Forged steel roll

Abstract

A forged steel roll according to the present disclosure has a chemical composition consisting of, 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: 0.2000% or less, O: 0.0050% or less, Cr: 2.80 to 8.00%, Mo: 0.40 to 3.00%, Cu: 0.100% or less, B: 0.0100% or less, Ni: 0 to 1.20%, V: 0 to 2.00%, and Nb: 0 to 0.50%, with the balance being Fe and impurities. A large-angle grain boundary length per 1 mm2 in an outer layer of the forged steel roll is 2500 to 9000 mm.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a forged steel roll, and more particularly relates to a forged steel roll which is suitable for use for cold rolling.BACKGROUND ART

[0002] Forged steel rolls are used as mill rolls, particularly in rolls for cold rolling. When a forged steel roll is used for cold rolling for an extended period, the degree of roughness on the surface of the forged steel roll gradually decreases due to the use for an extended period. If the degree of roughness on the surface of a forged steel roll decreases, in some cases a slip may occur between the forged steel roll and the workpiece. If a slip occurs, in some cases poor engagement may occur. Furthermore, in some cases scoring may also occur between the forged steel roll and the workpiece during rolling.

[0003] If a slip and / or scoring occurs, thermal shock is applied to the roll surface. In some cases a crack may initiate in the outer layer of the forged steel roll due to such thermal shock. If usage of a forged steel roll in which a crack has initiated is continued, the crack will gradually propagate. If the crack continues to propagate, in some cases a part of the outer layer of the roll may peel off. This phenomenon is known as “spalling”.

[0004] If a crack is confirmed in a forged steel roll, in order to suppress the occurrence of spalling, the outer layer of the roll is ground down by an amount corresponding to the depth of the crack so as to remove the crack. If the crack is deep, the amount by which the roll is ground down to remove the crack will be large. If the amount by which the roll is ground down is large, the specific roll consumption (kg / ton) will decrease. Therefore, there is a need for a forged steel roll which is capable of suppressing crack initiation due to thermal shock and also capable of suppressing crack propagation. Hereunder, in the present description, having the ability to suppress crack initiation and propagation is also referred to as “excellent crack resistance”.

[0005] Techniques for increasing resistance to thermal shock in a forged steel roll are proposed in Japanese Patent Application Publication No. 2-185928 (Patent Literature 1), Japanese Patent Application Publication No. 1-234548 (Patent Literature 2), Japanese Patent Application Publication No. 5-086439 (Patent Literature 3), Japanese Patent Application Publication No. 5-132738 (Patent Literature 4), and Japanese Patent Application Publication No. 2010-242166 (Patent Literature 5).

[0006] Patent Literature 1 discloses a method for producing a forged steel roll in which the outer layer of a forged steel material containing C: 0.7 to 1.0%, Si: 0.15 to 1.5%, Mn: 0.15 to 1.5%, Cr: 3.0 to 6.0%, Mo: 3.0 to 5.0%, and V: 1.2% or less is subjected to quenching. The quenched forged steel material is subjected to a cryogenic treatment. The forged steel material after the cryogenic treatment is subjected to tempering at a temperature of 180° C. or more. According to Patent Literature 1, by increasing the temper softening resistance, the tempering temperature is increased by 40° C. or more in comparison to the conventional methods. Therefore, the resistance to cracks is increased.

[0007] Patent Literature 2 discloses a forged steel roll that contains, by weight percent, C: 0.45 to 0.95%, Mn: 1.0% or less, Cr: 4.5 to 6.0%, Mo: 0.3 to 0.7%, and Ni: 0.6 to 2.0%, with the balance being Fe and impurities, and in which the content of Si is kept down to less than 0.1%. According to Patent Literature 2, by keeping the content of Si as an impurity to less than 0.1% and making the content of Ni 0.6 to 2.0%, the spalling resistance and the thermal shock crack resistance of the forged steel roll are increased.

[0008] Patent Literature 3 discloses a forged steel roll that contains, C: 0.90 to 1.10 wt %, Si: 0.5 to 1.0 wt %, Mn: 0.1 to 1.0 wt %, Cr: 4.0 to 6.0 wt %, Mo: 3.0 to 6.0 wt %, V: 0.5 to 2.0 wt %, and Co: 1.0 to 3.0 wt %, with the balance being Fe and impurities. According to Patent Literature 3, by the forged steel roll having the aforementioned chemical composition, the thermal shock resistance of the forged steel roll is increased.

[0009] Patent Literature 4 discloses a forged steel roll that contains, by weight percent, C: 0.7 to 1.4%, Si: 0.8 to 2.5%, Mn: 0.8 to 2.5%, Ni: 0.5 to 2.5%, Cr: 2.5 to 6.5%, Mo: 2.5 to 8.5%, W: 0.3 to 3.0%, and V: 0.5 to 4.5%, with the balance being Fe and impurities. The amount of retained austenite formed due to tempering after cryogenic treatment contained in the forged steel roll is more than 15% to 40%. According to Patent Literature 4, by containing retained austenite, propagation of cracks is suppressed.

[0010] Patent Literature 5 discloses a forged steel roll containing, in mass %, C: 0.6 to 1.2%. Si: 0.4 to 0.8%, Mn: 0.4 to 1.0%, Ni: 0.4 to 1.0%, Cr: 3.0 to 6.0%, and Mo: 0.2 to 0.5%, with the balance being Fe and impurities. In the forged steel roll, the average particle size of carbides dispersed in the steel microstructure of the outer layer of the roll within a region from the roll surface to a depth of 50 mm is 1 μm or less. In addition, an area fraction of the dispersed carbides is 5 to 30%. According to Patent Literature 5, by causing the aforementioned carbides to be dispersed in the outer layer of the roll, the crack initiation is suppressed.CITATION LISTPatent LiteraturePatent Literature 1: Japanese Patent Application Publication No. 2-185928

[0012] Patent Literature 2: Japanese Patent Application Publication No. 1-234548

[0013] Patent Literature 3: Japanese Patent Application Publication No. 5-086439

[0014] Patent Literature 4: Japanese Patent Application Publication No. 5-132738

[0015] Patent Literature 5: Japanese Patent Application Publication No. 2010-242166SUMMARY OF INVENTIONTechnical Problem

[0016] However, both the initiation of cracks due to thermal shock and the propagation of cracks may also be suppressed by means other than those described in Patent Literature 1 to Patent Literature 5.

[0017] An objective of the present invention is to provide a forged steel roll, in which the initiation of cracks due to thermal shock can be suppressed and the propagation of cracks can also be suppressed.Solution to Problem

[0018] A forged steel roll of the present disclosure has a chemical composition consisting of, in mass %,

[0019] C: 0.70 to 1.50%,

[0020] Si: 0.20 to 1.50%,

[0021] Mn: 0.20 to 1.50%,

[0022] P: 0.030% or less,

[0023] S: 0.0200% or less,

[0024] Al: 0.050% or less,

[0025] N: 0.2000% or less,

[0026] O: 0.0050% or less,

[0027] Cr: 2.80 to 8.00%,

[0028] Mo: 0.40 to 3.00%,

[0029] Cu: 0.100% or less,

[0030] B: 0.0100% or less,

[0031] Ni: 0 to 1.20%,

[0032] V: 0 to 2.00%,

[0033] Nb: 0 to 0.50%, and

[0034] the balance: Fe and impurities,

[0035] wherein:

[0036] a large-angle grain boundary length per 1 mm2 in an outer layer of the forged steel roll is 2500 to 9000 mm.Advantageous Effects of Invention

[0037] In the forged steel roll according to the present invention, the crack initiation due to thermal shock can be suppressed, and the propagation of cracks can also be suppressed.BRIEF DESCRIPTION OF DRAWINGS

[0038] FIG. 1 is a configuration diagram of a drop-weight-friction thermal shock tester used in a crack resistance test in the examples.

[0039] FIG. 2 is a schematic diagram of a contact region obtained in a crack resistance test using the drop-weight-friction thermal shock tester illustrated in FIG. 1.

[0040] FIG. 3 is a cross-sectional view of the contact region illustrated in FIG. 2.DESCRIPTION OF EMBODIMENTS

[0041] First, the present inventors have conducted studies from the viewpoint of the chemical composition with respect to a forged steel roll that is excellent in crack resistance. As a result, the present inventors have considered that if a forged steel roll has a chemical composition 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: 0.2000% or less, O: 0.0050% or less, Cr: 2.80 to 8.00%, Mo: 0.40 to 3.00%, Cu: 0.100% or less, B: 0.0100% or less, Ni: 0 to 1.20%, V: 0 to 2.00%, and Nb: 0 to 0.50%, with the balance being Fe and impurities, there is a possibility that excellent crack resistance can be obtained.

[0042] Therefore, with respect to a forged steel roll which satisfies the chemical composition described above, the present inventors have conducted further investigations regarding the mechanism by which cracks initiate and propagate if thermal shock is applied to an outer layer of the forged steel roll.

[0043] Here, the present inventors focused their attention on grain boundary length in the outer layer of the forged steel roll. The present inventors have considered that the grain boundary length in the outer layer influences the initiation or propagation of cracks. Therefore, the present inventors studied and investigated the relation between the initiation of cracks due to thermal shock and propagation of cracks and the large-angle grain boundary length per 1 mm2 in the outer layer. Here, the phrase “large-angle grain boundary length” means the length of a grain boundary where an orientation difference between adjacent grains is 150 or more. As the result of such studies and investigations, the present inventors obtained the following findings.

[0044] The large-angle grain boundary length per 1 mm2 influences both the initiation of cracks due to thermal shock and propagation of the cracks. Specifically, if the large-angle grain boundary length is too small, although propagation of cracks will be suppressed, the frequency of crack initiation will increase. On the other hand, if the large-angle grain boundary length is too large, although the frequency of crack initiation will be suppressed, cracks will easily propagate. Therefore, in a forged steel roll having the chemical composition described above, in order to suppress the initiation of cracks due to thermal shock and also suppress the propagation of such cracks, it is effective to adjust the large-angle grain boundary length per 1 mm2 to be controlled within an appropriate range.

[0045] The present inventors have conducted further studies based on the above findings. As a result, the present inventors have discovered that in a forged steel roll satisfying the chemical composition described above, if the large-angle grain boundary length per 1 mm2 in the outer layer of the forged steel roll is 2500 to 9000 mm, even if the forged steel roll is subjected to thermal shock during use, the initiation of cracks can be suppressed and the propagation of cracks can also be suppressed, and excellent crack resistance is obtained.

[0046] A forged steel roll of the present embodiment, which has been completed based on the findings described above, is as follows.

[0047] A forged steel roll according to a first configuration has a chemical composition consisting of, in mass %,

[0048] C: 0.70 to 1.50%,

[0049] Si: 0.20 to 1.50%,

[0050] Mn: 0.20 to 1.50%,

[0051] P: 0.030% or less,

[0052] S: 0.0200% or less,

[0053] Al: 0.050% or less,

[0054] N: 0.2000% or less,

[0055] O: 0.0050% or less,

[0056] Cr: 2.80 to 8.00%,

[0057] Mo: 0.40 to 3.00%,

[0058] Cu: 0.100% or less,

[0059] B: 0.0100% or less,

[0060] Ni: 0 to 1.20%,

[0061] V: 0 to 2.00%,

[0062] Nb: 0 to 0.50%, and

[0063] the balance: Fe and impurities,

[0064] wherein:

[0065] a large-angle grain boundary length per 1 mm2 in an outer layer of the forged steel roll is 2500 to 9000 mm.

[0066] A forged steel roll according to a second configuration is in accordance with the forged steel roll of the first configuration, wherein the chemical composition contains one or more types of element selected from a group consisting of:

[0067] Ni: 0.01 to 1.20%,

[0068] V: 0.01 to 2.00%, and

[0069] Nb: 0.01 to 0.50%.

[0070] A forged steel roll according to a third configuration is in accordance with the forged steel roll of the first and second configurations, wherein: a content of N is 0.0200% or less in percent by mass.

[0071] A forged steel roll according to a fourth configuration is in accordance with the forged steel roll of any one of the first to third configurations, wherein:

[0072] a content of Si is 0.40 to 1.50% in percent by mass,

[0073] the chemical composition satisfies Formula (1), and

[0074] a Vickers hardness at 400° C. is 400 HV or more:4.5≤Cr+Mo+V+Nb≤13.5(1)where, a content of a corresponding element in percent by mass is substituted for each symbol of an element in Formula (1), and if an element is not contained, “0” is substituted for the corresponding symbol of an element.

[0076] Hereunder, the forged steel roll of the present embodiment is described in detail. The symbol “%” in relation to an element means “mass percent” unless otherwise specified.[Features of Forged Steel Roll of Present Embodiment]

[0077] The forged steel roll of the present embodiment satisfies the following feature 1 and feature 2.(Feature 1)

[0078] The chemical composition consists of, 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: 0.2000% or less, O: 0.0050% or less, Cr: 2.80 to 8.00%, Mo: 0.40 to 3.00%, Cu: 0.100% or less, B: 0.0100% or less, Ni: 0 to 1.20%, V: 0 to 2.00%, and Nb: 0 to 0.50%, with the balance being Fe and impurities.(Feature 2)

[0079] A large-angle grain boundary length per 1 mm2 in an outer layer of the forged steel roll is 2500 to 9000 mm.

[0080] Feature 1 and feature 2 are described hereunder.[(Feature 1) Regarding Chemical Composition]

[0081] The chemical composition of the forged steel roll of the present embodiment contains the following elements.C: 0.70 to 1.50%

[0082] Carbon (C) increases the hardness of the outer layer of the forged steel roll. If the content of C is less than 0.70%, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of C is more than 1.50%, coarse carbides will form. In such case, sufficient hardness will not be obtained in the outer layer of the forged steel roll in some cases. Therefore, the content of C is 0.70 to 1.50%.

[0083] A preferable lower limit of the content of C is 0.75%, more preferably is 0.80%, further preferably is 0.85%, and further preferably is 0.90%.

[0084] A preferable upper limit of the content of C is 1.45%, more preferably is 1.40%, further preferably is 1.35%, further preferably is 1.20%, further preferably is 1.15%, further preferably is 1.10%, and further preferably is 1.05%.Si: 0.20 to 1.50%

[0085] Silicon (Si) deoxidizes the steel at the stage of molten steel. Si also increases the hardenability of the steel. If the content of Si is less than 0.20%, the aforementioned advantageous effects will not be sufficiently obtained. On the other hand, if the content of Si is more than 1.50%, the toughness of the forged steel roll will decrease. Therefore, the content of Si is 0.20 to 1.50%.

[0086] A preferable lower limit of the content of Si is 0.25%, more preferably is 0.30%, further preferably is 0.40%, further preferably is 0.45%, further preferably is 0.50%, further preferably is 0.55%, further preferably is 0.60%, further preferably is 0.65%, further preferably is 0.70%, and further preferably is 0.75%.

[0087] Note that, when the content of Si is 0.40% or more, a sufficient amount of dissolved Si will be obtained. In such case, on the precondition that Formula (1) which is described later is satisfied, the high temperature hardness can be increased in the outer layer of the forged steel roll. Therefore, the crack resistance increases further.

[0088] A preferable upper limit of the content of Si is 1.45%, more preferably is 1.40%, further preferably is 1.35%, further preferably is 1.30%, further preferably is 1.25%, further preferably is 1.20%, and further preferably is 1.15%.Mn: 0.20 to 1.50%

[0089] Manganese (Mn) increases the hardenability of the steel. If the content of Mn is less than 0.20%, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of Mn is more than 1.50%, the toughness of the forged steel roll will decrease. Therefore, the content of Mn is 0.20 to 1.50%.

[0090] A preferable lower limit of the content of Mn is 0.25%, more preferably is 0.30%, further preferably is 0.35%, and further preferably is 0.40%.

[0091] A preferable upper limit of the content of Mn is 1.45%, more preferably is 1.40%, further preferably is 1.35%, further preferably is 1.30%, and further preferably is 1.25%.P: 0.030% or Less

[0092] Phosphorus (P) is an impurity. P segregates to grain boundaries and thereby decreases the toughness of the forged steel roll. Therefore, the content of P is 0.030% or less.

[0093] The content of P is preferably as low as possible. However, excessively reducing the content of P will raise the production cost. Therefore, when ordinary industrial production is taken into consideration, a preferable lower limit of the content of P is more than 0%, more preferably is 0.001%, and further preferably is 0.002%.

[0094] A preferable upper limit of the content of P is 0.025%, and more preferably is 0.020%.S: 0.0200% or Less

[0095] Sulfur (S) is an impurity. S segregates to grain boundaries and thereby decreases the toughness and hot workability of the forged steel roll. Therefore, the content of S is 0.0200% or less.

[0096] The content of S is preferably as low as possible. However, excessively reducing the content of S will raise the production cost. Therefore, when ordinary industrial production is taken into consideration, a preferable lower limit of the content of S is more than 0%, more preferably is 0.0001%, further preferably is 0.0002%, and further preferably is 0.0003%.

[0097] A preferable upper limit of the content of S is 0.0050%, more preferably is 0.0040%, and further preferably is 0.0030%.Al: 0.050% or Less

[0098] Aluminum (Al) deoxidizes the steel at the stage of molten steel. However, if the content of Al is more than 0.050%, coarse Al nitrides will form. In such case, the toughness of the steel material will decrease. Therefore, the content of Al is 0.050% or less.

[0099] A preferable lower limit of the content of Al is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.005%, and further preferably is 0.010%.

[0100] A preferable upper limit of the content of Al is 0.040%, more preferably is 0.035%, further preferably is 0.030%, and further preferably is 0.025%. Note that, in the present description, the term “content of Al” means the total content of Al (total Al) in the steel.N: 0.2000% or Less

[0101] Nitrogen (N) increases the strength of the forged steel roll by solid-solution strengthening. In particular, N increases the high temperature hardness of the forged steel roll. However, if the content of N is more than 0.2000%, coarse nitrides will form. In such case, the toughness of the forged steel roll will decrease. Therefore, the content of N is 0.2000% or less.

[0102] A preferable lower limit of the content of N is more than 0%, more preferably is 0.0001%, further preferably is 0.0005%, further preferably is 0.0010%, further preferably is 0.0025%, further preferably is 0.0035%, and further preferably is 0.0045%.

[0103] A preferable upper limit of the content of N is 0.1900%, more preferably is 0.1800%, further preferably is 0.1500%, further preferably is 0.1200%, further preferably is 0.1000%, further preferably is 0.0500%, further preferably is 0.0200%, further preferably is 0.0150%, and further preferably is 0.0100%.O: 0.0050% or Less

[0104] Oxygen (O) is an impurity. O forms oxides and thereby decreases the toughness of the forged steel roll. Therefore, the content of O is 0.0050% or less. The content of O is preferably as low as possible. However, excessively reducing the content of O will raise the production cost. Therefore, when ordinary industrial production is taken into consideration, a preferable lower limit of the content of O is more than 0%, more preferably is 0.0001%, further preferably is 0.0005%, further preferably is 0.0007%, and further preferably is 0.0010%. A preferable upper limit of the content of O is 0.0040%, more preferably is 0.0035%, and further preferably is 0.0030%.Cr: 2.80 to 8.00%

[0105] Chromium (Cr) forms carbides, and thereby increases the wear resistance of the forged steel roll. Cr also increases the temper softening resistance of the steel by dissolving, thus increasing the high temperature hardness of the outer layer of the forged steel roll. If the content of Cr is less than 2.80%, the aforementioned advantageous effects will not be sufficiently obtained. On the other hand, if the content of Cr is more than 8.00%, coarse carbides will form. In such case, the grindability and toughness of the forged steel roll will decrease. Therefore, the content of Cr is 2.80 to 8.00%.

[0106] A preferable lower limit of the content of Cr is 2.85%, more preferably is 3.00%, further preferably is 3.50%, and further preferably is 4.00%.

[0107] A preferable upper limit of the content of Cr is 7.50%, more preferably is 7.00%, further preferably is 6.50%, further preferably is 6.00%, and further preferably is 5.50%.Mo: 0.40 to 3.00%

[0108] Molybdenum (Mo) forms carbides, and thereby increases the wear resistance of the forged steel roll. Mo also increases the high temperature hardness of the outer layer of the forged steel roll by dissolving. If the content of Mo is less than 0.40%, the aforementioned advantageous effects will not be sufficiently obtained. On the other hand, if the content of Mo is more than 3.00%, coarse carbides will form. In such case, the grindability and toughness of the forged steel roll will decrease. Therefore, the content of Mo is 0.40 to 3.00%.

[0109] A preferable lower limit of the content of Mo is 0.45%, more preferably is 0.50%, and further preferably is 0.55%.

[0110] A preferable upper limit of the content of Mo is 2.80%, more preferably is 2.60%, further preferably is 2.40%, further preferably is 2.20%, further preferably is 2.00%, further preferably is 1.90%, further preferably is 1.80%, further preferably is 1.70%, and further preferably is 1.60%.Cu: 0.100% or Less

[0111] Copper (Cu) is an impurity. Cu decreases hot workability of the steel. Therefore, the content of Cu is 0.100% or less.

[0112] The content of Cu is preferably as low as possible. However, excessively reducing the content of Cu will raise the production cost. Therefore, when ordinary industrial production is taken into consideration, a preferable lower limit of the content of Cu is more than 0%, more preferably is 0.001%, further preferably is 0.005%, further preferably is 0.010%, and further preferably is 0.015%.

[0113] A preferable upper limit of the content of Cu is 0.095%, more preferably is 0.090%, further preferably is 0.085%, further preferably is 0.080%, further preferably is 0.075%, and further preferably is 0.070%.B: 0.0100% or Less

[0114] Boron (B) is an impurity. B decreases the toughness of the forged steel roll. Therefore, the content of B is 0.0100% or less.

[0115] The content of B is preferably as low as possible. However, excessively reducing the content of B will raise the production cost. Therefore, when ordinary industrial production is taken into consideration, a preferable lower limit of the content of B is more than 0%, more preferably is 0.0001%, further preferably is 0.0005%, further preferably is 0.0010%, further preferably is 0.0015%, and further preferably is 0.0020%.

[0116] A preferable upper limit of the content of B is 0.0090%, more preferably is 0.0085%, and further preferably is 0.0080%.

[0117] The balance of the chemical composition of the forged steel roll according to the present embodiment is Fe and impurities. Here, the term “impurities” means substances which are mixed in from ore and scrap used as raw material or from the production environment or the like when industrially producing the forged steel roll, and which are permitted within a range that does not adversely affect the forged steel roll of the present embodiment.[Regarding Optional Elements]

[0118] The chemical composition of the forged steel roll of the present embodiment may further contain, in lieu of a part of Fe, one or more types of element selected from the group consisting of:

[0119] Ni: 0 to 1.20%,

[0120] V: 0 to 2.00%, and

[0121] Nb: 0 to 0.50%.

[0122] Hereunder, the optional elements of respective groups are described.[First Group: Ni]Ni: 1.20% or Less

[0123] Nickel (Ni) is an optional element, and does not have to be contained. That is, the content of Ni may be 0%. When contained, that is, when the content of Ni is more than 0%, Ni increases the hardenability of the steel. If even a small amount of Ni is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Ni is more than 1.20%, retained austenite will be excessively formed. In such case, the hardness of the forged steel roll will decrease. Therefore, the content of Ni is 0 to 1.20%, and when contained, the content of Ni is 1.20% or less.

[0124] A preferable lower limit of the content of Ni is 0.010%, more preferably is 0.05%, further preferably is 0.10%, further preferably is 0.15%, and further preferably is 0.20%.

[0125] A preferable upper limit of the content of Ni is 1.15%, more preferably is 1.10%, further preferably is 1.05%, further preferably is 1.00%, further preferably is 0.95%, further preferably is 0.90%, further preferably is 0.85%, further preferably is 0.80%, further preferably is 0.75%, and further preferably is 0.70%.[Second Group: V and Nb]V: 2.00% or Less

[0126] Vanadium (V) is an optional element, and does not have to be contained. That is, the content of V may be 0%. When contained, that is, when the content of V is more than 0%, V forms carbides and thereby increases the wear resistance of the forged steel roll. V also increases the high temperature hardness of the forged steel roll by dissolving. If even a small amount of V is contained, the aforementioned advantageous effects will be obtained to a certain extent. However, if the content of V is more than 2.00%, coarse carbides will form. In such case, the grindability and toughness of the forged steel roll will decrease. Therefore, the content of V is 0 to 2.00%, and when contained, the content of V is 2.00% or less.

[0127] A preferable lower limit of the content of V is 0.01%, more preferably is 0.05%, further preferably is 0.10%, further preferably is 0.15%, further preferably is 0.20%, and further preferably is 0.25%.

[0128] A preferable upper limit of the content of V is 1.80%, more preferably is 1.60%, further preferably is 1.50%, further preferably is 1.40%, further preferably is 1.30%, and further preferably is 1.20%.Nb: 0.50% or Less

[0129] Niobium (Nb) is an optional element, and does not have to be contained. That is, the content of Nb may be 0%. When contained, that is, when the content of Nb is more than 0%, Nb forms carbides and thereby increases the wear resistance of the forged steel roll. Nb also increases the high temperature hardness of the forged steel roll by dissolving. If even a small amount of Nb is contained, the aforementioned advantageous effects will be obtained to a certain extent. However, if the content of Nb is more than 0.50%, coarse carbides will form. In such case, the grindability and toughness of the forged steel roll will decrease. Therefore, the content of Nb is 0 to 0.50%, and when contained, the content of Nb is 0.50% or less.

[0130] A preferable lower limit of the content of Nb is 0.01%, more preferably is 0.05%, further preferably is 0.10%, further preferably is 0.15%, further preferably is 0.20%, and further preferably is 0.25%.

[0131] A preferable upper limit of the content of Nb is 0.45%, more preferably is 0.42%, further preferably is 0.40%, further preferably is 0.38%, and further preferably is 0.36%.[(Feature 2) Regarding Large-Angle Grain Boundary Length Per 1 mm2 in Outer Layer]

[0132] In the forged steel roll of the present embodiment, in addition, the large-angle grain boundary length per 1 mm2 in the outer layer is 2500 to 9000 mm. Here, the term “large-angle grain boundary length” means the length of a grain boundary where an orientation difference between adjacent grains is 15° or more.

[0133] If the large-angle grain boundary length per 1 mm2 is more than 9000 mm, the large-angle grain boundary length is too long. In such case, the resistance to sliding of grain boundaries increases. Therefore, the frequency of crack initiation with respect to cracks that occur along grain boundaries is suppressed. However, at the moment when a crack initiates, excessive energy that has been accumulated is released, which causes grain boundaries to slide significantly. Consequently, the depth of the crack becomes excessively large and the crack resistance decreases.

[0134] On the other hand, if the large-angle grain boundary length per 1 mm2 is less than 2500 mm, the large-angle grain boundary length is too short. In such case, propagation of cracks is suppressed. However, because the resistance to sliding of grain boundaries is small, the frequency of crack initiation increases. Consequently, cracks initiate excessively and the crack resistance decreases.

[0135] If the large-angle grain boundary length per 1 mm2 in the outer layer is 2500 to 9000 mm, the frequency of crack initiation will be sufficiently suppressed, and even if cracks initiate, propagation of the cracks will be sufficiently suppressed. As a result, sufficient crack resistance is obtained.

[0136] A preferable lower limit of the large-angle grain boundary length per 1 mm2 is 3000 mm, more preferably is 3200 mm, further preferably is 3300 mm, further preferably is 3400 mm, and further preferably is 3500 mm.

[0137] A preferable upper limit of the large-angle grain boundary length per 1 mm2 is 8800 mm, more preferably is 8600 mm, further preferably is 8400 mm, further preferably is 8300 mm, further preferably is 8200 mm, further preferably is 8100 mm, and further preferably is 8000 mm.[Method for Measuring Large-Angle Grain Boundary Length Per 1 mm2 in Outer Layer]

[0138] The large-angle grain boundary length per 1 mm2 in the outer layer of the forged steel roll of the present embodiment is determined by the following method.

[0139] Five test specimens which each include a position at a depth of 1 mm from the surface of the body part of the forged steel roll are taken. Among the surfaces of each test specimen, a surface that includes the position at a depth of 1 mm from the surface of the forged steel roll is defined as an observation surface. The observation surface of each test specimen is mirror-polished with diamond paste. After mirror polishing, the observation surface is chemically polished with colloidal silica solution to remove the machining-affected layer introduced during mirror polishing.

[0140] On the polished observation surface, an observation field corresponding to the position at a depth of 1 mm from the surface of the forged steel roll is observed at a magnification of 5000× using SEM / EBSD / OIM. The observation field is set to a size of 20 μm×20 μm, and the measurement interval between adjacent measurement points is set to 0.03 μm. In the EBSD measurement, the acceleration voltage is set to 25 kV.

[0141] The crystal orientation at each measurement point in the observation field is obtained using EBSD analysis software. Based on the crystal orientation at each measurement point, a grain boundary where the crystal orientation difference between adjacent grains is 15° or more is defined as a “large-angle grain boundary”. The total length of large-angle grain boundaries in the observation field is measured. Then, based on the observation field area and the total length of large-angle grain boundaries measured, the large-angle grain boundary length per 1 mm2 is determined for the observation field of each test specimen. The arithmetic average value of the large-angle grain boundary lengths obtained in the five test specimens is defined as the large-angle grain boundary length per 1 mm2 (mm) in the outer layer.

[0142] Note that, it suffices to use well-known software as the EBSD analysis software. Well-known EBSD analysis software is, for example, software with the trade name “OIM Analysis” made by TSL Solutions Co. Ltd. In order to eliminate the influence of measurement errors caused by noise on the data, cleanup processing is not carried out, and data having an Image Quality (IQ) value, which is also measured, of 11000 or less is excluded from the analysis.[Advantageous Effects of Forged Steel Roll of Present Embodiment]

[0143] The forged steel roll of the present embodiment satisfies feature 1 and feature 2. Therefore, excellent crack resistance is obtained in the forged steel roll of the present embodiment. Specifically, when the forged steel roll is used, the frequency of crack initiation is sufficiently suppressed, and even if cracks initiate, propagation of the cracks is sufficiently suppressed. As a result, excellent crack resistance is obtained.[Preferable Form of Forged Steel Roll of Present Embodiment]

[0144] Preferably the forged steel roll of the present embodiment also satisfies the following feature 3.(Feature 3)

[0145] In the chemical composition, the content of Si is 0.40 to 1.50% in percent by mass, and the chemical composition satisfies Formula (1). In addition, the Vickers hardness at 400° C. is 400 HV or more.4.5≤Cr+Mo+V+Nb≤13.5(1)where, a content of a corresponding element in percent by mass is substituted for each symbol of an element in Formula (1), and if an element is not contained, “0” is substituted for the corresponding symbol of an element.

[0147] If the forged steel roll of the present embodiment also satisfies feature 3, and not only feature 1 and feature 2, the high temperature hardness of the outer layer of the forged steel roll will further increase. As a result, the frequency of crack initiation during use of the forged steel roll will be further suppressed, and the crack resistance will increase further. Feature 3 includes the following three requirements.

[0148] (Requirement 1) The content of Si is 0.40 to 1.50% in percent by mass.

[0149] (Requirement 2) The chemical composition satisfies Formula (1).4.5≤Cr+Mo+V+Nb≤13.5(1)where, a content of a corresponding element in percent by mass is substituted for each symbol of an element in Formula (1), and if an element is not contained, “0” is substituted for the corresponding symbol of an element.

[0151] Note that, if the chemical composition of the forged steel roll does not contain V and Nb, which are optional elements, in other words, if the chemical composition of the forged steel roll consists of only the essential elements, Formula (1) is defined as follows:4.5≤Cr+Mo≤13.5(1)where, a content of a corresponding element in percent by mass is substituted for each symbol of an element in Formula (1).

[0153] (Requirement 3) The Vickers hardness at 400° C. is 400 HV or more.

[0154] Hereunder, requirement 1 to requirement 3 of feature 3 are described.[(Requirement 1) Regarding Content of Si]

[0155] If the content of Si is 0.40% or more, the amount of dissolved Si in the forged steel roll will increase. As a result, the high temperature hardness of the forged steel roll will increase. In such case, the frequency of crack initiation in the outer layer of the forged steel roll will be further suppressed. Therefore, a preferable content of Si is 0.40 to 1.50%.[(Requirement 2) Regarding Formula (1)]

[0156] F1 is defined as follows.F⁢1=Cr+Mo+V+Nb

[0157] F1 is an index of the high temperature hardness of the forged steel roll. Note that, if the chemical composition of the forged steel roll does not contain V and Nb, which are optional elements, in other words, if the chemical composition of the forged steel roll consists of only the essential elements, F1 is defined as follows.F⁢1=Cr+Mo

[0158] Cr, Mo, V, and Nb form carbides, and thereby increase the wear resistance of the forged steel roll. In addition, Cr, Mo, V, and Nb dissolve, and thereby increase the high temperature hardness of the forged steel roll. That is, Cr, Mo, V, and Nb increase wear resistance by forming carbides, and increase the high temperature hardness by dissolving.

[0159] If F1 is 4.50 or more, not only will Cr, Mo, V, and Nb be distributed as carbide forming elements, but will also be distributed as dissolved elements. Therefore, if F1 is 4.50 or more, the high temperature hardness of the forged steel roll will further increase. As a result, the frequency of crack initiation during use of the forged steel roll will be further suppressed and the crack resistance will further increase.

[0160] On the other hand, if F1 is equal to or less than 13.50 that is the total of the upper limit values of the contents of Cr, Mo, V, and Nb, coarsening of carbides of Cr, Mo, V, and Nb will be sufficiently suppressed. Therefore, sufficient grindability and sufficient toughness will be maintained in the forged steel roll.

[0161] A more preferable lower limit of F1 is 4.70, further preferably is 4.90, further preferably is 5.10, further preferably is 5.30, further preferably is 5.50, further preferably is 5.60, further preferably is 5.70, further preferably is 5.80, and further preferably is 5.90. A more preferable upper limit of F1 is 13.40, further preferably is 13.30, further preferably is 13.20, further preferably is 13.10, further preferably is 13.00, further preferably is 12.90, further preferably is 12.80, further preferably is 12.70, and further preferably is 12.60.

[0162] Note that, if F1=Cr+Mo, a more preferable upper limit is 11.00, further preferably is 10.80, and further preferably is 10.50.[(Requirement 3) Regarding Vickers Hardness at 400° C.]

[0163] In conventional forged steel rolls, when the temperature rises from room temperature, there is a sharp decrease in hardness at around 400° C. Consequently, sufficient crack resistance is not obtained. If the hardness at 400° C. is high, the frequency of crack initiation in the outer layer of the forged steel roll will be further suppressed. In the case of a forged steel roll that satisfies feature 1 and 2, if requirement 1 and requirement 2 are satisfied, and in addition, the Vickers hardness at 400° C. is 400 HV or more (that is, if feature 3 is satisfied), the high temperature hardness of the forged steel roll will further increase. As a result, the frequency of crack initiation will be further suppressed and the crack resistance will further increase.

[0164] A preferable lower limit of the Vickers hardness at 400° C. is 405 HV, more preferably is 410 HV, further preferably is 415 HV, further preferably is 420 HV, and further preferably is 425 HV.

[0165] The upper limit of the Vickers hardness at 400° C. is not particularly limited. A preferable upper limit of the Vickers hardness at 400° C. is 600 HV, more preferably is 550 HV, further preferably is 520 HV, and further preferably is 500 HV. In this case, the toughness of the forged steel roll increases further.[Method for Measuring Vickers Hardness at 400° C.]

[0166] The Vickers hardness of the forged steel roll at 400° C. is determined by the following method.

[0167] A test specimen that includes a region from the surface of the forged steel roll to a position at a depth of 1 mm is taken. The size of the test specimen is to be 5 mm×5 mm×10 mm, and a surface of 5 mm×10 mm is to be taken as a measurement surface. The measurement surface is to be a cross section that is perpendicular to the surface of the forged steel roll and that includes the surface of the forged steel roll and a position at a depth of 1 mm from the surface. The test specimen on which a thermocouple is suspended and an indenter are heated from room temperature to 400° C. in a vacuum (3×10−5 Torr) and held at 400° C. for five minutes. The Vickers hardness in an outer layer region of the surface of the test specimen after being held at 400° C. for five minutes is determined by a high-temperature Vickers hardness test method in accordance with JIS Z 2252: 1991. Specifically, on the measurement surface of the test specimen, the Vickers hardness is measured at an arbitrary five points which are each at a position at a depth of 1 mm from the surface of the forged steel roll. At such time, the test force is set to 300 gf. The arithmetic average value of the obtained five Vickers hardness values is taken as the Vickers hardness (unit is HV) at 400° C.[Regarding Microstructure of Outer Layer of Forged Steel Roll of Present Embodiment]

[0168] The microstructure of the outer layer of the forged steel roll of the present embodiment is mainly composed of martensite and / or bainite. The phrase “mainly composed of martensite and / or bainite” means that the total area fraction of martensite and bainite is 85% or more. In the microstructure, the structures other than martensite and bainite are, for example, retained austenite and carbides.[Method for Measuring Total Area Fraction of Martensite and Bainite in Outer Layer of Forged Steel Roll]

[0169] The total area fraction of martensite and bainite in the outer layer of the forged steel roll of the present embodiment is determined by the following method.

[0170] A test specimen including a position at a depth of 1 mm from the surface of the body part of the forged steel roll is taken. Among the surfaces of the test specimen, a surface that includes the position at a depth of 1 mm from the surface of the forged steel roll is defined as an observation surface. The observation surface is mirror-polished. After mirror polishing, the observation surface is etched using 2% nitric acid-alcohol (nital etching reagent). On the etched observation surface, an arbitrary five observation fields (400 μm×600 μm) at a position at a depth of 1 mm from the surface of the body part of the forged steel roll are observed using an optical microscope with a magnification of 200×.

[0171] Each observation field is etched with Murakami's reagent. Murakami's reagent colors the carbides in the observation field. By this means, carbides can be easily distinguished from other phases based on contrast. The carbides in each observation field are then identified. The area fraction of carbides is determined based on the total area of carbides identified in all the observation fields and the total area of all the observation fields.

[0172] In addition, the area fraction of retained austenite is determined by the following X-ray diffraction method. A test specimen including a position at a depth of 1 mm from the surface of the body part of the forged steel roll is taken. Although not particularly limited, the size of the test specimen is, for example, 15 mm×15 mm×10 mm in thickness. 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 α phase, the (211) plane of α phase, the (220) plane of α phase, the (200) plane of γ phase, the (220) plane of γ phase, and the (311) plane of γ phase is measured to calculate an integrated intensity of each plane. In the measurement of the X-ray diffraction intensity, the target of the X-ray diffraction apparatus is Co (Co Kα ray), and the output is 40 kV-135 mA. After calculation, the volume ratio Vγ (%) of retained austenite is calculated using Formula (I) for combinations (3×3=9 pairs) of each plane of the α phase and each plane of the γ phase. Then, an average value of the volume ratios Vγ of retained austenite of the nine pairs is defined as the volume ratio (%) of retained austenite:V⁢γ=100 / {1+(I⁢α×R⁢γ) / (I⁢γ×R⁢α)}(I)where, Iα is the integrated intensity of α phase. Rα is the crystallographic theoretical value of α phase. Iγ is the integrated intensity of γ phase. Rγ is the crystallographic theoretical value of γ phase. Note that, in the present description, Rα in the (200) plane of α phase is 9.9, Rα in the (211) plane of α phase is 20.4, Rα in the (220) plane of α phase is 9.3, Rγ in the (200) plane of γ phase is 23.8, Rγ in the (220) plane of γ phase is 12.5, and Rγ in the (311) plane of γ phase is 17.2. Note that, a value obtained by rounding off the second decimal place of the obtained numerical value is adopted as the volume ratio of retained austenite. In the measurement of the X-ray diffraction intensity, in the test specimen, the X-ray is directed at a position corresponding to a position at a depth of 1 mm from the surface of the body part of the forged steel roll.

[0174] The volume ratio (%) of retained austenite obtained by the X-ray diffraction method described above is regarded as the area fraction (%) of retained austenite. The total area fraction of martensite and bainite in the outer layer of the forged steel roll is then determined by the following equation.Total area fraction of martensite and bainite in outer layer of forged steel roll=100.0−(area fraction of carbides+area fraction of retained austenite)[Uses of Forged Steel Roll of Present Embodiment]

[0175] The forged steel roll of the present embodiment can be widely applied as a roll for rolling. In particular, the forged steel roll of the present embodiment is suitable as a roll for cold rolling of steel sheets. The roll for cold rolling is, for example, a work roll for a cold tandem rolling mill or a cold reverse rolling mill, or a work roll for skin pass rolling (temper rolling).[Method for Producing Forged Steel Roll]

[0176] An example of a method for producing the forged steel roll of the present embodiment will now be described. The forged steel roll of the present embodiment may also be produced by a production method other than the production method described hereunder. However, the production method described hereunder is a preferable example of a method for producing the forged steel roll of the present embodiment.

[0177] One example of a method for producing the forged steel roll of the present embodiment includes the following processes.

[0178] (Process 1) Steelmaking process

[0179] (Process 2) Hot forging process

[0180] (Process 3) Annealing process

[0181] (Process 4) Rough machining process

[0182] (Process 5) Quenching process

[0183] (Process 6) Tempering process

[0184] (Process 7) Finishing machining process

[0185] Each process is described hereunder.[(Process 1) Steelmaking Process]

[0186] In the steelmaking process, a molten steel satisfying feature 1, or a molten steel satisfying feature 1 and requirement 1 and requirement 2 is used to produce an ingot by a known casting process. The known casting process is, for example, the bottom pouring ingot casting method. An electroslag remelting (ESR) process in which the cast ingot is used as an electrode or the like may also be performed.[(Process 2) Hot Forging Process]

[0187] In the hot forging process, first, the ingot is heated in a heating furnace. After being heated, the ingot is subjected to hot forging to produce a roll near-net-shape material that has a rough shape (hereinafter, referred to as “intermediate shaped material”). In the hot forging process, the following essential condition 1 and essential condition 2 are satisfied.

[0188] (Essential condition 1) Temperature T1 of ingot immediately before start of hot forging: 1100 to 1200° C.

[0189] (Essential condition 2) Forging ratio S: 1.3 to 3.2

[0190] Each essential condition is described hereunder.[(Essential Condition 1) Regarding Temperature T1 of Ingot Immediately Before Start of Hot Forging]

[0191] The temperature T1 of the ingot immediately before the start of hot forging is set to 1100 to 1200° C. If the temperature T1 is less than 1100° C., the temperature T1 will be too low. In such case, cracks may occur in the ingot during hot forging. If the temperature T1 is more than 1200° C., cracks may occur in the ingot during hot forging. Therefore, the temperature T1 is set to 1100 to 1200° C.[(Essential Condition 2) Regarding Forging Ratio S]

[0192] In the hot forging process, in addition, the forging ratio S is set to 1.3 to 3.2. In the present description, the term “forging ratio S” means the forging ratio in solid forging. The forging ratio S is defined by the following equation.Forging ratio S=cross-sectional area of ingot before hot forging / cross-sectional area of intermediate shaped material body part after hot forging

[0193] Here, the phrase “cross-sectional area of ingot before hot forging” means the cross-sectional area at a cross section perpendicular to the longitudinal direction of the ingot. Further, the phrase “cross-sectional area of intermediate shaped material body part after hot forging” means the cross-sectional area at a cross section perpendicular to the longitudinal direction of the body part of the intermediate shaped material.

[0194] The forging ratio S influences the large-angle grain boundary length per 1 mm2 in the forged steel roll after production. If the forging ratio S is less than 1.3, the large-angle grain boundary length per 1 mm2 in the forged steel roll after production will be too short. On the other hand, if the forging ratio S is more than 3.2, the large-angle grain boundary length per 1 mm2 in the forged steel roll after production will be too long. When the forging ratio S is made to fall within the range of 1.3 to 3.2, the large-angle grain boundary length per 1 mm2 in the forged steel roll after production can be adjusted to an appropriate size.[Preferable Conditions in Hot Forging Process (Optional Conditions)]

[0195] Preferably, in the hot forging process, in addition, the following conditions are satisfied. These conditions are optional conditions.

[0196] (Optional condition 1) Heating temperature TO: 1200 to 1300° C.

[0197] (Optional condition 2) Holding time t0 at heating temperature TO: 10 hours or more

[0198] (Optional condition 3) Temperature difference ΔT=heating temperature T0−temperature T1 of ingot immediately before start of hot forging: 50 to 150° C.

[0199] Hereunder, optional condition 1 to optional condition 3 are described.[(Optional Condition 1 and Optional Condition 2) Regarding Heating Temperature T0 and Holding Time t0]

[0200] If the heating temperature T0 is set to 1200 to 1300° C., and the holding time t0 is set to 10 hours or more, Si precipitates such as Si carbides in the intermediate shaped material will sufficiently dissolve in the annealing process that is a subsequent process. Therefore, a sufficient amount of dissolved Si can be secured in the produced forged steel roll. As a result, in the forged steel roll, the Vickers hardness at 400° C. will be 400 HV or more.[(Optional Condition 3) Regarding Temperature Difference ΔT]

[0201] In addition, if a temperature difference ΔT obtained by subtracting the temperature T1 of the ingot immediately before the start of hot forging from the heating temperature T0 is within the range of 50 to 150° C., Si precipitates such as Si carbides in the intermediate shaped material will sufficiently dissolve in the annealing process that is a subsequent process. Therefore, a sufficient amount of dissolved Si can be secured in the produced forged steel roll. As a result, in the forged steel roll, the Vickers hardness at 400° C. will be 400 HV or more. A more preferable temperature difference ΔT is 60 to 100° C.

[0202] Note that, if the steel material temperature during hot forging drops to 900° C., the steel material is to be heated again in a heating furnace. Then, after the steel material has been reheated, hot forging is to be resumed. The heating temperature T0 and the temperature T1 in the case of reheating and resuming hot forging are both to satisfy essential condition 1. In addition, preferably the holding time t0 satisfies a condition of being 1 to 10 hours. Note that, if the steel material is heated again in a heating furnace during hot forging, the temperature difference ΔT means the temperature difference at the time of the first heating, and the temperature difference ΔT at the time of the second heating and a subsequent heating does not have to satisfy optional condition 3.[(Process 3) Annealing Process]

[0203] In the annealing process, the intermediate shaped material produced in the hot forging process is subjected to annealing. Performing the annealing process makes it easier to grind the intermediate shaped material in the rough machining process that is the next process. It suffices to perform annealing under well-known conditions using an electric furnace or a gas furnace. The annealing temperature is, for example, 500 to 1000° C. The holding time is, for example, 10 to 50 hours.[(Process 4) Rough Machining Process]

[0204] In the rough machining process, rough machining is performed on the intermediate shaped material after the annealing process, and the intermediate shaped material is further subjected to rough machining to make the intermediate shaped material into a shape that is near to the final roll shape. The rough machining is, for example, grinding. It suffices to perform the rough machining under well-known conditions.[(Process 5) Quenching Process]

[0205] In the quenching process, the intermediate shaped material after the rough machining process is subjected to quenching. In the quenching process, the following essential condition 3 to essential condition 5 are satisfied.

[0206] (Essential condition 3) Quenching temperature T2: 950 to 1100° C.

[0207] (Essential condition 4) Holding time t2 at quenching temperature T2: 0.02 to 0.22 hours

[0208] (Essential condition 5) FA that is defined by the following formula is: 5.00 to 12.00FA=S / (T⁢2×(2⁢6+log1⁢0(5×t⁢2))×1⁢0-5)where, the forging ratio S is substituted for “S” in FA. The quenching temperature (° C.) is substituted for “T2”. The holding time (hrs) is substituted for “t2”.

[0210] Hereunder, essential condition 3 to essential condition 5 are described.[(Essential Condition 3 and Essential Condition 4) Regarding Quenching Temperature T2 and Holding Time t2]

[0211] The quenching temperature T2 and the holding time t2 influence the large-angle grain boundary length in the produced forged steel roll. If the quenching temperature T2 is too low or if the holding time t2 is too short, the large-angle grain boundary length per 1 mm2 in the produced forged steel roll will be too long. On the other hand, if the quenching temperature T2 is too high or if the holding time t2 is too long, the large-angle grain boundary length per 1 mm2 in the produced forged steel roll will be too short. If the quenching temperature T2 is set to 950 to 1100° C., and the holding time t2 is set to 0.02 to 0.22 hours, the large-angle grain boundary length per 1 mm2 in the produced forged steel roll can be adjusted to an appropriate size.[(Essential Condition 5) Regarding FA]

[0212] The forging ratio S, and the quenching temperature T2 and holding time t2 influence the large-angle grain boundary length per 1 mm2 in the forged steel roll in relation to each other. If FA, which is the relational expression between the forging ratio S and the quenching temperature T2 and holding time t2, is less than 5.00, the large-angle grain boundary length per 1 mm2 in the produced forged steel roll will be too short. On the other hand, if FA is more than 12.00, the large-angle grain boundary length per 1 mm2 in the produced forged steel roll will be too long. If FA is made to be fall within the range of 5.00 to 12.00, the large-angle grain boundary length per 1 mm2 in the produced forged steel roll can be adjusted to an appropriate size.[Preferable Condition in Quenching Process (Optional Condition)]

[0213] Preferably, the following condition is also satisfied in the quenching process.(Optional Condition 4) Cooling Time CT from Quenching Temperature T2 to 800° C.: 30 to 300 Seconds

[0214] If the cooling time CT from the quenching temperature T2 to 800° C. is 30 to 300 seconds, formation of Si precipitates by dissolved Si can be sufficiently suppressed, and carbides consisting of alloys other than Si can be sufficiently formed. Therefore the high temperature hardness of the forged steel roll can be increased.

[0215] Note that, a cryogenic treatment may be performed on the intermediate shaped material at a timing after the quenching process and before the tempering process. The cooling temperature in the cryogenic treatment may be in a known range, for example, −30 to −196° C.[(Process 6) Tempering Process]

[0216] In the tempering process, the intermediate shaped material after the quenching process is subjected to tempering. The hardness of the outer layer of the forged steel roll is adjusted by the tempering. The tempering temperature is, for example, to be set in the range of 100 to 600° C.[(Process 7) Finishing Machining Process]

[0217] In the finishing machining process, the intermediate shaped material after the tempering process is subjected to finishing machining. The finishing machining is, for example, grinding using a grinder. The intermediate shaped material is processed into the final product shape by the finishing machining. The forged steel roll of the present embodiment can be produced by the above process.

[0218] Hereunder, the present invention is described in further detail by way of examples. The present invention is not limited in any way by these examples.Examples[Production of Forged Steel Rolls]

[0219] Forged steel rolls having the chemical compositions shown in Table s were produced by the following production method.TABLE 1Chemical Composition (unit is mass %; balance is Fe and impurities)OptionalEssential ElementsElementsFormulaNumberCSiMnPSAlNOCrMoCuBNiVNb(1)11.130.220.250.0120.00900.0190.00510.00274.990.750.0440.0053———5.7420.710.371.180.0140.00680.0180.00770.00262.841.890.0480.0003———4.7330.960.321.490.0140.01880.0240.01580.00214.531.200.0330.0066———5.7341.480.290.550.0070.01200.0180.00570.00257.962.980.0290.0065———0.9451.300.260.820.0170.01770.0150.00580.00316.170.430.0200.0079———6.6061.041.160.220.0160.00540.0180.00480.00344.230.950.0980.0054———5.1871.250.431.450.0140.01680.0460.00700.00346.340.430.0360.0073———6.7781.491.010.740.0160.01610.0110.01960.00247.982.970.0570.0096———0.9590.731.480.910.0100.00090.0190.00560.00282.851.690.0290.0059———4.54100.920.610.430.0170.01960.0150.00800.00255.020.530.0450.0022———5.55110.890.240.390.0180.01510.0200.00760.00275.000.510.0330.00660.51——5.51120.950.590.420.0280.01690.0120.00660.00284.960.470.0350.00521.18——5.43130.880.370.450.0160.01240.0150.00540.00305.010.530.0390.0058—0.99—6.53140.900.630.410.0170.01450.0190.00630.00145.050.490.0910.0066—1.96—7.50150.940.290.380.0110.01960.0170.00670.00304.990.490.0230.0079——0.225.70160.930.590.400.0050.01720.0470.01850.00475.020.550.0630.0061——0.486.05170.870.310.370.0060.01400.0080.00530.00275.030.520.0570.00600.94—0.295.84180.910.600.440.0140.01660.0230.00620.00174.970.500.0230.00940.310.52—5.99191.380.580.420.0100.01310.0200.01950.00157.922.970.0410.00510.461.950.4813.32201.130.240.270.0130.00860.0160.10160.00285.990.850.0340.0059———6.84210.970.590.410.0090.01870.0160.00780.00253.920.470.0600.0047———4.39220.890.680.390.0110.01560.0310.00980.00173.640.410.0470.0039—0.36—4.41231.020.660.400.0130.01290.0290.00610.00295.000.510.0780.0057———5.51240.950.570.380.0070.01690.0150.00740.00304.960.540.0580.00420.56——5.50250.740.731.130.0140.00780.0200.19240.00312.821.760.0450.0052———4.58260.990.600.450.0100.01370.0220.00380.00245.050.430.0410.0050———5.48270.870.590.490.0160.01190.0340.00540.00185.020.530.0350.0028——0.215.76280.930.610.360.0090.00870.0170.00800.00204.990.570.0570.0069———5.56291.070.670.450.0110.01220.0220.00770.00234.940.470.0690.00490.380.64—6.05300.840.580.410.0150.01710.0180.00510.00215.030.490.0440.0041———5.52310.960.550.430.0060.01530.0260.00680.00375.080.460.0390.00380.75—0.375.91321.000.630.560.0120.01670.0180.00690.00274.970.540.0510.0054———5.51330.940.620.460.0110.01060.0140.00810.00245.010.510.0590.00480.500.530.376.42340.940.520.430.0120.01380.0170.00680.00275.000.520.0470.0048———5.52350.880.630.380.0080.01450.0210.00500.00154.970.480.0350.0050—0.41—5.86360.900.570.410.0150.01060.0150.00780.00395.010.500.0290.0047———5.51370.890.590.450.0180.01590.0180.00740.00225.010.490.0220.0063——0.445.94380.860.640.430.0100.01600.0190.00730.00245.040.480.0390.0054———5.52390.890.630.360.0160.01650.0160.00740.00165.010.480.0340.00470.37——5.49400.920.570.390.0090.00880.0150.00590.00214.980.460.0350.0060———5.44410.880.610.410.0180.01630.0140.00620.00345.020.510.0380.0031—0.46—5.99420.870.550.380.0180.01470.0110.00660.00184.990.470.0320.0068———5.46430.900.560.400.0170.01420.0150.01320.00254.980.530.0390.0065——0.385.89440.910.580.450.0110.01560.0140.00580.00224.960.490.0310.0091———5.45450.900.600.390.0180.01510.0200.00660.00315.000.520.0200.00580.95—0.405.92460.880.590.420.0090.01590.0140.00790.00465.030.480.0560.0040———5.51470.930.570.380.0160.00570.0120.00610.00335.030.540.0330.00570.710.72—6.29480.920.620.410.0180.01800.0240.00600.00285.010.510.0930.0014———5.52490.890.590.430.0070.01420.0200.01940.00254.970.500.0730.0051—0.270.366.10500.950.630.370.0150.01610.0150.01880.00244.980.470.0520.0055———5.45510.920.610.400.0170.00750.0160.00530.00234.990.490.0380.00480.770.530.356.36520.910.580.440.0160.00100.0140.00840.00195.040.520.0500.0050———5.56530.900.580.390.0110.01070.0420.00560.00215.020.480.0240.00530.59——5.50

[0220] Specifically, ingots were cast from molten steels by a bottom pouring ingot casting method. Each of the produced ingots was subjected to an electroslag remelting (ESR) process to remelt the ingot and produce an ingot to serve as a starting material. Each produced ingot was subjected to a hot forging process. Essential condition 1 (forging temperature T1 (° C.)) and essential 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 were as shown in Table 2. For each test number, an intermediate shaped material having a roll shape in which a roll body part had a diameter 4 of 700 mm, a body length of 2100 mm, and an overall length of 4100 mm was produced by the hot forging process.TABLE 2Hot Forging ProcessOptionalOptionalCondition 1OptionalEssentialCondition 3Quenching ProcessHeatingCondition 2Condition 1TemperatureEssentialEssentialOptionalTemperatureHolding TimeTemperatureDifferenceEssentialCondition 3Condition 4Condition 4T0 (° C.)t0 (hrs)T1 (° C.)ΔT (° C.)Condition 2QuenchingHoldingEssentialCoolingFirstSecondFirstSecondFirstSecondFirstForgingTemperatureTime t2Condition 5Time CTNumberTimeTimeTimeTimeTimeTimeTimeRatio ST2 (° C.)(hrs)FA(secs)112201180157112011801002.010400.227.38150212901150202114011501501.59800.105.963031250110010412001100502.710600.189.81300412301120128110011201301.710900.056.141205130011101910118011101202.49800.139.496061200117013311201170803.210500.1911.7327071210120012511301200802.611000.079.2590812801160176117011601101.310000.055.124591240114015111501140901.89500.157.322851012601110119111011101502.210000.108.56180111270119011212001190702.510500.199.171351212501150146114011501102.410200.029.41255131220115024811601150601.69700.146.3830141230113016711801130503.09900.1711.692401513001120209118011201201.510100.215.7130016128012001010115012001302.810600.0710.34105171210119022411301190802.19600.048.651501812901140151117011401202.310300.118.68195191240115019611501150902.910400.1610.76602012301170148113011401002.110200.217.911202113001100168116011001401.69900.176.232102212301160127112011601102.110800.117.55270231180112010211301120502.99700.0511.77180241160117023811001170602.410400.138.9490251170118018911501120201.79900.126.66902613101150146118011501301.39600.155.231202713401110159120011101402.010100.227.60240281250112081115011201003.110700.0911.2922529127011406311901140802.710200.1910.19285301220118015511901180302.210000.188.4845311200113016711801130201.49800.125.54753212901200116113012001602.510300.109.441653312801110144111011101701.810500.166.62105341250118015211701180802.29700.148.7820351200119010611201190803.010800.1910.6925361260116019111901160701.710400.096.373303713001180173116011801402.510000.119.71360381260114020311701140901.29500.035.021203912301130137111011301201.29500.025.051504012001150151011401150603.310700.1811.88210411210116010111501160603.411000.1611.93304212701120225112011201502.59300.1810.362704312201170126110011701201.79400.117.031354412901190187119011901001.811200.206.18150451280110019212001100802.911100.0910.18904612501140169113011401202.210400.018.56604712601160135116011601001.69800.016.612404812301180201011401180902.410000.249.20135491230112016611701120603.010600.2310.86455012901150127115011501401.410900.204.942105112501130189114011301101.310500.154.78240521210117020311301170803.19600.1312.51120531270113014411801130903.29500.2212.93105

[0221] Note that, in the forging process, each ingot was heated at a heating temperature T0 and held for a holding time t0. Then, after the ingot temperature was decreased to a forging temperature T1, hot forging was performed. Note that, in all of the test numbers, the ingot temperature decreased to 900° C. during hot forging. Therefore, hot forging was temporarily interrupted, and the ingot was heated again in a heating furnace (second heating). After the ingot was reheated, the hot forging was resumed. The first and second heating temperatures T0, holding times t0, and temperatures T1, and the first temperature difference ΔT are shown in Table 2.

[0222] An annealing process was performed on the intermediate shaped material after the hot forging process. In the annealing process, the intermediate shaped material was held at 900° C. for 10 hours. Thereafter, in addition, the intermediate shaped material was held at 600° C. for 15 hours. The intermediate shaped material after the annealing process was subjected to a rough machining process. Specifically, for each test number, the intermediate shaped material was subjected to grinding to produce an intermediate shaped material having a roll shape in which a roll body part had a diameter b of 650 mm, a body length of 2000 mm, and an overall length of 4000 nm.

[0223] The intermediate shaped material after the rough machining process was subjected to a quenching process. Essential condition 3 (quenching temperature T2 (° C.)), essential condition 4 (holding time t2 (hours)), essential condition 5 (FA), and optional condition 4 (cooling time CT (secs)) in the quenching process are shown in Table 2. Note that, in the quenching process, the intermediate shaped material was heated by induction heating, and thereafter was water-cooled.

[0224] The intermediate shaped material after the quenching process was subjected to a cryogenic treatment. In the cryogenic treatment, the intermediate shaped material was cooled to −60 to −140° C. After undergoing the cryogenic treatment, the intermediate shaped material was subjected to tempering at 100 to 600° C., and thereafter was subjected to a finishing machining process. In the finishing machining process, the intermediate shaped material was subjected to grinding and finished into a final roll shape having a roll body part with a diameter φ of 645 mm, a body length of 1950 mm, and an overall length of 3950 mm. A forged steel roll of each test number was produced by the above production process. Note that, the total area fraction of martensite and bainite in the outer layer of the forged steel roll of each test number was determined based on the method described above in the section [Method for measuring total area fraction of martensite and bainite in outer layer of forged steel roll]. As a result, it was determined that in the forged steel roll of each test number, the total area fraction of martensite and bainite in the outer layer was 85% or more.[Evaluation Tests]

[0225] The following evaluation tests were carried out using the forged steel roll of each test number.

[0226] (Test 1) Test to measure large-angle grain boundary length per 1 mm2

[0227] (Test 2) Vickers hardness at 400° C. test

[0228] (Test 3) Crack resistance test

[0229] Test 1 to test 3 are described hereunder.[(Test 1) Test to Measure Large-Angle Grain Boundary Length Per 1 mm2]

[0230] The large-angle grain boundary length per 1 mm2 (mm / mm2) in the outer layer of the forged steel roll of each test number was determined in accordance with the method described above in the section [Method for measuring large-angle grain boundary length per 1 mm2 in outer layer]. The obtained large-angle grain boundary length per 1 mm2 is shown in the column “Large-angle Grain boundary Length (mm / mm2)” in Table 3.TABLE 3Large-angleGrainVickersCrack ResistanceboundaryHardnessMaximumNumberLengthat 400° C.Crackof CracksNumber(mm / mm2)(HV)Depth (μm)(Cracks)Remarks1345038023328Inventive Example2274039219422Inventive Example3583038732526Inventive Example4278038919923Inventive Example5532038730625Inventive Example6897054139710Inventive Example7516040630318Inventive Example8252048615111Inventive Example933405882308Inventive Example10415041526814Inventive Example11481038629724Inventive Example12529041731515Inventive Example13293039020223Inventive Example14866041339017Inventive Example15260038617425Inventive Example16650041934312Inventive Example17448038828123Inventive Example18455041227911Inventive Example19696041135610Inventive Example20364040822826Inventive Example21289038120029Inventive Example22348039223827Inventive Example23874038838823Inventive Example24483037629524Inventive Example25290039419622Inventive Example26252039616727Inventive Example27338038323425Inventive Example28752038033222Inventive Example29637039133926Inventive Example30424038627329Inventive Example31254038517628Inventive Example32537039232125Inventive Example33297038420523Inventive Example34435038527525Inventive Example35703039236127Inventive Example36293038120823Inventive Example37562038832824Inventive Example38245041512935Comparative Example39248041813433Comparative Example40914041042712Comparative Example4190604124189Comparative Example42903041341114Comparative Example4392404144337Comparative Example44247041613333Comparative Example45243041512138Comparative Example46903041741610Comparative Example47919041242211Comparative Example48243041829936Comparative Example49239041735837Comparative Example50248041414732Comparative Example51245041512434Comparative Example52904041141411Comparative Example53909041341913Comparative Example[(Test 2) Vickers Hardness at 400° C. Test]

[0231] The Vickers hardness (HV) at 400° C. of the forged steel roll of each test number was determined in accordance with the method described above in the section [Method for measuring Vickers hardness at 400° C.]. Note that, a high temperature microhardness tester with the model name QM-2 manufactured by Nikon Corporation was used for the measurement. The obtained Vickers hardness is shown in the column “Vickers Hardness (HV) at 400° C.” in Table 3.[(Test 3) Crack Resistance Test]

[0232] To evaluate the crack resistance, a thermal shock test was conducted using a drop-weight-friction thermal shock tester 10 illustrated in FIG. 1. Specifically, a test specimen 13 with dimensions of 20 mm in the depth direction, 20 mm in the circumferential direction, and 30 mm in the longitudinal direction was taken from the surface at a central position in the longitudinal direction of the body part of the forged steel roll of each test number.

[0233] A rod-shaped biting material 12 of 5 mm in diameter and 10 mm in length was attached to the outer circumference of a pinion 11 of the drop-weight-friction thermal shock tester 10. The biting material 12 was a low carbon steel wire rod SWRM6 prescribed in JIS G 3505: 2017. The tip portion of the biting material 12 was bent, and the tip portion was embedded in the radial direction of the pinion 11 from the outer peripheral surface of the pinion 11. As illustrated in FIG. 1, a surface 13A of the test specimen 13 was brought into contact with the biting material 12. At such time, the biting material was arranged so that the portion of the biting material 12 other than the tip portion extended upward. The test specimen 13 and the biting material were arranged so that the bent part of the biting material 12 contacted the upper part of the surface 13A. The surface 13A had a size of 20 mm×30 mm, and was arranged so that the 30 mm side was in the vertical direction. The surface 13A was a surface corresponding to the surface of the forged steel roll. After the test specimen 13 and the biting material 12 were arranged as described above, the pinion 11 was rotated using a weight, and the biting material 12 was caused to slide strongly on the surface 13A to apply thermal shock to the surface 13A.

[0234] FIG. 2 is a schematic diagram of the surface 13A after the biting material 12 had passed over the surface 13A. As illustrated in FIG. 2, a contact region 100 that was a portion where the biting material 12 had slid was formed in the longitudinal direction on the surface 13A. Therefore, as illustrated in FIG. 2, the contact region 100 was cut at approximately the center of its width along a plane CS including the longitudinal direction and the normal direction of the surface 13A.

[0235] The entire area of the contact region 100 on the cut surface was observed with an optical microscope with a magnification of 100×, and the vicinity of the surface of the entire contact region 100 was sequentially photographed to generate a plurality of sequential photographic images. FIG. 3 is a cross-sectional view illustrating a portion of the sequential photographic images. The number of cracks 50 which had initiated throughout the entire contact region 100 and the maximum depth of the cracks 50 were identified using the sequential photographic images. In each photographic image, a length NL of each crack 50 in the normal direction N of the surface 13A was recognized as the crack depth. If the crack depth was 10 μm or more, the relevant crack 50 was recognized as a crack. The number of recognized cracks was determined. In addition, the maximum crack depth among the recognized cracks was determined. The determined maximum depth (μm) of the cracks and the determined number of cracks are shown in the column “Maximum Crack Depth (μm)” and the column “Number of Cracks (Cracks)” in Table 3.Evaluation Results

[0236] Referring to Table 1, Table 2, and Table 3, in Test Nos. 1 to 37 the chemical composition was appropriate and the essential conditions 1 to 5 during the production process were satisfied. Therefore, the large-angle grain boundary length per 1 mm2 was 2500 to 9000 mm. Therefore, in these test numbers, excellent crack resistance was obtained. Specifically, in these test numbers, in the crack resistance test, the maximum crack depth was suppressed to 400 μm or less and the number of cracks was 30 or less.

[0237] Among Test Nos. 1 to 37, in Test Nos. 6 to 10, 12, 14, 16, 18 and 19, furthermore, the content of Si was 0.40 to 1.50%, and Formula (1) was satisfied. In addition, the Vickers hardness at 400° C. was 400 HV or more. As a result, in the crack resistance test, the number of cracks was 20 or less, and thus the frequency of crack initiation was further suppressed and further excellent crack resistance was obtained.

[0238] On the other hand, in Test Nos. 38 and 39, the forging ratio S was too low. Consequently, the large-angle grain boundary length per 1 mm2 was too short. As a result, the number of cracks was more than 30, and thus sufficient crack resistance was not obtained.

[0239] In Test Nos. 40 and 41, the forging ratio S was too high. Consequently, the large-angle grain boundary length per 1 mm2 was too long. As a result, the maximum crack depth was more than 400 μm, and thus sufficient crack resistance was not obtained.

[0240] In Test Nos. 42 and 43, the quenching temperature T2 was too low. Consequently, the large-angle grain boundary length per 1 mm2 was too long. As a result, the maximum crack depth was more than 400 μm, and thus sufficient crack resistance was not obtained.

[0241] In Test Nos. 44 and 45, the quenching temperature T2 was too high. Consequently, the large-angle grain boundary length per 1 mm2 was too short. As a result, the number of cracks was more than 30, and thus sufficient crack resistance was not obtained.

[0242] In Test Nos. 46 and 47, the holding time t2 at the quenching temperature was too short. Consequently, the large-angle grain boundary length per 1 mm2 was too long. As a result, the maximum crack depth was more than 400 μm, and thus sufficient crack resistance was not obtained.

[0243] In Test Nos. 48 and 49, the holding time t2 was too long. Consequently, the large-angle grain boundary length per 1 mm2 was too short. As a result, the number of cracks was more than 30, and thus sufficient crack resistance was not obtained.

[0244] In Test Nos. 50 and 51, the value of FA was too low. Consequently, the large-angle grain boundary length per 1 mm2 was too short. As a result, the number of cracks was more than 30, and thus sufficient crack resistance was not obtained.

[0245] In Test Nos. 52 and 53, the value of FA was too high. Consequently, the large-angle grain boundary length per 1 mm2 was too long. As a result, the maximum crack depth was more than 400 μm, and thus sufficient crack resistance was not obtained.

[0246] An embodiment of the present disclosure has been described above. However, the embodiment described above is merely an example for carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment within a range that does not depart from the gist of the present disclosure.

Claims

1. A forged steel roll, having a chemical composition consisting of, 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: 0.2000% or less,O: 0.0050% or less,Cr: 2.80 to 8.00%,Mo: 0.40 to 3.00%,Cu: 0.100% or less,B: 0.0100% or less,Ni: 0 to 1.20%,V: 0 to 2.00%,Nb: 0 to 0.50%, andthe balance: Fe and impurities,wherein:a large-angle grain boundary length per 1 mm2 in an outer layer of the forged steel roll is 2500 to 9000 mm.

2. The forged steel roll according to claim 1, wherein the chemical composition contains one or more types of element selected from a group consisting of:Ni: 0.01 to 1.20%,V: 0.01 to 2.00%, andNb: 0.01 to 0.50%.

3. The forged steel roll according to claim 1, wherein:a content of N is 0.0200% or less in percent by mass.

4. The forged steel roll according to claim 2, wherein:a content of N is 0.0200% or less in percent by mass.

5. The forged steel roll according to claim 1, wherein:a content of Si is 0.40 to 1.50% in percent by mass,the chemical composition satisfies Formula (1), anda Vickers hardness at 400° C. is 400 HV or more:4.5≤Cr+Mo+V+Nb≤13.5(1)where, a content of a corresponding element in percent by mass is substituted for each symbol of an element in Formula (1), and if an element is not contained, “0” is substituted for the corresponding symbol of an element.

6. The forged steel roll according to claim 2, wherein:a content of Si is 0.40 to 1.50% in percent by mass,the chemical composition satisfies Formula (1), anda Vickers hardness at 400° C. is 400 HV or more:4.5≤Cr+Mo+V+Nb≤13.5(1)where, a content of a corresponding element in percent by mass is substituted for each symbol of an element in Formula (1), and if an element is not contained, “0” is substituted for the corresponding symbol of an element.

7. The forged steel roll according to claim 3, wherein:a content of Si is 0.40 to 1.50% in percent by mass,the chemical composition satisfies Formula (1), anda Vickers hardness at 400° C. is 400 HV or more:4.5≤Cr+Mo+V+Nb≤13.5(1)where, a content of a corresponding element in percent by mass is substituted for each symbol of an element in Formula (1), and if an element is not contained, “0” is substituted for the corresponding symbol of an element.

8. The forged steel roll according to claim 4, wherein:a content of Si is 0.40 to 1.50% in percent by mass,the chemical composition satisfies Formula (1), anda Vickers hardness at 400° C. is 400 HV or more:4.5≤Cr+Mo+V+Nb≤13.5(1)where, a content of a corresponding element in percent by mass is substituted for each symbol of an element in Formula (1), and if an element is not contained, “0” is substituted for the corresponding symbol of an element.

Images