ROLL OUTER LAYER MATERIAL TO BE HOT-ROLLED, AND COMPOSITE ROLL TO BE HOT-ROLLED

Abstract

 To provide a composite hot-rolling roll and an outer layer material for a hot-rolling roll that have a wear resistance and fatigue resistance comparable to or higher than those of the related art and in which porosities and shrinkage cavities are reduced.
An outer layer material for a hot-rolling roll, the outer layer material having a chemical composition that contains, in mass%, C: 1.6 to 2.5%, Si: 0.2 to 1.5%, Mn: 0.2 to 1.6%, Cr: 4.5 to 7.0%, Mo: 1.0 to 5.0%, V: 4.0 to 6.0%, and Nb: 0.5 to 2.5%, in which a sum of contents of N and O of the chemical composition is 100 to 400 mass ppm, and a balance of the chemical composition is Fe and incidental impurities.

Description

Technical Field

[0001] The present invention relates to a composite hot-rolling roll and, in particular, to a composite hot-rolling roll and an outer layer material for a hot-rolling roll that are suitable for use in a hot-rolling finishing mill for steel sheets.

Background Art

[0002] In recent years, usage environments for rolls have been becoming more severe as the technology for hot rolling of steel sheets has advanced, and in addition, the volume of production of steel sheets for which a rolling load is high, such as high-strength steel sheets and thin thickness products, has been increasing. As a result, the level of quality required of a work roll for rolling has become high, and, accordingly, there is a need for a work roll for rolling that is free of casting defects, such as segregation, porosities, and shrinkage cavities.

[0003] Patent Literature 1, for example, proposes an outer layer material for such a work roll for rolling. The outer layer material of a rolling roll contains C: 1.5 to 3.5%, Si: 1.5% or less, Mn: 1.2% or less, Ni: 5.5% or less, Cr: 5.5 to 12.0%, Mo: 2.0 to 8.0%, V: 3.0 to 10.0%, and Nb: 0.5 to 7.0%, in which Nb and V are present such that the contents of Nb, V, and C satisfy a specific relationship, and a ratio between Nb and V is within a specific range. It is stated that in this outer layer material, segregation of hard carbides is inhibited even in an instance in which a centrifugal casting method is used, and, therefore, the outer layer material of a rolling roll has excellent wear resistance and crack resistance.

[0004] Furthermore, Patent Literature 2 proposes an outer layer material of a rolling roll, the outer layer material containing C: 1.5 to 3.5%, Si: 1.5% or less, Mn: 1.2% or less, Cr: 5.5 to 12.0%, Mo: 2.0 to 8.0%, V: 3.0 to 10.0%, and Nb: 0.5 to 7.0%, in which Nb and V are present such that the contents of Nb, V, and C satisfy a specific relationship, and a ratio between Nb and V is within a specific range. It is stated that in this outer layer material, segregation of hard carbides is inhibited even in an instance in which a centrifugal casting method is used, and, therefore, wear resistance and crack resistance are improved; thus, the outer layer material significantly contributes to improving productivity in hot rolling. Furthermore, Patent Literature 3 proposes an outer layer material of a rolling roll, the outer layer material containing C: 1.5 to 3.5%, Si: 0.1 to 2.0%, Mn: 0.1 to 2.0%, Cr: 5 to 25%, Mo: 2 to 12%, V: 3 to 10%, and Nb: 0.5 to 5%, in which a ratio between Mo and Cr is within a specific range, and the outer layer material has a carbide amount distribution in which the difference between adjacent maximum and minimum values is less than or equal to 20% of an average value, regarding a region extending 30 mm from a surface in a roll radius direction. It is stated that since laminated segregation is accordingly reduced, formation of segregation patterns is inhibited, and, therefore, the outer layer material of a rolling roll has excellent surface quality.

Citation List

Patent Literature

[0005] 

PTL 1: Japanese Unexamined Patent Application Publication No. 04-365836

PTL 2: Japanese Unexamined Patent Application Publication No. 05-1350

PTL 3: Japanese Unexamined Patent Application Publication No. 2000-239779

Summary of Invention

Technical Problem

[0006]  As described in Patent Literature mentioned above, outer layer materials of a rolling roll in which segregation of carbides is reduced by ensuring chemical components are present within appropriate ranges have been proposed. However, no effective countermeasures for porosities and shrinkage cavities have been clarified. Furthermore, in severe roll usage environments in recent years, in instances in which a high-Cr-content outer layer material of a rolling roll, such as those described in Patent Literature mentioned above, is used, a deep crack due to hot-rolling contact fatigue may form in a surface of the roll. Accordingly, there is a need for an outer layer material of a rolling roll, in which porosities and shrinkage cavities in the outer layer material are reduced and which has excellent fatigue resistance.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a composite hot-rolling roll and an outer layer material for a hot-rolling roll that have a wear resistance and fatigue resistance comparable to or higher than those of the related art and in which porosities and shrinkage cavities are reduced.

[0008] Note that in the present invention, "having a wear resistance comparable to or higher than those of the related art", as stated above, refers to instances in which a wear ratio, as measured by the method described below, is 0.97 or greater.

<1> In a two-disc slip rolling method (see Fig. 3), a wear test piece (outside diameter: 60 mmϕ, thickness: 10 mm, chamfered) taken from an outer layer material of a roll and an opposing piece are used. The wear test piece 5 is rotated at 700 rpm while being water-cooled with cooling water.

<2> In a state in which the opposing piece 8 (material: S45C, outside diameter: 190 mmϕ, width: 15 mm, chamfer: C1), which has been heated to 800°C by a high-frequency induction heating coil 7, is in contact with the rotating wear test piece 5 at a load of 980 N, the opposing piece 8 is rolled at a slip ratio of 9%.

<3> A wear test that lasts for 300 minutes, in which the opposing piece 8 is replaced with a new one every 50 minutes, is conducted. In the wear test, a wear ratio is obtained in a manner in which a related-art example is used as a reference, and a ratio of an amount of wear of the test piece with respect to the value of the reference (wear ratio = (amount of wear of the reference piece)/(amount of wear of the test piece)) is measured. The related-art example is shown in Table 1 (presented later) as No. 35, which is an outer layer material of a roll, the outer layer material having a chemical composition that contains, in mass%, C: 2.0%, Si: 0.5%, Mn: 0.5%, Cr: 6.0%, Mo: 5.0%, V: 7.0%, and Nb: 0.4%, in which the sum of the contents of N and O of the chemical composition is 430 mass ppm, and the balance of the chemical composition is Fe and incidental impurities.

[0009] Furthermore, in the present invention, "having a fatigue resistance comparable to or higher than those of the related art", as stated above, refers to instances in which a hot-rolling fatigue life, as measured by the method described below, is greater than three hundred and fifty thousand cycles (350,000 cycles).

<1> A notch (depth t: 1.2 mm, circumferential length L: 0.8 mm) is introduced at two locations of the outer circumferential surface of a hot-rolling fatigue test piece (outside diameter: 60 mmϕ, thickness: 10 mm), which is taken from an outer layer material of a roll, by using an electro-discharge machining (wire-cutting) method that uses a 0.2-mmϕ wire (see Fig. 6) .

<2> The edges of the rolling contact surface of the hot-rolling fatigue test piece 5 are chamfered to have a chamfer of 1.2 C.

<3> In the two-disc slip rolling method, a hot-rolling fatigue test piece 5 having notches and an opposing piece 8, which has been heated, are used, and the hot-rolling fatigue test piece 5 is rotated at 700 rpm while being water-cooled with cooling water 6.

<4> In a state in which the opposing piece 8 (material: S45C, outside diameter: 190 mmϕ, thickness: 15 mm), which has been heated to 800°C by a high-frequency induction heating coil 7, is pressed against the rotating test piece 5 at a load of 980 N, the opposing piece 8 is rolled at a slip ratio of 9%.

<5> The rolling is continued until the two notches, 9, introduced into the hot-rolling fatigue test piece 5 are broken. The number of rolling cycles until breakage occurs is determined for each of the notches 9, and the average value thereof is measured and designated as a hot-rolling fatigue life.

[0010] Furthermore, in the present invention, "porosities and shrinkage cavities are reduced", as stated above, refers to the following instances: when an X-ray CT measurement is performed on the surface of an outer layer material of a roll after irregularities and scales (an oxide layer) are removed therefrom by grinding, the measurement being performed with a maximum tube voltage of 225 kV, a tube voltage of 150 kV, and a tube current of 80 µA, a circle circumscribing a porosity or a shrinkage cavity in a captured image has a diameter of less than or equal to 0.50 mm.

Solution to Problem

[0011] The present inventors thoroughly investigated relationships between porosities and shrinkage cavities present in a hot-rolling roll and chemical components. As a result, it was discovered that porosities and shrinkage cavities are present near eutectic carbides (which are typically M₂C-based, M₆C-based, M₇C₃-based, and M₂₃C₆-based carbides), and the formation of porosities and shrinkage cavities is related to amounts of N, O, Al, and eutectic carbides. That is, the following knowledge, which did not exist in the related art, was obtained: by adjusting the amounts of N, O, Al, and eutectic carbides of an outer layer material of a roll to fall within specific ranges, a porosity- and shrinkage cavity-free outer layer material for a hot-rolling roll can be obtained.

[0012] Now, the results of an experiment that formed a basis of the present study will be described. A ring-shaped roll material (outside diameter: 250 mmϕ, thickness: 65 mm, wall thickness: 55 mm) corresponding to an outer layer material of a roll was cast from molten metal by using a centrifugal casting method. The molten metal was prepared in a high-frequency induction furnace. The chemical composition of the molten metal contained, in mass%, C: 2.2%, Si: 0.7%, Mn: 0.6%, Cr: 7.0%, Mo: 1.0%, V: 4.0%, Nb: 1.5%, and P: 0.019%. In the chemical composition, the content of Al and the content of N+O were varied over a range of 0.01 to 0.5 mass% and a range of 100 to 600 mass ppm, respectively. The balance of the chemical composition was Fe and incidental impurities. Note that a pouring temperature was 1,500°C, and a centrifugal force in multiples of gravity for the outer circumference of the ring-shaped roll material was 150 G. After the casting, a quenching process and a tempering process were performed. The quenching process was carried out by heating the ring-shaped roll material to a heating temperature of 1,030°C and then cooling the ring-shaped roll material by air cooling. Furthermore, the tempering process was performed two or three times, depending on the components, at a temperature of 500°C so that an amount of retained austenite could be less than 10% in terms of vol.%.

[0013] After irregularities and scales (an oxide layer) on the surface of the obtained ring-shaped roll material were removed by grinding, three X-ray CT measurement test pieces (20 × 20 × 50 mm) were taken therefrom and subjected to an X-ray CT measurement to investigate the presence or absence of porosities and shrinkage cavities. As illustrated in Fig. 1, the three X-ray CT measurement test pieces 2 were taken from portions of a ring-shaped test material 1, which were spaced from one another by 120° at a width-wise center. Fig. 2 is an example of a shrinkage cavity 3 in a test piece, which was identified in the X-ray CT measurement. In the X-ray CT, 100 transmission images of the test piece were acquired every 0.5 mm in a longitudinal direction thereof, and the diameters of circles 4, which circumscribed respective porosities or shrinkage cavities identified in the transmission images, were measured; in instances in which the maximum value of the diameters of the circumscribed circles 4 of a test piece was greater than 0.50 mm, it was determined that a defect was present, and in instances in which the maximum value was less than or equal to 0.50 mm, it was determined that no defect was present.

[0014] Regarding the results obtained, a relationship between the diameter of the circle circumscribing a porosity or a shrinkage cavity and the sum (N+O) of the contents of N and O is shown in Fig. 4, and a relationship between an amount of Al and wear resistance is shown in Fig. 5.

[0015] From Fig. 4, it can be seen that when N+O is less than or equal to 400 mass ppm, the diameter of the circumscribed circle is less than or equal to 0.50 mm, which is a size that does not present a problem in terms of quality.

[0016] Porosities form when N and O present in molten metal become gas in the process in which the molten metal is solidified and cooled to room temperature, and, therefore, a size of porosities can be reduced by reducing an amount of N and an amount of O. Thus, the amount of N and the amount of O mentioned here relate to N and O dissolved in the matrix and do not relate to N and O that are present as inclusions (nitrides and oxides) in steel.

[0017] Shrinkage cavities are shrinkage cavities. By ensuring that the amount of eutectic carbides falls within an appropriate range, a size of shrinkage cavities can be reduced.

[0018] Furthermore, from Fig. 5, it can be seen that when the amount of Al is within the range of the present invention, particularly excellent wear resistance is exhibited. When coarse porosities and/or shrinkage cavities are present, surrounding portions fall off during rolling as if they were peeled off, that is, wear resistance is degraded.

[0019] Accordingly, improving wear resistance requires adjusting N+O and the amount of eutectic carbides to fall within appropriate ranges, thereby reducing the sizes of porosities and shrinkage cavities. N and O may be originally present in a raw material, and N and O may be unintentionally incorporated in molten metal during the melting of raw materials as a result of contact of molten metal with air. Accordingly, by adjusting the raw materials to be used and/or covering a surface with an inert gas (e.g., Ar) to prevent contact of the molten metal with air during melting, N+O can be adjusted. In addition, since N and O easily combine with Al to form a nitride and an oxide, the adjustment can also be made with the Al content. Furthermore, the amount of eutectic carbides can be adjusted with the contents of Mo, Cr, and C, which form eutectic carbides.

[0020] The present invention was completed based on the knowledge described above, and a summary thereof is as follows.

[1] An outer layer material for a hot-rolling roll, the outer layer material having a chemical composition that contains, in mass%, C: 1.6 to 2.5%, Si: 0.2 to 1.5%, Mn: 0.2 to 1.6%, Cr: 4.5 to 7.0%, Mo: 1.0 to 5.0%, V: 4.0 to 6.0%, and Nb: 0.5 to 2.5%, wherein a sum of contents of N and O of the chemical composition is 100 to 400 mass ppm, and a balance of the chemical composition is Fe and incidental impurities.

[2] The outer layer material for a hot-rolling roll according to [1], wherein the chemical composition further contains, in mass%, Al: 0.01 to 0.30%.

[3] The outer layer material for a hot-rolling roll according to [1] or [2], wherein the chemical composition further contains, in mass%, P: 0.010 to 0.040%

[4] The outer layer material for a hot-rolling roll according to any one of [1] to [3], wherein contents of C, V, Mo, and Nb satisfy formula (1) and formula (2) below,



where %C, %V, %Nb, and %Mo are the contents (mass%) of respective corresponding elements.

[5] A composite hot-rolling roll, the composite hot-rolling roll having a three-layer structure that includes an outer layer, an intermediate layer, and an inner layer or having a two-layer structure that includes an outer layer and an inner layer, the outer layer including the outer layer material for a hot-rolling roll according to any one of [1] to [4] .

Advantageous Effects of Invention

[0021] With the present invention, it is possible to produce a composite hot-rolling roll and an outer layer material for a hot-rolling roll in which the formation of porosities and shrinkage cavities is reduced and which have excellent wear resistance and fatigue resistance. As a result, the effects of achieving an improvement in surface quality of a material to be rolled and achieving an improvement in roll life are also produced.

Brief Description of Drawings

[0022] 

[Fig. 1] Fig. 1 is a schematic diagram illustrating a test piece (X-ray CT test piece) used in an X-ray CT measurement.

[Fig. 2] Fig. 2 is an example of a shrinkage cavity in a test piece, which is identified in a transmission image obtained in an X-ray CT measurement.

[Fig. 3] Fig. 3 is a schematic diagram illustrating a configuration of a testing apparatus used in a hot-rolling contact wear test and illustrating a test piece for a hot-rolling contact wear test (wear test piece).

[Fig. 4] Fig. 4 is a graph illustrating a relationship between a diameter of a circumscribed circle of a porosity or a shrinkage cavity and N+O.

[Fig. 5] Fig. 5 is a graph illustrating a relationship between an amount of Al and wear resistance.

[Fig. 6] Fig. 6 is a schematic diagram illustrating a configuration of a testing apparatus used in a hot-rolling contact fatigue test and illustrating a test piece for a hot-rolling contact fatigue test (fatigue test piece) and a shape and dimensions of notches introduced into an outer circumferential surface of the test piece for a hot-rolling contact fatigue test (fatigue test piece).

[Fig. 7] Fig. 7 is a graph illustrating a relationship between wear resistance and fatigue resistance according to the present invention.

Description of Embodiments

[0023] An outer layer material of a roll of the present invention is produced by a casting method, such as a known centrifugal casting method or continuous pouring process for cladding. While the outer layer material can be directly used as a ring roll or a sleeve roll, the outer layer material is used as an outer layer material for a composite hot-rolling roll that is suitable for hot finish rolling. Furthermore, a composite hot-rolling roll of the present invention is formed of an outer layer and an inner layer, which is integrally fused with the outer layer. Note that an intermediate layer may be disposed between the outer layer and the inner layer. That is, an intermediate layer integrally fused with the outer layer, and, an inner layer integrally fused with the intermediate layer may be employed in place of an inner layer integrally fused with the outer layer.

[0024] The outer layer material for a hot-rolling roll of the present invention has a chemical composition that contains, in mass%, C: 1.6 to 2.5%, Si: 0.2 to 1.5%, Mn: 0.2 to 1.6%, Cr: 4.5 to 7.0%, Mo: 1.0 to 5.0%, V: 4.0 to 6.0%, and Nb: 0.5 to 2.5%, in which the sum of the contents of N and O of the chemical composition is 100 to 400 mass ppm, and the balance of the chemical composition is Fe and incidental impurities.

[0025] First, reasons for the limitations on the chemical composition of the outer layer material for a hot-rolling roll of the present invention will be described. Note that hereinafter, unless otherwise specified, "mass% " is expressed simply as "%", and "mass ppm" is expressed simply as "ppm".

C: 1.6 to 2.5%

[0026] C has functions of increasing a hardness of a matrix by being dissolved into the matrix and of forming a hard carbide by combining with a carbide-forming element, thereby improving the wear resistance of an outer layer material of a roll. If a C content is less than 1.6%, an amount of carbides is insufficient, and, therefore, wear resistance is degraded. In addition, an amount of solidified eutectic crystals is low, and, therefore, shrinkage cavities form. On the other hand, if C is present in an amount greater than 2.5%, carbides are coarsened, and an amount of eutectic carbides is excessively increased; as a result, an outer layer material of a roll is hard and brittle, the formation and growth of fatigue cracks are promoted therein, and, therefore, fatigue resistance is degraded. Accordingly, the C content is limited to a range of 1.6 to 2.5%. Note that, preferably, the C content is greater than or equal to 1.7%. Furthermore, preferably, the C content is less than or equal to 2.4%.

Si: 0.2 to 1.5%

[0027] Si is an element that acts as a deoxidation agent and improves the castability of molten metal. Furthermore, Si has a function of strengthening a matrix by being dissolved into the matrix. Producing these effects requires the presence of Si in an amount of 0.2% or greater. If a Si content is less than 0.2%, the function of strengthening a matrix is not significantly exhibited, and, therefore, wear resistance is degraded. On the other hand, even if Si is present in an amount greater than 1.5%, the effects no longer increase, and, therefore, effects comparable to the content cannot be expected, which is economically disadvantageous; in addition, a structure of the matrix may be brittle, and, therefore, fatigue resistance may be degraded. Accordingly, the Si content is limited to 0.2 to 1.5%. Note that, preferably, the Si content is greater than or equal to 0.3%. Furthermore, preferably, the Si content is less than or equal to 1.3%.

Mn: 0.2 to 1.6%

[0028] Mn is an element that has a function of rendering S harmless by forming MnS, thereby immobilizing S, and since a portion of Mn is dissolved in a structure of a matrix, Mn has an effect of improving hardenability. Furthermore, Mn has a function of strengthening (solid-solution-strengthening) a matrix by being dissolved into the matrix. Producing these effects requires the presence of Mn in an amount of 0.2% or greater. If a Mn content is less than 0.2%, the function of strengthening a matrix is not significantly exhibited, and, therefore, wear resistance is degraded. On the other hand, even if Mn is present in an amount greater than 1.6%, the effects no longer increase, and, therefore, effects comparable to the content cannot be expected; in addition, a material may be brittle, and, therefore, fatigue resistance may be degraded. Accordingly, the Mn content is limited to 0.2 to 1.6%. Note that, preferably, the Mn content is greater than or equal to 0.3%. Furthermore, preferably, the Mn content is less than or equal to 1.4%.

Cr: 4.5 to 7.0%

[0029] Cr is an element that has functions of combining with C to primarily form a eutectic carbide, thereby improving wear resistance, and of reducing frictional force associated with a steel sheet, during rolling, thereby reducing surface damage in rolls to stabilize rolling. Producing these effects requires the presence of Cr in an amount of 4.5% or greater. If a Cr content is less than 4.5%, the amount of eutectic carbides is low, and, therefore, wear resistance is degraded. On the other hand, if Cr is present in an amount greater than 7.0%, coarse eutectic carbides are increased, and, therefore, fatigue resistance is degraded. Accordingly, when the Cr content is within a range of 4.5 to 7.0%, an outer layer material of a rolling roll that is obtained has excellent fatigue resistance. Note that, preferably, the Cr content is greater than or equal to 4.7%. Furthermore, preferably, the Cr content is less than or equal to 6.5%.

Mo: 1.0 to 5.0%

[0030] Mo is an element that combines with C to form a hard carbide, thereby improving wear resistance. Furthermore, Mo strengthens carbides by being dissolved into hard MC-type carbides in which V or Nb is bound to C. In addition, Mo is also dissolved into eutectic carbides, which results in an increase in the fracture resistance of the carbides. Via these functions, Mo improves the wear resistance and fatigue resistance of an outer layer material of a roll. Producing these effects requires the presence of Mo in an amount of 1.0% or greater. On the other hand, if Mo is present in an amount greater than 5.0%, Mo-based hard and brittle carbides form; as a result, hot-rolling contact fatigue resistance is degraded, and, therefore, fatigue resistance is degraded. Accordingly, a Mo content is limited to a range of 1.0 to 5.0%. Note that, preferably, the Mo content is greater than or equal to 1.2%. Furthermore, preferably, the Mo content is less than or equal to 4.9%.

V: 4.0 to 6.0%

[0031] V is an important element in the present invention, in terms of ensuring both a wear resistance and a fatigue resistance of a roll. V is an element that forms a very hard carbide (MC-type carbide), thereby improving wear resistance, and that effectively acts to enable eutectic carbides to be divided and dispersedly crystallized, thereby improving hot-rolling contact fatigue resistance; hence, V is an element that significantly improves the fatigue resistance of an outer layer material of a roll. These effects are prominent when V is present in an amount of 4.0% or greater. On the other hand, if V is present in an amount greater than 6.0%, MC-type carbides are coarsened, and, therefore, characteristics of a rolling roll are unstable. Accordingly, a V content is limited to a range of 4.0 to 6.0%. Note that, preferably, the V content is greater than or equal to 4.3%. Furthermore, preferably, the V content is less than or equal to 5.9%.

Nb: 0.5 to 2.5%

[0032] Nb strengthens MC-type carbides by being dissolved into the MC-type carbides and, via the function of increasing the fracture resistance of MC-type carbides, improves wear resistance and, in particular, fatigue resistance. When both Nb and Mo are dissolved in carbides, fatigue resistance, as well as wear resistance, is noticeably improved. Furthermore, Nb is an element that has a function of promoting the division of eutectic carbides, thereby inhibiting breakage of the eutectic carbides; hence, Nb is an element that improves the fatigue resistance of an outer layer material of a roll. Furthermore, Nb also has a function of inhibiting segregation of MC-type carbides that may occur during centrifugal casting. These effects are prominent when Nb is present in an amount of 0.5% or greater. On the other hand, if a Nb content is greater than 2.5%, the growth of MC-type carbides in molten metal is promoted, and, therefore, hot-rolling contact fatigue resistance is degraded. Accordingly, the Nb content is limited to a range of 0.5 to 2.5%. Note that, preferably, the Nb content is greater than or equal to 0.8%. Furthermore, preferably, the Nb content is less than or equal to 2.0%.

N+O: 100 to 400 mass ppm

[0033] N and O are unintentionally incorporated into molten metal when nitrogen and oxygen present in a raw material and nitrogen and oxygen present in air are absorbed. Accordingly, amounts of N and O in molten metal can be adjusted by reducing amounts of nitrogen and oxygen present in a raw material; blocking air during the melting of raw materials (e.g., by covering the surface of molten metal with an inert gas, such as argon gas, to block air); reducing entrainment of air that may occur during the casting of molten metal that uses, for example, a centrifugal casting method or a continuous pouring process for cladding; and/or the like. By ensuring that the sum (N+O) of the contents of N and O is less than or equal to 400 mass ppm, porosities can be reduced. However, it is economically disadvantageous to attempt to achieve a sum of the contents of N and O of less than 100 mass ppm. Furthermore, when the sum of the contents of N and O is less than 100 mass ppm, fatigue resistance may be degraded. Accordingly, N+O is limited to a range of 100 to 400 mass ppm. Note that, preferably, N+O is greater than or equal to 120 mass ppm, and more preferably, greater than or equal to 150 mass ppm. Furthermore, preferably, N+O is less than or equal to 370 mass ppm, and more preferably, less than or equal to 350 mass ppm.

Balance Fe and Incidental Impurities

[0034] In the present invention, the balance, other than the chemical composition described above, is Fe and incidental impurities. Examples of the incidental impurities include S, Ni, Cu, Ca, Sb, Ti, Zr, and B. These are impurities present in a raw material and/or impurities unintentionally incorporated from a refractory material or the like. Regarding these incidental impurities, preferably, S: 0.05% or less, Ni: 0.15% or less, Cu: 0.20% or less, Ca: 0.01% or less, Sb: 0.01% or less, Ti: 0.05% or less, Zr: 0.05% or less, and B: 0.008% or less may be present. When a total amount of these incidental impurities is less than or equal to 0.5%, neither wear resistance nor thermal fatigue resistance is adversely affected. Accordingly, it is sufficient that the total amount be less than or equal to 0.5%. Note that, more preferably, the total amount is less than or equal to 0.4%. Furthermore, as incidental impurities, Al and P may be unintentionally incorporated. The contents of these are Al: less than 0.01% and P: less than 0.010%.

[0035]  Furthermore, in the present invention, in addition to the chemical composition described above, Al: 0.01 to 0.30% and/or P: 0.010 to 0.040% may be present.

Al: 0.01 to 0.30%

[0036] Al is an element that forms an oxide and a nitride by combining with nitrogen and oxygen in molten metal and is, therefore, an element that inhibits the formation of porosities and shrinkage cavities. In terms of producing the effect, it is preferable to ensure the presence of Al in an amount of 0.01% or greater. On the other hand, if Al is present in an amount greater than 0.30%, a large amount of oxides or nitrides may form, and, therefore, hot-rolling contact fatigue resistance may be degraded. Accordingly, in an instance in which Al is to be present, a preferred range of an Al content is 0.01 to 0.30%. Note that, more preferably, the Al content is greater than or equal to 0.02%. Furthermore, more preferably, the Al content is less than or equal to 0.25%.

P: 0.010 to 0.040%

[0037] It has been believed that P, which is unintentionally incorporated from a raw material or the like in the process of production, causes degradation of mechanical properties. However, the inventors diligently performed studies and uncovered that the presence of a small amount of P produces an effect of improving hardness and tensile/compressive strength. It is believed that the function of P of increasing strength (increasing hardness) is solid solution strengthening resulting from the dissolution of P in a structure of the matrix. When a P content is 0.010 to 0.040%, an effect of improving wear resistance by increasing the strength of the structure of the matrix is produced. However, when P is present in an amount greater than 0.040%, mechanical properties may be degraded. Accordingly, in an instance in which P is to be present, it is preferable that the P content be within a range of 0.010 to 0.040%. Note that, more preferably, the P content is greater than or equal to 0.012%. Furthermore, preferably, the P content is less than or equal to 0.035%.

[0038] Furthermore, in the present invention, it is preferable that the contents of C, V, Nb, and Mo satisfy formula (1) and formula (2) below.

[0039] In the formulae, %C, %V, %Nb, and %Mo are the contents (mass%) of the respective corresponding elements. In instances in which the contents of V, Nb, and Mo are within the range of formula (1), Mo is dissolved in MC-type carbides, so that solid solution strengthening is achieved, and, therefore, wear resistance is improved. Furthermore, when the contents of V, Nb, and C are within the range of formula (2), segregation of carbides is inhibited, and, therefore, wear resistance and fatigue resistance are improved. It is believed that a reason for the improvement in wear resistance and fatigue resistance is that in instances in which the contents of V, Nb, and C satisfy the range of formula (2), a process for the formation of a structure in the solidification of molten metal changes.

[0040] A preferred method for producing the composite hot-rolling roll of the present invention will now be described.

[0041] In the present invention, it is preferable that a method for producing the outer layer material of a roll be a method that uses a known casting method, such as a centrifugal casting method or a continuous pouring process for cladding. Note that, as will be appreciated, the present invention is not limited to these methods.

[0042] In an instance in which the outer layer material of a roll is to be cast by using a centrifugal casting method, the centrifugal casting is to be performed by, first, pouring molten metal having a chemical composition corresponding to the above-described chemical composition of the outer layer material of a roll into a rotating mold in a manner such that a predetermined wall thickness is achieved; the mold has, on an inner surface thereof, a coating of a refractory material primarily made of zircon or the like, and a thickness of the coating is 1 to 5 mm. In this instance, it is preferable that a rotational speed of the mold be such that the multiples of gravity applied to an outer surface of a roll be within a range of 100 to 200 G. Furthermore, in an instance in which an intermediate layer is to be formed, it is preferable that the centrifugal casting be performed by, during the solidification of the outer layer material of a roll or after complete solidification thereof, pouring molten metal having a chemical composition corresponding to a chemical composition of the intermediate layer, while the mold is rotated. For the formation of a composite roll, it is preferable that after complete solidification of the outer layer or the intermediate layer, the rotation of the mold be stopped, the mold be raised, and thereafter, static casting be performed to form an inner layer material. As a result, the inner surface side of the outer layer material of a roll is remelted, and, accordingly, a composite roll is formed in which the outer layer and the inner layer, or, the outer layer and the intermediate layer plus the intermediate layer and the inner layer are integrally fused together.

[0043] In the present invention, the chemical compositions of the inner layer and the intermediate layer are not particularly limited. It is preferable that the inner layer, which is formed by static casting, be formed from spheroidal graphite cast iron (ductile cast iron) or compacted vermicular graphite cast iron (CV cast iron), which has excellent castability and mechanical properties, or, from forged steel or the like. In a roll produced by centrifugal casting, since the outer layer and the inner layer are integrally fused, the inner layer includes a component of the outer layer material unintentionally incorporated therein in an amount of approximately 1 to 8 %. In instances in which a carbide-forming element present in the outer layer material, such as Cr or V, is unintentionally incorporated into the inner layer, the inner layer becomes brittle. Accordingly, it is preferable that a ratio of unintentional incorporation of a component of the outer layer into the inner layer be limited to less than 6%.

[0044] Furthermore, in instances in which an intermediate layer is to be formed, it is preferable that a material for the intermediate layer be graphitized steel, high-carbon steel (C: 1.5 to 3.0 mass%), hypoeutectic cast iron, or the like. Similarly, since the intermediate layer and the outer layer are integrally fused, the intermediate layer includes a component of the outer layer unintentionally incorporated therein in an amount ranging from 10 to 95%. From the standpoint of limiting an amount of unintentional incorporation of a component of the outer layer into the inner layer, it is important to reduce the amount of unintentional incorporation of a component of the outer layer into the intermediate layer as much as possible.

[0045] It is preferable that the composite hot-rolling roll of the present invention be subjected to a heat treatment after being cast. In the heat treatment, it is preferable that a step of performing heating at 950 to 1,100°C and then performing air cooling or air blast cooling be carried out, and further, a step of performing heating and holding at 480 to 570°C and subsequently performing cooling be carried out two or more times.

[0046] Note that, a preferred hardness of the composite hot-rolling roll of the present invention is 79 to 88 HS (Shore hardness), and a more preferred hardness thereof is 80 to 86 HS. If the hardness is less than 79 HS, wear resistance is degraded, and on the other hand, if the hardness is greater than 88 HS, cracks that formed in the surface of the hot-rolling roll during hot rolling cannot be easily removed by grinding. A hardness as described above can be achieved by adjusting the heat treatment temperatures mentioned above.

EXAMPLES

[0047] Molten metal having a chemical composition corresponding to the chemical composition of an outer layer material of a roll shown in Table 1 was prepared in a high-frequency induction furnace, and then, a ring-shaped test material (ring roll, outside diameter: 250 mmϕ, thickness: 65 mm, wall thickness: 55 mm) was formed by using a centrifugal casting method. Note that a pouring temperature was 1,500°C, and a centrifugal force in multiples of gravity for the outer circumference of the ring-shaped roll material was 150 G. After the casting, a quenching process was performed in which the ring-shaped test material was heated to 1,030°C and cooled by air cooling. Subsequently, a tempering process was performed two or three times, depending on the components, by using a temperature of 500°C so that an amount of retained austenite could be less than 10% in terms of vol.%. For cooling from the tempering temperature, furnace cooling was used. A wear test piece and an X-ray CT measurement test piece were taken from the obtained ring-shaped test material, and a wear test and an X-ray CT measurement were performed.

[Table 1]

No.	Chemical composition (mass% (mass ppm for N+O))										Satisfaction of formula (1)	Satisfaction of formula (2)	Notes
	C	Si	Mn	V	Cr	Mo	Nb	N+O (ppm)	Al	P
1	1.9	0.4	0.3	4.5	5.7	2.2	1.0	155	-	-	Yes	No	Invention example
2	1.6	0.3	0.5	5.6	6.5	5.0	0.5	237	-	-	No	Yes	Invention example
3	2.5	0.2	1.6	5.9	4.5	3.1	1.8	103	-	0.025	Yes	No	Invention example
4	2.4	1.5	0.2	6.0	6.0	5.0	1.0	397	-	-	No	No	Invention example
5	2.2	0.7	0.6	4.0	7.0	1.0	1.5	351	-	-	No	Yes	Invention example
6	1.8	0.5	0.3	5.1	6.3	3.8	0.9	286	-	-	No	Yes	Invention example
7	2.3	0.4	0.4	5.7	4.6	3.0	1.7	211	0.010	-	Yes	No	Invention example
8	2.1	0.3	0.6	4.9	5.1	3.6	0.6	184	0.100	-	No	Yes	Invention example
9	2.4	0.5	0.4	5.2	5.8	2.9	2.0	134	0.200	-	Yes	No	Invention example
10	1.7	0.8	0.3	5.7	6.9	4.5	1.4	108	0.300	-	No	Yes	Invention example
11	2.2	0.7	0.7	4.9	6.0	2.9	1.5	128	0.200	-	Yes	Yes	Invention example
12	2.4	1.0	1.0	5.0	4.5	3.1	1.9	116	0.250	0.014	Yes	Yes	Invention example
13	2.0	0.9	1.2	5.0	6.0	2.5	1.0	165	0.100	-	Yes	Yes	Invention example
14	3.0	0.4	0.3	5.9	6.4	2.8	2.4	164	-	-	Yes	No	Comparative example
15	1.5	0.8	0.7	4.6	5.2	3.6	1.9	397	-	-	Yes	No	Comparative example
16	2.5	1.6	1.0	4.7	4.6	1.7	0.6	364	-	-	Yes	Yes	Comparative example
17	2.1	0.1	0.4	5.1	6.8	1.6	1.1	265	-	-	No	Yes	Comparative example
18	2.4	0.4	1.7	6.0	6.1	2.3	0.9	292	-	-	Yes	No	Comparative example
19	2.3	0.2	0.1	5.5	5.9	2.7	1.6	246	-	-	Yes	No	Comparative example
20	2.5	0.3	0.5	7.0	6.2	4.1	2.2	199	-	-	Yes	No	Comparative example
21	2.2	0.5	0.4	3.5	5.1	4.5	0.7	203	-	-	No	No	Comparative example
22	2.0	0.5	0.5	5.2	7.5	3.6	1.8	225	-	-	Yes	Yes	Comparative example
23	2.3	0.2	0.3	5.0	4.0	3.2	2.5	219	-	-	Yes	No	Comparative example
24	2.5	0.6	0.6	5.3	6.7	6.0	2.0	256	-	-	No	No	Comparative example
25	2.1	0.3	1.0	4.3	5.3	0.9	1.6	244	-	-	No	Yes	Comparative example
26	1.9	0.5	0.7	5.8	5.9	5.0	3.5	188	-	-	Yes	No	Comparative example
27	1.8	0.7	0.5	5.0	5.0	4.7	0.4	208	-	-	No	No	Comparative example
28	2.0	0.6	0.6	4.9	5.8	4.2	1.2	572	-	-	No	Yes	Comparative example
29	2.2	1.0	1.1	5.7	6.3	3.9	0.7	453	-	-	Yes	Yes	Comparative example
30	1.7	0.9	0.4	4.7	6.6	3.9	1.7	89	-	-	Yes	Yes	Comparative example
31	2.1	0.7	1.1	5.7	6.5	1.3	1.0	101	0.350	0.220	No	Yes	Comparative example
32	2.3	1.3	1.3	6.0	7.0	4.4	0.6	553	0.010	0.040	No	No	Comparative example
33	1.9	0.6	0.8	5.2	5.3	3.6	2.3	237	-	0.042	Yes	Yes	Comparative example
34	1.6	1.2	0.4	4.8	5.5	3.9	1.9	218	-	-	Yes	Yes	Invention example
35	2.0	0.5	0.5	7.0	6.0	5.0	0.4	430	-	-	No	No	Related-art example

•The balance, other than the above components, is Fe and incidental impurities. •The underline indicates the value is outside the range of the present invention. •The symbol "-" in the table indicates the element was not added.

[0048]  After irregularities and scales (an oxide layer) were removed by grinding from the surface of the obtained ring-shaped roll material, three X-ray CT measurement test pieces (20 × 20 × 50 mm) were taken therefrom and subjected to an X-ray CT measurement to investigate the presence or absence of porosities and shrinkage cavities. As illustrated in Fig. 1, three pieces of the X-ray CT measurement test piece 2, were taken from portions of a ring-shaped test material 1, which were spaced from one another about a width-wise center by 120°. The X-ray CT instrument which has a maximum tube voltage of 225 kV was used, and transmission images of the entire test piece were acquired at a tube voltage of 150 kV and a tube current of 80 µA. In instances in which a diameter of a circle circumscribing a detected individual porosity or shrinkage cavity was greater than 0.50 mm, it was determined that a defect was present, and in instances in which the diameter was less than or equal to 0.50 mm, it was determined that no defect was present.

[0049] The method for the wear test was as follows. A wear test piece (outside diameter: 60 mmϕ, thickness: 10 mm, chamfered) was taken from the obtained ring-shaped test material. The wear test was conducted by using a two-disc slip rolling method, in which the test piece and an opposing piece were used, as illustrated in Fig. 3. The test piece 5 was rotated at 700 rpm while being water-cooled with cooling water 6. In a state in which the opposing piece 8 (material: S45C, outside diameter: 190 mmϕ, thickness: 15 mm, chamfer: C1), which had been heated to 800°C by a high-frequency induction heating coil 7, was in contact with the rotating test piece 5 at a load of 980 N, the opposing piece 8 was rolled at a slip ratio of 9%. The wear test was conducted for 300 minutes, in which the opposing piece was replaced with a new one every 50 minutes. Thus, the test was conducted. A related-art example was used as a reference. A ratio of an amount of wear of the test piece with respect to the value of the reference was evaluated as follows: wear ratio (= (amount of wear of the reference piece)/(amount of wear of the test piece)). In instances in which the wear ratio was 0.97 or greater, it was determined that the test piece had a wear resistance comparable to or higher than those of the related art, and in instances in which the wear ratio was less than 0.97, it was determined that the test piece had a low wear resistance.

[0050] Furthermore, a hot-rolling fatigue test piece (outside diameter: 60 mmϕ, thickness: 10 mm) was taken from the obtained ring-shaped roll material, and a hot-rolling fatigue test, which, according to the disclosure of Japanese Unexamined Patent Application Publication No. 2010-101752, can reproducibly evaluate the fatigue resistance of an actual work roll for hot rolling, was conducted. Note that a notch (depth t: 1.2 mm, circumferential length L: 0.8 mm) as illustrated in Fig. 6 was introduced at two locations of the outer circumferential surface of the fatigue test piece, by using an electro-discharge machining (wire-cutting) method that uses a 0.2-mmϕ wire. Furthermore, the edges of the rolling contact surface of the fatigue test piece were chamfered to have a chamfer 1.2 C. As illustrated in Fig. 6, the hot-rolling fatigue test was conducted by using a two-disc slip rolling method, in which the test piece 5 (hot-rolling fatigue test piece 5) having notches and the opposing piece 8, which had been heated, were used. That is, as illustrated in Fig. 6, the test piece 5 (hot-rolling fatigue test piece 5) was rotated at 700 rpm while being water-cooled with cooling water 6, and then, in a state in which the opposing piece 8 (material: S45C, outside diameter: 190 mmϕ, thickness: 15 mm), which had been heated to 800°C by a high-frequency induction heating coil 7, was pressed against the rotating test piece 5 at a load of 980 N, the opposing piece 8 was rolled at a slip ratio of 9%. The rolling was continued until the two notches 9 introduced into the hot-rolling fatigue test piece 5 were broken. The number of rolling cycles until breakage occurred was determined for each of the notches, and the average value thereof was designated as a hot-rolling fatigue life. In instances in which the hot-rolling fatigue life was greater than three hundred and fifty thousand cycles, an evaluation that significantly excellent fatigue life was achieved was made.

[0051] The results obtained are shown in Table 2.

[Table 2]

No.	Diameter of circumscribed circle (mm)	Presence or absence of defect	Wear resistance	Evaluation	Fatigue resistance (thousand cycles)	Evaluation	Overall evaluation	Notes
1	0.13	Absent	1.00	○	386	○	○	Invention example
2	0.28	Absent	0.97	○	374	○	○	Invention example
3	0.10	Absent	1.10	○	385	○	○	Invention example
4	0.50	Absent	1.08	○	360	○	○	Invention example
5	0.45	Absent	1.02	○	367	○	○	Invention example
6	0.32	Absent	1.00	○	371	○	○	Invention example
7	0.23	Absent	1.11	○	382	○	○	Invention example
8	0.20	Absent	1.14	○	393	○	○	Invention example
9	0.17	Absent	1.31	○	401	○	○	Invention example
10	0.11	Absent	1.26	○	409	○	○	Invention example
11	0.16	Absent	1.37	○	417	⊙	⊙	Invention example
12	0.12	Absent	1.38	○	436	⊙	⊙	Invention example
13	0.19	Absent	1.33	○	411	⊙	⊙	Invention example
14	0.20	Absent	1.19	○	298	×	×	Comparative example
15	0.54	Present	0.95	×	367	○	×	Comparative example
16	0.50	Absent	1.08	○	314	×	×	Comparative example
17	0.31	Absent	0.96	×	377	○	×	Comparative example
18	0.39	Absent	1.05	○	332	×	×	Comparative example
19	0.31	Absent	0.95	×	383	○	×	Comparative example
20	0.25	Absent	1.26	○	305	×	×	Comparative example
21	0.26	Absent	0.93	×	355	○	×	Comparative example
22	0.27	Absent	1.16	○	296	×	×	Comparative example
23	0.28	Absent	0.91	×	354	○	×	Comparative example
24	0.31	Absent	1.14	○	263	×	×	Comparative example
25	0.33	Absent	0.92	×	362	○	×	Comparative example
26	0.22	Absent	1.11	○	320	×	×	Comparative example
27	0.25	Absent	0.95	×	368	○	×	Comparative example
28	0.90	Present	0.90	×	329	×	×	Comparative example
29	0.76	Present	0.92	×	356	○	×	Comparative example
30	0.09	Absent	1.26	○	252	×	×	Comparative example
31	0.12	Absent	1.08	○	273	×	×	Comparative example
32	0.86	Present	0.96	×	359	○	×	Comparative example
33	0.29	Absent	1.24	○	310	×	×	Comparative example
34	0.28	Absent	1.02	○	352	○	○	Invention example
35	0.61	Present	1 (Reference)	○	350	×	×	Related-art example

[0052] For the wear resistance, in comparison with the related art, test pieces having a wear ratio of 0.97 or greater were rated as "○" (pass), and test pieces having a wear ratio of less than 0.97 were rated as "×" (fail). For the fatigue resistance, test pieces having greater than four hundred and ten thousand cycles (410,000 cycles) were rated as "⊙" (pass, particularly excellent), test pieces having greater than three hundred and fifty thousand cycles and four hundred and ten thousand or fewer cycles (350,001 to 410,000 cycles) were rated as "○" (pass), and test pieces having fewer than or equal to three hundred and fifty thousand cycles (350,000 cycles) were rated as "×" (fail). Based on these, an overall evaluation was made.

[0053] In the overall evaluation, test pieces that were free of the defects of porosities and shrinkage cavities, had a rating of "○" (pass) for wear resistance, and had a rating of "○" (pass) for fatigue resistance were rated as "○" (pass) .

[0054] Furthermore, test pieces that were free of the defects of porosities and shrinkage cavities, had a rating of "○" (pass) for wear resistance, and had a rating of "⊙" (pass, particularly excellent) for fatigue resistance were rated as "⊙" (pass, particularly excellent).

[0055] Furthermore, test pieces that had one or more of the following evaluations were rated as "×" (fail) for the overall evaluation: the defect of a porosity or a shrinkage cavity was "present"; the rating of "×" (fail) was given for wear resistance; and the rating of "×" (fail) was given for fatigue resistance.

[0056] It is apparent that in Invention Examples, wear resistance comparable to or higher than that of the related-art example was achieved, and porosities and shrinkage cavities were significantly reduced. In particular, in Examples (Nos. 7 to 13) that had an Al content within a suitable range, the wear ratio was high, which indicates excellent wear resistance was achieved. A reason for this is as follows. If a porosity and/or a shrinkage cavity are present, surrounding portions fall off during a wear test as if they were peeled off, which results in a significant reduction in the weight of the test piece, but when the Al content is within a preferred range, sizes of porosities and shrinkage cavities are significantly reduced, which reduces variations in the mass of the test piece during a wear test.

[0057] Furthermore, as shown in Fig. 7, it is apparent that in instances (Nos. 11 to 13) in which C, V, Nb, and Mo satisfied formula (1) and formula (2), excellent wear resistance and fatigue resistance were achieved compared with the related-art example and the invention examples that did not satisfy formula (1) and formula (2), while the formation of porosities and shrinkage cavities was inhibited.

[0058] Accordingly, with the present invention, it is possible to produce a composite hot-rolling roll and an outer layer material for a hot-rolling roll in which the formation of porosities and shrinkage cavities is reduced and which have excellent wear resistance and fatigue resistance. As a result, the effects of achieving an improvement in surface quality of a material to be rolled and achieving an improvement in roll life are also produced.

Reference Signs List

[0059] 

1 Ring-shaped test material

2 Test piece (X-ray CT measurement test piece)

3 Porosity or shrinkage cavity

4 Circumscribed circle

5 Test piece (wear test piece, hot-rolling fatigue test piece)

6 Cooling water

7 High-frequency induction heating coil

8 Opposing piece

9 Notch

Claims

1. An outer layer material for a hot-rolling roll, the outer layer material having a chemical composition that contains, in mass%,

C: 1.6 to 2.5%,

Si: 0.2 to 1.5%,

Mn: 0.2 to 1.6%,

Cr: 4.5 to 7.0%,

Mo: 1.0 to 5.0%,

V: 4.0 to 6.0%, and

Nb: 0.5 to 2.5%, wherein

a sum of contents of N and O of the chemical composition is 100 to 400 mass ppm, and a balance of the chemical composition is Fe and incidental impurities.

2. The outer layer material for a hot-rolling roll according to Claim 1, wherein the chemical composition further contains, in mass%, Al: 0.01 to 0.30%.

3. The outer layer material for a hot-rolling roll according to Claim 1 or 2, wherein the chemical composition further contains, in mass%, P: 0.010 to 0.040%.

4. The outer layer material for a hot-rolling roll according to any one of Claims 1 to 3, wherein contents of C, V, Mo, and Nb satisfy formula (1) and formula (2) below,



where %C, %V, %Nb, and %Mo are the contents (mass%) of respective corresponding elements.

5. A composite hot-rolling roll, the composite hot-rolling roll having a three-layer structure that includes an outer layer, an intermediate layer, and an inner layer or having a two-layer structure that includes an outer layer and an inner layer,
the outer layer comprising the outer layer material for a hot-rolling roll according to any one of Claims 1 to 4.

Drawing