Outer layer material for hot rolling rolls, centrifugal casting mold for casting outer layer material for hot rolling rolls, method for manufacturing outer layer material for hot rolling rolls, and composite roll for hot rolling.

Abstract

The present invention provides a hot-rolling roll outer layer material that suppresses variations in the amount of crystallized carbides along the length direction and exhibits excellent wear resistance over its entire length, a centrifugal casting die for casting the hot-rolling roll outer layer material, a method for manufacturing the hot-rolling roll outer layer material, and a hot-rolling composite roll. [Solution] Contains C: 1.2~2.6%, Si: 0.1~2.2%, Mn: 0.1~2.2%, Ni: 0.4~5.5%, Cr: 2.0~10.5%, Mo: 2.0~12.0%, V: 2.0~9.5%, W: 0.5~8.5%, P: 0.01~0.06%, S: 0.001~0.060%, with the content of Mo, W, V, and Cr satisfying formula (1), the total crystallized carbide area ratio being 8.5% or more and 19.0% or less, each coefficient of variation being 16.0 or less, the maximum difference in the total crystallized carbide area ratio being 6.0% or less, the maximum difference between each coefficient of variation being 10.0 or less, and the crystallized M2C type carbide area ratio being 1.5% or more and 8.0% or less.

Description

Technical Field

[0001] The present invention relates to a roll outer layer material for hot rolling, a centrifugal casting mold for casting the roll outer layer material for hot rolling, a method for manufacturing the roll outer layer material for hot rolling, and a composite roll for hot rolling.

Background Art

[0002] In recent years, the demand for high-quality steel sheets has been increasing. Along with this, improvement in the hot rolling technology of steel sheets has been demanded. Specifically, improvement in the characteristics of hot rolling rolls used in hot rolling equipment, particularly improvement in wear resistance, has been strongly demanded.

[0003] As hot rolling rolls, currently, HiCr cast steel rolls and high-speed rolls are used. In HiCr cast steel rolls, Cr-based M7C3 carbides are introduced into the structure to improve wear resistance. High-speed rolls contain carbide-forming elements such as V, Cr, Mo, and W based on high-speed steel, and a large amount of hard carbides such as V-based MC carbides, Mo and W-based M2C carbides, and Cr-based M7C3 carbides are introduced into the structure. Note that the above-mentioned high-speed steel is a kind of tool steel. M means a metal element forming a carbide.

[0004] Various technologies are being considered to improve the wear resistance of hot rolling rolls. For example, Patent Document 1 lists the following by mass%, C:0.7~3.6%, Si:0.2~2.5%, Mn:0.2~2.0%, Cr:2.0~10%, Mo:0.2~10%, V:2.0~10%, B:0.001~0.50%, Al:0.001~0.50%, Ti:0.001~0.50%, Zr:0.001~0.50%, Cu:0. An outer layer material for a centrifugal casting rolling mill composite roll is disclosed, containing 0.001-0.50%, Mg:0.001-0.50%, Ca:0.001-0.50%, with the remainder being Fe and unavoidable impurities, and further containing one or more of Ni:0.1-10%, W:0.2-10%, Nb:0.2-10%, and Co:0.2-10%. By doing so, an outer layer material having a microstructure in which MC carbides crystallize finely, uniformly, and spherically can be obtained, and the wear resistance of the outer layer material is improved.

[0005] Patent Document 2 discloses a rolling mill roll characterized by being an Fe-based alloy containing C: 1.0-2.6%, Cr: 4.0-10.0%, Mo: 5.0-10.0%, W: less than 5.0%, and V: 3.0-8.0%, satisfying 12.0% ≤ 2Mo + W ≤ 20.0%, 2Mo / W ≥ 3.0, and 0.2% ≤ C - 0.24V ≤ 0.7%. It states that by doing so, MC, M4C3, M2C, and M6C carbides, which particularly contribute to wear resistance, can be generated in an optimal range on the rolling mill roll, thereby improving the wear resistance of the rolling mill roll. Patent Document 2 also discloses the manufacture of a rolling mill roll by a centrifugal casting machine.

[0006] Patent Document 3 contains, by mass, C: 1.50~2.70%, Si: 0.3~3%, Mn: 0.1~3%, Ni: 0.1~2.5%, Cr: 4.0~7.0%, Mo: 4.1~8.0%, V: 5.0~10.0%, W: 0~0.4%, Nb: 0.1~3.0%, N: 0.005~0.15%, B: 0~0.05%, and further contains Co: 0.1~5%, Zr: 0.01~0.5%, Ti: 0.05~0.5%, and Al: 0.001~0.5%. Disclosed is an outer layer material for centrifugal casting composite rolls for rolling, characterized in that it contains at least one selected from the group, with the remainder being substantially Fe and unavoidable impurities, the ratio V / Nb of V content (mass%) to Nb content (mass%) is 1 to 20.0, and C-bal, expressed by the following formula: C-bal = C%-0.2×V%-0.06×Cr%-0.063×Mo%-0.033×W%-0.13×Nb%, is 0 to 0.28. In the above formula, C%, V%, Cr%, Mo%, W%, and Nb% are the mass%) content of C, V, Cr, Mo, W, and Nb, respectively. It is stated that this improves the wear resistance of the outer layer material for centrifugal casting composite rolls for rolling. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2002-161331 [Patent Document 2] Japanese Patent Publication No. 2004-183085 [Patent Document 3] Japanese Patent Publication No. 2020-22989 [Overview of the project] [Problems that the invention aims to solve]

[0008] The outer layer material for centrifugal casting composite rolls disclosed in Patent Document 1 is manufactured by injecting molten metal of the outer layer material into a cylindrical die rotating at high speed, and then cooling and solidifying the molten metal. Specifically, sand molds are provided on both sides of the cylindrical die in the longitudinal direction. An opening is formed on one side of the sand mold in the longitudinal direction, and the molten metal of the outer layer material is injected into the cylindrical die through this opening. The other side of the sand mold is closed and forms the bottom. Both ends of the cylindrical die are cooled by the sand molds, and one end of the cylindrical die is exposed to the outside air. Therefore, the ends of the cylindrical die cool more easily than the center of the cylindrical die. This can result in a difference in the solidification rate (sometimes referred to as the cooling rate) of the molten metal between the center and both ends of the cylindrical die in the longitudinal direction. In addition, the microstructure of carbides that improve the wear resistance of the outer layer material crystallizes when the molten metal solidifies inside the cylindrical die. Therefore, if differences in solidification rates occur as described above, variations may occur in the amount of carbides crystallized on the outer layer material along its length. Furthermore, this may cause variations in the amount of wear on the outer layer material along its length. Moreover, in areas with high wear, the roll having the outer layer material becomes more susceptible to grinding, potentially shortening the roll's lifespan. Thus, the outer layer material for centrifugal casting composite rolls disclosed in Patent Document 1 still had room for improvement in terms of wear resistance. The same applies to Patent Document 2, which describes the manufacture of rolling rolls using a centrifugal casting machine, and Patent Document 3, which describes the manufacture of the outer layer material for centrifugal casting composite rolls for rolling using a centrifugal casting machine.

[0009] The present invention was made to solve the above-mentioned problems, and aims to provide a hot rolling roll outer layer material that has excellent wear resistance over its entire length by suppressing variations in the amount of crystallized carbides in the longitudinal direction, a centrifugal casting die for casting the hot rolling roll outer layer material, a method for manufacturing the hot rolling roll outer layer material, and a hot rolling composite roll.

[0010] Here, "excellent wear resistance" means that the amount of wear measured by the Okoshi rapid wear tester is 0.0025g or less. The Okoshi rapid wear tester will be explained in the examples below. [Means for solving the problem]

[0011] To solve the above-mentioned problems, the inventors conducted a detailed investigation into the microstructure, carbides, chemical composition, and wear amount of the hot rolling roll outer layer material, as well as the manufacturing conditions of the hot rolling roll outer layer material by centrifugal casting. As a result, they found that the wear resistance of the hot rolling roll outer layer material can be improved by optimizing the chemical composition and manufacturing conditions of the hot rolling roll outer layer material so that the total amount of crystallized carbides over the entire length of the hot rolling roll outer layer material is within a specific range. Specifically, they found a Mo,W-based crystallized M2C type carbide that can improve wear resistance even in small amounts. Furthermore, they found that the wear resistance of the hot rolling roll outer layer material can be improved by optimizing the chemical composition of the hot rolling roll outer layer material so that the amount of crystallized M2C carbides is within a specific range.

[0012] This invention was completed based on these findings and further investigations. The gist of this invention is as follows: [1] Contains, by mass%, C: 1.2~2.6%, Si: 0.1~2.2%, Mn: 0.1~2.2%, Ni: 0.4~5.5%, Cr: 2.0~10.5%, Mo: 2.0~12.0%, V: 2.0~9.5%, W: 0.5~8.5%, P: 0.01~0.06%, S: 0.001~0.060%, with the remainder being Fe and unavoidable impurities, and having a component composition in which the content of Mo, W, V, and Cr satisfies the following formula (1), both in the length direction A roll outer layer material for hot rolling, wherein the total crystallized carbide area ratio at the ends and the central part is 8.5% or more and 19.0% or less, the coefficient of variation for each is 16.0 or less, the maximum difference between the total crystallized carbide area ratios at both ends and the central part is 6.0% or less, the maximum difference between the coefficients of variation for each is 10.0 or less, and the crystallized M2C type carbide area ratio is 1.5% or more and 8.0% or less. 0.14≦(0.063[%Mo]+0.033[%W]) / (0.177[%V]+0.099[%Cr]+0.063[%Mo]+0.033[%W])≦0.55 ···(1) In equation (1), [%V], [%Cr], [%Mo], and [%W] represent the mass percentage of each element. A centrifugal casting die for casting the outer layer material for hot rolling rolls described in [2][1], wherein the centrifugal casting die has a refractory layer covering the inner circumferential surface of the centrifugal casting die, and the ends of the centrifugal casting die are defined as being located at positions from both ends of the centrifugal casting die to 8 to 25% of the total length of the centrifugal casting die in the longitudinal direction of the centrifugal casting die, and the thickness of the refractory layer at these ends is 1.1 times or more and 2.2 times or less than the thickness of the refractory layer in the other parts. A method for manufacturing an outer layer material for hot rolling rolls as described in [3][1], wherein when a molten metal having the above-mentioned component composition is injected into a centrifugal casting die to centrifugal cast the outer layer material for hot rolling rolls, the inner circumferential surface of the centrifugal casting die is covered with a refractory material, and the ends of the centrifugal casting die are defined as being at positions from 8 to 25% of the total length of the centrifugal casting die from both ends in the longitudinal direction of the centrifugal casting die, and the thickness of the refractory material at these ends is 1.1 times or more and 2.2 times or less than the thickness of the refractory material at the other parts. [4] A composite roll for hot rolling having a two-layer structure of an outer layer and an inner layer, or a three-layer structure of an outer layer, an intermediate layer and an inner layer, wherein the outer layer is the hot rolling roll outer layer material described in [1]. [Effects of the Invention]

[0013] According to the present invention, it is possible to suppress variations in the lengthwise amount of crystallized carbides, thereby improving the wear resistance of the outer layer material for hot-rolling rolls. Furthermore, it is possible to improve the lifespan of hot-rolling composite rolls. Consequently, it is possible to improve the productivity of hot-rolled steel sheets manufactured using the outer layer material for hot-rolling rolls and the hot-rolling composite rolls. [Brief explanation of the drawing]

[0014] [Figure 1] It is a diagram showing a centrifugal casting machine to which the manufacturing method of the roll outer layer material for hot rolling according to this embodiment can be applied. [Figure 2] It is a cross-sectional view showing a part of the cylindrical mold enlarged. [Figure 3] It is a diagram for explaining the manufacturing method of the composite roll for hot rolling according to this embodiment. [Figure 4] It is a diagram for explaining the position where a test piece for tissue observation is taken from the ring-shaped test material. [Figure 5] It is a diagram for explaining the wear test by the Ogoshi-type rapid wear testing machine.

Mode for Carrying Out the Invention

[0015] Hereinafter, embodiments of the present invention (hereinafter referred to as "this embodiment") will be described. Note that the following description shows an example of this embodiment, and the present invention is not limited to the following embodiments. The roll outer layer material for hot rolling according to this embodiment is manufactured by the centrifugal casting method described later and can be used as a ring roll or a sleeve roll as it is. Alternatively, the roll outer layer material for hot rolling according to this embodiment can be applied as the outer layer (outer layer material) of a composite roll for hot rolling suitable for a hot rough rolling stand.

[0016] (Component Composition of Roll Outer Layer Material for Hot Rolling) First, the reasons for limiting the component composition of the roll outer layer material for hot rolling according to this embodiment (hereinafter simply referred to as "outer layer material") will be described. Hereinafter, unless otherwise specified, "mass%" is simply denoted as "%".

[0017] C: 1.2 to 2.6% Carbon (C) combines with elements such as V, Cr, Mo, and W to form hard crystallized carbides, thereby protecting the matrix structure and contributing to improved wear resistance. Here, hard crystallized carbides refer to MC-type carbides, M2C-type carbides, and M7C3-type carbides. In addition, carbon (C) has the effect of improving the hardness of the matrix through solid solution strengthening. If the carbon content is less than 1.2%, the amount of crystallized carbides is insufficient, and excellent wear resistance cannot be obtained. On the other hand, if the carbon content exceeds 2.6%, excessive crystallized carbides are produced, reducing surface roughness resistance and crack resistance. Furthermore, coarse carbides are produced, and the shedding of these coarse carbides actually reduces wear resistance. For this reason, the carbon content is limited to 1.2% or more and 2.6% or less. The carbon content is preferably 1.3% or more, more preferably 1.4% or more. Also, the carbon content is preferably 2.4% or less, more preferably 2.2% or less. The C content is more preferably 1.3% to 2.4%, and even more preferably 1.4% to 2.2%.

[0018] Si: 0.1~2.2% Si acts as a deoxidizing agent in molten metal, improving its fluidity and preventing casting defects. Below 0.1% Si content, the deoxidizing effect is insufficient. On the other hand, above 2.2% Si content, the deoxidizing effect saturates. Therefore, the Si content is limited to between 0.1% and 2.2%. The Si content is preferably 0.2% or more, more preferably 0.4% or more. Furthermore, the Si content is preferably 2.0% or less, more preferably 1.8% or less. A Si content of 0.2% to 2.0% is more preferable, and a Si content of 0.4% to 1.8% is even more preferable.

[0019] Mn: 0.1~2.2% Mn has the effect of fixing and removing sulfur (S), which has adverse effects on the product, as MnS. When the Mn content is less than 0.1%, the effect of fixing S as MnS is not observed. On the other hand, when the Mn content exceeds 2.2%, this effect saturates. Therefore, the Mn content is limited to 0.1% or more and 2.2% or less. The Mn content is preferably 0.2% or more, more preferably 0.3% or more. Furthermore, the Mn content is preferably 2.0% or less, more preferably 1.8% or less. The Mn content is more preferably 0.2% or more and 2.0%, and even more preferably 0.3% or more and 1.8% or less.

[0020] Ni: 0.4~5.5% Ni is an element that improves the hardenability of the matrix and has the effect of improving the hardness of the matrix. If the Ni content is less than 0.4%, the effect of improving the hardness of the matrix is ​​insufficient. On the other hand, if the Ni content exceeds 5.5%, it promotes the retention of austenite, which reduces the hardness. For this reason, the Ni content is limited to 0.4% or more and 5.5% or less. The Ni content is preferably 0.6% or more, more preferably 0.8% or more. Furthermore, the Ni content is preferably 5.2% or less, more preferably 5.0% or less. The Ni content is more preferably 0.6% or more and 5.2% or less, and even more preferably 0.8% or more and 5.2% or less.

[0021] Cr: 2.0~10.5% Cr is a carbide-forming element that combines with C to form M7C3 type carbides. M7C3 type carbides are hard carbides and therefore have the effect of improving wear resistance. If the Cr content is less than 2.0%, the amount of M7C3 type carbides is insufficient, and wear resistance decreases. On the other hand, if the Cr content exceeds 10.5%, coarse M7C3 type carbides are formed, and wear resistance actually worsens. For this reason, the Cr content is limited to 2.0% or more and 10.5% or less. The Cr content is preferably 2.5% or more, more preferably 3.0% or more. Also, the Cr content is preferably 9.0% or less, more preferably 8.5% or less. The Cr content is more preferably 2.5% or more and 9.0%, and even more preferably 3.0% or more and 8.5% or less.

[0022] Mo: 2.0~12.0% Mo is a carbide-forming element that combines with C to form M2C-type carbides. M2C-type carbides are harder than M7C3-type carbides, crystallize in a planar and lamellar manner, and even in small amounts, efficiently improve wear resistance. If the Mo content is less than 2.0%, the amount of M2C-type carbides is insufficient, and the effect of improving wear resistance is inadequate. On the other hand, if the Mo content exceeds 12.0%, coarse lamellar M2C-type carbides are formed, leading to chipping and actually worsening wear resistance, as well as a decrease in toughness. Therefore, the Mo content is limited to 2.0% or more and 12.0% or less. The Mo content is preferably 2.5% or more, more preferably 3.0% or more. Furthermore, the Mo content is preferably 11.0% or less, more preferably 10.0% or less. The Mo content is preferably 2.5% to 11.0%, and more preferably 3.0% to 10.0%.

[0023] V: 2.0~9.5% V is a carbide-forming element that combines with C to form MC-type carbides. MC-type carbides have a Vickers hardness of approximately 2800 Hv, making them one of the hardest carbides and improving wear resistance. If the V content is less than 2.0%, the amount of MC-type carbides is insufficient, and the effect of improving wear resistance is not adequate. On the other hand, if the V content exceeds 9.5%, VC, which has a lower specific gravity than molten iron, concentrates on the inside of the outer layer material (in the case of composite rolls described later, this becomes the "outer layer," and the same applies hereafter) due to the centrifugal force during centrifugal casting, causing segregation. Therefore, the V content is limited to between 2.0% and 9.5%. VC refers to vanadium carbide, which is a type of MC-type carbide. The V content is preferably 2.5% or more, more preferably 3.0% or more. Also, the V content is preferably 8.5% or less, and more preferably 7.0% or less. The V content is more preferably 2.5% to 8.5%, and even more preferably 3.0% to 7.0%.

[0024] W: 0.5~8.5% W is a carbide-forming element and, like Mo, combines with C to form hard carbides such as hard M2C-type carbides, thereby improving wear resistance. If the W content is less than 0.5%, the effect is insufficient and wear resistance deteriorates. On the other hand, if the W content exceeds 8.5%, coarse M2C-type eutectic carbides are formed, and wear resistance actually deteriorates. Therefore, the W content is limited to 0.5% or more and 8.5% or less. The W content is preferably 0.6% or more, more preferably 0.8% or more. Furthermore, the W content is preferably 7.0% or less, more preferably 6.0% or less. The W content is more preferably 0.6% or more and 7.0%, and even more preferably 0.8% or more and 6.0% or less.

[0025] P: 0.01~0.06% While phosphorus (P) has been thought to be introduced during the roll manufacturing process and degrade mechanical properties, our inventors have diligently investigated and found that a small amount of P improves wear resistance. Below 0.01% P content, the improvement in wear resistance is insufficient. On the other hand, above 0.06% P content, mechanical properties deteriorate. Therefore, the P content is limited to 0.01% or more and 0.06% or less. Preferably, the P content is 0.02% or more. Preferably, the P content is 0.05% or less. More preferably, the P content is 0.02% or more and 0.05% or less.

[0026] S: 0.001~0.060% S is usually treated as a harmful element in iron-based alloys and its content is limited to below a certain amount, but within that range, MnS has a lubricating effect. Furthermore, as a result of our diligent research, we have revealed that trace amounts of MnS have the effect of improving wear resistance. If the S content is less than 0.001%, the lubricating effect and the effect of improving wear resistance are not sufficient. On the other hand, if the S content is too high, that is, above 0.060%, the material becomes brittle. Therefore, the S content is limited to 0.001% or more and 0.060% or less. The S content is preferably 0.002% or more. Also, the S content is preferably 0.050% or less. The S content is more preferably 0.002% or more and 0.050% or less.

[0027] Furthermore, the present invention is characterized in that, in addition to the content of C, Cr, Mo, V, W, Ni, Mn, and Si being within the above-mentioned range, it also satisfies the following formula (2).

[0028] 0.14≦(0.063[%Mo]+0.033[%W]) / (0.177[%V]+0.099[%Cr]+0.063[%Mo]+0.033[%W])≦0.55 ···(2) In equation (2), [%V], [%Cr], [%Mo], and [%W] represent the content (mass %) of each element, respectively.

[0029] Regarding ((0.063[%Mo]+0.033[%W]) / (0.177[%V]+0.099[%Cr]+0.063[%Mo]+0.033[%W])) in equation (2): This parameter represents the ratio of the content of M2C-type carbide-forming elements (Mo, W) to the total carbide-forming elements (Mo, W, V, Cr). By adjusting the ratio of these content to satisfy equation (2), the proportion of crystallized M2C-type carbides is optimized, and wear resistance is improved.

[0030] Furthermore, if the lower limit of the content ratio is less than 0.14, the proportion of crystallized M2C-type carbides will be insufficient, resulting in reduced wear resistance. On the other hand, if the upper limit of the content ratio exceeds 0.55, the proportion of M2C-type carbides crystallizing in a planar and lamellar manner may become too high. When this occurs, stress concentrates on the M2C-type carbides, which have low toughness, causing cracks to form and chipping, which actually worsens wear resistance. For this reason, the value of the content ratio mentioned above was limited to 0.14 or more and 0.55 or less. It is more preferable that the value of the content ratio mentioned above is 0.20 or more and 0.50 or less.

[0031] Remainder: Fe and unavoidable impurities The remainder of the mixture, other than the components mentioned above, consists of Fe and unavoidable impurities. Examples of unavoidable impurities include Mg, Zr, REM (rare earth metals), Sn, As, Sb, Bi, Pb, Zn, N, and O. However, within limits that do not impair the effects of the present invention, the mixture may contain 0.03% or less of Mg, Zr, and REM, 0.04% or less of Sn, As, and Sb, and 0.01% or less of Bi, Pb, Zn, N, and O. The REM mentioned above is a collective term for Sc, Y, and a total of 17 lanthanide elements. One or more of these 17 elements may be included as unavoidable impurities, and the REM content refers to the total content of these elements.

[0032] (Microstructure of outer layer material for hot-rolling rolls) Next, the reason for the limitation of the microstructure (hereinafter simply referred to as "structure") of the hot rolling roll outer layer material of the present invention will be explained. Past research results have shown that the wear resistance of the hot rolling roll outer layer material tends to improve as the area ratio of crystallized carbides increases. However, it has also been shown that if the area ratio of crystallized carbides is excessively large, the wear resistance actually decreases. That is, when the area ratio of crystallized carbides is large, there are many hard carbides on the surface (sometimes referred to as the contact surface) of the hot rolling roll outer layer material. When this hot rolling roll outer layer material is applied to a rolling mill, even if the matrix structure of the hot rolling roll outer layer material wears down, carbides that are harder than the matrix structure remain on the surface of the hot rolling roll outer layer material without wearing down. Therefore, the carbides that are harder than the matrix structure support the contact load during rolling and suppress further wear of the matrix structure. Based on this principle, it is believed that when the area ratio of crystallized carbides is large, the wear resistance of the outer layer material of hot-rolling rolls improves.

[0033] On the other hand, if the area ratio of crystallized carbides is excessively large, a large amount of coarse carbides will be present on the surface of the hot rolling roll outer layer material. When this hot rolling roll outer layer material is applied to a rolling mill, the coarse crystallized carbides may detach from the matrix structure of the hot rolling roll outer layer material. This is because coarse crystallized carbides are brittle and therefore prone to cracking during rolling. As the cracks propagate, the coarse crystallized carbides break down and gradually become smaller. Furthermore, during rolling, the broken and smaller crystallized carbides repeatedly receive contact loads and chip off one after another. As a result, the carbides, which are harder than the matrix structure, cannot support the contact load during rolling, making the matrix structure of the hot rolling roll outer layer material more susceptible to wear. Based on this principle, it is believed that if the area ratio of crystallized carbides is excessively large, the wear resistance actually decreases.

[0034] Furthermore, during the centrifugal casting process of the outer layer material for hot rolling mills, differences in the solidification rate (sometimes referred to as the cooling rate) of the molten metal may occur along the length of the outer layer material. When such a situation occurs, microstructure irregularities may occur along the length of the outer layer material. Here, microstructure irregularities refer to variations in the amount of carbides crystallized along the length of the outer layer material and fluctuations in the area ratio of crystallized carbides. In other words, this means that the wear resistance is not uniform along the length of the outer layer material. The inventors of the present invention have diligently studied how to make the wear resistance of the outer layer material for hot rolling mills uniform along the length, and have discovered the following.

[0035] Total crystallized carbide area ratio: 8.5~19.0% Specifically, the hot-rolling roll outer layer material according to this embodiment is characterized by having a component composition within the numerical range described above. Furthermore, the hot-rolling roll outer layer material is characterized by having a total area ratio of 8.5% to 19.0% at both ends and the central part in the longitudinal direction. If the total area ratio of crystalline carbides is less than 8.5%, there will be insufficient hard carbides, and the wear resistance will deteriorate. Therefore, the area ratio of crystalline carbides is set to 8.5% or more. Preferably, the area ratio of crystalline carbides is 9.0% or more. On the other hand, if the area ratio of crystalline carbides exceeds 19.0%, coarse carbides will be generated, and the wear resistance will actually deteriorate as the coarse carbides with low toughness chip off. Therefore, the area ratio of crystalline carbides is set to 19.0% or less. Preferably, the area ratio of crystalline carbides is 18.0% or less. It is more preferable that the total crystallized carbide area ratio is between 9.0% and 18.0%.

[0036] Furthermore, the coefficient of variation of the total crystallized carbide area ratio at both ends and the central part is 16.0 or less. If the coefficient of variation of the total crystallized carbide area ratio at each of these points is greater than 16.0, the variation in the crystallized carbide area ratio will be large even within the positions of both ends or the central part, resulting in differences in the amount of wear. Preferably, it is 12.0 or less. Also, the maximum difference between the total crystallized carbide area ratios at both ends and the central part is 6.0% or less. If the maximum difference between the total crystallized carbide area ratios is greater than 6.0%, the variation in the crystallized carbide area ratio between both ends and the central part will be large, resulting in differences in the amount of wear. Preferably, the maximum difference between the total crystallized carbide area ratios is 3.0% or less. Furthermore, the maximum difference between the coefficients of variation is 10.0 or less. If the maximum difference between the coefficients of variation exceeds 10.0, the variation in the crystallized carbide area ratio between both ends and the central part will be large, resulting in differences in the amount of wear. The maximum difference between the coefficients of variation is preferably 7.0 or less.

[0037] Here, whole crystallized carbides refer to MC-type carbides, M2C-type carbides, and M7C3-type carbides. The coefficient of variation is the value obtained by dividing the standard deviation of the whole crystallized carbide area ratio at both ends and the central part, measured by the method described later, by the average value of the whole crystallized carbide area ratio, and is expressed by the following formula (3). Coefficient of variation = (Standard deviation of total crystallized carbide area ratio / Mean value of total crystallized carbide area ratio) × 100 ... (3)

[0038] The maximum difference between total crystallized carbide area ratios is the largest absolute value of the difference in total crystallized carbide area ratios between the two ends, or between each end and the center. That is, calculate the difference between the total crystallized carbide area ratio at one end of the hot rolling roll outer layer material and the total crystallized carbide area ratio at the other end. Similarly, calculate the difference between the total crystallized carbide area ratio at one end and the total crystallized carbide area ratio at the center, and the difference between the total crystallized carbide area ratio at the other end and the total crystallized carbide area ratio at the center. The largest absolute value of these three differences is the maximum difference in total crystallized carbide area ratios. The maximum difference between coefficients of variation is the largest absolute value of the difference in coefficients of variation between the two ends, or between the two ends and the center, of the hot rolling roll outer layer material. That is, calculate the difference between the coefficient of variation at one end of the hot rolling roll outer layer material and the coefficient of variation at the other end. Similarly, calculate the difference between the coefficient of variation at one end and the coefficient of variation at the center, and the difference between the coefficient of variation at the other end and the coefficient of variation at the center. The maximum absolute value of these differences is the maximum difference in coefficients of variation.

[0039] Crystallized M2C type carbide area ratio: 1.5~8.0% Furthermore, the outer layer material for hot rolling rolls according to this embodiment is characterized by having an area ratio of crystallized M2C type carbides of 1.5% or more and 8.0% or less. Crystallized M2C type carbides are the carbides that contribute the most to wear resistance among crystallized carbides. If the area ratio of crystallized M2C type carbides is less than 1.5%, there will be insufficient crystallized M2C type carbides, and wear resistance will deteriorate. For this reason, the area ratio of crystallized M2C type carbides should be 1.5% or more. Preferably, the area ratio of crystallized M2C type carbides is 2.0% or more. On the other hand, if the area ratio of crystallized M2C type carbides exceeds 8.0%, coarse M2C type carbides will be generated, and wear resistance will actually deteriorate as the coarse crystallized M2C type carbides with low toughness chip off. For this reason, the area ratio of crystallized M2C type carbides should be 8.0% or less. Preferably, the area ratio of crystallized M2C type carbides is 7.5% or less. It is more preferable that the area ratio of crystallized M2C type carbides is between 2.0% and 8.0%.

[0040] Furthermore, the aforementioned structural irregularities can be suppressed by controlling the thickness of the refractory layer of the cylindrical mold used when centrifugal casting the outer layer material for hot rolling, which will be described later.

[0041] (Method for observing the microstructure of the outer layer material of hot-rolling rolls) Next, we will explain the method for observing the microstructure of the outer layer material for hot rolling mills. Samples for microstructure observation were cut from the outer surfaces of both ends in the longitudinal direction and the center of the manufactured outer layer material for hot rolling mills. Samples for microstructure observation were also cut from both ends in the longitudinal direction and the center of the center (hereinafter referred to as the center of the thickness; sometimes also referred to as the Z-plane) of the outer layer material for hot rolling mills. These samples were mirror-polished and then etched with Nital solution. After that, the microstructure of the outer surface and the center of the thickness (Z-plane) of the outer layer material for hot rolling mills was observed using a digital microscope, and the area ratio of all crystallized carbides was measured by image analysis.

[0042] Next, using ImageJ as an image analysis tool, the photographs were binarized at a measurement magnification of 200x. This is because there is a difference in brightness between the matrix structure and the carbides in the photographs. By performing binarization, the matrix structure and carbides can be classified, and their respective areas can be determined. In this embodiment, for each sample, 30 photographs of the corroded surface were taken after polishing, and for each sample, the average area ratio of all crystallized carbides was calculated, with the entire image (roll structure) being set to 100%. This average value was taken as the "area ratio of all crystallized carbides (%)" in the range from the outer surface to the center of the thickness of the hot-rolling roll outer layer material. In addition, the standard deviation was calculated from the area ratio of all crystallized carbides in the 30 images of the outer surface of both ends and the center of the hot-rolling roll outer layer material taken at this time, and the coefficient of variation was calculated by dividing by the average value. Furthermore, the standard deviation was calculated from the total crystallized carbide area ratio of 30 images taken from the central part (Z-plane) of each thickness at both ends and the central part, and the coefficient of variation was calculated by dividing by the mean value.

[0043] To calculate the area ratio of crystallized M2C-type carbides, the crystal orientation of the carbides and the matrix structure was simultaneously measured using SEM-EBSD on the same sample used for microstructural observation. The images of the crystallized M2C-type carbides and the matrix structure were separated by utilizing the difference in crystal structure. The SEM-EBSD measurement conditions were: acceleration voltage 15kV, magnification 200x, measurement range 600×1100μm, and step size 0.5μm. Prior to the SEM-EBSD measurement, the OIM Data Collection software from TSL Solutions Co., Ltd. was configured to measure crystallized M2C-type carbides. The measurement data of crystallized M2C-type carbides obtained by SEM-EBSD measurement was processed using OIM Analysis from EDAX. ImageJ was used as the image analysis tool to measure the area ratio of crystallized M2C-type carbides only. Similar to the measurement of the total crystallized carbide area ratio described above, the average value of the crystallized M2C-type carbide area ratio obtained from 30 field-of-view images was calculated. This average value was defined as the "area percentage (%) of crystallized M2C type carbides" in the range from the outer surface to the center of the thickness of the outer layer material of the hot-rolling roll.

[0044] (Hardness of the outer layer material for hot-rolling rolls) The hardness of the outer layer material of the hot rolling roll according to this embodiment is preferably 44.0 HS to 52.0 HS in Shore hardness at 600°C, and 74.0 HS to 86.0 HS in Shore hardness at 20°C. If the Shore hardness at 20°C is less than 74.0 HS, the wear resistance deteriorates. If the Shore hardness at 20°C exceeds 86.0 HS, it becomes difficult to grind away cracks formed on the outer surface of the hot rolling roll during hot rolling. The temperature of the outer surface of the hot rolling roll during hot rolling is around 600°C, and if the Shore hardness at 600°C is less than 44.0 HS, plastic flow occurs, and the steel material is more likely to stick to the outer surface of the hot rolling roll. If the Shore hardness at 600°C exceeds 52.0 HS, the Shore hardness of the hot rolling roll is too high, making it easier for slip to occur during hot rolling. The hardness of the hot rolling roll outer layer material according to this embodiment can be stably ensured by heat-treating the hot rolling roll outer layer material according to this embodiment, which has the above-described component composition, so that the tempering parameter P, described later, is within the range of 12,000 to 19,000.

[0045] (Method for manufacturing outer layer material for hot-rolling rolls) A method for manufacturing the outer layer material for hot rolling rolls according to this embodiment will now be described. One such manufacturing method is centrifugal casting. In centrifugal casting, a cylindrical mold is rotated around its central axis, and molten metal of the outer layer material for hot rolling rolls according to this embodiment is injected into the inside of the cylindrical mold. This generates centrifugal force in the molten metal injected into the cylindrical mold, causing the molten metal to adhere closely to the inner surface of the cylindrical mold, thus centrifugal casting the outer layer material for hot rolling rolls.

[0046] Figure 1 shows a centrifugal casting machine 1 to which the method for manufacturing the outer layer material for hot-rolling rolls according to this embodiment can be applied. The centrifugal casting machine 1 shown in Figure 1 has a bottomed cylindrical mold 2, a rotation support part 3 that rotatably supports the cylindrical mold 2, and a rotation part (not shown) that rotates the cylindrical mold 2 around a predetermined point. The cylindrical mold 2 has an outer mold part 4 for centrifugal casting of the outer layer material, and shaft mold parts 5 and 6 located on both sides of the outer mold part 4 in the longitudinal direction of the cylindrical mold 2, for centrifugal casting of the shaft portion of the composite roll for hot-rolling. As shown in Figure 1, the inner diameters of the shaft mold parts 5 and 6 are smaller than the inner diameter of the outer mold part 4. The outer mold part 4 described above corresponds to the centrifugal casting mold of this embodiment.

[0047] The cylindrical mold 2 rotates around its central axis in conjunction with the rotation of the roller 11, which will be described later. An opening 7 is formed at one end of the cylindrical mold 2 in the axial direction. Molten outer layer material having the above-described component composition (hereinafter referred to as outer layer material molten metal) is poured into the cylindrical mold 2 through the opening 7. The other end of the cylindrical mold 2 in the axial direction is closed and forms the bottom. In the example shown in Figure 1, the outer layer material molten metal stored in the ladle 8 is temporarily received in the pouring vat 9. The pouring vat 9 has a spout 10 that extends in a cylindrical shape. The spout 10 is inserted into the cylindrical mold 2, and the tip of the spout 10 is positioned on the side of the opening 7 of the outer layer mold part 4. In this state, the outer layer material molten metal is poured into the cylindrical mold 2 from the spout 10.

[0048] The rotating support section 3 has rollers 11 that contact the outer circumferential surface of the cylindrical mold 2, and the cylindrical mold 2 is placed on these rollers 11. The rollers 11 are connected to an actuator (not shown) and are rotated by the actuator. The rollers 11 are also rotatably arranged on a support base (not shown).

[0049] The rotating part rotates the cylindrical mold 2 around a predetermined point on the bottom side of the cylindrical mold 2 so that the central axis of the cylindrical mold 2 is approximately parallel to the horizontal plane (referred to as the horizontal state). The rotating part also rotates the cylindrical mold around a predetermined point so that the central axis of the cylindrical mold 2 is perpendicular to the horizontal plane, thereby raising the cylindrical mold 2 upright. The centrifugal casting machine 1 rotates the horizontal cylindrical mold 2 using the actuator of the rotating support part 3 and injects the molten outer layer material into the rotating cylindrical mold 2. Figure 1 shows this state.

[0050] Figure 2 is an enlarged cross-sectional view showing a part of the cylindrical mold 2. As shown in Figure 2, the inner circumferential surfaces of the shaft mold sections 5 and 6 are covered with sand, and a sand layer (sometimes called a sand mold) 12 is formed on the inner circumferential surfaces of the shaft mold sections 5 and 6 by the sand. The inner circumferential surface of the outer layer mold section 4 is covered with refractory material, and a refractory layer (sometimes called a refractory mold) 13 is formed on the inner circumferential surface of the outer layer mold section 4 by the refractory material.

[0051] Furthermore, as shown in Figure 2, the thickness of the refractory layer 13 at both ends of the outer mold section 4 in the axial direction (length direction) of the cylindrical mold 2 is greater than the thickness of the refractory layer 13 at the center of the outer mold section 4. The reason for this is explained below. When the outer mold section 4 is formed by injecting molten outer material into the interior of the outer mold section 4, heat tends to accumulate more easily at the center of the outer mold section 4 compared to the ends. Therefore, the solidification rate of the molten metal at the center of the outer mold section 4 is slower than the solidification rate of the molten metal at both ends. This difference in the solidification rate of the molten outer material may cause a difference in the amount of carbides crystallized as the molten outer material solidifies between the center and both ends of the outer mold section 4. Therefore, by making the thickness of the refractory layer 13 at both ends of the outer mold section 4 greater than the thickness of the refractory layer 13 at the center, the ends of the outer mold section 4 are kept warm and cooling at the ends is suppressed. In other words, the solidification rate of the molten metal is made uniform throughout the entire length of the outer mold section 4. In the following description, the refractory layer 13 on the central side of the outer mold section 4 in the longitudinal direction will be referred to as the first refractory layer 13a, and the refractory layers 13 on both ends of the outer mold section 4 will be referred to as the second refractory layer 13b.

[0052] Specifically, in this embodiment, the thickness of the first refractory layer 13a is 1 to 5 mm, and the thickness of the second refractory layer 13b is between 1.1 and 2.2 times the thickness of the first refractory layer 13a. When the thickness of the second refractory layer 13b is less than 1.1 times the thickness of the first refractory layer 13a, the solidification rate of the molten outer layer material at both ends of the outer layer mold section 4 becomes faster than at the central end. As a result, the amount of carbides crystallized at both ends of the outer layer mold section 4 is less than at the central end. On the other hand, when the thickness of the second refractory layer 13b exceeds 2.2 times the thickness of the first refractory layer 13a, the solidification rate of the molten outer layer material at both ends of the outer layer mold section 4 becomes slower than at the central end. As a result, the amount of carbides crystallized at both ends of the outer layer mold section 4 becomes excessively large compared to the central end. Therefore, the thickness of the second refractory layer 13b was limited to 1.1 times or more and 2.2 times or less the thickness of the first refractory layer 13a. Here, the refractory material may be a conventionally known refractory material mainly composed of zirconium orthosilicate (ZrO2·SiO2). Furthermore, the ends of the outer layer mold section 4 refer to the outermost ends in the axial direction of the outer layer mold section 4, that is, from the boundary between the outer layer mold section 4 and the shaft mold sections 5 and 6 towards the center of the outer layer mold section 4, up to 8 to 25% of the total length of the outer layer mold section 4. In order to make the outer diameter of the outer layer material uniform over its entire length, the ends of the outer layer mold section 4 where the second refractory layer 13b is formed are radially recessed outward compared to the center of the outer layer mold section 4 where the first refractory layer 13a is formed. Therefore, the inner diameter of the first refractory layer 13a and the inner diameter of the second refractory layer 13b are the same.

[0053] (Composite rolls for hot rolling) The hot-rolling composite roll according to this embodiment can have a two-layer structure consisting of an outer layer (the outer layer material of the hot-rolling roll according to this embodiment) and an inner layer that is welded and integrated with the outer layer. Alternatively, the hot-rolling composite roll according to this embodiment may have a three-layer structure with an intermediate layer between the outer layer and the inner layer. In the three-layer hot-rolling composite roll, the intermediate layer is welded and integrated with the outer layer, and the inner layer is welded and integrated with the intermediate layer. It is preferable that the inner layer be manufactured by a static casting method.

[0054] (Method for manufacturing composite rolls for hot rolling) Figure 3 is a diagram illustrating the manufacturing method of a composite roll for hot rolling according to this embodiment. In the example shown in Figure 3, the cylindrical mold 2 is rotated while tilted by the tilting part of the centrifugal casting machine 1, and molten outer layer material is poured into the outer layer mold part 4 of the cylindrical mold 2. Figure 3(A) shows this state. Next, while maintaining the rotation of the cylindrical mold 2, the molten outer layer material is solidified to form the outer layer material in the outer layer mold part 4.

[0055] When forming an intermediate layer, as shown in Figure 3(B), the cylindrical mold 2 is tilted and rotated by the tilting mechanism. In this state, molten metal (hereinafter sometimes referred to as intermediate layer molten metal) having a different component composition from the outer layer molten metal is poured into the outer layer mold section 4 where the outer layer material is formed. The intermediate layer molten metal is solidified while maintaining the rotation of the cylindrical mold 2 to form the intermediate layer in the outer layer mold section 4. When forming an inner layer, as shown in Figure 3(C), the rotation of the cylindrical mold 2 is stopped and the cylindrical mold 2 is raised. In this state, molten metal (hereinafter sometimes referred to as inner layer molten metal) that forms the inner layer is poured into the inside of the cylindrical mold 2. The inner layer molten metal is solidified with the cylindrical mold 2 in the raised position to form the inner layer inside the cylindrical mold 2.

[0056] For the inner layer to be cast by static casting, it is preferable to use spheroidal graphite cast iron or cyanomorphic graphite cast iron (CV cast iron), which have excellent castability and mechanical properties. In a two-layer composite roll for hot rolling, the outer layer and inner layer are welded together, and components of the outer layer material are mixed into the inner layer during the welding process. When carbide-forming elements such as Cr and V contained in the outer layer material are mixed into the inner layer, the inner layer becomes weakened. Therefore, it is preferable to suppress the mixing of outer layer components into the inner layer as much as possible. There are various methods for suppressing the mixing of outer layer components into the inner layer, but one example is optimizing the casting temperature and casting timing. Alternatively, the mixing of outer layer components into the inner layer may be suppressed by forming an intermediate layer between the outer layer and the inner layer.

[0057] Furthermore, it is preferable to use graphite steel, high-carbon steel, or hypoeutectic cast iron as the intermediate layer. In a three-layer composite roll for hot rolling, the intermediate layer and the outer layer are welded together, and components of the outer layer material are mixed into the intermediate layer during the welding process. When the intermediate layer, which contains components of the outer layer material, is welded to the inner layer, the components of the outer layer material mixed into the intermediate layer move into the inner layer. Therefore, in order to suppress the mixing of components of the outer layer material into the inner layer, it is preferable to suppress the mixing of components of the outer layer material into the intermediate layer as much as possible. There are various methods for suppressing the mixing of components of the outer layer material into the intermediate layer, but one example is optimizing the casting temperature and casting timing.

[0058] The heat treatment for the composite roll for hot rolling according to this embodiment will now be described. In this heat treatment, quenching and tempering are performed in that order. In the quenching process, the composite roll for hot rolling is heated to 900 to 1100°C, and then air-cooled or blast-cooled. In the tempering process, the composite roll for hot rolling is heated and held so that the tempering parameter P described in the following formula (4) is within the range of 12000 to 19000, and then cooled. It is preferable to perform the tempering process two or more times. During such heat treatment, the quenching temperature, tempering parameter, and number of tempering cycles are adjusted according to the outer layer component, and by changing the tempering parameter within the range described, it is possible to obtain a structure with adjusted hardness of the outer layer material.

[0059] P = T(log(t) + A) ... (4) T is the tempering temperature (K), t is the tempering time (h), and A is a constant. In this embodiment, A = 20.

[0060] The temperature of the hot-rolling composite roll during heat treatment is the internal temperature of the hot-rolling composite roll at a position 5 mm below the outer surface in the radial direction of the hot-rolling composite roll. This temperature is measured by embedding a thermocouple at the aforementioned position in the hot-rolling composite roll.

[0061] Based on the above, a composite roll for hot rolling can be obtained having three layers: an outer layer, an intermediate layer, and an inner layer, or two layers: an outer layer and an inner layer.

[0062] (Effects / Actions) According to this embodiment, when centrifugal casting is performed to produce the outer layer material for hot rolling rolls, the second refractory layer 13b keeps both ends of the outer layer mold section 4 warm. This makes it possible to make the solidification rate of the molten outer layer material uniform over the entire length of the outer layer mold section 4. Furthermore, it is possible to suppress and make uniform variations in the amount of crystallized carbides over the entire length of the outer layer material for hot rolling rolls. As a result, the wear resistance of the outer layer material for hot rolling rolls and the composite rolls for hot rolling having said outer layer material can be improved. Their lifespan can also be improved. Ultimately, the productivity of hot-rolled steel sheets manufactured using the outer layer material for hot rolling rolls and composite rolls for hot rolling can be improved.

[0063] It should be noted that the present invention is not limited to the embodiments described above. For example, the centrifugal casting machine to which the method for manufacturing the outer layer material for hot rolling rolls according to this embodiment can be applied is not limited to the horizontal centrifugal casting machine 1 shown in Figure 1. A vertical centrifugal casting machine or an inclined centrifugal casting machine may be used instead of the horizontal centrifugal casting machine shown in Figure 1. When the method for manufacturing the outer layer material for hot rolling rolls according to this embodiment is applied to such centrifugal casting machines, the same effects and advantages as those of the embodiment described above can be obtained. [Examples]

[0064] The present invention will be described in more detail by the following examples. The present invention is not limited to the following examples.

[0065] Table 1 summarizes the chemical composition of the test material for the present invention example and the test material for the comparative example. The molten metal with the component composition of the outer layer material shown in Table 1 was melted in a high-frequency melting furnace, and a ring-shaped test material (outer diameter: φ250 mm, width: 65 mm, wall thickness: 55 mm) was prepared by centrifugal casting. The remainder other than the component composition shown in Table 1 is Fe and unavoidable impurities.

[0066] Next, after the ring-shaped test material had cooled, it was removed from the mold and hardened at 900-1100°C. Then, the ring-shaped test material was heated, held, and cooled three times so that the tempering parameter P, represented by equation (4) above, was within the range of 12000-19000. After that, a test piece was taken from each ring-shaped test material for microstructural observation and abrasion testing using the Okoshi rapid abrasion testing machine.

[0067] Specimens for microstructure observation were taken from both ends and the center of the ring-shaped test material in the longitudinal direction. Figure 4 is a diagram illustrating the positions for taking specimens for microstructure observation from the ring-shaped test material. As shown in Figure 4, specimens for microstructure observation were taken from the center of the ring-shaped test material in the thickness direction, at both ends and the center of the ring-shaped test material. In addition, a specimen for evaluating the amount of wear (60 × 25 × 5 mm, with a test surface of 60 × 25 mm) was taken from the ring-shaped test material so that it was 60 mm in the longitudinal direction and 5 mm in the thickness direction. In the Okoshi rapid wear tester, the wear resistance of the test surface is evaluated by sliding contact of a mating material with the test surface and wearing down the test surface by the amount of wear. In this wear test, the mating material was slid into contact with the test surface at a position 5 mm from both ends and in the center.

[0068] [Table 1]

[0069] The abrasion tests using the Okoshi rapid abrasion testing machine were conducted as follows. Test pieces for abrasion testing were taken from each of the ring-shaped test materials No. 1 to 25. Okoshi abrasion test pieces (60 × 25 × 5 mm) were taken from the test pieces of each of the invention examples 1 to 15 and each of the comparative examples 16 to 25. Figure 5 is a diagram illustrating the abrasion test using the Okoshi rapid abrasion testing machine 14. As shown in Figure 5, in the Okoshi rapid abrasion testing machine 14, the mating material 16 is brought into sliding contact with each test piece 15 without lubrication. In this way, each test piece 15 was abraded.

[0070] The abrasion resistance of each test piece 15 was evaluated by the volume of abrasion particles (hereinafter referred to as abrasion volume) W. Specifically, a ring-shaped S45C tempered material with a diameter of φ60 mm and a width of 3 mm was used as the mating material 16. The ring width of the mating material 16 refers to the length in the direction along the central axis of the ring. The mating material 16, rotated at 0.61 m / s, was brought into contact with the test piece 15, which was fixed to the Okoshi-type rapid abrasion testing machine 14, at room temperature and without lubrication, with a sliding distance of 600 m and a final load of 12.6 kgf (123.48 N). In this way, the test piece 15 was abraded, and the abrasion volume W (mm³) of the test piece 15 was evaluated. 3 The abrasion resistance of test piece 15 was evaluated by the following method. The abrasion volume W was calculated using the following approximate formula (5).

[0071] W = (Bb³) / 12r ... (5) r: Average value of the ring radius of the mating material before and after the abrasion test, B: Ring width of the mating material, b: Length of the abrasion marks formed on the test piece. Also, the abrasion volume (mm) 3 ) The density of iron is 7.85 × 10 -3 (g / mm 3 The amount of wear (g) was calculated by multiplying by ). The results of the wear test are shown in Table 2. In Table 2, of the ends of the centrifugal-cast outer layer material, the end on the opening side of the cylindrical mold is referred to as one end, and the end on the bottom side of the cylindrical mold is referred to as the other end.

[0072] [Table 2]

[0073] In this example, a wear amount of 0.0025g or less was evaluated as "acceptable" for excellent wear resistance. Furthermore, a maximum difference in wear amount of 0.0010g or less was evaluated as "acceptable" for having no or small difference in wear amount over the entire length of the outer layer material. As shown in Table 2, it was confirmed that Examples 1 to 15 of the present invention exhibited superior wear resistance over the entire length of the outer layer material compared to Comparative Examples 16 to 25, and that there was no or small difference in wear amount. [Explanation of Symbols]

[0074] 1. Centrifugal casting machine 2. Cylindrical mold 3. Rotating support section 4. Outer layer mold section (mold for centrifugal casting of outer layer material for hot-rolled rolls) 5. The axial mold section at the bottom of the cylindrical mold. 6. Shaft mold portion on the opening side of the cylindrical mold 7 Opening 8 Ladle 9 Casting tub 10 Spout 11 Rollers 12 sand layer 13 Refractory layer 13a 1st refractory layer 13b Second refractory layer 14. Okoshi-type rapid abrasion testing machine 15 test specimens 16 Opposing material

Claims

1. In mass percent, C: 1.2-2.6%, Si: 0.1-2.2%, Mn: 0.1 to 2.2%, Ni: 0.4 to 5.5%, Cr: 2.0 to 10.5%, Mo: 2.0-12.0%, V: 2.0-9.5%, W: 0.5-8.5%, P: 0.01-0.06%, S: Contains 0.001 to 0.060%, The remainder consists of Fe and unavoidable impurities, The composition has a component structure in which the content of Mo, W, V, and Cr satisfies the following formula (1): The total crystallized carbide area ratio at both ends and the central part in the longitudinal direction is 8.5% or more and 19.0% or less, Furthermore, the coefficient of variation for each is 16.0 or less. Furthermore, the maximum difference between the total crystallized carbide area ratios at both ends and the central portion is 6.0% or less. Furthermore, the maximum difference among the differences between each coefficient of variation is 10.0 or less. Crystallization M ２ A roll outer layer material for hot rolling, having a C-type carbide area ratio of 1.5% to 8.0%. 0.14≦(0.063[%Mo]+0.033[%W]) / (0.177[%V]+0.099[%Cr]+0.063[%Mo]+0.033[%W])≦0.55...(1) In equation (1), [%V], [%Cr], [%Mo], and [%W] represent the content (mass%) of each element.

2. A centrifugal casting die for casting the outer layer material of a hot-rolling roll according to claim 1, The centrifugal casting mold has a refractory layer covering the inner circumferential surface, A centrifugal casting mold for a hot rolling roll outer layer material, wherein the ends of the centrifugal casting mold are defined as being located at positions from both ends of the centrifugal casting mold to 8 to 25% of the total length of the centrifugal casting mold, and the thickness of the refractory layer at these ends is 1.1 times or more and 2.2 times or less than the thickness of the refractory layer in the other parts.

3. A method for manufacturing an outer layer material for hot-rolling rolls according to claim 1, A method for manufacturing an outer layer material for a hot rolling roll, comprising: injecting molten metal having the above-mentioned component composition into a centrifugal casting die to centrifugal cast the outer layer material for the hot rolling roll, covering the inner circumferential surface of the centrifugal casting die with refractory material; defining the ends of the centrifugal casting die as being located at positions from 8 to 25% of the total length of the centrifugal casting die from both ends in the longitudinal direction of the centrifugal casting die, and setting the thickness of the refractory material at these ends to be 1.1 times or more and 2.2 times or less than the thickness of the refractory material at the other parts.

4. A composite roll for hot rolling having a two-layer structure of an outer layer and an inner layer, or a three-layer structure of an outer layer, an intermediate layer, and an inner layer, A composite roll for hot rolling, wherein the outer layer is the hot rolling roll outer layer material described in claim 1.

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