Hot rolling roll outer layer material and method for producing the same, and composite roll for hot rolling

Abstract

To provide a hot rolling roll outer layer material, a method for producing the outer layer material, and a composite roll for hot rolling.SOLUTION: A hot rolling roll outer layer material of the present invention contains C: 1.2% to 2.4%, Si: 0.1% to 2.0%, Mn: 0.1% to 2.5%, Ni: 0.5% to 7.0%, Cr: 1.5% to 8.5%, Mo: 2.5% to 18.5%, V: 1.5% to 8.5%, W: 0.4% to 4.5%, P: 0.01% to 0.08%, and S: 0.001% to 0.030%, the balance being Fe and unavoidable impurities, and has a component composition in which contents of C, Mo, W, V, Cr, Si, Mn, and Ni satisfy a predetermined relational expression, has an outer layer material structure in which a total precipitated carbide area ratio is 7.5% to 20.5%, a ratio of a precipitated M2C-type carbide area ratio to the total precipitated carbide area ratio is 0.15 to 0.55, and a eutectic cell size is 40 μm to 150 μm, and has thermophysical property values in which a difference ΔTLS between a liquidus temperature and a solidus temperature is 200°C to 380°C.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a roll outer layer material for hot rolling, which is particularly suitable for use in rough rolling of steel sheets, a method for producing the same, and a composite roll for hot rolling. [Background technology]

[0002] In recent years, the demand for high-quality steel sheets has increased, and this has led to a demand for improvements in hot rolling technology for steel sheets. Therefore, there is a strong demand for improvements in the properties of hot rolling rolls used in hot rolling equipment, specifically improvements in wear resistance, slip resistance, etc.

[0003] Therefore, in order to improve wear resistance, HiCr cast steel rolls are used, in which Cr-based M7C3 type carbides are introduced into the structure, and high-speed steel rolls are based on high-speed steel, a type of tool steel, and contain carbide-forming elements such as V, Cr, Mo, and W, and have large amounts of hard carbides such as V-based MC type carbides, Mo-based W-based M2C type carbides, and Cr-based M7C3 type carbides (where M represents a metal element that forms carbides) introduced into the structure.

[0004] When a large amount of carbide is introduced into the structure to improve the wear resistance, the hot rolling roll becomes hard and has good wear resistance, but this results in a decrease in the surface roughness of the roll and a decrease in the coefficient of friction during rolling, making slippage more likely to occur.

[0005] Various techniques have been disclosed to solve such problems, for example, the techniques disclosed in Patent Documents 1 to 3.

[0006] Patent Document 1 describes the following composition in mass percent: C: 0.7 to 3.6%, Si: 0.2 to 2.5%, Mn: 0.2 to 2.0%, Cr: 2.0 to 10%, Mo: 0.2 to 10%, V: 2.0 to 10%, B: 0.001 to 0.50%, Al: 0.001 to 0.50%, Ti: 0.001 to 0.50%, Zr: 0.001 to 0.50%, Cu: 0. An outer layer material for a rolling composite roll has been proposed, which contains 0.001-0.50% of Ni, 0.001-0.50% of Mg, 0.001-0.50% of Ca, and one or more of 0.1-10% of Ni, 0.2-10% of W, 0.2-10% of Nb, and 0.2-10% of Co, with the balance being Fe and unavoidable impurities. This is said to result in a microstructure in which MC carbides are crystallized finely, uniformly, and spherically, resulting in an outer layer material for a rolling composite roll with excellent wear resistance.

[0007] Patent Document 2 describes a steel sheet containing, by mass, 1.50 to 2.70% C, 0.3 to 3% Si, 0.1 to 3% Mn, 0.1 to 2.5% Ni, 4.0 to 7.0% Cr, 4.1 to 8.0% Mo, 5.0 to 10.0% V, 0 to 0.4% W, 0.1 to 3.0% Nb, 0.005 to 0.15% N, and 0 to 0.05% B, and further containing 0.1 to 5% Co, 0.01 to 0.5% Zr, 0.05 to 0.5% Ti, and 0.05 to 0.5% A. The present inventors have proposed a centrifugally cast composite roll for rolling, which contains at least one selected from the group consisting of V: ​​0.001 to 0.5%, with the remainder consisting essentially of an Fe alloy consisting of Fe and unavoidable impurities, in which V / Nb, which represents the ratio of the V content (mass%) to the Nb content (mass%), is 1 to 20.0, and C-bal, which is expressed by the following formula, is 0 to 0.28. This results in a centrifugally cast composite roll for rolling with excellent wear resistance. The following formula: C-bal=C%-0.2×V%-0.06×Cr%-0.063×Mo%-0.033×W%-0.13×Nb% In the formula, C%, V%, Cr%, Mo%, W%, and Nb% represent the contents (mass%) of C, V, Cr, Mo, W, and Nb, respectively.

[0008] Patent Document 3 describes a steel sheet 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 at least one of V: ​​0.05 to 0.50%, Ti: 0.05 to 0.50%, and Nb: 0.02 to 0.20%, and A work roll for a hot rolling roughing mill stand has been proposed, which has an outer shell made of cast steel having a molten metal composition that satisfies (A) formula: 8≦Cr / C≦14, (B) formula: 3.0≦12.3C+0.55Cr-15.2≦7.0, and (C) formula: 26.0≦15.5C+Cr, with the balance being Fe and unavoidable impurities, and a core material made of ductile cast iron cast inside the outer shell layer, integrated via an intermediate layer. This results in a work roll for a hot rolling roughing mill stand that is both wear-resistant and slip-resistant by adding appropriate amounts of C and Cr and controlling the amount of M7C3 carbide. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-161331 [Patent Document 2] Japanese Patent Application Publication No. 2020-022989 [Patent Document 3] Japanese Patent Publication No. 2020-63485 Summary of the Invention [Problem to be solved by the invention]

[0010] As described above, with the increasing demand for high-quality steel sheets and the improvement of hot rolling technology for steel sheets, the properties required for hot rolling rolls are becoming increasingly strict, and in particular, improved wear resistance is being strongly demanded. However, in conventional chemical composition design, wear resistance is generally improved by increasing the amount of carbides, and when rolls are designed to meet the demand for wear resistance, slippage and seizure tend to occur. As a result, there is a problem of an increased frequency of roll troubles.

[0011] Furthermore, the conventional hot rolling rolls described in Patent Documents 1 to 3 are insufficient in either wear resistance or slip resistance, and are unable to achieve both.

[0012] Therefore, an object of the present invention is to provide an outer layer material for a hot rolling roll that has excellent wear resistance and slip resistance and that has solved the above problems, a method for producing the same, and a composite roll for hot rolling.

[0013] Here, "excellent wear resistance" in the present invention means that the total amount of wear measured by the hot rolling wear test described below is 0.84 g or less.

[0014] In addition, in the present invention, "excellent slip resistance" means that the ratio of the friction coefficient in the first test to the friction coefficient in the fifth test (i.e., μ5th test / μ1st test), measured by the hot rolling wear test described below, is 0.78 or more and 1.10 or less.

[0015] The above hot rolling wear test will be described in detail in the examples below. [Means for solving the problem]

[0016] In order to solve the above-mentioned problems, the present inventors have conducted detailed investigations into the structure, carbides, chemical components (composition), thermal properties, wear amount and friction coefficient during testing of hot rolling rolls, as well as manufacturing conditions for hot rolling rolls by centrifugal casting.

[0017] As a result, they optimized the chemical composition so that the total amount of crystallized carbides was within a specific range, and discovered Mo- and W-based crystallized M2C carbides that can efficiently improve wear resistance even in small amounts within that chemical composition. They also found that by optimizing the amount of crystallized M2C carbides and optimizing the chemical composition and manufacturing conditions so that the eutectic cell size was within a specific range, it was possible to achieve both improved wear resistance and slip resistance.

[0018] The present invention was completed based on these findings and further investigations. That is, the gist of the present invention is as follows. [1] In mass%, C: 1.2 to 2.4%, Si: 0.1 to 2.0%, Mn: 0.1 to 2.5%, Ni: 0.5 to 7.0%, Cr: 1.5 to 8.5%, Mo: 2.5 to 18.5%, V: 1.5 to 8.5%, W: 0.4 to 4.5%, P: 0.01 to 0.08%, S: 0.001 to 0.030%, and the remainder being Fe and unavoidable impurities; And the contents of C, Mo, W, V, and Cr satisfy formula (1), And the contents of V, Cr, Mo, W, Si, Mn, and Ni satisfy the formula (2), The area ratio of all crystallized carbides is 7.5 to 20.5%, the relationship between the area ratio of all crystallized carbides and the area ratio of crystallized M2C-type carbides satisfies formula (3), and the outer layer material has a structure in which the eutectic cell size is 40 to 150 μm, A roll outer layer material for hot rolling, characterized in that a thermophysical property value expressed by the difference between the liquidus temperature and the solidus temperature satisfies formula (4). 0.4≦[%C]×((0.063[%Mo]+0.033[%W]) / (0.177[%V]+0.099[%Cr]))≦1.5 (1) 1.6≦([%V]+[%Cr]+[%Mo]+[%W]) / ([%Si]+[%Mn]+[%Ni])≦9.4 (2) 0.15≦Crystallized M2C type carbide area ratio (%) / Total crystallized carbide area ratio (%)≦0.55 (3) 200≦ΔT LS (℃)≦380 (4) Here, [%C], [%V], [%Cr], [%Mo], [%W], [%Si], [%Mn], and [%Ni] in equations (1) and (2) are the contents (mass%) of each element, and ΔT LS (°C) is the value of liquidus temperature (°C) - solidus temperature (°C). [2] A method for producing an outer layer material for a hot rolling roll according to [1], When centrifugal casting is performed on the poured molten metal having the above-mentioned component composition of the outer layer material, A, which represents the vibration acceleration of the mold during centrifugal casting, satisfies equation (5), and wherein the relationship between the pouring temperature and the mold temperature during pouring satisfies formula (6). 5≦A(mm / s 2 )≦45 (5) 7.0≦Pouring temperature (℃) / Mold temperature (℃)≦22.5 (6) [3] A composite roll for hot rolling having two layers, an outer layer and an inner layer, or three layers, an outer layer, an intermediate layer, and an inner layer, A composite roll for hot rolling, characterized in that the outer layer is made of the outer layer material for hot rolling roll according to [1]. [Effects of the Invention]

[0019] The present invention provides a hot rolling roll outer layer material having excellent wear resistance and slip resistance, a method for manufacturing the same, and a hot rolling composite roll. The present invention has the effect of improving the life of the hot rolling roll and, accordingly, improving the productivity of hot-rolled steel sheets. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a diagram showing a schematic configuration of the testing machine used in the hot rolling wear test. DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention will be described in detail below. Note that the following description shows a preferred embodiment of the present invention, but the present invention is not limited to this embodiment.

[0022] First, the reasons for limiting the component composition of the outer layer material for a hot rolling roll of the present invention (hereinafter, sometimes simply referred to as "outer layer material") will be explained. Hereinafter, unless otherwise specified, "mass %" will be simply referred to as "%".

[0023] C: 1.2 to 2.4% Carbon combines with V, Cr, Mo, W, etc. to form hard crystallized carbides, thereby protecting the matrix structure and contributing to improved wear resistance. Here, hard crystallized carbides refer to MC-type carbides, M2C-type carbides, and M7C3-type carbides. Furthermore, carbon has the effect of improving the matrix hardness by solid solution strengthening. If the carbon content is less than 1.2%, the amount of crystallized carbides is insufficient, making it impossible to obtain excellent wear resistance. On the other hand, if the carbon content exceeds 2.4%, excessive crystallized carbides are formed, resulting in excessively high hardness, which reduces the roll surface roughness and the coefficient of friction, making slippage more likely. Therefore, the carbon content is limited to 1.2% or more and 2.4% or less. The carbon content is preferably 1.3% or more, more preferably 1.4% or more. The carbon content is preferably 2.3% or less, more preferably 2.2% or less.

[0024] Si: 0.1 to 2.0% Si acts as a deoxidizer in the molten metal, improving the fluidity of the molten metal and preventing casting defects. If the Si content is less than 0.1%, the deoxidizing effect is insufficient. On the other hand, if the Si content exceeds 2.0%, the deoxidizing effect saturates. Therefore, the Si content is limited to 0.1% or more and 2.0% or less. The Si content is preferably 0.2% or more, and more preferably 0.4% or more. Furthermore, the Si content is preferably 1.8% or less, and more preferably 1.6% or less.

[0025] Mn: 0.1 to 2.5% Mn has the effect of fixing and removing S, which has a negative effect on products, as MnS. If the Mn content is less than 0.1%, the effect of fixing S as MnS is not observed. On the other hand, if the Mn content exceeds 2.5%, this effect saturates. Therefore, the Mn content is limited to 0.1% or more and 2.5% or less. The Mn content is preferably 0.2% or more, and more preferably 0.3% or more. Furthermore, the Mn content is preferably 2.4% or less, and more preferably 2.2% or less.

[0026] Ni: 0.5 to 7.0% Ni is an element that improves the hardenability of the matrix, and has the effect of improving the matrix hardness and wear resistance. If the Ni content is less than 0.5%, the effect of improving the matrix hardness is insufficient. On the other hand, if the Ni content exceeds 7.0%, the hardness decreases because it promotes the retention of austenite. Therefore, the Ni content is limited to 0.5% or more and 7.0% or less. The Ni content is preferably 0.8% or more, and more preferably 1.0% or more. Furthermore, the Ni content is preferably 6.0% or less, and more preferably 5.0% or less.

[0027] Cr: 1.5 to 8.5% Cr is a carbide-forming element and combines with C to form M7C3 carbides. M7C3 carbides are hard carbides and therefore have the effect of improving wear resistance. If the Cr content is less than 1.5%, the amount of M7C3 carbides is insufficient, resulting in reduced wear resistance. On the other hand, if the Cr content exceeds 8.5%, coarse M7C3 carbides are formed, which actually worsens wear resistance. In addition, the surface roughness of the roll becomes smaller, which reduces the coefficient of friction during testing and makes slippage more likely. Therefore, the Cr content is limited to 1.5% or more and 8.5% or less. The Cr content is preferably 2.5% or more, more preferably 3.0% or more. The Cr content is also preferably 8.0% or less, more preferably 7.5% or less.

[0028] Mo: 2.5 to 18.5% Mo is a carbide-forming element and combines with C to form M2C-type carbides. M2C-type carbides are harder than M7C3-type carbides and crystallize in a planar and lamellar shape. Even a small amount of M2C-type carbides effectively improves wear resistance. If the Mo content is less than 2.5%, the amount of M2C-type carbides is insufficient, resulting in an insufficient effect on improving wear resistance. On the other hand, if the Mo content exceeds 18.5%, coarse lamellar M2C-type carbides are formed, which can cause chipping and thus worsen wear resistance and reduce toughness. Therefore, the Mo content is limited to 2.5% or more and 18.5% or less. The Mo content is preferably 3.0% or more, more preferably 3.5% or more. The Mo content is also preferably 17.0% or less, more preferably 16.0% or less.

[0029] V: 1.5 to 8.5% V is a carbide-forming element and combines with C to form MC-type carbides. MC-type carbides have a Vickers hardness of approximately 2800 Hv, making them one of the hardest carbides and effective in improving wear resistance. If the V content is less than 1.5%, the amount of MC-type carbides is insufficient, resulting in insufficient wear resistance. On the other hand, if the V content exceeds 8.5%, VC, which has a lower specific gravity than molten iron, is concentrated inside the outer layer material (note that in the case of a composite roll described below, this will be referred to as the "outer layer"; the same applies hereinafter) due to the centrifugal force during centrifugal casting, causing segregation. Therefore, the V content is limited to 1.5% or more and 8.5% or less. The V content is preferably 2.0% or more, more preferably 2.5% or more. The V content is also preferably 7.0% or less, more preferably 6.5% or less.

[0030] W: 0.4 to 4.5% W is a carbide-forming element, and like Mo, it combines with C to form hard carbides such as hard M2C carbides, thereby improving wear resistance. If the W content is less than 0.4%, this effect is insufficient, and wear resistance deteriorates. On the other hand, if the W content exceeds 4.5%, coarse M2C eutectic carbides are formed, which actually deteriorates wear resistance. Therefore, the W content is limited to 0.4% or more and 4.5% or less. The W content is preferably 0.6% or more, and more preferably 0.8% or more. Furthermore, the W content is preferably 4.0% or less, and more preferably 3.5% or less.

[0031] P: 0.01 to 0.08% It has been thought that P is mixed in during the roll manufacturing process and reduces mechanical properties, but as a result of extensive research by the present inventors, it has been found that the inclusion of a small amount of P has the effect of improving wear resistance. If the P content is less than 0.01%, the effect of improving wear resistance is insufficient, while if the P content exceeds 0.08%, mechanical properties deteriorate. Therefore, the P content is limited to 0.01% or more and 0.08% or less. The P content is preferably 0.02% or more. Furthermore, the P content is preferably 0.06% or less.

[0032] S: 0.001 to 0.030% S is usually considered a harmful element in iron-based alloys and its content is limited to a certain amount, but MnS has a lubricating effect within that range. Furthermore, as a result of extensive research by the present inventors, it has been revealed that a small amount of MnS has the effect of improving wear resistance. On the other hand, a high S content makes the material brittle. Therefore, the S content is limited to 0.001% or more and 0.030% or less. The S content is preferably 0.002% or more. Furthermore, the S content is preferably 0.025% or less.

[0033] In the present invention, the contents of C, Cr, Mo, V, W, Ni, Mn, and Si are within the above ranges, and furthermore, the following formulas (1) and (2) are satisfied. 0.4≦[%C]×((0.063[%Mo]+0.033[%W]) / (0.177[%V]+0.099[%Cr]))≦1.5 (1) 1.6≦([%V]+[%Cr]+[%Mo]+[%W]) / ([%Si]+[%Mn]+[%Ni])≦9.4 (2) Here, [%C], [%V], [%Cr], [%Mo], [%W], [%Si], [%Mn], and [%Ni] shown in formulas (1) and (2) are the contents (mass%) of each element.

[0034] The parameter "[%C] × ((0.063[%Mo] + 0.033[%W]) / (0.177[%V] + 0.099[%Cr]))" in formula (1) indicates the ratio of the content of M2C carbide-forming elements (Mo, W) to the content of other carbide-forming elements (V, Cr). By adjusting this parameter to satisfy formula (1), the proportion of crystallized M2C carbide is optimized, resulting in improved wear resistance and slip resistance.

[0035] If the value of "[%C] × ((0.063 [%Mo] + 0.033 [%W]) / (0.177 [%V] + 0.099 [%Cr]))" in formula (1) is less than 0.4, the proportion of crystallized M2C carbides will be insufficient, resulting in reduced wear resistance. On the other hand, if this value exceeds 1.5, the proportion of planar and lamellar crystallized M2C carbides will be too high, causing stress to concentrate on the M2C carbides, which have low toughness. This will cause cracks due to the M2C carbides, which will then chip, thereby worsening wear resistance. Therefore, the value of "[%C] × ((0.063 [%Mo] + 0.033 [%W]) / (0.177 [%V] + 0.099 [%Cr]))" in formula (1) is limited to 0.4 or more and 1.5 or less. The value is preferably 0.6 or more and preferably 1.2 or less.

[0036] The parameter "([%V] + [%Cr] + [%Mo] + [%W]) / ([%Si] + [%Mn] + [%Ni])" in formula (2) indicates the ratio of carbide-forming elements (V, Cr, Mo, W) to non-carbide-forming elements (Si, Mn, Ni). By adjusting this parameter to satisfy formula (2), the amount and size of carbides formed are optimized, improving wear resistance.

[0037] If the value of "([%V] + [%Cr] + [%Mo] + [%W]) / ([%Si] + [%Mn] + [%Ni])" in formula (2) is less than 1.6, the amount of carbide formed is insufficient and sufficient wear resistance cannot be obtained. On the other hand, if this value exceeds 9.4, the carbides become coarse, and wear resistance actually decreases. Therefore, the value of "([%V] + [%Cr] + [%Mo] + [%W]) / ([%Si] + [%Mn] + [%Ni])" in formula (2) is limited to 1.6 or more and 9.4 or less. This value is preferably 1.8 or more and preferably 9.0 or less.

[0038] Remainder: Fe and unavoidable impurities The balance other than the above components consists of Fe and unavoidable impurities.

[0039] Examples of unavoidable impurities include Mg, Zr, REM (rare earth metals), Sn, As, Sb, Bi, Pb, Zn, N, and O. However, within the scope of the present invention, when one or more of the above elements are contained, Mg, Zr, and REM may each be contained in an amount of 0.03% or less, Sn, As, and Sb in an amount of 0.04% or less, and Bi, Pb, Zn, N, and O in an amount of 0.01% or less, as long as the effects of the present invention are not impaired. The content of the above elements may be 0%. Note that REM is a collective term for 17 elements, including Sc, Y, and lanthanoid elements. One or more of these 17 elements may be contained as unavoidable impurities, and the REM content refers to the total content of these elements.

[0040] Next, the reasons for limiting the structure of the outer layer material for hot rolling rolls of the present invention will be explained.

[0041] The following is known from past research results: The larger the area ratio of crystallized carbides, the more hard carbides there are on the contact surface of the roll surface. As a result, even if the roll matrix wears, the carbides around the matrix, which are several times harder than the matrix, remain on the contact surface and support the contact load. This suppresses further wear of the matrix, and therefore tends to improve wear resistance. On the other hand, it is said that the larger the area ratio of crystallized carbides, the harder they are and the less susceptible they are to wear, which reduces surface roughness and makes slip more likely to occur. However, the relationship between slip and roll surface properties has mostly been discussed when there is no scale on the roll surface.

[0042] Here, we consider the rolls used in the hot rolling of steel sheets as an example. In a hot rolling mill, rolls come into contact with a high-temperature slab, and therefore scale adheres to the roll surface. When scale adheres to the roll surface, it is not the roll material structure that comes into direct contact with the steel sheet, but the scale adhered to the roll material surface. Therefore, when discussing the contact surface during rolling in an actual hot rolling environment, it is important to consider the presence of scale on the roll surface and the surface quality when the scale adheres. However, the relationship between the scale on the roll surface and the roll structure and surface quality has not been discussed much up to now.

[0043] Therefore, the present inventors have conducted extensive research into the relationship between the scale on the roll surface and the roll structure and surface properties, and have discovered the following.

[0044] Specifically, it is important that the outer layer material for a hot rolling roll of the present invention, which has a component composition within the above-mentioned range, has a structure in the range (region) from the outer surface of the outer layer material to the center of the wall thickness such that the total crystallized carbides are present in an area ratio of 7.5 to 20.5%.

[0045] [Total crystallized carbide area ratio: 7.5~20.5%] "All crystallized carbides" refers to MC, M2C, and M7C3 carbides. Among the crystallized carbide types, M2C carbides are less likely to oxidize, resulting in poor adhesion to scale transferred from the rolled material. The inventors have confirmed that the difference in height between the matrix structure, where scale is easily attached, and the crystallized M2C carbides, where scale is less likely to attach, forms surface irregularities, contributing to improved slip resistance. The MC carbides have properties between those of the matrix structure and M2C carbides.

[0046] If the area ratio of all crystallized carbides is less than 7.5%, there will be a shortage of hard carbides, resulting in poor wear resistance. Therefore, the area ratio of all crystallized carbides is set to 7.5% or more. The area ratio of all crystallized carbides is preferably 8.0% or more. On the other hand, if the area ratio of all crystallized carbides exceeds 20.5%, coarse carbides will be generated, and the coarse carbides with low toughness will chip off, thereby worsening wear resistance. Therefore, the area ratio of all crystallized carbides is set to 20.5% or less. The area ratio of all crystallized carbides is preferably 19.5% or less.

[0047] Furthermore, it is also important that the outer layer material structure of the roll of the present invention has an area ratio of all crystallized carbides within the above range, and that the relationship between the area ratio of all crystallized carbides and the area ratio of crystallized M2C type carbides satisfies the following formula (3). 0.15≦Crystallized M2C type carbide area ratio (%) / Total crystallized carbide area ratio (%)≦0.55 (3) The relationship shown in equation (3) defines the ratio between the area fraction of all crystallized carbides and the area fraction of crystallized M2C carbides. If the value of "area fraction of crystallized M2C carbides (%) / area fraction of all crystallized carbides (%)" shown in equation (3) is less than 0.15, the amount of crystallized M2C carbides will be insufficient, resulting in reduced wear resistance. In addition, the roll surface will have less unevenness, making slippage more likely to occur.

[0048] On the other hand, if the value exceeds 0.55, the amount of crystallized M2C carbides becomes excessive, which causes excessive unevenness on the roll surface and increases the coefficient of friction during testing, but also makes it easier for cracks to occur due to the crystallized M2C carbides, thereby deteriorating wear resistance.

[0049] Therefore, the value of "area ratio of crystallized M2C type carbides (%) / area ratio of all crystallized carbides (%)" is limited to 0.15 or more and 0.55 or less, and preferably 0.18 or more and 0.50 or less.

[0050] Thus, contrary to conventional knowledge, the inventors have confirmed that even when the area fraction of crystallized carbides is small, the morphology of the planar crystallized M2C carbides contributes to improved wear resistance as long as the ratio of the area fraction of the crystallized M2C carbides to the area fraction of all crystallized carbides is controlled to an appropriate amount. Furthermore, since the crystallized M2C carbides are resistant to oxidation and have low adhesion to the scale of the rolled material, the crystallized M2C carbides are concave compared to the matrix structure, etc., and a difference in height occurs on the roll surface. It has also been confirmed that when the density of the concaves and convexes on the roll surface is appropriate, slip resistance is improved.

[0051] Furthermore, it is important that the structure of the outer layer material of the roll of the present invention ranging from the outer surface to the center of the wall thickness satisfies the above-mentioned requirements for the total crystallized carbide area ratio and the crystallized M2C type carbide area ratio, and also that the eutectic cell size is 40 μm or more and 150 μm or less.

[0052] [Eutectic cell size: 40~150μm] "Eutectic cells" refer to a eutectic consisting of a matrix and eutectic carbides, where the matrix is ​​surrounded by eutectic carbides, or the eutectic carbide network opens up, solidifying into a cellular colony structure. It has been confirmed that when scale forms on the roll surface, the smaller the eutectic cell size (also referred to as the "distance between eutectic carbides"), the higher the friction coefficient. Because the adhesion of the matrix, MC carbides, M2C carbides, and M7C3 carbides to the scale of the rolled material varies, each structure generates a height difference (unevenness) on the roll surface. When the eutectic cell size is less than 40 μm, the unevenness on the roll surface becomes minute, increasing the friction coefficient during testing and making seizure more likely. On the other hand, when the eutectic cell size exceeds 150 μm, the unevenness on the roll surface becomes rough, decreasing the friction coefficient and making slippage more likely.

[0053] Therefore, from the viewpoint of seizure resistance and slip resistance, the eutectic cell size is set to 40 μm or more and 150 μm or less, preferably 50 μm or more and 120 μm or less.

[0054] This eutectic cell size can be controlled by the thermal properties of the material, which will be described later, and the vibration acceleration during centrifugal casting in the manufacturing method.

[0055] Next, the method for observing the structure will be described below.

[0056] [Area ratio of total crystallized carbides] First, specimens were cut from the outer surface and the center of the wall thickness (Z-face) of the obtained outer layer material. The specimens were mirror-polished and etched with nital solution. The structure of the outer surface and the center of the wall thickness (Z-face) of the roll outer layer material were then observed using a digital microscope, and the area ratio of all crystallized carbides was measured using image analysis. Next, images were binarized at 200x magnification using ImageJ as an image analysis tool. Because the brightness of the matrix and carbides in the photographs differed, binarization allowed for classification of the matrix and carbides and the area of ​​each to be determined. Here, 10 photographs were taken of the polished and etched surface of each specimen. The average area ratio of all crystallized carbides was calculated for each specimen, with the entire image (roll structure) taken as 100%. This average value was taken as the "area ratio (%) of all crystallized carbides" for the area from the outer surface to the center of the wall thickness of the roll outer layer material.

[0057] [Area ratio of crystallized M2C type carbides] To calculate the area fraction of the crystallized M2C carbides, the same specimens as described above were simultaneously measured for the crystal orientations of the carbides and the matrix using SEM-EBSD. Images of the crystallized M2C carbides were then separated based on the differences in their crystal structures. The SEM-EBSD measurement conditions were an acceleration voltage of 15 kV, a magnification of 200x, a measurement range of 600 × 1100 μm, and a step size of 0.5 μm. The crystal orientation of the crystallized M2C carbides was analyzed using the Mo2C database in the OIM Analysis 7 software (TSL). Iron (alpha) was used to represent the crystal orientation of the matrix structure, and Iron (gamma) was used to represent the crystal orientation of the MC carbides. ImageJ was used as an image analysis tool to measure the area fraction of the crystallized M2C carbides alone. As described above, the average area fraction was calculated from 10 fields of view. This average value is defined as the "area ratio (%) of crystallized M2C type carbides" in the range from the outer surface of the outer layer material of the roll to the center of the wall thickness.

[0058] [Eutectic cell size] Samples were cut from the outer surface and the center of the wall thickness (Z-plane) of the obtained outer layer material. The samples were mirror-polished and etched with nital solution. The structure of the outer surface and the center of the wall thickness (Z-plane) of the roll outer layer material were then observed using a digital microscope, and the eutectic cell size was measured. Here, the diameter of the eutectic cell was measured from a digital microscope image taken at a measurement magnification of 100x. In this example, 10 fields of view were photographed from the sample, and a total of six lines were drawn randomly vertically and / or horizontally in each obtained image. As shown in Equation (6) below, the eutectic cell size in one image was calculated using the "length of the i-th line (Li)," the "number of intersections between the i-th line and the eutectic cell (Ni)," and the "number of lines."

[0059] Specifically, the length of the first line (L1) was first divided by the number of intersections (N1) between the first line and eutectic carbides with a particle size exceeding 5.0 μm, and then the length of the second line (L2) was divided by the number of intersections (N2) between the second line and eutectic carbides with a particle size exceeding 5.0 μm. This process was repeated for the number of lines (6). The sum of the obtained values ​​divided by the total number of lines was determined as the "eutectic cell size." Here, the average value of the eutectic cell sizes obtained in this way for 10 fields of view was used. This average value was used as the "eutectic cell size (μm)" in the range from the outer surface of the roll outer layer material to the center of the wall thickness.

[0060] The eutectic cell size is determined by the above method, taking advantage of the fact that eutectic carbides with a grain size exceeding 5.0 μm exist at the boundary of the eutectic cell.

[0061] Eutectic cell size = ((L1 / N1) + (L2 / N2) + (L3 / N3) + (L4 / N4) + (L5 / N5) + (L6 / N6)) / 6 (6) In formula (6), Li is the length of the i-th line, and Ni is the number of intersections between the i-th line and eutectic carbides with particle sizes exceeding 5.0 μm, where i is a natural number from 1 to 6.

[0062] Next, the reasons for limiting the physical property values ​​of the outer layer material for hot rolling rolls of the present invention will be explained.

[0063] The roll of the present invention has a ΔT value representing the thermal property of the outer layer material having the component composition and structure within the above ranges. LS However, it is important to satisfy the following equation (4). 200≦ΔT LS ≦380 (4) Here, ΔT shown in equation (4) LS (°C) is the value of liquidus temperature (°C) - solidus temperature (°C).

[0064] Equation (4) shows the relationship between the difference between the liquidus temperature and solidus temperature of a material (hereinafter, the value of "liquidus temperature (°C) - solidus temperature (°C)" will be referred to as "ΔT (unit: °C)"), and represents the temperature range from the start to the end of solidification. ΔT LS If ΔT is less than 200°C, the temperature range from the start of solidification to the end of solidification is narrow, which significantly suppresses the growth of the eutectic cell size, resulting in a smaller eutectic cell size than the above range. LS When ΔT exceeds 380°C, the temperature range from the start to the end of solidification is wide, which causes the growth of eutectic cells to become significant, resulting in a larger eutectic cell size than the above range. LS is between 200℃ and 380℃. LS is preferably 220°C or higher and preferably 350°C or lower.

[0065] Next, the method for measuring the liquidus temperature and solidus temperature will be described below.

[0066] A sample (φ5 x 5 mm) cut from the center of the outer layer material was measured using an ULVAC-RIKO differential thermogravimetric analyzer (TGD-9800). The temperature was raised from 1000°C to 1530°C, and then cooled from 1530°C to 1000°C. An alumina container was used as the measurement container, and the temperature was raised at a rate of 5°C / min in an Ar atmosphere. The liquidus and solidus temperatures were measured by DTA. Each temperature was the average of three samples.

[0067] According to the roll of the present invention having the above-described configuration, it is possible to provide an outer layer material for a hot rolling roll that has both excellent wear resistance and excellent slip resistance. In order to further improve the properties, the roll may further have the following configuration.

[0068] [Shore hardness] (optimal conditions) The hardness of the outer layer material for hot rolling rolls of the present invention is preferably 44.0 HS or more and 52.0 HS or less in Shore hardness at 600°C, and 74.0 HS or more and 86.0 HS or less in Shore hardness at 20°C.

[0069] Because the roll surface temperature during hot rolling is around 600°C, if the Shore hardness at 600°C is less than 44.0HS, plastic flow occurs and the steel material is likely to seize onto the roll surface. On the other hand, if the Shore hardness at 600°C exceeds 52.0HS, the roll hardness is too high and slippage is likely to occur during hot rolling.

[0070] If the Shore hardness at 20°C is less than 74.0 HS, the roll hardness is insufficient, resulting in poor wear resistance. On the other hand, if the Shore hardness at 20°C is more than 86.0 HS, the roll hardness is too high, resulting in a significant increase in roll grinding time.

[0071] Such hardness can be reliably ensured by heat treating a roll having the components of the present invention so that the tempering parameter P, which will be described later, falls within the range of 13,000 to 18,000.

[0072] Next, one embodiment of the method for producing an outer layer material for a hot rolling roll of the present invention will be described.

[0073] In the method for producing the outer layer material for a hot rolling roll of the present invention, a molten metal having the component composition of the outer layer material for a hot rolling roll (hereinafter simply referred to as "molten metal for outer layer material") is poured into a rotating mold whose inner surface is coated with a refractory material mainly made of zircon or the like to a thickness of 1 to 5 mm, so as to obtain a predetermined thickness, and centrifugal casting is performed.

[0074] In order to make the eutectic cell size 40 to 150 μm in the outer layer constituting the hot rolling composite roll (i.e., the outer layer material of the hot rolling roll), centrifugal casting is performed under the conditions that A, which represents the vibration acceleration of the mold during centrifugal casting, satisfies the formula (5), and the relationship between the pouring temperature and the mold temperature during pouring satisfies the formula (6). 5≦A(mm / s 2 )≦45 (5) 7.0≦Pouring temperature (℃) / Mold temperature (℃)≦22.5 (6) [Vibration acceleration of the mold during centrifugal casting: 5 mm / s 2 ≦A≦45mm / s 2 〕 The vibration can be measured by the following method.

[0075] First, acceleration sensors are installed in the center of the mold in the longitudinal and width directions to measure vibration during centrifugal casting. There are no particular restrictions on the method for generating vibration and controlling the vibration acceleration within an appropriate range, but in this case, vibration is controlled by partially changing the mold material and shifting the center of gravity from the axis by 2 to 4%. Applying appropriate vibration accelerates the solidification of the molten metal, which in turn reduces the size of the eutectic cells, resulting in the above-mentioned structure with excellent wear resistance.

[0076] The vibration acceleration of the mold during centrifugal casting of the outer layer material for hot rolling rolls is 5 mm / s 2 If the vibration acceleration is less than 5 mm / s, the increase in the solidification rate will be insufficient, and as a result, the effect of refining the eutectic cell size will not be obtained. 2 From the viewpoint of miniaturizing the eutectic cell size, the vibration acceleration is preferably 8 mm / s 2 More preferably, 10 mm / s or more 2 That's all.

[0077] On the other hand, if the vibration acceleration is 45 mm / s 2 If the vibration acceleration is more than 45mm / s, casting defects will occur due to vibration, and the roll quality will worsen. 2 From the viewpoint of roll quality, the vibration acceleration is preferably 42 mm / s2 less than 40 mm / s, and more preferably 2 The following is the result.

[0078] [Pouring temperature (℃) / Mold temperature (℃): 7.0 to 22.5] If the ratio of the pouring temperature to the mold temperature when pouring the outer layer material for a hot rolling roll (i.e., pouring temperature (°C) / mold temperature (°C)) is less than 7.0, the temperature difference between the molten metal of the outer layer material and the mold (metal mold) is small, so the solidification rate does not increase sufficiently, and as a result, the effect of refining the eutectic cell size cannot be obtained. Therefore, the ratio of the pouring temperature to the mold temperature when pouring is set to 7.0 or more. From the viewpoint of the effect of refining the eutectic cell size, the ratio of the pouring temperature to the mold temperature when pouring is preferably 8.0 or more, and more preferably 9.0 or more.

[0079] On the other hand, if the ratio of the pouring temperature to the mold temperature during pouring exceeds 22.5, the eutectic cell size becomes excessively fine, and the irregularities on the roll surface become too fine, resulting in a high coefficient of friction during testing. As a result, seizure becomes more likely to occur. Therefore, the ratio of the pouring temperature to the mold temperature during pouring is set to 22.5 or less. From the perspective of preventing seizure due to an increase in the coefficient of friction, the ratio of the pouring temperature to the mold temperature during pouring is preferably 20.5 or less, and more preferably 18.5 or less.

[0080] The pouring temperature was measured using a thermocouple on the surface of the molten metal in the ladle before pouring. The mold temperature was measured at the center of the mold surface using a thermocouple.

[0081] The inventors have found that by adjusting the above-mentioned "pouring temperature / mold temperature" and controlling the solidification rate, it is possible to change the area fraction of crystallized carbides, and in particular to control the area fraction of the crystallized MC carbides that crystallize first. This control method makes it possible to control the "total area fraction of crystallized carbides (%)" and "area fraction of crystallized M2C carbides (%) / area fraction of total crystallized carbides (%)" within appropriate ranges.

[0082] In the present invention, the above-mentioned effects can be obtained by centrifugal casting under the above-mentioned casting conditions, and as a result, the above-mentioned structure having excellent wear resistance and slip resistance can be obtained.

[0083] It is desirable to carry out a heat treatment after centrifugal casting.

[0084] For example, as a heat treatment, it is preferable to carry out a quenching treatment in which the centrifugal cast hot rolling roll outer layer material is heated to a temperature of 900 to 1100°C, followed by air cooling or air blast cooling, and then a tempering treatment in which the material is heated and held so that the value of the tempering parameter (P) expressed by equation (7) is within the range of 13000 to 18000, followed by cooling, twice or more times. In this case, the quenching temperature, tempering parameters, and number of tempering times can be changed within the ranges described depending on the components to obtain the above-mentioned structure.

[0085] P = T(log(t) + A) (7) In equation (7), T is the tempering temperature (K), t is the tempering time (h), and A is a constant (A=20 in the present invention).

[0086] The temperature during the heat treatment is the internal temperature at the center of the roll material, 5 mm below the surface of the roll material, measured by embedding a thermocouple.

[0087] Next, the composite roll for hot rolling and the method for producing the same of the present invention will be described.

[0088] The hot rolling composite roll of the present invention (hereinafter sometimes simply referred to as "composite roll") has two layers, an outer layer and an inner layer, or three layers, an outer layer, an intermediate layer, and an inner layer. The outer layer of the composite roll is made of the hot rolling outer layer material of the present invention, which has the above-mentioned component composition and structure and exhibits the above-mentioned thermal property values. As a result, a composite roll with improved wear resistance and slip resistance can be obtained, as described above. The component composition, structure, and thermal property values ​​of the hot rolling outer layer material have already been explained, so they will not be described here.

[0089] Next, a preferred example of the method for producing the composite roll for hot rolling of the present invention will be described.

[0090] In the method for producing a hot rolling composite roll of the present invention, first, a hot rolling roll outer layer material is cast by centrifugal casting under the above-mentioned casting conditions. Note that the method for producing the hot rolling roll outer layer material that becomes the outer layer has already been explained, so a detailed explanation will be omitted.

[0091] Next, an inner layer is formed inside the outer layer formed by centrifugal casting. Alternatively, instead of the inner layer being integrated with the outer layer by welding, an intermediate layer may be formed between the outer layer and the inner layer.

[0092] As described above, in one embodiment, when a two-layer structure consisting of an outer layer and an inner layer is used, the composite roll for hot rolling of the present invention has a centrifugally cast outer layer and an inner layer welded and integrated with the outer layer. In this case, the inner layer is preferably produced by static casting. For example, after the outer layer material has completely solidified, the rotation of the mold is stopped, the mold is erected, and then a molten metal having the composition of the inner layer material is poured into the mold for static casting. This remelts the inner surface side of the outer layer material, resulting in a composite roll in which the outer layer and inner layer are welded and integrated.

[0093] For the inner layer to be statically cast, it is preferable to use spheroidal graphite cast iron, worm-shaped graphite cast iron (CV cast iron), or the like, which has excellent castability and mechanical properties, as the inner layer material. The reason for this is as follows: In a centrifugal cast roll, the outer layer and the inner layer are welded together, and components of the outer layer material are mixed into the inner layer. If carbide-forming elements such as Cr and V contained in the outer layer material are mixed into the inner layer, the inner layer will become brittle. For this reason, it is preferable to minimize the mixing rate of these elements into the outer layer components.

[0094] In another embodiment, when a three-layer structure is formed by disposing an intermediate layer between an outer layer and an inner layer, the composite roll for hot rolling of the present invention has a centrifugally cast outer layer, an intermediate layer welded and integrated with the outer layer, and an inner layer welded and integrated with the intermediate layer. In this case, the intermediate layer is preferably produced by centrifugal casting. For example, during or after the outer layer material has solidified completely, a molten metal having a composition for the intermediate layer material is poured into a rotating mold and centrifugal cast. Thereafter, a molten metal having a composition for the inner layer material is statically cast using the same static casting method as described above. This remelts the inner surface of the outer layer material, resulting in a composite roll in which the outer layer and intermediate layer are welded and integrated with the intermediate layer and inner layer.

[0095] When forming an intermediate layer, it is preferable to use graphite steel, high-carbon steel, hypoeutectic cast iron, etc. as the intermediate layer material. The reason for this is as follows: the intermediate layer and the outer layer are welded together, and components of the outer layer material are mixed into the intermediate layer. Also, since the intermediate layer and the inner layer are welded together, it is preferable to minimize the mixing rate of the outer layer material into the intermediate layer in order to suppress the mixing rate of the outer layer material into the inner layer via the intermediate layer.

[0096] Through the above manufacturing steps, the composite roll for hot rolling of the present invention having three layers, an outer layer, an intermediate layer and an inner layer, or two layers, an outer layer and an inner layer, can be obtained.

[0097] In the method for producing a composite roll for hot rolling of the present invention, it is desirable to subject the obtained composite roll for hot rolling to the same heat treatment as described above. [Example]

[0098] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples.

[0099] First, a molten metal having the chemical composition of the outer layer material for a hot rolling roll shown in Table 1 was melted in a high-frequency melting furnace. Next, a ring-shaped test material (outer diameter: φ250 mm, width: 65 mm, thickness: 55 mm) was produced by centrifugal casting using the vibration acceleration during centrifugal casting and the ratio of the pouring temperature to the mold temperature during pouring shown in Table 2. The balance other than the chemical composition shown in Table 1 was Fe and unavoidable impurities.

[0100] Next, a heat treatment was performed. Specifically, after cooling, the ring-shaped test material was removed from the mold and quenched at 900 to 1100°C. Subsequently, the ring-shaped test material was subjected to a tempering treatment three times, in which the ring-shaped test material was heated and held thereafter cooled, so that the tempering parameter P expressed by the above formula (7) was within the range of 13,000 to 18,000.

[0101] Thereafter, for each of the obtained ring-shaped test pieces, (1) microstructural observation, (2) evaluation of wear volume and measurement of friction coefficient by hot rolling wear test, and (3) measurement of liquidus temperature and solidus temperature were performed by the methods described below. Note that the test pieces for microstructural observation and the test pieces for hot rolling wear test were taken from the center in the longitudinal direction of each ring-shaped test piece and the center in the plate thickness direction (hereinafter referred to as "center of thickness").

[0102] As described above, the composite roll for hot rolling of the present invention uses the above outer layer material for hot rolling roll as the outer layer of the composite roll, and therefore is evaluated in the same manner as the outer layer material in this example.

[0103] [Table 1]

[0104] (1) Microstructure observation Using each of the heat-treated ring-shaped test pieces, the "area ratio of all crystallized carbides," "area ratio of crystallized M2C-type carbides," and "eutectic cell size" were measured using the above-mentioned measurement methods, and the values ​​are shown in Table 2. In the present invention, the area ratio of all crystallized carbides must be the above-mentioned range from the outer layer surface to the center of the wall thickness, and therefore, in this example, the structure of the center of the wall thickness is observed as a representative example.

[0105] (2) Hot rolling wear test The hot rolling wear test method was as follows.

[0106] Test specimens (outer diameter: 60 mm, width: 10 mm, C1 chamfered) for hot rolling wear tests were prepared from each of the ring-shaped test pieces. The hot rolling wear tests were performed using a two-disk sliding rolling method, as shown in Figure 1. Test specimen 1 was rotated at 700 rpm while being cooled with cooling water 2. A counterpiece 4 (material: S45C, outer diameter: 190 mm, width: 15 mm, C1 chamfered) heated to 800°C by a high-frequency induction heating coil 3 was placed against the rotating test specimen 1, and a load of 686 N was applied in the load direction 7 indicated by the thick arrow in Figure 1, causing the test specimens to roll while in contact with each other. The rotation direction 5 of test specimen 1 and the rotation direction 6 of counterpiece 4 were set so that the tangent at the contact point between test specimen 1 and counterpiece 4 was aligned. The abrasion test was carried out for 450 minutes, and the mating piece was replaced with a new one every 45 minutes (31,500 rotations of the test piece), for a total of five tests (total number of rotations of the test piece: 157,500 rotations). The total amount of wear per test (i.e., the amount of wear per 157,500 rotations of the test piece (unit: g)) was calculated after five tests. The results are shown in the "Total amount of wear (g) / 5 times" column in Table 2.

[0107] The friction coefficient was measured using a hot rolling wear tester as follows.

[0108] Similar to the hot rolling wear test, a test specimen (outer diameter: 60 mm, width: 10 mm, C1 chamfered) was prepared for the hot rolling wear test. The wear test was performed using a two-disk sliding rolling method, as shown in Figure 1. Test specimen 1 was rotated at 76 rpm while being cooled with cooling water 2. A counterpiece 4 (material: S45C, outer diameter: 190 mm, width: 15 mm, C1 chamfered) heated to 1000°C by a high-frequency induction heating coil 3 was placed in contact with the rotating test specimen 1 and rolled against it under a load of 686 N in the load direction 7. The rotation direction 5 of test specimen 1 and the rotation direction 6 of counterpiece 4 were set so that the tangent at the contact point between test specimen 1 and counterpiece 4 was aligned. The counterpiece 4 was replaced with a new one every hour, and the hot rolling wear test was repeated. This was repeated a total of five times, and the hot rolling wear test was carried out for a total of five hours on the test piece 1. The torque and load were measured during the test, and the friction coefficient was calculated from the following formula (8).

[0109] Furthermore, the ratio of the average value of the friction coefficients obtained in the first test (i.e., the first hour of the hot rolling wear test) for the last minute before the end of the test to the average value of the friction coefficients obtained in the fifth test (i.e., the last hour of the hot rolling wear test) for the last minute before the end of the test was calculated, i.e., "the above average value of the friction coefficients in the fifth test (μ5th test) / the above average value of the friction coefficients in the first test (μ1st test)." Two sets of hot rolling wear tests for a total of five hours were conducted for each ring-shaped test material (test piece 1), and the average value of the above ratio obtained in each hot rolling wear test was calculated.

[0110] The results are shown in the "(μ5th / μ1st)" column of Table 2.

[0111] μ = T / P × L (8) Here, μ is the friction coefficient, T is the torque (kgf m), P is the load (kgf), and L is the radius of the test piece (m) in equation (8).

[0112] Since the above-mentioned hot rolling wear test is a test that simulates continuous hot rolling operation, the mating piece 4 is heated to 1000°C, and the friction coefficient and seizure state are evaluated after 1 to 5 hours. The presence or absence of seizure was determined by visually inspecting the surface of the test piece 1 after 5 hours, and cases where there was transfer of the mating piece 4 material were recorded as "seizure present," and cases where there was no transfer were recorded as "seizure absent," and these are shown in the "seizure" column in Table 2.

[0113] Here, the evaluation was as follows: The value of "Total abrasion amount (g) / 5 times" in Table 2, which indicates the amount of abrasion, was rated as "pass" (i.e., excellent abrasion resistance) if it was 0.84 g or less, and as "fail" if it exceeded 0.84 g.

[0114] Furthermore, the value of "μ5th / μ1st" in Table 2, which represents the ratio of the coefficients of friction, was rated as "pass" when it was 0.78 or more, and as "fail" when it was less than 0.78. Furthermore, since excessive coefficients of friction can cause seizure, the ratio of the coefficients of friction was rated as "pass" when it was 1.10 or less, and as "fail" when it was more than 1.10. In other words, when the ratio of the coefficients of friction was within the range of 0.78 to 1.10, it was evaluated as having excellent wear resistance.

[0115] (3) Liquidus temperature, solidus temperature Using samples cut out from the outer layer material of each heat-treated ring-shaped test material, the "liquidus temperature" and "solidus temperature" were measured using the above-mentioned measurement method, and the above-mentioned thermophysical property values ​​(△T LS ) was calculated. The obtained values ​​are shown in Table 2.

[0116] [Table 2]

[0117] As is clear from Table 2, it was confirmed that the examples of the present invention had both excellent wear resistance and slip resistance compared to the comparative examples.

[0118] Therefore, according to the present invention, it is possible to manufacture a hot rolling outer layer material and a hot rolling composite roll that are excellent in wear resistance and slip resistance. As a result, the life of the hot rolling roll is improved and time loss during rolling interruptions due to roll trouble is reduced, thereby improving the rolling efficiency of the hot rolling roll and improving the productivity of hot rolled steel sheets. [Explanation of symbols]

[0119] 1 test piece 2 Cooling water 3 High frequency induction heating coil 4 Counterpart 5. Rotation direction of test piece 6 Rotation direction of the mating piece 7 Load direction

Claims

1. The alloy contains, in mass%, C: 1.2 to 2.4%, Si: 0.1 to 2.0%, Mn: 0.1 to 2.5%, Ni: 0.5 to 7.0%, Cr: 1.5 to 8.5%, Mo: 2.5 to 18.5%, V: 1.5 to 8.5%, W: 0.4 to 4.5%, P: 0.01 to 0.08%, S: 0.001 to 0.030%, and the balance being Fe and unavoidable impurities; And the contents of C, Mo, W, V, and Cr satisfy formula (1), and the contents of V, Cr, Mo, W, Si, Mn, and Ni satisfy the formula (2), The area ratio of all crystallized carbides is 7.5 to 20.5%, and the area ratio of all crystallized carbides and the crystallized M ２ The relationship between the area ratio of C-type carbides satisfies formula (3), and the structure of the outer layer material has a eutectic cell size of 40 to 150 μm, A hot rolling roll outer layer material, characterized in that a thermophysical property value expressed by the difference between the liquidus temperature and the solidus temperature satisfies formula (4). 0.4≦[%C]×((0.063[%Mo]+0.033[%W]) / (0.177[%V]+0.099[%Cr]))≦1.5 (1) 1.6≦([%V]+[%Cr]+[%Mo]+[%W]) / ([%Si]+[%Mn]+[%Ni])≦9.4...(2) 0.15≦ Crystallization M ２ C-type carbide area ratio (%) / total crystallized carbide area ratio (%) ≤ 0.55 ・・・(3) 200≦ΔT ＬＳ (℃)≦380 ・・・(4) Here, [%C], [%V], [%Cr], [%Mo], [%W], [%Si], [%Mn], and [%Ni] shown in formulas (1) and (2) are the contents (mass%) of each element, and ΔT shown in formula (4) ＬＳ (°C) is the value of liquidus temperature (°C) - solidus temperature (°C).

2. A method for producing an outer layer material for a hot rolling roll according to claim 1, When centrifugal casting is performed on the poured molten metal having the above-mentioned component composition of the outer layer material, A, which represents the vibration acceleration of the mold during centrifugal casting, satisfies equation (5), and wherein the relationship between the pouring temperature and the mold temperature during pouring satisfies formula (6). 5≦A(mm / s ２ )≦45 ・・・(5) 7.0≦Pouring temperature (°C) / Mold temperature (°C)≦22.5 (6)

3. A composite roll for hot rolling having two layers, an outer layer and an inner layer, or three layers, an outer layer, an intermediate layer, and an inner layer, 2. A composite roll for hot rolling, wherein the outer layer comprises the outer layer material for hot rolling roll according to claim 1.

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