Forged steel roll

Abstract

Provided is a forged steel roll having excellent wear resistance. The forged steel roll according to the present disclosure has a chemical composition containing, in terms of mass%, 0.85%-1.05% of C, 0.60%-1.20% of Si, 0.30%-0.60% of Mn, 0.020% or less of P, 0.020% or less of S, 0.001%-0.050% of Al, 0.001%-0.020% of N, 0.0050% or less of O, 4.00%-6.00% of Cr, 0.20% to less than 1.00% of Mo, 1.00%-2.00% of V, 0.40% or less of Cu, and 0.30%-0.60% of Ni, with the balance being Fe and impurities. The forged steel roll satisfies formula (1), and the number ratio of an MC-type carbide in the total carbide having an equivalent circle diameter of 0.5-5.0 μm is 30% or more.　(1): (V + 2Mo) / (Cr + Ni) ≥ 0.400 The content in mass% of the corresponding element in the chemical composition is plugged in for each chemical symbol in formula (1).

Description

Forged steel rolls

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

[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 occurs 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] A technique for improving the wear resistance of forged steel rolls is proposed in Japanese Patent Laid-Open Publication No. 2003-1307 (Patent Document 1).

[0005] The forged steel roll disclosed in Patent Document 1 contains 0.8 to 1.2 mass% C, 0.3 to 0.5 mass% Si, 0.4 to 0.6 mass% Mn, 2.5 to 4.0 mass% Cr, 0.3 to 0.5 mass% Mo, and 0.3 mass% or less V, with the balance consisting essentially of Fe and unavoidable impurities. This forged steel roll further has a Vickers hardness of 900 HV or more in a surface layer extending from the surface to a depth of 4 to 8 mm toward the center, and a Vickers hardness of less than 900 HV in a portion deeper than the surface layer, thereby achieving excellent wear resistance.

[0006] Japanese Patent Application Laid-Open No. 2003-1307

[0007] However, the wear resistance of the forged steel roll may be increased by a means different from that described in Patent Document 1.

[0008] An object of the present disclosure is to provide a forged steel roll having excellent wear resistance.

[0009] The forged steel roll of the present disclosure has a chemical composition, in mass%, of 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: 0.001 to 0.050%, N: 0.001 to 0.020%, 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 to 0.050%, B: 0 to 0.0100%, The alloy contains 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 balance being Fe and impurities, and satisfies formula (1). The ratio of the number of MC type carbides to all carbides having an equivalent circle diameter of 0.5-5.0 μm is 30% or more. (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.

[0010] The forged steel roll of the present disclosure provides excellent wear resistance.

[0011] Fig. 1 is a schematic diagram of a two-cylinder rolling wear tester used in a wear resistance evaluation test, and Fig. 2 is a front view of the rolled material test piece in Fig. 1.

[0012] The present inventors first studied forged steel rolls having excellent wear resistance from the viewpoint of chemical composition, and as a result, the present inventors found that the forged steel rolls have the following composition, in 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: 0.001 to 0.050%, N: 0.001 to 0.020%, 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: 0.85 to 1.05 ... and Nb: 0.85 to 1.05%. It was considered that excellent wear resistance could be obtained if the steel sheet had a chemical composition containing the following: 0 to 0.050%, B: 0 to 0.0100%, W: 0 to 0.50%, Co: 0 to 0.50%, Sn: 0 to 0.10%, Sb: 0 to 0.05%, As: 0 to 0.05%, Zr: 0 to 0.05%, Bi: 0 to 0.10%, Se: 0 to 0.10%, Te: 0 to 0.05%, Pb: 0 to 0.09%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, with the balance being Fe and impurities.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] The carbides contained in the forged steel roll having the above-mentioned chemical composition are mainly composed of M, which is mainly composed of Cr. ７ C ３ The carbides contain Mn-type carbides and MC-type carbides mainly composed of V and Mo. ７ C ３ M-type carbides tend to crystallize out of the liquid phase during solidification in the steelmaking process. ７ C ３ On the other hand, MC type carbides tend to become coarse. ７ C ３ Compared with M-type carbides, MC-type carbides are less likely to crystallize from the liquid phase during solidification in the steelmaking process. Therefore, MC-type carbides are less likely to become coarse. ７ C ３ It was thought that if MC type carbides could be preferentially obtained over MC type carbides, the generation of coarse crystallized carbides could be suppressed.

[0017] Furthermore, MC type carbides are ７ C ３ Therefore, when using forged steel rolls, MC type carbide is harder than M type carbide. ７ C ３Therefore, the inventors have found that M ７ C ３ It was thought that if it were possible to preferentially obtain MC type carbides, which are harder than MC type carbides, the wear resistance would be improved.

[0018] The present inventors have ７ C ３ In order to obtain MC type carbides preferentially over MC type carbides, it was thought that it was necessary to adjust the V content, Mo content, and Cr content, which are components constituting these carbides. Furthermore, Ni tends to remain in the liquid phase together with Cr, and M ７ C ３ Therefore, the inventors have found that adjusting the Ni content in addition to the V content, Mo content, and Cr content can improve the M content. ７ C ３ It was thought that MC type carbides would be obtained preferentially over MC type carbides.

[0019] Based on the above findings, the present inventors have investigated the relationship between the V content, Mo content, Cr content, and Ni content. As a result, the present inventors have found that in a forged steel roll having the above-mentioned chemical composition, excellent wear resistance can be obtained if formula (1) is satisfied: (V + 2Mo) / (Cr + Ni) ≧ 0.400 (1) Here, the content of the corresponding element in the chemical composition in mass % is substituted for each element symbol in formula (1).

[0020] 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.

[0021] To obtain excellent wear resistance, it is preferable to increase the area ratio of MC carbides on the surface of the forged steel roll as much as possible, 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), it was found that many coarse MC carbides were present at the grain boundaries. On the other hand, it was found that in the microstructure of such a forged steel roll, there were regions depleted of MC carbides 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 coarse MC carbides present around them. This is thought to be the reason for the deterioration of the wear resistance of the forged steel roll.

[0022] 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.

[0023] Therefore, we considered that reducing the number of coarse MC carbides and suppressing the formation of MC carbide-deficient regions inside the crystal grains would be effective in improving the wear resistance of forged steel rolls. As described above, suppressing the formation of MC carbide-deficient regions increases the proportion of MC carbides in 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 in carbides of an appropriate size as much as possible.

[0024] Based on the above findings, the present inventors have conducted further studies and have 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 number to all carbides having an equivalent circle diameter of 0.5 to 5.0 μm is 30% or more.

[0025] The forged steel roll of this embodiment has been completed based on the above technical concept and has the following configuration.

[0026] A forged steel roll of a first configuration has a chemical composition, in mass %, of 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: 0.001 to 0.050%, N: 0.001 to 0.020%, 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 to 0.050%, B: 0 to 0.0100%, The alloy contains 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 balance being Fe and impurities, and satisfies formula (1). The ratio of the number of MC type carbides to all carbides having an equivalent circle diameter of 0.5-5.0 μm is 30% or more. (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.

[0027] The forged steel roll of the second configuration is the forged steel roll of the first configuration, having a chemical composition, in mass %, of Ti: 0.001 to 0.050%, Nb: 0.001 to 0.050%, B: 0.0001 to 0.0100%, W: 0.01 to 0.50%, Co: 0.01 to 0.50%, Sn: 0.01 to 0.10%, Sb: 0.01 to 0.05%, As: 0.01 to 0.05%, Zr: 0.01 to 0.05%, Bi: 0.01 to 0.10%, Se: 0.01 to 0.10%, Te: 0.01 to 0.05%, Pb: 0.01 to 0.09%, It contains one or more elements selected from the group consisting of Ca: 0.0001 to 0.0050%, and Mg: 0.0001 to 0.0050%.

[0028] The forged steel roll of this embodiment will be described in detail below. Unless otherwise specified, "%" for elements means mass %.

[0029] [Configuration of the forged steel roll of this embodiment] The forged steel roll of this embodiment includes a barrel portion and a pair of shank portions. The barrel portion is cylindrical and includes a pair of end faces and a circumferential surface (hereinafter also simply referred to as "surface") disposed between the pair of end faces. The circumferential 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.

[0030] 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.

[0031] [Features of the forged steel roll of this embodiment] The forged steel roll of this embodiment satisfies the following features 1 to 3. (Feature 1) The chemical composition is, in 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: 0.001 to 0.050%, N: 0.001 to 0.020%, 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. 0.050%, 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 balance consisting of Fe and impurities. (Feature 2) The chemical composition satisfies the following formula (1): (V+2Mo) / (Cr+Ni)≧0.400 (1) Here, the content of the corresponding element in the chemical composition in mass % is substituted for each element symbol in formula (1). (Feature 3) 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. Features 1 to 3 are explained below.

[0032] [(Feature 1) Chemical Composition] The forged steel roll of this embodiment contains the following elements.

[0033] 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%, the above effect cannot be sufficiently obtained. On the other hand, if the C content exceeds 1.05%, coarse carbides are formed. 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 preferred lower limit of the C content is 0.87%, more preferably 0.90%, and even more preferably 0.95%. The preferred upper limit of the C content is 1.03%, more preferably 1.00%, and even more preferably 0.98%.

[0034] Si: 0.60 to 1.20% Silicon (Si) deoxidizes steel at the molten steel stage. Si also improves the hardenability of steel. If the Si content is less than 0.60%, the above effect 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%. A preferred lower limit of the Si content is 0.65%, more preferably 0.70%, and even more preferably 0.75%. A preferred upper limit of the Si content is 1.15%, more preferably 1.10%, and even more preferably 1.05%.

[0035] Mn: 0.30 to 0.60% Manganese (Mn) improves the hardenability of steel. If the Mn content is less than 0.30%, the above 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%. A preferred lower limit of the Mn content is 0.33%, more preferably 0.35%, and even more preferably 0.40%. A preferred upper limit of the Mn content is 0.57%, more preferably 0.55%, and even more preferably 0.50%.

[0036] 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 of the P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the P content is more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the P content is 0.018%, even more preferably 0.015%, and even more preferably 0.010%.

[0037] 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 manufacturing costs. Therefore, considering 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%, even more preferably 0.015%, and even more preferably 0.010%.

[0038] Al: 0.001 to 0.050% Aluminum (Al) deoxidizes steel during the molten steel stage. If the Al content is less than 0.001%, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content exceeds 0.050%, coarse Al nitrides are formed. In this case, the toughness of the steel material decreases during the manufacturing process of the forged steel roll. Therefore, the Al content is 0.001 to 0.050%. A preferred lower limit of the Al content is 0.002%, more preferably 0.005%, and even more preferably 0.010%. A preferred upper limit of the Al content is 0.040%, more preferably 0.035%, even more preferably 0.030%, and even more preferably 0.025%. In this specification, the Al content refers to the total Al content in the steel.

[0039] N: 0.001 to 0.020% Nitrogen (N) increases the hardness of the forged steel roll through solid solution strengthening. If the N content is less than 0.001%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content exceeds 0.020%, coarse nitrides are formed. In this case, the toughness of the forged steel roll decreases. Therefore, the N content is 0.001 to 0.020%. The preferred lower limit of the N content is 0.003%, and more preferably 0.005%. The preferred upper limit of the N content is 0.015%, more preferably 0.010%, and even more preferably 0.008%.

[0040] O: 0.0050% or less Oxygen (O) is an impurity. If the O content exceeds 0.0050%, O generates oxides and reduces 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, considering normal industrial production, the preferred lower limit of the O content is more than 0%, more preferably 0.0001%, even more preferably 0.0005%, even more preferably 0.0007%, and even more preferably 0.0010%. The preferred upper limit of the O content is 0.0040%, even more preferably 0.0035%, and even more preferably 0.0030%.

[0041] Cr: 4.00 to 6.00% Chromium (Cr) is M ７ C ３ Cr forms type carbides to improve the wear resistance of the forged steel roll. Cr also improves the temper softening resistance of the steel by solid solution, and increases the hardness of the surface layer of the forged steel roll. 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 M ７ C ３In this case, type carbides are formed. In this case, the wear resistance of the forged steel roll is reduced. Therefore, the Cr content is 4.00 to 6.00%. The preferred lower limit of the Cr content is 4.05%, more preferably 4.10%, and even more preferably 4.15%. The preferred upper limit of the Cr content is 5.95%, more preferably 5.90%, and even more preferably 5.85%.

[0042] Mo: 0.20 to less than 1.00% Molybdenum (Mo) promotes the formation of MC-type carbides, thereby improving 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 fully achieved. 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 roll are reduced. Therefore, the Mo content is 0.20 to less than 1.00%. The preferred lower limit of the Mo content is 0.25%, more preferably 0.30%, and even more preferably 0.35%. The preferred upper limit of the Mo content is 0.95%, more preferably 0.90%, and even more preferably 0.85%.

[0043] V: 1.00 to 2.00% Vanadium (V) forms MC-type carbides together with Mo, 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 roll are reduced. Therefore, the V content is 1.00 to 2.00%. The preferred lower limit of the V content is 1.05%, more preferably 1.10%, and even more preferably 1.15%. The preferred upper limit of the V content is 1.90%, more preferably 1.80%, and even more preferably 1.70%.

[0044] 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 of the Cu content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the Cu content is more than 0%, more preferably 0.01%, even more preferably 0.02%, even more preferably 0.03%, and even more preferably 0.04%. The preferred upper limit of the Cu content is less than 0.40%, even 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%.

[0045] Ni: 0.30 to 0.60% Nickel (Ni) is M ７ C ３ Ni promotes the formation of carbides, thereby improving the wear resistance of the forged steel roll. Ni also improves the hardenability of the steel. If the Ni content is less than 0.30%, the above effect cannot be sufficiently obtained. On the other hand, if the Ni content exceeds 0.60%, coarse M ７ C ３ Ni-type carbides are formed. If the Ni content exceeds 0.60%, retained austenite is formed in excess. In this case, the wear resistance of the forged steel roll is reduced. Therefore, the Ni content is 0.30 to 0.60%. The preferred lower limit of the Ni content is 0.33%, more preferably 0.35%, and even more preferably 0.40%. The preferred upper limit of the Ni content is 0.57%, more preferably 0.55%, and even more preferably 0.50%.

[0046] 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.

[0047] [Optional Elements] The chemical composition of the forged steel roll of this embodiment further includes, in place of a portion of Fe, one or more elements selected from the group consisting of: Ti: 0 to 0.050%, Nb: 0 to 0.050%, B: 0 to 0.0100%, W: 0 to 0.50%, Co: 0 to 0.50%, Sn: 0 to 0.10%, Sb: 0 to 0.05%, As: 0 to 0.05%, Zr: 0 to 0.05%, Bi: 0 to 0.10%, Se: 0 to 0.10%, Te: 0 to 0.05%, Pb: 0 to 0.09%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%. All of these elements are optional elements. These optional elements will be explained below.

[0048] [First Group: Ti, Nb, B, W, and Co] The chemical composition of the forged steel roll of this embodiment may further contain one or more elements selected from the first group described above in place of a portion of Fe. All of these elements increase the hardness of the surface layer of the forged steel roll. Each element in the first group will be described below.

[0049] Ti: 0 to 0.050% Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When titanium 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 even a small amount of Ti is contained, 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 during 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%. The preferred lower limit of the Ti content is 0.001%, more preferably 0.002%, and even more preferably 0.004%. The preferred upper limit of the Ti content is 0.040%, more preferably 0.035%, and even more preferably 0.030%.

[0050] Nb: 0 to 0.050% Niobium (Nb) is an optional element and does not necessarily need to be contained. That is, the Nb content may be 0%. When contained, that is, when the Nb content exceeds 0%, Nb forms precipitates such as carbides or nitrides, thereby increasing 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%. The preferred lower limit of the Nb content is 0.001%, more preferably 0.002%, and even more preferably 0.004%. The preferred upper limit of the Nb content is 0.040%, more preferably 0.035%, and even more preferably 0.030%.

[0051] B: 0 to 0.0100% Boron (B) is an optional element and does not necessarily need to be contained. In other words, the B content may be 0%. When 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 decreases 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%. A preferred lower limit of the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. A preferred upper limit of the B content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0052] W: 0 to 0.50% Tungsten (W) is an optional element and does not necessarily need to 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%. The preferred lower limit of the W content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the W content is 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0053] 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 decreases 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%. The preferred lower limit of the Co content is 0.01%, more preferably 0.05%, and even more preferably 0.08%. The preferred upper limit of the Co content is 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0054] [Second Group: 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 one or more elements selected from the above-mentioned second group in place of a portion of Fe. All of these elements improve the grindability of the forged steel roll. Each element of the second group will be described below.

[0055] Sn: 0 to 0.10% Tin (Sn) is an optional element and does not necessarily need to be contained. That is, the Sn content may be 0%. When Sn is contained, that is, when the Sn content exceeds 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%. The preferred lower limit of the Sn content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Sn content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0056] Sb: 0 to 0.05% Antimony (Sb) is an optional element and does not necessarily need to be contained. That is, the Sb content may be 0%. When contained, that is, when the Sb content exceeds 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%. A preferable lower limit of the Sb content is 0.01%. A preferable upper limit of the Sb content is 0.04%.

[0057] As: 0 to 0.05% Arsenic (As) is an optional element and does not necessarily need to be contained. That is, the As content may be 0%. When As is contained, that is, when the As content exceeds 0%, As improves the grindability of the forged steel roll. Even if even 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%. A preferable lower limit of the As content is 0.01%. A preferable upper limit of the As content is 0.04%.

[0058] Zr: 0 to 0.05% Zirconium (Zr) is an optional element and does not necessarily need to be contained. That is, the Zr content may be 0%. When contained, that is, when the Zr content exceeds 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 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 Zr content is 0 to 0.05%. The preferable lower limit of the Zr content is 0.01%. The preferable upper limit of the Zr content is 0.04%.

[0059] Bi: 0 to 0.10% Bismuth (Bi) is an optional element and does not necessarily need to be contained. That is, the Bi content may be 0%. When Bi is 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 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 Bi content is 0 to 0.10%. The preferred lower limit of the Bi content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Bi content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0060] Se: 0 to 0.10% Selenium (Se) is an optional element and does not necessarily need to 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 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 Se content is 0 to 0.10%. The preferred lower limit of the Se content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Se content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0061] Te: 0 to 0.05% 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 exceeds 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%. A preferable lower limit of the Te content is 0.01%. A preferable upper limit of the Te content is 0.04%.

[0062] Pb: 0 to 0.09% Lead (Pb) is an optional element and does not necessarily need to be contained. That is, the Pb content may be 0%. When Pb is contained, that is, when the Pb content exceeds 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%. The preferred lower limit of the Pb content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Pb content is 0.08%, more preferably 0.07%, and even more preferably 0.06%.

[0063] Ca: 0 to 0.0050% 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 exceeds 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 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 Ca content is 0 to 0.0050%. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the Ca content is 0.0047%, more preferably 0.0045%, and even more preferably 0.0040%.

[0064] Mg: 0 to 0.0050% 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 exceeds 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 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 Mg content is 0 to 0.0050%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the Mg content is 0.0047%, more preferably 0.0045%, and even more preferably 0.0040%.

[0065] [(Feature 2) Formula (1)] The forged steel roll of this embodiment further has a chemical composition that satisfies formula (1): (V+2Mo) / (Cr+Ni)≧0.400 (1) Here, the content of the corresponding element in the chemical composition, expressed in mass %, is substituted for each element symbol in formula (1).

[0066] Fn1 is defined as follows: Fn1 = (V + 2Mo) / (Cr + Ni)

[0067] Fn1 corresponds to the left side of formula (1). Fn1 is the ratio of the MC type carbides and M contained in the forged steel roll that satisfies the characteristic 1. ７ C ３ As described above, the MC type carbide is an index for adjusting the ratio of M ７ C ３ MC type carbides are harder than M type carbides. ７ C ３ Therefore, in order to improve the wear resistance of forged steel rolls, it is necessary to use M ７ C ３ It is preferable to increase the proportion of MC type carbides relative to M type carbides. ７ C ３ The Cr content that forms the M type carbide, and ７ C ３By increasing the contents of V and Mo, which form MC type carbides, relative to the Ni content, which promotes the formation of MC type carbides, M ７ C ３ Therefore, MC type carbides can be formed preferentially over M type carbides.

[0068] If Fn1 is less than 0.400, the V content and Mo content are too small relative to the Cr content and Ni content. In this case, M is more likely to be present than MC type carbides. ７ C ３ Therefore, carbide-deficient regions tend to be formed 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 cannot have sufficient wear resistance.

[0069] If Fn1 is 0.400 or more, in a forged steel roll that satisfies Feature 1 and further satisfies Feature 3 described later, M ７ C ３ MC type carbides are preferentially obtained over MC type carbides, resulting in excellent wear resistance of the forged steel roll.

[0070] 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 Feature 1 and Feature 3 described below, the upper limit of Fn1 is, for example, 0.800.

[0071] [(Feature 3) Number Ratio of MC Type Carbides NR] Furthermore, in the forged steel roll of this embodiment, the number ratio of MC type carbides to all carbides having an equivalent circle diameter of 0.5 to 5.0 μm is 30% or more.

[0072] The number ratio of MC type carbides among all carbides with an equivalent circle diameter of 0.5 to 5.0 μm is defined as the MC type carbide number ratio NR. As described above, in order to improve the wear resistance of forged steel rolls, it is effective to suppress the formation of MC type carbide depleted regions inside crystal grains rather than increasing the number of coarse MC type 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 type carbides among all carbides with an equivalent circle diameter of 0.5 to 5.0 μm.

[0073] When the MC carbide number ratio NR 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.

[0074] If the MC carbide number ratio NR 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 and 2. Therefore, in the forged steel roll of this embodiment, the MC carbide number ratio NR is 30% or more.

[0075] 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 number ratio NR of MC type carbides is not particularly limited. When the chemical composition of the forged steel roll satisfies Features 1 and 2, the upper limit of the number ratio NR of MC type carbides is, for example, 90%.

[0076] [Method for measuring the number ratio NR of MC type carbides] The number ratio NR of MC type carbides can be measured by the following method. A test piece is taken from the surface layer of the barrel of a forged steel roll, with the observation surface being a surface 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. On the mirror-polished observation surface, 10 arbitrary observation fields are determined, each centered at a position 1 mm deep from the surface of the barrel of the forged steel roll. The size of the observation field is 240 μm × 180 μm. A field emission scanning electron microscope (FE-SEM) is used to photograph the 10 observation fields as Z-contrast images, so-called COMPO images, of a backscattered electron detector. The observation magnification is 500x. In a COMPO image, carbides have a dark contrast compared to the matrix, which is mainly composed of iron, because they contain a lot of carbon, which has a small atomic number. Therefore, the matrix and carbides can be distinguished by their contrast.

[0077] 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. In the quantitative analysis using EDS, quantitative analysis was performed on elements other than Fe to eliminate noise from Fe, which is the main component of the matrix. Note that carbides with a V content of 30% or more by mass are defined as MC carbides. Carbides with a V content of less than 30% by mass are defined as other carbides. In the EDS analysis, the acceleration voltage was set to 15 kV. The EDS analysis time was set so that the X-ray count was 500 counts or more.

[0078] Here, by appropriately setting the contrast in the COMPO image, it is possible to distinguish between MC carbides, which have a high carbon content among carbides, and other carbides, which have a lower carbon content than the MC carbides. Specifically, if the contrast is appropriately set, the MC carbides will be displayed with a darker contrast than the other carbides. The contrast of the COMPO image is adjusted so that the five MC carbides identified by the quantitative analysis using EDS can be distinguished from the five other carbides. In this way, the observation field is photographed with settings that allow the parent phase, MC carbides, and other carbides to be distinguished, and a photographic image is generated.

[0079] 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.

[0080] Based on the total number of carbides having 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 having an equivalent circle diameter of 0.5 to 5.0 μm is determined. The obtained number ratio (%) is defined as the MC carbide number ratio NR (%). The MC carbide number ratio NR (%) is an integer value obtained by rounding the determined value to one decimal place.

[0081] [Effects of the Forged Steel Roll of the Present Embodiment] The forged steel roll of the present embodiment satisfies Features 1 to 3. Therefore, the forged steel roll of the present embodiment has excellent wear resistance.

[0082] [Regarding the Microstructure of the Surface Layer of the Forged Steel Roll of the Present Embodiment] The microstructure of the surface layer of the forged steel roll of the present 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. In the microstructure, structures other than martensite and bainite include, for example, pearlite, retained austenite, and carbides.

[0083] [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 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. The mirror-polished observation surface is etched using 2% nitric acid alcohol (nital etchant). Five observation fields (240 μm × 180 μm) of the etched observation surface, centered at a position 1 mm deep from the surface of the barrel of the forged steel roll, are observed with 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 is observed as a structure that is darker than martensite and bainite, making it easy to distinguish between them. The total area of ​​pearlite in the observation field is determined by known image processing, and the area ratio of pearlite is determined based on the total area of ​​pearlite identified in all the observation fields and the total area of ​​all the observation fields.

[0084] The area ratio of carbides 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, centered at a position 1 mm deep from the surface of the barrel of the forged steel roll, and observed with an FE-SEM to obtain Z-contrast images, or so-called COMPO images, using a backscattered electron detector. The observation magnification is 500x. All carbides with a circle-equivalent diameter of 0.5 μm or more are identified from the photographic images. Carbides with a circle-equivalent diameter of less than 0.5 μm can be ignored as they are considered to be part of martensite and bainite. Furthermore, the Z-contrast image makes it easy to distinguish between martensite and bainite structures, which are primarily composed of iron, and carbides containing a large amount of carbon. The area ratio of carbides is calculated based on the total area of ​​carbides identified in all the observation fields and the total area of ​​all the observation fields.

[0085] Furthermore, the area ratio of retained austenite is determined by the following X-ray diffraction method. A test specimen is taken from the barrel of the forged steel roll, including a depth of 1 mm from the surface. The size of the test specimen is not particularly limited, but is, for example, 15 mm x 15 mm x 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 retained austenite is calculated using formula (I) for each combination (4 × 3 = 12 pairs) of each face of the α phase and each face of the γ phase. The average value of the volume fraction Vγ of the 12 pairs of retained austenite is then defined as the volume fraction (%) of retained austenite. Vγ = 100 / {1 + (Iα × Rγ) / (Iγ × Rα)} (I) Here, Iα is the integrated intensity of the α phase. Rα is the crystallographically theoretically calculated value of the α phase. Iγ is the integrated intensity of the γ phase. Rγ is the crystallographically theoretically calculated value of the γ phase. In this specification, Rα at the (110) plane of the α phase is 100, Rα at the (200) plane of the α phase is 14.0, Rα at the (211) plane of the α phase is 25.6, Rα at the (220) plane of the α phase is 8.4, Rγ at the (200) plane of the γ phase is 34.0, Rγ at the (220) plane of the γ phase is 17.9, and Rγ at 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.

[0086] 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 fraction of martensite and bainite in the surface layer of the forged steel roll = 100 - (area fraction of pearlite + area fraction of carbide + area fraction of retained austenite)

[0087] [Applications 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 cold tandem rolling mill or a cold reverse rolling mill, or work rolls for skin pass (temper rolling).

[0088] [Method for manufacturing forged steel roll] 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.

[0089] An example of the method for manufacturing a forged steel roll according to the present embodiment includes the following steps: (Step 1) Steelmaking step (Step 2) Hot forging step (Step 3) Annealing step (Step 4) Rough machining step (Step 5) Quenching step (Step 6) Tempering step (Step 7) Finish machining step Each step will be described below.

[0090] [(Step 1) Steelmaking Step] In the steelmaking step, an ingot is produced by a known casting method using molten steel that satisfies Features 1 and 2. Known casting methods include bottom pouring and ingot casting. Electroslag remelting (ESR) is performed using the cast ingot (electrode ingot) as an electrode.

[0091] The reason for performing the ESR method in the steelmaking process of the forged steel roll of this embodiment is as follows. During the solidification process in a general casting method, alloy 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. V and Mo, which are the main components of MC carbides, are deficient around the coarse MC carbides. Therefore, precipitation of MC carbides is suppressed inside crystal grains where coarse MC carbides exist at the grain boundaries. It is believed that in this way, MC-type carbide-depleted regions are formed inside the crystal grains.

[0092] In the ESR method, the electrode ingot is remelted by the Joule heat of the molten slag. The molten electrode ingot forms 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, an ingot for forging is obtained.

[0093] 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 for the forged steel roll of this embodiment.

[0094] The ESR method carried out 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.

[0095] [Condition 1: Solidification Rate SR] 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 (in a plan view). The ESR method allows for stricter 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. As a result, coarse MC carbides are more likely to crystallize and form. 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.

[0096] If the solidification rate SR in the ESR method is 3.0 mm / min or more, excessive growth of MC type carbides can be suppressed. 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.

[0097] [(Step 2) Hot Forging Step] In the hot forging step, first, the forging ingot obtained by the ESR method is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1000 to 1200°C. Hot forging is then performed on the heated forging ingot. 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.

[0098] The hot forging process satisfies the following condition 2: (Condition 2) The cumulative time t at 1000 to 1100° C. is 30 hours or less. Condition 2 will be explained below.

[0099] [Condition 2: Cumulative Time t] In the hot forging process, the cumulative time during which the surface temperature of the forged material (the forged ingot or the roll preform) is 1000 to 1100°C from the time the forging ingot is carried into the heating furnace until the roll preform is cooled to room temperature after forging is completed is defined as the cumulative time t (hours) at 1000 to 1100°C. The surface temperature of the forged material is measured using a non-contact radiation thermometer. When the chemical composition of the forged material satisfies Feature 1, workability significantly decreases when the surface temperature of the forged material falls 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 preform having the desired shape is obtained. In other words, the cumulative time t at 1000 to 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.

[0100] 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.

[0101] 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 restriction on the lower limit of the cumulative time t at 1000 to 1100°C. Considering normal industrial production, the lower limit of the cumulative time t at 1000 to 1100°C is, for example, 8 hours.

[0102] [(Step 3) Annealing Step] In the annealing step, the intermediate blank produced in the hot forging step is annealed. By performing the annealing step, the intermediate blank is easier to grind in the next rough processing step. 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.

[0103] [(Step 4) Roughing Step] In the roughing step, the intermediate preform after the annealing step is subjected to roughing to further roughly process the intermediate preform into a shape close to the final roll shape. The roughing is, for example, grinding. The roughing may be performed under well-known conditions.

[0104] [(Step 5) Quenching Step] In the quenching step, the surface layer of the intermediate preform after the rough machining step is subjected to a well-known quenching method. 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.

[0105] 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.

[0106] [(Step 6) Tempering Step] In the tempering step, the intermediate preform after the quenching step is tempered. Tempering reduces the amount of retained austenite remaining at the time of quenching. Furthermore, tempering adjusts the hardness of the surface layer of the forged steel roll. The tempering temperature is, for example, 100 to 200°C.

[0107] [(Step 7) Finishing Step] In the finishing step, the intermediate preform after the tempering step is subjected to finishing. The finishing step is, for example, grinding using a grinding machine. By the finishing step, the intermediate preform is processed into the shape of the final product.

[0108] 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.

[0109] 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.

[0110] Forged steel rolls having the chemical compositions shown in Tables 1A and 1B were manufactured by the following method.

[0111]

[0112]

[0113] Specifically, electrode ingots were cast from molten steel by bottom pouring ingot casting. Electroslag remelting (ESR) was carried out using the produced electrode ingots as electrodes. Condition 1 (solidification rate SR (mm / min)) in the ESR process was as shown in Table 2. A hot forging process was carried out on the forging ingots obtained by the ESR process. Condition 2 (cumulative time t (hours) at 1000 to 1100°C) in the hot forging process was as shown in Table 2. By the hot forging process, intermediate blanks in the shape of rolls with a roll barrel diameter of 700 mm, a barrel length of 2100 mm, and a total length of 4100 mm were produced for each test number.

[0114]

[0115] The intermediate preforms after the hot forging process were subjected to an annealing process. In the annealing process, the preforms were held at 800°C for 10 hours. Thereafter, the preforms were further held at 600°C for 15 hours. The intermediate preforms after the annealing process were subjected to a rough machining process. Specifically, for each test number, grinding was performed on the intermediate preforms to manufacture intermediate preforms in the shape of rolls having a roll barrel diameter of 650 mm, a barrel length of 2000 mm, and a total length of 4000 mm.

[0116] 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.

[0117] 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 having 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 roll of each test number.

[0118] [Evaluation Tests] The following evaluation tests were carried out on the manufactured forged steel rolls of each test number: (Test 1) Measurement test of the MC type carbide number ratio NR (Test 2) Wear resistance evaluation test Each test will be described below.

[0119] [(Test 1) Measurement test of MC type carbide number ratio NR] Based on the method described in the above [Method for measuring MC type carbide number ratio NR], the MC type carbide number ratio NR of the forged steel roll of each test number was determined. The obtained MC type carbide number ratio NR is shown in the "MC type carbide number ratio NR (%)" column in Table 2.

[0120] [(Test 2) Wear Resistance Evaluation Test] The wear resistance of the forged steel rolls of each test number was evaluated using a twin-cylinder rolling wear tester. FIG. 1 is a schematic diagram of a twin-cylinder rolling wear tester 10. A cylindrical roll test piece 12 was collected from the surface layer of each forged steel roll (a region extending from the surface of the barrel to 80 mm in the depth direction). 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 position on the central axis of the roll test piece 12, which was the center position in the width direction, corresponded to a position 10 mm deep from the surface of the barrel of the forged steel roll. The outer peripheral surface of the roll test piece 12 was polished.

[0121] A two-cylinder rolling wear test was carried out using the collected roll test piece 12. In the two-cylinder rolling wear test, a rolled material test piece 11 rotating in the opposite direction was pressed against the rotating roll test piece 12 with a load F described below. A front view of the rolled material test piece 11 is shown in FIG. 2. The numerical values ​​in FIG. 2 indicate dimensions (unit: mm). "R7.5" in FIG. 2 indicates that the radius of curvature of the outer peripheral surface was 7.5 mm. As shown in FIG. 2, the diameter of the rolled material test piece 11 was 160 mm and the width was 15 mm. The rolled material test piece 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 FIG. 2 and polishing the outer peripheral surface.

[0122] 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 slip ratio between the roll test piece 12 and the rolled material test piece 11 was 5%. The slip ratio (%) is defined by the following formula: Slip ratio (%) = (Peripheral 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). In addition, the roll test piece was water-cooled for cooling. The test was terminated after 20,000 cycles.

[0123] 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 depression 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 to each other across 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. When the wear cross-sectional area of ​​the roll test piece 12 was 2,000 μm ２ When the wear cross-sectional area of ​​the roll test piece 12 was 2000 μm or less, the test piece was rated as "E (Excellent)" and it was determined that excellent wear resistance was obtained (indicated by "E" in the "Wear Resistance" column in Table 2). ２ If the value was greater than 100%, the evaluation was rated "B (Bad)" and it was determined that excellent abrasion resistance was not obtained (indicated by "B" in the "Abrasion resistance" column in Table 2).

[0124] [Test Results] Referring to Tables 1A, 1B and 2, the forged steel rolls of test numbers 1 to 20 satisfied characteristics 1 to 3. Therefore, excellent wear resistance was obtained.

[0125] 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.

[0126] In Test Nos. 23 and 24, the cumulative time t at 1000 to 1100°C in the hot forging step was too long. Therefore, the forged steel rolls did not satisfy Feature 3. As a result, excellent wear resistance was not obtained.

[0127] In test numbers 25 and 26, Fn1 was too low, and therefore the forged steel rolls did not satisfy characteristic 2. As a result, excellent wear resistance was not obtained.

[0128] 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. Chemical composition, in mass%, is: 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: 0.001 to 0.050%, N: 0.001 to 0.020%, 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 to 0.050%, B: 0 to 0.0100%, W: 0 to 0.50%, 1. A forged steel roll comprising: Co: 0 to 0.50%, Sn: 0 to 0.10%, Sb: 0 to 0.05%, As: 0 to 0.05%, Zr: 0 to 0.05%, Bi: 0 to 0.10%, Se: 0 to 0.10%, Te: 0 to 0.05%, Pb: 0 to 0.09%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, with the balance being Fe and impurities; 2. A forged steel roll comprising: a 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. A forged steel roll according to claim 1, wherein the chemical composition is, in mass %, Ti: 0.001 to 0.050%, Nb: 0.001 to 0.050%, B: 0.0001 to 0.0100%, W: 0.01 to 0.50%, Co: 0.01 to 0.50%, Sn: 0.01 to 0.10%, Sb: 0.01 to 0.05%, As: 0.01 to 0.05%, Zr: 0.01 to 0.05%, Bi: 0.01 to 0.10%, Se: 0.01 to 0.10%, Te: 0.01 to 0.05%, Pb: 0.01 to 0.09%, Ca: 0.0001 to 0.0050%, and A forged steel roll comprising one or more elements selected from the group consisting of: Mg: 0.0001 to 0.0050%.