Forged steel rolls

Abstract

To provide a forged steel roll that has sufficient wear resistance and can sufficiently suppress crack formation. [Solution] The forged steel roll of 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: 0.20~1.0 It contains less than 0%, V: 1.00-2.00%, Cu: 0.40% or less, Ni: 0.30-0.60%, with the remainder being Fe and impurities, satisfying formula (1), and in the surface layer of the body, the number ratio 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 amount of solid-dissolved carbon in the matrix is ​​0.80% or less by mass%. (V+2Mo) / (Cr+Ni)≧0.400 (1)

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. As a result, the material is rolled into the desired shape. On the other hand, the surface of the body of the forged steel roll (hereinafter simply referred to as 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 roll gradually decreases. If the surface roughness of the forged steel roll decreases, slippage will occur between the forged steel roll and the material to be rolled. In this case, there is a possibility that the material to be rolled will not be properly gripped, or that seizure will occur in the material to be rolled or the forged steel roll.

[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, sufficient wear resistance is required for the forging roll.

[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 of 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 of the interior beyond the surface layer is less than 900 HV, thereby increasing 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] Incidentally, when the aforementioned slip occurs during the use of forging rolls, thermal shock due to friction is applied to the surface of the forging roll. This thermal shock can cause cracks to form on the surface of the forging roll. If the forging roll with cracks continues to be used, the repeated load applied to the surface will cause the cracks to propagate. As the cracks propagate in this way, a portion of the surface of the forging roll may eventually peel off. This peeling phenomenon is called spalling.

[0008] To suppress spalling, if a crack is detected in the forging roll, the surface of the forging roll is ground down according to the depth of the crack to remove it. If the crack is deep, more of the forging roll needs to be ground down to remove the crack. As a result, the lifespan of the forging roll is shortened. In other words, to extend the lifespan of the forging roll, it is desirable not only to improve wear resistance but also to suppress crack formation.

[0009] An object of the present disclosure is to provide a forged steel roll having sufficient wear resistance and capable of sufficiently suppressing the occurrence of cracks.

Means for Solving the Problems

[0010] The forged steel roll of the present disclosure has a cylindrical body portion and a pair of shaft portions, and the chemical composition of the forged steel roll is, in mass%, C: 0.85 to 1.05%, Si: 0.60 to 1.20%, Mn: 0.30 to 0.60%, P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00 to 6.00%, Mo: less than 0.20 to 1.00%, V: 1.00 to 2.00%, Cu: 0.40% or less, and Ni: 0.30 to 0.60%, and contains the balance consists of Fe and impurities, <000009​​​​​​​​​​​​​​​​​​​​​​​​Si: 0.60~1.20%, Mn: 0.30~0.60%, P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00~6.00%, Mo: 0.20-1.00% 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. Satisfying equation (1), On the surface of the body, The proportion of MC-type carbides in the total carbides with an equivalent circular diameter of 0.5 to 5.0 μm is 30% or more. The amount of dissolved carbon in the matrix phase is 0.80% or less by mass. [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. (V+2Mo) / (Cr+Ni)≧0.400 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element in the chemical composition. [Effects of the Invention]

[0012] The forged steel rolls of this disclosure have sufficient wear resistance and can sufficiently suppress the occurrence of cracks. [Brief explanation of the drawing]

[0013] [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]

[0014] The inventors first investigated means of improving the wear resistance of forged steel rolls from the perspective of chemical composition. As a result, the inventors 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:0.20~1.00%, V:1.00~2.00%, Cu:0.40% or less, Ni:0.30~0.60%, Ti:0~0.050%, Nb:0~0 We considered that a chemical composition containing 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, could potentially provide sufficient wear resistance.

[0015] However, even forged steel rolls that met the above-mentioned chemical composition sometimes failed to provide sufficient wear resistance. Therefore, the inventors investigated the cause of the lack of sufficient wear resistance by focusing on the carbides contained in the forged steel rolls. As a result, the following findings were obtained.

[0016] Generally, carbides are harder than the matrix phase of a forged steel roll. Here, the matrix phase of a forged steel roll refers to the iron-dominant phase in the microstructure of the forged steel roll, excluding precipitates and inclusions. The carbides contained in a forged steel roll having the above chemical composition mainly consist of MC-type carbides, which are mainly composed of V and Mo, and M7C3-type carbides, which are mainly composed of Cr. MC-type carbides are even harder than M7C3-type carbides. Therefore, in order to improve the wear resistance of the forged steel roll, it is preferable to obtain MC-type carbides preferentially over M7C3-type carbides.

[0017] To preferentially obtain MC-type carbides over M7C3-type carbides, it is effective to adjust the content of V, Mo, and Cr, which are components of these carbides. Furthermore, in the steelmaking process of the manufacturing process of forged steel rolls, Ni remains in the liquid phase along with Cr during the solidification process of molten steel, promoting the crystallization of M7C3-type carbides. Based on these findings, the inventors investigated the relationship between V content, Mo content, Cr content, and Ni content. As a result, the inventors found that in forged steel rolls having the above chemical composition, if equation (1) is also satisfied, MC-type carbides are preferentially obtained over M7C3-type carbides, and wear resistance is improved. (V+2Mo) / (Cr+Ni)≧0.400 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element in the chemical composition.

[0018] To further enhance wear resistance, it is effective to increase the area ratio of MC-type carbides on the surface of the forged steel roll as much as possible. 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, even in forged steel rolls that did not exhibit sufficient wear resistance, observation of their microstructure revealed that a large number of coarse MC-type carbides were present 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 than 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, in order to improve the wear resistance of forged steel rolls, we considered it 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 sufficient 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, the inventors found that sufficient wear resistance can be obtained 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 in the surface layer of the body of a forged steel roll having the above chemical composition.

[0023] The inventors then investigated means to further suppress crack initiation in forged steel rolls that have sufficient wear resistance. As a result, they obtained the following findings.

[0024] In forged steel rolls having the chemical composition described above, 0.85 to 1.05% carbon by mass is contained to increase wear resistance by generating carbides. Therefore, even in forged steel rolls where sufficient carbides have been generated as described above, a large amount of carbon is dissolved in the matrix. When such forged steel rolls are subjected to thermal shock, the carbon dissolved in the matrix may be released as carbides. In the regions where carbon has been released, the lattice constant in the crystal structure decreases, and the volume shrinks. As a result, localized tensile stress is generated between the region where carbon has been released and the adjacent region where carbon has not been released. This tensile stress acts as a driving force, causing cracks to form in the forged steel roll.

[0025] Considering this mechanism, it may be possible to suppress crack initiation when thermal shock is applied by reducing the amount of dissolved carbon in the matrix phase of a forged steel roll. Therefore, the inventors investigated the relationship between the amount of dissolved carbon in the matrix phase of a forged steel roll and the susceptibility to crack initiation. As a result, the inventors found that crack initiation can be sufficiently suppressed by further adjusting the amount of dissolved carbon in the matrix phase to 0.80% or less by mass in the surface layer of the body of a forged steel roll having the above chemical composition.

[0026] The forged steel roll of this embodiment was completed based on the above technical concept and has the following configuration.

[0027] 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: 0.20-1.00% 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. Satisfying equation (1), On the surface of the body, The proportion of MC-type carbides in the total carbides with an equivalent circular diameter of 0.5 to 5.0 μm is 30% or more. The amount of dissolved carbon in the matrix phase is 0.80% or less by mass. (V+2Mo) / (Cr+Ni)≧0.400 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element in the chemical composition.

[0028] 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: 0.20-1.00% 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. Satisfying equation (1), On the surface of the body, The proportion of MC-type carbides in the total carbides with an equivalent circular diameter of 0.5 to 5.0 μm is 30% or more. The amount of dissolved carbon in the matrix phase is 0.80% or less by mass. [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. (V+2Mo) / (Cr+Ni)≧0.400 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element in the chemical composition.

[0029] The third configuration of the forged steel roll is A forged steel roll of the second configuration, The chemical composition contains the first group.

[0030] 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.

[0031] The fifth configuration of the forged steel roll is, A forged steel roll having one of the first to fourth configurations, On the surface of the body of the forged steel roll, The amount of dissolved carbon in the matrix phase is 0.65% or more by mass.

[0032] 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.

[0033] [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.

[0034] 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.

[0035] [Features of the forged steel roll of this embodiment] The forged steel roll of this embodiment satisfies the following features 1 to 4. (Feature 1) The chemical composition, in mass%, is 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: 0.20-1.00%, V: 1.00-2.00%, Cu: 0.40% or less, Ni: 0.30-0.60%, Ti: 0-0.0% It contains 50% Nb: 0-0.050%, B: 0-0.0100%, W: 0-0.50%, Co: 0-0.50%, Sn: 0-0.10%, Sb: 0-0.05%, As: 0-0.05%, Zr: 0-0.05%, Bi: 0-0.10%, Se: 0-0.10%, Te: 0-0.05%, Pb: 0-0.09%, Ca: 0-0.0050%, and Mg: 0-0.0050%, with the remainder being Fe and impurities. (Feature 2) The chemical composition satisfies the following equation (1). (V+2Mo) / (Cr+Ni)≧0.400 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element in the chemical composition. (Feature 3) 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 4) In the surface layer of the body, the amount of dissolved carbon in the matrix phase is 0.80% or less by mass. Features 1 through 4 are explained below.

[0036] [(Feature 1) Regarding chemical composition] The forged steel roll of this embodiment contains the following elements:

[0037] 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 0.95%. The preferred upper limit for the C content is 1.03%, more preferably 1.00%, and even more preferably 0.98%.

[0038] 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%.

[0039] 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%.

[0040] 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%.

[0041] 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%.

[0042] 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.

[0043] 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 0.003%, and more preferably 0.005%. The preferred upper limit for the N content is 0.015%, more preferably 0.010%, and even more preferably 0.008%.

[0044] 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%.

[0045] Cr: 4.00~6.00% Chromium (Cr) generates 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 chromium content exceeds 6.00%, coarse carbides are formed. In this case, the toughness 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%.

[0046] Mo: Less than 0.20-1.00% Molybdenum (Mo) generates MC-type carbides, thereby 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 0.20%, the above effects cannot be fully obtained. On the other hand, if the Mo content is 1.00% or higher, coarse carbides will be formed. In this case, the toughness of the forged steel roll will decrease. Therefore, the Mo content is less than 0.20-1.00%. The preferred lower limit for the Mo content is 0.25%, more preferably 0.30%, and even more preferably 0.35%. The preferred upper limit for the Mo content is 0.99%, more preferably 0.95%, even more preferably 0.90%, and even more preferably 0.85%.

[0047] V: 1.00~2.00% Vanadium (V) generates MC-type carbides, thereby 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 will not be sufficiently obtained. On the other hand, if the V content exceeds 2.00%, coarse carbides are formed. In this case, the toughness 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%.

[0048] 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 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%.

[0049] 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%.

[0050] 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.

[0051] [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.

[0052] [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.

[0053] 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%.

[0054] 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%.

[0055] 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%.

[0056] 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%.

[0057] 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%.

[0058] [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.

[0059] 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%.

[0060] 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%.

[0061] 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%.

[0062] 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%.

[0063] 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 present, i.e., if the Bi content is greater than 0%, Bi improves the grindability of forged steel rolls. Even a small amount of Bi will provide some degree of the above effect. 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%.

[0064] 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%.

[0065] 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%.

[0066] 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%.

[0067] 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%.

[0068] 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%.

[0069] [Regarding (Feature 2) Equation (1)] The forged steel roll of this embodiment further satisfies formula (1). (V+2Mo) / (Cr+Ni)≧0.400 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element in the chemical composition.

[0070] Fn1 is defined as follows: Fn1 = (V + 2Mo) / (Cr + Ni)

[0071] Fn1 corresponds to the left side of equation (1). Fn1 is an index for preferentially generating MC type carbides over M7C3 type carbides in forged steel rolls that satisfy characteristic 1. As mentioned above, MC type carbides are harder than M7C3 type carbides. Therefore, in order to improve the wear resistance of forged steel rolls, it is preferable to preferentially generate MC type carbides over M7C3 type carbides. By increasing the content of V and Mo, which generate MC type carbides, relative to the Cr content that generates M7C3 type carbides and the Ni content that promotes the generation of M7C3 type carbides, it is possible to preferentially generate MC type carbides over M7C3 type carbides.

[0072] If Fn1 is less than 0.400, the V and Mo content is too low compared to the Cr and Ni content. In this case, M7C3 type carbides precipitate preferentially over MC type carbides. Therefore, the proportion of hard MC type carbides in the total carbides becomes small. As a result, even forged steel rolls that satisfy features 1 and 3 cannot obtain sufficient wear resistance.

[0073] If Fn1 is 0.400 or higher, MC type carbides are preferentially formed over M7C3 type carbides in the forged steel roll. As a result, assuming that the forged steel roll satisfies features 1 and 3, sufficient wear resistance can be obtained.

[0074] A preferred lower limit for Fn1 is 0.405, more preferably 0.410, and even more preferably 0.415. The upper limit of Fn1 is not particularly limited. If the forged steel roll satisfies features 1, 3, and 4, the upper limit of Fn1 is, for example, 0.800.

[0075] [(Feature 3) Regarding the Quantity Ratio NR] 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.

[0076] 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.

[0077] 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, sufficient wear resistance cannot be obtained.

[0078] 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 features 1 and 2, sufficient wear resistance can be obtained. Therefore, in the forged steel roll of this embodiment, the number ratio NR is 30% or more.

[0079] 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 percentage NR. If the forged steel roll satisfies features 1, 2, and 4, the upper limit to the number percentage NR is, for example, 90%.

[0080] [Method for measuring the number of items (NR)] The number proportion NR can be measured by the following method. 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] [(Feature 4) Regarding the amount of dissolved carbon in the matrix phase] In the forged steel roll of this embodiment, the amount of dissolved carbon in the matrix phase in the surface layer of the body is 0.80% or less by mass.

[0086] The amount of dissolved carbon in the matrix phase at the surface of the body is the amount of dissolved carbon [C]. S Defined as (mass%). As mentioned above, when thermal shock is applied to a forged steel roll, carbon dissolved in the matrix may be released as carbides. As a result, tensile stress is generated in the forged steel roll, promoting crack initiation. Solid solution carbon content [C] SIf it exceeds 0.80%, precipitation of carbides due to thermal shock is likely to occur. In this case, the occurrence of cracks in the forged steel roll cannot be sufficiently suppressed.

[0087] On the other hand, the amount of carbon in solid solution [C] S If it is 0.80% or less, even if a thermal shock is applied to the forged steel roll, carbides are unlikely to precipitate. In this case, the generation of tensile stress in the forged steel roll is suppressed. As a result, on the premise that the forged steel roll satisfies Feature 1, the occurrence of cracks can be sufficiently suppressed. Therefore, in the forged steel roll of the present embodiment, the amount of carbon in solid solution [C] S is 0.80% or less.

[0088] The amount of carbon in solid solution [C] S The preferable upper limit is 0.78%, more preferably 0.76%, and even more preferably 0.74%.

[0089] In the forged steel roll of the present embodiment, further, on the surface layer of the body part of the forged steel roll, it is preferable that the amount of carbon in solid solution in the matrix phase is 0.65% or more in mass%.

[0090] The higher the amount of carbon in solid solution in the matrix phase, the higher the hardness of the matrix phase due to solid solution strengthening. On the surface layer of the body part of the forged steel roll, if the hardness of the matrix phase increases, wear of the matrix phase is suppressed, and thus shedding of hard carbides is also suppressed. In this way, the wear resistance of the forged steel roll is further enhanced. If the amount of carbon in solid solution [C] S is 0.65% or more, the matrix phase is sufficiently hard. As a result, in the forged steel roll of the present embodiment, excellent wear resistance can be obtained.

[0091] The amount of carbon in solid solution [C] S The more preferable lower limit is 0.66%, more preferably 0.67%, and even more preferably 0.68%.

[0092] [Measurement method of the amount of carbon in solid solution [C] S The amount of carbon in solid solution [C] S can be measured by the following method. ​A rectangular parallelepiped specimen measuring 20 mm × 15 mm × 2 mm is taken from the surface of the body of the forged steel roll. The direction parallel to the 20 mm side of the specimen is defined as the long side direction, the direction parallel to the 15 mm side is defined as the short side direction, and the direction parallel to the 2 mm side is defined as the thickness direction. The surface containing the 20 mm side and the 15 mm side of the specimen is defined as the bottom surface. The long side direction of the specimen is parallel to the axial direction of the body of the forged steel roll. The bottom surface of the specimen is perpendicular to the radial direction of the body at the center of the bottom surface. The center of the specimen is the axial center of the body, which coincides with a position 2 mm radially deep from the surface of the body.

[0093] Constant current electrolysis is performed on the collected test specimens using a 10% AA system solution (a mixed solution of 10% by volume of acetylacetone and 1% by mass of tetramethylammonium chloride / methanol solution). Specifically, the above-mentioned 10% AA system solution is prepared. Then, approximately 1 g of the test specimen is subjected to constant current electrolysis using the 10% AA system solution at room temperature. The current density during constant current electrolysis is 20 mA / cm². 2 The procedure is as follows: After constant current electrolysis, the test specimen is immersed in an alcohol solution and ultrasonic cleaning is performed. The 10% AA solution used for constant current electrolysis and the alcohol solution used for subsequent ultrasonic cleaning are filtered through a silver membrane filter with a pore size of 0.2 μm, and the residue is collected.

[0094] The carbon concentration of the recovered residue will be measured using the combustion-infrared absorption method based on JIS G 1211-3 (2018). For the combustion-infrared absorption method, for example, a device such as the CSLS600 manufactured by LECO Corporation can be used. The measured carbon concentration in the residue in mass % will be subtracted from the carbon content in the chemical composition of the forged steel roll to determine the dissolved carbon content [C]. S This is defined as follows. Note that the amount of dissolved carbon [C] S This value is rounded to two decimal places, obtained by rounding the third decimal place of the obtained value.

[0095] [Effects of the forged steel roll of this embodiment] The forged steel roll of this embodiment satisfies features 1 to 4. Therefore, the forged steel roll of this embodiment has sufficient wear resistance and can sufficiently suppress crack initiation.

[0096] [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.

[0097] [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.

[0098] 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.

[0099] 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.

[0100] 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)

[0101] [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).

[0102] [Method for manufacturing forged steel rolls] An example of a manufacturing method for the forged steel roll of this embodiment will be described. The forged steel roll of this embodiment may be manufactured by a manufacturing method other than the one described below. However, the manufacturing method described below is a preferred example of a manufacturing method for the forged steel roll of this embodiment.

[0103] 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.

[0104] [(Process 1) Steelmaking Process] In the steelmaking process, ingots are manufactured using molten steel that satisfies features 1 and 2, by a known casting method. Known casting methods include, for example, the bottom-pouring ingot method. The cast ingots (electrode ingots) are used as electrodes, and the electroslag remelting (ESR) method is performed.

[0105] 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 features 1 and 2, 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.

[0106] 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.

[0107] 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 can be obtained in which the generation of coarse MC-type carbides and MC-type carbide-deficient regions is suppressed. For this reason, the ESR method is implemented in the steelmaking process of the forging rolls in this embodiment.

[0108] 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.

[0109] [Regarding Condition 1] Here, the solidification rate SR in the ESR method 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. However, 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. Furthermore, if the solidification rate SR in the ESR method is too slow, the growth of MC-type carbides is 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.

[0110] If the solidification rate SR in the ESR method is 3.0 mm / min or higher, the excessive growth of MC-type carbides can be suppressed. In this case, assuming that the manufacturing process satisfies condition 3 described later, it is possible to adjust the ratio NR of the number of forged steel rolls produced to 30% or higher. Therefore, the coagulation rate (SR) in the ESR method should be 3.0 mm / min or higher.

[0111] [(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. Hot forging is then performed on the heated forging ingot. If the temperature of the forging ingot decreases during hot forging, the forging ingot may be heated again in the heating furnace. Then, hot forging may be resumed on the reheated forging ingot. In this way, a rough-shaped roll material (hereinafter referred to as intermediate material) is manufactured.

[0112] The hot forging process satisfies conditions 2 and 3 below. (Condition 2) The heating temperature T1 should be 1100°C or lower. (Condition 3) The cumulative time t1 at temperatures between 1000 and 1100°C is 30 hours or less. Conditions 2 and 3 will be explained below.

[0113] [Regarding Condition 2] In forging ingots obtained by an ESR process that satisfies condition 1, the formation of coarse carbides is suppressed. When such forging ingots are heated to temperatures exceeding 1100°C, the dissolution of carbides in the forging ingot into the matrix is ​​excessively promoted. In this case, the amount of dissolved carbon [C] in the forging rolls produced is increased. S It is not possible to adjust it to below 0.80%.

[0114] In the hot forging process, the temperature of the heating furnace used to heat the forging ingot is defined as the heating temperature T1 (°C). If the heating temperature T1 in the hot forging process is 1100°C or lower, excessive dissolution of carbides into the matrix can be suppressed. In this case, assuming that the quenching process satisfies condition 4 described later, the amount of dissolved carbon [C] in the forged steel rolls produced is... S It is possible to adjust this to 0.80% or less. Therefore, the heating temperature T1 should be 1100°C or lower. Furthermore, when reheating a forging ingot in a heating furnace during hot forging, the heating temperature T1 must also be 1100°C or lower. The lower limit of the heating temperature T1 (°C) is not particularly limited. Considering typical industrial production, the lower limit of the heating temperature T1 is, for example, 1000°C.

[0115] [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 t1 (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 characteristics 1 and 2, 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 t1 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.

[0116] 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 t1 at 1000-1100°C during the hot forging process.

[0117] If the cumulative time t1 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. In this case, assuming that the steelmaking process satisfies condition 1, it is possible to adjust the ratio NR of the number of forged steel rolls produced to 30% or more. Therefore, the cumulative time t1 at 1000-1100°C during the hot forging process is 30 hours or less. The cumulative time t1 at 1000-1100°C is not particularly limited. Considering typical industrial production, the lower limit of the cumulative time t1 at 1000-1100°C is, for example, 8 hours.

[0118] [(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.

[0119] [(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, for example, grinding. Rough machining can be performed under well-known conditions.

[0120] [(Step 5) Hardening process] In the hardening process, high-frequency induction hardening is performed on the surface layer of the intermediate material after the rough machining process using a high-frequency induction hardening apparatus. The high-frequency induction hardening apparatus comprises, for example, a well-known annular high-frequency induction heating device (hereinafter also simply referred to as the "heating device") and a well-known annular cooling device arranged coaxially with the heating device. In high-frequency induction hardening, the intermediate material, which is arranged coaxially with the annular heating and cooling devices, is moved axially and passed continuously through the heating and cooling devices.

[0121] The quenching process satisfies the following condition 4. (Condition 4) When the heating temperature in the quenching process is T2 (°C) and the holding time at heating temperature T2 is t2 (hours), the quenching parameter C, defined by equation (A), is 26000 or less. C=(T2+273.15)×(log(t2)+20) (A) Note that in equation (A), log refers to the common logarithm with base 10. The following explains condition 4.

[0122] [Regarding Condition 4] The quenching parameter C is an indicator of the amount of heat imparted to the intermediate material during the quenching process. In the quenching process, a larger quenching parameter C results in a greater amount of heat being imparted to the intermediate material. When the quenching parameter C exceeds 26000, the dissolution of carbides in the intermediate material into the matrix is ​​excessively promoted during the quenching process. In this case, regardless of the tempering process conditions, the amount of dissolved carbon [C] in the manufactured forged steel roll is significantly increased. S It is not possible to adjust it to below 0.80%.

[0123] If the quenching parameter C is 26000 or less, excessive dissolution of carbides into the matrix can be suppressed. In this case, assuming that the hot forging process satisfies condition 2, the amount of dissolved carbon [C] in the forged steel rolls produced is S It is possible to adjust this to 0.80% or less. Therefore, the hardening parameter C shall be 26000 or less. The heating temperature T2 (°C) shall be the temperature of the heating device provided in the high-frequency induction hardening apparatus. The holding time t2 (hours) at heating temperature T2 shall be the time from when one end of the intermediate material overlaps with the annular heating device, when observed radially, until it no longer overlaps with the heating device.

[0124] In the manufacturing process of the forged steel roll of this embodiment, it is further preferable that the quenching parameter C is 23500 or higher.

[0125] If the quenching parameter C is 23500 or higher, the carbides in the intermediate material can be dissolved into the matrix to some extent. In this case, the amount of dissolved carbon [C] in the forging roll S It is possible to adjust this to 0.65% or higher.

[0126] [(Step 6) Tempering process] In the tempering process, tempering is performed on the intermediate material after the quenching process. Tempering reduces the amount of retained austenite remaining after quenching. Furthermore, tempering adjusts the hardness of the surface layer of the forged steel roll. The tempering temperature is, for example, 100 to 200°C. Sub-zero treatment may be performed on the intermediate material after the quenching process but before the tempering process. The cooling temperature for sub-zero treatment can be within a well-known range, for example, -30 to -196°C.

[0127] [(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. Through this finishing process, the intermediate material is shaped into the final product form.

[0128] The forged steel roll of this embodiment is manufactured through the above process. [Examples]

[0129] The effects of the forged steel roll of this embodiment will be further explained in detail by the following examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effects of the forged steel roll of this embodiment. Therefore, the forged steel roll of this embodiment is not limited to this one example of conditions.

[0130] Forged steel rolls having the chemical compositions shown in Tables 1A and 1B were manufactured by the following method.

[0131] [Table 1A]

[0132] [Table 1B]

[0133] Specifically, electrode ingots were cast from molten steel using the bottom-pouring ingot casting method. The manufactured electrode ingots were then used as electrodes for the electroslag remelting (ESR) method. The solidification rate SR (mm / min) in the ESR method is shown in Table 2.

[0134] [Table 2]

[0135] A hot forging process was performed on the forging ingots obtained by the ESR method. The heating temperature T1 (°C) and the cumulative time t1 (hours) at 1000-1100°C during the hot forging process are shown in Table 2. Through the hot forging process, intermediate raw materials with a roll shape, a roll body diameter of φ700 mm, a roll body length of 2100 mm, and an overall length of 4100 mm were manufactured for each test number.

[0136] 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, grinding 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.

[0137] A quenching process was performed on the intermediate raw material after the rough machining process. The quenching process was carried out using a high-frequency induction hardening apparatus in which a circular high-frequency induction heating device and a circular cooling device were arranged coaxially. The heating temperature T2 (°C), the holding time t2 (hours) at heating temperature T2, and the quenching parameter C calculated from these are shown in Table 2. The cooling method used in the cooling device provided with the high-frequency induction hardening apparatus was water cooling.

[0138] Sub-zero treatment was performed on the intermediate raw material after the quenching process. In the sub-zero treatment, the intermediate raw material was cooled to -60 to -140°C. After the sub-zero treatment, the intermediate raw material was tempered at 100 to 200°C, and then the finishing process was performed. In the finishing process, the intermediate raw material was ground to produce the 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 the forged steel rolls for each test number 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 of the forged steel rolls for all test numbers was 85% or more.

[0139] [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) Solid solution carbon content [C] S Measurement test (Test 3) Abrasion Resistance Evaluation Test (Test 4) Crack Generation Inhibition Ability Evaluation Test The following describes each test.

[0140] [(Test 1) Measurement Test of Number Ratio NR] Based on the method described in the above [Measurement Method of Number Ratio NR], the number ratio NR of MC-type carbides in all carbides with an equivalent circle diameter of 0.5 to 5.0 μm on the surface layer of the forged steel roll of each test number was obtained. The obtained number ratio NR is shown in the column of "Number Ratio NR (%)" in Table 3.

[0141] [Table 3]

[0142] [(Test 2) Measurement Test of Solute Carbon Content [C]] S Based on the method described in the above [Measurement Method of Solute Carbon Content [C]] S , the solute carbon content [C] in the matrix phase on the surface layer of the forged steel roll of each test number was obtained. S The obtained solute carbon content [C] S is shown in the column of "Solute Carbon Content [C] S (%)" in Table 3.

[0143] [(Test 3) Abrasion Resistance Evaluation Test] The abrasion resistance of the forged steel roll of each test number was evaluated using a two-cylinder rolling wear test machine. Figure 1 is a schematic diagram of the two-cylinder rolling wear test machine 10. A cylindrical roll test piece 12 was taken from the surface layer of the forged steel roll of each test number (the region from the surface of the barrel part to a depth of 40 mm in the depth direction). The diameter of the roll test piece 12 was 40 mm, and the width was 10 mm. The central axis of the roll test piece 12 was parallel to the radial direction of the forged steel roll. The outer peripheral surface of the roll test piece 12 was polished.

[0144] ​Using the taken roll test piece 12, a two-cylinder type rolling wear test was carried out. In the two-cylinder type rolling wear test, a rolling material test piece 11 rotating in the reverse direction was pressed against the rotating roll test piece 12 with a load F described later. A front view of the rolling material test piece 11 is shown in Fig. 2. The numerical values in Fig. 2 indicate dimensions (unit: mm). "R7.5" in Fig. 2 indicates that the radius of curvature of the outer peripheral surface was 7.5 mm. As shown in Fig. 2, the diameter of the rolling material test piece 11 was 160 mm and the width was 15 mm. The rolling material test piece 11 was made by processing a steel material having a chemical composition corresponding to S45C specified in JIS G 4051 (2018) into the shape shown in Fig. 2 and polishing the outer peripheral surface.

[0145] In the two-cylinder type rolling wear test, the contact load between the roll test piece 12 and the rolling material test piece 11 was 700 N, and no lubricant was used. The rotational speed of the roll test piece 12 was set to 2000 rpm, and the slip ratio between the roll test piece 12 and the rolling material test piece 11 was set to 5%. The slip ratio (%) is defined by the following formula. Slip ratio (%) = (Peripheral speed of rolling material test piece - Peripheral speed of roll test piece) / Peripheral speed of roll test piece × 100 In the two-cylinder type rolling wear test, the rolling material test piece 11 was maintained at 200 °C by high-frequency induction heating using an induction heating coil not shown. Also, the roll test piece was water-cooled for cooling. The number of test cut-off times was set to 20,000 times.

[0146] For the roll test piece 12 after 20,000 rolling times, the wear shape of the outer peripheral surface was measured with a laser microscope (manufactured by Keyence Corporation, product name: Shape Analysis Laser Microscope VK-X250). Specifically, at an arbitrary position on the outer peripheral surface of the roll test piece 12, a surface shape scan was performed in the width direction. The scanned range was the entire length in the width direction of the outer peripheral surface. From the obtained surface shape profile, the wear cross-sectional area (the area of the depression generated by the wear of the outer peripheral surface in the cross section including the width direction and the radial direction of the roll test piece 12) was calculated. The same wear cross-sectional area measurement was also performed on the outer peripheral surface at positions directly opposite to each other across the center of the roll test piece 12. The arithmetic mean value of the wear cross-sectional areas obtained from the two measurements was taken as the wear cross-sectional area of the roll test piece 12.

[0147] The wear cross-sectional area of ​​the roll test piece 12 was 3500 μm². 2 The following conditions were met: The evaluation was given as "G (Good)," indicating that sufficient wear resistance was achieved (indicated as "G" in the "Wear Resistance" column of Table 3). Furthermore, the wear cross-sectional area of ​​the roll test piece 12 was 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 3). On the other hand, the wear cross-sectional area of ​​roll test piece 12 was 3500 μm². 2 If the result was "B" (Bad), it was determined that sufficient wear resistance was not achieved (indicated as "B" in the "Wear Resistance" column in Table 3).

[0148] [(Test 4) Crack Initiation Suppression Performance Evaluation Test] The extent to which crack initiation was suppressed in the forged steel rolls for each test number was evaluated using the following method. A rectangular parallelepiped specimen measuring 20 mm × 15 mm × 2 mm was taken from the surface layer of the body of each forged steel roll for each test number. The direction parallel to the 20 mm side of the specimen was defined as the long side direction, the direction parallel to the 15 mm side as the short side direction, and the direction parallel to the 2 mm side as the thickness direction. Of the two faces of the specimen that include the 20 mm side and the 15 mm side, the face furthest from the central axis of the body was designated as the evaluation surface. The long side direction of the specimen was parallel to the axial direction of the body of the forged steel roll. The evaluation surface of the specimen was perpendicular to the radial direction of the body at the center of the evaluation surface. The center position of the specimen was the axial center of the body, coinciding with a position 2 mm radially deep from the surface of the body.

[0149] The evaluation surface of the collected test specimen was polished to a mirror finish by buffing. To simulate the thermal shock applied to the forging roll by slip, a thermocouple was attached to the mirror-finished evaluation surface and a laser was irradiated onto it. The laser was scanned 15 mm in the short-side direction from the center of one long side of the evaluation surface to the center of the other long side. The laser irradiation conditions were: irradiation power of 0.6 kW, focusing diameter of 0.9 mm, scanning speed of 15 m / min, and defocus (DF) of 50 mm. Argon was used as the shielding gas. By irradiating with a laser under these conditions, the laser-irradiated area of ​​the evaluation surface was heated to 500°C ± 50°C.

[0150] The laser-irradiated area on the evaluation surface was observed at 50x magnification using an optical microscope to identify cracks. A person skilled in the art could easily identify cracks generated in the laser-irradiated area under these conditions. If the number of identified cracks was 10 or less, the evaluation was "E (Excellent)," indicating that crack generation was sufficiently suppressed (indicated as "E" in the "Crack Generation Suppression Ability" column in Table 3). If the number of identified cracks was 11 or more, the evaluation was "B (Bad)," indicating that crack generation was not sufficiently suppressed (indicated as "B" in the "Crack Generation Suppression Ability" column in Table 3).

[0151] [Test Results] Referring to Tables 1A, 1B, 2, and 3, the forged steel rolls for test numbers 1 to 21 met features 1 to 4. Therefore, sufficient wear resistance was obtained, and crack initiation was sufficiently suppressed. In particular, for test numbers 1 to 19, the amount of dissolved carbon [C] S The percentage was 0.65% or higher. As a result, excellent wear resistance was achieved.

[0152] On the other hand, in tests 22 and 23, Fn1 was too low. As a result, sufficient wear resistance could not be obtained.

[0153] In tests 24 and 25, the solidification rate SR in the ESR method used in the steelmaking process was too slow. As a result, the number percentage NR (%) was too low. Consequently, sufficient wear resistance could not be obtained.

[0154] In tests 26 and 27, the heating temperature T1 in the hot forging process was too high. Therefore, the amount of dissolved carbon [C] S The pressure was too high. As a result, crack initiation could not be adequately suppressed.

[0155] In tests 28 and 29, the cumulative time t at 1000-1100°C during the hot forging process was too long. As a result, the number of defective pieces NR (%) was too low. Consequently, sufficient wear resistance was not achieved.

[0156] In tests 30 and 31, the quenching parameter C in the quenching process was too high. Therefore, the amount of dissolved carbon [C] S The pressure was too high. As a result, crack initiation could not be adequately suppressed.

[0157] 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: 0.20 to less than 1.00%, V: 1.00-2.00%, Cu: 0.40% or less, It contains Ni: 0.30-0.60%, The remainder consists of Fe and impurities. Satisfying equation (1), In the surface layer of the aforementioned body, The number proportion of MC-type carbides in the total carbides with an equivalent circular diameter of 0.5 to 5.0 μm is 30% or more. The amount of dissolved carbon in the matrix phase is 0.80% or less by mass. Forged steel roll. (V+2Mo) / (Cr+Ni)≧0.400 (1) Here, each element symbol in formula (1) is substituted with the mass percentage content of the corresponding element in the aforementioned chemical composition.

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: 0.20 to less than 1.00%, 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. Satisfying equation (1), In the surface layer of the aforementioned body, The number proportion of MC-type carbides in the total carbides with an equivalent circular diameter of 0.5 to 5.0 μm is 30% or more. The amount of dissolved carbon in the matrix phase is 0.80% or less by mass. 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. (V+2Mo) / (Cr+Ni)≧0.400 (1) Here, each element symbol in formula (1) is substituted with the mass percentage content of the corresponding element in the aforementioned chemical composition.

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.

5. A forging roll according to any one of claims 1 to 4, On the surface layer of the body of the forged steel roll, The amount of dissolved carbon in the matrix phase is 0.65% or more by mass. Forged steel roll.

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