Forged steel rolls

Abstract

To provide a forged steel roll that offers excellent wear resistance and excellent grinding properties. [Solution] The forged steel roll according to this disclosure has a chemical composition in mass percent of: 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% It contains V: 1.00~2.00%, Cu: 0.40% or less, and Ni: 0.30~0.60%, with the remainder being Fe and impurities. In the surface layer of the body, the number proportion of MC-type carbides in the total carbides with an equivalent circular diameter of 0.5~5.0 μm is 30% or more, and the number density of MC-type carbides with a radial length of the body exceeding 5.0 μm is 100 pieces / mm². 2 The following applies:

Description

[Technical Field]

[0001] This disclosure relates to forging rolls, and more particularly to forging rolls suitable for cold rolling applications. [Background technology]

[0002] Forged steel rolls are used as rolling rolls, typified by cold rolling rolls. Forged steel rolls apply a load to the material to be rolled, such as steel, thereby rolling the material into the desired shape. On the other hand, the surface of the forged steel roll wears down due to contact with the material to be rolled during rolling. Therefore, if forged steel rolls are used for a long period of time, the surface roughness of the forged steel rolls gradually decreases. If the surface roughness of the forged steel rolls decreases, slippage will occur between the forged steel rolls and the material to be rolled. In this case, poor gripping of the material to be rolled may occur, or seizure of the material to be rolled or the forged steel rolls may occur.

[0003] To suppress slippage between the forging roll and the rolled material, it is necessary to periodically grind the surface of the forging roll to ensure that the surface roughness does not fall below a certain value. However, if the number of grinding cycles per unit of operating time of the forging roll is high, the lifespan of the forging roll will be shortened. To extend the lifespan of the forging roll, it is desirable to suppress wear on the forging roll and reduce the number of grinding cycles per unit of operating time as much as possible. Therefore, forging rolls require excellent wear resistance.

[0004] A technique for improving the wear resistance of forged steel rolls is proposed in Japanese Patent Publication No. 2003-1307 (Patent Document 1).

[0005] The forged steel roll disclosed in Patent Document 1 contains C: 0.8-1.2 mass%, Si: 0.3-0.5 mass%, Mn: 0.4-0.6 mass%, Cr: 2.5-4.0 mass%, Mo: 0.3-0.5 mass%, and V: 0.3 mass% or less, with the remainder being substantially Fe and unavoidable impurities. In this forged steel roll, the Vickers hardness in the surface layer from the surface to a depth of 4-8 mm toward the center is 900 HV or higher, while the Vickers hardness in the interior beyond the surface layer is less than 900 HV. As a result, this forged steel roll provides excellent wear resistance. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2003-1307 [Overview of the project] [Problems that the invention aims to solve]

[0007] Generally, as the wear resistance of a forged steel roll increases, its grindability decreases. However, as mentioned above, forged steel rolls need to be ground periodically. Therefore, forged steel rolls are required to have both excellent wear resistance and excellent grindability. Patent Document 1 examines means to improve the wear resistance of forged steel rolls, but does not examine means to improve their grindability.

[0008] The purpose of this disclosure is to provide a forged steel roll having excellent wear resistance and excellent grindability. [Means for solving the problem]

[0009] The forged steel rolls of this disclosure are A cylindrical body, It comprises a pair of shafts, The chemical composition of the forged steel roll is, in 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%, and contains the balance consists of Fe and impurities, in the surface layer of the barrel part, the number ratio of MC-type carbides in all carbides with an equivalent circle diameter of 0.5 - 5.0 μm is 30% or more, in the surface layer of the barrel part, the number density of MC-type carbides with a radial length of more than 5.0 μm in the barrel part is 100 pieces / mm or less.

[0010] The forged steel roll of the present disclosure has a cylindrical barrel part and a pair of shaft parts, and the chemical composition of the forged steel roll is, in 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%, and contains further contains one or more selected from the group consisting of Group 1 and Group 2, and the balance consists of Fe and impurities, <In the surface layer of the body, the number proportion of MC-type carbides among the total carbides with an equivalent circular diameter of 0.5 to 5.0 μm is 30% or more. On the surface of the body, the number density of MC-type carbides with a radial length of 5.0 μm or more is 100 particles / mm². 2 The following applies: [Group 1] Ti: 0.050% or less, Nb: 0.050% or less, B: 0.0100% or less, W: 0.50% or less, and, One or more species selected from the group consisting of Co: 0.50% or less. [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, One or more selected from the group consisting of Mg: 0.0050% or less. [Effects of the Invention]

[0011] The forged steel rolls of this disclosure provide excellent wear resistance and excellent grindability. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 is a schematic diagram of a two-cylinder rolling abrasion testing machine used for abrasion resistance evaluation tests. [Figure 2] Figure 2 is a front view of the rolled material test specimen shown in Figure 1. [Modes for carrying out the invention]

[0013] The inventors of the present invention conducted the following studies on forged steel rolls that provide excellent wear resistance and excellent grinding performance.

[0014] The inventors initially focused on carbides contained in forged steel rolls and investigated means to improve wear resistance.

[0015] The microstructure of typical forged steel rolls contains carbides. These carbides are harder than the base material of the forged steel roll. Therefore, promoting the formation and growth of carbides and increasing the surface area ratio of carbides on the surface of the forged steel roll will improve the wear resistance of the roll.

[0016] Among the carbides contained in forged steel rolls, MC-type carbides, which mainly consist of V and Mo, are even harder than other carbides contained in forged steel rolls (M7C3-type carbides, cementite, etc.). Therefore, in order to improve the wear resistance of forged steel rolls, it is preferable to increase the proportion of MC-type carbides in the total carbides. Accordingly, the inventors investigated the chemical composition of forged steel rolls that would actively yield MC-type carbides. As a result, the inventors have found that forged steel rolls have the following composition in 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, Ni: 0.30-0.60%, Ti: 0-0.050%, Nb: 0-0.050%, B: 0-0. We hypothesized that a chemical composition containing 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, would increase the proportion of MC-type carbides in the total carbides, potentially resulting in excellent wear resistance.

[0017] However, even forged steel rolls that met the above-mentioned chemical composition sometimes failed to exhibit excellent wear resistance. Therefore, the inventors investigated the reasons why excellent wear resistance could not be obtained. As a result, the inventors obtained the following findings.

[0018] To obtain excellent wear resistance, it is effective to maximize the area ratio of MC-type carbides on the surface of forged steel rolls. To achieve this, it seems preferable to promote the formation and growth of MC-type carbides and increase the number of coarse MC-type carbides.

[0019] However, observation of the microstructure of the forged steel rolls that did not exhibit excellent wear resistance revealed the presence of numerous coarse MC-type carbides at the grain boundaries. On the other hand, in the microstructure of such forged steel rolls, regions deficient in MC-type carbides existed within the grains near the grain boundaries where the coarse MC-type carbides were present. This is thought to be because there is a deficiency of V and Mo, which are responsible for generating MC-type carbides, around the coarse MC-type carbides. These MC-type carbide-deficient regions are significantly more susceptible to wear compared to regions where MC-type carbides are sufficiently dispersed. Furthermore, as wear progresses in the MC-type carbide-deficient regions, the detachment of coarse MC-type carbides surrounding these regions is also promoted. In this way, the wear resistance of the forged steel rolls is thought to be reduced.

[0020] In contrast, the microstructure of the forged steel rolls exhibiting superior wear resistance showed fewer coarse MC-type carbides at the grain boundaries. Instead, the proportion of MC-type carbides among the finely dispersed carbides within the grains was high. In other words, the formation of MC-type carbide-deficient regions was sufficiently suppressed.

[0021] Therefore, to improve the wear resistance of forged steel rolls, it was considered effective to suppress the formation of MC-type carbide-deficient regions within the crystal grains, rather than increasing the number of coarse MC-type carbides. As mentioned above, suppressing the formation of MC-type carbide-deficient regions increases the proportion of MC-type carbides among the total carbides finely dispersed within the crystal grains. In other words, to obtain excellent wear resistance, it is desirable to increase the proportion of MC-type carbides among carbides of appropriate size as much as possible.

[0022] Based on the above findings, the inventors conducted further investigations. As a result, they found that in the surface layer of the body of a forged steel roll having the above chemical composition, if the number ratio of MC-type carbides among the total carbides with an equivalent circular diameter of 0.5 to 5.0 μm is 30% or more, excellent wear resistance can be obtained.

[0023] Next, the inventors investigated means to improve the grindability of the above-mentioned forged steel roll with excellent wear resistance.

[0024] As mentioned above, the MC-type carbides contained in forged steel rolls are even harder than other carbides. Alumina abrasives are usually used for grinding forged steel rolls. Alumina abrasives are harder than the base material of the forged steel roll, but softer than the MC-type carbides. Therefore, when grinding the surface of the forged steel roll, if the alumina abrasives collide with the ultra-hard MC-type carbides, the alumina abrasives will break. If the alumina abrasives are broken, the grinding of the surface of the forged steel roll will not progress. In this way, the MC-type carbides present on the surface of the forged steel roll may reduce the grindability of the forged steel roll.

[0025] Therefore, the inventors investigated means to avoid collisions between MC-type carbides in forged steel rolls and alumina abrasive grains. During the grinding of forged steel rolls, the base material of the forged steel roll is scraped off and removed as grinding powder. At this time, it was thought that if the MC-type carbides in the forged steel rolls were encapsulated in the grinding powder and removed together with the grinding powder, collisions between the MC-type carbides and abrasive grains would be suppressed.

[0026] Grinding powder is generally in the form of strips, and the thickness direction of the grinding powder corresponds to the radial direction of the body of the forging roll (hereinafter also referred to as the "body radial direction"). In other words, if the length of the MC-type carbide in the body radial direction is shorter than the thickness of the grinding powder, the MC-type carbide is more easily embedded in the grinding powder and removed during grinding of the forging roll. Specifically, it has been found that if the length of the MC-type carbide in the body radial direction in the forging roll is 5.0 μm or less, the MC-type carbide is more easily embedded in the grinding powder. On the other hand, if the length of the MC-type carbide in the body radial direction exceeds 5.0 μm, it becomes less likely to be embedded in the grinding powder, and the MC-type carbide and alumina abrasive grains are more likely to collide. As a result, the alumina abrasive grains are destroyed, and the grinding performance decreases. In other words, in order to improve the grinding performance of forging rolls, it is effective to reduce the number density of MC-type carbides with a length in the body radial direction exceeding 5.0 μm as much as possible.

[0027] Based on the above findings, the inventors investigated and examined the relationship between the number density of MC-type carbides with a length exceeding 5.0 μm in the radial direction of the body and grindability. As a result, the number density of MC-type carbides with a length exceeding 5.0 μm in the radial direction of the body exceeding 100 particles / mm² on the surface layer of the body of the forged steel roll was found to be 100 particles / mm². 2 We found that, under the following conditions, even better grindability can be obtained in the aforementioned forged steel rolls with excellent wear resistance.

[0028] The forged steel roll of this embodiment was completed based on the above technical concept and has the following configuration. Note that the above mechanism is a hypothesis. Therefore, it is possible that the forged steel roll of this embodiment may achieve excellent wear resistance and excellent grindability through a mechanism different from the one described above. However, it has been proven in the later examples that a forged steel roll having the following configuration can achieve excellent wear resistance and excellent grindability.

[0029] The forged steel roll of the first configuration is A cylindrical body, It comprises a pair of shafts, The chemical composition of the forged steel roll is, in 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, It contains Ni: 0.30~0.60%, The remainder consists of Fe and impurities. In the surface layer of the body, the number proportion of MC-type carbides among the total carbides with an equivalent circular diameter of 0.5 to 5.0 μm is 30% or more. On the surface of the body, the number density of MC-type carbides with a radial length of 5.0 μm or more is 100 particles / mm². 2 The following applies:

[0030] The second configuration of the forged steel roll is A cylindrical body, It comprises a pair of shafts, The chemical composition of the forged steel roll is, in 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, It contains Ni: 0.30~0.60%, Furthermore, it contains one or more selected from the group consisting of Group 1 and Group 2, The remainder consists of Fe and impurities. In the surface layer of the body, the number proportion of MC-type carbides among the total carbides with an equivalent circular diameter of 0.5 to 5.0 μm is 30% or more. On the surface of the body, the number density of MC-type carbides with a radial length of 5.0 μm or more is 100 particles / mm². 2 The following applies: [Group 1] Ti: 0.050% or less, Nb: 0.050% or less, B: 0.0100% or less, W: 0.50% or less, and, One or more species selected from the group consisting of Co: 0.50% or less. [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, One or more selected from the group consisting of Mg: 0.0050% or less.

[0031] The third configuration of the forged steel roll is A forged steel roll of the second configuration, The chemical composition contains the first group.

[0032] The forged steel roll of the fourth configuration is A forged steel roll having a second or third configuration, The chemical composition contains the second group of substances.

[0033] The forged steel rolls of this embodiment will be described in detail below. Unless otherwise specified, the "%" in relation to elements refers to mass percentage.

[0034] [Configuration of the forged steel roll in this embodiment] The forged steel roll of this embodiment comprises a body and a pair of shafts. The body is cylindrical and includes a pair of end faces and a circumferential surface (hereinafter also simply referred to as the "surface") positioned between the pair of end faces. The circumferential surface comes into contact with the material to be rolled during rolling. The shafts are cylindrical and are provided on the pair of end faces of the body such that their central axes coincide with the central axes of the body. The diameter of the body is greater than the diameter of the shafts.

[0035] In this embodiment of the forged steel roll, the area extending 40 mm in depth from the surface of the body 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 the area that is exposed to the outer surface after grinding and can newly come into contact with the rolled material as the surface of the body.

[0036] [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 as follows: 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% It contains 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) In the surface layer of the body, the proportion of MC-type carbides among the total carbides with an equivalent circular diameter of 0.5 to 5.0 μm is 30% or more. (Feature 3) On the surface of the body, the number density of MC-type carbides with a radial length of 5.0 μm or more is 100 particles / mm². 2 The following applies: Features 1 to 3 are explained below.

[0037] [(Feature 1) Regarding chemical composition] The forged steel roll of this embodiment contains the following elements:

[0038] C: 0.85~1.05% Carbon (C) increases the hardness of the surface layer of forged steel rolls. If the C content is less than 0.85%, the above effect will not be sufficiently obtained. On the other hand, if the carbon content exceeds 1.05%, coarse carbides will be formed. In this case, sufficient hardness may not be obtained on the surface of the forged steel roll. Therefore, the C content is 0.85-1.05%. The preferred lower limit for the C content is 0.87%, more preferably 0.90%, and even more preferably 0.92%. The preferred upper limit for the C content is 1.03%, more preferably 1.00%, and even more preferably 0.98%.

[0039] Si: 0.60~1.20% Silicon (Si) deoxidizes steel during the molten steel stage. Furthermore, Si increases the tempering softening resistance of the steel and enhances the surface hardness of forged steel rolls. If the Si content is less than 0.60%, these effects are not 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 0.60-1.20%. The preferred lower limit for the Si content is 0.65%, more preferably 0.70%, and even more preferably 0.75%. The preferred upper limit for the Si content is 1.15%, more preferably 1.10%, and even more preferably 1.05%.

[0040] Mn: 0.30~0.60% Manganese (Mn) enhances the hardenability of forged steel rolls. If the Mn content is less than 0.30%, the above effect cannot be fully 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 0.30-0.60%. The preferred lower limit for the Mn content is 0.33%, more preferably 0.35%, and even more preferably 0.40%. The preferred upper limit for the Mn content is 0.57%, more preferably 0.55%, and even more preferably 0.50%.

[0041] P:0.020% or less Phosphorus (P) is an impurity. If the P content exceeds 0.020%, P segregates at the grain boundaries, reducing the toughness of the forged steel roll. Therefore, the P content is 0.020% or less. A low phosphorus (P) content is preferable. However, excessive reduction of the P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the P content is greater than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the P content is 0.018%, more preferably 0.015%, and even more preferably 0.010%.

[0042] S: 0.020% or less Sulfur (S) is an impurity. If the S content exceeds 0.020%, S segregates at the grain boundaries, reducing the toughness of the forged steel rolls and the hot workability of the steel material during the manufacturing process of the forged steel rolls. Therefore, the sulfur content is 0.020% or less. A low sulfur (S) content is preferable. However, excessive reduction of the S content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the S content is greater than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the S content is 0.018%, more preferably 0.015%, and even more preferably 0.010%.

[0043] 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. In this case, the toughness of the steel decreases during the manufacturing process of forged steel rolls. Therefore, the Al content is 0.050% or less. The preferred lower limit of the Al content is greater than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.005%, and more preferably 0.010%. The preferred upper limit for the Al content is 0.040%, more preferably 0.035%, more preferably 0.030%, and still more preferably 0.025%. In this specification, Al content refers to the total Al content in the steel.

[0044] 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. In this case, the toughness of the forged steel roll decreases. Therefore, the N content is 0.020% or less. The preferred lower limit of the N content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%. The preferred upper limit for the N content is 0.015%, more preferably 0.010%, and even more preferably 0.008%.

[0045] O: 0.0050% or less Oxygen (O) is an impurity. If the O content exceeds 0.0050%, the O forms oxides, reducing the toughness of the forged steel rolls. Therefore, the O content is 0.0050% or less. A low oxygen content is preferable. However, excessive reduction of the oxygen content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the oxygen content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, more preferably 0.0007%, and more preferably 0.0010%. The preferred upper limit for the O content is 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0046] Cr: 4.00~6.00% Chromium (Cr) forms carbides, thereby increasing the wear resistance of forged steel rolls. Furthermore, Cr increases the tempering softening resistance of the steel, thereby increasing the surface hardness of the forged steel rolls. If the Cr content is less than 4.00%, the above effects cannot be fully obtained. On the other hand, if the Cr content exceeds 6.00%, the proportion of M7C3 type carbides in the total carbides increases. In this case, the forged steel roll cannot satisfy characteristic 2. As a result, the wear resistance of the forged steel roll decreases. Therefore, the Cr content is 4.00-6.00%. The preferred lower limit for the Cr content is 4.05%, more preferably 4.10%, and even more preferably 4.15%. The preferred upper limit for the Cr content is 5.95%, more preferably 5.90%, and even more preferably 5.85%.

[0047] Mo: 1.00~2.50% Molybdenum (Mo) forms MC-type carbides, improving the wear resistance of forged steel rolls. Furthermore, Mo increases the tempering softening resistance of the steel, thereby increasing the surface hardness of the forged steel rolls. If the Mo content is less than 1.00%, the above effects cannot be fully obtained. On the other hand, if the Mo content exceeds 2.50%, coarse MC-type carbides are formed. In this case, the toughness of the forged steel roll decreases. Furthermore, in this case, the forged steel roll cannot satisfy characteristic 3. As a result, the grindability of the forged steel roll decreases. Therefore, the Mo content is 1.00 to 2.50%. The preferred lower limit for the Mo content is 1.05%, more preferably 1.10%, and even more preferably 1.15%. The preferred upper limit for the Mo content is 2.45%, more preferably 2.40%, and even more preferably 2.35%.

[0048] V: 1.00~2.00% Vanadium (V) forms MC-type carbides, improving the wear resistance of forged steel rolls. V also increases the tempering softening resistance of the steel, thereby increasing the surface hardness of the forged steel rolls. If the V content is less than 1.00%, the above effects are not sufficiently obtained. As a result, the proportion of MC-type carbides in the total carbides decreases. In this case, the forged steel rolls cannot satisfy characteristic 2. Consequently, the wear resistance of the forged steel rolls decreases. On the other hand, if the V content exceeds 2.00%, coarse MC-type carbides are formed. In this case, the toughness of the forged steel roll decreases. Furthermore, in this case, the forged steel roll cannot satisfy characteristic 3. As a result, the grindability of the forged steel roll decreases. 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 for the V content is 1.90%, more preferably 1.80%, and even more preferably 1.70%.

[0049] Cu: 0.40% or less Copper (Cu) is an impurity. If the Cu content exceeds 0.40%, it reduces the hot workability of the steel material in the manufacturing process of forged steel rolls. Therefore, the copper content is 0.40% or less. A low Cu content is preferable. However, excessive reduction of the Cu content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the Cu content is greater than 0%, more preferably 0.01%, more preferably 0.02%, more preferably 0.03%, and more preferably 0.04%. The preferred upper limit for the Cu content is less than 0.40%, more preferably 0.39%, more preferably 0.35%, more preferably 0.30%, more preferably 0.25%, and more preferably 0.20%.

[0050] Ni: 0.30~0.60% Nickel (Ni) improves the hardenability of forged steel rolls. If the Ni content is less than 0.30%, the above effect will not be fully achieved. On the other hand, if the Ni content exceeds 0.60%, excess retained austenite is formed. In this case, the hardness of the forged steel roll decreases. Therefore, the Ni content is 0.30-0.60%. The preferred lower limit for the Ni content is 0.33%, more preferably 0.35%, and even more preferably 0.40%. The preferred upper limit for the Ni content is 0.57%, more preferably 0.55%, and even more preferably 0.50%.

[0051] The remainder of the chemical composition of the forged steel roll according to this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from the ore, scrap, or manufacturing environment used as raw materials during the industrial manufacture of the forged steel roll, and are acceptable within a range that does not adversely affect the forged steel roll according to this embodiment.

[0052] [About Optional Elements] The chemical composition of the forged steel roll in this embodiment may further include one or more elements selected from the groups consisting of Group 1 and Group 2, in place of a portion of Fe. [Group 1] Ti: 0.050% or less, Nb: 0.050% or less, B: 0.0100% or less, W: 0.50% or less, and, One or more species selected from the group consisting of Co: 0.50% or less. [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, One or more selected from the group consisting of Mg: 0.0050% or less. The following describes these arbitrary elements.

[0053] [Group 1: Ti, Nb, B, W, and Co] The chemical composition of the forged steel roll in this embodiment may further include the elements of the first group described above, in place of some of the Fe. These elements are arbitrary and all enhance the hardness of the surface layer of the forged steel roll. The elements of the first group will be described below.

[0054] Ti: 0.050% or less Titanium (Ti) is an optional element and does not need to be included. In other words, the Ti content may be 0%. If Ti is present, that is, if the Ti content is greater than 0%, Ti will form precipitates that are carbides or nitrides, thereby increasing the hardness of the surface layer of the forged steel roll. Even if only a small amount of Ti is present, the above effect can be obtained to some extent. On the other hand, if the Ti content exceeds 0.050%, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material will decrease in the manufacturing process of the forged steel rolls. Therefore, the Ti content is between 0 and 0.050%, and if present, the Ti content is 0.050% or less. The preferred lower limit for the Ti content is 0.001%, more preferably 0.002%, and even more preferably 0.004%. The preferred upper limit for the Ti content is 0.040%, more preferably 0.035%, and even more preferably 0.030%.

[0055] Nb: 0.050% or less Niobium (Nb) is an optional element and does not need to be included. In other words, the Nb content may be 0%. If Nb is present, that is, if the Nb content is greater than 0%, Nb forms precipitates that are carbides or nitrides, thereby increasing the hardness of the surface layer of the forged steel roll. Even if only a small amount of Nb is present, the above effect can be obtained to some extent. On the other hand, if the Nb content exceeds 0.050%, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material will decrease in the manufacturing process of the forged steel rolls. Therefore, the Nb content is 0-0.050%, and if present, the Nb content is 0.050% or less. 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 for the Nb content is 0.040%, more preferably 0.035%, and even more preferably 0.030%.

[0056] B: 0.0100% or less Boron (B) is an optional element and does not need to be included. In other words, the B content may be 0%. If present, i.e., if the B content is greater than 0%, B enhances the hardenability of the forged steel roll and increases the hardness of the surface layer of the forged steel roll. Even if only a small amount of B is present, the above effects can be obtained to some extent. On the other hand, if the B content exceeds 0.0100%, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material will decrease in the manufacturing process of the forged steel rolls. Therefore, the B content is between 0 and 0.0100%, and if present, the B content is 0.0100% or less. The preferred lower limit for the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit for the B content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0057] W: 0.50% or less Tungsten (W) is an optional element and does not need to be included. In other words, the W content may be 0%. When present, i.e., when the W content is greater than 0%, W enhances the hardenability of the forged steel roll and increases the hardness of the surface layer of the forged steel roll. Even a small amount of W will provide some of the above effects. On the other hand, if the W content exceeds 0.50%, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material will decrease in the manufacturing process of the forged steel rolls. Therefore, the W content is 0-0.50%, and if present, the W content is 0.50% or less. 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 for the W content is 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0058] Co:0.50% or less Cobalt (Co) is an optional element and does not need to be included. In other words, the Co content may be 0%. When present, i.e., when the Co content is greater than 0%, Co enhances the hardenability of forged steel rolls and increases the hardness of the surface layer of the forged steel rolls. Even a small amount of Co will provide some of the above effects. On the other hand, if the Co content exceeds 0.50%, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material will decrease in the manufacturing process of the forged steel rolls. Therefore, the Co content is between 0% and 0.50%, and if present, the Co content is 0.50% or less. The preferred lower limit for the Co content is 0.01%, more preferably 0.05%, and even more preferably 0.08%. The preferred upper limit for the Co content is 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0059] [Group 2: Sn, Sb, As, Zr, Bi, Se, Te, Pb, Ca, and Mg] The chemical composition of the forged steel roll in this embodiment may further include the elements of the second group described above, in place of some of the Fe. These elements are optional and all enhance the grindability of the forged steel roll. The elements of the second group will be described below.

[0060] Sn: 0.10% or less Tin (Sn) is an optional element and does not need to be included. In other words, the Sn content may be 0%. If present, i.e., if the Sn content is greater than 0%, Sn improves the grindability of forged steel rolls. Even a small amount of Sn present will provide some degree of the above effect. On the other hand, if the Sn content exceeds 0.10%, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material will decrease in the manufacturing process of the forged steel rolls. Therefore, the Sn content is 0-0.10%, and if present, the Sn content is 0.10% or less. The preferred lower limit for the Sn content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the Sn content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0061] Sb: 0.05% or less Antimony (Sb) is an optional element and does not need to be included. In other words, the Sb content may be 0%. If Sb is present, that is, if the Sb content is greater than 0%, Sb improves the grindability of forged steel rolls. Even if only a small amount of Sb is present, the above effect can be obtained to some extent. On the other hand, if the Sb content exceeds 0.05%, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material will decrease in the manufacturing process of the forged steel rolls. Therefore, the Sb content is 0-0.05%, and if present, the Sb content is 0.05% or less. The preferred lower limit for Sb content is 0.01%. The preferred upper limit for Sb content is 0.04%.

[0062] As: 0.05% or less Arsenic (As) is an optional element and may not be present. In other words, the As content may be 0%. If present, i.e., if the As content is greater than 0%, As improves the grindability of forged steel rolls. Even a small amount of As will provide some degree of the above effect. On the other hand, if the As content exceeds 0.05%, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material will decrease in the manufacturing process of the forged steel rolls. Therefore, the As content is 0-0.05%, and if present, the As content is 0.05% or less. The preferred lower limit for As content is 0.01%. The preferred upper limit for As content is 0.04%.

[0063] Zr: 0.05% or less Zirconium (Zr) is an optional element and may not be present. In other words, the Zr content may be 0%. If Zr is present, that is, if the Zr content is greater than 0%, Zr improves the grindability of forged steel rolls. Even if only a small amount of Zr is present, the above effect can be obtained to some extent. On the other hand, if the Zr content exceeds 0.05%, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material will decrease in the manufacturing process of the forged steel rolls. Therefore, the Zr content is 0-0.05%, and if present, the Zr content is 0.05% or less. The preferred lower limit for Zr content is 0.01%. The preferred upper limit for Zr content is 0.04%.

[0064] Bi:0.10% or less Bismuth (Bi) is an optional element and does not need to be included. In other words, the Bi content may be 0%. If Bi is present, that is, if the Bi content is greater than 0%, Bi improves the grindability of forged steel rolls. Even if only a small amount of Bi is present, the above effect can be obtained to some extent. On the other hand, if the Bi content exceeds 0.10%, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material will decrease in the manufacturing process of the forged steel rolls. Therefore, the Bi content is 0-0.10%, and if present, the Bi content is 0.10% or less. The preferred lower limit for the Bi content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the Bi content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0065] Se: 0.10% or less Selenium (Se) is an optional element and does not need to be present. In other words, the Se content may be 0%. If present, i.e., if the Se content is greater than 0%, Se improves the grindability of forged steel rolls. Even if only a small amount of Se is present, the above effect can be obtained to some extent. On the other hand, if the Se content exceeds 0.10%, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material will decrease in the manufacturing process of forged steel rolls. Therefore, the Se content is 0-0.10%, and if present, the Se content is 0.10% or less. The preferred lower limit for the Se content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the Se content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0066] Te: 0.05% or less Tellurium (Te) is an optional element and does not need to be included. In other words, the Te content may be 0%. If present, i.e., if the Te content is greater than 0%, Te improves the grindability of forged steel rolls. Even a small amount of Te will provide some degree of the above effect. On the other hand, if the Te content exceeds 0.05%, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material will decrease in the manufacturing process of the forged steel rolls. Therefore, the Te content is 0-0.05%, and if present, the Te content is 0.05% or less. The preferred lower limit for Te content is 0.01%. The preferred upper limit for Te content is 0.04%.

[0067] Pb: 0.09% or less Lead (Pb) is an optional element and does not need to be included. In other words, the Pb content may be 0%. If Pb is present, that is, if the Pb content is greater than 0%, Pb improves the grindability of forged steel rolls. Even if only a small amount of Pb is present, the above effect can be obtained to some extent. On the other hand, if the Pb content exceeds 0.09%, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material will decrease in the manufacturing process of the forged steel rolls. Therefore, the Pb content is 0-0.09%, and if present, the Pb content is 0.09% or less. 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 for the Pb content is 0.08%, more preferably 0.07%, and even more preferably 0.06%.

[0068] Ca: 0.0050% or less Calcium (Ca) is an optional element and may not be present. In other words, the Ca content may be 0%. If calcium is present, that is, if the calcium content is greater than 0%, calcium improves the grindability of forged steel rolls. Even if only a small amount of calcium is present, the above effect can be obtained to some extent. On the other hand, if the Ca content exceeds 0.0050%, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material will decrease in the manufacturing process of the forged steel rolls. Therefore, the Ca content is between 0 and 0.0050%, and if present, the Ca content is 0.0050% or less. The preferred lower limit for the Ca content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit for the Ca content is 0.0047%, more preferably 0.0045%, and even more preferably 0.0040%.

[0069] Mg: 0.0050% or less Magnesium (Mg) is an optional element and does not need to be included. In other words, the Mg content may be 0%. If magnesium is present, i.e., if the magnesium content is greater than 0%, then magnesium improves the grindability of forged steel rolls. Even if only a small amount of magnesium is present, the above effect can be obtained to some extent. On the other hand, if the Mg content exceeds 0.0050%, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material will decrease in the manufacturing process of the forged steel rolls. Therefore, the Mg content is between 0 and 0.0050%, and if present, the Mg content is 0.0050% or less. 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 for the Mg content is 0.0047%, more preferably 0.0045%, and even more preferably 0.0040%.

[0070] [(Feature 2) Regarding the NR ratio of MC-type carbides] In the forged steel roll of this embodiment, the proportion of MC-type carbides among the total carbides with an equivalent circular diameter of 0.5 to 5.0 μm in the surface layer of the body is 30% or more.

[0071] 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-deficient regions within the crystal grains rather than increasing the number of coarse MC-type carbides. Most of the carbides finely dispersed within the 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 contribute little 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. Here, the number ratio of MC-type carbides among all carbides with an equivalent circle diameter of 0.5 to 5.0 μm on the surface layer of the body is defined as the number ratio NR.

[0072] When the number ratio NR is less than 30%, the formation of MC-type carbide-deficient regions within the crystal grains cannot be suppressed. In this case, excellent wear resistance cannot be obtained.

[0073] If the number ratio NR is 30% or more, MC-type carbides of sufficient size to enhance wear resistance are sufficiently dispersed within the crystal grains. In this case, assuming that the forged steel roll satisfies characteristic 1, excellent wear resistance can be obtained. Therefore, in the forged steel roll of this embodiment, the number ratio NR is 30% or more.

[0074] The preferred lower limit of the number ratio NR is 35%, more preferably 40%, and even more preferably 45%. There is no particular upper limit to the number ratio NR. If the forged steel roll satisfies features 1 and 3, the upper limit to the number ratio NR is, for example, 90%.

[0075] [Method for measuring the number of items (NR)] The number percentage NR of MC-type carbides in the total carbides with an equivalent circular diameter of 0.5 to 5.0 μm on the surface layer of the body can be measured by the following method. A test specimen is taken from the surface of the body of a forged steel roll, including a surface perpendicular to the axial direction of the body of the forged steel roll as the observation surface. The observation surface includes a position 20 mm deep from the surface of the forged steel roll. The observation surface is mirror-polished. Ten arbitrary observation fields are selected from the mirror-polished observation surface, centered at a position 20 mm deep from the surface of the body of the forged steel roll. The size of each observation field is 240 μm × 180 μm. Z-contrast images, also known as COMPO images, are taken from the backscattered electron detector of the ten observation fields using an electrolytic emission scanning electron microscope (FE-SEM). The observation magnification is 500x. In the COMPO image, the carbide has a darker contrast compared to the matrix phase, which is mainly composed of iron, because it contains many carbon atoms with a lower atomic number. Therefore, the matrix phase and the carbide can be distinguished by contrast.

[0076] Furthermore, several carbides in the observation field will be quantitatively analyzed using energy-dispersive X-ray spectroscopy (EDS) attached to the FE-SEM to identify five MC-type carbides and five other carbides. Carbides with a V content of 30% or more by mass will be defined as MC-type carbides. Carbides with a V content of less than 30% by mass will be defined as other carbides. The acceleration voltage for EDS analysis will be set to 15kV. The EDS analysis time will be set so that the X-ray count is 500 counts or more.

[0077] Here, in the COMPO image as well, by appropriately setting the contrast, it is possible to distinguish between MC-type carbides, which have a high carbon content, and other carbides, which have a lower carbon content compared to MC-type carbides. Specifically, if the contrast setting is appropriate, MC-type carbides will be displayed with a darker contrast than other carbides. The contrast of the COMPO image is adjusted so that the five MC-type carbides identified by quantitative analysis using EDS and the five other carbides can be distinguished. In this way, the observation field is captured and a photographic image is generated with settings that allow the matrix, MC-type carbides, and other carbides to be distinguished.

[0078] From the obtained photographic images, all carbides with an equivalent circle diameter of 0.5 to 5.0 μm within the entire observation field are identified, and their total number is determined. Here, the equivalent circle diameter refers to the diameter (μm) when the area of ​​the carbide is converted to a circle. From the identified carbides with an equivalent circle diameter of 0.5 to 5.0 μm, MC-type carbides are further identified, and their total number is determined.

[0079] Based on the total number of carbides with an equivalent circle diameter of 0.5 to 5.0 μm and the total number of MC-type carbides, the percentage (%) of MC-type carbides in the total carbides with an equivalent circle diameter of 0.5 to 5.0 μm is determined. The obtained percentage (%) is defined as the percentage NR (%). Note that the quantity percentage NR(%) is an integer value obtained by rounding the calculated number to the first decimal place.

[0080] [(Feature 3) Regarding the number density ND] In the forged steel roll of this embodiment, the number density of MC-type carbides with a radial length of 5.0 μm or more on the surface layer of the body is 100 particles / mm². 2 The following applies:

[0081] The number density of MC-type carbides with a radial length of the barrel part exceeding 5.0 μm on the surface layer of the barrel part is defined as the number density ND. As described above, MC-type carbides with a radial length of the barrel part exceeding 5.0 μm are difficult to be removed together with grinding powder during the grinding of the forged steel roll. The MC-type carbides that are not removed collide with the abrasive grains during grinding. Since MC-type carbides are particularly hard among carbides, the abrasive grains may be broken by the impact during the collision with the MC-type carbides. If the abrasive grains are broken, the grinding of the base material of the forged steel roll does not proceed, and the assumed grinding amount cannot be obtained. As a result, the grindability deteriorates.

[0082] When the number density ND exceeds 100 pieces / mm 2 there are too many MC-type carbides with a radial length of the barrel part exceeding 5.0 μm. Therefore, the collision between the MC-type carbides and the abrasive grains cannot be sufficiently suppressed, and excellent grindability cannot be obtained.

[0083] On the other hand, when the number density ND is 100 pieces / mm 2 or less, during the grinding of the forged steel roll, there are sufficiently few MC-type carbides with a size that cannot be included in the grinding powder. Therefore, the MC-type carbides are easily removed together with the grinding powder during grinding. As a result, assuming that the forged steel roll satisfies Feature 1, excellent grindability can be obtained.

[0084] The lower limit of the number density ND is not particularly limited. When the forged steel roll satisfies Feature 1 and Feature 2, the lower limit of the number density ND is, for example, 30 pieces / mm 2 or the like. The preferable upper limit of the number density ND is 95 pieces / mm 2 more preferably 90 pieces / mm 2 more preferably 85 pieces / mm 2 or the like.

[0085] [Measurement method of number density ND] The number density ND of MC-type carbides with a radial length of the barrel part exceeding 5.0 μm on the surface layer of the barrel part can be measured by the following method. A test specimen is taken from the surface of the body of the forged steel roll, including a surface perpendicular to the axial direction of the body of the forged steel roll as the observation surface. The size of the observation surface is a rectangle with a long side of 20 mm and a short side of 10 mm, including a position 20 mm deep from the surface of the forged steel roll. The long side of the observation surface is perpendicular to the radial direction of the body at the center of the long side. The observation surface is mirror polished. From the mirror polished observation surface, 10 arbitrary observation fields are determined, centered at a position 20 mm deep from the surface of the body of the forged steel roll. The size of each observation field is 240 μm long and 180 μm short. The long side of the observation field is parallel to the long side of the observation surface. Here, the diameter of the body is extremely large compared to the size of the observation surface and the observation fields. Therefore, it is assumed that the long side of all observation fields is perpendicular to the radial direction of the body, and the short side of all observation fields is parallel to the radial direction of the body. Ten observation fields were observed using FE-SEM, and photographic images that could identify MC-type carbides were obtained using the same method as described in [Method for measuring the number ratio NR] above.

[0086] Specifically, first, COMPO images of each observation field are taken with settings that allow for distinction between the matrix phase and the carbides. The observation magnification is set to 500x. Then, quantitative analysis is performed on several carbides in each observation field using the EDS attached to the FE-SEM to identify five MC-type carbides and five other carbides. Note that carbides with a V content of 30% or more by mass are defined as MC-type carbides. Carbonides with a V content of less than 30% by mass are defined as other carbides. For EDS analysis, the acceleration voltage is set to 15kV. The EDS analysis time is set so that the X-ray count is 500 counts or more. Furthermore, the contrast of the COMPO image is adjusted so that the five MC-type carbides and the five other carbides identified by the quantitative analysis using EDS can be distinguished. In this way, each observation field is taken with settings that allow for distinction between the matrix phase, MC-type carbides, and other carbides, and photographic images are generated.

[0087] From the obtained photographic images, all MC-type carbides with a radial length exceeding 5.0 μm within the entire observation field are identified, and their total number is determined. Here, the radial length of the MC-type carbide is defined as the maximum distance between any two straight lines perpendicular to the radial direction that are in contact with the MC-type carbide.

[0088] The total number of MC-type carbides with a radial length exceeding 5.0 μm obtained, and the total area of ​​the 10 observation fields (0.432 mm²) 2 Based on this, the number density ND (pieces / mm²) 2 ) Note that the number density is ND (pieces / mm²). 2 ) is the integer value obtained by rounding the calculated number to the first decimal place.

[0089] [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 can achieve both excellent wear resistance and excellent grindability.

[0090] [Microstructure of the surface layer of the forged steel roll of this embodiment] The microstructure of the surface layer of the forged steel roll in this embodiment consists mainly of martensite and / or bainite. "Mainly consisting of martensite and / or bainite" means that the total area ratio of martensite and bainite is 85% or more. Other microstructures besides martensite and bainite include, for example, pearlite, retained austenite, and carbides.

[0091] [Method for measuring the total area ratio of martensite and bainite on the surface of forged steel rolls] The total area ratio of martensite and bainite on the surface of the forged steel roll in 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 body of the forged steel roll, with the observation surface being a plane perpendicular to the axial direction of the body of the forged steel roll. The observation surface includes a position 20 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 etching solution). Of the etched observation surface, five arbitrary observation fields (240 μm × 180 μm) centered at a position 20 mm deep from the surface of the body of the forged steel roll are observed with a 500x optical microscope. If pearlite is present, it is more strongly etched by the Nital etching solution than martensite and bainite. Therefore, pearlite is observed as a darker structure than martensite and bainite, making it easy to distinguish between the two. The total area of ​​pearlite in the observation field is determined using known image processing. The area ratio of pearlite is determined based on the total area of ​​pearlite identified in all observation fields and the total area of ​​all observation fields.

[0092] The area ratio of carbides is determined by the following FE-SEM observation. A specimen is taken from the surface of the body of the forged steel roll, including a surface perpendicular to the axial direction of the body of the forged steel roll as the observation surface. The observation surface includes a position 20 mm deep from the surface of the forged steel roll. The observation surface is mirror-polished. From the mirror-polished observation surface, five arbitrary observation fields (240 μm × 180 μm) centered at a position 20 mm deep from the surface of the body of the forged steel roll are observed with an FE-SEM, and a Z-contrast image, also known as a COMPO image, is captured by the backscattered electron detector. The observation magnification is 500x. All carbides with an equivalent circle diameter of 5 μm or more are identified from the captured photographic images. Carbides with an equivalent circle diameter of less than 5 μm can be ignored as they are part of martensite and bainite. Furthermore, in the Z-contrast image, martensite and bainite structures, which are mainly composed of iron, can be easily distinguished from carbides containing a large amount of carbon. The area ratio of carbides is defined as the area ratio calculated based on the total area of ​​carbides with an equivalent circle diameter of 5 μm or more identified in all observation fields, and the total area of ​​all observation fields.

[0093] Furthermore, the area ratio of retained austenite is determined by the following X-ray diffraction method. A test specimen is taken from the surface of the body of the forged steel roll, including a position 20 mm deep. 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 set to Cu (CuKα rays), and the output is set to 40 kV-400 mA. After calculation, the volume fraction Vγ (%) of retained austenite is calculated for each combination (4 × 3 = 12 pairs) of each face of the α phase and each face of the γ phase using equation (I). 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 crystallographic theoretical calculation value of the α phase. Iγ is the integrated intensity of the γ phase. Rγ is the crystallographic theoretical calculation value of the γ phase. In this specification, Rα at the (110) plane of the α phase is set to 100, Rα at the (200) plane of the α phase is set to 14.0, Rα at the (211) plane of the α phase is set to 25.6, Rα at the (220) plane of the α phase is set to 8.4, Rγ at the (200) plane of the γ phase is set to 34.0, Rγ at the (220) plane of the γ phase is set to 17.9, and Rγ at the (311) plane of the γ phase is set to 20.5. The volume fraction of retained austenite is rounded to the first decimal place of the obtained value. In the measurement of X-ray diffraction intensity, X-rays are irradiated onto the test specimen at a position corresponding to a depth of 20 mm from the surface of the body of the forged steel roll.

[0094] The volume fraction (%) of retained austenite obtained by the above-mentioned X-ray diffraction method is considered to be the area fraction (%) of retained austenite. Then, the total area fraction of martensite and bainite on the surface of the forged steel roll is calculated using the following formula. The total area ratio of martensite and bainite on the surface of a forged steel roll = 100 - (area ratio of pearlite + area ratio of carbides + area ratio of retained austenite)

[0095] [Applications of the forged steel roll of this embodiment] The forged steel roll of this embodiment is widely applicable as a rolling roll. The forged steel roll of this embodiment is particularly suitable as a roll for cold rolling thin steel sheets. Cold rolling rolls include, for example, work rolls for cold tandem rolling mills, cold reverse rolling mills, or work rolls for skin pass (temper rolling).

[0096] [Method for manufacturing forged steel rolls] An example of a method for manufacturing the forged steel roll of this embodiment will be described. The forged steel roll of this embodiment may be manufactured by methods other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing the forged steel roll of this embodiment.

[0097] An example of the manufacturing method for the forged steel roll of this embodiment includes the following steps. (Process 1) Steelmaking process (Process 2) Hot forging process (Step 3) Annealing process (Process 4) Rough processing process (Step 5) Hardening process (Process 6) Tempering process (Process 7) Finishing process The following describes each step.

[0098] [(Process 1) Steelmaking Process] In the steelmaking process, ingots are manufactured using molten steel that satisfies characteristic 1, by known casting methods. Known casting methods include, for example, the bottom pouring ingot method. The cast ingots (electrode ingots) are used as electrodes, and electroslag remelting (ESR) or the like is performed.

[0099] The reason for implementing the ESR method in the steelmaking process of the forging rolls in this embodiment is as follows: In the solidification process of general casting methods, alloying elements may concentrate in the liquid phase, leading to solidification segregation. The forged steel roll of this embodiment satisfies Feature 1, and is therefore adjusted to have a chemical composition that facilitates the formation of MC-type carbides. Consequently, when solidification segregation occurs during the solidification of the forged steel roll of this embodiment, MC-type carbides preferentially crystallize from the remaining liquid phase. The MC-type carbides produced by crystallization are coarser than those produced by precipitation. Furthermore, the regions where the liquid phase remained at the end of solidification become grain boundaries after solidification is complete. In this way, coarse MC-type carbides are formed at the grain boundaries. Around the coarse MC-type carbides, V and Mo, the main components of MC-type carbides, are deficient. Therefore, within the grains where coarse MC-type carbides exist at the grain boundaries, the precipitation of MC-type carbides is suppressed. In this way, it is believed that MC-type carbide-deficient regions are formed within the grains.

[0100] In the ESR method, the electrode ingot is remelted by the Joule heating of the molten slag. The molten electrode ingot settles in the molten slag as droplets and accumulates in a mold of any shape, solidifying in layers. When the electrode ingot is completely melted and the molten steel has solidified completely up to the top, a forging ingot is obtained.

[0101] Furthermore, in the ESR method, the molten steel stored in the mold solidifies while maintaining a relatively shallow pool of molten steel. Therefore, solidification segregation can be suppressed. As a result, the crystallization of MC-type carbides can be suppressed. In other words, by implementing the ESR method, coarse MC-type carbides contained in the electrode ingot can be remelted, and a forging ingot with suppressed formation of MC-type carbide-deficient regions can be obtained. For this reason, the ESR method is implemented in the steelmaking process of the forging rolls in this embodiment.

[0102] The ESR method used in the steelmaking process satisfies the following condition 1. (Condition 1) The coagulation rate (SR) is 3.0 mm / min or higher. Condition 1 will be explained below.

[0103] [Regarding Condition 1] In the ESR method, the solidification rate SR refers to the rate at which the solidification interface rises (mm / min) at the center of the mold when viewed from above (planar view). Compared to general casting methods, the ESR method allows for precise control of the solidification rate of molten steel. On the other hand, if the solidification rate SR in the ESR method is too slow, solidification segregation is actually promoted. As a result, coarse MC-type carbides are more likely to be formed by crystallization. Consequently, the formation of MC-type carbide-deficient regions is promoted. If the solidification rate SR is too slow, the growth of MC-type carbides is further excessively promoted. In this case, the growth of MC-type carbides is Ostwald growth. Therefore, fine MC-type carbides within the grains are incorporated into and disappear as coarser MC-type carbides. As a result, the number ratio NR decreases.

[0104] If the solidification rate SR in the ESR method is 3.0 mm / min or higher, the formation of MC-type carbide-deficient regions can be suppressed. Furthermore, the disappearance of fine MC-type carbides within the grains can be suppressed. As a result, the forged steel rolls produced can satisfy characteristic 2. Therefore, the coagulation rate (SR) in the ESR method is 3.0 mm / min or higher.

[0105] [(Process 2) Hot Forging Process] In the hot forging process, first, the forging ingot obtained by the ESR method is heated in a heating furnace at a heating temperature T and held for a predetermined time. Hot forging is then performed on the ingot after heating and holding. In this way, a rough-shaped roll material (hereinafter referred to as an intermediate material) is manufactured.

[0106] The hot forging process satisfies conditions 2 and 3 below. (Condition 2) The heating temperature T before the start of hot forging shall be between 1125 and less than 1200°C, and the holding time t1 at the heating temperature T (°C) shall be 1.0 hour or longer. (Condition 3) The cumulative time t2 at 1000-1100°C is 30 hours or less. Conditions 2 and 3 will be explained below.

[0107] [Regarding Condition 2] At temperatures above 1125°C, the solid solution of MC-type carbides is promoted. As a result, the MC-type carbides become finer. In particular, MC-type carbides extending in a certain direction have a large specific surface area and are easily dissolved. Therefore, by heating the forging ingot to 1125°C or higher before starting hot forging, it is possible to effectively refine the MC-type carbides extending in the diameter direction of the body within the forging ingot. On the other hand, if the forging ingot is heated to 1200°C or higher, a portion of the forging ingot may melt, creating a liquid phase, from which coarse crystallized carbides may be formed. Therefore, the heating temperature T before starting hot forging should be between 1125°C and less than 1200°C.

[0108] If the holding time t1 at the heating temperature T (°C) before the start of hot forging is 1.0 hour or longer, MC-type carbides with a large radial length in the body can be sufficiently refined. As a result, the forged steel rolls produced can satisfy characteristic 3. Therefore, the holding time t1 at the heating temperature T (°C) before the start of hot forging should be 1.0 hour or more.

[0109] Furthermore, there is no particular upper limit to the holding time t1 at the heating temperature T (°C) before the start of hot forging. Considering normal industrial production, the upper limit of the holding time t1 is, for example, 20 hours.

[0110] [Regarding Condition 3] In the hot forging process, the cumulative time during which the surface temperature of the workpiece (forging ingot or roll material) is between 1000 and 1100°C, from the time the forging ingot is brought into the heating furnace until the roll material cools to room temperature after forging, is defined as the cumulative time t2 (hours) at 1000-1100°C. The surface temperature of the workpiece is measured using a non-contact radiation thermometer. If the chemical composition of the workpiece satisfies characteristic 1, the workability decreases significantly when the surface temperature of the workpiece falls below 1000°C. In this case, forging is interrupted and the workpiece is reheated in the heating furnace. After that, the reheated workpiece is removed from the heating furnace and forging is resumed. In the hot forging process, this operation is repeated until a roll material of the desired shape is obtained. In other words, the cumulative time t2 at 1000-1100°C includes the time from when the forging process is interrupted until the workpiece is brought into the heating furnace, as well as the time the workpiece is heated in the furnace.

[0111] In the temperature range of 1000-1100°C, the growth of MC-type carbides is excessively accelerated. As mentioned above, the growth of MC-type carbides in this case is also Ostwald growth. Therefore, fine MC-type carbides within the grains are incorporated into coarser MC-type carbides and disappear. As a result, the number ratio NR decreases. For this reason, it is preferable to have a short cumulative time t2 at 1000-1100°C during the hot forging process.

[0112] If the cumulative time t2 at 1000-1100°C during the hot forging process is 30 hours or less, the disappearance of fine MC-type carbides within the grain can be suppressed. As a result, the forged steel rolls produced can satisfy characteristic 2. Therefore, the cumulative time t2 at 1000-1100°C during the hot forging process is 30 hours or less. There is no particular lower limit to the cumulative time t2 at 1000-1100°C. Considering typical industrial production, the lower limit to the cumulative time t2 at 1000-1100°C is, for example, 8 hours.

[0113] [(Step 3) Annealing process] In the annealing process, the intermediate material produced in the hot forging process is annealed. By performing the annealing process, the intermediate material becomes easier to grind in the subsequent rough machining process. Annealing can be carried out using an electric furnace or a gas furnace under well-known conditions. The annealing temperature is, for example, 500 to 800°C. The holding time is, for example, 10 to 50 hours.

[0114] [(Process 4) Rough processing process] In the rough machining process, the intermediate material after the annealing process is subjected to rough machining to further shape it closer to the final roll shape. Rough machining can be performed, for example, by cutting. Rough machining can be carried out under well-known conditions.

[0115] [(Step 5) Hardening process] In the quenching process, a well-known quenching treatment is performed on the surface layer of the intermediate material after the rough machining process. Specifically, the intermediate material is heated and held at 900-1100°C, and then rapidly cooled. The rapid cooling method is, for example, water cooling.

[0116] [(Step 6) Tempering process] In the tempering process, tempering is performed on the intermediate material after the quenching process. 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 a sub-zero treatment may be performed on the intermediate material after the quenching process but before the tempering process. The cooling temperature in the sub-zero treatment may be within a well-known range, for example, -30 to -196°C.

[0117] [(Process 7) Finishing Process] In the finishing process, finishing is performed on the intermediate material after the tempering process. Finishing is done, for example, by grinding using a grinding machine. The finishing process shapes the intermediate material into the final product shape. By following the above steps, the forged steel roll of this embodiment can be manufactured.

[0118] The present invention will be described in more detail below with reference to examples. The present invention is not limited in any way to these examples. [Examples]

[0119] Forged steel rolls having the chemical compositions shown in Tables 1A and 1B were manufactured by the following manufacturing method.

[0120] [Table 1A]

[0121] [Table 1B]

[0122] Specifically, electrode ingots were cast from molten steel using the bottom-pour ingot casting method. The manufactured electrode ingots were used as electrodes, and the electroslag remelting (ESR) method was performed. The solidification rate SR (mm / min) in the ESR method is shown in Table 2. A hot forging process was performed on the forging ingots obtained by the ESR method. The heating temperature T before the start of the hot forging process was between 1125 and less than 1200°C. The holding time t1 (hours) at the heating temperature T (°C) before the start of hot forging for each test number is shown in Table 2. The cumulative time t2 (hours) at 1000 to 1100°C during the hot forging process is also shown in Table 2. Through the hot forging process, intermediate raw materials with a roll shape of roll body diameter φ700 mm, body length 2100 mm, and total length 4100 mm were manufactured for each test number.

[0123] [Table 2]

[0124] An annealing process was performed on the intermediate raw material after the hot forging process. In the annealing process, the material was held at 800°C for 10 hours. After that, it was held at 600°C for another 15 hours. A rough machining process was performed on the intermediate raw material after the annealing process. Specifically, for each test number, machining was performed on the intermediate raw material to produce a roll-shaped intermediate raw material with a roll body diameter of φ650 mm, a roll body length of 2000 mm, and an overall length of 4000 mm.

[0125] A quenching process was performed on the intermediate material after the rough machining process. In the quenching process, the intermediate material was heated to 900-1100°C by induction heating and held for 10 minutes. After that, the heated intermediate material was water-cooled.

[0126] After the quenching process, the intermediate raw material was tempered at 100-600°C, followed by a finishing process. In the finishing process, the intermediate raw material was ground to produce a final roll shape with a roll body diameter of φ645 mm, a roll body length of 1950 mm, and an overall length of 3950 mm. The forged steel rolls for each test number were manufactured using the above manufacturing process. The total area ratio of martensite and bainite on the surface of each forged steel roll was determined based on the method described in [Method for measuring the total area ratio of martensite and bainite on the surface of forged steel rolls] above. As a result, the total area ratio of martensite and bainite on the surface was 85% or more for all forged steel rolls of each test number.

[0127] [About the evaluation test] The following evaluation tests were performed on each manufactured forged steel roll with a test number. (Test 1) Measurement test of the number of NRs (Test 2) Measurement test of number density ND (Test 3) Abrasion resistance evaluation test (Test 4) Grinding performance evaluation test The following describes each test.

[0128] [(Test 1) Measurement test of the number ratio NR] Based on the method described in [Method for Measuring Number Ratio NR] above, the number ratio NR of MC-type carbides with an equivalent circular diameter of 0.5 to 5.0 μm was determined in the total carbides on the surface of each test numbered forged steel roll. The obtained number ratio NR is shown in the "Number Ratio NR (%)" column of Table 2.

[0129] [(Test 2) Measurement test of number density ND] Based on the method described in [Method for Measuring Number Density ND] above, the number density ND of MC-type carbides with a radial length exceeding 5.0 μm was determined on the surface layer of each test number of forged steel roll. The obtained number density ND is shown in Table 2 as "Number Density ND (pieces / mm²)". 2 This is shown in the ")" column.

[0130] [(Test 3) Abrasion Resistance Evaluation Test] The wear resistance of the forged steel rolls for each test number was evaluated using a two-cylinder rolling wear testing machine. Figure 1 is a schematic diagram of the two-cylinder rolling wear testing machine 10. A cylindrical roll test specimen 12 was taken from the surface layer (the area from the surface of the body to a depth of 40 mm) of each forged steel roll with a given test number. The roll test specimen 12 had a diameter of 40 mm and a width of 10 mm. The central axis of the roll test specimen 12 was parallel to the radial direction of the forged steel roll. The outer surface of the roll test specimen 12 was polished.

[0131] A two-cylinder rolling abrasion test was conducted using the collected roll test specimen 12. In the two-cylinder rolling abrasion test, a rolled material test specimen 11 rotating in the opposite direction was pressed against the rotating roll test specimen 12 with a load F described later. A front view of the rolled material test specimen 11 is shown in Figure 2. The numbers in Figure 2 indicate dimensions (in mm). "R7.5" in Figure 2 indicates that the radius of curvature of the outer surface was 7.5 mm. As shown in Figure 2, the diameter of the rolled material test specimen 11 was 160 mm and the width was 15 mm. The rolled material test specimen 11 was manufactured by processing steel material having a chemical composition equivalent to S45C as specified in JIS G 4051 (2018) into the shape shown in Figure 2 and polishing the outer surface.

[0132] In the two-cylinder rolling abrasion test, the contact load between the roll specimen 12 and the rolled material specimen 11 was set to 700 N, and no lubricant was used. The rotation speed of the roll specimen 12 was set to 2000 rpm, and the slip ratio between the roll specimen 12 and the rolled material specimen 11 was set to 5%. The slip ratio (%) is defined by the following formula: Slip ratio (%) = (Peripheral speed of rolled material specimen - Peripheral speed of roll specimen) / Peripheral speed of roll specimen × 100 In the two-cylinder rolling abrasion test, the rolled material specimen 11 was maintained at 200°C by high-frequency induction heating using an induction heating coil (not shown). The roll specimen was water-cooled for cooling. The test was terminated after 20,000 cycles.

[0133] The wear shape of the outer surface of the roll test specimen 12 after 20,000 rolling cycles was measured using a laser microscope (Keyence Corporation, product name: Shape Analysis Laser Microscope VK-X250). Specifically, a scan of the surface shape was performed in the width direction at an arbitrary position on the outer surface of the roll test specimen 12. The scanned area was the entire width of the outer surface. From the obtained surface shape profile, the wear cross-sectional area (the area of ​​the depression caused by wear on the outer surface in a cross-section including the width and radial directions of the roll test specimen 12) was calculated. A similar wear cross-sectional area measurement was performed on the outer surface of the roll test specimen 12 at a position directly opposite the center. The arithmetic mean of the wear cross-sectional areas obtained from these two measurements was taken as the wear cross-sectional area of ​​the roll test specimen 12. The wear cross-sectional area of ​​the roll test piece 12 is 2000 μm². 2 If the following conditions were met, the evaluation was set to "E (Excellent)," indicating that excellent wear resistance was achieved (indicated as "E" in the "Wear Resistance" column of Table 2). On the other hand, the wear cross-sectional area of ​​roll test piece 12 was 2000 μm². 2 If the result was "B" (Bad), it was determined that excellent wear resistance was not achieved (indicated as "B" in the "Wear Resistance" column in Table 2).

[0134] [(Test 4) Grindability Evaluation Test] Grindability evaluation tests were conducted using the forged steel rolls for each test number in the following manner. A cylindrical test specimen measuring φ40 mm in diameter and 200 mm in length was taken from the surface layer (the area from the surface of the body down to a depth of 40 mm) of the forged steel roll for each test number. The central axis of the test specimen was parallel to the longitudinal direction of the body and corresponded to a depth of 20 mm from the surface of the body.

[0135] A cylindrical grinding machine was used for the grinding performance evaluation test. The grinding wheel specifications followed JIS R 6242:2023. Specifically, a grinding wheel was used in which alumina abrasive material was bonded with a vitrified binder. The abrasive material had a grit size of 100, a bonding degree of F, and an abrasive grain ratio of 38%. The dimensions of the grinding wheel were φ400 mm in diameter and 15 mm in width.

[0136] In the grinding performance evaluation test using the grinding wheel described above, the grinding method was wet traverse grinding, and the feed rate was set to 1500 mm / min. Furthermore, the peripheral speed of the grinding wheel was set to 33 m / sec, and the peripheral speed of the test piece was set to 0.75 m / sec. The depth of cut per pass was set to 0.01 mm, and the target depth of cut (hereinafter also referred to as the target depth of cut) was set to 0.1 mm. In other words, a total of 10 passes were performed in the grinding performance evaluation test.

[0137] The diameter (mm) of each test specimen was measured after the grindability evaluation test. Based on this diameter, the depth of cut obtained from the grindability evaluation test (hereinafter also referred to as the actual depth of cut) was calculated. Specifically, it was calculated using the following formula. Actual depth of cut (mm) = (40mm - (Diameter of each test piece after grindability evaluation test (mm)) / 2 From the actual depth of cut obtained by the above formula, the material removal rate (%) for each test number in the grindability evaluation test was determined using the following formula. Material removal rate (%) = Actual cutting depth (mm) / Target cutting depth (mm) If the material removal rate was 60% or higher, the evaluation was "E (Excellent)," indicating that excellent grinding performance was achieved (indicated as "E" in the "Grinding Performance" column of Table 2). On the other hand, if the material removal rate was less than 60%, the evaluation was "B (Bad)," indicating that excellent grinding performance was not achieved (indicated as "B" in the "Grinding Performance" column of Table 2).

[0138] [Test Results] Referring to Tables 1A, 1B, and 2, the forged steel rolls for test numbers 1 to 19 met features 1 to 3. Therefore, excellent wear resistance and excellent grindability were obtained.

[0139] On the other hand, in tests 20 and 21, the solidification rate SR in the ESR method performed during the steelmaking process was too slow. As a result, the forged steel rolls did not meet characteristic 2. Consequently, excellent wear resistance could not be obtained.

[0140] In tests 22 and 23, the holding time t1 at the heating temperature T before the start of hot forging was too short. As a result, the forged steel rolls did not meet characteristic 3. Consequently, excellent grindability was not achieved.

[0141] In tests 24 and 25, the cumulative time t2 at 1000-1100°C during the hot forging process was too long. As a result, the forged steel rolls did not meet characteristic 2. Consequently, excellent wear resistance was not achieved.

[0142] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. It is a forged steel roll, A cylindrical body, It comprises a pair of shafts, The chemical composition of the aforementioned forged steel roll is, in 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, It contains Ni: 0.30-0.60%, The remainder consists of Fe and impurities. In the surface layer of the body, the number proportion of MC-type carbides among the total carbides with an equivalent circular diameter of 0.5 to 5.0 μm is 30% or more. In the surface layer of the body, the number density of MC-type carbides with a radial length of 5.0 μm or more is 100 particles / mm². ２ The following is: Forged steel roll.

2. It is a forged steel roll, A cylindrical body, It comprises a pair of shafts, The chemical composition of the aforementioned forged steel roll is, in 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, It contains Ni: 0.30-0.60%, Furthermore, it contains one or more selected from the group consisting of Group 1 and Group 2, The remainder consists of Fe and impurities. In the surface layer of the body, the number proportion of MC-type carbides among the total carbides with an equivalent circular diameter of 0.5 to 5.0 μm is 30% or more. In the surface layer of the body, the number density of MC-type carbides with a radial length of 5.0 μm or more is 100 particles / mm². ２ The following is: 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, One or more selected from the group consisting of Co: 0.50% or less. [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, One or more selected from the group consisting of Mg: 0.0050% or less.

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

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

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