Forged steel roll

Abstract

To provide a forged steel roll capable of suppressing the occurrence of a crack due to thermal shock and also suppressing the progress of the crack.SOLUTION: A forged steel roll according to the present disclosure has a chemical composition consisting of, by mass%, C: 0.70 to 1.50%, Si: 0.20 to 1.50%, Mn: 0.20 to 1.50%, P: 0.030% or less, S: 0.0200% or less, Al: 0.050% or less, N: 0.0200 to 0.2000%, O: 0.0050% or less, Cr: 2.80 to 8.00%, Mo: 0.20 to 3.00%, V: 0.10 to 2.00%, Cu: 0. 100% or less, B: 0.0100% or less, and a balance of Fe and impurities, A number ratio of M (C, N) carbonitrides to carbides and carbonitrides having an equivalent circle diameter of 0.50 μm or more is 5.0% or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to forged steel rolls. [Background technology]

[0002] Forged steel rolls are known as rolling rolls used in rolling processes. In rolling processes using forged steel rolls, the surface of the forged steel roll wears due to contact with the material being rolled during rolling. Therefore, when forged steel rolls are used for a long period of time, the surface roughness of the forged steel roll gradually decreases. When the surface roughness decreases, slippage is more likely to occur between the forged steel roll and the material being rolled. On the other hand, when slippage occurs, poor engagement may occur. Furthermore, when slippage occurs, seizure may occur between the forged steel roll and the material being rolled during rolling.

[0003] When slippage or seizure occurs, a thermal shock is applied to the surface of the forged steel roll. This thermal shock may cause cracks to form in the surface layer of the forged steel roll. On the other hand, if a forged steel roll with a crack continues to be used, the crack will gradually grow. Furthermore, as the crack grows, a part of the surface layer of the roll may peel off. This phenomenon is called spalling.

[0004] Therefore, when a crack is found in a forged steel roll, the roll surface is ground according to the depth of the crack to remove the crack in order to suppress the occurrence of spalling. That is, the deeper the crack, the greater the amount of roll grinding required to remove the crack. Therefore, the greater the amount of roll grinding, the worse the roll consumption rate (kg / ton). Therefore, there is a demand for a forged steel roll that can suppress the occurrence and propagation of cracks due to thermal shock. Hereinafter, in this specification, the ability to suppress the occurrence and propagation of cracks due to thermal shock is also referred to as "excellent crack resistance."

[0005] Techniques for improving the resistance to thermal shock in forged steel rolls have been proposed in Japanese Patent Laid-Open Publication Nos. 2-185928 (Patent Document 1), 1-234548 (Patent Document 2), 5-086439 (Patent Document 3), 5-132738 (Patent Document 4), and 2010-242166 (Patent Document 5).

[0006] In the method for manufacturing a forged steel roll disclosed in Patent Document 1, the surface layer of a forged steel material containing 0.7 to 1.0% C, 0.15 to 1.5% Si, 0.15 to 1.5% Mn, 3.0 to 6.0% Cr, 3.0 to 5.0% Mo, and 1.2% or less V is quenched. The quenched forged steel material is subjected to sub-zero treatment. The forged steel material after the sub-zero treatment is tempered at a temperature of 180°C or higher. In Patent Document 1, the tempering temperature is increased by 40°C or more compared to conventional methods by increasing the tempering softening resistance. In this way, Patent Document 1 improves the crack resistance of the forged steel roll.

[0007] The forged steel roll disclosed in Patent Document 2 contains, by weight, C: 0.45 to 0.95%, Mn: 1.0% or less, Cr: 4.5 to 6.0%, Mo: 0.3 to 0.7%, Ni: 0.6 to 2.0%, the balance being Fe and impurities, and further the Si content is restricted to less than 0.1%. In Patent Document 2, by restricting the Si content as an impurity to less than 0.1% and setting the Ni content to 0.6 to 2.0%, the spalling resistance and thermal shock crack resistance of the forged steel roll are improved.

[0008] The forged steel roll disclosed in Patent Document 3 contains 0.90-1.10 wt% C, 0.5-1.0 wt% Si, 0.1-1.0 wt% Mn, 4.0-6.0 wt% Cr, 3.0-6.0 wt% Mo, 0.5-2.0 wt% V, and 1.0-3.0 wt% Co, with the remainder consisting of Fe and impurities. In Patent Document 3, the forged steel roll has the above-mentioned chemical composition, thereby improving the thermal shock resistance of the forged steel roll.

[0009] The forged steel roll disclosed in Patent Document 4 contains, by weight, 0.7 to 1.4% C, 0.8 to 2.5% Si, 0.8 to 2.5% Mn, 0.5 to 2.5% Ni, 2.5 to 6.5% Cr, 2.5 to 8.5% Mo, 0.3 to 3.0% W, 0.5 to 4.5% V, and the balance being Fe and impurities. This forged steel roll contains more than 15% to 40% retained austenite obtained by tempering after sub-zero treatment. In Patent Document 4, the retained austenite suppresses crack propagation.

[0010] The forged steel roll disclosed in Patent Document 5 contains, by mass%, 0.6 to 1.2% C, 0.4 to 0.8% Si, 0.4 to 1.0% Mn, 0.4 to 1.0% Ni, 3.0 to 6.0% Cr, and 0.2 to 0.5% Mo, with the remainder consisting of Fe and impurities. In this forged steel roll, the average particle size of carbides dispersed in the metal structure of the roll surface layer within 50 mm from the roll surface is 1 μm or less. Furthermore, the area fraction of the dispersed carbides is 5 to 30%. In Patent Document 5, the occurrence of cracks is suppressed by dispersing the above-mentioned carbides in the roll surface layer. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Publication No. 2-185928 [Patent Document 2] Japanese Patent Application Publication No. 1-234548 [Patent Document 3] Japanese Patent Application Publication No. 5-086439 [Patent Document 4] Japanese Patent Application Publication No. 5-132738 [Patent Document 5] Japanese Patent Application Laid-Open No. 2010-242166 Summary of the Invention [Problem to be solved by the invention]

[0012] The thermal shock resistance of forged steel rolls can be improved by the means described in Patent Documents 1 to 5. However, it is also possible to suppress both the occurrence and propagation of cracks due to thermal shock by means other than those described in Patent Documents 1 to 5.

[0013] An object of the present disclosure is to provide a forged steel roll that can suppress the occurrence of cracks due to thermal shock and also suppress the propagation of the cracks. [Means for solving the problem]

[0014] The forged steel roll according to the present disclosure comprises: The chemical composition is, in mass%, C: 0.70~1.50%, Si: 0.20 to 1.50% Mn: 0.20 to 1.50% P: 0.030% or less, S: 0.0200% or less, Al: 0.050% or less, N: 0.0200~0.2000%, O: 0.0050% or less, Cr: 2.80~8.00%, Mo: 0.20~3.00%, V: 0.10~2.00%, Cu: 0.100% or less, B: 0.0100% or less, and the balance being Fe and impurities, Of the carbides and carbonitrides having an equivalent circle diameter of 0.50 μm or more, the number ratio of M(C,N) type carbonitrides is 5.0% or more.

[0015] The forged steel roll according to the present disclosure comprises: The chemical composition is, in mass%, C: 0.70~1.50%, Si: 0.20 to 1.50% Mn: 0.20 to 1.50% P: 0.030% or less, S: 0.0200% or less, Al: 0.050% or less, N: 0.0200~0.2000%, O: 0.0050% or less, Cr: 2.80~8.00%, Mo: 0.20~3.00%, V: 0.10~2.00%, Cu: 0.100% or less, and B: 0.0100% or less, Ni: 1.20% or less, Co: 0.50% or less, Nb: 0.50% or less, Ti: 0.050% or less, and W: 0.50% or less, containing one or more elements selected from the group consisting of, the balance being Fe and impurities, Of the carbides and carbonitrides having an equivalent circle diameter of 0.50 μm or more, the number ratio of M(C,N) type carbonitrides is 5.0% or more. [Effects of the Invention]

[0016] The forged steel roll according to the present disclosure can suppress the occurrence of cracks due to thermal shock, and can also suppress the propagation of cracks. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a diagram showing the relationship between the proportion (%) of the number of M(C,N) type carbonitrides and the number of cracks (numbers), which is an index of crack resistance. [Figure 2] FIG. 2 is a diagram showing the configuration of a drop weight frictional thermal shock tester used in the crack resistance evaluation test in the examples. [Figure 3] FIG. 3 is a schematic diagram of the contact area obtained in the crack resistance evaluation test using the drop weight frictional thermal shock tester shown in FIG. [Figure 4] FIG. 4 is a cross-sectional view of the contact area shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present inventors first investigated forged steel rolls with excellent crack resistance from the viewpoint of chemical composition. As a result, they thought that if the nitrogen (N) content could be increased to form a large number of carbonitrides in the forged steel roll, excellent crack resistance could be obtained. As a result of the investigation by the present inventors, it was found that if the N content was increased to 0.0200 to 0.2000% by mass, carbonitrides could be stably formed in the forged steel roll.

[0019] Based on the above findings, the present inventors have proposed a forged steel roll containing, in mass %, C: 0.70 to 1.50%, Si: 0.20 to 1.50%, Mn: 0.20 to 1.50%, P: 0.030% or less, S: 0.0200% or less, Al: 0.050% or less, N: 0.0200 to 0.2000%, O: 0.0050% or less, Cr: 2.80 to 8.00%, Mo: 0. It was thought that excellent crack resistance could be obtained if the chemical composition was as follows: 0.20-3.00%, V: 0.10-2.00%, Cu: 0.100% or less, B: 0.0100% or less, Ni: 0-1.20%, Co: 0-0.50%, Nb: 0-0.50%, Ti: 0-0.050%, W: 0-0.50%, and the remainder consisting of Fe and impurities.

[0020] On the other hand, even when a forged steel roll having the above-mentioned chemical composition is used, there are cases where excellent crack resistance cannot be obtained. Therefore, the present inventors have further investigated and studied in detail a method for suppressing the occurrence and propagation of cracks when a thermal shock is applied to the surface layer of a forged steel roll having the above-mentioned chemical composition.

[0021] As described above, in a forged steel roll having the above-mentioned chemical composition, carbonitrides are formed in the forged steel roll, which may improve the crack resistance. On the other hand, simply forming carbonitrides may not always provide excellent crack resistance, as described above. Therefore, the present inventors have focused on the type of carbonitrides to study ways to improve the crack resistance of forged steel rolls. As a result, the present inventors have found that, among carbonitrides, M(C,N)-type carbonitrides having an equivalent circle diameter of 0.50 μm or more can particularly improve the crack resistance of forged steel rolls. Furthermore, in a forged steel roll having the above-mentioned chemical composition, carbides may also be formed in addition to carbonitrides. In other words, increasing the proportion of M(C,N)-type carbonitrides among carbides and carbonitrides having an equivalent circle diameter of 0.50 μm or more may improve the crack resistance of forged steel rolls.

[0022] Therefore, the present inventors have further studied in detail the relationship between the number fraction (%) of M(C,N) type carbonitrides among carbides and carbonitrides having an equivalent circle diameter of 0.50 μm or more and crack resistance. Hereinafter, in this specification, the number fraction (%) of M(C,N) type carbonitrides among carbides and carbonitrides having an equivalent circle diameter of 0.50 μm or more will also be simply referred to as the "number fraction (%) of M(C,N) type carbonitrides." As a result of the studies by the present inventors, it has become clear that in a forged steel roll having the above-mentioned chemical composition, excellent crack resistance can be stably obtained if the number fraction of M(C,N) type carbonitrides is 5.0% or more. This point will be specifically explained using the drawings.

[0023] Fig. 1 is a diagram showing the relationship between the percentage (%) of M(C,N) type carbonitrides and the number of cracks (numbers), which is an index of crack resistance. Fig. 1 was created using the percentage (%) of M(C,N) type carbonitrides and the number of cracks (numbers), which were determined by a method described later, for a forged steel roll having the above-mentioned chemical composition in the examples described later. In this example, the number of cracks is an index of crack occurrence, which is an index of crack resistance.

[0024] 1, it can be seen that in the forged steel roll having the above-mentioned chemical composition, when the proportion of M(C,N) type carbonitrides is 5.0% or more, the number of cracks drops sharply to 20 or less, demonstrating excellent crack resistance. Therefore, the forged steel roll according to this embodiment has the above-mentioned chemical composition and the proportion of M(C,N) type carbonitrides is 5.0% or more. As a result, the forged steel roll according to this embodiment has excellent crack resistance.

[0025] The details of why increasing the proportion of M(C,N) carbonitrides to 5.0% or more in a forged steel roll having the above-mentioned chemical composition improves crack resistance are not clear. However, the present inventors speculate as follows: As described above, if slippage or seizure occurs during use as a rolling roll, a thermal shock is applied to the surface of the forged steel roll. At this time, in a forged steel roll having the above-mentioned chemical composition, M(C,N) carbonitrides with a circle equivalent diameter of 0.50 μm or more may combine with oxygen (O) to form oxides. The oxides formed in this way have a low friction coefficient. Therefore, if oxides are formed during sliding, frictional heat generation is reduced. As a result, thermal shock during use as a rolling roll may be suppressed, potentially reducing the occurrence of cracks.

[0026] It is possible that the crack resistance of a forged steel roll having the above-mentioned chemical composition and in which the number ratio of M(C,N) type carbonitrides is 5.0% or more may be enhanced by a mechanism different from that speculated by the inventors. However, the fact that a forged steel roll having the above-mentioned chemical composition and in which the number ratio of M(C,N) type carbonitrides is 5.0% or more among carbides and carbonitrides having an equivalent circle diameter of 0.50 μm or more has excellent crack resistance is proven by the examples described later.

[0027] The forged steel roll according to this embodiment, which has been completed based on the above findings, has the following configuration.

[0028] The forged steel roll of the first configuration is The chemical composition is, in mass%, C: 0.70~1.50%, Si: 0.20 to 1.50% Mn: 0.20 to 1.50% P: 0.030% or less, S: 0.0200% or less, Al: 0.050% or less, N: 0.0200~0.2000%, O: 0.0050% or less, Cr: 2.80~8.00%, Mo: 0.20~3.00%, V: 0.10~2.00%, Cu: 0.100% or less, B: 0.0100% or less, and the balance being Fe and impurities, Of the carbides and carbonitrides having an equivalent circle diameter of 0.50 μm or more, the number ratio of M(C,N) type carbonitrides is 5.0% or more.

[0029] The second configuration of forged steel rolls is The chemical composition is, in mass%, C: 0.70~1.50%, Si: 0.20 to 1.50% Mn: 0.20 to 1.50% P: 0.030% or less, S: 0.0200% or less, Al: 0.050% or less, N: 0.0200~0.2000%, O: 0.0050% or less, Cr: 2.80~8.00%, Mo: 0.20~3.00%, V: 0.10~2.00%, Cu: 0.100% or less, and B: 0.0100% or less, Ni: 1.20% or less, Co: 0.50% or less, Nb: 0.50% or less, Ti: 0.050% or less, and W: 0.50% or less, containing one or more elements selected from the group consisting of, the balance being Fe and impurities, Of the carbides and carbonitrides having an equivalent circle diameter of 0.50 μm or more, the number ratio of M(C,N) type carbonitrides is 5.0% or more.

[0030] The forged steel roll of this embodiment will be described in detail below. Unless otherwise specified, "%" for elements means mass %.

[0031] [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 C: 0.70 to 1.50%, Si: 0.20 to 1.50%, Mn: 0.20 to 1.50%, P: 0.030% or less, S: 0.0200% or less, Al: 0.050% or less, N: 0.0200 to 0.2000%, O: 0.0050% or less, Cr: 2.80 to 8.00%, Mo: 0.20 to 3.00%, V: 0.10 to 2.00%, Cu: 0.100% or less, B: 0.0100% or less, Ni: 0 to 1.20%, Co: 0 to 0.50%, Nb: 0 to 0.50%, Ti: 0 to 0.050%, W: 0 to 0.50%, and the balance being Fe and impurities. (Feature 2) Of the carbides and carbonitrides having an equivalent circle diameter of 0.50 μm or more, the number ratio of M(C,N) type carbonitrides is 5.0% or more. Feature 1 and Feature 2 will be explained below.

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

[0033] C: 0.70 to 1.50% Carbon (C) increases the hardness of the surface layer of the forged steel roll. Furthermore, C forms M(C,N) type carbonitrides to suppress the occurrence of cracks in the forged steel roll. As a result, the crack resistance of the forged steel roll is improved. If the C content is too low, the above effect cannot be sufficiently obtained. On the other hand, if the C content is too high, coarse carbides are formed, and sufficient hardness may not be obtained in the surface layer of the forged steel roll. Therefore, the C content is 0.70 to 1.50%. The lower limit of the C content is preferably 0.75%, more preferably 0.80%, even more preferably 0.85%, and still more preferably 0.90%. The upper limit of the C content is preferably 1.45%, more preferably 1.40%, even more preferably 1.35%, even more preferably 1.20%, even more preferably 1.15%, even more preferably 1.10%, and even more preferably 1.05%.

[0034] Si: 0.20 to 1.50% Silicon (Si) deoxidizes steel. Si also improves the hardenability of steel. If the Si content is too low, the above effects cannot be sufficiently obtained. On the other hand, if the Si content is too high, the toughness of the forged steel roll decreases. Therefore, the Si content is 0.20 to 1.50%. The lower limit of the Si content is preferably 0.25%, more preferably 0.30%, even more preferably 0.40%, and still more preferably 0.45%. The upper limit of the Si content is preferably 1.45%, more preferably 1.40%, even more preferably 1.35%, even more preferably 1.30%, even more preferably 1.25%, and even more preferably 1.20%.

[0035] Mn: 0.20 to 1.50% Manganese (Mn) improves the hardenability of steel. If the Mn content is too low, this effect cannot be sufficiently obtained. On the other hand, if the Mn content is too high, the toughness of the forged steel roll decreases. Therefore, the Mn content is 0.20 to 1.50%. The lower limit of the Mn content is preferably 0.25%, more preferably 0.30%, even more preferably 0.35%, and still more preferably 0.40%. The upper limit of the Mn content is preferably 1.45%, more preferably 1.40%, even more preferably 1.35%, even more preferably 1.30%, and even more preferably 1.25%.

[0036] P:0.030% or less Phosphorus (P) is an impurity. P segregates at grain boundaries and reduces the toughness of forged steel rolls. Therefore, the P content is limited to 0.030% or less. The P content is preferably as low as possible. However, excessive reduction in the P content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the P content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.002%. The upper limit of the P content is preferably 0.025%, and more preferably 0.020%.

[0037] S: 0.0200% or less Sulfur (S) is an impurity. S segregates at grain boundaries and reduces the toughness and hot workability of forged steel rolls. Therefore, the S content is 0.0200% or less. The S content is preferably as low as possible. However, excessive reduction of the S content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the S content is preferably more than 0%, more preferably 0.0001%, even more preferably 0.0002%, and even more preferably 0.0003%. The upper limit of the S content is preferably 0.0150%, more preferably 0.0125%, and even more preferably 0.0100%.

[0038] Al: 0.050% or less Aluminum (Al) deoxidizes steel. On the other hand, if the Al content is too high, coarse Al nitrides are formed, reducing the toughness of the steel. Therefore, the Al content should be 0.050% or less. The lower limit of the Al content is preferably more than 0%, more preferably 0.001%, even more preferably 0.002%, even more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Al content is preferably 0.040%, more preferably 0.035%, even more preferably 0.030%, and still more preferably 0.025%. In this specification, the Al content means the total Al content in the steel.

[0039] N: 0.0200 to 0.2000% Nitrogen (N) forms M(C,N) type carbonitrides to suppress the occurrence of cracks in forged steel rolls. As a result, the crack resistance of the forged steel rolls is improved. If the N content is too low, the above effect cannot be sufficiently obtained. On the other hand, if the N content is too high, coarse M(C,N) type carbonitrides are formed, which accelerates the propagation of cracks in the forged steel rolls. As a result, the crack resistance of the forged steel rolls is actually reduced. Therefore, the N content is 0.0200 to 0.2000%. The lower limit of the N content is preferably more than 0.0200%, more preferably 0.0205%, even more preferably 0.0225%, and still more preferably 0.0250%. The upper limit of the N content is preferably 0.1900%, more preferably 0.1700%, even more preferably 0.1500%, even more preferably 0.1250%, even more preferably 0.1000%, even more preferably 0.0960%, even more preferably 0.0900%, and even more preferably 0.0850%. If the N content is 0.1000% or less, the formation of coarse M(C,N) type carbonitrides is suppressed, and the propagation of cracks can be further suppressed, resulting in further improved crack resistance of the forged steel roll.

[0040] O: 0.0050% or less Oxygen (O) is an impurity. O forms oxides and reduces the toughness of forged steel rolls. Therefore, the O content is 0.0050% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the O content is preferably more than 0%, more preferably 0.0001%, even more preferably 0.0005%, even more preferably 0.0007%, and even more preferably 0.0010%. The upper limit of the O content is preferably 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0041] Cr: 2.80~8.00% Chromium (Cr) forms carbides to improve the wear resistance of forged steel rolls. Cr also improves the tempering softening resistance of steel through solid solution, and increases the high-temperature hardness of the surface layer of the forged steel roll. If the Cr content is too low, the above effects cannot be sufficiently obtained. On the other hand, if the Cr content is too high, coarse carbides are formed, which reduces the grindability and toughness of the forged steel roll. Therefore, the Cr content is 2.80 to 8.00%. The lower limit of the Cr content is preferably 2.85%, more preferably 3.00%, even more preferably 3.50%, and still more preferably 4.00%. The upper limit of the Cr content is preferably 7.50%, more preferably 7.00%, even more preferably 6.50%, even more preferably 6.00%, and even more preferably 5.50%.

[0042] Mo: 0.20 to 3.00% Molybdenum (Mo) assists in the formation of M(C,N) type carbonitrides and suppresses the occurrence of cracks in forged steel rolls. As a result, the crack resistance of the forged steel rolls is improved. If the Mo content is too low, the above effect cannot be sufficiently obtained. On the other hand, if the Mo content is too high, coarse carbides are formed, which reduces the grindability and toughness of the forged steel rolls. Therefore, the Mo content is 0.20 to 3.00%. The lower limit of the Mo content is preferably 0.25%, more preferably 0.30%, even more preferably 0.40%, and still more preferably 0.45%. The upper limit of the Mo content is preferably 2.80%, more preferably 2.60%, even more preferably 2.50%, and still more preferably 2.30%.

[0043] V: 0.10~2.00% Vanadium (V) forms M(C,N) type carbonitrides to suppress the occurrence of cracks in forged steel rolls. As a result, the crack resistance of the forged steel rolls is improved. If the V content is too low, the above effect cannot be sufficiently obtained. On the other hand, if the V content is too high, coarse carbides are formed, which reduces the grindability and toughness of the forged steel rolls. Therefore, the V content is 0.10 to 2.00%. The lower limit of the V content is preferably 0.15%, more preferably 0.20%, even more preferably 0.25%, even more preferably 0.30%, even more preferably 0.35%, and even more preferably 0.40%. The upper limit of the V content is preferably 1.80%, more preferably 1.60%, even more preferably 1.50%, even more preferably 1.40%, even more preferably 1.30%, and even more preferably 1.20%.

[0044] Cu:0.100% or less Copper (Cu) is an impurity that reduces the hot workability of steel. Therefore, the Cu content is limited to 0.100% or less. The Cu content is preferably as low as possible. However, excessive reduction in the Cu content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the Cu content is preferably more than 0%, more preferably 0.001%, even more preferably 0.005%, even more preferably 0.010%, and even more preferably 0.015%. The upper limit of the Cu content is preferably 0.095%, more preferably 0.090%, even more preferably 0.085%, even more preferably 0.080%, even more preferably 0.075%, and even more preferably 0.070%.

[0045] B: 0.0100% or less Boron (B) is an impurity that reduces the toughness of forged steel rolls. Therefore, the B content is 0.0100% or less. The B content is preferably as low as possible. However, excessive reduction in the B content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the B content is preferably more than 0%, more preferably 0.0001%, even more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0015%, and even more preferably 0.0020%. The upper limit of the B content is preferably 0.0090%, more preferably 0.0085%, and even more preferably 0.0080%.

[0046] The balance of the chemical composition of the forged steel roll according to this embodiment is composed of Fe and impurities. Here, the impurities refer to substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, during industrial production of the forged steel roll, and are acceptable within a range that does not adversely affect the forged steel roll of this embodiment.

[0047] [About optional elements] The chemical composition of the forged steel roll of this embodiment further contains, in place of a portion of Fe, Ni: 0-1.20% Co: 0 to 0.50% Nb: 0 to 0.50% Ti: 0 to 0.050%, and W: 0 to 0.50%. The optional elements in each group will be explained below.

[0048] [Group 1: Ni and Co] Ni: 1.20% or less Nickel (Ni) is an optional element and may not be contained. In other words, the Ni content may be 0%. When nickel is contained, that is, when the Ni content is more than 0%, Ni improves the hardenability of the steel and increases the hardness of the surface layer of the forged steel roll. Even if even a small amount of Ni is contained, the above effects can be obtained to some extent. However, if the Ni content is too high, excessive retained austenite is formed. In this case, the hardness of the forged steel roll decreases. Therefore, the Ni content is 0 to 1.20%, and when nickel is contained, the Ni content is 1.20% or less. The lower limit of the Ni content is preferably 0.01%, more preferably 0.05%, even more preferably 0.10%, even more preferably 0.15%, and even more preferably 0.20%. The upper limit of the Ni content is preferably 1.15%, more preferably 1.10%, even more preferably 1.05%, even more preferably 1.00%, even more preferably 0.95%, even more preferably 0.90%, even more preferably 0.85%, even more preferably 0.80%, even more preferably 0.75%, and even more preferably 0.70%.

[0049] Co:0.50% or less Cobalt (Co) is an optional element and may not be contained. In other words, the Co content may be 0%. When it is contained, that is, when the Co content is more than 0%, Co improves the hardenability of the steel and increases the hardness of the surface layer of the forged steel roll. Even if even a small amount of Co is contained, the above effects can be obtained to some extent. However, if the Co content is too high, the hot workability of the steel material decreases in the manufacturing process of the forged steel roll. Therefore, the Co content is 0 to 0.50%, and when it is contained, it is 0.50% or less. The lower limit of the Co content is preferably 0.01%, more preferably 0.05%, and even more preferably 0.08%. The upper limit of the Co content is preferably 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0050] [Group 2: Nb, Ti and W] Nb: 0.50% or less Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, that is, when the Nb content exceeds 0%, Nb assists in the formation of M(C,N) type carbonitrides and suppresses the occurrence of cracks in the forged steel roll. As a result, the crack resistance of the forged steel roll is improved. Even if even a small amount of Nb is contained, the above effects can be obtained to some extent. However, if the Nb content is too high, coarse carbides are formed. In this case, the grindability and toughness of the forged steel roll are reduced. Therefore, the Nb content is 0 to 0.50%, and when contained, the Nb content is 0.50% or less. The lower limit of the Nb content is preferably 0.01%, more preferably 0.05%, even more preferably 0.10%, even more preferably 0.15%, even more preferably 0.20%, and even more preferably 0.25%. The upper limit of the Nb content is preferably 0.45%, more preferably 0.42%, even more preferably 0.40%, even more preferably 0.38%, and still more preferably 0.36%.

[0051] Ti: 0.050% or less Titanium (Ti) is an optional element and may not be contained. In other words, the Ti content may be 0%. When titanium is contained, that is, when the Ti content exceeds 0%, Ti assists in the formation of M(C,N) type carbonitrides and suppresses the occurrence of cracks in the forged steel roll. As a result, the crack resistance of the forged steel roll is improved. Even if even a small amount of Ti is contained, the above effect can be obtained to some extent. However, if the Ti content is too high, the hot workability of the steel material is reduced in the manufacturing process of the forged steel roll. Therefore, the Ti content is 0 to 0.050%, and when contained, the Ti content is 0.050% or less. The lower limit of the Ti content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.004%. The upper limit of the Ti content is preferably 0.040%, more preferably 0.035%, and even more preferably 0.030%.

[0052] W: 0.50% or less Tungsten (W) is an optional element and may not be contained. In other words, the W content may be 0%. When W is contained, that is, when the W content exceeds 0%, W assists in the formation of M(C,N) type carbonitrides and suppresses the occurrence of cracks in the forged steel roll. As a result, the crack resistance of the forged steel roll is improved. Even if even a small amount of W is contained, the above effect can be obtained to some extent. However, if the W content is too high, the hot workability of the steel material is reduced in the manufacturing process of the forged steel roll. Therefore, the W content is 0 to 0.50%, and when W is contained, it is 0.50% or less. The lower limit of the W content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the W content is preferably 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0053] [(Feature 2) The ratio of the number of M(C,N) type carbonitrides] In the forged steel roll according to this embodiment, the number ratio (%) of M(C,N) type carbonitrides among the carbides and carbonitrides having an equivalent circle diameter of 0.50 μm or more in the forged steel roll is 5.0% or more. As described above, in this specification, the number ratio (%) of M(C,N) type carbonitrides among the carbides and carbonitrides having an equivalent circle diameter of 0.50 μm or more is also simply referred to as the "number ratio (%) of M(C,N) type carbonitrides." In other words, the forged steel roll according to this embodiment has the chemical composition described in Feature 1, and the number ratio of M(C,N) type carbonitrides is 5.0% or more.

[0054] Here, the M(C,N) type carbonitride is a carbonitride composed of a metal (M), carbon (C), and nitrogen (N). Since the forged steel roll according to this embodiment has the above-mentioned chemical composition, vanadium (V) is essentially contained as the metal (M) of the M(C,N) type carbonitride.

[0055] As described above, M(C,N) carbonitrides having an equivalent circle diameter of 0.50 μm or more form oxides during use as a rolling roll, suppressing the occurrence of cracks in the forged steel roll and improving the crack resistance of the forged steel roll. Therefore, in the forged steel roll according to this embodiment, the number ratio of M(C,N) carbonitrides among carbides and carbonitrides having an equivalent circle diameter of 0.50 μm or more is set to 5.0% or more. The upper limit of the number ratio of M(C,N) carbonitrides is not particularly limited and may be 100.0%. The upper limit of the number ratio of M(C,N) carbonitrides may be 98.0% or 97.0%. The lower limit of the number ratio of M(C,N) carbonitrides is preferably 10.0%, more preferably 15.0%, and even more preferably 20.0%.

[0056] [Method for measuring the number ratio of M(C,N) type carbonitrides] The number ratio of M(C,N) type carbonitrides can be measured by the following method. A test specimen was taken from the surface of the barrel of the forged steel roll, with the observation surface being a plane perpendicular to the axial direction of the barrel. The observation surface included a position 1 mm deep from the surface of the forged steel roll. The obtained observation surface was mirror-polished. After mirror-polishing, 10 observation fields were identified, each with a center 1 mm deep from the surface of the barrel of the forged steel roll. The size of each observation field was 240 μm × 180 μm. For each of the 10 observation fields, a field emission scanning electron microscope (FE-SEM) was used to capture Z-contrast images, or so-called COMPO images, using a backscattered electron detector. The observation magnification was 500x. The matrix was primarily composed of iron, and the carbides and carbonitrides contained a large amount of carbon, which has a small atomic number. Therefore, the carbides and carbonitrides were identified as dark contrast in the COMPO image. In other words, the matrix and the carbides and carbonitrides could be identified as particles based on their contrast.

[0057] Furthermore, quantitative analysis of the identified particles was performed using energy dispersive X-ray spectroscopy (EDS) attached to the FE-SEM, and five M(C,N)-type carbonitrides and five other carbides and / or carbonitrides were identified. Among the particles identified from the contrast, those with a V content of 30% or more by mass were defined as M(C,N)-type carbonitrides. In other words, those with a V content of less than 30% by mass were defined as other carbides or carbonitrides. The acceleration voltage for the EDS analysis was 15 kV. The EDS analysis time was set so that the X-ray count was 500 or more.

[0058] Here, even in the COMPO image, by setting the contrast appropriately, it is possible to distinguish the M(C,N)-type carbonitrides with a high nitrogen content from other particles. Specifically, if the contrast setting is appropriate, the M(C,N)-type carbonitrides will appear with an even darker contrast than the other particles. The contrast of the COMPO image is adjusted so that the five M(C,N)-type carbonitrides identified by quantitative analysis using EDS can be distinguished from the five other particles. In this way, the observation field is photographed with settings that allow for distinction between the matrix, M(C,N)-type carbonitrides, and other particles, and a photographic image is generated.

[0059] From the obtained photographic image of the entire observation field, all particles with a circle-equivalent diameter of 0.50 μm or more are identified and the total number is counted. Here, the circle-equivalent diameter means the diameter (μm) when the area of ​​a particle is converted into a circle. In this embodiment, the upper limit of the circle-equivalent diameter of a particle is about 50.00 μm. That is, in this embodiment, particles with a circle-equivalent diameter of 0.50 to 50.00 μm are identified and counted. Furthermore, for the identified particles with a circle-equivalent diameter of 0.50 μm or more, M(C,N) type carbonitrides are identified by the above-mentioned method. The total number of M(C,N) type carbonitrides in the photographic image of the entire observation field is counted.

[0060] The total number of M(C,N) type carbonitrides having an equivalent circle diameter of 0.50 μm or more obtained is divided by the total number of particles having an equivalent circle diameter of 0.50 μm or more to determine the number ratio (%) of M(C,N) type carbonitrides to carbides and carbonitrides having an equivalent circle diameter of 0.50 μm or more. The number ratio (%) of M(C,N) type carbonitrides is the value obtained by rounding the obtained value to one decimal place.

[0061] [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 has excellent crack resistance. Specifically, when the forged steel roll is used, the frequency of crack occurrence is sufficiently suppressed, and even if a crack occurs, the propagation of the crack is sufficiently suppressed. As a result, excellent crack resistance is obtained.

[0062] [Microstructure of the surface layer of the forged steel roll of this embodiment] The microstructure of the surface layer of the forged steel roll of this embodiment is mainly composed of martensite and / or bainite. "Mainly composed of martensite and / or bainite" means that the total area ratio of martensite and bainite is 85% or more. In the microstructure, structures other than martensite and bainite include, for example, retained austenite and carbides.

[0063] [Method for measuring the total area ratio of martensite and bainite in the surface layer of a forged steel roll] The total area ratio of martensite and bainite in the surface layer of the forged steel roll of this embodiment is determined by the following method. A test piece is taken from the surface of the barrel of the forged steel roll, including a position 1 mm deep from the surface. Of the surfaces of the test piece, the surface including a position 1 mm deep from the surface of the forged steel roll is defined as the observation surface. 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 (400 μm × 600 μm) at a position 1 mm deep from the surface of the barrel of the forged steel roll are observed using an optical microscope at 200x magnification.

[0064] Each observation field is etched with Murakami's reagent. Murakami's reagent colors the carbides and carbonitrides in the observation field. This allows the carbides and carbonitrides to be easily distinguished from other phases by contrast. Therefore, the carbides and carbonitrides are identified in each observation field. The area ratio of carbides and carbonitrides is calculated based on the total area of ​​the carbides and carbonitrides identified in all observation fields and the total area of ​​all observation fields.

[0065] Furthermore, the area fraction of retained austenite is determined by the following X-ray diffraction method. A test specimen is taken from the barrel surface of the forged steel roll, including a depth of 1 mm. The size of the test specimen is not particularly limited, but for example, it is 15 mm × 15 mm × 10 mm thick. In this case, the thickness direction of the test specimen is the radial direction of the forged steel roll. Using the obtained test specimen, the X-ray diffraction intensity of each of the (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 Co (CoKα radiation), and the output is 40 kV-135 mA. After calculation, the volume fraction Vγ (%) of retained austenite is calculated using formula (I) for each combination (3 × 3 = 9 pairs) of each plane of the α phase and each plane of the γ phase. The average value of the nine sets of volume fractions Vγ of retained austenite is defined as the volume fraction (%) of retained austenite. Vγ=100 / {1+(Iα×Rγ) / (Iγ×Rα)} (I) Here, Iα is the integrated intensity of the α phase. Rα is the crystallographically calculated value of the α phase. Iγ is the integrated intensity of the γ phase. Rγ is the crystallographically calculated value of the γ phase. In this specification, Rα for the (200) plane of the α phase is 9.9, Rα for the (211) plane of the α phase is 20.4, Rα for the (220) plane of the α phase is 9.3, Rγ for the (200) plane of the γ phase is 23.8, Rγ for the (220) plane of the γ phase is 12.5, and Rγ for the (311) plane of the γ phase is 17.2. The volume fraction of retained austenite is rounded to one decimal place. In measuring the X-ray diffraction intensity, X-rays are irradiated on the test specimen at a position corresponding to a depth of 1 mm from the surface of the barrel of the forged steel roll.

[0066] The volume fraction (%) of retained austenite obtained by the above-mentioned X-ray diffraction method is regarded as the area fraction (%) of retained austenite. Then, the total area fraction of martensite and bainite in the surface layer of the forged steel roll is calculated by the following formula. Total area ratio of martensite and bainite in the surface layer of the forged steel roll = 100.0 - (area ratio of carbides and carbonitrides + area ratio of retained austenite)

[0067] [Use of the forged steel roll of this embodiment] The forged steel roll of this embodiment is widely applicable as a rolling roll for rolling processing. The forged steel roll of this embodiment is particularly suitable as a roll for cold rolling of thin steel sheets. Examples of rolls for cold rolling include work rolls for a cold tandem rolling mill or a cold reverse rolling mill, or work rolls for skin pass (temper rolling).

[0068] [Manufacturing method of forged steel rolls] An example of a method for manufacturing a forged steel roll according to this embodiment will be described. The forged steel roll according to this embodiment may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of the method for manufacturing a forged steel roll according to this embodiment.

[0069] An example of a method for manufacturing a forged steel roll according to this embodiment includes the following steps. (Process 1) Steelmaking process (Process 2) Hot forging process (Process 3) Annealing process (Process 4) Rough processing process (Process 5) Quenching process (Process 6) Tempering process (Process 7) Finishing process Each step will be described below.

[0070] [(Process 1) Steelmaking process] In the steelmaking process, molten steel satisfying Feature 1 is used to produce ingots by a known casting method, such as bottom pouring. The cast ingot (electrode ingot) is used as an electrode to carry out electroslag remelting (ESR).

[0071] The reason why the ESR method is carried out in the steelmaking process of the forged steel roll of this embodiment is as follows. In the solidification process in a general casting method, alloy elements may concentrate in the liquid phase, causing solidification segregation. Since the forged steel roll of this embodiment satisfies Feature 1, carbides and carbonitrides are likely to be generated. Therefore, if solidification segregation occurs during solidification of the forged steel roll of this embodiment, carbides and carbonitrides will preferentially crystallize from the remaining liquid phase. In this case, the proportion of M(C,N) type carbonitrides formed by precipitation will decrease, and the crack resistance of the manufactured forged steel roll will decrease.

[0072] In the ESR method, the electrode ingot is remelted by the Joule heat of the molten slag. The molten electrode ingot becomes droplets that sink through the molten slag and solidify in layers while being stored in a mold of any shape. When the electrode ingot is completely melted and the molten steel solidifies all the way to the top, the ingot for forging is obtained.

[0073] Furthermore, in the ESR method, the molten steel stored in the mold solidifies while maintaining a relatively shallow molten steel pool. Therefore, solidification segregation can be suppressed. As a result, the crystallization of carbides can be suppressed. In other words, by performing the ESR method, coarse carbides contained in the electrode ingot can be remelted, and an ingot for forging in which the formation of coarse carbides is suppressed can be obtained. Therefore, the ESR method is performed in the steelmaking process of the forged steel roll of this embodiment.

[0074] The ESR method used in the steelmaking process satisfies the following condition 1. (Condition 1) The solidification rate SR is 3.0 mm / min or more. Condition 1 will be explained below.

[0075] [Regarding Condition 1] Here, the solidification rate SR in the ESR method refers to the rising speed (mm / min) of the solidification interface at a position corresponding to the center of the mold when viewed from above (plan view). The ESR method allows for more precise control of the solidification rate of molten steel than conventional casting methods. However, if the solidification rate SR in the ESR method is too slow, solidification segregation is actually promoted. As a result, coarse carbides and other particles are more likely to be formed by crystallization. As a result, the precipitation of M(C,N) type carbonitrides is suppressed, and the number ratio of M(C,N) type carbonitrides may decrease.

[0076] If the solidification rate SR in the ESR method is 3.0 mm / min or more, the crystallization of carbides can be suppressed. As a result, the forged steel roll can satisfy Feature 2. Therefore, the solidification rate SR in the ESR method is 3.0 mm / min or more.

[0077] [(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. The heating temperature is not particularly limited, but is, for example, 1000 to 1200°C. The heated forging ingot is then subjected to hot forging. If the temperature of the forging ingot drops during hot forging, the forging ingot may be heated again in the heating furnace. Then, hot forging may be resumed on the reheated forging ingot. In this manner, a roughly shaped roll blank (hereinafter referred to as an intermediate blank) is produced.

[0078] In the hot forging process, the following condition 2 is satisfied. (Condition 2) The cumulative time t at 1000 to 1100°C is 30 hours or less. Condition 2 will be explained below.

[0079] [Regarding Condition 2] In the hot forging process, the cumulative time t (hours) during which the surface temperature of the forged material (the forged ingot or roll blank) is 1000-1100°C from the time the forging ingot is carried into the heating furnace until the roll blank is cooled to room temperature after forging is completed is defined as the cumulative time t (hours) at 1000-1100°C. When the chemical composition of the forged material satisfies Feature 1, workability significantly decreases when the surface temperature of the forged material drops below 1000°C. In this case, forging is interrupted and the forged material is reheated in the heating furnace. The reheated forged material is then removed from the heating furnace, and forging is resumed. In the hot forging process, this operation is repeated until a roll blank with the desired shape is obtained. In other words, the cumulative time t at 1000-1100°C includes the time during which the forged material is heated in the heating furnace and the time during which the heated forged material is transported.

[0080] In the temperature range of 1000 to 1100°C, the growth of M(C,N) type carbonitrides is easily promoted. In this temperature range, fine M(C,N) type carbonitrides are absorbed into coarse M(C,N) type carbonitrides and disappear. Therefore, if the time maintained in this temperature range is too long, the number density of M(C,N) type carbonitrides decreases, and the number ratio of M(C,N) type carbonitrides decreases. Therefore, it is preferable that the cumulative time t at 1000 to 1100°C in the hot forging process is short.

[0081] Specifically, if the cumulative time t at 1000 to 1100°C in the hot forging process is 30 hours or less, excessive growth of M(C,N) type carbonitrides can be suppressed. As a result, the forged steel roll can satisfy Feature 2. Therefore, the cumulative time t at 1000 to 1100°C in the hot forging process is 30 hours or less. There are no particular limitations on the cumulative time t at 1000 to 1100° C. Considering normal industrial production, the lower limit of the cumulative time t at 1000 to 1100° C. is, for example, 8 hours.

[0082] [(Process 3) Annealing process] In the annealing process, the intermediate blank produced in the hot forging process is annealed. By performing the annealing process, the intermediate blank is easier to grind in the next rough processing process. Annealing may be performed under well-known conditions using an electric furnace or a gas furnace. The annealing temperature is, for example, 500 to 800°C. The holding time is, for example, 10 to 50 hours.

[0083] [(Process 4) Rough processing process] In the rough processing step, the intermediate preform after the annealing step is subjected to rough processing to further roughly process the intermediate preform into a shape close to the final roll shape. The rough processing is, for example, grinding. The rough processing may be performed under well-known conditions.

[0084] [(Process 5) Quenching process] In the quenching process, the surface layer of the intermediate preform after the rough machining process is subjected to a well-known quenching process. Specifically, the intermediate preform is heated to and held at 900 to 1100°C, and then rapidly cooled. The rapid cooling method is, for example, water cooling.

[0085] After the quenching step and before the tempering step, the intermediate material may be subjected to sub-zero treatment. The cooling temperature in the sub-zero treatment may be in a known range, for example, -30 to -196°C.

[0086] [(Step 6) Tempering process] In the tempering process, the intermediate blank after the quenching process is tempered. The amount of retained austenite remaining at the time of quenching is reduced by tempering. Furthermore, the hardness of the surface layer of the forged steel roll is adjusted by tempering. The tempering temperature is, for example, 100 to 600°C.

[0087] [(Process 7) Finishing process] In the finishing process, the intermediate blank after the tempering process is subjected to finishing, such as grinding using a grinding machine, to form the intermediate blank into the shape of the final product.

[0088] The forged steel roll of this embodiment is manufactured by the above steps. The above-mentioned manufacturing method is a preferred example of the manufacturing method of the forged steel roll according to this embodiment. Therefore, the forged steel roll having the above-mentioned configuration may be manufactured by a manufacturing method other than the above-mentioned manufacturing method. In short, the manufacturing method is not particularly limited as long as it can manufacture the forged steel roll of this embodiment having the above-mentioned configuration. [Example]

[0089] [Manufacturing forged steel rolls] Forged steel rolls having the chemical compositions shown in Tables 1-1 and 1-2 were manufactured by the following manufacturing method.

[0090] [Table 1-1]

[0091] [Table 1-2]

[0092] Specifically, electrode ingots were cast from molten steel by the bottom pouring ingot casting method. Electroslag remelting (ESR) was performed using the produced electrode ingots as electrodes. Condition 1 for the ESR method (solidification rate SR (mm / min)) was as shown in Table 2. A hot forging process was performed on the forging ingots obtained by the ESR method. Condition 2 for the hot forging process (cumulative time t (hours) at 1000-1100°C) was as shown in Table 2. By the hot forging process, intermediate blanks in the shape of rolls with a roll barrel diameter of φ700 mm, a barrel length of 2100 mm, and a total length of 4100 mm were produced for each test number.

[0093] [Table 2]

[0094] The intermediate blanks after the hot forging process were subjected to an annealing process. In the annealing process, they were held at 800°C for 10 hours, and then further held at 600°C for 15 hours. The intermediate blanks after the annealing process were subjected to a rough machining process. Specifically, for each test number, grinding was performed on the intermediate blanks to produce intermediate blanks in the shape of rolls with a roll barrel diameter of 650 mm, a barrel length of 2000 mm, and a total length of 4000 mm.

[0095] The intermediate preform after the rough machining step was subjected to a quenching step in which the intermediate preform was heated to and held at 900 to 1100°C, and then water-cooled.

[0096] The intermediate preform after the quenching process was subjected to sub-zero treatment. In the sub-zero treatment, the intermediate preform was cooled to -60 to -140°C. The sub-zero treated intermediate preform was then subjected to a tempering process at 100 to 200°C, followed by a finishing process. In the finishing process, the intermediate preform was ground to a final roll shape with a roll barrel diameter of 645 mm, a barrel length of 1950 mm, and a total length of 3950 mm. The forged steel rolls of each test number were manufactured by the above manufacturing process. The total area ratio of martensite and bainite in the surface layer of the forged steel roll of each test number was determined by the method described in the above-mentioned "Method for measuring the total area ratio of martensite and bainite in the surface layer of a forged steel roll." As a result, the total area ratio of martensite and bainite in the surface layer was 85% or more in the forged steel rolls of all test numbers.

[0097] [About the evaluation test] The following evaluation tests were carried out on the manufactured forged steel rolls with each test number. (Test 1) Measurement test of the number ratio of M(C,N) type carbonitrides (Test 2) Crack resistance evaluation test Each test will be explained below.

[0098] [(Test 1) Measurement test of the number ratio of M(C,N) type carbonitrides] The number proportion (%) of M(C,N) type carbonitrides in the forged steel roll of each test number was determined according to the method described above in [Method for measuring the number proportion of M(C,N) type carbonitrides]. The number proportions of M(C,N) type carbonitrides obtained are shown in Table 2.

[0099] [(Test 2) Crack resistance evaluation test] To evaluate the crack resistance, a thermal shock test was carried out using a drop weight friction thermal shock tester 10 shown in Fig. 2. Specifically, a test piece 13 measuring 20 mm in the depth direction, 20 mm in the circumferential direction, and 30 mm in the longitudinal direction was taken from the surface of the longitudinal center position of the barrel of each forged steel roll of each test number.

[0100] A rod-shaped biting member 12 with a diameter of 5 mm and a length of 10 mm was attached to the outer periphery of the pinion 11 of a drop-weight frictional thermal shock tester 10. The biting member 12 was made of mild steel wire SWRM6 as specified in JIS G 3505:2017. The tip of the biting member 12 was bent and embedded radially into the pinion 11 from the outer periphery of the pinion 11. As shown in FIG. 2 , the surface 13A of the test piece 13 was brought into contact with the biting member 12. The biting member 12 was positioned so that the portion of the biting member 12 other than the tip extended upward. The test piece 13 and the biting member 12 were positioned so that the bent portion of the biting member 12 contacted the upper part of the surface 13A. The surface 13A was 20 mm × 30 mm and was positioned so that the 30 mm side was vertical. The surface 13A corresponded to the surface of the forged steel roll. After the test piece 13 and the biting member 12 were arranged as described above, the pinion 11 was rotated using a weight, causing the biting member 12 to slide strongly on the surface 13A, thereby applying a thermal shock to the surface 13A.

[0101] Fig. 3 is a schematic diagram of the surface 13A after the biting material 12 has passed over the surface 13A. As shown in Fig. 3, a contact area 100, which is the portion where the biting material 12 has slid, was formed in the longitudinal direction on the surface 13A. Therefore, as shown in Fig. 3, the contact area 100 was cut at approximately the center of the width along a plane CS including the longitudinal direction and normal direction of the surface 13A.

[0102] The entire contact area 100 of the cut surface was observed with a 100x optical microscope, and the surface vicinity of the entire contact area 100 was continuously photographed to generate multiple continuous photographic images. Figure 4 is a cross-sectional view showing a portion of the continuous photographic images. Using the continuous photographic images, the number of cracks 50 occurring throughout the entire contact area 100 and the maximum depth of the cracks 50 were confirmed. In each photographic image, the length NL of the crack 50 in the normal direction N of the surface 13A was recognized as the crack depth (μm). A crack depth of 10 μm or greater was recognized as a crack. The number of recognized cracks was calculated. Furthermore, the maximum crack depth of the recognized cracks was calculated. The obtained maximum crack depth (μm) and number of cracks are shown in the "Maximum crack depth (μm)" and "Number of cracks (cracks)" columns in Table 3.

[0103] [Evaluation results] With reference to Tables 1-1, 1-2, and 2, test numbers 1 to 19 had appropriate chemical compositions and were manufactured using the above-described preferred manufacturing process. Therefore, the number ratio of M(C,N) type carbonitrides was 5.0% or more. As a result, in the crack resistance evaluation test, these test numbers had a maximum crack depth of 300 μm or less and the number of cracks was 20 or less. In other words, these test numbers had excellent crack resistance.

[0104] Furthermore, the N content was 0.1000% or less in test numbers 1, 2, 4, 6, 9, 11, 13 to 15, and 17 to 19. As a result, in the crack resistance evaluation test, these test numbers had a maximum crack depth of 200 μm or less, and had even better crack resistance.

[0105] On the other hand, test number 20 had an excessively high N content, and as a result, in the crack resistance evaluation test, this test number had a maximum crack depth of more than 300 μm, indicating that it did not have excellent crack resistance.

[0106] In test numbers 21 to 23, the solidification rate (SR) of the ESR method in the steelmaking process was too slow. As a result, the proportion of M(C,N) type carbonitrides was less than 5.0%. As a result, in the crack resistance evaluation test, these test numbers had more than 20 cracks, indicating that they did not have excellent crack resistance.

[0107] For test numbers 24 to 26, the cumulative time t at 1000 to 1100°C in the hot forging process was too long. As a result, the proportion of M(C,N) type carbonitrides was less than 5.0%. As a result, these test numbers had more than 20 cracks in the crack resistance evaluation test, indicating that they did not have excellent crack resistance.

[0108] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. The chemical composition, in mass%, is C: 0.70-1.50%, Si: 0.20-1.50%, Mn: 0.20-1.50%, P: 0.030% or less, S: 0.0200% or less, Al: 0.050% or less, N: 0.0200-0.2000%, O: 0.0050% or less, Cr: 2.80-8.00%, Mo: 0.20-3.00%, V: 0.10-2.00%, Cu: 0.100% or less, B: 0.0100% or less, and the balance being Fe and impurities; Among carbides and carbonitrides having an equivalent circle diameter of 0.50 μm or more, the number ratio of M(C,N) type carbonitrides is 5.0% or more; Forged steel roll.

2. The chemical composition, in mass%, is C: 0.70-1.50%, Si: 0.20-1.50%, Mn: 0.20-1.50%, P: 0.030% or less, S: 0.0200% or less, Al: 0.050% or less, N: 0.0200-0.2000%, O: 0.0050% or less, Cr: 2.80-8.00%, Mo: 0.20-3.00%, V: 0.10-2.00%, Cu: 0.100% or less, and B: 0.0100% or less, Ni: 1.20% or less, Co: 0.50% or less, Nb: 0.50% or less, Ti: 0.050% or less, and W: 0.50% or less, containing one or more elements selected from the group consisting of the balance being Fe and impurities; Among carbides and carbonitrides having an equivalent circle diameter of 0.50 μm or more, the number ratio of M(C,N) type carbonitrides is 5.0% or more; Forged steel roll.

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