Outer layer material for hot rolling roll and hot rolling composite roll

Abstract

To provide an outer layer material of a hot-rolling roll excellent in seizure resistance and slip resistance more than ever and a composite roll for hot rolling.SOLUTION: An outer layer material for a hot-rolling roll comprises: in mass%, C: 1.2 to 2.5%, Si: 0.15 to 2.50%, Mn: 0.15 to 2.50%, Ni: 0.2 to 8.0%, Cr: 1.5 to 10.0%, Mo: 3.5 to 12.0%, V: 2.0 to 7.5%, W: 0.1 to 6.0%, P: 0.01 to 0.04%, S: 0.001 to 0.010%, the balance consisting of Fe and unavoidable impurities, and has a composition in which the contents of C, Cr, Mo, V, and W satisfy a specific formula, carbides with a grain size of 1 μm or more are present in an area ratio of 8.0 to 20.0%, and the Shore hardness at 20°C is 75.0 HS or more and 85.0 HS or less, and the Shore hardness at 600°C is 45.0 HS or more and 50.0 HS or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a roll outer layer material for hot rolling and a composite roll for hot rolling, and particularly to a roll outer layer material for hot rolling and a composite roll for hot rolling that are suitable for application to the latter stands of rough rolling of steel plates.

Background Art

[0002] In recent years, the demand for high-quality steel plates has been increasing, and accordingly, there is a demand for improving the hot rolling technology of steel plates. Therefore, there is a strong demand for improving the characteristics of hot rolling rolls used in hot rolling facilities, specifically, improving wear resistance, seizure resistance, slip resistance, etc. To improve wear resistance, HiCr cast steel rolls into which Cr-based M 7 C 3 carbides are introduced, and high-speed steel, which is a kind of tool steel, contains carbide-forming elements such as V, Cr, Mo, and W, and contains V-based MC carbides, Mo, W-based M 2 C carbides, Cr-based M 7 C 3 high rolls into which a large amount of hard carbides such as carbides (M represents a metal element forming the carbide) are introduced are used. However, when a large amount of carbides are introduced to improve wear resistance, the hardness is high and the wear resistance is good, so the surface roughness of the roll becomes small, and the friction coefficient during rolling becomes small, so slip is likely to occur. On the other hand, when the friction coefficient during rolling becomes large, seizure, which is a phenomenon in which a part of the rolled material adheres to the roll material, occurs, and there is a problem that the quality of the product rolled after seizure occurs deteriorates.

[0003] Various techniques have been disclosed to solve such problems. For example, Patent Document 1 proposes a composite roll for hot rolling, characterized in that the chemical composition of the outer layer contains, by mass ratio, C: 1.0 - 3.0%, Si: 0.2 - 2.0%, Mn: 0.2 - 2.0%, V: 3.0 - 10.0%, Cr: 3.0 - 10.0%, and one or two of Mo and W: 2.0 - 10.0%, or further contains one or two of Ni: 0.2 - 5.0% or Co: 0.2 - 10.0%, with the balance being Fe and inevitable impurities. By crystallizing a large amount of hard, fine, and granular MC-type carbides, it is claimed to be a composite roll for hot rolling that can ensure a highly stable friction coefficient with the rolled steel material.

[0004] Also, Patent Document 2 proposes a composite roll for hot rolling, which contains, by mass%, C: 0.8 - 4.0%, Si: 0.2 - 2.0%, Mn: 0.2 - 2.0%, Cr: 3.0 - 15%, V: 3.0 - 15%, one or two of Mo and W: ≥2%, and Mo + 0.5W: ≥6.1%, or further contains one or two or more of Ni: 0.2 - 5%, Co: 0.5 - 10%, Nb: 0.50 - 5.0%, and one or more of Al, Ti, Zr: ≤0.5%. The metal structure has carbides with an area ratio of 5 - 30%, and the outer layer material of the composite roll for hot rolling is provided with an outer layer material in which the average gap between adjacent carbides in the distribution of each carbide is 20 μm or less. By finely dispersing an appropriate amount of granular carbides and reducing the adjacent gaps between each carbide, it is claimed to be a composite roll for hot rolling with improved slip resistance and seizure resistance.

[0005] Patent Document 3 discloses a molten metal composition containing, by mass%, C: 0.90 to 1.40%, Si: 0.50 to 1.50%, Mn: 0.50 to 1.50%, Ni: 0.5 to 2.0%, Cr: 9.0 to 16.0%, Mo: 1.00 to 3.00%, Al: 0.010 to 0.030%, and further containing at least one of V: 0.05 to 0.50%, Ti: 0.05 to 0.50%, Nb: 0.02 to 0.20%, and satisfying the following formulas: (1) 8 ≤ Cr / C ≤ 14, (2) 3.0 ≤ 12.3C + 0.55Cr - 15.2 ≤ 7.0, and (3) 26.0 ≤ 15.5C + Cr. The outer shell layer is made of cast steel with the balance being Fe and inevitable impurities, and an axial core made of ductile cast iron cast inside the outer shell layer is integrated via an intermediate layer. A work roll for a hot rolling rough rolling stand has been proposed. By appropriately adding the amounts of C and Cr and controlling the amount of 7 C 3 carbides, it is claimed that a work roll for a hot rolling rough rolling stand can achieve both wear resistance and slip resistance.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, as the demand for high-quality steel plates increases and the hot rolling technology of steel plates improves, the characteristics required for hot rolling rolls are becoming increasingly stringent, and in particular, higher wear resistance is strongly demanded. Therefore, when designing a roll to meet the wear resistance requirement, it becomes easy to slip, and conversely, when increasing the friction coefficient to prevent slipping, it becomes easy to seize, increasing the frequency of roll troubles. In the conventional hot rolling rolls described in Patent Documents 1 to 3, the seizure resistance and slip resistance are not sufficient.

[0008] Therefore, an object of the present invention is to provide an outer layer material for a hot rolling roll and a composite hot rolling roll that solve the above problems and are excellent in seizure resistance and slip resistance.

Means for Solving the Problems

[0009] The present inventor investigated in detail the relationship between the base structure, carbide, hardness, friction coefficient, and chemical composition of a hot rolling roll. As a result, it was found that the seizure resistance and slip resistance are improved by optimizing the chemical composition so that the area ratio and high-temperature hardness of the carbide are within a specific range. The present invention was completed through further consideration based on these findings. That is, the gist of the present invention is as follows. [1] By mass%, C: 1.2 to 2.5%, Si: 0.15 to 2.50%, Mn: 0.15 to 2.50%, Ni: 0.2 to 8.0%, Cr: 1.5 to 10.0%, Mo: 3.5 to 12.0%, V: 2.0 to 7.5%, W: 0.1 to 6.0%, P: 0.01 to 0.04%, S: 0.001 to 0.010% are contained, It consists of the remaining Fe and inevitable impurities, has a composition in which the contents of C, Cr, Mo, V, and W satisfy the following formulas (1) and (2), carbides with a particle size of 1 μm or more are present at an area ratio of 8.0 to 20.0%, the Shore hardness at 20 °C is 75.0 HS or more and 85.0 HS or less, and the Shore hardness at 600 °C is 45.0 HS or more and 50.0 HS or less. An outer layer material for hot rolling rolls. 20.0 ≤ [%C] × ([%V] + [%Cr] + [%Mo] + [%W]) ≤ 35.0 (1) 1.00 ≤ [%C] × ((0.177 × [%V]) / (0.099 × [%Cr] + 0.063 × [%Mo] + 0.033 × [%W])) ≤ 3.00 (2) Here, in formulas (1) and (2), [%C], [%Cr], [%Mo], [%V], and [%W] represent the contents (mass %) of the respective elements. [2] A composite roll for hot rolling having two layers of an outer layer and an inner layer, or three layers of an outer layer, an intermediate layer, and an inner layer, wherein the outer layer is in mass %, C: 1.2 to 2.5%, Si: 0.15 to 2.50%, Mn: 0.15 to 2.50%, Ni: 0.2 to 8.0%, Cr: 1.5 to 10.0%, Mo: 3.5 to 12.0%, V: 2.0 to 7.5%, W: 0.1 to 6.0%, P: 0.01 to 0.04%, S: 0.001 to 0.010% is contained, It consists of the remaining Fe and inevitable impurities, has a composition in which the contents of C, Cr, Mo, V, and W satisfy the following formulas (1) and (2), carbides with a particle size of 1 μm or more are present at an area ratio of 8.0 to 20.0%, the Shore hardness at 20 °C is 75.0 HS or more and 85.0 HS or less, and the Shore hardness at 600 °C is 45.0 HS or more and 50.0 HS or less. A composite roll for hot rolling. 20.0 ≤ [%C] × ([%V] + [%Cr] + [%Mo] + [%W]) ≤ 35.0 (1) 1.00 ≤ [%C] × ((0.177 × [%V]) / (0.099 × [%Cr] + 0.063 × [%Mo] + 0.033 × [%W])) ≤ 3.00 (2) Here, in formulas (1) and (2), [%C], [%Cr], [%Mo], [%V], and [%W] represent the content (mass %) of each element.

Advantages of the Invention

[0010] According to the present invention, it is possible to provide an outer layer material for a hot rolling roll and a composite roll for hot rolling, which are excellent in seizure resistance and slip resistance. As a result, the time loss due to roll trouble and the resulting rolling interruption is reduced, improving the rolling efficiency of the hot rolling roll, and accordingly, the productivity of the hot rolled steel sheet is also improved.

Brief Description of the Drawings

[0011]

Figure 1

Modes for Carrying Out the Invention

[0012] The reason for limiting the composition of the outer layer material of the hot rolling roll of the present invention will be described. Hereinafter, mass % will be simply denoted as % unless otherwise specified.

[0013] C: 1.2 to 2.5% C combines with V, Cr, Mo, W, etc. to form hard carbides, contributing to the improvement of wear resistance. Also, it dissolves in the matrix to increase the hardness. If C is less than 1.2%, the amount of carbides is insufficient, and excellent wear resistance cannot be obtained. Also, since the hardness is low, plastic flow occurs, increasing the friction coefficient and making seizure likely. On the other hand, if C exceeds 2.5%, excessive carbides are generated, reducing the skin pass resistance and crack resistance. Also, when the hardness increases, the surface roughness of the roll decreases, and slip occurs due to the decrease in the friction coefficient. Therefore, C is limited to 1.2% or more and 2.5% or less. The content of C is preferably 1.5% or more and 2.2% or less.

[0014] Si: 0.15 - 2.50% Si acts as a deoxidizer in the molten metal, improves the fluidity of the molten metal, and has the effect of preventing casting defects. If Si is less than 0.15%, the deoxidation effect is insufficient. On the other hand, even if Si exceeds 2.50%, the effect saturates. Therefore, Si is limited to 0.15% or more and 2.50% or less. Incidentally, the content of Si is preferably 0.30% or more and 1.50% or less.

[0015] Mn: 0.15 - 2.50% Mn has the effect of deoxidizing the molten metal and fixing S that has an adverse effect as MnS. If Mn is less than 0.15%, the addition effect is insufficient. On the other hand, even if Mn exceeds 2.50%, the effect saturates. Therefore, Mn is limited to 0.15% or more and 2.50% or less. Incidentally, the content of Mn is preferably 0.30% or more and 1.50% or less.

[0016] Ni: 0.2 - 8.0% Ni has the effect of improving the hardenability of the matrix and improving the hardness of the matrix. If Ni is less than 0.2%, the effect hardly appears. On the other hand, if Ni exceeds 8.0%, austenite tends to remain and the hardness decreases. Therefore, Ni is limited to 0.2% or more and 8.0% or less. Incidentally, the content of Ni is preferably 1.0% or more and 4.0% or less.

[0017] Cr: 1.5 - 10.0% Cr is a carbide - forming element and combines with C to form M 7 C 3 carbide. Since it is a hard carbide, it has the effect of improving wear resistance. If Cr is less than 1.5%, the amount of M 7 C 3 carbide is insufficient and the wear resistance decreases. On the other hand, if Cr exceeds 10.0%, coarse M 7 C 3 carbide is generated and, conversely, the wear resistance deteriorates. Therefore, Cr is limited to 1.5% or more and 10.0% or less. Incidentally, the content of Cr is preferably 3.0% or more and 7.0% or less.

[0018] Mo: 3.5 - 12.0% Mo is a carbide - forming element and combines with C to form M 2 C carbides. Since they are hard carbides, they have the effect of improving wear resistance. If Mo is less than 3.5%, these effects are insufficient. On the other hand, when Mo exceeds 12.0%, coarse M 2 C carbides are generated and the toughness decreases. Therefore, Mo is limited to 3.5% or more and 12.0% or less. The Mo content is preferably 5.5% or more and 9.5% or less.

[0019] V: 2.0 - 7.5% V is a carbide - forming element and combines with C to form MC carbides. The MC carbides have a Vickers hardness Hv value of 2800 and are one of the hardest carbides. If V is less than 2.0%, the crystallization and precipitation amount of MC carbides is insufficient and the wear resistance deteriorates. On the other hand, when V exceeds 7.5%, VC carbides, which are lighter in specific gravity than the molten iron, are concentrated inside the inner layer due to the centrifugal force during centrifugal casting, resulting in segregation. Therefore, V is limited to 2.0% or more and 7.5% or less. The V content is preferably 3.0% or more and 6.5% or less.

[0020] W: 0.1 - 6.0% W is a carbide - forming element and combines with C to form hard M 2 C and other hard carbides are generated, increasing the hardness of the outer layer and having the effect of improving wear resistance. If W is less than 0.1%, the effect is insufficient and the wear resistance deteriorates. On the other hand, when W exceeds 6.0%, coarse M 2 C carbides are generated and the wear resistance deteriorates instead. Therefore, W is limited to 0.1% or more and 6.0% or less. The W content is preferably 1.0% or more and 4.0% or less.

[0021] P: 0.01 - 0.04% P has been considered to be mixed in during the manufacturing process and to reduce mechanical properties. However, as a result of the inventors' intensive studies, it has been clarified that the inclusion of a small amount of P has the effect of improving hardness and wear resistance. If the amount of P is less than 0.01%, the effect is not sufficient. On the other hand, if the amount of P exceeds 0.04%, the mechanical properties deteriorate. Therefore, the amount of P is limited to 0.01% or more and 0.04% or less. Incidentally, the P content is preferably 0.02% or more and 0.03% or less.

[0022] S: 0.001 - 0.010% S is usually treated as a harmful element in iron-based alloys and is restricted to a content of a certain amount or less. Within that range, MnS has the effect of a lubricant. On the other hand, if the content is high, the material becomes brittle. Therefore, the amount of S is limited to 0.001% or more and 0.010% or less. Incidentally, the S content is preferably 0.002% or more and 0.006% or less.

[0023] Inevitable impurities The remainder other than the components described above consists of Fe and inevitable impurities.

[0024] Further, in the present invention, it is characterized in that the contents of C, Cr, Mo, V, and W are within the above ranges and, in addition, the following formula (1) and the following formula (2) are satisfied. 20.0 ≤ [%C] × ([%V] + [%Cr] + [%Mo] + [%W]) ≤ 35.0 (1) 1.00 ≤ [%C] × ((0.177 × [%V]) / (0.099 × [%Cr] + 0.063 × [%Mo] + 0.033 × [%W])) ≤ 3.00 (2) Here, in formula (1) and formula (2), [%C], [%Cr], [%Mo], [%V], and [%W] represent the contents (mass%) of the respective elements.

[0025] Regarding [%C]×([%V]+[%Cr]+[%Mo]+[%W]), this parameter indicates the relationship between carbon and carbide-forming elements. By adjusting it to satisfy the above (1), the amount of carbide and the C content in the matrix are optimized, the hardness is improved, and thereby the wear resistance is enhanced. When the value of [%C]×([%V]+[%Cr]+[%Mo]+[%W]) is less than 20.0, due to insufficient carbide amount or low C content in the matrix, sufficient hardness cannot be obtained and the wear resistance decreases. Also, plastic flow is likely to occur during rolling, and the steel material is prone to seizure. On the other hand, when it exceeds 35.0, due to an increase in the amount of carbide or high C content in the matrix, excessive hardness is obtained. Therefore, the friction coefficient decreases due to the small roll surface roughness during rolling, and slip is likely to occur. Thus, the value of [%C]×([%V]+[%Cr]+[%Mo]+[%W]) is limited to 20.0 or more and 35.0 or less. Further, it is preferably 25.0 or more and 30.0 or less.

[0026] Regarding [%C]×((0.177×[%V]) / (0.099×[%Cr]+0.063×[%Mo]+0.033×[%W])), this parameter represents the ratio of the amount of MC carbide to the amount of (M 2 C carbide amount + M 7 C 3 carbide amount). By adjusting it to satisfy the above (2), the ratio of each carbide amount is optimized, and it becomes possible for the friction coefficient during rolling to take a value at which seizure and slip do not occur. When the value of [%C]×((0.177×[%V]) / (0.099×[%Cr]+0.063×[%Mo]+0.033×[%W])) is less than 1.00, M 2 C, M 7 C 3The proportion of carbide increases. Since these crystallize widely in a planar shape, the roll surface becomes flattened, the surface roughness decreases, and the friction coefficient during rolling decreases, making slip more likely to occur. On the other hand, when it exceeds 3.00, the proportion of MC carbide increases. This is fine granular carbide, and the increase in the protrusions on the roll surface increases the friction coefficient during rolling, making seizure more likely to occur. Therefore, the value of [%C]×((0.177×[%V]) / (0.099×[%Cr]+0.063×[%Mo]+0.033×[%W])) is limited to 1.00 or more and 3.00 or less. More preferably, it is 1.20 or more and 2.00 or less. Next, the reasons for limiting the structure of the outer layer material of the hot rolling roll of the present invention will be described.

[0027] The outer layer material of the hot rolling roll of the present invention is characterized by having a composition within the above range and a structure in which carbides having a particle size of 1 μm or more are present in an area ratio of 8.0 to 20.0%. Carbides are MC carbides, M 2 C carbides, M 7 C 3 carbides that crystallize during solidification. Here, the matrix is preferably martensite or bainite. Although various studies have been conducted on carbides, most of them are related to wear resistance. As a result of the inventors' intensive studies, in order to make the amounts of each carbide appropriate, by limiting to a structure having a composition within the above range and in which carbides having a particle size of 1 μm or more are present in an area ratio of 8.0 to 20.0%, it was discovered that the seizure resistance and slip resistance are greatly improved. By paying attention not only to the hardness of each carbide but also to its morphology, and adjusting the amount and ratio of granular and fine MC carbides and M 2 C and M 7 C 3 carbides that crystallize in a planar shape, it is considered that the friction coefficient value during rolling can be controlled, and accordingly, the seizure resistance and slip resistance are improved. Next, a preferred manufacturing method of the outer layer material of the hot rolling roll and the composite hot rolling roll of the present invention will be described.

[0028] In the present invention, as a method for manufacturing the roll outer layer material, a centrifugal casting method, a continuous surfacing casting method, etc. are preferable. However, from the viewpoint of manufacturing cost, the centrifugal casting method is more preferable. Note that the present invention is not limited to these manufacturing methods.

[0029] When the composite roll for hot rolling of the present invention casts the roll outer layer material by the centrifugal casting method, it is composed of a centrifugally cast outer layer and an inner layer welded and integrated with the outer layer. Note that an intermediate layer may be disposed between the outer layer and the inner layer. That is, instead of the inner layer welded and integrated with the outer layer, an intermediate layer welded and integrated with the outer layer and an inner layer welded and integrated with the intermediate layer may be used. Note that it is preferable to manufacture the inner layer by a static casting method.

[0030] For the inner layer manufactured by the static casting method, it is preferable to use spheroidal graphite cast iron, vermicular graphite cast iron (CV cast iron), etc. which are excellent in castability and mechanical properties. In the centrifugally cast roll, the outer layer and the inner layer are welded and integrated, and the components of the outer layer material are mixed into the inner layer. When carbide forming elements such as Cr and V contained in the outer layer material are mixed into the inner layer, the inner layer is weakened. Therefore, it is preferable to suppress the mixing rate of the outer layer material components into the inner layer as much as possible.

[0031] Further, when forming the intermediate layer, as the intermediate layer material, it is preferable to use graphite steel, high carbon steel, hypoeutectic cast iron, etc. The intermediate layer and the outer layer are welded and integrated, and the components of the outer layer material are mixed into the intermediate layer. In order to suppress the mixing rate of the outer layer material components into the inner layer, it is preferable to suppress the mixing rate of the outer layer material into the intermediate layer as much as possible.

[0032] The composite roll for hot rolling of the present invention is preferably heat-treated after casting. The heat treatment includes a quenching treatment of heating to 900 to 1100 °C and air-cooling or blast air-cooling, and further preferably performing a tempering treatment of heating and holding and then cooling so that the tempering parameter P described in the following formula (3) is in the range of 10000 to 20000 two or more times. At this time, by changing the quenching temperature, the tempering parameter, and the number of tempering times within the described range according to the components, it is possible to obtain the above-described structure. P = T(log(t) + A) (3) Here, T is the tempering temperature (K), t is the tempering time (h), and A is a constant. (In the present invention, A = 20 is used).

[0033] In addition, the preferable hardness of the hot rolling composite roll of the present invention is 75.0 HS to 85.0 HS in Shore hardness at 20°C and 45.0 HS to 50.0 HS in Shore hardness at 600°C. If the Shore hardness at 20°C is less than 75.0 HS, the wear resistance deteriorates. On the other hand, if the hardness exceeds 85.0 HS, it becomes difficult to grind and remove the cracks formed on the surface of the hot rolling roll during hot rolling. Also, the roll surface temperature during hot rolling is around about 600°C. If the Shore hardness at 600°C is less than 45.0 HS, plastic flow occurs and the steel material is likely to adhere to the roll surface. On the other hand, if the hardness exceeds 50.0 HS, the roll hardness is too high, and slip is likely to occur during rolling. Such hardness can be stably ensured by heat-treating the roll having the components of the present invention so that the tempering parameter P is in the range of 10,000 to 20,000.

Examples

[0034] The present invention will be described in more detail with the following examples, but the present invention is not limited to these examples.

[0035] With the chemical compositions of the hot rolling roll outer layer materials shown in Table 1 (the balance is Fe and inevitable impurities), each test material of Examples 1 to 8 of the present invention and each test material of Comparative Examples 9 to 20 were heated and melted up to 1450 to 1550°C and cast into a Y-type keel block mold (rectangular parallelepiped part: thickness 35 mm, width 230 mm, height 120 mm). After cooling, the ingot was taken out, quenched at 900°C to 1100°C, and then subjected to a tempering treatment of heating and holding and then cooling three times so that the tempering parameter P was in the range of 10,000 to 20,000. Thereafter, microstructure observation, hardness measurement, and friction coefficient measurement using a hot rolling rolling wear tester were performed. Note that the test pieces for microstructure observation and hardness measurement were taken from the center of the wall thickness.

[0036]

Table 1

[0037] For each sample cut from the ingots of the invention examples and comparative examples, the Vickers hardness HV at 20°C and 600°C was measured five times each using a Vickers hardness tester (test force: 1 kgf), and the average value was calculated. In the high-temperature (600°C) Vickers measurement, it conformed to JIS Z2252 "High-Temperature Vickers Hardness Test Method". A diamond indenter was used, and a testing machine of Nikon QM-2 (simultaneous heating type for the indenter and test piece) was used. The experiment was conducted in an argon gas atmosphere, with a heating rate of 20°C / min and a load holding time of 10 seconds. The obtained Vickers hardness was converted to Shore hardness using the calculation formula of JIS B 7731.

[0038] The method for measuring the friction coefficient using a hot rolling wear tester was as follows. Hot rolling wear test pieces (outer diameter 60 mmφ, width 10 mm, with C1 chamfer) were taken from the ingots of each obtained invention example and each comparative example. As shown in Fig. 1, the wear test was conducted in a two-disc sliding rolling manner between the test piece and the counter piece. The test piece 1 was rotated at 76 rpm while being water-cooled with cooling water 2, and a counter piece (outer diameter 190 mmφ, width 15 mm, with C1 chamfer) 4 heated to 1000°C by a high-frequency induction heating coil 3 was brought into contact with the rotating test piece 1 while applying a load of 180 N in the load direction 7 and rolling. The rotation direction 5 of the test piece and the rotation direction 6 of the counter piece were such that the tangents at the contact point between the test piece 1 and the counter piece 4 were in the same direction. The friction coefficient measurement method was to conduct the test for 300 minutes, and the counter piece was renewed with a new one every 60 minutes, for a total of 5 tests. The torque and load during the test were measured, and the friction coefficient was calculated from the following formula (4). μ=T / P×L (4) Here, μ is the friction coefficient, T is the torque (kgf·m), P is the load (kgf), and L is the radius (m) of the test piece.

[0039] Since the above test assumes a continuous operation of hot rolling, the counter piece was heated to 1000°C, and the friction coefficient and seizure situation after 300 minutes were evaluated. The presence or absence of seizure was visually confirmed on the surface of the test piece after 300 minutes. Those with transfer of the material of the counter piece were considered to have seizure, and those without transfer were considered to have no seizure.

[0040] The hot rolling wear test (hereinafter also referred to as the wear test) method was as follows. Similar to the friction coefficient measurement test, hot rolling wear test pieces (outer diameter 60 mmφ, width 10 mm, with chamfer on C1 surface) were taken from the ingots of each inventive example and each comparative example obtained. As shown in Fig. 1, the wear test was conducted in a two-disk sliding rolling manner between test piece 1 and mating piece 4. While cooling test piece 1 with cooling water 2, it was rotated at 700 rpm. To the rotating test piece 1, a mating piece (outer diameter 190 mmφ, width 15 mm, with chamfer on C1 surface) 4 heated to 800 °C by high-frequency induction heating coil 3 was brought into contact while applying a load of 686 N in the load direction 7 and rolled. The rotation direction 5 of the test piece and the rotation direction 6 of the mating piece are the rotation directions in which the tangents at the contact point between test piece 1 and mating piece 4 are in the same direction. The wear test was carried out for 135 minutes. The mating piece was renewed with a new one every 45 minutes (31,500 rotations of the test piece), and a total of 3 tests (94,500 rotations of the test piece) were conducted. The mass reduction amount of the test piece before and after the test, that is, the wear amount, was measured.

[0041] For each sample after heat treatment, after mirror polishing and corrosion with nital solution, microstructure observation was carried out with a digital microscope. Photographs were taken in a field of view where 200 or more eutectic cells could be confirmed within the field of view to be photographed. Also, using an image analysis tool (ImageJ), binarization processing was performed on a photograph with a measurement magnification of 200 times. Since there is a difference in brightness between the matrix structure and carbides in the photograph, by performing binarization processing, the matrix structure and carbides can be classified and the area can be obtained. Five photographs of each sample were taken, and the average value of the area ratio of carbides was calculated. Here, the area ratio of the carbides is the area ratio of carbides with a particle size of 1 μm or more. The particle size of the carbides was measured from the diameter of the carbides in the digital microscope image taken at 500 times magnification, and this was taken as the particle size of the carbides. For rod-shaped carbides, the length of the short side was taken as the particle size of the carbides. Also, when the shape of the carbides was elliptical or the like, the minimum diameter was taken as the particle size.

[0042] The results obtained are shown in Table 2.

[0043]

Table 2

[0044] The friction coefficient in Table 2 was considered acceptable in the range of 0.15 to 0.30, and values less than 0.15 or greater than 0.30 were considered unacceptable. When the friction coefficient was less than 0.15, it was considered that the anti-slip property was insufficient because the friction coefficient during the test was small. Also, when it was greater than 0.30, seizure occurred because the friction coefficient during the test was high. It is considered that the appropriate high-temperature hardness of the base and the ratio of each carbide amount made the friction coefficient during the test fall within an appropriate range, improving the seizure resistance and anti-slip property. Also, the wear amount was 0.50 g or less in the inventive examples, whereas it exceeded 0.50 g in Sample Nos. 16 to 20 of the comparative examples where the friction coefficient was greater than 0.30. Furthermore, the wear amount also exceeded 0.50 g in Sample No. 11 of the comparative example containing an excessive amount of Cr compared to the specified component. It is considered that the wear resistance was improved by the friction coefficient being below the upper limit and the appropriate component composition.

[0045] Therefore, according to the present invention, it becomes possible to manufacture a hot-rolling roll outer layer material and a composite roll excellent in seizure resistance and anti-slip property. As a result, time loss during rolling interruption due to roll trouble such as roll biting failure due to slip and surface roughness such as slip marks on the material to be rolled is reduced, improving the rolling efficiency of the hot-rolling roll, and accordingly, the productivity of the hot-rolled steel sheet is also improved.

Explanation of Reference Numerals

[0046] 1: Test piece 2: Cooling water 3: High-frequency induction heating coil 4: Counter piece 5: Rotation direction of test piece 6: Rotation direction of counter piece 7: Direction of load

Claims

1. by mass%, C: 1.2 to 2.5%, Si: 0.15 to 2.50%, Mn: 0.15 to 2.50%, Ni: 0.2 to 8.0%, Cr: 1.5 to 10.0%, Mo: 3.5 to 12.0%, V: 2.0 to 7.5%, W: 0.1 to 6.0%, P: 0.01 to 0.04%, S: 0.001 to 0.010%, and the balance consists of Fe and inevitable impurities, and has a composition in which the contents of C, Cr, Mo, V, and W satisfy the following formulas (1) and (2), carbides having a particle size of 1 μm or more are present in an area ratio of 8.0 to 20.0%, the Shore hardness at 20°C is 75.0 HS or more and 85.0 HS or less, and the Shore hardness at 600°C is 45.0 HS or more and 50.0 HS or less. An outer layer material for a hot rolling roll. 20.0 ≤ [%C] × ([%V] + [%Cr] + [%Mo] + [%W]) ≤ 30.0 (1) 1.00 ≤ [%C] × ((0.177 × [%V]) / (0.099 × [%Cr] + 0.063 × [%Mo] + 0.033 × [%W])) ≤ 3.00 (2) Here, in formulas (1) and (2), [%C], [%Cr], [%Mo], [%V], and [%W] represent the contents (mass%) of the respective elements.

2. A composite roll for hot rolling having two layers of an outer layer and an inner layer, or three layers of an outer layer, an intermediate layer, and an inner layer, wherein the outer layer is by mass%, C: 1.2 to 2.5%, Si: 0.15 to 2.50%, Mn: 0.15 to 2.50%, Ni: 0.2 to 8.0%, Cr: 1.5 to 10.0%, Mo: 3.5 to 12.0%, V: 2.0 to 7.5%, W: 0.1 to 6.0%, P: 0.01 to 0.04%, S: 0.001 to 0.010%, and the balance consists of Fe and inevitable impurities, and has a composition in which the contents of C, Cr, Mo, V, and W satisfy the following formulas (1) and (2), carbides having a particle size of 1 μm or more are present in an area ratio of 8.0 to 20.0%, the Shore hardness at 20°C is 75.0 HS or more and 85.0 HS or less, and the Shore hardness at 600°C is 45.0 HS or more and 50.0 HS or less. A composite roll for hot rolling. 20.0 ≤ [%C] × ([%V] + [%Cr] + [%Mo] + [%W]) ≤ 30.0 (1) 1.00 ≤ [%C] × ((0.177 × [%V]) / (0.099 × [%Cr] + 0.063 × [%Mo] + 0.033 × [%W])) ≤ 3.00 (2) Here, in formulas (1) and (2), [%C], [%Cr], [%Mo], [%V], and [%W] represent the contents (mass %) of the respective elements.

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