Forged steel roll

Abstract

To provide a forged steel roll having superior wear resistance and grindability.SOLUTION: A forged steel roll comprises a chemical composition containing, by mass%, C: 0.85-1.05%, Si: 0.60-1.20%, Mn: 0.30-0.60%, P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00-6.00%, Mo: 0.20 to less than 1.00%, V: 1.00-2.00%, Cu: 0.40% or less, and Ni: 0.30-0.60%, with the reminder of Fe and impurities. The composition satisfies the formula (V+2Mo) / (Cr+Ni)≥0.400 (1). In the total carbides, the number fraction of MC-type carbides with an equivalent circle diameter of 0.5-5.0 μm is 30% or more, and the number density of carbides greater than 5.0 μm in equivalent circle diameter is 100 / mm2 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to forged steel rolls, and more particularly to forged steel rolls suitable for cold rolling applications. [Background technology]

[0002] Forged steel rolls are used as rolling rolls, typically for cold rolling. The forged steel rolls apply a load to the material being rolled, typically steel, thereby rolling the material into a desired shape. However, the surface of the forged steel roll wears due to contact with the material being rolled during rolling. Therefore, when the forged steel roll is used for a long period of time, the surface roughness of the forged steel roll gradually decreases. If the surface roughness of the forged steel roll decreases, slippage occurs between the forged steel roll and the material being rolled. In this case, there is a possibility that poor engagement of the material being rolled occurs, or that seizure or the like may occur in the material being rolled or the forged steel roll.

[0003] In order to suppress slippage between the forged steel roll and the material being rolled, it is necessary to periodically grind the surface of the forged steel roll so that the surface roughness of the forged steel roll does not fall below a certain value. However, if the number of times the forged steel roll is ground per hour of use is large, the life of the forged steel roll will be shortened. In order to extend the life of the forged steel roll, it is desirable to suppress the wear of the forged steel roll and reduce the number of times it is ground per hour of use as much as possible. Therefore, forged steel rolls are required to have excellent wear resistance.

[0004] On the other hand, when the wear resistance is increased, the hardness of the surface layer of the forged steel roll increases. If the surface layer of the forged steel roll becomes excessively hard, the grindability of the forged steel roll may decrease. However, as described above, the surface of the forged steel roll is ground periodically. Therefore, the forged steel roll is required to have both excellent wear resistance and excellent grindability.

[0005] A technique for improving the wear resistance and grindability of forged steel rolls is proposed in Japanese Patent Laid-Open Publication No. 5-195159 (Patent Document 1).

[0006] The forged steel roll disclosed in Patent Document 1 contains 0.8-1.0 wt% C, 0.5-1.0 wt% Si, 0.1-1.0 wt% Mn, 0.1-0.5 wt% Ni, 4.5-10.0 wt% Cr, 6.0-12.0 wt% Mo, and 1.5 wt% or less V, with the balance being Fe and unavoidable impurities. By satisfying the above chemical composition, this forged steel roll can obtain excellent wear resistance and grindability. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 5-195159 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the wear resistance and grindability of the forged steel roll may be improved by a means different from that described in Patent Document 1.

[0009] An object of the present disclosure is to provide a forged steel roll having excellent wear resistance and abrasiveness. [Means for solving the problem]

[0010] The forged steel roll of the present disclosure comprises: The chemical composition is, in mass%, C: 0.85-1.05%, Si: 0.60 to 1.20% Mn: 0.30 to 0.60% P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00~6.00%, Mo: 0.20 to less than 1.00% V: 1.00~2.00%, Cu: 0.40% or less, and Ni: 0.30 to 0.60%; the balance being Fe and impurities, Formula (1) is satisfied, The number ratio of MC type carbides to all carbides having a circle equivalent diameter of 0.5 to 5.0 μm is 30% or more, The number density of carbides with a circle equivalent diameter exceeding 5.0 μm is 100 / mm 2 The following is the result. (V + 2Mo) / (Cr + Ni) ≥ 0.400 (1) Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element in the chemical composition.

[0011] The forged steel roll of the present disclosure comprises: The chemical composition is, in mass%, C: 0.85~1.05%, Si: 0.60 to 1.20% Mn: 0.30 to 0.60% P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00~6.00%, Mo: 0.20 to less than 1.00% V: 1.00~2.00%, Cu: 0.40% or less, and Ni: 0.30 to 0.60%; Further, it contains one or more selected from the group consisting of Group 1 and Group 2, the balance being Fe and impurities, Satisfying equation (1), The number ratio of MC type carbides to all carbides having a circle equivalent diameter of 0.5 to 5.0 μm is 30% or more, The number density of carbides with a circle equivalent diameter exceeding 5.0 μm is 100 / mm 2 The following is the result. [Group 1] Ti: 0.050% or less, Nb: 0.050% or less, B: 0.0100% or less, W: 0.50% or less, and Co: 0.50% or less, one or more selected from the group consisting of [Group 2] Sn: 0.10% or less, Sb: 0.05% or less, As: 0.05% or less, Zr: 0.05% or less, Bi: 0.10% or less, Se: 0.10% or less, Te: 0.05% or less, Pb: 0.09% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of (V + 2Mo) / (Cr + Ni) ≥ 0.400 (1) Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element in the chemical composition. [Effects of the Invention]

[0012] The forged steel roll of the present disclosure provides excellent wear resistance and abrasiveness. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram of a two-cylinder rolling wear tester used in a wear resistance evaluation test. [Figure 2] FIG. 2 is a front view of the rolled material test piece in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present inventors first studied forged steel rolls having excellent wear resistance from the viewpoint of chemical composition, and as a result, they found that the forged steel rolls have the following composition by mass: C: 0.85 to 1.05%, Si: 0.60 to 1.20%, Mn: 0.30 to 0.60%, P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00 to 6.00%, Mo: 0.20 to less than 1.00%, V: 1.00 to 2.00%, Cu: 0.40% or less, Ni: 0.30 to 0.60%, Ti: 0 to 0.050%, Nb: 0.0050% or less It was thought that excellent wear resistance could be obtained if the alloy had a chemical composition containing the following elements: 0.050%, B: 0-0.0100%, W: 0-0.50%, Co: 0-0.50%, Sn: 0-0.10%, Sb: 0-0.05%, As: 0-0.05%, Zr: 0-0.05%, Bi: 0-0.10%, Se: 0-0.10%, Te: 0-0.05%, Pb: 0-0.09%, Ca: 0-0.0050%, and Mg: 0-0.0050%, with the remainder consisting of Fe and impurities.

[0015] However, even when a forged steel roll satisfies the above-mentioned chemical composition, there are cases in which excellent wear resistance is not obtained. Therefore, the present inventors investigated the cause of the failure to obtain excellent wear resistance. As a result, the present inventors have found the following.

[0016] The microstructure of a forged steel roll contains carbides. These carbides are harder than the base material of the forged steel roll. Therefore, it seems that the wear resistance of the forged steel roll would be improved by promoting the generation and growth of carbides and increasing the area ratio of carbides on the surface of the forged steel roll. However, when observing the microstructure of a forged steel roll that did not exhibit excellent wear resistance, it was found that although carbides were sufficiently generated and grown, there were regions inside the crystal grains where carbides were depleted. Such carbide-depleted regions are significantly more susceptible to wear than the base material in which carbides are dispersed. Furthermore, the progression of wear in the carbide-depleted regions also promotes the shedding of carbides present around them. This is thought to be the reason for the deterioration of the wear resistance of the forged steel roll.

[0017] The mechanism by which carbide-deficient regions are formed inside crystal grains is thought to be as follows. Carbides large enough to contribute to improved wear resistance are mainly formed during the steelmaking process from solidification to the hot forging process. Carbides obtained in these manufacturing processes include crystallized carbides, which crystallize from the liquid phase and are mainly formed at grain boundaries, and precipitated carbides, which are formed within crystal grains after solidification. Crystallized carbides are coarser than precipitated carbides. Therefore, around the crystallized carbides, much C is consumed for the formation of the crystallized carbides, resulting in an extremely small amount of solute C. In this case, the amount of solute C inside the crystal grains is reduced due to the influence of the crystallized carbides present at the grain boundaries. This suppresses the precipitation of carbides inside the crystal grains. As a result, carbide-deficient regions are formed inside the crystal grains. Therefore, the inventors believed that if the formation of the above-mentioned crystallized carbides could be suppressed, the carbide-deficient regions could be suppressed and the wear resistance of forged steel rolls could be improved. Therefore, the present inventors have investigated means for suppressing the formation of crystallized carbides.

[0018] The carbides contained in the forged steel roll having the above-mentioned chemical composition mainly contain M7C3 type carbides containing Cr as the main component and MC type carbides containing V and Mo as the main components. Among these carbides, M7C3 type carbides are likely to crystallize from the liquid phase during solidification in the steelmaking process. Therefore, M7C3 type carbides are likely to become coarse. On the other hand, MC type carbides are less likely to crystallize from the liquid phase during solidification in the steelmaking process compared to M7C3 type carbides. Therefore, MC type carbides are less likely to become coarse. Therefore, it was thought that if MC type carbides could be preferentially obtained over M7C3 type carbides, the generation of coarse crystallized carbides could be suppressed.

[0019] Furthermore, MC carbides are harder than M7C3 carbides. Therefore, when a forged steel roll is used, MC carbides are less likely to break than M7C3 carbides. Therefore, the inventors thought that if MC carbides, which are harder than M7C3 carbides, could be preferentially obtained, the wear resistance would be improved.

[0020] The present inventors believed that in order to obtain MC carbides preferentially over M7C3 carbides, it is necessary to adjust the V content, Mo content, and Cr content, which are components constituting these carbides. Furthermore, Ni, together with Cr, tends to remain in the liquid phase, promoting the formation of M7C3 carbides. Therefore, the present inventors believed that by adjusting the Ni content in addition to the V content, Mo content, and Cr content, MC carbides would be obtained preferentially over M7C3 carbides.

[0021] Based on the above findings, the present inventors have investigated the relationship between the V content, Mo content, Cr content and Ni content, and have found that excellent wear resistance can be obtained in a forged steel roll having the above chemical composition if formula (1) is satisfied. (V + 2Mo) / (Cr + Ni) ≥ 0.400 (1) Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element in the chemical composition.

[0022] However, even when the forged steel roll satisfies the above-mentioned chemical composition and formula (1), there are cases where excellent wear resistance is not obtained. Therefore, the present inventors further investigated the cause of the failure to obtain excellent wear resistance. As a result, the present inventors have obtained the following findings.

[0023] To obtain excellent wear resistance, it is preferable to maximize the area ratio of MC carbides on the surface of the forged steel roll, as described above. However, when observing the microstructure of a forged steel roll that did not exhibit excellent wear resistance and satisfied the above-mentioned chemical composition and formula (1), numerous coarse MC carbides were present at the grain boundaries. On the other hand, in the microstructure of such a forged steel roll, it was found that regions depleted of MC carbides existed within the grains near the grain boundaries where the coarse MC carbides were present. This is thought to be due to a lack of V and Mo, which form MC carbides, around the coarse MC carbides. The MC carbide-depleted regions are significantly more susceptible to wear than regions where the MC carbides are sufficiently dispersed. Furthermore, the progression of wear in the MC carbide-depleted regions also promotes the shedding of the surrounding coarse MC carbides. This is thought to be the reason for the deterioration of the wear resistance of the forged steel roll.

[0024] In contrast, in the microstructure of the forged steel roll that provided excellent wear resistance, the number of coarse MC carbides present at the grain boundaries was small. Instead, the proportion of MC carbides among the finely dispersed carbides within the grains was high. In other words, there were almost no MC carbide-depleted regions.

[0025] Therefore, we considered that reducing the number of coarse MC carbides and suppressing the formation of MC carbide-poor regions inside the crystal grains would be effective in improving the wear resistance of forged steel rolls. As mentioned above, suppressing the formation of MC carbide-poor regions increases the proportion of MC carbides among all carbides finely dispersed within the crystal grains. In other words, in order to obtain excellent wear resistance, it is desirable to increase the proportion of MC carbides among carbides of an appropriate size as much as possible.

[0026] Based on the above findings, the present inventors have further investigated and found that, in a forged steel roll having the above chemical composition, excellent wear resistance can be obtained if the proportion of MC type carbides in the total carbides having an equivalent circle diameter of 0.5 to 5.0 μm is 30% or more.

[0027] The present inventors then investigated means for obtaining a forged steel roll having excellent grindability in addition to the above-mentioned forged steel roll having excellent wear resistance, and as a result, the present inventors discovered the following.

[0028] Forged steel rolls have excellent wear resistance. The carbides, including MC-type carbides, contained in the forged steel roll are exposed on the surface of the forged steel roll, suppressing contact between the rolled material and the base material of the forged steel roll. This increases the wear resistance of the forged steel roll. However, the carbides exposed on the surface of the forged steel roll may result in reduced grindability. Alumina abrasive grains are typically used in grinding forged steel rolls. Alumina abrasive grains are harder than the base material of the forged steel roll but softer than carbides. Therefore, when grinding the surface layer of the forged steel roll, if the alumina abrasive grains collide with the carbides, the alumina abrasive grains are destroyed. If the alumina abrasive grains are destroyed, grinding of the surface layer of the forged steel roll will not proceed. As described above, carbides on the surface of the forged steel roll increase wear resistance but reduce grindability. Therefore, in order to achieve both wear resistance and grindability, it is necessary to suppress the destruction of the alumina abrasive grains during grinding, even if carbides are present on the surface of the forged steel roll.

[0029] The present inventors have studied means for avoiding collisions between carbides in a forged steel roll and alumina abrasive grains. In grinding a forged steel roll, the base material of the forged steel roll is ground off and removed as grinding powder. It was thought that if the carbides in the forged steel roll were encapsulated in the grinding powder and removed together with the grinding powder, collisions between the carbides and abrasive grains would be suppressed.

[0030] In other words, if the size of the carbides is small enough to be included in the grinding powder, the carbides are easily removed together with the grinding powder when the forged steel roll is ground. Therefore, the present inventors investigated the size of the carbides in the forged steel roll. As a result, it was found that if the equivalent circle diameter of the carbides in the forged steel roll is 0.5 to 5.0 μm, the carbides are easily included in the grinding powder when the forged steel roll is ground. On the other hand, if the equivalent circle diameter of the carbides in the forged steel roll exceeds 5.0 μm, the carbides are too large and protrude from the grinding powder. In other words, the carbides cannot be included in the grinding powder, and the carbides are difficult to remove together with the grinding powder. In this case, the carbides are likely to collide with the alumina abrasive grains, causing the alumina abrasive grains to break. As a result, grinding of the surface layer of the forged steel roll does not proceed.

[0031] Based on the above findings, the inventors investigated and examined the number density of coarse carbides at a level that would not excessively destroy the alumina abrasive grains during grinding. As a result, it was found that the number density of carbides with a circle equivalent diameter of more than 5.0 μm was 100 pieces / mm 2 It has been found that if the thickness is less than or equal to the above, the forged steel roll can have excellent wear resistance and can also have excellent grindability.

[0032] The forged steel roll of this embodiment has been completed based on the above technical concept and has the following configuration.

[0033] The forged steel roll of the first configuration is The chemical composition is, in mass%, C: 0.85-1.05%, Si: 0.60 to 1.20% Mn: 0.30 to 0.60% P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00~6.00%, Mo: 0.20 to less than 1.00% V: 1.00~2.00%, Cu: 0.40% or less, and Ni: 0.30 to 0.60%; the balance being Fe and impurities, Satisfying equation (1), The number ratio of MC type carbides to all carbides having a circle equivalent diameter of 0.5 to 5.0 μm is 30% or more, The number density of carbides with a circle equivalent diameter exceeding 5.0 μm is 100 / mm 2 The following is the result. (V + 2Mo) / (Cr + Ni) ≥ 0.400 (1) Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element in the chemical composition.

[0034] The second configuration of forged steel rolls is The chemical composition is, in mass%, C: 0.85-1.05%, Si: 0.60 to 1.20% Mn: 0.30 to 0.60% P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00~6.00%, Mo: 0.20 to less than 1.00% V: 1.00~2.00%, Cu: 0.40% or less, and Ni: 0.30 to 0.60%; Further, it contains one or more selected from the group consisting of Group 1 and Group 2, the balance being Fe and impurities, Satisfying equation (1), The number ratio of MC type carbides to all carbides having a circle equivalent diameter of 0.5 to 5.0 μm is 30% or more, The number density of carbides with a circle equivalent diameter exceeding 5.0 μm is 100 / mm 2 The following is the result. [Group 1] Ti: 0.050% or less, Nb: 0.050% or less, B: 0.0100% or less, W: 0.50% or less, and Co: 0.50% or less, one or more selected from the group consisting of [Group 2] Sn: 0.10% or less, Sb: 0.05% or less, As: 0.05% or less, Zr: 0.05% or less, Bi: 0.10% or less, Se: 0.10% or less, Te: 0.05% or less, Pb: 0.09% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of (V + 2Mo) / (Cr + Ni) ≥ 0.400 (1) Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element in the chemical composition.

[0035] The third configuration of forged steel rolls is A forged steel roll of a second configuration, The chemical composition contains the first group.

[0036] The fourth configuration of forged steel rolls is A forged steel roll of the second or third configuration, The chemical composition contains the second group.

[0037] The forged steel roll of this embodiment will be described in detail below. Unless otherwise specified, "%" for elements means mass %.

[0038] [Configuration of the forged steel roll of this embodiment] The forged steel roll of this embodiment comprises a barrel portion and a pair of shank portions. The barrel portion is cylindrical and includes a pair of end faces and a peripheral surface (hereinafter also simply referred to as the surface) disposed between the pair of end faces. The peripheral surface comes into contact with the material to be rolled during rolling. The shank portions are cylindrical and are provided on the pair of end faces of the barrel portion, respectively, so that the central axis of the shank portions coincides with the central axis of the barrel portion. The diameter of the barrel portion is larger than the diameter of the shank portions.

[0039] In the forged steel roll of this embodiment, the region extending from the surface of the barrel portion to a depth of 80 mm is further defined as the surface layer. As described above, the forged steel roll is used while repeatedly grinding the surface as the roughness decreases. The surface layer of the forged steel roll is exposed to the outer surface by grinding, and is a region that can newly come into contact with the rolled material as the surface of the barrel portion.

[0040] [Features of the forged steel roll of this embodiment] The forged steel roll of this embodiment satisfies the following features 1 to 4. (Feature 1) The chemical composition, in mass%, is C: 0.85-1.05%, Si: 0.60-1.20%, Mn: 0.30-0.60%, P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00-6.00%, Mo: 0.20-less than 1.00%, V: 1.00-2.00%, Cu: 0.40% or less, Ni: 0.30-0.60%, Ti: 0-0.0 50%, Nb: 0-0.050%, B: 0-0.0100%, W: 0-0.50%, Co: 0-0.50%, Sn: 0-0.10%, Sb: 0-0.05%, As: 0-0.05%, Zr: 0-0.05%, Bi: 0-0.10%, Se: 0-0.10%, Te: 0-0.05%, Pb: 0-0.09%, Ca: 0-0.0050%, and Mg: 0-0.0050%, with the remainder being Fe and impurities. (Feature 2) A forged steel roll satisfying characteristic 1 satisfies the following formula (1). (V + 2Mo) / (Cr + Ni) ≥ 0.400 (1) Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element in the chemical composition. (Feature 3) The proportion of MC type carbides in all carbides with an equivalent circle diameter of 0.5 to 5.0 μm is 30% or more. (Feature 4) The number density of carbides with a circle equivalent diameter exceeding 5.0 μm is 100 / mm 2 The following is the result. Features 1 to 4 will be explained below.

[0041] [(Feature 1) Chemical composition] The forged steel roll of this embodiment contains the following elements.

[0042] C: 0.85 to 1.05% Carbon (C) increases the hardness of the surface layer of the forged steel roll. If the C content is less than 0.85%, this effect cannot be sufficiently obtained. On the other hand, if the C content exceeds 1.05%, coarse carbides are formed, and in this case, sufficient hardness may not be obtained in the surface layer of the forged steel roll. Therefore, the C content is 0.85 to 1.05%. The lower limit of the C content is preferably 0.87%, more preferably 0.90%, and even more preferably 0.95%. The upper limit of the C content is preferably 1.03%, more preferably 1.00%, and even more preferably 0.98%.

[0043] Si: 0.60 to 1.20% Silicon (Si) deoxidizes steel during the molten steel stage. Si also improves the hardenability of steel. If the Si content is less than 0.60%, the above effects cannot be sufficiently obtained. On the other hand, if the Si content exceeds 1.20%, the toughness of the forged steel roll decreases. Therefore, the Si content is 0.60 to 1.20%. The lower limit of the Si content is preferably 0.65%, more preferably 0.70%, and even more preferably 0.75%. The upper limit of the Si content is preferably 1.15%, more preferably 1.10%, and even more preferably 1.05%.

[0044] Mn: 0.30 to 0.60% Manganese (Mn) improves the hardenability of steel. If the Mn content is less than 0.30%, this effect cannot be sufficiently obtained. On the other hand, if the Mn content exceeds 0.60%, the toughness of the forged steel roll decreases. Therefore, the Mn content is 0.30 to 0.60%. The lower limit of the Mn content is preferably 0.33%, more preferably 0.35%, and even more preferably 0.40%. The upper limit of the Mn content is preferably 0.57%, more preferably 0.55%, and even more preferably 0.50%.

[0045] P:0.020% or less Phosphorus (P) is an impurity. If the P content exceeds 0.020%, P segregates at grain boundaries and reduces the toughness of the forged steel roll. Therefore, the P content is 0.020% or less. The P content is preferably as low as possible. However, excessive reduction in the P content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the P content is preferably more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The upper limit of the P content is preferably 0.018%, more preferably 0.015%, and even more preferably 0.010%.

[0046] S: 0.020% or less Sulfur (S) is an impurity. If the S content exceeds 0.020%, S segregates at grain boundaries, reducing the toughness of the forged steel roll and the hot workability of the steel material during the manufacturing process of the forged steel roll. Therefore, the S content is 0.020% or less. The S content is preferably as low as possible. However, excessive reduction of the S content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the S content is preferably more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The upper limit of the S content is preferably 0.018%, more preferably 0.015%, and even more preferably 0.010%.

[0047] Al: 0.050% or less Aluminum (Al) deoxidizes steel during the molten steel stage. However, if the Al content exceeds 0.050%, coarse Al nitrides are formed, which reduces the toughness of the steel material during the manufacturing process of forged steel rolls. Therefore, the Al content is 0.050% or less. The lower limit of the Al content is preferably more than 0%, more preferably 0.001%, even more preferably 0.002%, even more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Al content is preferably 0.040%, more preferably 0.035%, even more preferably 0.030%, and still more preferably 0.025%. In this specification, the Al content means the total Al content in the steel.

[0048] N: 0.020% or less Nitrogen (N) increases the hardness of forged steel rolls through solid solution strengthening. However, if the N content exceeds 0.020%, coarse nitrides are formed, which reduces the toughness of the forged steel rolls. Therefore, the N content is 0.020% or less. The lower limit of the N content is preferably more than 0%, more preferably 0.001%, even more preferably 0.003%, and still more preferably 0.005%. The upper limit of the N content is preferably 0.015%, more preferably 0.010%, and even more preferably 0.008%.

[0049] O: 0.0050% or less Oxygen (O) is an impurity. If the O content exceeds 0.0050%, O forms oxides that reduce the toughness of the forged steel roll. Therefore, the O content is 0.0050% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the O content is preferably more than 0%, more preferably 0.0001%, even more preferably 0.0005%, even more preferably 0.0007%, and even more preferably 0.0010%. The upper limit of the O content is preferably 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0050] Cr: 4.00~6.00% Chromium (Cr) forms M7C3 type carbides to enhance the wear resistance of forged steel rolls. Cr also enhances the temper softening resistance of steel through solid solution, thereby increasing the hardness of the surface layer of forged steel rolls. If the Cr content is less than 4.00%, the above effects cannot be sufficiently obtained. On the other hand, if the Cr content exceeds 6.00%, coarse M7C3 type carbides are formed. In this case, the wear resistance and grindability of the forged steel rolls are reduced. Therefore, the Cr content is 4.00 to 6.00%. The lower limit of the Cr content is preferably 4.05%, more preferably 4.10%, and even more preferably 4.15%. The upper limit of the Cr content is preferably 5.95%, more preferably 5.90%, and even more preferably 5.85%.

[0051] Mo: 0.20 to less than 1.00% Molybdenum (Mo) promotes the formation of MC type carbides and improves the wear resistance of forged steel rolls. Mo also increases the hardness of the surface layer of forged steel rolls through solid solution. If the Mo content is less than 0.20%, the above effect cannot be sufficiently obtained. On the other hand, if the Mo content is 1.00% or more, coarse MC type carbides are formed. In this case, the grindability and toughness of the forged steel rolls are reduced. Therefore, the Mo content is 0.20 to less than 1.00%. The lower limit of the Mo content is preferably 0.25%, more preferably 0.30%, and even more preferably 0.35%. The upper limit of the Mo content is preferably 0.95%, more preferably 0.90%, and even more preferably 0.85%.

[0052] V: 1.00~2.00% Vanadium (V) forms MC type carbides together with Mo, thereby improving the wear resistance of forged steel rolls. V also increases the hardness of forged steel rolls by solid solution. If the V content is less than 1.00%, the above effect cannot be sufficiently obtained. On the other hand, if the V content exceeds 2.00%, coarse MC type carbides are formed. In this case, the grindability and toughness of the forged steel rolls are reduced. Therefore, the V content is 1.00 to 2.00%. The lower limit of the V content is preferably 1.05%, more preferably 1.10%, and even more preferably 1.15%. The upper limit of the V content is preferably 1.90%, more preferably 1.80%, and even more preferably 1.70%.

[0053] Cu: 0.40% or less Copper (Cu) is an impurity. If the Cu content exceeds 0.40%, Cu reduces the hot workability of the steel material during the manufacturing process of forged steel rolls. Therefore, the Cu content is 0.40% or less. The Cu content is preferably as low as possible. However, excessive reduction in the Cu content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the Cu content is preferably more than 0%, more preferably 0.01%, even more preferably 0.02%, even more preferably 0.03%, and even more preferably 0.04%. The upper limit of the Cu content is preferably less than 0.40%, more preferably 0.39%, even more preferably 0.35%, even more preferably 0.30%, even more preferably 0.25%, and even more preferably 0.20%.

[0054] Ni: 0.30 to 0.60% Nickel (Ni) promotes the formation of M7C3 type carbides to increase the wear resistance of forged steel rolls. Ni also improves the hardenability of steel. If the Ni content is less than 0.30%, the above effects cannot be sufficiently obtained. On the other hand, if the Ni content exceeds 0.60%, coarse M7C3 type carbides are formed, which reduces the wear resistance and grindability of the forged steel roll. If the Ni content exceeds 0.60%, retained austenite is also formed in excess, which reduces the wear resistance of the forged steel roll. Therefore, the Ni content is 0.30 to 0.60%. The lower limit of the Ni content is preferably 0.33%, more preferably 0.35%, and even more preferably 0.40%. The upper limit of the Ni content is preferably 0.57%, more preferably 0.55%, and even more preferably 0.50%.

[0055] The balance of the chemical composition of the forged steel roll according to this embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, during industrial production of the forged steel roll, and are acceptable within a range that does not adversely affect the forged steel roll according to this embodiment.

[0056] [About optional elements] The chemical composition of the forged steel roll of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of the first group and the second group. [Group 1] Ti: 0.050% or less, Nb: 0.050% or less, B: 0.0100% or less, W: 0.50% or less, and Co: 0.50% or less, one or more selected from the group consisting of [Group 2] Sn: 0.10% or less, Sb: 0.05% or less, As: 0.05% or less, Zr: 0.05% or less, Bi: 0.10% or less, Se: 0.10% or less, Te: 0.05% or less, Pb: 0.09% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of These optional elements will be explained below.

[0057] [Group 1: Ti, Nb, B, W and Co] The chemical composition of the forged steel roll of this embodiment may further contain the above-mentioned elements of Group 1 in place of a portion of Fe. These elements are optional elements, and all of them increase the hardness of the surface layer of the forged steel roll. Each element of Group 1 will be described below.

[0058] Ti: 0.050% or less Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0%. When Ti is contained, that is, when the Ti content exceeds 0%, Ti forms precipitates such as carbides or nitrides, thereby increasing the hardness of the surface layer of the forged steel roll. Even if Ti is contained even a small amount, the above effect can be obtained to some extent. On the other hand, if the Ti content exceeds 0.050%, the hot workability of the steel material deteriorates in the manufacturing process of the forged steel roll, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ti content is 0 to 0.050%, and when Ti is contained, the Ti content is 0.050% or less. The lower limit of the Ti content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.004%. The upper limit of the Ti content is preferably 0.040%, more preferably 0.035%, and even more preferably 0.030%.

[0059] Nb: 0.050% or less Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content exceeds 0%, Nb forms precipitates such as carbides or nitrides, which increase the hardness of the surface layer of the forged steel roll. Even if even a small amount of Nb is contained, the above effect can be obtained to some extent. On the other hand, if the Nb content exceeds 0.050%, the hot workability of the steel material deteriorates in the manufacturing process of the forged steel roll, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Nb content is 0 to 0.050%, and when Nb is contained, the Nb content is 0.050% or less. The lower limit of the Nb content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.004%. The upper limit of the Nb content is preferably 0.040%, more preferably 0.035%, and even more preferably 0.030%.

[0060] B: 0.0100% or less Boron (B) is an optional element and may not be contained, that is, the B content may be 0%. When B is contained, that is, when the B content exceeds 0%, B improves the hardenability of the forged steel roll and increases the hardness of the surface layer of the forged steel roll. Even if even a small amount of B is contained, the above effects can be obtained to some extent. On the other hand, if the B content exceeds 0.0100%, the hot workability of the steel material deteriorates in the manufacturing process of the forged steel roll, even if the contents of other elements are within the ranges of this embodiment. Therefore, the B content is 0 to 0.0100%, and if B is contained, it is 0.0100% or less. The lower limit of the B content is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The upper limit of the B content is preferably 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0061] W: 0.50% or less Tungsten (W) is an optional element and may not be contained, that is, the W content may be 0%. When W is contained, that is, when the W content exceeds 0%, W improves the hardenability of the forged steel roll and increases the hardness of the surface layer of the forged steel roll. Even if even a small amount of W is contained, the above effects can be obtained to some extent. On the other hand, if the W content exceeds 0.50%, the hot workability of the steel material deteriorates in the manufacturing process of the forged steel roll, even if the contents of other elements are within the ranges of this embodiment. Therefore, the W content is 0 to 0.50%, and if W is contained, it is 0.50% or less. The lower limit of the W content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the W content is preferably 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0062] Co:0.50% or less Cobalt (Co) is an optional element and may not be contained, that is, the Co content may be 0%. When Co is contained, that is, when the Co content exceeds 0%, Co improves the hardenability of the forged steel roll and increases the hardness of the surface layer of the forged steel roll. Even if even a small amount of Co is contained, the above effects can be obtained to some extent. On the other hand, if the Co content exceeds 0.50%, the hot workability of the steel material deteriorates in the manufacturing process of the forged steel roll, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Co content is 0 to 0.50%, and if Co is contained, it is 0.50% or less. The lower limit of the Co content is preferably 0.01%, more preferably 0.05%, and even more preferably 0.08%. The upper limit of the Co content is preferably 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0063] [Group 2: Sn, Sb, As, Zr, Bi, Se, Te, Pb, Ca and Mg] The chemical composition of the forged steel roll of this embodiment may further contain the above-mentioned second group elements in place of a portion of Fe. These elements are optional elements, and all of them improve the grindability of the forged steel roll. Each element of the second group will be described below.

[0064] Sn: 0.10% or less Tin (Sn) is an optional element and may not be contained, that is, the Sn content may be 0%. When Sn is contained, that is, when the Sn content is more than 0%, Sn improves the grindability of the forged steel roll. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. On the other hand, if the Sn content exceeds 0.10%, the hot workability of the steel material deteriorates in the manufacturing process of the forged steel roll, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0 to 0.10%, and when Sn is contained, the Sn content is 0.10% or less. The lower limit of the Sn content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the Sn content is preferably 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0065] Sb: 0.05% or less Antimony (Sb) is an optional element and may not be contained, that is, the Sb content may be 0%. When Sb is contained, that is, when the Sb content is more than 0%, Sb improves the grindability of the forged steel roll. Even if even a small amount of Sb is contained, the above effect can be obtained to some extent. On the other hand, if the Sb content exceeds 0.05%, the hot workability of the steel material deteriorates in the manufacturing process of the forged steel roll, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sb content is 0 to 0.05%, and if Sb is contained, the Sb content is 0.05% or less. The preferred lower limit of the Sb content is 0.01%. The preferred upper limit of the Sb content is 0.04%.

[0066] As: 0.05% or less Arsenic (As) is an optional element and may not be contained, that is, the As content may be 0%. When contained, that is, when the As content exceeds 0%, As improves the grindability of the forged steel roll. Even if only a small amount of As is contained, the above effect can be obtained to some extent. On the other hand, if the As content exceeds 0.05%, the hot workability of the steel material deteriorates in the manufacturing process of the forged steel roll, even if the contents of other elements are within the ranges of this embodiment. Therefore, the As content is 0 to 0.05%, and when As is contained, the As content is 0.05% or less. The preferred lower limit of the As content is 0.01%. The preferred upper limit of the As content is 0.04%.

[0067] Zr: 0.05% or less Zirconium (Zr) is an optional element and may not be contained, that is, the Zr content may be 0%. When contained, that is, when the Zr content is more than 0%, Zr improves the grindability of the forged steel roll. Even if even a small amount of Zr is contained, the above effect can be obtained to some extent. On the other hand, if the Zr content exceeds 0.05%, the hot workability of the steel material will be reduced in the manufacturing process of the forged steel roll, even if the contents of the other elements are within the ranges of this embodiment. Therefore, the Zr content is 0 to 0.05%, and if Zr is contained, the Zr content is 0.05% or less. The preferred lower limit of the Zr content is 0.01%. The preferred upper limit of the Zr content is 0.04%.

[0068] Bi:0.10% or less Bismuth (Bi) is an optional element and may not be contained, that is, the Bi content may be 0%. When contained, that is, when the Bi content exceeds 0%, Bi improves the grindability of the forged steel roll. Even if even a small amount of Bi is contained, the above effect can be obtained to some extent. On the other hand, if the Bi content exceeds 0.10%, the hot workability of the steel material will be reduced in the manufacturing process of the forged steel roll, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Bi content is 0 to 0.10%, and when Bi is contained, the Bi content is 0.10% or less. The lower limit of the Bi content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the Bi content is preferably 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0069] Se: 0.10% or less Selenium (Se) is an optional element and may not be contained, that is, the Se content may be 0%. When contained, that is, when the Se content exceeds 0%, Se improves the grindability of the forged steel roll. Even if even a small amount of Se is contained, the above effect can be obtained to some extent. On the other hand, if the Se content exceeds 0.10%, the hot workability of the steel material will be reduced in the manufacturing process of the forged steel roll, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Se content is 0 to 0.10%, and if Se is contained, the Se content is 0.10% or less. The lower limit of the Se content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the Se content is preferably 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0070] Te: 0.05% or less Tellurium (Te) is an optional element and may not be contained, that is, the Te content may be 0%. When contained, that is, when the Te content is more than 0%, Te improves the grindability of the forged steel roll. Even if even a small amount of Te is contained, the above effect can be obtained to some extent. On the other hand, if the Te content exceeds 0.05%, the hot workability of the steel material deteriorates in the manufacturing process of the forged steel roll, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Te content is 0 to 0.05%, and if Te is contained, the Te content is 0.05% or less. The lower limit of the Te content is preferably 0.01%. The upper limit of the Te content is preferably 0.04%.

[0071] Pb: 0.09% or less Lead (Pb) is an optional element and may not be contained, that is, the Pb content may be 0%. When Pb is contained, that is, when the Pb content is more than 0%, Pb improves the grindability of the forged steel roll. Even if even a small amount of Pb is contained, the above effect can be obtained to some extent. On the other hand, if the Pb content exceeds 0.09%, the hot workability of the steel material deteriorates in the manufacturing process of the forged steel roll, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Pb content is 0 to 0.09%, and if Pb is contained, the Pb content is 0.09% or less. The lower limit of the Pb content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the Pb content is preferably 0.08%, more preferably 0.07%, and even more preferably 0.06%.

[0072] Ca:0.0050% or less Calcium (Ca) is an optional element and may not be contained, that is, the Ca content may be 0%. When contained, that is, when the Ca content is more than 0%, Ca improves the grindability of the forged steel roll. Even if even a small amount of Ca is contained, the above effect can be obtained to some extent. On the other hand, if the Ca content exceeds 0.0050%, the hot workability of the steel material will be reduced in the manufacturing process of the forged steel roll, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ca content is 0 to 0.0050%, and when Ca is contained, the Ca content is 0.0050% or less. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The upper limit of the Ca content is preferably 0.0047%, more preferably 0.0045%, and even more preferably 0.0040%.

[0073] Mg: 0.0050% or less Magnesium (Mg) is an optional element and may not be contained, that is, the Mg content may be 0%. When contained, that is, when the Mg content is more than 0%, Mg improves the grindability of the forged steel roll. Even if even a small amount of Mg is contained, the above effect can be obtained to some extent. On the other hand, if the Mg content exceeds 0.0050%, the hot workability of the steel material will be reduced in the manufacturing process of the forged steel roll, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mg content is 0 to 0.0050%, and when Mg is contained, the Mg content is 0.0050% or less. The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The upper limit of the Mg content is preferably 0.0047%, more preferably 0.0045%, and even more preferably 0.0040%.

[0074] [(Feature 2) Regarding Formula (1)] Furthermore, the chemical composition of the forged steel roll of this embodiment satisfies formula (1). (V + 2Mo) / (Cr + Ni) ≥ 0.400 (1) Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element in the chemical composition.

[0075] Fn1 is defined as follows: Fn1=(V+2Mo) / (Cr+Ni)

[0076] Fn1 corresponds to the left side of formula (1). Fn1 is an index for adjusting the ratio of MC carbides and M7C3 carbides among all carbides contained in a forged steel roll that satisfies Feature 1. As described above, MC carbides are harder than M7C3 carbides. Furthermore, MC carbides are less likely to form as coarse crystallized carbides than M7C3 carbides. Therefore, in order to improve the wear resistance of a forged steel roll, it is preferable to increase the ratio of MC carbides relative to M7C3 carbides. By increasing the contents of V and Mo, which form MC carbides, relative to the Cr content, which forms M7C3 carbides, and the Ni content, which promotes the formation of M7C3 carbides, it is possible to preferentially form MC carbides over M7C3 carbides.

[0077] If Fn1 is less than 0.400, the V content and Mo content are too low relative to the Cr content and Ni content. In this case, M7C3 type carbides precipitate preferentially over MC type carbides. Therefore, carbide-deficient regions are likely to form in the forged steel roll. Furthermore, the amount of hard MC type carbides in the forged steel roll decreases. As a result, the forged steel roll does not have sufficient wear resistance.

[0078] When Fn1 is 0.400 or more, MC type carbides are obtained preferentially over M7C3 type carbides in a forged steel roll that satisfies Feature 1 and further satisfies Features 3 and 4 described below, and as a result, excellent wear resistance of the forged steel roll is obtained.

[0079] The lower limit of Fn1 is preferably 0.405, more preferably 0.410, and even more preferably 0.415. The upper limit of Fn1 is not particularly limited. When the forged steel roll satisfies the characteristic 1 and the characteristics 3 and 4 described below, the upper limit of Fn1 is, for example, 0.800.

[0080] [(Feature 3) MC type carbide number ratio NR] Furthermore, in the forged steel roll of this embodiment, the proportion of MC type carbides in the total carbides having an equivalent circle diameter of 0.5 to 5.0 μm is 30% or more.

[0081] The number ratio of MC carbides to all carbides with an equivalent circle diameter of 0.5 to 5.0 μm is defined as the number ratio of MC carbides (NR). As described above, in order to improve the wear resistance of forged steel rolls, it is effective to suppress the formation of MC carbide-depleted regions inside crystal grains rather than increasing the number of coarse MC carbides. Most of the carbides finely dispersed inside crystal grains have an equivalent circle diameter of 5.0 μm or less. On the other hand, carbides with an equivalent circle diameter of less than 0.5 μm hardly contribute to improving wear resistance. Therefore, in order to improve the wear resistance of forged steel rolls, it is preferable to increase the number ratio of MC carbides to all carbides with an equivalent circle diameter of 0.5 to 5.0 μm.

[0082] When the number ratio NR of MC carbides is less than 30%, the formation of MC carbide-depleted regions inside the crystal grains cannot be suppressed, and in this case, excellent wear resistance cannot be obtained.

[0083] If the number ratio of MC carbides is 30% or more, MC carbides having a size sufficient to enhance wear resistance are sufficiently dispersed inside the crystal grains. In this case, excellent wear resistance can be obtained on the premise that the forged steel roll satisfies Features 1, 2, and 4 described later. Therefore, in the forged steel roll of this embodiment, the number ratio NR of MC type carbides is 30% or more.

[0084] The lower limit of the number ratio NR of MC type carbides is preferably 35%, more preferably 40%, and even more preferably 45%. The upper limit of the ratio NR of the number of MC type carbides is not particularly limited. When the forged steel roll satisfies Features 1, 2, and 4 described below, the upper limit of the ratio NR of the number of MC type carbides is, for example, 90%.

[0085] [Method for measuring the number ratio of MC type carbides (NR)] The number ratio of MC type carbides, NR, can be measured by the following method. A test specimen was taken from the surface of the barrel of the forged steel roll, with the observation surface being a plane perpendicular to the axial direction of the barrel. The observation surface included a position 1 mm deep from the surface of the forged steel roll. The observation surface was mirror-polished. After mirror-polishing, 10 observation fields were arbitrarily selected, centered at a position 1 mm deep from the surface of the barrel of the forged steel roll. Each observation field measured 240 μm × 180 μm. Using a field emission scanning electron microscope (FE-SEM), Z-contrast images of the 10 observation fields were taken with a backscattered electron detector, known as COMPO images. The observation magnification was 500x. In the COMPO image, the carbides contained a large amount of carbon, which has a small atomic number, compared to the matrix, which is primarily composed of iron, resulting in a dark contrast. Therefore, the matrix and carbides can be distinguished by their contrast.

[0086] Furthermore, quantitative analysis was performed on several carbides in the observation field using energy dispersive X-ray spectroscopy (EDS) attached to the FE-SEM, and five MC carbides and five other carbides were identified. MC carbides are defined as carbides with a V content of 30% or more by mass. Other carbides are defined as carbides with a V content of less than 30% by mass. The acceleration voltage for EDS analysis was 15 kV. The EDS analysis time was set so that the X-ray count was 500 counts or more.

[0087] Here, even in the COMPO image, by setting the contrast appropriately, it is possible to distinguish between MC carbides, which have a high carbon content among carbides, and other carbides with a lower carbon content than MC carbides. Specifically, if the contrast setting is appropriate, MC carbides will appear with a darker contrast than other carbides. The contrast of the COMPO image is adjusted so that the five MC carbides identified by quantitative analysis using EDS can be distinguished from the five other carbides. In this way, the observation field is photographed with settings that allow for distinction between the matrix, MC carbides, and other carbides, and a photographic image is generated.

[0088] From the obtained photographic images, all carbides with a circle-equivalent diameter of 0.5 to 5.0 μm within the entire observation field are identified, and their total number is calculated. Here, the circle-equivalent diameter means the diameter (μm) when the area of ​​the carbide is converted into a circle. From the identified carbides with a circle-equivalent diameter of 0.5 to 5.0 μm, MC type carbides are further identified, and their total number is calculated.

[0089] Based on the total number of carbides with an equivalent circle diameter of 0.5 to 5.0 μm and the total number of MC carbides, the number ratio (%) of MC carbides to all carbides with an equivalent circle diameter of 0.5 to 5.0 μm is calculated. The obtained number ratio (%) is defined as the number ratio NR (%) of MC carbides. The MC type carbide number ratio NR (%) is an integer value obtained by rounding off the obtained value to one decimal place.

[0090] [(Feature 4) Carbide number density ND] In the forged steel roll of this embodiment, the number density of carbides having an equivalent circle diameter of more than 5.0 μm is 100 pieces / mm 2 The following is the result.

[0091] The number density of carbides with a circle equivalent diameter exceeding 5.0 μm is defined as the carbide number density ND. Carbides with a circle equivalent diameter exceeding 5.0 μm are difficult to remove together with grinding powder when grinding forged steel rolls. Carbides that are not removed may collide with abrasive grains during grinding, destroying the abrasive grains due to the impact. If the abrasive grains are destroyed, the grinding of the base material of the forged steel roll will not progress, and the expected amount of grinding will not be obtained, resulting in a decrease in grindability.

[0092] Carbide number density ND is 100 pieces / mm 2 If the diameter exceeds 5.0 μm, there are too many carbides with an equivalent circle diameter exceeding 5.0 μm. Carbides with an equivalent circle diameter exceeding 5.0 μm are too large. As a result, the carbides cannot be contained in the grinding powder, making it difficult to remove them along with the grinding powder. In this case, the carbides that are not removed collide with the abrasive grains during grinding, easily destroying the abrasive grains due to the impact. As a result, grinding of the base material of the forged steel roll does not progress, and excellent grindability cannot be obtained.

[0093] Carbide number density ND is 100 pieces / mm 2 If the thickness is less than the above, there are few carbides of a size that cannot be included in the grinding powder when the forged steel roll is ground. Therefore, the carbides are easily removed together with the grinding powder during grinding. As a result, excellent grindability can be obtained on the premise that the forged steel roll satisfies Features 1 to 3. Therefore, in the forged steel roll of this embodiment, the carbide number density ND is 100 pieces / mm 2 The following is the result.

[0094] The preferred upper limit of the carbide number density ND is 95 pieces / mm 2 and more preferably 90 pieces / mm 2 and more preferably 85 pieces / mm 2 is. The lower limit of the carbide number density ND is not particularly limited. When the forged steel roll satisfies the characteristics 1 to 3, the lower limit of the carbide number density ND is, for example, 50 pieces / mm 2 is.

[0095] [Method for measuring carbide number density ND] The carbide number density ND can be measured by the following method. A test specimen is taken from the surface of the barrel of the forged steel roll, with the observation surface being a plane perpendicular to the axial direction of the barrel of the forged steel roll. The observation surface includes a position 1 mm deep from the surface of the forged steel roll. The observation surface is mirror-polished. After mirror-polishing, 10 observation fields are arbitrarily selected, with their centers at a position 1 mm deep from the surface of the barrel of the forged steel roll. The size of each observation field is 240 μm × 180 μm. COMPO images of the 10 observation fields are taken using an FE-SEM. The observation magnification is 500x. In the COMPO image, compared to the matrix, which is mainly composed of iron, the carbides contain a large amount of carbon, which has a small atomic number, resulting in a dark contrast. This makes it possible to distinguish between the matrix and the carbides. The observation fields are photographed under settings that allow for differentiation between the matrix and the carbides, and photographic images are generated.

[0096] From the photographic images obtained, all carbides with a circle-equivalent diameter of more than 5.0 μm within the entire observation field were identified and their total number was calculated. Here, the circle-equivalent diameter means the diameter (μm) when the area of ​​the carbide is converted into a circle.

[0097] The total number of carbides with a circle equivalent diameter exceeding 5.0 μm and the total area (0.432 mm) of the 10 observation fields were calculated. 2 ) based on the number density ND (pieces / mm 2 ) is found. The carbide number density ND (pieces / mm 2 ) is the integer value obtained by rounding the calculated number to the nearest tenth.

[0098] [Effects of the forged steel roll of this embodiment] The forged steel roll of this embodiment satisfies Features 1 to 4. Therefore, the forged steel roll of this embodiment has excellent wear resistance and grindability.

[0099] [Microstructure of the surface layer of the forged steel roll of this embodiment] The microstructure of the surface layer of the forged steel roll of this embodiment is mainly composed of martensite and / or bainite. "Mainly composed of martensite and / or bainite" means that the total area ratio of martensite and bainite is 85% or more. The microstructure, other than martensite and bainite, includes, for example, pearlite, retained austenite, and carbides.

[0100] [Method for measuring the total area ratio of martensite and bainite in the surface layer of a forged steel roll] The total area ratio of martensite and bainite in the surface layer of the forged steel roll of this embodiment is determined by the following method. First, the area ratio of pearlite is determined. A test specimen is taken from the surface of the barrel of the forged steel roll, with the observation surface being perpendicular to the axial direction of the barrel. The observation surface includes a position 1 mm deep from the surface of the forged steel roll. The observation surface is mirror-polished. After mirror-polishing, the observation surface is etched using 2% nitric acid alcohol (Nital etchant). Five observation fields (240 μm × 180 μm) randomly selected from the etched observation surface, centered at a position 1 mm deep from the surface of the barrel of the forged steel roll, are observed under an optical microscope at 500x magnification. If pearlite is present, it is corroded more strongly by the Nital etchant than martensite and bainite. Therefore, pearlite appears as a darker structure than martensite and bainite, making it easy to distinguish between them. The total area of ​​pearlite in the observation field is determined using well-known image processing. The area ratio of pearlite is calculated based on the total area of ​​pearlite identified in all observation fields and the total area of ​​all observation fields.

[0101] The carbide area ratio is determined by the following FE-SEM observation. A test specimen is taken from the surface layer of the barrel of the forged steel roll, with the observation surface being a plane perpendicular to the axial direction of the barrel of the forged steel roll. The observation surface includes a position 1 mm deep from the surface of the forged steel roll. The observation surface is mirror-polished. After mirror-polishing, five observation fields (240 μm × 180 μm) are arbitrarily selected from the observation surface, centered at a position 1 mm deep from the surface of the barrel of the forged steel roll, and observed with the FE-SEM. Z-contrast images, so-called COMPO images, are taken with a backscattered electron detector. The observation magnification is 500x. In the Z-contrast image, martensite and bainite structures, which are mainly composed of iron, can be easily distinguished from carbides containing large amounts of carbon. The carbide area ratio is determined based on the total area of ​​carbides identified in all observation fields and the total area of ​​all observation fields.

[0102] Furthermore, the area ratio of retained austenite is determined by the following X-ray diffraction method. A test specimen is taken from the barrel surface of the forged steel roll, including a depth of 1 mm. The size of the test specimen is not particularly limited, but for example, it is 15 mm × 15 mm × 10 mm thick. In this case, the thickness direction of the test specimen is the radial direction of the forged steel roll. Using the obtained test specimen, the X-ray diffraction intensity of each of the (110) plane of the α phase, the (200) plane of the α phase, the (211) plane of the α phase, the (220) plane of the α phase, the (200) plane of the γ phase, the (220) plane of the γ phase, and the (311) plane of the γ phase is measured, and the integrated intensity of each plane is calculated. In measuring the X-ray diffraction intensity, the target of the X-ray diffractometer is Cu (CuKα radiation), and the output is 40 kV-400 mA. After the calculation, the volume fraction Vγ (%) of the retained austenite is calculated for each combination (4 × 3 = 12 pairs) of each α phase surface and each γ phase surface using formula (I). The average value of the volume fraction Vγ of the retained austenite for the 12 pairs is then defined as the volume fraction (%) of the retained austenite. Vγ=100 / {1+(Iα×Rγ) / (Iγ×Rα)} (I) Here, Iα is the integrated intensity of the α phase. Rα is the crystallographically calculated value of the α phase. Iγ is the integrated intensity of the γ phase. Rγ is the crystallographically calculated value of the γ phase. In this specification, Rα for the (110) plane of the α phase is 100, Rα for the (200) plane of the α phase is 14.0, Rα for the (211) plane of the α phase is 25.6, Rα for the (220) plane of the α phase is 8.4, Rγ for the (200) plane of the γ phase is 34.0, Rγ for the (220) plane of the γ phase is 17.9, and Rγ for the (311) plane of the γ phase is 20.5. The volume fraction of retained austenite is rounded to the nearest tenth. In measuring the X-ray diffraction intensity, X-rays are irradiated onto the test specimen at a position corresponding to a depth of 1 mm from the surface of the barrel of the forged steel roll.

[0103] The volume fraction (%) of retained austenite obtained by the above-mentioned X-ray diffraction method is regarded as the area fraction (%) of retained austenite. Then, the total area fraction of martensite and bainite in the surface layer of the forged steel roll is calculated by the following formula. Total area ratio of martensite and bainite in the surface layer of the forged steel roll = 100 - (area ratio of pearlite + area ratio of carbide + area ratio of retained austenite)

[0104] [Use of the forged steel roll of this embodiment] The forged steel roll of this embodiment is widely applicable as a roll for rolling. The forged steel roll of this embodiment is particularly suitable as a roll for cold rolling of thin steel sheets. Examples of rolls for cold rolling include work rolls for a tandem cold rolling mill, a reverse cold rolling mill, or a work roll for skin pass (temper rolling).

[0105] [Manufacturing method of forged steel rolls] An example of a method for manufacturing the forged steel roll of this embodiment will be described. The forged steel roll of this embodiment may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of the method for manufacturing the forged steel roll of this embodiment.

[0106] An example of the method for manufacturing the forged steel roll of this embodiment includes the following steps. (Process 1) Steelmaking process (Process 2) Hot forging process (Process 3) Annealing process (Process 4) Rough processing process (Process 5) Quenching process (Process 6) Tempering process (Process 7) Finishing process Each step will be described below.

[0107] [(Process 1) Steelmaking process] In the steelmaking process, molten steel that satisfies Features 1 and 2 is used to produce ingots by a known casting method, such as bottom pouring. The cast ingot (electrode ingot) is used as an electrode to perform electroslag remelting (ESR).

[0108] The reason why the ESR method is carried out in the steelmaking process of the forged steel roll of this embodiment is as follows. During the solidification process in a typical casting method, alloying elements may concentrate in the liquid phase, causing solidification segregation. The forged steel roll of this embodiment satisfies Features 1 and 2, and is therefore adjusted to have a chemical composition that facilitates the formation of MC carbides. Therefore, if solidification segregation occurs during solidification of the forged steel roll of this embodiment, MC carbides preferentially crystallize from the remaining liquid phase. The MC carbides formed by crystallization are coarser than the MC carbides formed by precipitation. Furthermore, regions where the liquid phase remained at the final stage of solidification become grain boundaries after solidification is completed. In this way, coarse MC carbides are formed at the grain boundaries. Around the coarse MC carbides, V and Mo, the main components of MC carbides, are deficient. Therefore, precipitation of MC carbides is suppressed inside crystal grains where coarse MC carbides exist at the grain boundaries. It is believed that this is how MC carbide-poor regions are formed inside crystal grains.

[0109] In the ESR method, the electrode ingot is remelted by the Joule heat of the molten slag. The molten electrode ingot becomes droplets that sink through the molten slag and solidify in layers while being stored in a mold of any shape. When the electrode ingot is completely melted and the molten steel solidifies all the way to the top, the ingot for forging is obtained.

[0110] Furthermore, in the ESR method, the molten steel stored in the mold solidifies while maintaining a relatively shallow molten steel pool. Therefore, solidification segregation can be suppressed. As a result, the crystallization of MC carbides can be suppressed. In other words, by performing the ESR method, coarse MC carbides contained in the electrode ingot can be remelted, and an ingot for forging can be obtained in which the generation of coarse MC carbides and MC carbide-depleted regions is suppressed. Therefore, the ESR method is performed in the steelmaking process of the forged steel roll of this embodiment.

[0111] The ESR method used in the steelmaking process satisfies the following condition 1. (Condition 1) The solidification rate SR is 3.0 mm / min or more. Condition 1 will be explained below.

[0112] [Regarding Condition 1] Here, the solidification rate (SR) in the ESR method refers to the rising speed (mm / min) of the solidification interface at a position corresponding to the center of the mold when viewed from above (plan view). The ESR method allows for more precise control of the solidification rate of molten steel compared to conventional casting methods. However, if the solidification rate (SR) in the ESR method is too slow, solidification segregation is promoted. This makes it easier for coarse MC carbides to crystallize. As a result, the formation of MC carbide-depleted regions is promoted. Furthermore, if the solidification rate (SR) in the ESR method is too slow, the growth of MC carbides is excessively promoted. In this case, the growth of MC carbides is Ostwald ripening. Therefore, fine MC carbides within the grains are absorbed into the coarse MC carbides and disappear. As a result, the MC carbide number ratio (NR) decreases.

[0113] If the solidification rate SR in the ESR method is 3.0 mm / min or more, the excessive growth of MC type carbides can be suppressed, and as a result, the forged steel roll can satisfy Feature 3. Therefore, the solidification rate SR in the ESR method is 3.0 mm / min or more.

[0114] [(Process 2) Hot forging process] In the hot forging process, first, a forging ingot obtained by the ESR method is heated in a heating furnace at a heating temperature T. The heated forging ingot is then subjected to hot forging. If the temperature of the forging ingot drops during hot forging, the forging ingot may be heated again in the heating furnace. Then, hot forging may be resumed on the reheated forging ingot. In this manner, a roughly shaped roll blank (hereinafter referred to as an intermediate blank) is produced.

[0115] In the hot forging process, the following conditions 2 and 3 are satisfied. (Condition 2) Heating temperature just before the start of hot forging: 1125-1200°C (Condition 3) The cumulative time t at 1000 to 1100°C is 30 hours or less. Conditions 2 and 3 will be explained below.

[0116] [Regarding Condition 2] If the heating temperature T immediately before the start of hot forging is set to 1125 to 1200°C, the carbides precipitated in the forging ingot will dissolve. This will prevent the precipitation of coarse carbides. As a result, the size of the carbides in the forged steel roll can be appropriately adjusted.

[0117] If the heating temperature T of the forging ingot immediately before the start of hot forging is less than 1125°C, the heating temperature T is too low. Therefore, the carbides in the forging ingot are unlikely to dissolve. In this case, coarse carbides tend to remain in the forged steel roll after production. As a result, the forged steel roll does not satisfy Feature 4. If the heating temperature T of the forging ingot just before the start of hot forging exceeds 1200°C, the heating temperature T is too high. In this case, a part of the forging ingot melts, forming a liquid phase, which is likely to produce coarse crystallized carbides. As a result, the forged steel roll does not satisfy Feature 4.

[0118] If the heating temperature T immediately before the start of hot forging is 1125 to 1200° C., the precipitation of coarse carbides can be suppressed. As a result, the forged steel roll can satisfy Feature 4. Therefore, the temperature T of the forging ingot immediately before the start of hot forging is 1125 to 1200°C.

[0119] The time for which the ingot for forging is held at temperature T immediately before the start of hot forging is not particularly limited. However, taking into consideration a normal manufacturing process, the time for which the ingot is held at temperature T is, for example, 20 hours or less.

[0120] [Regarding condition 3] In the hot forging process, the cumulative time t (hours) during which the surface temperature of the forged material (the forged ingot or roll blank) is 1000-1100°C from the time the forging ingot is carried into the heating furnace until the roll blank is cooled to room temperature after forging is completed is defined as the cumulative time t (hours) at 1000-1100°C. When the chemical composition of the forged material satisfies Feature 1, workability significantly decreases when the surface temperature of the forged material drops below 1000°C. In this case, forging is interrupted and the forged material is reheated in the heating furnace. The reheated forged material is then removed from the heating furnace, and forging is resumed. In the hot forging process, this operation is repeated until a roll blank with the desired shape is obtained. In other words, the cumulative time t at 1000-1100°C includes the time during which the forged material is heated in the heating furnace and the time during which the heated forged material is transported.

[0121] In the temperature range of 1000 to 1100°C, the growth of MC carbides is excessively promoted. In this case, the growth of MC carbides also occurs by Ostwald ripening, as described above. Therefore, the fine MC carbides within the grains are absorbed into the coarse MC carbides and disappear. As a result, the number ratio NR of MC carbides decreases. Therefore, it is preferable that the cumulative time t at 1000 to 1100°C in the hot forging process is short.

[0122] If the cumulative time t at 1000 to 1100°C in the hot forging process is 30 hours or less, excessive growth of MC type carbides can be suppressed. As a result, the forged steel roll can satisfy Feature 3. Therefore, the cumulative time t at 1000 to 1100°C in the hot forging process is 30 hours or less. There is no particular limitation on the cumulative time t at 1000 to 1100° C. Taking into consideration normal industrial production, the lower limit of the cumulative time t at 1000 to 1100° C. is, for example, 8 hours.

[0123] [(Process 3) Annealing process] In the annealing process, the intermediate blank produced in the hot forging process is annealed. By performing the annealing process, the intermediate blank is easier to grind in the next rough processing process. Annealing may be performed under well-known conditions using an electric furnace or a gas furnace. The annealing temperature is, for example, 500 to 800°C. The holding time is, for example, 10 to 50 hours.

[0124] [(Process 4) Rough processing process] In the rough processing step, the intermediate preform after the annealing step is subjected to rough processing to further roughly process the intermediate preform into a shape close to the final roll shape. The rough processing is, for example, grinding. The rough processing may be performed under well-known conditions.

[0125] [(Process 5) Quenching process] In the quenching process, the surface layer of the intermediate preform after the rough machining process is subjected to a well-known quenching process. Specifically, the intermediate preform is heated to and held at 900 to 1100°C, and then rapidly cooled. The rapid cooling method is, for example, water cooling.

[0126] After the quenching step and before the tempering step, the intermediate material may be subjected to sub-zero treatment. The cooling temperature in the sub-zero treatment may be in a known range, for example, -30 to -196°C.

[0127] [(Step 6) Tempering process] In the tempering process, the intermediate blank after the quenching process is tempered. The amount of retained austenite remaining at the time of quenching is reduced by tempering. Furthermore, the hardness of the surface layer of the forged steel roll is adjusted by tempering. The tempering temperature is, for example, 100 to 200°C.

[0128] [(Process 7) Finishing process] In the finishing process, the intermediate blank after the tempering process is subjected to finishing, such as grinding using a grinding machine, to form the intermediate blank into the shape of the final product.

[0129] The forged steel roll of this embodiment is manufactured by the above steps. The above-mentioned manufacturing method is a preferred example of the manufacturing method of the forged steel roll according to this embodiment. Therefore, the forged steel roll having the above-mentioned configuration may be manufactured by a manufacturing method other than the above-mentioned manufacturing method. In short, the manufacturing method is not particularly limited as long as it can manufacture the forged steel roll of this embodiment having the above-mentioned configuration. [Example]

[0130] The effects of the forged steel roll of this embodiment will be explained more specifically with reference to examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the forged steel roll of this embodiment. Therefore, the forged steel roll of this embodiment is not limited to this one example of conditions.

[0131] Forged steel rolls having the chemical compositions shown in Tables 1A and 1B were manufactured by the following method.

[0132] [Table 1A]

[0133] [Table 1B]

[0134] Specifically, electrode ingots were cast from molten steel by the bottom pouring ingot casting method. Electroslag remelting (ESR) was performed using the produced electrode ingots as electrodes. Condition 1 (solidification rate SR (mm / min)) in the ESR method was as shown in Table 2. The forging ingots obtained by the ESR method were subjected to a hot forging process. Condition 2 (heating temperature T (°C)) and Condition 3 (cumulative time t (hours) at 1000-1100°C) in the hot forging process were as shown in Table 2. The holding time under Condition 2 was 20 hours or less. For each test number, intermediate roll shapes with a roll barrel diameter of 700 mm, a barrel length of 2100 mm, and a total length of 4100 mm were produced by the hot forging process.

[0135] [Table 2]

[0136] The intermediate blanks after the hot forging process were subjected to an annealing process. In the annealing process, they were held at 800°C for 10 hours, and then further held at 600°C for 15 hours. The intermediate blanks after the annealing process were subjected to a rough machining process. Specifically, for each test number, grinding was performed on the intermediate blanks to produce intermediate blanks in the shape of rolls with a roll barrel diameter of 650 mm, a barrel length of 2000 mm, and a total length of 4000 mm.

[0137] The intermediate preform after the rough machining step was subjected to a quenching step in which the intermediate preform was heated to and held at 900 to 1100°C, and then water-cooled.

[0138] The intermediate preform after the quenching process was subjected to sub-zero treatment. In the sub-zero treatment, the intermediate preform was cooled to -60 to -140°C. The sub-zero treated intermediate preform was tempered at 100 to 200°C and then subjected to a finish processing process. In the finish processing process, the intermediate preform was ground to a final roll shape with a roll barrel diameter of 645 mm, a barrel length of 1950 mm, and a total length of 3950 mm. The forged steel rolls of each test number were manufactured by the above manufacturing process. The total area ratio of martensite and bainite in the surface layer of the forged steel roll of each test number was determined by the method described in the above-mentioned "Method for measuring the total area ratio of martensite and bainite in the surface layer of a forged steel roll." As a result, the total area ratio of martensite and bainite in the surface layer was 85% or more in the forged steel rolls of all test numbers.

[0139] [About the evaluation test] The following evaluation tests were carried out on the manufactured forged steel rolls with each test number. (Test 1) Measurement test of the number ratio of MC type carbides (NR) (Test 2) Measurement test of carbide number density ND (Test 3) Abrasion resistance evaluation test (Test 4) Grindability evaluation test Each test will be explained below.

[0140] [(Test 1) Measurement test of the number ratio of MC type carbides (NR)] The MC type carbide number ratio NR of the forged steel roll of each test number was determined based on the method described in the above [Method for measuring the MC type carbide number ratio NR]. The obtained MC type carbide number ratio NR is shown in the "MC type carbide number ratio NR (%)" column in Table 2.

[0141] [(Test 2) Measurement test of carbide number density ND] The carbide number density ND of the forged steel roll of each test number was determined based on the method described in the above [Method for measuring carbide number density ND]. The obtained carbide number density ND is listed in the "Carbide number density ND (number / mm 2 ) column.

[0142] [(Test 3) Wear resistance evaluation test] The wear resistance of the forged steel rolls of each test number was evaluated using a two-cylinder rolling wear tester. Figure 1 is a schematic diagram of a two-cylinder rolling wear tester 10. A cylindrical roll test piece 12 was taken from the surface layer (region extending from the surface of the barrel to 80 mm in the depth direction) of the forged steel roll of each test number. The diameter of the roll test piece 12 was 80 mm and the width was 10 mm. The central axis of the roll test piece 12 was parallel to the radial direction of the forged steel roll. The outer peripheral surface of the roll test piece 12 was polished.

[0143] A two-cylinder rolling wear test was conducted using the collected roll test specimen 12. In the two-cylinder rolling wear test, a rolled material test specimen 11 rotating in the opposite direction was pressed against the rotating roll test specimen 12 with a load F (described below). Figure 2 shows a front view of the rolled material test specimen 11. The numbers in Figure 2 indicate dimensions (unit: mm). "R7.5" in Figure 2 indicates that the radius of curvature of the outer surface was 7.5 mm. As shown in Figure 2, the diameter of the rolled material test specimen 11 was 160 mm and the width was 15 mm. The rolled material test specimen 11 was prepared by processing a steel material having a chemical composition equivalent to S45C specified in JIS G 4051 (2018) into the shape shown in Figure 2 and polishing the outer surface.

[0144] In the two-cylinder rolling wear test, the contact load between the roll test piece 12 and the rolled material test piece 11 was 700 N, and no lubricant was used. The rotation speed of the roll test piece 12 was 2000 rpm, and the sliding ratio between the roll test piece 12 and the rolled material test piece 11 was 5%. The sliding ratio (%) is defined by the following formula. Slippage ratio (%) = (circumferential speed of rolled material test piece - peripheral speed of roll test piece) / peripheral speed of roll test piece × 100 In the two-cylinder rolling wear test, the rolled material test piece 11 was maintained at 200°C by high-frequency induction heating using an induction heating coil (not shown). The roll test piece was water-cooled. The test was terminated after 20,000 cycles.

[0145] The wear shape of the outer peripheral surface of the roll test piece 12 after 20,000 rolling cycles was measured using a laser microscope (Keyence Corporation, product name: Shape Analysis Laser Microscope VK-X250). Specifically, the surface shape was scanned in the width direction at an arbitrary position on the outer peripheral surface of the roll test piece 12. The scanned range was the entire width direction length of the outer peripheral surface. From the obtained surface shape profile, the wear cross-sectional area (the area of ​​the depressions caused by wear on the outer peripheral surface in a cross section including the width direction and radial direction of the roll test piece 12) was calculated. Similar wear cross-sectional area measurements were also performed on the outer peripheral surface at positions diametrically opposite the center of the roll test piece 12. The arithmetic mean value of the wear cross-sectional areas obtained by the measurements at the two locations was taken as the wear cross-sectional area of ​​the roll test piece 12. The wear cross-sectional area of ​​the roll test piece 12 is 2000 μm 2 If the wear cross-sectional area of ​​the roll test piece 12 was 2000 μm or less, it was evaluated as "E (Excellent)" and it was determined that excellent wear resistance was obtained (indicated as "E" in the "Wear Resistance" column in Table 2). 2 If the value was over 100%, it was rated as "B (Bad)" and it was determined that excellent abrasion resistance was not obtained (indicated as "B" in the "Abrasion resistance" column in Table 2).

[0146] [(Test 4) Grindability Evaluation Test] Using the forged steel rolls of each test number, a grindability evaluation test was carried out in the following manner. A cylindrical test piece with a diameter of 100 mm and a length of 200 mm was taken, with the center axis at the D / 4 position of the forged steel roll body, where D is the diameter of the forged steel roll body for each test number. The longitudinal direction of the test piece coincided with the longitudinal direction of the forged steel roll for each test number, and the radial direction coincided with the radial direction of the forged steel roll body for each test number. A cylindrical grinding machine was used for the grinding performance evaluation test. The grinding wheel conditions were in accordance with JIS R 6242:2023. Specifically, a grinding wheel made of alumina abrasive bonded with a vitrified bond was used. The abrasive grain size was 100, the bonding degree was F, and the abrasive grain ratio was 38%. The grinding wheel dimensions were 400 mm in diameter and 15 mm in width.

[0147] In the grindability evaluation test using the above grinding wheel, the grinding method was wet traverse grinding, and the feed rate was 1500 mm / min. Furthermore, the peripheral speed of the grinding wheel was 33 m / sec, and the peripheral speed of the test piece was 0.75 m / sec. The cutting depth per pass was 0.01 mm, and the target cutting depth (hereinafter also referred to as the target cutting depth) was 0.1 mm. In other words, a total of 10 passes were performed in the grindability evaluation test.

[0148] After the grindability evaluation test, the diameter (mm) of the test piece of each test number was measured. Based on the diameter, the depth of cut obtained in the grindability evaluation test (hereinafter also referred to as the actual depth of cut) was calculated. Specifically, it was calculated using the following formula. Actual cutting depth (mm) = (100 mm - (diameter (mm) of each test piece after grindability evaluation test)) / 2 The workpiece removal rate (%) for each test number in the grindability evaluation test was calculated using the following formula from the actual cutting depth obtained by the above formula. Workpiece removal rate (%) = Actual cutting depth (mm) / Target cutting depth (mm) When the workpiece removal rate was 60% or more, the evaluation was given as "E (Excellent)" and it was determined that excellent grinding performance was obtained (indicated as "E" in the "Grindability" column in Table 2). On the other hand, when the workpiece removal rate was less than 60%, the evaluation was given as "B (Bad)" and it was determined that excellent grinding performance was not obtained (indicated as "B" in the "Grindability" column in Table 2).

[0149] [Test Results] Referring to Tables 1A, 1B and 2, the forged steel rolls of test numbers 1 to 20 satisfied characteristics 1 to 4. Therefore, excellent wear resistance and grindability were obtained.

[0150] On the other hand, in Test Nos. 21 and 22, the solidification rate (SR) in the ESR method performed in the steelmaking process was too slow, and therefore the forged steel rolls did not satisfy Feature 3. As a result, excellent wear resistance was not obtained.

[0151] In test numbers 23 and 24, the heating temperature T in the hot forging process was too low, and therefore the forged steel rolls did not satisfy feature 4. As a result, excellent grindability was not obtained.

[0152] In test numbers 25 and 26, the cumulative time t at 1000 to 1100°C in the hot forging process was too long, and therefore the forged steel rolls did not satisfy feature 3. As a result, excellent wear resistance was not obtained.

[0153] In test numbers 27 and 28, Fn1 was too low, and therefore the forged steel rolls did not satisfy characteristic 2. As a result, excellent wear resistance was not obtained.

[0154] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. The chemical composition, in mass%, is C: 0.85-1.05%, Si: 0.60-1.20%, Mn: 0.30-0.60%, P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00-6.00%, Mo: 0.20 to less than 1.00% V: 1.00-2.00%, Cu: 0.40% or less, and Ni: 0.30 to 0.60%; the balance being Fe and impurities; Formula (1) is satisfied, the proportion of MC type carbides in all carbides having a circle equivalent diameter of 0.5 to 5.0 μm is 30% or more, The number density of carbides with a circle equivalent diameter exceeding 5.0 μm is 100 pieces / mm ２ Below is the Forged steel roll. (V+2Mo) / (Cr+Ni)≧0.400 (1) Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element in the chemical composition.

2. The chemical composition, in mass%, is C: 0.85-1.05%, Si: 0.60-1.20%, Mn: 0.30-0.60%, P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00-6.00%, Mo: 0.20 to less than 1.00% V: 1.00-2.00%, Cu: 0.40% or less, and Ni: 0.30 to 0.60%; Further, the composition contains one or more selected from the group consisting of Group 1 and Group 2, the balance being Fe and impurities; Formula (1) is satisfied, the proportion of MC type carbides in all carbides having a circle equivalent diameter of 0.5 to 5.0 μm is 30% or more, The number density of carbides with a circle equivalent diameter exceeding 5.0 μm is 100 pieces / mm ２ Below is the Forged steel roll. [Group 1] Ti: 0.050% or less, Nb: 0.050% or less, B: 0.0100% or less, W: 0.50% or less, and Co: 0.50% or less, one or more selected from the group consisting of [Group 2] Sn: 0.10% or less, Sb: 0.05% or less, As: 0.05% or less, Zr: 0.05% or less, Bi: 0.10% or less, Se: 0.10% or less, Te: 0.05% or less, Pb: 0.09% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of (V+2Mo) / (Cr+Ni)≧0.400 (1) Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element in the chemical composition.

3. 3. The forged steel roll according to claim 2, the chemical composition contains the first group; Forged steel roll.

4. 3. The forged steel roll according to claim 2, The chemical composition contains the second group. Forged steel roll.

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