Forged steel roll

Abstract

To provide a forged steel roll having superior wear resistance.SOLUTION: A forged steel roll contains, in a chemical composition, 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: 1.00-2.50%, V: 1.00-2.00%, Cu: 0.40% or less, and Ni: 0.30-0.60%, with the remainder comprising Fe and impurities. The composition satisfies the following expressions: C-0.25×V≤0.70 (1) and (3×V+Mo) / Cr≥1.00 (2). In the expressions (1) and (2), the symbols of the respective elements are replaced with the mass percentages of the corresponding elements contained in the chemical composition.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] 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 remainder consisting essentially of Fe and unavoidable impurities. Furthermore, in this forged steel roll, the Vickers hardness of a surface layer extending from the surface to a depth of 4 to 8 mm toward the center is 900 HV or more, and the Vickers hardness of a portion deeper than this surface layer is less than 900 HV. This provides excellent wear resistance for this forged steel roll. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-1307 Summary of the Invention [Problem to be solved by the invention]

[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. [Means for solving the problem]

[0009] 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: 1.00-2.50% 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) and formula (2) are satisfied. C-0.25×V≦0.70 (1) (3×V+Mo) / Cr≧1.00 (2) Here, the content of the corresponding element in the chemical composition in terms of mass % is substituted for each element symbol in formula (1) and formula (2).

[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: 1.00-2.50% 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, Formula (1) and formula (2) are satisfied. [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 C-0.25×V≦0.70 (1) (3×V+Mo) / Cr≧1.00 (2) Here, the content of the corresponding element in the chemical composition in terms of mass % is substituted for each element symbol in formula (1) and formula (2). [Effects of the Invention]

[0011] The forged steel roll of the present disclosure provides excellent wear resistance. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present inventors first investigated forged steel rolls with excellent wear resistance from the viewpoint of chemical composition, and as a result, they found that the forged steel rolls contain, 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: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00 to 6.00%, Mo: 1.00 to 2.50%, V: 1.00 to 2.00%, Cu: 0.40% or less, Ni: 0.30 to 0.60%, Ti: 0 to 0.050%, and Nb: 0 to 0. It was thought that excellent wear resistance could be obtained if the alloy had a chemical composition containing the following elements: 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 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 formation 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 a sufficient amount of fully grown carbides was present, 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 the carbide-depleted regions. 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-poor regions form within crystal grains is believed to be as follows. Carbides large enough to contribute to improved wear resistance are primarily formed between solidification in the steelmaking process and the hot forging process. A high C content in the chemical composition of a forged steel roll lowers the solidification point of the matrix, making it easier for a liquid phase to remain during solidification. As a result, solidification segregation is promoted, and carbides crystallize from the remaining liquid phase. Regions where liquid phase remained at the end of solidification become grain boundaries after solidification is complete. In this way, crystallized carbides form at the grain boundaries. Crystallized carbides are generally coarser than precipitated carbides formed within the grains after solidification. Therefore, around the crystallized carbides, C is consumed in the formation of the crystallized carbides, resulting in extremely low amounts of solute C. Therefore, carbide precipitation is suppressed within the grains where crystallized carbides exist at the grain boundaries. In this way, carbide-poor regions form within the grains.

[0016] In order to improve the wear resistance of forged steel rolls, it is desirable to suppress the formation of carbide-poor regions inside crystal grains. Here, the inventors focused on the content of V, which easily combines with C to form carbides. If the C content is too high relative to the V content, C tends to concentrate in the liquid phase. As a result, as described above, carbides tend to crystallize from the liquid phase at the end of solidification. On the other hand, if the C content is sufficiently low relative to the V content, a sufficient amount of V is incorporated into the solid phase during solidification and becomes solid-solution state. In this case, the solute V combines with the solute C inside the crystal grains after solidification, allowing precipitated carbides to form inside the crystal grains. As a result, the concentration of C in the remaining liquid phase is suppressed, and the formation of crystallized carbides can also be suppressed. In this way, the formation of carbide-poor regions inside crystal grains can be suppressed.

[0017] The inventors also investigated the type of carbide. The carbides contained in the forged steel roll having the above-mentioned chemical composition are mainly composed of MC-type carbides containing V and Mo as the main components and M7C3-type carbides containing Cr as the main component. MC-type carbides are harder than M7C3-type carbides. Furthermore, the driving force for carbide formation of MC-type carbides is stronger than that of M7C3-type carbides. Therefore, the amount of alloying elements required to obtain the same amount of carbide can be reduced. As a result, solidification segregation, which is a factor in the formation of carbide-deficient regions, is suppressed. In other words, in order to improve the wear resistance of the forged steel roll, it is effective to adjust the V content, Mo content, and Cr content so that MC-type carbides are preferentially formed over M7C3-type carbides.

[0018] Based on the above findings, the inventors further investigated the relationship between the C content and the V content for sufficiently suppressing the formation of carbide depleted regions, and the relationship between the V content, the Mo content, and the Cr content for preferentially obtaining MC type carbides. As a result, they found that excellent wear resistance can be obtained in a forged steel roll having the above chemical composition if formulas (1) and (2) are satisfied. C-0.25×V≦0.70 (1) (3×V+Mo) / Cr≧1.00 (2) Here, the content of the corresponding element in the chemical composition in terms of mass % is substituted for each element symbol in formula (1) and formula (2).

[0019] The forged steel roll of this embodiment has been completed based on the above technical concept and has the following configuration. Note that the above mechanism is a presumption. Therefore, the forged steel roll of this embodiment may have excellent wear resistance due to a mechanism different from the above. However, it has been proven in the examples described later that a forged steel roll having the following configuration can have excellent wear resistance.

[0020] 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: 1.00-2.50% 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) and formula (2) are satisfied. C-0.25×V≦0.70 (1) (3×V+Mo) / Cr≧1.00 (2) Here, the content of the corresponding element in the chemical composition in terms of mass % is substituted for each element symbol in formula (1) and formula (2).

[0021] 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: 1.00-2.50% 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, Formula (1) and formula (2) are satisfied. [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 C-0.25×V≦0.70 (1) (3×V+Mo) / Cr≧1.00 (2) Here, the content of the corresponding element in the chemical composition in terms of mass % is substituted for each element symbol in formula (1) and formula (2).

[0022] The third configuration of forged steel rolls is A forged steel roll of a second configuration, The chemical composition contains the first group.

[0023] 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.

[0024] The forged steel roll of this embodiment will be described in detail below. Unless otherwise specified, "%" for elements means mass %.

[0025] [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.

[0026] 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.

[0027] [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, 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: 1.00-2.50%, V: 1.00-2.00%, Cu: 0.40% or less, Ni: 0.30-0.60%, Ti: 0-0.05%. 0%, 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) The chemical composition further satisfies formula (1). C-0.25×V≦0.70(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 chemical composition further satisfies formula (2). (3×V+Mo) / Cr≧1.00 (2) Here, each element symbol in formula (2) is substituted with the content in mass % of the corresponding element in the chemical composition. Features 1 to 3 will be explained below.

[0028] [(Feature 1) Chemical composition] The forged steel roll of this embodiment contains the following elements.

[0029] 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 0.92%. The upper limit of the C content is preferably 1.03%, more preferably 1.00%, and even more preferably 0.98%.

[0030] Si: 0.60 to 1.20% Silicon (Si) deoxidizes steel during the molten steel stage. Si also increases the tempering softening resistance of steel materials and increases the hardness of the surface layer of forged steel rolls. If the Si content is less than 0.60%, the above effects cannot be fully achieved. 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%.

[0031] Mn: 0.30 to 0.60% Manganese (Mn) improves the hardenability of forged steel rolls. 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%.

[0032] 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%.

[0033] 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 in 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%.

[0034] 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.

[0035] 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 0.002%, and still more preferably 0.003%. The upper limit of the N content is preferably 0.015%, more preferably 0.010%, and even more preferably 0.008%.

[0036] O: 0.0050% or less Oxygen (O) is an impurity. If the O content exceeds 0.0050%, O forms oxides, which 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, 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%.

[0037] Cr: 4.00~6.00% Chromium (Cr) forms carbides to enhance the wear resistance of forged steel rolls. Cr also enhances the tempering softening resistance of steel materials and increases 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 carbides are formed, which reduces the toughness of the forged steel roll. 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%.

[0038] Mo: 1.00-2.50% Molybdenum (Mo) forms carbides to increase the wear resistance of forged steel rolls. Mo also increases the tempering softening resistance of steel materials and increases the hardness of the surface layer of forged steel rolls. If the Mo content is less than 1.00%, the above effects cannot be sufficiently obtained. On the other hand, if the Mo content exceeds 2.50%, coarse carbides are formed, which reduces the toughness of the forged steel roll. Therefore, the Mo content is 1.00 to 2.50%. The lower limit of the Mo content is preferably 1.05%, more preferably 1.10%, and even more preferably 1.15%. The upper limit of the Mo content is preferably 2.45%, more preferably 2.40%, and even more preferably 2.35%.

[0039] V: 1.00~2.00% Vanadium (V) forms carbides to increase the wear resistance of forged steel rolls. V also increases the tempering softening resistance of steel materials and increases the hardness of the surface layer of forged steel rolls. If the V content is less than 1.00%, the above effects cannot be sufficiently obtained. On the other hand, if the V content exceeds 2.00%, coarse carbides are formed, which reduces the toughness of the forged steel roll. 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%.

[0040] Cu: 0.40% or less Copper (Cu) is an impurity. If the Cu content exceeds 0.40%, the hot workability of the steel material decreases 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%.

[0041] Ni: 0.30 to 0.60% Nickel (Ni) improves the hardenability of the forged steel roll. If the Ni content is less than 0.30%, this effect cannot be sufficiently obtained. On the other hand, if the Ni content exceeds 0.60%, retained austenite is formed in excess, which reduces the hardness of the surface layer 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%.

[0042] 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.

[0043] [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.

[0044] [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.

[0045] 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%.

[0046] 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%.

[0047] 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, the B content 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%.

[0048] 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 when W is contained, the W content 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%.

[0049] 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 when Co is contained, the Co content 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%.

[0050] [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.

[0051] 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%.

[0052] 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%.

[0053] 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%.

[0054] 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%.

[0055] 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%.

[0056] 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%.

[0057] 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%.

[0058] 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%.

[0059] 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%.

[0060] 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%.

[0061] [(Feature 2) Regarding Formula (1)] Furthermore, the chemical composition of the forged steel roll of this embodiment satisfies formula (1). C-0.25×V≦0.70 (1) Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element in the chemical composition.

[0062] Fn1 is defined as follows: Fn1=C-0.25×V

[0063] Fn1 corresponds to the left side of equation (1). As mentioned above, the presence of carbide-poor regions inside crystal grains promotes wear of forged steel rolls. In order to suppress the formation of carbide-poor regions, it is effective to suppress the crystallization of carbides in solidification segregation areas during the steelmaking process. Therefore, it is desirable to sufficiently reduce the content of C, which is prone to solidification segregation and promotes the crystallization of carbides, in accordance with the V content.

[0064] When Fn1 is 0.70 or less, the C content relative to the V content is sufficiently low. Therefore, the crystallization of carbides in solidification segregation areas can be suppressed in the final stage of solidification in the steelmaking process. As a result, the formation of carbide-poor regions within crystal grains can be suppressed. Therefore, on the premise that the forged steel roll satisfies Features 1 and 3, excellent wear resistance can be obtained.

[0065] The lower limit of Fn1 is not particularly limited. When the chemical composition of the forged steel roll satisfies Feature 1, the lower limit of Fn1 is, for example, 0.35. The upper limit of Fn1 is preferably 0.68, more preferably 0.66, and even more preferably 0.64. Note that Fn1 is a value obtained by rounding the obtained numerical value to two decimal places.

[0066] [(Feature 3) Regarding formula (2)] Furthermore, the chemical composition of the forged steel roll of this embodiment satisfies formula (2). (3×V+Mo) / Cr≧1.00 (2) Here, each element symbol in formula (2) is substituted with the content in mass % of the corresponding element in the chemical composition.

[0067] Let Fn2 be defined as follows: Fn2=(3×V+Mo) / Cr

[0068] Fn2 corresponds to the left side of formula (2). Fn2 is an index for adjusting the ratio of MC carbides and the ratio of M7C3 carbides in all carbides contained in the forged steel roll. As described above, MC carbides are harder than M7C3 carbides. Furthermore, MC carbides are less likely to form as crystallized carbides than M7C3 carbides. Therefore, in order to improve the wear resistance of the 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, MC carbides can be preferentially formed over M7C3 carbides.

[0069] When Fn2 is 1.00 or more, the formation of MC carbides is prioritized over M7C3 carbides. Therefore, the ratio of MC carbides to all carbides contained in the forged steel roll is sufficiently increased. As a result, excellent wear resistance can be obtained, provided that the forged steel roll satisfies Features 1 and 2.

[0070] The lower limit of Fn2 is preferably 1.02, more preferably 1.05, and even more preferably 1.10. The upper limit of Fn2 is not particularly limited. When the chemical composition of the forged steel roll satisfies Feature 1, the upper limit of Fn2 is, for example, 2.13. Note that Fn2 is a value obtained by rounding the obtained numerical value to two decimal places.

[0071] [Effects of the forged steel roll of this embodiment] The forged steel roll of this embodiment satisfies Features 1 to 3. Therefore, the forged steel roll of this embodiment has excellent wear resistance.

[0072] [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.

[0073] [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 is observed as a structure with a lower brightness (black color) 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.

[0074] The carbide area ratio is determined by the following FE-SEM (Field Emission Scanning Electron Microscope) 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 mirror-polished 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 primarily 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.

[0075] 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.

[0076] 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)

[0077] [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).

[0078] [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 is manufactured by a well-known manufacturing method. The example of the method for manufacturing the forged steel roll of this embodiment includes, for example, a steelmaking process, a hot forging process, an annealing process, a rough processing process, a quenching process, a tempering process, and a finish processing process.

[0079] In the steelmaking process, molten steel satisfying Features 1 to 3 is used to produce an ingot by a known casting method. Examples of known casting methods include bottom pouring ingot casting. Electroslag remelting (ESR) may also be carried out using the cast ingot as an electrode.

[0080] In the hot forging process, first, the ingot is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1000 to 1200°C. The heated ingot is then subjected to hot forging. If the temperature of the ingot drops during hot forging, the ingot may be heated again in the heating furnace. Then, hot forging may be resumed on the reheated ingot. In this manner, a roughly shaped roll blank (hereinafter referred to as an intermediate blank) is produced.

[0081] 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 1000°C. The holding time is, for example, 10 to 50 hours.

[0082] 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, cutting. The rough processing may be performed under well-known conditions.

[0083] 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.

[0084] In the tempering process, the intermediate preform after the quenching process is tempered. The tempering temperature is, for example, 100 to 600°C. In order to reduce the amount of retained austenite and further increase the total area ratio of martensite and bainite, the tempering treatment may be performed multiple times, or the intermediate preform may be subjected to sub-zero treatment after the quenching process and before the tempering process. The cooling temperature in the sub-zero treatment may be in a known range, for example, -30 to -196°C.

[0085] 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. The forged steel roll of this embodiment can be manufactured by the above steps.

[0086] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way. [Example]

[0087] Forged steel rolls having the chemical compositions shown in Tables 1A and 1B were manufactured by the following manufacturing method.

[0088] [Table 1A]

[0089] [Table 1B]

[0090] Specifically, ingots were cast from molten steel using the bottom pouring ingot casting method. The produced ingots were remelted using the electroslag remelting (ESR) method to produce the ingots that would become the raw materials. The produced ingots were then subjected to a hot forging process. The heating temperature in the hot forging process was 1000 to 1200°C. Through the hot forging process, intermediate roll-shaped blanks with a roll barrel diameter of φ700 mm, a barrel length of 2100 mm, and a total length of 4100 mm were produced for each test number.

[0091] The intermediate blanks after the hot forging process were subjected to an annealing process. In the annealing process, they were held at 900°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, cutting was performed on the intermediate blanks to manufacture 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.

[0092] The intermediate preform after the rough machining process was subjected to a quenching process. In the quenching process, the intermediate preform was heated to 900 to 1100°C by induction heating and held at that temperature for 20 minutes. Thereafter, the heated intermediate preform was water-cooled.

[0093] The intermediate preform after the quenching process was subjected to a tempering process at 100 to 600°C, and then a finishing process was carried out. In the finishing process, the intermediate preform was ground to form 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.

[0094] [About the evaluation test] Using the forged steel rolls of each test number, a wear resistance evaluation test was carried out in the following manner. A cylindrical test piece with a diameter of 20 mm and a height of 20 mm was taken from the forged steel roll of each test number. One side of the circular bottom of the test piece was used as the measurement surface. The measurement surface was perpendicular to the radial direction of the forged steel roll. The center position of the measurement surface corresponded to a depth of 1 mm from the surface of the barrel of the forged steel roll. The weight (mg) of the test piece of each test number was measured before the wear test.

[0095] An abrasion test was carried out using the collected test specimen. Specifically, the measurement surface of the test specimen was pressed perpendicularly against rotating sandpaper at a pressure of 20 psi. The measurement surface was pressed against the sandpaper for 4 minutes. The roughness (grit) of the sandpaper was #120, and the rotation speed was 200 rpm. When the measurement surface was pressed against the sandpaper, the center of the measurement surface was located 18 mm radially from the center of rotation of the sandpaper. The abrasion distance in the abrasion test was 90.8 m.

[0096] The weight (mg) of the test piece of each test number after the abrasion test was measured. Then, the amount of wear (mg) of the test piece of each test number in the abrasion test was calculated using the following formula: Abrasion amount (mg) = weight of test piece before abrasion test (mg) - weight of test piece after abrasion test (mg) When the amount of wear was 90 mg or less, the evaluation was "E (Excellent)" and it was determined that excellent wear resistance was obtained (indicated by "E" in the "Wear Resistance" column in Table 2). On the other hand, when the amount of wear was more than 90 mg, the evaluation was "B (Bad)" and it was determined that excellent wear resistance was not obtained (indicated by "B" in the "Wear Resistance" column in Table 2).

[0097] [Table 2]

[0098] [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.

[0099] On the other hand, in test numbers 21 and 22, Fn1 was too high, and as a result, excellent wear resistance was not obtained.

[0100] In test numbers 23 and 24, Fn2 was too low, and as a result, excellent wear resistance was not obtained.

[0101] 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: 1.00-2.50%, V: 1.00-2.00%, Cu: 0.40% or less, and Ni: 0.30 to 0.60%; the balance being Fe and impurities; Satisfying formula (1) and formula (2), Forged steel roll. C-0.25×V≦0.70 (1) (3×V+Mo) / Cr≧1.00 (2) Here, the content in mass % of the corresponding element in the chemical composition is substituted for each element symbol in formula (1) and formula (2).

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: 1.00-2.50%, 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; Satisfying formula (1) and formula (2), 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 C-0.25×V≦0.70 (1) (3×V+Mo) / Cr≧1.00 (2) Here, the content in mass % of the corresponding element in the chemical composition is substituted for each element symbol in formula (1) and formula (2).

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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