Forged steel roller

Abstract

Provided is a forged steel roller capable of obtaining excellent crack resistance. This forged steel roller has a chemical composition containing, in mass%, 0.70 to 1.50% of C, 0.20 to 1.50% of Si, 0.20 to 1.50% of Mn, 0.030% or less of P, 0.0200% or less of S, 0.050% or less of Al, 0.2000% or less of N, 0.0050% or less of O, 2.80 to 8.00% of Cr, 0.40 to 3.00% of Mo, 0.100% or less of Cu, 0.0100% or less of B, 0 to 1.20% of Ni, 0 to 2.00% of V, and 0 to 0.50% of Nb, with the remainder being Fe and impurities, and the surface layer of the forged steel roller has an average large-angle grain boundary length per 1 mm2 of 2500 to 9000 mm.

Description

Technical Field

[0001] The present invention relates to a forged steel roll, and more particularly to a forged steel roll suitable for cold rolling. Background Art

[0002] Forged steel rolls are used as rolling rollers, particularly for cold rolling. When forged steel rolls are used for long-term cold rolling, their surface roughness gradually decreases. This reduction in surface roughness can lead to slippage between the forged steel roll and the rolled material. This slippage can cause poor engagement. Furthermore, during the rolling process, seizure can occur between the forged steel roll and the rolled material.

[0003] When slippage or seizure occurs, the roller surface is subjected to thermal shock. This thermal shock can cause cracks to appear on the surface of the forged steel roller. Continued use of a cracked forged steel roller will gradually expand. If the crack continues to grow, parts of the roller surface may peel off. This phenomenon is called spalling.

[0004] When cracks are detected in forged steel rolls, the roll surface must be ground down to remove the cracks, depending on the crack depth, to prevent spalling. Deeper cracks increase the amount of roller grinding required to remove them. This increased grinding also degrades roll unit consumption (kg / ton). Therefore, there is a need for forged steel rolls that can suppress the initiation and growth of cracks caused by thermal shock. Hereinafter, the ability to suppress both the initiation and growth of cracks caused by thermal shock will be referred to as "excellent crack resistance."

[0005] Japanese Patent Application Publication No. 2-185928 (Patent Document 1), Japanese Patent Application Publication No. 1-234548 (Patent Document 2), Japanese Patent Application Publication No. 5-086439 (Patent Document 3), Japanese Patent Application Publication No. 5-132738 (Patent Document 4), and Japanese Patent Application Publication No. 2010-242166 (Patent Document 5) propose technologies for improving the resistance of forged steel rolls to thermal shock.

[0006] In the method for manufacturing a forged steel roll disclosed in Patent Document 1, the surface layer of a forged steel material containing 0.7-1.0% C, 0.15-1.5% Si, 0.15-1.5% Mn, 3.0-6.0% Cr, 3.0-5.0% Mo, and 1.2% or less V is quenched. The quenched forged steel material is then cryogenically treated. The cryogenically treated forged steel material is then tempered at a temperature of 180°C or higher. Patent Document 1 raises the tempering temperature by 40°C or higher compared to conventional methods by increasing the resistance to temper softening. This results in improved crack resistance.

[0007] The forged steel roll disclosed in Patent Document 2 contains, by weight, 0.45-0.95% C, 1.0% or less Mn, 4.5-6.0% Cr, 0.3-0.7% Mo, and 0.6-2.0% Ni, with the balance being Fe and impurities. Furthermore, the Si content is suppressed to less than 0.1%. Patent Document 2 improves the spalling resistance and thermal shock cracking resistance of the forged steel roll by suppressing the Si content as an impurity to less than 0.1% and setting the Ni content to 0.6-2.0%.

[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 balance being Fe and impurities. Patent Document 3 discloses that the forged steel roll has improved thermal shock resistance by having this chemical composition.

[0009] The forged steel roll disclosed in Patent Document 4 contains, by weight, 0.7-1.4% C, 0.8-2.5% Si, 0.8-2.5% Mn, 0.5-2.5% Ni, 2.5-6.5% Cr, 2.5-8.5% Mo, 0.3-3.0% W, 0.5-4.5% V, with the balance being Fe and impurities. After deep cooling and subsequent tempering, the forged steel roll contains retained austenite in an amount exceeding 15% and not exceeding 40%. Patent Document 4 suppresses crack growth by including retained austenite.

[0010] The forged steel roll disclosed in Patent Document 5 contains, by mass, 0.6-1.2% C, 0.4-0.8% Si, 0.4-1.0% Mn, 0.4-1.0% Ni, 3.0-6.0% Cr, and 0.2-0.5% Mo, with the balance being Fe and impurities. In this forged steel roll, the average particle size of carbides dispersed in the metallographic structure of the roll surface within 50 mm from the roll surface is 1 μm or less. Furthermore, the area fraction of the dispersed carbides is 5-30%. Patent Document 5 suppresses cracking by dispersing these carbides in the roll surface.

[0011] Prior art literature

[0012] Patent Literature

[0013] Patent Document 1: Japanese Patent Application Laid-Open No. 2-185928

[0014] Patent Document 2: Japanese Patent Application Laid-Open No. 1-234548

[0015] Patent Document 3: Japanese Patent Application Laid-Open No. 5-086439

[0016] Patent Document 4: Japanese Patent Application Laid-Open No. 5-132738

[0017] Patent Document 5: Japanese Patent Application Laid-Open No. 2010-242166 Summary of the Invention

[0018] Problems to be solved by the invention

[0019] However, it is also possible to simultaneously suppress crack initiation and crack expansion due to thermal shock by a means different from the means described in Patent Documents 1 to 5.

[0020] An object of the present invention is to provide a forged steel roll capable of suppressing the occurrence of cracks due to thermal shock and also suppressing the expansion of the cracks.

[0021] Solutions for solving problems

[0022] The chemical composition of the forged steel roll of the present invention is expressed in mass %.

[0023] C: 0.70~1.50%,

[0024] Si: 0.20-1.50%,

[0025] Mn: 0.20-1.50%,

[0026] P: 0.030% or less,

[0027] S: 0.0200% or less,

[0028] Al: 0.050% or less,

[0029] N: 0.2000% or less,

[0030] O: 0.0050% or less,

[0031] Cr: 2.80~8.00%,

[0032] Mo: 0.40~3.00%,

[0033] Cu: 0.100% or less,

[0034] B: 0.0100% or less,

[0035] Ni: 0-1.20%,

[0036] V: 0~2.00%

[0037] Nb: 0-0.50%, and

[0038] Balance: Fe and impurities,

[0039] The average thickness of the surface layer of the forged steel roller is 1 mm. 2 The length of the high-angle grain boundary is 2500 to 9000 mm.

[0040] Effects of the Invention

[0041] The forged steel roll of the present invention can suppress the occurrence of cracks due to thermal shock and can also suppress the expansion of the cracks. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 4 is a structural diagram of a drop-weight friction thermal shock tester used in the crack resistance evaluation test in Examples.

[0043] Figure 2 is used Figure 1 Schematic diagram of the contact area obtained in the crack resistance evaluation test of the drop weight friction thermal shock tester.

[0044] Figure 3 yes Figure 2 Cross-sectional view of the contact area shown. DETAILED DESCRIPTION

[0045] The present inventors first studied forged steel rolls with excellent crack resistance from the perspective of chemical composition. The inventors concluded that excellent crack resistance could be achieved if the forged steel roll had the following chemical composition: 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: 0.2000% or less, O: 0.0050% or less, Cr: 2.80-8.00%, Mo: 0.40-3.00%, Cu: 0.100% or less, B: 0.0100% or less, Ni: 0-1.20%, V: 0-2.00%, and Nb: 0-0.50%, with the balance being Fe and impurities.

[0046] Therefore, the present inventors further investigated and studied the mechanism of crack generation and crack propagation when a thermal shock is applied to the surface layer of a forged steel roll satisfying the above-mentioned chemical composition.

[0047] Here, the present inventors focused on the grain boundary length of the surface layer of the forged steel roll. The present inventors believed that the grain boundary length of the surface layer would affect the generation or expansion of cracks. Therefore, the present inventors studied the generation and expansion of cracks caused by thermal shock and the average length of the surface layer per 1 mm. 2 The relationship between the length of high-angle grain boundaries and the length of high-angle grain boundaries was investigated and studied. Here, "high-angle grain boundary length" refers to the length of grain boundaries where the orientation difference between adjacent grains is 15° or more. Through investigation and research, the present inventors have obtained the following insights.

[0048] Average per 1mm 2 The length of the high-angle grain boundary affects both the generation and expansion of cracks caused by thermal shock. Specifically, if the high-angle grain boundary length is too short, the expansion of cracks will be suppressed, but the frequency of crack generation will increase. On the other hand, if the high-angle grain boundary length is too long, the frequency of crack generation will be suppressed, but the cracks will easily expand. Therefore, in order to suppress the generation of cracks caused by thermal shock in the forged steel roll having the above chemical composition and to suppress the expansion of cracks, the average length of the high-angle grain boundary per 1mm is set to 100mm. 2 It is effective to adjust the high-angle grain boundary length to an appropriate range.

[0049] Based on the above findings, the present inventors conducted further research. As a result, the present inventors found that in a forged steel roll that satisfies the above chemical composition, if the average thickness of the forged steel roll surface is 1 mm 2 When the length of the high-angle grain boundary is 2500 to 9000 mm, even if a thermal shock is applied during use, the generation of cracks can be suppressed, and the expansion of cracks can also be suppressed, thereby obtaining excellent crack resistance.

[0050] The forged steel roll of the present embodiment, which was completed based on the above findings, has the following configuration.

[0051] The chemical composition of the forged steel roll of the first structure is expressed in mass %.

[0052] C: 0.70~1.50%,

[0053] Si: 0.20-1.50%,

[0054] Mn: 0.20-1.50%,

[0055] P: 0.030% or less,

[0056] S: 0.0200% or less,

[0057] Al: 0.050% or less,

[0058] N: 0.2000% or less,

[0059] O: 0.0050% or less,

[0060] Cr: 2.80~8.00%,

[0061] Mo: 0.40~3.00%,

[0062] Cu: 0.100% or less,

[0063] B: 0.0100% or less,

[0064] Ni: 0-1.20%,

[0065] V: 0~2.00%

[0066] Nb: 0-0.50%, and

[0067] Balance: Fe and impurities,

[0068] The average thickness of the surface of the forged steel roll is 1 mm. 2 The length of the high-angle grain boundary is 2500 to 9000 mm.

[0069] The forged steel roll of the second structure is the forged steel roll of the first structure,

[0070] The above chemical composition contains

[0071] Ni: 0.01~1.20%,

[0072] V: 0.01 to 2.00%, and

[0073] Nb: One or more kinds selected from the group consisting of 0.01 to 0.50%.

[0074] The forged steel roll of the third configuration is the forged steel roll of the first and second configurations, wherein:

[0075] The N content is 0.0200% or less in mass%.

[0076] The forged steel roller of the fourth structure is

[0077] A forged steel roller of any one of the first to third structures,

[0078] Furthermore, the Si content is 0.40 to 1.50% by mass.

[0079] The above chemical composition satisfies formula (1),

[0080] The forged steel roll has a Vickers hardness of 400 HV or higher at 400°C.

[0081] 4.50≤Cr+Mo+V+Nb≤13.50 (1)

[0082] Here, the content of each element in the formula is substituted for the symbol of each element in mass %. If the element is not contained, "0" is substituted for the symbol of the corresponding element.

[0083] Hereinafter, the forged steel roll of the present embodiment will be described in detail. Unless otherwise specified, "%" for an element represents mass %.

[0084] [Features of the Forged Steel Roll of the Present Embodiment]

[0085] The forged steel roll of the present embodiment satisfies the following features 1 and 2.

[0086] (Feature 1)

[0087] The chemical composition contains, in mass%, 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: 0.2000% or less, O: 0.0050% or less, Cr: 2.80-8.00%, Mo: 0.40-3.00%, Cu: 0.100% or less, B: 0.0100% or less, Ni: 0-1.20%, V: 0-2.00%, and Nb: 0-0.50%, with the balance being Fe and impurities.

[0088] (Feature 2)

[0089] The average thickness of the forged steel roller surface is 1mm 2 The length of the high-angle grain boundary is 2500 to 9000 mm.

[0090] Hereinafter, feature 1 and feature 2 will be described.

[0091] [Feature 1: Chemical composition]

[0092] The chemical composition of the forged steel roll of the present embodiment contains the following elements.

[0093] C: 0.70~1.50%

[0094] Carbon (C) increases the hardness of the forged steel roll surface. If the C content is less than 0.70%, this effect is not fully realized. On the other hand, if the C content exceeds 1.50%, coarse carbides are formed. In this case, the forged steel roll surface may not achieve sufficient hardness. Therefore, the C content is set between 0.70% and 1.50%.

[0095] The lower limit of the C content is preferably 0.75%, more preferably 0.80%, further preferably 0.85%, and further preferably 0.90%.

[0096] The upper limit of the C content is preferably 1.45%, more preferably 1.40%, further preferably 1.35%, further preferably 1.20%, further preferably 1.15%, further preferably 1.10%, further preferably 1.05%.

[0097] Si: 0.20~1.50%

[0098] Silicon (Si) deoxidizes steel in the molten steel stage. It also improves the hardenability of steel. If the Si content is less than 0.20%, these effects are not fully achieved. On the other hand, if the Si content exceeds 1.50%, the toughness of the forged steel roll decreases. Therefore, the Si content is preferably between 0.20% and 1.50%.

[0099] The preferred lower limit of the Si content is 0.25%, more preferably 0.30%, more preferably 0.40%, more preferably 0.45%, more preferably 0.50%, more preferably 0.55%, more preferably 0.60%, more preferably 0.65%, more preferably 0.70%, and more preferably 0.75%.

[0100] It should be noted that when the Si content is 0.40% or more, a sufficient amount of solid-solution Si can be obtained. In this case, the high-temperature hardness of the forged steel roll surface layer can be increased, provided that the equation (1) described below is satisfied. Consequently, crack resistance is further improved.

[0101] The upper limit of the Si content is preferably 1.45%, more preferably 1.40%, more preferably 1.35%, more preferably 1.30%, more preferably 1.25%, more preferably 1.20%, and more preferably 1.15%.

[0102] Mn: 0.20~1.50%

[0103] Manganese (Mn) improves the hardenability of steel. If the Mn content is less than 0.20%, this effect cannot be fully achieved. On the other hand, if the Mn content exceeds 1.50%, the toughness of the forged steel roll decreases. Therefore, the Mn content is 0.20-1.50%.

[0104] The lower limit of the Mn content is preferably 0.25%, more preferably 0.30%, further preferably 0.35%, further preferably 0.40%.

[0105] The upper limit of the Mn content is preferably 1.45%, more preferably 1.40%, more preferably 1.35%, more preferably 1.30%, and still more preferably 1.25%.

[0106] P: 0.030% or less

[0107] Phosphorus (P) is an impurity. P segregates at grain boundaries, reducing the toughness of forged steel rolls. Therefore, the P content is limited to 0.030% or less.

[0108] It is preferable to keep the P content as low as possible. However, excessively reducing the P content increases manufacturing costs. Therefore, considering typical industrial production, the preferred lower limit of the P content is greater than 0%, more preferably 0.001%, and even more preferably 0.002%.

[0109] The upper limit of the P content is preferably 0.025%, more preferably 0.020%.

[0110] S: 0.0200% or less

[0111] Sulfur (S) is an impurity. It segregates at grain boundaries, reducing the toughness and hot workability of forged steel rolls. Therefore, the S content is limited to 0.0200% or less.

[0112] It is preferable to keep the S content as low as possible. However, excessively reducing the S content increases manufacturing costs. Therefore, considering typical industrial production, the preferred lower limit of the S content is greater than 0%, more preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0113] The upper limit of the S content is preferably 0.0050%, more preferably 0.0040%, and further preferably 0.0030%.

[0114] Al: 0.050% or less

[0115] Aluminum (Al) deoxidizes steel in the molten steel stage. However, if the Al content exceeds 0.050%, coarse Al nitrides will form, reducing the toughness of the steel. Therefore, the Al content should be kept below 0.050%.

[0116] The lower limit of the Al content is preferably greater than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.005%, and even more preferably 0.010%.

[0117] The upper limit of the Al content is preferably 0.040%, more preferably 0.035%, further preferably 0.030%, and further preferably 0.025%.

[0118] It should be noted that, in this specification, the Al content refers to the content of all Al (Total Al) in steel.

[0119] N: 0.2000% or less

[0120] Nitrogen (N) increases the strength of forged steel rolls through solid solution strengthening. In particular, it improves the high-temperature hardness of forged steel rolls. However, if the N content exceeds 0.2000%, coarse nitrides will form, reducing the toughness of the forged steel rolls. Therefore, the N content should be kept below 0.2000%.

[0121] The lower limit of the N content is preferably greater than 0%, more preferably 0.0001%, more preferably 0.0005%, more preferably 0.0010%, more preferably 0.0025%, more preferably 0.0035%, and more preferably 0.0045%.

[0122] The upper limit of the N content is preferably 0.1900%, more preferably 0.1800%, more preferably 0.1500%, more preferably 0.1200%, more preferably 0.1000%, more preferably 0.0500%, more preferably 0.0200%, more preferably 0.0150%, and even more preferably 0.0100%.

[0123] O: 0.0050% or less

[0124] Oxygen (O) is an impurity. O forms oxides, which reduce the toughness of forged steel rolls. Therefore, the O content is 0.0050% or less.

[0125] It is preferable to keep the O content as low as possible. However, excessively reducing the O content increases manufacturing costs. Therefore, considering typical industrial production, the preferred lower limit of the O content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, more preferably 0.0007%, and even more preferably 0.0010%.

[0126] The upper limit of the O content is preferably 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0127] Cr: 2.80~8.00%

[0128] Chromium (Cr) forms carbides, improving the wear resistance of forged steel rolls. Cr also increases the steel's resistance to temper softening through solid solution and enhances the high-temperature hardness of the forged steel roll surface. If the Cr content is less than 2.80%, these effects cannot be fully achieved. On the other hand, if the Cr content exceeds 8.00%, coarse carbides form. In this case, the grindability and toughness of the forged steel roll are reduced. Therefore, the Cr content is between 2.80% and 8.00%.

[0129] The lower limit of the Cr content is preferably 2.85%, more preferably 3.00%, further preferably 3.50%, further preferably 4.00%.

[0130] The upper limit of the Cr content is preferably 7.50%, more preferably 7.00%, more preferably 6.50%, more preferably 6.00%, and still more preferably 5.50%.

[0131] Mo: 0.40~3.00%

[0132] Molybdenum (Mo) forms carbides, improving the wear resistance of forged steel rolls. Mo also increases the high-temperature hardness of the forged steel roll surface through solid solution. If the Mo content is less than 0.40%, these effects cannot be fully achieved. On the other hand, if the Mo content exceeds 3.00%, coarse carbides will form. In this case, the grindability and toughness of the forged steel roll will be reduced. Therefore, the Mo content is between 0.40% and 3.00%.

[0133] The lower limit of the Mo content is preferably 0.45%, more preferably 0.50%, and even more preferably 0.55%.

[0134] The upper limit of the Mo content is preferably 2.80%, more preferably 2.60%, more preferably 2.40%, more preferably 2.20%, more preferably 2.00%, more preferably 1.90%, more preferably 1.80%, more preferably 1.70%, and more preferably 1.60%.

[0135] Cu: 0.100% or less

[0136] Copper (Cu) is an impurity. Cu reduces the hot workability of steel. Therefore, the Cu content is 0.100% or less.

[0137] It is preferable to keep the Cu content as low as possible. However, excessively reducing the Cu content increases manufacturing costs. Therefore, considering typical industrial production, the preferred lower limit of the Cu content is greater than 0%, more preferably 0.001%, more preferably 0.005%, more preferably 0.010%, and even more preferably 0.015%.

[0138] The upper limit of the Cu content is preferably 0.095%, more preferably 0.090%, more preferably 0.085%, more preferably 0.080%, more preferably 0.075%, and even more preferably 0.070%.

[0139] B: 0.0100% or less

[0140] Boron (B) is an impurity. B reduces the toughness of the forged steel roll. Therefore, the B content is 0.0100% or less.

[0141] It is preferable to keep the B content as low as possible. However, excessively reducing the B content increases manufacturing costs. Therefore, considering typical industrial production, the preferred lower limit of the B content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, more preferably 0.0010%, more preferably 0.0015%, and even more preferably 0.0020%.

[0142] The upper limit of the B content is preferably 0.0090%, more preferably 0.0085%, further preferably 0.0080%.

[0143] The balance of the chemical composition of the forged steel roll of this embodiment is Fe and impurities. Impurities are substances that enter the forged steel roll during industrial production, such as from raw material ores and scrap, or from the manufacturing environment. These substances are permissible as long as they do not adversely affect the forged steel roll of this embodiment.

[0144] About Optional Elements

[0145] The chemical composition of the forged steel roll of this embodiment may further contain a compound selected from the group consisting of

[0146] Ni: 0-1.20%,

[0147] V: 0-2.00%, and

[0148] Nb: One or more kinds selected from the group consisting of 0 to 0.50% replace a portion of Fe.

[0149] The optional elements of each group are described below.

[0150] [Group 1: Ni]

[0151] Ni: 1.20% or less

[0152] Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When contained, that is, when the Ni content is greater than 0%, Ni improves the hardenability of the steel. Even if a small amount of Ni is contained, the above-mentioned effects can be achieved to a certain extent. However, if the Ni content exceeds 1.20%, excessive retained austenite will be formed. In this case, the hardness of the forged steel roll will be reduced. Therefore, the Ni content is 0 to 1.20%, and when contained, the Ni content is less than 1.20%.

[0153] The lower limit of the Ni content is preferably 0.01%, more preferably 0.05%, more preferably 0.10%, more preferably 0.15%, and even more preferably 0.20%.

[0154] The preferred upper limit of the Ni content is 1.15%, more preferably 1.10%, more preferably 1.05%, more preferably 1.00%, more preferably 0.95%, more preferably 0.90%, more preferably 0.85%, more preferably 0.80%, more preferably 0.75%, and more preferably 0.70%.

[0155] [Group 2: V and Nb]

[0156] V: 2.00% or less

[0157] Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When contained, that is, when the V content is greater than 0%, V forms carbides, thereby improving the wear resistance of the forged steel roll. In addition, V improves the high-temperature hardness of the forged steel roll by solid solution. Even if a small amount of V is contained, the above effect can be obtained to a certain extent. However, if the V content exceeds 2.00%, coarse carbides will be generated. In this case, the grindability and toughness of the forged steel roll will be reduced. Therefore, the V content is 0 to 2.00%, and when contained, the V content is less than 2.00%.

[0158] The lower limit of the V content is preferably 0.01%, more preferably 0.05%, more preferably 0.10%, more preferably 0.15%, more preferably 0.20%, and even more preferably 0.25%.

[0159] The upper limit of the V content is preferably 1.80%, more preferably 1.60%, more preferably 1.50%, more preferably 1.40%, more preferably 1.30%, and more preferably 1.20%.

[0160] Nb: 0.50% or less

[0161] 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 is greater than 0%, Nb forms carbides, thereby improving the wear resistance of the forged steel roll. In addition, Nb improves the high-temperature hardness of the forged steel roll by solid solution. Even if a small amount of Nb is contained, the above effect can be achieved to a certain extent. However, if the Nb content exceeds 0.50%, coarse carbides will be generated. In this case, the grindability and toughness of the forged steel roll will be reduced. Therefore, the Nb content is 0 to 0.50%, and when contained, the Nb content is less than 0.50%.

[0162] The lower limit of the Nb content is preferably 0.01%, more preferably 0.05%, more preferably 0.10%, more preferably 0.15%, more preferably 0.20%, and still more preferably 0.25%.

[0163] The upper limit of the Nb content is preferably 0.45%, more preferably 0.42%, more preferably 0.40%, more preferably 0.38%, and even more preferably 0.36%.

[0164] [(Feature 2) About the average per 1mm of the surface 2 high-angle grain boundary length]

[0165] Furthermore, in the forged steel roll of this embodiment, the average thickness per 1 mm of the surface layer is2 The high-angle grain boundary length is 2500 to 9000 mm. Here, the "high-angle grain boundary length" refers to the length of the grain boundary where the orientation difference between adjacent grains is 15° or more.

[0166] Average per 1mm 2 When the high-angle grain boundary length exceeds 9000 mm, the high-angle grain boundary length is too long. In this case, the resistance to grain boundary sliding increases. Therefore, the frequency of cracks along the grain boundaries is suppressed. However, at the moment of crack formation, the accumulated excess energy is released, causing large-scale grain boundary sliding. As a result, the crack depth becomes excessive, and crack resistance decreases.

[0167] On the other hand, the average 2 When the high-angle grain boundary length is less than 2500 mm, the high-angle grain boundary length is too short. In this case, crack expansion is suppressed. However, since the resistance to grain boundary sliding is low, the frequency of crack generation increases. As a result, excessive cracks are generated, and crack resistance decreases.

[0168] If the average surface area is 1 mm 2 If the high-angle grain boundary length is 2500 to 9000 mm, the frequency of crack generation is sufficiently suppressed, and even if cracks occur, the expansion of the cracks is sufficiently suppressed. As a result, sufficient crack resistance is obtained.

[0169] Average per 1mm 2 The preferred lower limit of the high-angle grain boundary length is 3000 mm, more preferably 3200 mm, more preferably 3300 mm, more preferably 3400 mm, and more preferably 3500 mm.

[0170] Average per 1mm 2 The preferred upper limit of the high-angle grain boundary length is 8800 mm, more preferably 8600 mm, more preferably 8400 mm, more preferably 8300 mm, more preferably 8200 mm, more preferably 8100 mm, and more preferably 8000 mm.

[0171] [The average surface per 1mm 2 Method for determination of high-angle grain boundary length]

[0172] The average thickness per mm of the surface layer of the forged steel roll of this embodiment is 2 The high-angle grain boundary length is calculated by the following method.

[0173] Five test pieces were collected, each containing a depth of 1 mm from the surface of the forged steel roll. The surface of each test piece containing a depth of 1 mm from the forged steel roll surface was defined as the observation surface. The observation surface of each test piece was mirror-polished using diamond paste. The mirror-polished observation surface was then chemically polished using colloidal silica liquid to remove the process-induced deterioration layer introduced during mirror polishing.

[0174] Using SEM / EBSD / OIM, the polished observation surface was observed at a magnification of 5000x, covering a field of view equivalent to a depth of 1 mm from the forged steel roller surface. The observation field was set to 20 μm × 20 μm, and the measurement interval between adjacent measurement points was set to 0.03 μm. The accelerating voltage for EBSD measurement was set to 25 kV.

[0175] Use EBSD analysis software to obtain the crystal orientation of each measurement in the observation field. Based on the crystal orientation of each measurement point, the grain boundary with a crystal orientation difference of 15° or more between adjacent grains is defined as a "high-angle grain boundary". Measure the total length of the high-angle grain boundaries in the observation field. Based on the observation field area and the total length of the measured high-angle grain boundaries, calculate the average length per 1mm of each observation field of each test piece. 2 The arithmetic mean of the high-angle grain boundary lengths obtained in 5 test pieces is defined as the average length per 1 mm of the surface layer. 2 Length of high-angle grain boundaries (mm).

[0176] It should be noted that the EBSD analysis software used can be any known software. An example of known EBSD analysis software is OIM Analysis, manufactured by TSLSolutions. To eliminate the influence of measurement errors caused by noise on the data, data with an image quality (IQ) value of 11,000 or less, measured together, are excluded from analysis without cleaning.

[0177] [Effects of the Forged Steel Roll of the Present Embodiment]

[0178] The forged steel roll of this embodiment satisfies both Features 1 and 2. Therefore, the forged steel roll of this embodiment can achieve excellent crack resistance. Specifically, when using the forged steel roll, the frequency of crack occurrence is significantly reduced, and even if cracks do occur, their expansion is also significantly suppressed. As a result, excellent crack resistance can be achieved.

[0179] [Preferred Modes of the Forged Steel Roll of the Present Embodiment]

[0180] The forged steel roll of the present embodiment further preferably satisfies the following feature 3.

[0181] (Feature 3)

[0182] The chemical composition has a Si content of 0.40 to 1.50% by mass, and the chemical composition satisfies the formula (1). Furthermore, the Vickers hardness at 400° C. is 400 HV or higher.

[0183] 4.50≤Cr+Mo+V+Nb≤13.50 (1)

[0184] Here, the content of each element in the formula is substituted for the symbol of each element in mass %. If the element is not contained, "0" is substituted for the symbol of the corresponding element.

[0185] If the forged steel roll of this embodiment satisfies not only Features 1 and 2 but also Feature 3, the high-temperature hardness of the forged steel roll surface layer will be further improved. As a result, the frequency of cracks occurring during use of the forged steel roll is further suppressed, further improving crack resistance. Feature 3 includes the following three conditions.

[0186] (Condition 1) The Si content is 0.40 to 1.50% by mass.

[0187] (Condition 2) The chemical composition satisfies formula (1).

[0188] 4.50≤Cr+Mo+V+Nb≤13.50 (1)

[0189] Here, the content of each element in the formula is substituted for the symbol of each element in mass %. If the element is not contained, "0" is substituted for the symbol of the corresponding element.

[0190] It should be noted that when the chemical composition of the forged steel roll does not contain V and Nb as optional elements, that is, when the chemical composition of the forged steel roll consists only of essential elements, formula (1) is defined as follows.

[0191] 4.50≤Cr+Mo≤13.50 (1)

[0192] Here, the content of the corresponding element in mass % is substituted for each element symbol in the formula.

[0193] (Condition 3) The Vickers hardness at 400° C. is 400 HV or higher.

[0194] Conditions 1 to 3 of Feature 3 are described below.

[0195] [(Condition 1) Regarding Si content]

[0196] If the Si content is 0.40% or higher, the amount of dissolved Si in the forged steel roll increases. Consequently, the high-temperature hardness of the forged steel roll increases. This further reduces the frequency of cracks in the surface layer of the forged steel roll. Therefore, the preferred Si content is 0.40-1.50%.

[0197] [(Condition 2) Regarding formula (1)]

[0198] F1 is defined as follows.

[0199] F1=Cr+Mo+V+Nb

[0200] F1 is an index of the high-temperature hardness of the forged steel roll. When the chemical composition of the forged steel roll does not contain V and Nb as optional elements, that is, when the chemical composition of the forged steel roll consists only of essential elements, F1 is defined as follows.

[0201] F1=Cr+Mo

[0202] Cr, Mo, V, and Nb form carbides, improving the wear resistance of forged steel rolls. Cr, Mo, V, and Nb further dissolve in solid solution, increasing the high-temperature hardness of forged steel rolls. In other words, Cr, Mo, V, and Nb improve wear resistance by forming carbides and increase high-temperature hardness by dissolving in solid solution.

[0203] When F1 is 4.50 or higher, Cr, Mo, V, and Nb are distributed not only as carbide-forming elements but also as solid-solution elements. Therefore, when F1 is 4.50 or higher, the high-temperature hardness of the forged steel roll is further improved. As a result, the frequency of cracks during use of the forged steel roll is further suppressed, further improving crack resistance.

[0204] On the other hand, if F1 is 13.50 or less, which is the sum of the upper limits of the contents of Cr, Mo, V, and Nb, the coarsening of carbides of Cr, Mo, V, and Nb is sufficiently suppressed, so that the forged steel roll maintains sufficient grindability and toughness.

[0205] A more preferred lower limit of F1 is 4.70, more preferably 4.90, more preferably 5.10, more preferably 5.30, more preferably 5.50, more preferably 5.60, more preferably 5.70, more preferably 5.80, and more preferably 5.90. A more preferred upper limit of F1 is 13.40, more preferably 13.30, more preferably 13.20, more preferably 13.10, more preferably 13.00, more preferably 12.90, more preferably 12.80, more preferably 12.70, and more preferably 12.60.

[0206] In addition, when F1=Cr+Mo, the upper limit is more preferably 11.00, more preferably 10.80, and even more preferably 10.50.

[0207] [(Condition 3) Vickers hardness at 400°C]

[0208] Conventional forged steel rolls experience a sharp drop in hardness at around 400°C when heated from room temperature. Consequently, sufficient crack resistance cannot be achieved. If the hardness at 400°C is high, the frequency of cracks occurring in the surface layer of the forged steel roll is further suppressed. In the case of forged steel rolls that meet characteristics 1 and 2, if conditions 1 and 2 are met and the Vickers hardness at 400°C is 400 HV or higher (i.e., if characteristic 3 is met), the high-temperature hardness of the forged steel roll is further improved. As a result, the frequency of cracks occurring is further suppressed, further improving crack resistance.

[0209] The lower limit of the Vickers hardness at 400° C. is preferably 405 HV, more preferably 410 HV, more preferably 415 HV, more preferably 420 HV, and even more preferably 425 HV.

[0210] The upper limit of the Vickers hardness at 400° C. is not particularly limited. The upper limit of the Vickers hardness at 400° C. is preferably 600 HV, more preferably 550 HV, more preferably 520 HV, and even more preferably 500 HV. In this case, the toughness of the forged steel roll is further improved.

[0211] [Method for measuring Vickers hardness at 400°C]

[0212] The Vickers hardness of the forged steel roll at 400° C. was determined by the following method.

[0213] A test piece was collected from the surface of the forged steel roll to a depth of 1 mm. The size of the test piece was set to 5 mm × 5 mm × 10 mm, and the 5 mm × 10 mm was set as the measurement surface. The measurement surface was a cross section perpendicular to the surface of the forged steel roll, and was set to include the surface of the forged steel roll and the depth of 1 mm from the surface. The test piece with the thermocouple hanging and the indenter were placed in a vacuum (3 × 10 -5 Torr) from room temperature to 400°C and maintained at 400°C for 5 minutes. The Vickers hardness of the surface layer of the test piece after holding was determined using the high-temperature Vickers hardness test method according to JIS Z 2252:1991. Specifically, the Vickers hardness was measured at any five points on the measuring surface of the test piece at a depth of 1 mm from the surface of the forged steel roller. At this time, the test force was set to 300 gf. The arithmetic mean of the five Vickers hardnesses obtained was taken as the Vickers hardness (in HV) at 400°C.

[0214] [Microstructure of the Surface Layer of the Forged Steel Roll of the Present Embodiment]

[0215] The microstructure of the surface layer of the forged steel roll of this embodiment is primarily composed of martensite and / or bainite. "Mainly composed of martensite and / or bainite" means that the combined area ratio of martensite and bainite is 85% or greater. Structures other than martensite and bainite in the microstructure include, for example, retained austenite and carbides.

[0216] [Method for measuring the total area ratio of martensite and bainite in the surface layer of a forged steel roll]

[0217] The total area ratio of martensite and bainite in the surface layer of the forged steel roll according to the present embodiment is determined by the following method.

[0218] Collect a test piece that includes a depth of 1 mm from the surface of the forged steel roller. The surface of the test piece that includes a depth of 1 mm from the surface of the forged steel roller is defined as the observation surface. Mirror-polish the observation surface. Etch the mirror-polished observation surface with 2% nitric acid (Nital). Observe any five observation fields (400 μm × 600 μm) at a depth of 1 mm from the surface of the forged steel roller using a 200x optical microscope.

[0219] Each observation field is etched with Murakami's reagent. Murakami's reagent colors the carbides in the observation field. This allows for easy distinction between carbides and other phases by comparison. Carbides are then identified in each observation field. The area ratio of carbides is calculated based on the total area of ​​carbides identified in all observation fields and the total area of ​​all observation fields.

[0220] Furthermore, the area ratio of retained austenite is determined by the following X-ray diffraction method. A test piece is collected at a depth of 1 mm from the surface of the forged steel roll body. The size of the test piece is not particularly limited, and for example, it is 15 mm × 15 mm × 10 mm thick. At this time, the thickness direction of the test piece is the radial direction of the forged steel roll. Using the obtained test piece, 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 the measurement of the X-ray diffraction intensity, the target of the X-ray diffraction device is set to Co (CoKα ray) and the output is set to 40 kV-135 mA. After calculation, for each combination of each plane of the α phase and each plane of the γ phase (3×3=9 groups), the volume ratio Vγ (%) of retained austenite is calculated using formula (1). Then, the average value of the volume fractions Vγ of the nine groups of retained austenite is defined as the volume fraction (%) of retained austenite.

[0221] Vγ=100 / {1+(Iα×Rγ) / (Iγ×Rα)}(I)

[0222] Wherein, 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. It should be noted that, in this specification, Rα of the (200) plane of the α phase is set to 9.9, Rα of the (211) plane of the α phase is set to 20.4, Rα of the (220) plane of the α phase is set to 9.3, Rγ of the (200) plane of the γ phase is set to 23.8, Rγ of the (220) plane of the γ phase is set to 12.5, and Rγ of the (311) plane of the γ phase is set to 17.2. It should be noted that, for the volume fraction of retained austenite, the obtained value is rounded to the second decimal place. It should be noted that, in the measurement of the X-ray diffraction intensity, in the test piece, X-rays are irradiated at a position corresponding to a depth position of 1 mm from the surface of the forged steel roll body.

[0223] The volume fraction (%) of retained austenite obtained by the above X-ray diffraction method was 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 was calculated by the following formula.

[0224] Total area ratio of martensite and bainite in the surface layer of forged steel roll = 100.0 - (area ratio of carbides + area ratio of retained austenite)

[0225] [Application of the Forged Steel Roll of the Present Embodiment]

[0226] The forged steel roll of this embodiment can be used as a rolling roll in a wide range of applications. It is particularly suitable for use as a roll for cold rolling thin steel sheets. Examples of cold rolling rolls include work rolls in tandem cold mills, reversing cold mills, and work rolls for temper rolling (quench and temper rolling).

[0227] [Method for manufacturing forged steel roller]

[0228] 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 can also be manufactured by other manufacturing methods besides the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a forged steel roll according to this embodiment.

[0229] An example of a method for manufacturing a forged steel roll according to the present embodiment includes the following steps.

[0230] (Process 1) Steelmaking process

[0231] (Process 2) Hot Forging Process

[0232] (Process 3) Annealing process

[0233] (Process 4) Rough machining process

[0234] (Process 5) Quenching process

[0235] (Process 6) Tempering process

[0236] (Process 7) Finishing process

[0237] Hereinafter, each step will be described.

[0238] [(Process 1) Steelmaking process]

[0239] In the steelmaking process, molten steel satisfying characteristic 1, or molten steel satisfying characteristic 1, conditions 1, and conditions 2, is used to cast an ingot using a known casting method. Examples of known casting methods include the bottom casting method. The cast ingot can be used as an electrode to perform electroslag remelting (ESR) or other methods.

[0240] [(Process 2) Hot Forging Process]

[0241] In the hot forging process, the ingot is first heated in a heating furnace. Then, the heated ingot is hot forged to produce a rough roll blank (hereinafter referred to as an intermediate blank). The hot forging process satisfies the following requirements 1 and 2.

[0242] (Requirement 1) Ingot temperature T1 immediately before hot forging: 1100-1200°C

[0243] (Requirement 2) Forging ratio S: 1.3 to 3.2

[0244] The following describes each necessary condition.

[0245] [(Requirement 1) Regarding the temperature T1 of the ingot immediately before hot forging]

[0246] The ingot temperature T1 immediately before hot forging is set to 1100-1200°C. If T1 is below 1100°C, it is too low. In this case, cracks may form in the ingot during hot forging. If T1 exceeds 1200°C, cracks may also form in the ingot during hot forging. Therefore, T1 is set to 1100-1200°C.

[0247] [(Requirement 2) Regarding the forging ratio S]

[0248] Furthermore, in the hot forging process, the forging ratio S is set to 1.3 to 3.2. In this specification, the forging ratio S refers to the forging ratio during solid forging. The forging ratio S is defined by the following formula.

[0249] Forging ratio S = cross-sectional area of ​​ingot before hot forging / cross-sectional area of ​​intermediate billet after hot forging

[0250] Here, the cross-sectional area of ​​the ingot before hot forging refers to the cross-sectional area of ​​the ingot perpendicular to the longitudinal direction. In addition, the cross-sectional area of ​​the intermediate billet body after hot forging refers to the cross-sectional area of ​​the intermediate billet body perpendicular to the longitudinal direction.

[0251] Forging ratio S will affect the average thickness of the forged steel roll after manufacture. 2 If the forging ratio S is less than 1.3, the average length of the forged steel roll per 1mm after manufacture is 2 On the other hand, if the forging ratio S exceeds 3.2, the average length of the forged steel roll per 1 mm after manufacture will be too short. 2 The length of the high-angle grain boundary will become too long. If the forging ratio S is set to 1.3 to 3.2, the average length of the forged steel roll per 1 mm can be 2 The length of the high-angle grain boundary is adjusted to an appropriate size.

[0252] [Preferred Conditions (Optional Conditions) in Hot Forging Process]

[0253] Preferably, the hot forging step further satisfies the following conditions. These conditions are optional.

[0254] (Optional condition 1) Heating temperature T0: 1200~1300℃

[0255] (Optional condition 2) Holding time t0 at heating temperature T0: 10 hours or more

[0256] (Optional condition 3) Temperature difference ΔT = heating temperature T0 - ingot temperature T1 immediately before hot forging: 50-150°C

[0257] Optional conditions 1 to 3 are described below.

[0258] [(Optional Condition 1 and Optional Condition 2) Regarding Heating Temperature T0 and Holding Time t0]

[0259] If the heating temperature T0 is set to 1200-1300°C and the holding time t0 is set to 10 hours or longer, Si carbides and other Si precipitates in the intermediate steel are fully dissolved in the subsequent annealing step. This ensures a sufficient amount of dissolved Si in the produced forged steel roll. As a result, the forged steel roll achieves a Vickers hardness of 400 HV or higher at 400°C.

[0260] [(Optional Condition 3) Regarding Temperature Difference ΔT]

[0261] Furthermore, if the temperature difference ΔT (subtracting the ingot temperature T1 immediately before hot forging from the heating temperature T0) is within the range of 50-150°C, Si carbides and other Si precipitates in the intermediate bar will be fully dissolved during the subsequent annealing step. This ensures a sufficient amount of dissolved Si in the produced forged steel roll. As a result, the forged steel roll achieves a Vickers hardness of 400 HV or higher at 400°C. A more preferred temperature difference ΔT is 60-100°C.

[0262] It should be noted that when the temperature of the steel during hot forging drops to 900°C, the steel is heated again in a heating furnace. Then, the reheated steel is hot forged again. The heating temperature T0 and the temperature T1 during reheating and hot forging again both meet the necessary condition 1. In addition, the preferred holding time t0 is 1 to 10 hours. It should be noted that when the steel during hot forging is heated again in a heating furnace, the temperature difference ΔT refers to the temperature difference during the first heating, and the temperature difference ΔT for the second and subsequent times may not meet the optional condition 3.

[0263] [(Step 3) Annealing step]

[0264] In the annealing process, the intermediate billet produced in the hot forging process is annealed. This annealing process makes the intermediate billet easier to grind in the subsequent rough machining process. Annealing can be performed in an electric furnace or a gas furnace under known conditions. The annealing temperature is, for example, 500-1000°C, and the holding time is, for example, 10-50 hours.

[0265] [(Process 4) Rough machining process]

[0266] In the rough machining process, the intermediate blank after the annealing process is subjected to rough machining to form a shape closer to the final roll shape. Rough machining is, for example, grinding. Rough machining can be performed under known conditions.

[0267] [(Process 5) Quenching process]

[0268] In the quenching step, the intermediate billet after the rough working step is quenched. The following requirements 3 to 5 are satisfied in the quenching step.

[0269] (Necessary condition 3) Quenching temperature T2: 950~1100℃

[0270] (Requirement 4) Holding time t2 at quenching temperature T2: 0.02 to 0.22 hours

[0271] (Requirement 5) FA defined as follows: 5.00 to 12.00

[0272] FA=S / (T2×(26+log 10 (5×t2))×10 -5 )

[0273] Here, "S" in FA is substituted with the forging ratio S. "T2" is substituted with the quenching temperature (°C). "t2" is substituted with the holding time (hours).

[0274] The following describes the requirements 3 to 5.

[0275] [(Requirement 3 and Requirement 4) Regarding Quenching Temperature T2 and Holding Time t2]

[0276] The quenching temperature T2 and holding time t2 will affect the length of the large-angle grain boundary of the forged steel roll after manufacture. If the quenching temperature T2 is too low or the holding time t2 is too short, the average length of the large-angle grain boundary of the forged steel roll after manufacture will be 2 On the other hand, if the quenching temperature T2 is too high or the holding time t2 is too long, the average length of the forged steel roll per 1 mm after manufacture will be too long. 2 If the quenching temperature T2 is set to 950-1100°C and the holding time t2 is set to 0.02-0.22 hours, the average length of the forged steel roll per 1 mm can be shortened. 2 The length of the high-angle grain boundary is adjusted to an appropriate size.

[0277] [(Requirement 5)FA]

[0278] Forging ratio S is related to quenching temperature T2 and holding time t2, thus affecting the average per 1mm of forged steel roll. 2 The length of high-angle grain boundary. When the relationship between forging ratio S, quenching temperature T2 and holding time t2 is less than 5.00, the average length of the forged steel roll after manufacturing is 1mm. 2 On the other hand, if FA exceeds 12.00, the average length of the forged steel roll per 1 mm after manufacture is too short. 2 If FA is set to 5.00 to 12.00, the average length of the forged steel roll per 1 mm after manufacture can be increased. 2 The length of the high-angle grain boundary is adjusted to an appropriate size.

[0279] [Preferred Conditions (Optional Conditions) in the Quenching Step]

[0280] Preferably, the following conditions are further satisfied in the quenching step.

[0281] (Optional condition 4) Cooling time CT from quenching temperature T2 to 800°C: 30 to 300 seconds

[0282] If the cooling time CT from the quenching temperature T2 to 800°C is 30 to 300 seconds, the formation of dissolved Si as Si precipitates can be sufficiently suppressed, and carbides composed of alloys other than Si can be sufficiently formed. This can improve the high-temperature hardness of the forged steel roll.

[0283] It should be noted that after the quenching step and before the tempering step, the intermediate piece may be subjected to a cryogenic treatment. The cooling temperature in the cryogenic treatment may be within a known range, for example, -30 to -196°C.

[0284] [(Process 6) Tempering process]

[0285] In the tempering step, the intermediate steel after the quenching step is tempered. The hardness of the surface layer of the forged steel roll is adjusted by tempering. The tempering temperature is set to 100 to 600°C, for example.

[0286] [(Process 7) Finishing process]

[0287] In the finishing process, the intermediate blank after the tempering process is subjected to finishing. Finishing can be performed, for example, by grinding using a grinder. This finishing process forms the intermediate blank into the final product shape. Through the above-described process, the forged steel roll of this embodiment can be manufactured.

[0288] The present invention will be described in more detail below with reference to the following examples, but the present invention is not limited to these examples in any way.

[0289] Example

[0290] [Manufacturing of forged steel rolls]

[0291] Forged steel rolls having the chemical compositions shown in Table 1 were produced by the following production method.

[0292] [Table 1]

[0293] Table 1

[0294]

[0295] Specifically, an ingot is cast from molten steel using a bottom casting method. The produced ingot is remelted using an electroslag remelting (ESR) method to produce an ingot as a billet. The produced ingot is subjected to a hot forging process. Necessary condition 1 (forging temperature T1 (°C)), necessary condition 2 (forging ratio S), and optional condition 1 (heating temperature T0 (°C)), optional condition 2 (holding time t0 (hours)), and optional condition 3 (temperature difference ΔT (°C)) in the hot forging process are shown in Table 2. Through the hot forging process, each test number produced an intermediate billet in the shape of a roller with a roller body diameter of φ700 mm, a roller body length of 2100 mm, and a total length of 4100 mm.

[0296] [Table 2]

[0297] Table 2

[0298]

[0299] It should be noted that in the forging process, the ingot is heated and maintained using the heating temperature T0 and the holding time t0. Then, the temperature of the ingot is lowered to the forging temperature T1, and hot forging is performed. It should be noted that in all test numbers, the ingot temperature dropped to 900°C during the hot forging process. Therefore, the hot forging was temporarily interrupted, and the ingot was heated again in the heating furnace (second heating). The reheated ingot was hot forged again. Table 2 shows the first and second heating temperatures T0, holding time t0, temperature T1, and the temperature difference ΔT of the first time.

[0300] After the hot forging process, the intermediate billet was annealed. During the annealing process, it was held at 900°C for 10 hours. It was then held at 600°C for 15 hours. The intermediate billet after the annealing process was then subjected to a rough machining process. Specifically, for each test number, the intermediate billet was ground to produce a roller-shaped intermediate billet with a barrel diameter of 650 mm, a barrel length of 2000 mm, and a total length of 4000 mm.

[0301] After the roughing process, the intermediate billet is subjected to a quenching process. Table 2 shows the required conditions 3 (quenching temperature T2 (°C)), 4 (holding time t2 (hours)), 5 (FA), and 4 (cooling time CT (seconds)) for the quenching process. It should be noted that in the quenching process, the intermediate billet is heated by induction heating and then water-cooled.

[0302] After the quenching process, the intermediate billet was cryogenically treated. During the cryogenic treatment, the intermediate billet was cooled to -60 to -140°C. The cryogenically treated intermediate billet was tempered at 100 to 600°C and then subjected to a finishing process. During the finishing process, the intermediate billet was ground to a final roll shape with a barrel diameter of φ645 mm, a barrel length of 1950 mm, and a total length of 3950 mm. Through the above manufacturing process, forged steel rolls of various test numbers were produced. It should be noted that the total area ratio of martensite and bainite in the surface layer of the forged steel rolls of each test number was determined based on the method described in the [Method for Determining the Total Area Ratio of Martensite and Bainite in the Surface Layer of Forged Steel Rolls] above. The results showed that the total area ratio of martensite and bainite in the surface layer of the forged steel rolls of all test numbers was 85% or greater.

[0303] [Evaluation test]

[0304] The following evaluation tests were performed using the forged steel rolls of the respective test numbers.

[0305] (Test 1) Average per 1mm 2 High-angle grain boundary length measurement test

[0306] (Test 2) Vickers hardness test at 400°C

[0307] (Test 3) Crack resistance evaluation test

[0308] Hereinafter, Tests 1 to 3 will be described.

[0309] [(Test 1) Average per 1mm 2 High-angle grain boundary length measurement test]

[0310] According to the above [the average surface per 1mm 2 The method described in [Method for measuring the length of high-angle grain boundaries] was used to determine the average length per 1 mm of the surface layer of the forged steel roll of each test number. 2 High-angle grain boundary length (mm / mm 2 The average value of each 1mm 2 The high-angle grain boundary length is shown in Table 3 as “High-angle grain boundary length (mm / mm 2 )” column.

[0311] [Table 3]

[0312] Table 3

[0313]

[0314] [(Test 2) Vickers hardness test at 400°C]

[0315] The Vickers hardness (HV) at 400°C of the forged steel rolls of each test number was determined according to the method described in the "Method for Measuring Vickers Hardness at 400°C" section above. Measurements were performed using a QM2 high-temperature microscope hardness tester manufactured by Nikon Corporation. The resulting Vickers hardness is shown in the "Vickers Hardness (HV) at 400°C" column in Table 3.

[0316] [(Test 3) Crack resistance evaluation test]

[0317] To evaluate the crack resistance, use Figure 1 The thermal shock test was conducted using the drop weight friction thermal shock tester 10. Specifically, a test piece 13 having a depth of 20 mm, a circumference of 20 mm, and a length of 30 mm was collected from the longitudinal center of the roll barrel of the forged steel roll of each test number.

[0318] A rod-shaped feed material 12 with a diameter of 5 mm and a length of 10 mm is mounted on the outer periphery of the pinion 11 of the drop weight friction thermal shock tester 10. The feed material 12 is a mild steel wire rod SWRM6 specified in JIS G 3505:2017. The front end of the feed material 12 is bent and inserted radially from the outer periphery of the pinion 11. Figure 1As shown, the surface 13A of the test piece 13 is brought into contact with the feed material 12. At this time, the feed material is arranged so that the portion of the feed material 12 other than the front end extends upward. The test piece 13 and the feed material are arranged so that the curved portion of the feed material 12 contacts the upper portion of the surface 13A. The surface 13A is set to 20 mm × 30 mm and is arranged so that the 30 mm side is in the vertical direction. The surface 13A is considered to be the surface corresponding to the surface of the forged steel roll. After the test piece 13 and the feed material 12 are arranged as described above, the pinion 11 is rotated using a weight, so that the feed material 12 slides strongly on the surface 13A, thereby applying a thermal shock to the surface 13A.

[0319] Figure 2 FIG. 1 is a schematic diagram of surface 13A after feed material 12 passes through surface 13A. Figure 2 As shown in FIG. 1 , a portion where the feed material 12 slides, that is, a contact area 100, is formed in the length direction of the surface 13A. Figure 2 As shown, the contact region 100 is cut at a substantially central portion of its width along a plane CS including the longitudinal direction and the normal direction of the surface 13A.

[0320] The entire contact region 100 in the cut surface was observed using a 100x optical microscope, and the vicinity of the surface of the entire contact region 100 was continuously photographed to generate a plurality of continuous photographic images. Figure 3 It is a cross-sectional view showing a portion of a continuous photographic image. Using the continuous photographic image, the number of cracks 50 appearing in the entire range of the contact area 100 and the maximum depth of the cracks 50 are confirmed. In each photographic image, the length NL of the crack 50 in the normal direction N of the surface 13A is regarded as the crack depth. When the crack depth is 10 μm or more, it is regarded as a crack. The number of cracks regarded as cracks is determined. In addition, the maximum crack depth among the cracks regarded as cracks is determined. The obtained maximum crack depth (μm) and the number of cracks are shown in the "Maximum crack depth (μm)" and "Number of cracks (cracks)" columns in Table 3.

[0321] [Evaluation results]

[0322] With reference to Table 1, Table 2 and Table 3, in test numbers 1 to 37, the chemical composition is appropriate and the necessary conditions 1 to 5 in the manufacturing process are satisfied. 2 The high-angle grain boundary length was 2500 to 9000 mm. Therefore, these test numbers achieved excellent crack resistance. Specifically, in the crack resistance evaluation test, the maximum crack depth in these test numbers was suppressed to 400 μm or less, and the number of cracks was 30 or less.

[0323] Among Test Nos. 1 to 37, Test Nos. 6 to 10, 12, 14, 16, 18, and 19 further had Si contents of 0.40% to 1.50%, satisfying Formula (1). Furthermore, the Vickers hardness at 400°C was 400 HV or higher. As a result, the number of cracks in the crack resistance evaluation test was 20 or fewer, further suppressing the frequency of cracking and achieving even better crack resistance.

[0324] On the other hand, in test numbers 38 and 39, the forging ratio S was too low. 2 The high-angle grain boundary length was too short, and as a result, the number of cracks exceeded 30, and sufficient crack resistance was not obtained.

[0325] In test numbers 40 and 41, the forging ratio S was too high. Therefore, the average 2 The length of the high-angle grain boundary was too long. As a result, the maximum crack depth exceeded 400 μm, and sufficient crack resistance was not obtained.

[0326] In test numbers 42 and 43, the quenching temperature T2 was too low. Therefore, the average 2 The length of the high-angle grain boundary was too long. As a result, the maximum crack depth exceeded 400 μm, and sufficient crack resistance was not obtained.

[0327] In test numbers 44 and 45, the quenching temperature T2 was too high. Therefore, the average 2 The high-angle grain boundary length was too short. As a result, the number of cracks exceeded 30, and sufficient crack resistance was not obtained.

[0328] In test numbers 46 and 47, the holding time t2 at the quenching temperature was too short. 2 The length of the high-angle grain boundary was too long. As a result, the maximum crack depth exceeded 400 μm, and sufficient crack resistance was not obtained.

[0329] In test numbers 48 and 49, the holding time t2 was too long. Therefore, the average 2 The high-angle grain boundary length was too short. As a result, the number of cracks exceeded 30, and sufficient crack resistance was not obtained.

[0330] In test numbers 50 and 51, the FA value is too low. Therefore, the average 2 The high-angle grain boundary length was too short. As a result, the number of cracks exceeded 30, and sufficient crack resistance was not obtained.

[0331] In test numbers 52 and 53, the FA value is too high. Therefore, the average 2 The length of the high-angle grain boundary was too long. As a result, the maximum crack depth exceeded 400 μm, and sufficient crack resistance was not obtained.

[0332] The embodiments of the present invention have been described above. However, the above embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above embodiments and can be implemented by appropriately modifying the above embodiments without departing from the scope of the present invention.

Claims

1. A forged steel roller, Its chemical composition is calculated by mass% as 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: 0.2000% or less, O: 0.0050% or less, Cr：2.80～8.00％、 Mo: 0.40~3.00%, Cu: 0.100% or less, B: 0.0100% or less, Ni: 0-1.20%, V：0～2.00％、 Nb: 0-0.50%, and Balance: Fe and impurities, The average thickness of the surface layer of the forged steel roller is 1 mm. 2 The length of the high-angle grain boundary is 2500 to 9000 mm.

2. The forged steel roller according to claim 1, wherein: The chemical composition contains Ni: 0.01~1.20%, V: 0.01 to 2.00%, and Nb: One or more kinds selected from the group consisting of 0.01 to 0.50%.

3. The forged steel roller according to claim 1, wherein: The N content is 0.0200% or less in mass%.

4. The forged steel roller according to claim 2, wherein: The N content is 0.0200% or less in mass%.

5. The forged steel roll according to any one of claims 1 to 4, wherein Furthermore, the Si content is 0.40 to 1.50% by mass. The chemical composition satisfies formula (1), The Vickers hardness of the forged steel roller at 400°C is 400 HV or more. 4.50≤Cr+Mo+V+Nb≤13.50 (1) Here, the content of each element in mass % is substituted for each element symbol in the formula. When the element is not contained, "0" is substituted for the corresponding element symbol.

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