Roll surface layer material for hot rolling with excellent fatigue resistance produced by centrifugal casting, and composite roll for hot rolling produced through centrifugal casting

Abstract

There is provided a roll surface layer material including a roll surface layer with excellent fatigue resistance. The roll surface layer material has a composition including, on a mass % basis, C: 2.3% to 2.9%, Si: 0.2% to 0.8%, Mn: 0.2% to 1.0%, Cr: 5.0% to 7.5%, Mo: 4.4% to 6.5%, V: 5.3% to 7.0%, Nb: 0.6% to 1.5%, and Co: 0.1% to 4.0% so as to satisfy 14.0(Mo+1.7V)17.0 (where Mo represents a content (mass %) of Mo and V represents a content (mass %) of V) and further including Al: 0.001% to 0.03% and/or REM: 0.001% to 0.03%, wherein a carbide is contained at an area fraction of 13% to 40%. A composite roll obtained by integrally welding a shaft member to the roll surface layer member is treated as a centrifugal cast roll that includes a surface layer with excellent fatigue resistance.

Claims

1. A roll surface layer material produced by centrifugal casting for a hot rolling mill, the roll surface layer material having excellent fatigue resistance and a compression 0.2% proof strength of 2000 MPa or more, and being used for a centrifugal cast roll for a hot rolling mill composite roll having a hardness of 79 to 88 HS, a composition of the roll surface layer material consisting of, on a mass % basis: Carbon (C): 2.3% to 2.9%; Silicon (Si): 0.2% to 0.8%; Manganese (Mn): 0.2% to 1.0%; Chromium (Cr): 5.0% to 7.5%; Molybdenum (Mo): 4.4% to 6.5%; Vanadium (V): 5.3% to 7.0%; Niobium (Nb): 0.6% to 1.5%; Cobalt (Co): 0.1% to 4.0%, and at least one of aluminum (Al) or rare earth metals (REM) in an amount of 0.001% to 0.03%, so as to satisfy formula (1) below, with the balance being iron (Fe) and incidental impurities including phosphor (P): 0.05% or less, sulfur (S): 0.05% or less and nitrogen (N): 0.06% or less, and wherein an area fraction of carbides in the roll surface layer material is in the range of 13% to 20%,
14.0(Mo+1.7V)17.0(1) wherein Mo represents a content (mass%) of molybdenum and V represents a content (mass%) of vanadium.

2. A centrifugal cast roll for a hot rolling mill composite roll having a hardness of 79 to 88 HS, the centrifugal cast roil having excellent fatigue resistance and a compression 0.2% proof strength of 2000 MPa or more, and including a surface layer and an internal layer integrally welded to the surface layer, a composition of the roll surface layer material consisting of, on a mass % basis: Carbon (C): 2.3% to 2.9%; Silicon (Si): 0.2% to 0.8%; Manganese (Mn): 0.2% to 1.0%; Chromium (Cr): 5.0% to 7.5%; Molybdenum (Mo): 4.4% to 6.5%; Vanadium (V): 5.3% to 7.0%; Niobium (Nb): 0.6% to 1.5%; Cobalt (Co): 0.1% to 4.0%, and at least one of aluminum (Al) or rare earth metals (REM) in an amount of 0.001% to 0.03%, so as to satisfy formula (1) below, with the balance being iron (Fe) and incidental impurities including phosphor (P): 0.05% or less, sulfur (S): 0.05% or less and nitrogen (N): 0.06% or less, and wherein an area fraction of carbides in the roll surface layer material is in the range of 13% to 20%,
14.0(Mo+1.7V)17.0(1) wherein Mo represents a content (mass%) of molybdenum and V represents a content (mass%) of vanadium.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is an explanatory view schematically showing a structure of a tester used in a hot rolling fatigue test.

(2) FIG. 2 is an explanatory view schematically showing the shape and size of a notch formed in a peripheral surface of a test specimen for a hot rolling fatigue test (fatigue test specimen) used in Examples.

(3) FIG. 3 is a graph showing the influence of REM and/or Al on the relationship between the ratio of the number of rotations of rolling motion leading breakage and the (Mo+1.7V) amount in the hot rolling fatigue test.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(4) A roll surface layer material of the present invention is made by centrifugal casting and can be directly used for ring rolls and sleeve rolls. The roll surface layer material is applied as a surface layer material of hot rolling mill composite roll suitable for hot finish rolling. The hot rolling mill composite roll of an embodiment of the present invention includes a surface layer made by centrifugal casting and an internal layer that is integrally welded to the surface layer. An intermediate layer may be disposed between the surface layer and the internal layer. In other words, the hot rolling mill composite roll may include, instead of the internal layer integrally welded to the surface layer, an intermediate layer integrally welded to the surface layer and an internal layer integrally welded to the intermediate layer. The internal layer is preferably made by a static casting method. In the present invention, the compositions of the internal layer and intermediate layer are not particularly limited, but the internal layer is preferably composed of spherical graphitic cast iron and the intermediate layer is composed of a high carbon material containing C: 1.5 to 3 mass %.

(5) The reasons for limiting the composition of the roll surface layer material (surface layer) will be described. Hereinafter, mass % is simply expressed as % unless otherwise specified.

(6) C: 2.3% to 2.9%

(7) C dissolves into a matrix and thus increases the hardness of the matrix and also bonds to a carbide-forming element and thus forms a hard carbide, thereby improving the wear resistance of the roll surface layer material. The amount of an eutectic carbide varies depending on the C content. The eutectic carbide affects the rolling characteristics. Therefore, at a C content of less than 2.3%, an insufficiently small amount of the eutectic carbide increases the friction force during rolling and causes unstable rolling, and also the compression 0.2% proof strength of the roll surface layer material decreases. On the other hand, at a C content exceeding 2.9%, the amount of the eutectic carbide excessively increases, the roll surface layer member becomes hard and brittle, the formation and growth of fatigue cracks are facilitated, and the fatigue resistance degrades. Accordingly, the C content is limited to the range of 2.3% to 2.9%.

(8) Si: 0.2% to 0.8%

(9) Si is an element that serves as a deoxidizer agent and that improves the castability of molten metal. To achieve such effects, 0.2% or more of Si needs to be contained. On the other hand, at a Si content exceeding 0.8%, the effects are saturated and effects corresponding to the content are not to be expected, which is economically disadvantageous. Accordingly, the Si content is limited to 0.2% to 0.8%.

(10) Mn: 0.2% to 1.0%

(11) Mn is an element that fixes S in the form of MnS, thereby rendering S harmless and that partly dissolves into a matrix, thereby improving the hardenability. To achieve such effects, 0.2% or more of Mn needs to be contained. At a Mn content exceeding 1.0%, the effects are saturated and effects corresponding to the content are not to be expected, and furthermore the material may become brittle. Accordingly, the Mn content is limited to 0.2% to 1.0%.

(12) Cr: 5.0% to 7.5%

(13) Cr is an element that bonds to C and mainly forms an eutectic carbide, thereby improving the wear resistance and that decreases the friction force with a steel sheet during rolling and thus reduces the damage to a roll surface, thereby stabilizing the rolling. To achieve such effects, 5.0% or more of Cr needs to be contained. At a Cr content exceeding 7.5%, the amount of a hard and brittle eutectic carbide excessively increases, which degrades the fatigue resistance. Accordingly, the Cr content is limited to the range of 5.0% to 7.5%.

(14) Mo: 4.4% to 6.5%

(15) Mo is an element that bonds to C and forms a hard carbide, thereby improving the wear resistance. Mo is also an element that dissolves into a hard MC carbide in which V and Nb bond to C, thereby reinforcing the carbide and that also dissolves into an eutectic carbide, thereby increasing the fracture resistance of the carbides. Through such actions, Mo improves the wear resistance and fatigue resistance of the roll surface layer member. To achieve such effects, 4.4% or more of Mo needs to be contained. At a Mo content exceeding 6.5%, a hard and brittle carbide mainly composed of Mo is formed. This degrades the resistance to hot rolling fatigue, which degrades the fatigue resistance. Accordingly, the Mo content is limited to the range of 4.4% to 6.5%.

(16) V: 5.3% to 7.0%

(17) V is an advantageous element in the present invention because V imparts both wear resistance and fatigue resistance required for a roll. V forms an extremely hard carbide (MC carbide) and thus improves the wear resistance and also effectively divides and disperses/crystallizes an eutectic carbide. V is also an element that improves the resistance to hot rolling fatigue, thereby considerably improving the fatigue resistance of the roll surface layer material. Such effects are significantly achieved at a V content of 5.3% or more. However, at a V content exceeding 7.0%, a coarse MC carbide is formed and the centrifugal casting segregation of the MC carbide is facilitated, which destabilizes various characteristics of a rolling mill roll. Accordingly, the V content is limited to the range of 5.3% to 7.0%.

(18) Nb: 0.6% to 1.5%

(19) Nb dissolves into an MC carbide and reinforces the MC carbide and thus increases the fracture resistance of the MC carbide, thereby further improving the wear resistance, in particular, the fatigue resistance. When both Nb and Mo are dissolved into a carbide, the wear resistance and the fatigue resistance are considerably improved. Nb is also an element that facilitates the division of an eutectic carbide and suppresses the fracture of the eutectic carbide, thereby improving the fatigue resistance of the roll surface layer material. Nb also suppresses the segregation of the MC carbide during centrifugal casting. Such effects are significantly achieved at a Nb content of 0.6% or more. However, at a Nb content exceeding 1.5%, the growth of the MC carbide in a molten metal is facilitated and the carbide segregation during centrifugal casting is promoted. Accordingly, the Nb content is limited to the range of 0.6% to 1.5%.

(20) Co: 0.1% to 4.0%

(21) Co is an element that dissolves into a matrix and reinforces the matrix, in particular, at high temperature, thereby improving the fatigue resistance. To achieve such effects, 0.1% or more of Co needs to be contained. On the other hand, at a Co content exceeding 4.0%, the effects are saturated and effects corresponding to the content are not to be expected, which is economically disadvantageous. Accordingly, the Co content is limited to the range of 0.1% to 4.0%. The Co content is preferably 0.2% to 3.0%.

(22) In the present invention, Mo and V are preferably contained in the above-described ranges and furthermore are preferably contained so as to satisfy formula (1) below.
14.0(Mo+1.7V)17.0(1)

(23) (where Mo represents a content (mass %) of Mo and V represents a content (mass %) of V)

(24) As shown in FIG. 3, when Al and/or REM is contained, by adding Mo and V so that (Mo+1.7V) satisfies the above formula (1), the number of rotations of rolling motion leading breakage is considerably increased compared with the reference (Conventional Example) and thus the resistance to hot rolling fatigue is considerably improved. The (Mo+1.7V) is an important factor for improving the resistance to hot rolling fatigue. Only when the (Mo+1.7V) is adjusted to be in the range of 14.0 to 17.0, excellent resistance to hot rolling fatigue can be maintained. In the case where the (Mo+1.7V) is outside the range of 14.0 to 17.0, even if Al and/or REM is contained, the resistance to hot rolling fatigue degrades. Accordingly, in the present invention, the contents of Mo and V can be adjusted so as to satisfy the formula (1).

(25) In the present invention, the contents of Mo and V can be adjusted so as to satisfy the formula (1) and Al and/or REM is essentially contained.

(26) Al: 0.001% to 0.03% and/or REM: 0.001% to 0.03%

(27) Only when Mo and V are contained so as to satisfy the formula (1), Al and/or REM considerably improves the resistance to hot rolling fatigue as shown in FIG. 3. To achieve such effects, 0.001% or more of each of Al and REM needs to be contained. On the other hand, even if more than 0.03% of each of Al and REM is contained, the effects are saturated and the castability is degraded by, for example, bubble formation and a decrease in fluidity of molten steel. Accordingly, the contents of Al and/or REM are limited to the range of Al: 0.001% to 0.03% and/or REM: 0.001% to 0.03%.

(28) The balance other than the above components is Fe and incidental impurities.

(29) Examples of the incidental impurities include P: 0.05% or less, S: 0.05% or less, and N: 0.06% or less. P segregates in a grain boundary and degrades the quality of a material. Therefore, in the present invention, the P content is desirably as low as possible, but a P content of 0.05% or less is permissible. S is present in the form of a sulfide inclusion and degrades the quality of a material. Therefore, the S content is preferably as low as possible, but a S content of 0.05% or less is permissible. N mixes in a concentration of about 0.06% or less through ordinary dissolution, but such a concentration does not affect the advantageous effects of the present invention. The N content is preferably less than 0.05% because N may form defects at a boundary between the surface layer and the intermediate layer or between the surface layer and the internal layer of a composite roll.

(30) In the roll surface layer material of the present invention, large amounts of Cr, V, Mo, and the like are contained and an extremely hard carbide (MC carbide) and an eutectic carbide are dispersed, whereby a desired hardness, a desired wear resistance, and the like are achieved. If the carbides have an area fraction of less than 13%, such a desired hardness, wear resistance, and the like are not easily achieved. On the other hand, if the carbides have an area fraction of more than 20%, the roll material may become brittle. Accordingly, the area fraction of the carbides is preferably limited to the range of 13% to 20%.

(31) A preferred method for producing a hot rolling mill composite roll of the present invention will now be described.

(32) In the present invention, the method for producing a roll surface layer member is a centrifugal casting method, which is performed with a low energy cost.

(33) A molten metal having the above roll surface layer material composition is poured into a rotatable mold whose internal surface is coated with a refractory mainly composed of zircon so that a predetermined wall thickness is achieved. The molten metal is then subjected to centrifugal casting. In the case where an intermediate layer is formed, the intermediate layer is preferably formed by the following method. During the solidification of the roll surface layer member or after the complete solidification of the roll surface layer member, a molten metal having an intermediate layer composition is poured into the mold while rotating the mold and then cast by centrifugal casting. After the surface layer or the intermediate layer is completely solidified, preferably, the rotation of the mold is stopped and the mold is put in a standing position, and then an internal layer material is cast by static casting to obtain a composite roll. Thus, the inner surface of the roll surface layer member is remelted to form a composite roll in which the surface layer and the internal layer are integrally welded or a composite roll in which the surface layer and the intermediate layer are integrally welded and the intermediate layer and the internal layer are integrally welded.

(34) The internal layer subjected to static casting is preferably composed of, for example, spherical graphitic cast iron or compacted vermicular graphitic cast iron (CV cast iron) having excellent castability and mechanical properties. Since the centrifugal cast roll includes the surface layer and the internal layer integrally welded to each other, about 1% to 8% of surface layer components mix in the internal layer. Cr, V, and the like contained in the surface layer member are powerful carbide-forming elements. The mixing of these elements in the internal layer causes the internal layer to be brittle. Accordingly, the proportion of the surface layer components mixed in the internal layer is preferably decreased to less than 6%.

(35) In the case where the intermediate layer is formed, for example, graphitic steel, high carbon steel, or hypoeutectic cast iron is preferably used for the intermediate layer material. The intermediate layer and the surface layer are integrally welded in a similar manner, and about 10% or more and 90% or less of the surface layer components mix in the intermediate layer. To suppress the amount of the surface layer components mixed in the internal layer, it is important to reduce the amount of the surface layer components mixed in the intermediate layer as much as possible.

(36) The hot rolling mill composite roll of the present invention is preferably heat treated after the casting. The heat treatment preferably includes performing a process in which the composite roll is heated to 950 C. to 1150 C. and cooled by air cooling or air blast cooling and performing, at least once, a process in which the composite roll is heated and held at 450 C. to 600 C. and then cooled.

(37) The hardness of the hot rolling mill composite roll of the present invention is preferably 79 to 88 HS and more preferably 80 to 86 HS. To stably achieve the hardness, it is recommended to adjust the heat treatment after the casting.

EXAMPLES

(38) A molten metal having a roll surface layer material composition shown in Table 1 was melted in a high frequency furnace and cast into a ring-shaped test member (ring roll; outer diameter: 250 mm, wall thickness: 55 mm) by a centrifugal casting method. The pouring temperature was 1380 C. to 1450 C. and the centrifugal force, expressed as multiples of gravity, was 176 G. After the casting, a quenching treatment in which the ring-shaped test member was reheated to a quenching temperature of 1050 C. and cooled by air cooling and a tempering treatment in which the ring-shaped test member was heated and held at a tempering temperature of 450 C. to 600 C. and cooled were performed to adjust the hardness to be 78 to 84 HS.

(39) A ring-shaped test member (ring roll) having a composition of a high-speed steel roll surface layer member made by centrifugal casting and used for a hot finish rolling mill (on a mass % basis, 2.1% C-0.4% Si-0.4% Mn-6.3% Cr-4.2% Mo-5.1% V-0.1% Nb-balance being Fe and incidental impurities) was cast by a centrifugal casting method and heat treated in the same manner to obtain a reference member (Conventional Example).

(40) A hardness test specimen, a compression test specimen, a hot rolling fatigue test specimen, and a test specimen for microstructure observation were taken from the obtained ring-shaped test member to perform a hardness test, a compression test, a hot rolling fatigue test, and a microstructure observation test. The test methods are as follows.

(41) (1) Hardness Test

(42) The Vickers hardness HV 50 of the prepared hardness test specimen was measured with a Vickers hardness tester (testing force: 50 kgf (490 kN)) in conformity with JIS Z 2244, and the Vickers hardness HV 50 was converted into Shore hardness HS using a JIS conversion table. The Vickers hardness HV 50 was measured at 10 positions for each specimen. The maximum value and the minimum value were taken away and the arithmetic mean was calculated. The arithmetic mean was defined as the hardness of the test member.

(43) (2) Compression Test

(44) A compression test was performed on the prepared compression test specimen (diameter 10 mmlength 20 mm) at room temperature. The number of repetitions was set to be two. In the compression test, a strain gage was attached to the central portion of the compression test specimen and a stress-strain curve was obtained. The 0.2% proof strength was read from the obtained stress-strain curve. The average of 0.2% proof strengths of two test specimens was defined as the 0.2% proof strength of each test member.

(45) (3) Hot Rolling Fatigue Test

(46) A hot rolling fatigue test specimen (outer diameter: 60 mm, wall thickness: 10 mm, chamfered) having a shape shown in FIG. 2 was taken from the obtained ring-shaped test member. In the hot rolling fatigue test specimen, a notch (depth t: 1.2 mm, length L in a circumferential direction: 0.8 mm) shown in FIG. 2 was formed at two positions (positions 180 apart from each other) of a peripheral surface of the specimen by an electro-discharge (wire cut) method that uses a wire with 0.20 mm. As shown in FIG. 1, the hot rolling fatigue test was conducted by a two-disc slipping/rolling method that uses a test specimen and an opposing specimen. The test specimen was rotated at 700 rpm while being cooled with water. An opposing specimen (material: S45C, outer diameter: 190 mm, width 15 mm, chamfered) heated to 790 C. was brought into contact with the rotating test specimen while applying pressure at a load of 980 N and the rolling motion was performed at a slip factor of 10%. The rolling motion was performed until the two notches formed in the hot rolling fatigue test specimen were broken. The number of rotations of rolling motion until each notch was broken was determined, and the average of the numbers of rotations of rolling motion was defined as the number of rotations of rolling motion leading breakage. The number of rotations of rolling motion leading breakage in Conventional Example was assumed to be a reference (1.0), and the ratio of the number of rotations of rolling motion leading breakage of each ring-shaped test member to the number of rotations of rolling motion leading breakage in Conventional Example, that is, (the number of rotations of rolling motion leading breakage of each ring-shaped test member)/(the number of rotations of rolling motion in Conventional Example) was calculated and used as an index of fatigue resistance. When the ratio of the numbers of rotations of rolling motion leading breakage was more than 1.5, the ring-shaped test member was evaluated to have excellent fatigue resistance.

(47) (4) Microstructure Observation Test

(48) The prepared test specimen for microstructure observation was polished and subjected to nital corrosion. The microstructure was observed using an image analyzer with an optical microscope at a magnification of 50 times. The obtained image was subjected to binary conversion to measure the area fraction of a carbide. The area fraction was treated as the amount of a carbide of each test member. Table 2 shows the results.

(49) TABLE-US-00001 TABLE 1 Test Chemical composition (mass %) member Mo + Satisfaction of No. C Si Mn P S Cr Mo V Nb Co REM Al 1.7V formula (1)* Remarks A 2.5 0.5 0.4 0.02 0.008 6.2 5.3 6 0.9 2.1 0.023 0.015 15.5 Yes Invention Example B 2.6 0.3 0.5 0.025 0.009 6.3 6.3 5.7 1.1 0.3 0.012 16 Yes Invention Example C 2.5 0.4 0.4 0.028 0.01 7.5 5.1 5.5 1.3 3.1 0.01 14.5 Yes Invention Example D 2.7 0.4 0.3 0.015 0.008 7.1 4.6 6.4 1 0.9 0.013 15.5 Yes Invention Example E 2.7 0.6 0.5 0.022 0.007 7 5.4 6 1.2 1.3 0.007 0.006 15.6 Yes Invention Example F 2.4 0.7 1 0.03 0.01 6.7 4.5 5.7 1.4 2 0.006 0.015 14.2 Yes Invention Example G 2.8 0.4 0.8 0.019 0.009 5.1 5.6 6.5 0.8 0.4 0.004 0.018 16.7 Yes Invention Example H 2.3 0.4 0.5 0.018 0.01 7.2 4.8 4.9 1.2 0.005 13.1 No Comparative Example I 2.1 0.3 0.4 0.019 0.008 6 6.1 6.9 1.2 0.021 17.9 No Comparative Example J 2.6 0.4 0.4 0.019 0.009 7.9 4.1 5.6 1.1 0.01 13.6 No Comparative Example K 3.1 0.4 0.4 0.024 0.009 7.3 5.6 7.3 1.1 0.004 0.006 18 No Comparative Example L 2.9 0.5 0.9 0.022 0.008 7.4 5.6 5.4 0.7 0.0004 14.8 Yes Comparative Example M 2.8 0.4 0.4 0.017 0.008 6.8 5.5 6.1 1 15.9 Yes Comparative Example N 2.6 0.6 0.6 0.021 0.009 6.5 4.8 5.6 1.1 14.3 Yes Comparative Example O 2.7 0.4 0.5 0.023 0.008 6.9 5.5 5.8 1.1 15.3 Yes Comparative Example P 2.1 0.4 0.4 0.026 0.011 6.3 4.2 5.1 0.1 12.9 No Conventional Example Q 2.7 0.3 0.3 0.021 0.009 9.8 6 5 1.3 14.5 Yes Comparative Example R 2.6 0.4 0.3 0.019 0.01 4.8 3.8 5.1 1 0.011 12.5 No Comparative Example S 2.7 0.3 0.3 0.018 0.008 7 6.9 6.2 1 17.5 No Comparative Example T 2.5 0.5 0.6 0.018 0.008 7.1 6 5.4 1 1.5 15.2 Yes Comparative Example U 2.7 0.4 0.5 0.025 0.01 7.3 5.7 6.4 1.1 2 16.6 Yes Comparative Example V 2.6 0.6 0.4 0.028 0.008 7 4 5.5 1.2 1 0.007 0.015 13.4 No Comparative Example W 2.5 0.5 0.5 0.018 0.006 6.9 6.9 6.2 0.9 1.5 0.009 0.018 17.4 No Comparative Example Satisfaction of formula (1)* 14.0 (Mo + 1.7V) 17.0 Underlined items are outside the scope of the present invention.

(50) TABLE-US-00002 TABLE 2 Amount of Strength Fatigue resistance Test carbide 0.2% proof Ratio of number of member Area Hardness strength rotations of rolling No. fraction (%) HS (MPa) motion leading breakage Remarks A 17.6 82 2215 2.6 Invention Example B 15.8 85 2263 1.7 Invention Example C 19 83 2145 2.2 Invention Example D 18.3 81 2140 1.9 Invention Example E 16.5 84 2190 2.5 Invention Example F 14.9 80 2110 2.1 Invention Example G 17 83 2142 2 Invention Example H 18.3 83 1916 1.2 Comparative Example I 11.2 82 1713 1.1 Comparative Example J 17.5 84 1983 1.2 Comparative Example K 22.7 84 1867 1.1 Comparative Example L 24.2 82 1867 1 Comparative Example M 16.7 83 1923 1.3 Comparative Example N 15.9 84 1996 1.2 Comparative Example O 16.9 84 1196 1 Comparative Example P 8.2 81 1823 1.0 (reference) Conventional Example Q 23.5 82 1910 1.2 Comparative Example R 13.6 82 1891 0.8 Comparative Example S 18.7 84 1872 1 Comparative Example T 16.8 81 1885 1.2 Comparative Example U 17.1 80 1925 1.3 Comparative Example V 18 83 1954 1.1 Comparative Example W 16.4 82 1863 1.2 Comparative Example Underlined items are outside the scope of the present invention.

(51) In Invention Examples, the number of rotations of rolling motion leading breakage was increased to more than 1.5 times the number of rotations of rolling motion leading breakage in Conventional Example (reference) and the resistance to hot rolling fatigue was considerably improved. Furthermore, the compression 0.2% proof strength was as high as 2000 MPa or more. Therefore, in Invention Examples, roll surface layer materials having excellent fatigue resistance and having both high compression 0.2% proof strength and excellent resistance to hot rolling fatigue were provided. In Comparative Examples which are outside the scope of the present invention, the compression 0.2% proof strength was degraded, the resistance to hot rolling fatigue was degraded, or both of them were degraded.