Forged steel rolls

Abstract

To provide a forged steel roll that can sufficiently suppress the occurrence of cracks. [Solution] The forged steel roll of this disclosure has a chemical composition in mass%, containing C: 0.85-1.05%, Si: 0.60-1.20%, Mn: 0.30-0.60%, P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00-6.00%, Mo: 1.00-2.50%, V: 1.00-2.00%, Cu: 0.40% or less, Ni: 0.30-0.60%, with the remainder being Fe and impurities, and the amount of solid dissolved carbon in the matrix phase in the surface layer of the body is 0.80% or less in mass%.

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] Furthermore, when slippage occurs, thermal shock is applied to the surface of the forging roll due to friction. 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.

[0004] 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 to sufficiently suppress the occurrence of cracks.

[0005] A technique for suppressing crack initiation in forged steel rolls has been proposed, for example, in International Publication No. 2023 / 012906 (Patent Document 1).

[0006] In the forged steel roll disclosed in Patent Document 1, the Vickers hardness Hv at 400°C is adjusted to 400 or higher. Patent Document 1 states that by increasing the resistance to stress applied to the forged steel roll due to thermal shock in this way, crack initiation can be suppressed. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] International Publication No. 2023 / 012906 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] However, crack formation in the forged steel roll may be suppressed by means other than those described in Patent Document 1.

[0009] The purpose of this disclosure is to provide a forged steel roll that can sufficiently suppress the occurrence of cracks. [Means for solving the problem]

[0010] The forged steel rolls of this disclosure are A cylindrical body, It comprises a pair of shafts, The chemical composition of the forged steel roll is, in mass%, C: 0.85~1.05%, Si: 0.60~1.20%, Mn: 0.30~0.60%, P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00~6.00%, Mo: 1.00~2.50%, V: 1.00~2.00%, Cu: not more than 0.40%, and Ni: 0.30 - 0.60%, and contains the balance consists of Fe and impurities, in the surface layer of the barrel part, the amount of carbon dissolved in the matrix phase is not more than 0.80% by mass.

[0011] The forged steel roll of the present disclosure has a cylindrical barrel part and a pair of shaft parts, and the chemical composition of the forged steel roll is, by mass, C: 0.85 - 1.05%, Si: 0.60 - 1.20%, Mn: 0.30 - 0.60%, P: not more than 0.020%, S: not more than 0.020%, Al: not more than 0.050%, N: not more than 0.020%, O: not more than 0.0050%, Cr: 4.00 - 6.00%, Mo: 1.00 - 2.50%, V: 1.00 - 2.00%, Cu: not more than 0.40%, and Ni: 0.30 - 0.60%, and contains further contains one or more selected from the group consisting of Group 1 and Group 2, the balance consists of Fe and impurities, in the surface layer of the barrel part, the amount of carbon dissolved in the matrix phase is not more than 0.80% by mass. [Group 1] Ti: not more than 0.050%, Nb: not more than 0.050%, B: not more than 0.0100%, W: not more than 0.50%, and Co: not more than 0.50%, one or more selected from the group consisting of [Group 2] Sn: not more than 0.10%, Sb: not more than 0.05%, As: not more than 0.05% Zr: 0.05% or less, Bi: 0.10% or less, Se: 0.10% or less, Te: 0.05% or less, Pb: 0.09% or less, Ca: 0.0050% or less, and, One or more selected from the group consisting of Mg: 0.0050% or less. [Effects of the Invention]

[0012] The forged steel roll of this disclosure 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 suppressing crack initiation in 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:1.00~2.50%, V:1.00~2.00%, Cu:0.40% or less, Ni:0.30~0.60%, Ti:0~0.050%, Nb:0~0.0 We considered that a chemical composition containing 50% 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 sufficiently suppress crack initiation.

[0015] However, even with forged steel rolls that met the above-mentioned chemical composition, crack initiation was sometimes not sufficiently suppressed. Therefore, the inventors investigated the reasons why crack initiation could not be sufficiently suppressed, focusing on the crack initiation mechanism. As a result, the following findings were obtained.

[0016] In forged steel rolls having the chemical composition described above, 0.85 to 1.05% carbon by mass is included to increase the hardness of the surface layer. Therefore, a large amount of carbon is dissolved in the matrix phase of the forged steel roll. When such a forged steel roll is subjected to thermal shock, the carbon dissolved in the matrix phase 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.

[0017] In light of this mechanism, it is considered effective in suppressing crack initiation by inhibiting the release of dissolved carbon from the matrix phase when thermal shock is applied, thereby suppressing the generation of localized tensile stress. Furthermore, in order to suppress the release of carbon from the matrix phase due to thermal shock, it is preferable to reduce the amount of dissolved carbon in the matrix phase.

[0018] Based on the above findings, 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 by 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, crack initiation can be sufficiently suppressed.

[0019] The forged steel roll of this embodiment was completed based on the above technical concept and has the following configuration.

[0020] The forged steel roll of the first configuration is A cylindrical body, It comprises a pair of shafts, The chemical composition of the forged steel roll is, in mass%, C: 0.85~1.05%, Si: 0.60~1.20%, Mn: 0.30~0.60%, P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00~6.00%, Mo: 1.00~2.50%, V: 1.00~2.00%, Cu: 0.40% or less, and, It contains Ni: 0.30~0.60%, The remainder consists of Fe and impurities. On the surface of the body, The amount of dissolved carbon in the matrix phase is 0.80% or less by mass.

[0021] The second configuration of the forged steel roll is A cylindrical body, It comprises a pair of shafts, The chemical composition of the forged steel roll is, in mass%, C: 0.85~1.05%, Si: 0.60~1.20%, Mn: 0.30~0.60%, P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00~6.00%, Mo: 1.00~2.50%, V: 1.00~2.00%, Cu: 0.40% or less, and, It contains Ni: 0.30~0.60%, Furthermore, it contains one or more selected from the group consisting of Group 1 and Group 2, The remainder consists of Fe and impurities. On the surface of the body, 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, and, One or more selected from the group consisting of Mg: 0.0050% or less.

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

[0023] The fourth configuration of the forged steel roll is, A forged steel roll having a second or third configuration, The chemical composition contains the second group of substances.

[0024] 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.

[0025] 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.

[0026] [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.

[0027] 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. The forged steel roll is used while repeatedly grinding its 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 come into contact with the rolled material as the surface of the body.

[0028] [Features of the forged steel roll of this embodiment] The forged steel roll of this embodiment satisfies the following features 1 and 2. (Feature 1) The chemical composition, in mass%, is as follows: C: 0.85~1.05%, Si: 0.60~1.20%, Mn: 0.30~0.60%, P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00~6.00%, Mo: 1.00~2.50%, V: 1.00~2.00%, Cu: 0.40% or less, Ni: 0.30~0.60%, Ti: 0~0.05% It contains 0%, Nb: 0-0.050%, B: 0-0.0100%, W: 0-0.50%, Co: 0-0.50%, Sn: 0-0.10%, Sb: 0-0.05%, As: 0-0.05%, Zr: 0-0.05%, Bi: 0-0.10%, Se: 0-0.10%, Te: 0-0.05%, Pb: 0-0.09%, Ca: 0-0.0050%, and Mg: 0-0.0050%, with the remainder being Fe and impurities. (Feature 2) In the surface layer of the body, the amount of dissolved carbon in the matrix phase is 0.80% or less by mass. Features 1 and 2 are described below.

[0029] [(Feature 1) Regarding chemical composition] The forged steel roll of this embodiment contains the following elements:

[0030] 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%.

[0031] 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%.

[0032] 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%.

[0033] 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%.

[0034] 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%.

[0035] 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.

[0036] 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%.

[0037] 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%.

[0038] 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%.

[0039] Mo: 1.00~2.50% Molybdenum (Mo) forms MC-type carbides, improving the wear resistance of forged steel rolls. Furthermore, Mo increases the tempering softening resistance of the steel, thereby increasing the surface hardness of the forged steel rolls. If the Mo content is less than 1.00%, the above effects cannot be fully obtained. On the other hand, if the Mo content exceeds 2.50%, coarse MC-type carbides are formed. In this case, the toughness of the forged steel roll decreases. Therefore, the Mo content is 1.00 to 2.50%. The preferred lower limit for the Mo content is 1.05%, more preferably 1.10%, and even more preferably 1.15%. The preferred upper limit for the Mo content is 2.45%, more preferably 2.40%, and even more preferably 2.35%.

[0040] 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%.

[0041] Cu: 0.40% or less Copper (Cu) is an impurity. If the Cu content exceeds 0.40%, the hot workability of the steel material will decrease 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%.

[0042] 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%.

[0043] 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.

[0044] [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, and, One or more selected from the group consisting of Mg: 0.0050% or less. The following describes these arbitrary elements.

[0045] [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.

[0046] 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%.

[0047] 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%.

[0048] 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%.

[0049] 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%.

[0050] 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 the forged steel roll and increases the hardness of its surface. 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%.

[0051] [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.

[0052] 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%.

[0053] 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%.

[0054] 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%.

[0055] 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%.

[0056] Bi:0.10% or less Bismuth (Bi) is an optional element and does not need to be included. In other words, the Bi content may be 0%. If Bi is present, that is, if the Bi content is greater than 0%, Bi improves the grindability of forged steel rolls. Even if only a small amount of Bi is present, the above effect can be obtained to some extent. On the other hand, if the Bi content exceeds 0.10%, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material will decrease in the manufacturing process of the forged steel rolls. Therefore, the Bi content is 0-0.10%, and if present, the Bi content is 0.10% or less. The preferred lower limit for the Bi content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the Bi content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0057] 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%.

[0058] 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%.

[0059] 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%.

[0060] 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%.

[0061] 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%.

[0062] [(Feature 2) 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.

[0063] 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] S If the precipitation rate exceeds 0.80%, carbide precipitation due to thermal shock is likely to occur. In this case, crack initiation in the forged steel roll cannot be adequately suppressed.

[0064] On the other hand, the amount of dissolved carbon [C] S If the carbide content is 0.80% or less, carbide precipitation is less likely to occur even if thermal shock is applied to the forged steel roll. In this case, the generation of tensile stress in the forged steel roll is suppressed. As a result, assuming that the forged steel roll satisfies characteristic 1, crack initiation can be sufficiently suppressed. Therefore, in the forged steel roll of this embodiment, the amount of dissolved carbon [C] S The percentage is 0.80% or less.

[0065] Solute carbon content [C] SThe preferable upper limit is 0.78%, more preferably 0.76%, and even more preferably 0.74%.

[0066] In the forged steel roll of this embodiment, further, in the surface layer of the body of the forged steel roll, the amount of carbon in solid solution in the matrix phase is preferably 0.65% or more by mass%.

[0067] It is necessary to periodically grind the surface of the forged steel roll so that the roughness of the surface decreases and slip does not occur. However, if the number of grindings per usage time of the forged steel roll is large, the life of the forged steel roll will be shortened. That is, in order to extend the life of the forged steel roll, it is desirable not only to suppress the generation of cracks, but also to suppress the wear of the forged steel roll and reduce the number of grindings per usage time as much as possible. Therefore, sufficient wear resistance is required for the forged steel roll.

[0068] 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. In the surface layer of the body of the forged steel roll, if the hardness of the matrix phase increases, the wear of the matrix phase is suppressed. If the wear of the matrix phase is suppressed, the detachment of hard carbides is further suppressed. In this way, the wear resistance of the forged steel roll is enhanced. Amount of carbon in solid solution [C] S If it is 0.65% or more, the matrix phase is sufficiently hard. As a result, in the forged steel roll of this embodiment, sufficient wear resistance can be obtained.

[0069] Amount of carbon in solid solution [C] S A more preferable lower limit of is 0.66%, more preferably 0.67%, and even more preferably 0.68%.

[0070] [Measurement method of amount of carbon in solid solution [C] S 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.

[0071] 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.

[0072] 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.

[0073] [Effects of the forged steel roll of this embodiment] The forged steel roll of this embodiment satisfies features 1 and 2. Therefore, the forged steel roll of this embodiment can sufficiently suppress crack formation.

[0074] [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.

[0075] [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.

[0076] The area ratio of carbides is determined by the following FE-SEM (Field Emission Scanning Electron Microscope) observation. A specimen is taken from the surface of the body of the forged steel roll, including a plane perpendicular to the axial direction of the body of the forged steel roll as the observation surface. The observation surface includes a depth of 20 mm 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 depth of 20 mm from the surface of the body of the forged steel roll are observed with the 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.

[0077] 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.

[0078] 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)

[0079] [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).

[0080] [Method for manufacturing forged steel rolls] An example of a method for manufacturing the forged steel roll of this embodiment will be described. The forged steel roll of this embodiment may be manufactured by methods other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing the forged steel roll of this embodiment.

[0081] 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.

[0082] [(Process 1) Steelmaking Process] In the steelmaking process, ingots are manufactured using molten steel that satisfies characteristic 1, by a well-known casting method. A well-known casting method is, for example, the bottom-pouring ingot method. The cast ingots may be used as electrodes to carry out electroslag remelting (ESR) or the like.

[0083] [(Process 2) Hot Forging Process] In the hot forging process, first, the ingot produced in the steelmaking process is heated in a heating furnace. Hot forging is then performed on the heated ingot. If the temperature of the ingot drops during hot forging, the ingot may be heated again in the heating furnace. Then, hot forging may be resumed on the reheated ingot. In this way, a rough-shaped roll material (hereinafter referred to as intermediate material) is produced.

[0084] The hot forging process satisfies the following condition 1. (Condition 1) The heating temperature T1 should be 1100°C or lower. Condition 1 will be explained below.

[0085] [Regarding Condition 1] When an ingot is heated to a temperature exceeding 1100°C, the dissolution of carbides in the ingot into the matrix is ​​excessively accelerated. In this case, the amount of dissolved carbon [C] in the forged steel roll produced is increased. S It is not possible to adjust it to below 0.80%.

[0086] In the hot forging process, the temperature of the heating furnace used to heat the 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 2 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 an 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.

[0087] [(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.

[0088] [(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.

[0089] [(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.

[0090] The quenching process satisfies the following condition 2. (Condition 2) 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. Condition 2 will be explained below.

[0091] [Regarding Condition 2] 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%.

[0092] 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 1, 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.

[0093] 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.

[0094] 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 forged steel roll S It is possible to adjust this to 0.65% or higher.

[0095] [(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 600°C. To further reduce the amount of retained austenite and increase the total area ratio of martensite and bainite, the tempering treatment may be performed multiple times, or a sub-zero treatment may be performed on the intermediate material after the quenching process but before the tempering process. The cooling temperature in the sub-zero treatment can be within a well-known range, for example, -30 to -196°C.

[0096] [(Process 7) Finishing Process] In the finishing process, finishing is performed on the intermediate material after the tempering process. Finishing is done, for example, by grinding using a grinding machine. The finishing process shapes the intermediate material into the final product shape.

[0097] The forged steel roll of this embodiment is manufactured through the above process. [Examples]

[0098] 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.

[0099] Forged steel rolls having the chemical compositions shown in Tables 1A and 1B were manufactured by the following method.

[0100] [Table 1A]

[0101] [Table 1B]

[0102] Specifically, electrode ingots were cast from molten steel using the bottom-pouring ingot casting method. The manufactured electrode ingots were then remelted using the electroslag remelting (ESR) method to produce forging ingots, which would serve as the raw material.

[0103] A hot forging process was performed on the forging ingots obtained by the ESR method. The heating temperature T1 (°C) during the hot forging process is 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.

[0104] [Table 2]

[0105] 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.

[0106] 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.

[0107] The intermediate raw material after the quenching process was tempered at 100-600°C. Following this, a finishing process was carried out. In the finishing process, the intermediate raw material was ground to achieve the final roll shape with a roll body diameter of φ645mm, a roll body length of 1950mm, and an overall length of 3950mm. The forged steel rolls for each test number were manufactured using the above process. The total area ratio of martensite and bainite on the surface of each forged steel roll was determined based on the method described in the above-mentioned [Method for Measuring the Total Area Ratio of Martensite and Bainite on the Surface of Forged Steel Rolls]. As a result, the total area ratio of martensite and bainite on the surface of all forged steel rolls was 85% or more.

[0108] [About the evaluation test] The following evaluation tests were performed on each manufactured forged steel roll with a test number. (Test 1) Solid solution carbon content [C] S Measurement test (Test 2) Crack Initiation Suppression Performance Evaluation Test (Test 3) Abrasion resistance evaluation test The following describes each test.

[0109] [(Test 1) Solid solution carbon content [C] S [Measurement test] The above [solid solution carbon content [C] S Based on the method described in [Measurement Method], the amount of dissolved carbon [C] in the matrix phase on the surface of each test number of forged steel rolls was measured. S The amount of dissolved carbon [C] obtained was calculated. S See "Solid dissolved carbon content [C]" in Table 2. S (%) is shown in the column.

[0110] [(Test 2) 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.

[0111] 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.

[0112] 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 2). 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 2).

[0113] [(Test 3) Abrasion Resistance Evaluation Test] The wear resistance of the forged steel rolls for each test number was evaluated using a two-cylinder rolling wear testing machine. Figure 1 is a schematic diagram of the two-cylinder rolling wear testing machine 10. A cylindrical roll test specimen 12 was taken from the surface layer (the area from the surface of the body to a depth of 40 mm) of each forged steel roll with a given test number. The roll test specimen 12 had a diameter of 40 mm and a width of 10 mm. The central axis of the roll test specimen 12 was parallel to the radial direction of the forged steel roll. The outer surface of the roll test specimen 12 was polished.

[0114] A two-cylinder rolling abrasion test was conducted using the collected roll test specimen 12. In the two-cylinder rolling abrasion test, a rolled material test specimen 11 rotating in the opposite direction was pressed against the rotating roll test specimen 12 with a load F described later. A front view of the rolled material test specimen 11 is shown in Figure 2. The numbers in Figure 2 indicate dimensions (in mm). "R7.5" in Figure 2 indicates that the radius of curvature of the outer surface was 7.5 mm. As shown in Figure 2, the diameter of the rolled material test specimen 11 was 160 mm and the width was 15 mm. The rolled material test specimen 11 was manufactured by processing steel material having a chemical composition equivalent to S45C as specified in JIS G 4051 (2018) into the shape shown in Figure 2 and polishing the outer surface.

[0115] In the two-cylinder rolling abrasion test, the contact load between the roll specimen 12 and the rolled material specimen 11 was set to 700 N, and no lubricant was used. The rotation speed of the roll specimen 12 was set to 2000 rpm, and the slip ratio between the roll specimen 12 and the rolled material specimen 11 was set to 5%. The slip ratio (%) is defined by the following formula. Slip ratio (%) = (Peripheral speed of rolled material test piece - Peripheral speed of roll test piece) / Peripheral speed of roll test piece × 100 In the two-cylinder rolling abrasion test, the rolled material specimen 11 was maintained at 200°C by high-frequency induction heating using an induction heating coil (not shown). The roll specimen was water-cooled for cooling. The test was terminated after 20,000 cycles.

[0116] The wear shape of the outer surface of the roll test specimen 12 after 20,000 rolling cycles was measured using a laser microscope (Keyence Corporation, product name: Shape Analysis Laser Microscope VK-X250). Specifically, a scan of the surface shape was performed in the width direction at an arbitrary position on the outer surface of the roll test specimen 12. The scanned area was the entire width of the outer surface. From the obtained surface shape profile, the wear cross-sectional area (the area of ​​the depression caused by wear on the outer surface in a cross-section including the width and radial directions of the roll test specimen 12) was calculated. A similar wear cross-sectional area measurement was performed on the outer surface of the roll test specimen 12 at a position directly opposite the center. The arithmetic mean of the wear cross-sectional areas obtained from these two measurements was taken as the wear cross-sectional area of ​​the roll test specimen 12.

[0117] The wear cross-sectional area of ​​the roll test piece 12 was 3500 μm². 2 If the following conditions were met, the evaluation was set to "E (Excellent)," and it was determined that sufficient wear resistance had been achieved (indicated as "E" in the "Wear Resistance" column in Table 2). 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 2).

[0118] [Test Results] Referring to Tables 1A, 1B, and 2, the forged steel rolls for test numbers 1-21 met features 1 and 2. Therefore, crack initiation was sufficiently suppressed. In test numbers 1-19, the amount of dissolved carbon [C] was also... S The percentage was 0.65% or higher. Therefore, sufficient wear resistance was achieved.

[0119] On the other hand, in tests 22 and 23, 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.

[0120] In tests 24 and 25, the quenching parameter C in the quenching process was too high. Therefore, the amount of dissolved carbon [C] SThe pressure was too high. As a result, crack initiation could not be adequately suppressed.

[0121] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. It is a forged steel roll, A cylindrical body, It comprises a pair of shafts, The chemical composition of the aforementioned forged steel roll is, in mass%, C: 0.85-1.05%, Si: 0.60-1.20%, Mn: 0.30-0.60%, P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00-6.00%, Mo: 1.00-2.50%, V: 1.00-2.00%, Cu: 0.40% or less, It contains Ni: 0.30-0.60%, The remainder consists of Fe and impurities. In the surface layer of the aforementioned body, The amount of dissolved carbon in the matrix phase is 0.80% or less by mass. Forged steel roll.

2. It is a forged steel roll, A cylindrical body, It comprises a pair of shafts, The chemical composition of the aforementioned forged steel roll is, in mass%, C: 0.85-1.05%, Si: 0.60-1.20%, Mn: 0.30-0.60%, P: 0.020% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.020% or less, O: 0.0050% or less, Cr: 4.00-6.00%, Mo: 1.00-2.50%, V: 1.00-2.00%, Cu: 0.40% or less, It contains Ni: 0.30-0.60%, Furthermore, it contains one or more selected from the group consisting of Group 1 and Group 2, The remainder consists of Fe and impurities. In the surface layer of the aforementioned body, 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.

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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