but not more than 3.0%, the balance being Fe and unavoidable impurities, with the proviso that 4.0%≦C+Si / 3+Cr / 7.5≦5.5%, and the metal structure of the peripheral surface used for rolling of the outer layer has an area ratio of cementite of 40% to 60%. However, the roll described in Patent Document 3 has significantly inferior wear resistance compared to a high-speed steel roll because the eutectic carbide is cementite only, and has problems with accident resistance because cementite is crystallized in excess.

[0007] Furthermore, Patent Document 4 relates to a cast composite roll suitable for use in the fifth to seventh stands of a hot rolling mill for steel sheets, which contains, by mass, 2.6 to 3.6% C, 0.1 to 3% Si, 0.3 to 2% Mn, 2.3 to 5.5% Ni, 0.5 to 3.2% Cr, 0.3 to 1.6% Mo, 0.2 to 3.4% V, 0.4 to 3% Nb, and 0.06% or less B, with 0.07≦V / Nb≦2.7, and a V equivalent (Veq=V+0.55Nb) of 2. Patent Document 4 discloses a centrifugally cast composite roll for hot rolling, having an outer layer material characterized in that 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 consisting of 50 mass% or more of Fe and the remainder being Fe and impurities, and the V equivalent of the outer layer satisfies the following formula: Veq1 / Veq2 = 1.1 to 5 (where Veq1 is the V equivalent in a region from the initial diameter to a depth of 30 mm in the radial direction, and Veq2 is the V equivalent at the discard diameter). The roll described in Patent Document 4 is suitable for use in the final seventh stand because of its high wear resistance, especially when close to the initial diameter, but is said to be suitable for use in the sixth and fifth stands when the diameter approaches the discard diameter through machining, as its heat crack resistance improves. However, the roll described in Patent Document 4 has a problem that the eutectic carbide is only cementite, and therefore the wear resistance is significantly inferior to that of a high-speed steel roll. In addition, the amount of MC carbide varies significantly in the radial direction, and therefore, particularly from the initial diameter to the intermediate diameter, segregation of the MC carbide is formed, causing surface roughness.

[0008] Japanese Patent No. 4483585 International Publication No. 2018 / 124101 Japanese Patent No. 6518314 Japanese Patent Application Laid-Open No. 2019-183276

[0009] As described above, the rolling roll described in Patent Document 1 has problems such as surface roughening being easily generated and a high risk of explosion due to a fast crack propagation rate. In addition, the rolling rolls described in Patent Documents 2 to 4 have problems such as significantly inferior wear resistance compared to high-speed steel rolls because the eutectic carbide is cementite only, and furthermore, there is room for further improvement in the composite rolling rolls, such as inferior accident resistance and surface roughening.

[0010] In view of the above circumstances, an object of the present invention is to provide a centrifugally cast composite roll for hot rolling which has excellent wear resistance and surface roughening resistance comparable to that of a high-speed steel roll, and also has accident resistance comparable to that of a high-alloy grain cast iron roll, and a method for manufacturing the same.

[0011] In order to solve the above problems, according to the present invention, there is provided a centrifugally cast composite roll for hot rolling having an outer layer and an inner layer, wherein the outer layer has a chemical composition, in mass ratio, of C: 2.0 to 3.5%, Si: 0.9 to 3.0%, Mn: 0.1 to 2.0%, Ni: 3.5 to 5.5%, Cr: 0.5 to 2.0%, Mo: 2.5 to 5.0%, W: 0.1 to 3.0%, V: 2.0 to 5.0%, Nb: 0.1 to 3.0%, P: 0.01 to 0.05%, N: 0.005 to 0.1%, with the balance being Fe and unavoidable impurities, and the chemical composition of the outer layer satisfies the following formula (1), ３ C eutectic carbide and M ２ A centrifugally cast composite roll for hot rolling is provided, characterized in that C eutectic carbide and X coexist in a quantity ratio satisfying the following formula (2): -0.55<Z-(0.05×CE)<-0.30 (1) 0.05<Y / X<1.0 (2) where Z=0.07[Cr%]+0.14[V%]+0.07[Nb%]-0.06[Ni%]-0.31[Si%]+0.02[Mo%]-0.01[W%], CE=[C%]+([Si%] / 3), and M is expressed as an area ratio. ３ C eutectic carbide is X (%), M ２ C eutectic carbide is Y (%).

[0012] According to the present invention, there is also provided a centrifugally cast composite roll for hot rolling having an outer layer, an intermediate layer and an inner layer, wherein the outer layer has a chemical composition, in mass ratio, of C: 2.0 to 3.5%, Si: 0.9 to 3.0%, Mn: 0.1 to 2.0%, Ni: 3.5 to 5.5%, Cr: 0.5 to 2.0%, Mo: 2.5 to 5.0%, W: 0.1 to 3.0%, V: 2.0 to 5.0%, Nb: 0.1 to 3.0%, P: 0.01 to 0.05%, N: 0.005 to 0.1%, with the balance being Fe and unavoidable impurities, and the chemical composition of the outer layer satisfies the following formula (1), ３ C eutectic carbide and M２ A centrifugally cast composite roll for hot rolling is provided, characterized in that C eutectic carbide and X coexist in a quantity ratio satisfying the following formula (2): -0.55<Z-(0.05×CE)<-0.30 (1) 0.05<Y / X<1.0 (2) where Z=0.07[Cr%]+0.14[V%]+0.07[Nb%]-0.06[Ni%]-0.31[Si%]+0.02[Mo%]-0.01[W%], CE=[C%]+([Si%] / 3), and M is expressed as an area ratio. ３ C eutectic carbide is X (%), M ２ C eutectic carbide is Y (%).

[0013] Furthermore, the outer layer may contain, in terms of mass ratio, one or more of the following chemical components: Ti: 0.001 to 0.5%, Co: 0.01 to 5.0%, B: 0.001 to 0.2%, and S: 0.3% or less.

[0014] Furthermore, the outer layer may contain 0.3% to 5.0% by area of ​​graphite and 1 to 20% by area of ​​MC carbide.

[0015] According to the present invention, there are provided a centrifugally cast composite roll for hot rolling which has excellent wear resistance and surface roughening resistance comparable to that of a high-speed steel roll and also has accident resistance comparable to that of a high-alloy grain cast iron roll, and a method for manufacturing the same.

[0016] 1 is a schematic explanatory diagram of a hot rolling wear tester. FIG. 2 is a schematic explanatory diagram of a friction heat quenching tester.

[0017] The present inventors aimed to provide a roll with crack propagation resistance (accident resistance) comparable to that of a high-alloy grain cast iron roll, crystallize graphite to prevent steel material from sticking to the roll surface in the event of a rolling accident during rolling, and improve wear resistance. As a result of intensive research by the present inventors, it was found that in a centrifugally cast composite roll for hot rolling in which an outer layer material containing V and Nb, elements that form hard MC carbides, is used to improve wear resistance, the crystallization of graphite becomes difficult when the alloy is made high, and an excessive amount of graphite crystallization causes surface roughness. However, by satisfying the following formula (1), it is possible to control the amount of graphite crystallization to an appropriate range of 0.3 to 5.0%.

[0018] Furthermore, the inventors have found that in order to achieve both excellent wear resistance and accident resistance, the eutectic carbide should not be limited to cementite but should be M ３ C eutectic carbide (cementite) and M ２ It has been found that it is effective to allow the C eutectic carbide to coexist in a quantity ratio that satisfies the following formula (2).

[0019] Hereinafter, embodiments of a centrifugally cast composite roll for hot rolling and a method for manufacturing the same according to the present invention will be described.

[0020] The centrifugally cast composite roll for rolling according to the present invention has an outer layer to be used in rolling. Furthermore, inside the outer layer, there is an intermediate layer and an inner layer, or a core material consisting of an inner layer. Examples of the inner layer material include high-grade cast iron, ductile cast iron, and other materials having high toughness, and examples of the intermediate layer material include adamite and graphite steel.

[0021] The centrifugally cast outer layer is formed from an Fe-based alloy containing, by mass, 2.0 to 3.5% C, 0.9 to 3.0% Si, 0.1 to 2.0% Mn, 3.5 to 5.5% Ni, 0.5 to 2.0% Cr, 2.5 to 5.0% Mo, 0.1 to 3.0% W, 2.0 to 5.0% V, 0.1 to 3.0% Nb, and 0.01 to 0.05% P, with the balance being essentially Fe and unavoidable impurities. The structure of the outer layer is composed of (a) MC-type carbides, (b) eutectic carbides, (c) a matrix, (d) graphite, and (e) others.

[0022] (Reasons for Limiting Components) First, the reasons for limiting the chemical components of the outer layer according to the present invention will be explained below. Note that, unless otherwise specified, "%" below indicates "% by mass."

[0023] C: 2.0 to 3.5% C primarily combines with Fe, Cr, Mo, Nb, V, W, etc. to form various hard carbides. It may also form graphite in some cases. Furthermore, it dissolves in the matrix to form pearlite, bainite, martensite, and other phases. The greater the content, the more effective it is in improving wear resistance. However, if it exceeds 3.5%, coarse carbides and graphite are formed, resulting in reduced toughness and rough surfaces. Furthermore, if it is less than 2.0%, the amount of carbides is small, it is difficult to ensure hardness, and wear resistance deteriorates. Therefore, the range is set to 2.0 to 3.5%. A more preferable range is 2.2 to 3.3%.

[0024] Si: 0.9 to 3.0% Si is necessary to suppress the generation of oxide defects by deoxidizing the molten metal. It also has the effect of improving the fluidity of the molten metal and preventing casting defects. Furthermore, when graphite is precipitated in high-alloy grain cast iron, etc., Si is necessary as an element that promotes graphite precipitation. Therefore, the content is set to 0.9% or more. However, if it exceeds 3.0%, it reduces toughness and causes a decrease in crack resistance. Therefore, the range is set to 0.9 to 3.0%. A more preferable range is 1.0 to 2.7%.

[0025] Mn: 0.1 to 2.0% Mn is added for the purpose of deoxidizing and desulfurizing. It also combines with S to form MnS. MnS has a lubricating effect, so it is effective in preventing seizure of the rolled material. For this reason, it is preferable to include MnS within a range that does not cause side effects. If Mn is less than 0.1%, these effects are insufficient. On the other hand, if Mn exceeds 2.0%, toughness is reduced. Therefore, the range is set to 0.1 to 2.0%. A more preferable range is 0.5 to 1.5%.

[0026] Ni: 3.5 to 5.5% Ni has the effect of improving the hardenability of the matrix, preventing the formation of pearlite during cooling, and promoting bainite transformation, making it an effective element for strengthening the matrix. Therefore, a content of 3.5% or more is necessary. However, if the content exceeds 5.5%, the amount of retained austenite becomes excessive, making it difficult to ensure hardness and causing deformation during hot rolling. Therefore, the range is set to 3.5 to 5.5%. A more preferable range is 4.0 to 5.2%.

[0027] Cr: 0.5 to 2.0% Cr is added to increase hardenability, hardness, temper softening resistance, and stabilize carbide hardness. However, if the content exceeds 2.0%, the amount of eutectic carbide becomes excessive, which reduces toughness and makes it difficult for graphite to crystallize. Therefore, the upper limit is set at 2.0%. On the other hand, if the content is less than 0.5%, the above effects cannot be obtained. Therefore, the range is set at 0.5 to 2.0%. A more preferable range is 0.8 to 1.5%.

[0028] Mo: 2.5 to 5.0% Mo mainly combines with C to form M ２ A minimum of 2.5% of C is required to form hard carbides, primarily C carbides, which contribute to improved wear resistance and improve the hardenability of the matrix. On the other hand, if the C content exceeds 5.0%, coarse carbides are formed, reducing surface roughening resistance and toughness. Therefore, the range is set to 2.5 to 5.0%. A more preferable range is 3.0 to 4.3%.

[0029] W: 0.1 to 3.0% W, like Mo, is dissolved in the matrix to strengthen the matrix, and also combines with C to form M. ２ It forms hard eutectic carbides, mainly C carbides, and contributes to improving wear resistance. To strengthen the matrix, a minimum of 0.1% or more of C is required, but if it exceeds 3.0%, coarse eutectic carbides are formed, reducing toughness. Therefore, the range is set to 0.1 to 3.0%.

[0030] V: 2.0 to 5.0% 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 carbides, which contribute greatly to wear resistance. If it is less than 2.0%, the amount of MC carbides is small, resulting in insufficient improvement in wear resistance. If it exceeds 5.0%, the MC carbides that crystallize at high temperatures segregate due to centrifugal force, and V is a strong graphitization inhibitor, making it difficult for graphite to crystallize. Therefore, the range is set to 2.0 to 5.0%. A more preferable range is 2.5 to 4.0%.

[0031] Nb: 0.1 to 3.0% Nb is hardly dissolved in the matrix, and most of it forms high-hardness MC carbides, improving wear resistance. In particular, the MC carbides formed by the addition of Nb have a smaller difference in density from the molten metal compared to the MC carbides formed by the addition of V, and therefore have the effect of reducing gravitational segregation during centrifugal casting. With regard to the Nb content, if it is less than 0.1%, this effect is insufficient, but if it is contained in excess of 3.0%, the MC carbides become coarse, leading to reduced toughness and segregation of the MC carbides. Therefore, the range is set to 0.1 to 3.0%.

[0032] P: 0.01 to 0.05% P has the effect of improving the fluidity of the outer layer material when it is cast. If the P content is less than 0.01%, this effect is insufficient, and if it is contained in excess of 0.05%, the toughness of the outer layer material is deteriorated. Therefore, the range is set to 0.01 to 0.05%.

[0033] N: 0.005 to 0.1% N has the effect of refining carbides, but it also bonds with V to form nitrides (VN) or carbonitrides (VCN). If it is less than 0.005%, the effect of refining carbides is insufficient, and if it is contained in excess of 0.1%, excess nitrides (VN) or carbonitrides (VCN) are formed, reducing toughness, so it is necessary to keep it to 0.1% or less. Therefore, if N is added, the range is set to 0.005 to 0.1%.

[0034] The basic components of the outer layer according to the present invention are as described above. However, depending on the size of the roll to which the outer layer is applied, the required usage characteristics of the roll, and the like, the outer layer may contain, in addition to the basic components described above, other chemical components selected as appropriate from the chemical components described below.

[0035] Ti: 0.001 to 0.5% The centrifugally cast composite roll for hot rolling according to the present invention can contain Ti in addition to the above essential elements. Ti is expected to have a degassing effect with N and O, and can also form TiCN or TiC, which can serve as crystallization nuclei for MC carbides. If the Ti content is less than 0.001%, this effect cannot be expected, and if it exceeds 0.5%, the viscosity of the molten metal increases, increasing the risk of inducing casting defects. Therefore, if Ti is added, the range is set to 0.001 to 0.5%. A more preferable range is 0.01 to 0.3%.

[0036] Co: 0.01 to 5.0% In addition to the above essential elements, the centrifugally cast composite roll for hot rolling according to the present invention can contain Co. Most of Co is dissolved in the matrix to strengthen the matrix. Therefore, it has the effect of improving hardness and strength at high temperatures. At less than 0.01%, the effect is insufficient, and at more than 5.0%, the effect saturates. Therefore, from an economical standpoint, the content of Co is set to 5.0% or less. Therefore, when Co is added, the range is set to 0.01 to 5.0%. Regarding the selection of whether or not to add Co, for example, when improved wear resistance is required and it is difficult to increase the amount of eutectic carbide, adding Co provides a significant effect.

[0037] B: 0.001 to 0.2% The centrifugally cast composite roll for hot rolling according to the present invention can contain B in addition to the above essential elements. B dissolves in carbides and forms carboborides. Carboborides have a lubricating effect and are effective in preventing seizure of the material being rolled. If the B content is less than 0.001%, the effect is insufficient, and if it exceeds 0.2%, toughness decreases. Therefore, the range is set to 0.001 to 0.2%.

[0038] S: 0.3% or less Normally, some amount of S is inevitably mixed in from the raw materials, but as mentioned above, it forms MnS, which has a lubricating effect and is effective in preventing seizure of the rolled material. On the other hand, excessive S content makes the material brittle, so it is preferable to limit it to 0.3% or less.

[0039] Inevitable Impurities The composition of the outer layer of the centrifugally cast composite roll for hot rolling according to the present invention consists essentially of Fe and unavoidable impurities in addition to the above elements. Elements such as Cu, Sb, Sn, Zr, Al, Te, and Ce may be contained as unavoidable elements within a range that does not impair the properties of the outer layer. In order not to impair the properties of the outer layer, the total amount of unavoidable impurities is preferably 0.6% or less.

[0040] (Relationship related to chemical composition) The chemical components (chemical composition) of the outer layer of the centrifugally cast composite roll for hot rolling according to the present invention must satisfy the following formula (1): −0.55<Z−(0.05×CE)<−0.30 (1), where Z=0.07[Cr%]+0.14[V%]+0.07[Nb%]−0.06[Ni%]−0.31[Si%]+0.02[Mo%]−0.01[W%], and CE=[C%]+([Si%] / 3). The parameter Z = 0.07 [Cr%] + 0.14 [V%] + 0.07 [Nb%] - 0.06 [Ni%] - 0.31 [Si%] + 0.02 [Mo%] - 0.01 [W%] in the above formula (1) is an index related to the crystallization of graphite present in the outer layer. CE is the so-called carbon equivalent. If formula (1) is less than -0.55, the amount of graphite crystallization in the outer layer material according to the present invention becomes excessive, resulting in reduced wear resistance. On the other hand, if formula (1) exceeds -0.30, the amount of graphite crystallization in the outer layer material according to the present invention becomes too small, resulting in reduced crack resistance.

[0041] The outer layer of the centrifugally cast composite roll for hot rolling according to the present invention is characterized in that it contains 0.3 to 5.0% of graphite and 1 to 20% of MC carbide as elements constituting the microstructure. ３ C eutectic carbide (cementite) and M ２The eutectic carbide and the carbide are coexisting in a quantity ratio that satisfies the following formula (2): 0.05<Y / X<1.0 (2) where M ３ C eutectic carbide X (%), M ２ The C eutectic carbide was represented as Y (%).

[0042] When the conditions of these formulas (1) and (2) are applied, the conventional technology has a problem that the amount of hard carbide-forming elements added increases, making it difficult to crystallize graphite. The present invention has a structure in which 1 to 20% of MC carbides are contained as elements constituting the microstructure of the outer layer, and M ３ C eutectic carbide (cementite) and M ２ It has been found that when the condition that the C eutectic carbide has a quantity ratio that satisfies the formula (2) is applied, graphite can be crystallized by satisfying the formula (1). This makes it possible to provide a roll that combines the wear resistance and surface roughening resistance of a high-speed steel roll with the accident resistance (crack resistance) of a high-alloy grain cast iron roll.

[0043] (Graphite Content) The outer layer of the centrifugally cast composite roll for hot rolling according to the present invention must contain 0.3 to 5.0% graphite by area ratio. The inventors investigated and studied the usage of centrifugally cast rolls used in hot rolling and found that in order to impart the same level of accident resistance (crack resistance) to a centrifugally cast composite roll for hot rolling as that of a high-alloy grain cast iron roll, it is effective to have graphite present in a predetermined ratio among the microstructural components of the outer layer. If the amount of graphite present in the outer layer is less than 0.3% by area ratio, crack resistance deteriorates. Furthermore, if the amount of graphite exceeds 5.0% by area ratio, the graphite crystallizes in a flake shape, which deteriorates surface roughening resistance and wear resistance. Therefore, the amount of graphite is specified to be 0.3 to 5.0% by area ratio.

[0044] (MC Carbide Content) The outer layer of the centrifugally cast composite roll for hot rolling according to the present invention must contain 1 to 20% MC carbide in area ratio. The inventors investigated and studied the usage of centrifugally cast rolls used in hot rolling, and found that in order to impart wear resistance comparable to that of a high-speed steel roll to a centrifugally cast composite roll for hot rolling, it is effective to have MC carbide present in a predetermined ratio among the microstructural components of the outer layer. When the amount of MC carbide present in the outer layer is less than 1% in area ratio, wear resistance deteriorates. Furthermore, when the amount of MC carbide exceeds 20% in area ratio, the MC carbide crystallizes coarsely, deteriorating surface roughening resistance. Therefore, the amount of MC carbide is specified to be 1 to 20% in area ratio.

[0045] (M ３ C eutectic carbide and M ２ The outer layer of the centrifugally cast composite roll for hot rolling according to the present invention contains M ３ C eutectic carbide and M ２ The inventors of the present invention have investigated and examined the use of centrifugally cast rolls for hot rolling, and have found that the amount of hard M eutectic carbide in the centrifugally cast composite rolls for hot rolling is insufficient. ２ C eutectic carbide is M ３ It is advantageous in improving wear resistance compared to C eutectic carbide. ２ When the amount of C eutectic carbide increases, it becomes difficult to crystallize graphite, and the accident resistance deteriorates. Therefore, in order to crystallize graphite and give the roll the same level of accident resistance (crack resistance) as that of high alloy grain cast iron rolls, and also to give the roll the same level of wear resistance as that of high speed steel rolls, it is necessary to ３ The area ratio X (%) of C eutectic carbide and M ２ It was found that it is effective to have a specific ratio (Y / X) of the area ratio Y (%) of C eutectic carbide. If the ratio (Y / X) is less than 0.05, wear resistance deteriorates. On the other hand, if the ratio (Y / X) exceeds 1.0, it becomes difficult for graphite to crystallize, which deteriorates crack resistance. Therefore, the ratio (Y / X) is specified to be 0.05 to 1.0.

[0046] (Effects) As explained above, in the centrifugally cast composite roll for hot rolling according to the present invention, the chemical composition of the outer layer is the above-mentioned predetermined components, satisfies the above formula (1), and further contains 0.3 to 5.0% by area ratio of graphite and 1 to 20% by area ratio of MC carbide, ３ C eutectic carbide and M ２ By configuring the roll so that the C eutectic carbide is contained within the range of satisfying the above formula (2), a centrifugally cast composite roll for hot rolling can be realized which has excellent wear resistance and surface roughening resistance comparable to that of a high-speed steel roll and also has accident resistance comparable to that of a high-alloy grain cast iron roll. Such a centrifugally cast composite roll for hot rolling is particularly suitable for application to a rear stand of hot finish rolling in a hot strip mill, where operational stability is required.

[0047] Although one embodiment of the present invention has been described above, the present invention is not limited thereto. It is clear that a person skilled in the art can conceive of various modifications and alterations within the scope of the ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

[0048] (Example 1) Using an experimental mold (inner diameter 100 mm, height 200 mm), test materials (test pieces) consisting of the chemical compositions shown in Table 1, i.e., Nos. 1 to 6 (inventive examples) and 7 to 14 (comparative examples), were cast. After tempering at 470°C, each test piece was subjected to M ２ C carbide and M ３ The results of evaluating the ratio of C carbide, the amount of graphite, and the amount of MC carbide are shown in Table 1. ２ C carbide and M ３ The amount of C carbide, the amount of graphite, and the amount of MC carbide were measured by observing the M in a field of view (magnification: 100 times) using an SEM (scanning electron microscope) or an optical microscope. ２ C carbide, M ３ Images of the extracted C carbide, graphite, and MC carbide were taken, and the area ratios of each were measured using image analysis software.

[0049] Next, in order to evaluate the abrasion resistance and the surface roughening resistance, an abrasion test was performed using a hot rolling abrasion tester shown in Figure 1. The hot rolling abrasion tester is composed of an S45C material (opposite disk) 1 corresponding to the rolled material, a test disk 2 made of a rolling roll material with an outer diameter of 80 mm and a thickness of 10 mm, an induction heating coil 3 for heating the S45C material 1, cooling water 4 for cooling the test disk 2, a pyrometer 5 for measuring the surface temperature of the S45C material 1, and a pyrometer 6 for measuring the surface temperature of the test disk 2. For the test, S45C material 1, which corresponds to the rolled material, was heated to 840-850°C, and a hot rolling wear test was carried out under the following conditions: test piece temperature 510-520°C, slip ratio 4.4%, load 400N, test piece peripheral speed 3.1m / s, and number of rolls 2500. After the hot rolling wear test, the wear loss and surface roughness of the contact surface with the mating material were measured for each test piece to evaluate wear resistance and surface roughness resistance. The test results were compared with those of conventional high-alloy grain roll material, and are shown in Table 1, with good results indicated as ○ and poor results indicated as ×.

[0050] Next, to evaluate crack resistance, a frictional heat generation quenching test was performed using the frictional heat generation quenching tester shown in Figure 2. The frictional heat generation quenching tester is composed of a rotating disk 7 with a diameter of 300 mm and a thickness of 30 mm, a roll test piece 8, and a water jet 9 for quenching the test piece after pressing it against the rotating disk 7. The crack resistance was evaluated by performing the frictional heat generation quenching test under the following test conditions: test piece size: 30 x 30 x 60 mm, load: 400 kg, disk rotation speed: 1500 rpm, pressing time: 5 seconds, and water cooling time: 30 seconds. After the frictional heat generation quenching test, the test piece was cut and the crack depth was measured to evaluate crack resistance. Compared to conventional high-alloy grain roll material, cases where the crack depth was equal to or less than that were marked with a circle, and cases where the crack depth was deeper were marked with an X. These are shown in Table 1.

[0051] As shown in Table 1, the test pieces Nos. 1 to 6 (inventive examples) showed good results in all of the wear resistance, surface roughening resistance, and crack resistance. In contrast, the test pieces Nos. 7 to 14 (comparative examples) were inferior in at least one of the wear resistance, surface roughening resistance, and crack resistance. From these results, it was confirmed that the centrifugally cast composite roll for hot rolling according to the present invention has excellent wear resistance, surface roughening resistance, and crack resistance.

[0052] Example 2 A composite roll having the chemical composition of Example No. 5 of the present invention shown in Table 1 was centrifugal cast to produce a composite roll for hot strip finishing stand rolling, having an inner layer diameter of 600 mm, an outer diameter of 800 mm, an outer layer thickness of 100 mm, and a barrel length of 2400 mm. The melting temperature was 1550°C, and the casting temperature was the solidification point + 100°C. After casting, the roll was tempered at 450°C for 50 hours. This roll was subjected to rolling in the finishing stand of an actual hot strip mill. As a result, it was confirmed that the roll consumption rate was approximately three times higher than that of conventional high-alloy grain rolls due to improved wear resistance, surface roughening resistance, and crack resistance.

[0053] The present invention is applicable to a centrifugally cast composite roll having excellent wear resistance, crack resistance and surface roughening resistance, which is used in a hot strip mill in a hot rolling process, and a method for manufacturing the same.

Claims

1. A centrifugally cast composite roll for hot rolling having an outer layer and an inner layer, wherein the chemical components of the outer layer are, by mass ratio, C: 2.0 to 3.5%, Si: 0.9 to 3.0%, Mn: 0.1 to 2.0%, Ni: 3.5 to 5.5%, Cr: 0.5 to 2.0%, Mo: 2.5 to 5.0%, W: 0.1 to 3.0%, V: 2.0 to 5.0%, Nb: 0.1 to 3.0%, P: 0.01 to 0.05%, N: 0.005 to 0.1%, and the balance is Fe and unavoidable impurities, and the chemical composition of the outer layer satisfies the following formula (1), M ３ C eutectic carbide and M ２ The centrifugally cast composite roll for hot rolling is characterized in that the C eutectic carbide coexists in a ratio satisfying the following formula (2). -0.55 < Z - (0.05 × CE) < -0.30... (1) 0.05 < Y / X < 1.0... (2) Here, Z = 0.07[Cr%] + 0.14[V%] + 0.07[Nb%] - 0.06[Ni%] - 0.31[Si%] + 0.02[Mo%] - 0.01[W%], and CE = [C%] + ([Si%] / 3), and by area ratio, M ３ The C eutectic carbide is X (%), M ２ The C eutectic carbide is Y (%).

2. A centrifugally cast composite roll for hot rolling having an outer layer, an intermediate layer, and an inner layer, wherein the chemical composition of the outer layer is, by mass ratio, C: 2.0 to 3.5%, Si: 0.9 to 3.0%, Mn: 0.1 to 2.0%, Ni: 3.5 to 5.5%, Cr: 0.5 to 2.0%, Mo: 2.5 to 5.0%, W: 0.1 to 3.0%, V: 2.0 to 5.0%, Nb: 0.1 to 3.0%, P: 0.01 to 0.05%, N: 0.005 to 0.1%, and the balance is Fe and unavoidable impurities, and the chemical composition of the outer layer satisfies the following formula (1), M ３ C eutectic carbide and M ２ The centrifugally cast composite roll for hot rolling is characterized in that the C eutectic carbide and M coexist in a ratio satisfying the following formula (2). -0.55 < Z - (0.05 × CE) < -0.30... (1) 0.05 < Y / X < 1.0... (2) Here, Z = 0.07[Cr%] + 0.14[V%] + 0.07[Nb%] - 0.06[Ni%] - 0.31[Si%] + 0.02[Mo%] - 0.01[W%], and CE = [C%] + ([Si%] / 3), and the area ratio of M ３ The C eutectic carbide is X (%), and M ２ The C eutectic carbide is Y (%).

3. Further, the outer layer contains, by mass ratio, any one or more of the following chemical components: Ti: 0.001 to 0.5%, Co: 0.01 to 5.0%, B: 0.001 to 0.2%, S: 0.3% or less. The centrifugally cast composite roll for hot rolling according to claim 1 or 2 is characterized by this.

4. Further, the outer layer has, by area ratio, 0.3% to 5.0% graphite and 1 to 20% MC carbide. The centrifugally cast composite roll for hot rolling according to claim 1 or 2 is characterized by this.

5. Further, the outer layer has, by area ratio, 0.3% to 5.0% graphite and 1 to 20% MC carbide. The centrifugally cast composite roll for hot rolling according to claim 3 is characterized by this.

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