Outer layer material for hot rolling roll, manufacturing method thereof, and composite roll for hot rolling

Abstract

To provide a roll outer layer material for hot-rolling and producing method thereof, as well as a composite roll for hot-rolling.SOLUTION: A hot-rolling roll outer layer of the present invention contains, in mass %, C: 1.3-2.8%, Si: 0.1-2.5%, Mn: 0.1-2.5%, Ni: 0.5-6.5%, Cr: 2.5-12.5%, Mo: 2.5-12.5%, V: 2.5-12.5%, W: 0.5-7.5%, P: 0.01-0.05%, S: 0.001-0.030%, the remaining portion has a component composition composed of Fe and unavoidable impurities, and a tissue on the outer surface of the outer layer material has an eutectic cell diameter of 250 μm or less, and an eutectic carbide area ratio of 1.0 μm or more with a particle size is 6.0-25.0%, and in a tissue in the region up to 50 mm from the outer surface of the outer layer material in the roll radius direction, a difference S of the maximum value and the minimum value of the eutectic carbide area rate is 18.0% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a hot rolling roll outer layer material and a hot rolling composite roll, and more particularly to a hot rolling roll outer layer material and a hot rolling composite roll that are suitably used for rough rolling of steel sheets. The present invention also relates to a method for manufacturing a hot rolling roll outer layer material. [Background technology]

[0002] In recent years, the demand for high-quality steel sheets has increased, necessitating the need 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, particularly in terms of wear resistance. To improve wear resistance, HiCr cast steel rolls incorporating Cr-based M7C3 carbides and high-speed steel rolls based on high-speed steel (a type of tool steel) containing carbide-forming elements such as V, Cr, Mo, and W, and incorporating large amounts of hard carbides such as V-based MC carbides, Mo-based W-based M2C carbides, and Cr-based M7C3 carbides (where M represents a carbide-forming metallic element) have been used.

[0003] On the other hand, centrifugally cast hot rolling rolls exhibit a morphology in which dendrite-enriched areas and eutectic carbide-enriched areas are alternately layered, resulting in the formation of lamination segregation, which forms band-like layers in the radial direction of the roll. The presence of lamination segregation can lead to the formation of deep heat cracks caused by eutectic carbides, making chipping more likely. In addition, wear differences occur between segregated and non-segregated areas, forming fine irregularities on the roll surface, which can be transferred to the rolled material, resulting in problems such as a deterioration in the surface quality of the rolled material.

[0004] There are various techniques for solving such problems, for example, the techniques disclosed in Patent Documents 1 to 3.

[0005] Patent Document 1 proposes a centrifugally cast outer layer material for a rolling mill roll and a rolling mill roll containing, by weight, 1.5-3.5% C, 0.1-2.0% Si, 0.1-2.0% Mn, 5-25% Cr, 2-12% Mo, 3-10% V, and 0.5-5% Nb, with a [%Mo] / [%Cr] ratio of 0.25-0.7, the remainder being Fe and unavoidable impurities. The carbide content distribution is such that the difference between adjacent maximum and minimum values ​​in a radial region extending from the surface to 30 mm is 20% or less of the average value. Patent Document 1 also proposes a method for producing a centrifugally cast outer layer material for a rolling mill roll and a rolling mill roll, in which the rotation speed of the mold is varied during the casting of a molten metal having the above chemical composition. These techniques provide a technology that achieves excellent roll characteristics, does not produce segregation patterns in the steel sheet, and does not cause rolling problems.

[0006] Patent Document 2 proposes a method for producing an outer layer material for a hot rolling roll, in which molten outer layer material is cast into a centrifugal casting mold to form the outer layer material for a rolling roll, in which the centrifugal casting mold has a refractory layer of 1 to 5 mm thick formed on the inner surface of the mold, the casting temperature of the molten outer layer material is in the range of (solidus temperature + 160°C) to (solidus temperature + 400°C), and the rotation speed of the centrifugal casting mold is adjusted so that the centrifugal force acting on the inner surface of the centrifugal casting mold is a gravity multiple of 160 to 200 G, and the method for producing an outer layer material for a hot rolling roll is proposed. The composition of the molten metal for the outer layer material includes, by mass, 1.5 to 4% C, 0.2 to 3% Si, 0.2 to 2% Mn, 1 to 30% Cr, and 0.5 to 10% Mo, or may further include one or more of the following: 6% or less Ni, 8% or less V, 3% or less Nb, 4% or less Co, 0.5% or less REM, and 0.3% or less B. This provides a technology that can suppress lamination segregation and achieve an extremely excellent roll surface.

[0007] Patent Document 3 proposes a roll outer layer material for hot rolling, which contains, by mass%, 1.5 to 3.0% C, 0.1 to 2.0% Si, 0.1 to 2.0% Mn, 5.0 to 15.0% Cr, 2.0 to 12.0% Mo, 3.0 to 10.0% V, 0.5 to 5.0% Nb, and one or more of 5.0% or less Co, 3.0% or less Ni, and 5.0% or less W, wherein the contents of Mo and Cr satisfy the following formula (A) with the remainder consisting of Fe and unavoidable impurities, has a carbide amount distribution in which the rate of change S of the carbide amount, expressed by the following formula (B), is 20% or less, and furthermore satisfies the hardness difference, expressed by the following formula (C), of 3.0 or less. 0.35≦[%Mo] / [%Cr]≦0.70 (A) Here, [%Mo] and [%Cr] are the contents (mass%) of each element. S=(X max -X min )×100 / X ave (B) where X max : Maximum value (area%) in the carbide amount distribution in the region from the outer surface to 30 mm in the roll radial direction, X min : The minimum value (area%) of the carbide amount distribution in the area from the outer surface to 30 mm in the roll radial direction, X ave : Average value (area %) of the carbide amount distribution in the region from the outer surface to 30 mm in the roll radial direction. ΔHS=HS Xmax -HS Xmin (C) Here, HS Xmax : Shore hardness at the maximum position in the carbide amount distribution in the area from the outer surface to 30 mm in the roll radial direction, HS Xmin : Shore hardness at the minimum value position in the carbide amount distribution in the region from the outer surface to 30 mm in the roll radial direction.

[0008] Patent Document 3 also proposes a method for producing a hot rolling roll outer layer material, in which, when pouring a molten metal having the above composition and centrifugal casting it, the rotation speed of the centrifugal casting mold is varied two or more times so that the centrifugal force on the outer surface of the roll outer layer material varies by 1.0 G / s or more, and the supply rate of the molten metal is set to 80 to 200 kg / s. These provide a technology that provides roll characteristics with no texture segregation and excellent surface quality. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-239779 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-331344 [Patent Document 3] Japanese Patent Application Publication No. 2020-139190 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. As a result, hot rolling rolls are required to suppress roll segregation such as lamination segregation while further improving their wear resistance. However, the conventional hot rolling rolls and their manufacturing methods described in Patent Documents 1 to 3 are insufficient in both or either one of segregation suppression and wear resistance, and are unable to achieve both.

[0011] Therefore, an object of the present invention is to provide a hot rolling roll outer layer material that solves the above problems, suppresses lamination segregation, and has excellent wear resistance, a method for manufacturing the same, and a hot rolling composite roll.

[0012] Here, "excellent wear resistance" in the present invention means that the average wear amount measured by the hot rolling wear test described later is 0.12 g or less. The hot rolling wear test will be described in detail in the examples described later. [Means for solving the problem]

[0013] In order to solve the above-mentioned problems, the present inventors have conducted detailed investigations into the base, carbides, wear amount, chemical components (composition) of hot rolling rolls and manufacturing conditions for hot rolling rolls by centrifugal casting.

[0014] As a result, they found that by optimizing the chemical composition so that the amount of eutectic carbide, which is carbide present in the gaps between colony structures (base structures) called eutectic cells, falls within a specific range, and by optimizing the manufacturing conditions so that segregation can be suppressed, it is possible to suppress lamination segregation and improve wear resistance at the same time.

[0015] The present invention was completed based on these findings and further investigations. That is, the gist of the present invention is as follows. [1] A composition containing, by mass%, C: 1.3 to 2.8%, Si: 0.1 to 2.5%, Mn: 0.1 to 2.5%, Ni: 0.5 to 6.5%, Cr: 2.5 to 12.5%, Mo: 2.5 to 12.5%, V: 2.5 to 12.5%, W: 0.5 to 7.5%, P: 0.01 to 0.05%, S: 0.001 to 0.030%, with the balance being Fe and unavoidable impurities; The structure on the outer surface of the outer layer material has a eutectic cell diameter of 250 μm or less, and an area ratio of eutectic carbides with a particle size of 1.0 μm or more of 6.0 to 25.0%, A hot rolling roll outer layer material, characterized in that the structure in the region from the outer surface of the outer layer material to 50 mm in the roll radial direction has a difference S in eutectic carbide area ratio expressed by equation (1) of 18.0% or less. S=S max -S min ···(1) where S max: The maximum eutectic carbide area ratio (%) among the eutectic carbide area ratios observed on each surface from the outer surface of the outer layer material to 50 mm in the roll radial direction, S min : The minimum eutectic carbide area ratio (%) among the eutectic carbide area ratios observed on each surface from the outer surface of the outer layer material to 50 mm in the roll radial direction. [2] A method for producing an outer layer material for a hot rolling roll according to the above [1], When centrifugal casting is performed on the poured molten metal having the above-mentioned component composition of the outer layer material, The centrifugal force on the outer surface of the outer layer material is set to 150 to 220 G in gravity multiples, And, the value of ΔT representing the difference between the pouring temperature of the molten metal and the liquidus temperature of the molten metal satisfies formula (2), and wherein A, which represents the vibration acceleration of the mold during centrifugal casting, satisfies formula (3). 30℃≦ΔT≦100℃ (2) 5mm / s 2 ≦A≦40mm / s 2 ···(3) [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] above. [Effects of the Invention]

[0016] The present invention can provide a hot rolling roll outer layer material and a hot rolling composite roll that are excellent in wear resistance and suppressed in lamination segregation, and can also provide a method for manufacturing a hot rolling roll outer layer material that combines these properties. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a diagram showing a schematic configuration of the testing machine used in the hot rolling wear test. [Figure 2]FIG. 2 is a diagram illustrating the maximum value (Smax) and minimum value (Smin) of the eutectic carbide area ratio in the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] 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.

[0019] 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 "%".

[0020] C: 1.3 to 2.8% Carbon combines with V, Cr, Mo, W, etc. to form hard carbides, thereby improving wear resistance. Here, hard carbides refer to MC, M2C, and M7C3 carbides. MC, M2C, and M7C3 carbides, which crystallize during solidification of the molten metal, are eutectic carbides. Carbon also improves matrix hardness by solid-solution strengthening. A carbon content of less than 1.3% results in an insufficient amount of eutectic carbide, making it impossible to achieve excellent wear resistance. On the other hand, a carbon content of more than 2.8% results in excessive carbide formation, promoting the segregation of eutectic carbides. Furthermore, coarse eutectic carbides are formed, which tend to fall off during rolling, resulting in reduced wear resistance. Therefore, the carbon content is limited to 1.3% or more and 2.8% or less. The C content is preferably 1.4% or more, more preferably 1.5% or more, and is preferably 2.5% or less, more preferably 2.2% or less.

[0021] Si: 0.1 to 2.5% 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 2.5%, the deoxidizing effect saturates. Therefore, the Si content is limited to 0.1% or more and 2.5% or less. The Si content is preferably 0.2% or more, and more preferably 0.4% or more. Furthermore, the Si content is preferably 2.2% or less, and more preferably 2.0% or less.

[0022] Mn: 0.1 to 2.5% Mn has the effect of fixing S, which has a negative effect on products, as MnS and removing it. 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 0.4% or more. Furthermore, the Mn content is preferably 2.2% or less, and more preferably 2.0% or less.

[0023] Ni: 0.5 to 6.5% 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 6.5%, the hardness decreases because it promotes the retention of austenite. Therefore, the Ni content is limited to 0.5% or more and 6.5% or less. The Ni content is preferably 1.0% or more, and more preferably 1.5% or more. Furthermore, the Ni content is preferably 5.0% or less, and more preferably 4.0% or less.

[0024] Cr: 2.5 to 12.5% 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 2.5%, the amount of M7C3 eutectic carbides will be insufficient, resulting in reduced wear resistance. On the other hand, if the Cr content exceeds 12.5%, coarse M7C3 eutectic carbides will form, which will actually worsen wear resistance. Therefore, the Cr content is limited to 2.5% or more and 12.5% ​​or less. The Cr content is preferably 3.5% or more, and more preferably 4.5% or more. The Cr content is also preferably 10.0% or less, and more preferably 8.0% or less.

[0025] Mo: 2.5 to 12.5% 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 therefore have the effect of further improving wear resistance. If the Mo content is less than 2.5%, the amount of M2C-type eutectic carbides is insufficient, and the effect of improving wear resistance is insufficient. On the other hand, if the Mo content exceeds 12.5%, coarse M2C-type eutectic carbides are formed, which deteriorates wear resistance and causes a decrease in toughness. Therefore, the Mo content is limited to 2.5% or more and 12.5% ​​or less. The Mo content is preferably 3.5% or more, more preferably 4.5% or more. The Mo content is also preferably 10.0% or less, more preferably 8.0% or less.

[0026] V: 2.5 to 12.5% 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 2.5%, the amount of MC-type eutectic carbides is insufficient, resulting in insufficient wear resistance. On the other hand, if the V content exceeds 12.5%, 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 2.5% or more and 12.5% ​​or less. The V content is preferably 3.5% or more, more preferably 4.5% or more. Furthermore, the V content is preferably 10.0% or less, more preferably 8.0% or less.

[0027] W: 0.5 to 7.5% W is a carbide-forming element, and like Mo, it bonds with C to form hard carbides such as hard M2C carbides, which increases the hardness of the outer layer material and improves its wear resistance. If the W content is less than 0.5%, this effect is insufficient, and wear resistance deteriorates. On the other hand, if the W content exceeds 7.5%, coarse M2C eutectic carbides are formed, which actually worsens wear resistance. Therefore, the W content is limited to 0.5% or more and 7.5% or less. The W content is preferably 1.0% or more, and more preferably 1.5% or more. The W content is also preferably 6.5% or less, and more preferably 5.5% or less.

[0028] P: 0.01 to 0.05% 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 hardness and wear resistance. If the P content is less than 0.01%, the effect of improving hardness and wear resistance is insufficient, while if the P content exceeds 0.05%, mechanical properties deteriorate. Therefore, the P content is limited to 0.01% or more and 0.05% or less. The P content is preferably 0.02% or more. Furthermore, the P content is preferably 0.04% or less.

[0029] 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 within that range, MnS has a lubricating effect. 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 0.020% or less.

[0030] Remainder: Fe and unavoidable impurities The balance other than the above components consists of Fe and unavoidable impurities. For example, unavoidable impurities include N and O. From the viewpoint of preventing porosity, it is preferable that the O content be 250 ppm by mass or less, and the N content be 1200 ppm by mass or less.

[0031] Next, the reasons for limiting the structure of the outer layer material for hot rolling rolls of the present invention will be explained.

[0032] Previous research has shown that the wear resistance tends to improve as the eutectic carbide area ratio increases. However, the range of the eutectic carbide area ratio that maintains excellent wear resistance while preventing lamination segregation during centrifugal casting has not yet been clarified. Therefore, the inventors have conducted extensive research and discovered the following.

[0033] Specifically, it is important that the outer layer material for a hot rolling roll of the present invention has a component composition within the above-mentioned range, and that the structure on the outer surface of the outer layer material has a eutectic cell diameter of 250 μm or less, a eutectic carbide area ratio of 6.0 to 25.0% having a particle size of 1.0 μm or more, and that the structure in the region from the outer surface of the outer layer material to 50 mm in the roll radial direction has a eutectic carbide area ratio difference S expressed by formula (1) of 18.0% or less. This is because when the outer layer material has this component composition and structure, it is possible to improve wear resistance while suppressing lamination segregation. S=S max -S min ···(1) where S max: The maximum eutectic carbide area ratio (%) among the eutectic carbide area ratios observed on each surface from the outer surface of the outer layer material to 50 mm in the roll radial direction, and S min : The minimum eutectic carbide area ratio (%) among the eutectic carbide area ratios observed on each surface from the outer surface of the outer layer material to 50 mm in the roll radial direction.

[0034] In the present invention, the term "eutectic carbide" refers to MC, M2C, and M7C3 carbides that are crystallized during the solidification of a molten metal, as described above. Furthermore, since the particle size of the precipitated carbides present in the matrix is ​​less than 1.0 μm, the term is limited to eutectic carbides with a particle size of 1.0 μm or more in the present invention.

[0035] Furthermore, "eutectic cells" refer to eutectic structures in which the matrix is ​​surrounded by eutectic carbides, or the eutectic carbide network is open, resulting in cellular solidification and a colony structure. Here, the "matrix" in the outer layer material is preferably martensite and / or bainite.

[0036] The "surface layer of the outer layer material" refers to the region extending from the outer surface of the outer layer material to 50 mm in the radial direction of the roll.

[0037] Eutectic cell: diameter 250 μm or less If the diameter of the eutectic cell is greater than 250 μm, the distance between eutectic carbides becomes longer, resulting in a deterioration in wear resistance. "The distance between eutectic carbides becomes longer" means that the area of ​​the matrix surrounded by the eutectic carbides is large (i.e., the cellular colony structure is coarse). If this distance becomes longer, the high-hardness eutectic carbides responsible for wear resistance cannot protect the matrix, and wear of the matrix progresses rapidly, resulting in a deterioration in wear resistance. Therefore, the diameter of the eutectic cell is set to 250 μm or less. From the viewpoint of wear resistance, the diameter of the eutectic cell is preferably 230 μm or less, more preferably 210 μm or less.

[0038] There is no particular lower limit for the diameter of the eutectic cell. As the diameter of the eutectic cell becomes smaller, the gaps between the eutectic cells increase. However, since the amount of eutectic carbide is mainly determined by the C content, the amount of eutectic carbide becomes insufficient, and microcavities tend to occur in the gaps between the eutectic cells. From the viewpoint of maintaining roll quality, the diameter of the eutectic cell is preferably 40 μm or more, and more preferably 60 μm or more.

[0039] The diameter of this eutectic cell can be controlled by the vibration acceleration during centrifugal casting in the manufacturing method described below.

[0040] Eutectic carbide area ratio: 6.0~25.0% In the present invention, the term "eutectic carbide area ratio" refers to the total area ratio of eutectic carbides with a particle size of 1.0 μm or more. If the eutectic carbide area ratio is less than 6.0%, the hard eutectic carbide responsible for wear resistance in the matrix is ​​small, making the matrix more susceptible to wear and resulting in a deterioration in wear resistance. On the other hand, if the eutectic carbide area ratio is greater than 25.0%, the eutectic carbide is more likely to fall off. In the areas where carbide falls off and spaces are created, plastic flow occurs in the matrix due to the action of the rolling torque acting in the circumferential direction of the roll, causing cracks to propagate in the circumferential direction and causing the matrix to fall off, thereby actually deteriorating wear resistance. The eutectic carbide area ratio is preferably 8.0% or more, more preferably 10.0% or more. The eutectic carbide area ratio is also preferably 24.0% or less, more preferably 22.0% or less.

[0041] In addition, since coarse eutectic carbides are prone to heat cracking and fall off, it is preferable that the particle size of the eutectic carbides be 50 μm or less.

[0042] This eutectic carbide area ratio can be controlled by the component composition and the gravity multiple and vibration acceleration (A) in the manufacturing method described below.

[0043] Eutectic carbide area ratio difference (S): 18.0% or less Lamination segregation is a layer formed by the accumulation of eutectic carbide, and occurs when the difference in eutectic carbide area ratio in the radial direction of the roll is large. Therefore, in the present invention, by specifying the "difference in eutectic carbide area ratio" expressed by formula (1), the range of eutectic carbide area ratio in which lamination segregation is unlikely to occur by centrifugal casting is controlled. Specifically, this difference in eutectic carbide area ratio is controlled by the component composition and an appropriate ΔT and gravity multiplier in the manufacturing method described below.

[0044] If this difference in eutectic carbide area ratio (S) is greater than 18.0%, the boundary between the eutectic carbide accumulation layer and the normal layer becomes clear to the naked eye, confirming lamination segregation. Therefore, in the present invention, this difference in eutectic carbide area ratio (S) is set to 18.0% or less. The difference in eutectic carbide area ratio is preferably 16.5% or less, and more preferably 15.0% or less.

[0045] It should be noted that the smaller the difference (S) in the eutectic carbide area ratio, the higher the quality. Therefore, this difference (S) may be 0%. In other words, it is preferably 0% or more.

[0046] As described above, the structure (base structure) of the present invention may contain martensite and / or bainite in an area ratio of 75.0 to 94.0% of the entire base structure in addition to the eutectic carbides. If the total area ratio of martensite and bainite is less than 75.0%, the carbide area ratio will be excessive, which may result in the eutectic carbide being more likely to fall off and, conversely, may result in a deterioration in wear resistance. On the other hand, if the total area ratio of martensite and bainite exceeds 94.0%, the hard eutectic carbide responsible for wear resistance will be insufficient, which may result in a deterioration in wear resistance.

[0047] Next, the method for observing the structure will be described below.

[0048] [Area ratio of eutectic carbide] First, the obtained outer layer material is mirror-polished and etched with nital solution, and then the structure of the roll surface (Z-plane) is observed using a digital microscope. In the present invention, a field of view in which 200 or more eutectic cells can be confirmed is defined as one field of view, and photographs are taken. Next, ImageJ is used as an image analysis tool to perform binarization of the photographs at a measurement magnification of 200x. Because there is a difference in brightness between the base structure and eutectic carbides in the photographs, binarization makes it possible to classify the base structure and eutectic carbides and determine their respective areas. Here, nine fields of view are photographed from the sample, and the average value of the area ratio of eutectic carbides is calculated. This average value is defined as the "area ratio (%) of eutectic carbides with a particle size of 1.0 μm or more" on the roll surface (the outer surface of the outer layer material).

[0049] As mentioned above, the present invention targets eutectic carbides with a particle size of 1.0 μm or more. Here, the "particle size" is measured as follows: The diameter of the eutectic carbide is measured from a digital microscope image taken at 200x magnification, and this diameter is taken as the particle size of the eutectic carbide. If the eutectic carbide has an elliptical or other shape, a perpendicular bisector is drawn across the longest line segment within the eutectic carbide, and the length between the two points where this perpendicular bisector intersects with the grain boundary of the eutectic carbide is taken as the particle size of the eutectic carbide.

[0050] In addition, the maximum value of the eutectic carbide area ratio in the roll radial direction (S max ) and minimum value (S min ) difference (S) is found as follows: Using the same method as above, the structure of the area from the roll surface (Z surface) to 50 mm in the roll radial direction (i.e., the surface layer of the outer layer material) is observed using a digital microscope. The roll is ground in 1 mm increments from the outer surface in the roll radial direction, and the eutectic carbide area ratio is measured each time using the above method, and the average value for 9 fields of view is calculated. This is repeated up to a position 50 mm from the outer surface in the roll radial direction. As shown in Figure 2, the largest of the average values ​​of the eutectic carbide area ratios obtained for each observation surface is taken as the maximum eutectic carbide area ratio (S max ), and the smallest one is the minimum value of the eutectic carbide area ratio (S minThe difference between this maximum and minimum value is the "eutectic carbide area ratio difference (S)" in the outer layer of the outer layer material.

[0051] [Base organization] As mentioned above, the matrix can be classified and its area determined by using ImageJ as an image analysis tool to perform binarization on a photograph taken at a measurement magnification of 200x. When binarized, the matrix and precipitated carbides appear black, and eutectic carbides appear white. Therefore, the area fraction (%) of the matrix structure (i.e., the total area fraction of martensite and bainite) is calculated by dividing the area fraction (%) of white eutectic carbides from 100%.

[0052] [Eutectic cell diameter] The diameter of the eutectic cells is measured by mirror-polishing the resulting outer layer material, etching it with nital solution, and then observing the structure of the roll surface (Z-plane) with a digital microscope. Here, the diameter of the eutectic cells is measured from digital microscope images taken at a measurement magnification of 100x. Here, three fields of view are photographed from the sample, and a total of six straight lines are drawn randomly vertically and / or horizontally on each obtained image. As shown in equation (4) below, the eutectic cell size in each image is calculated using the "length of the i-th straight line (Li)," the "number of intersections between the i-th straight line and the eutectic cell (Ni)," and the "number of straight lines."

[0053] Specifically, first, the length of the first line (L1) is 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) is divided by the number of intersections (N2) between the second line and eutectic carbides with a particle size exceeding 5.0 μm. This process is repeated for the number of lines (6). The sum of the obtained values ​​is divided by the total number of lines to determine the "eutectic cell size." The average value of the eutectic cell sizes in the three fields of view thus determined is defined as the "eutectic cell diameter (μm)" on the roll surface (outer surface of the outer layer material).

[0054] 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.

[0055] Eutectic cell size = ((L1 / N1) + (L2 / N2) + (L3 / N3) + (L4 / N4) + (L5 / N5) + (L6 / N6)) / 6 (4) In formula (4), 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.

[0056] Next, one embodiment of the method for producing an outer layer material for a hot rolling roll of the present invention will be described.

[0057] 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.

[0058] Specifically, when centrifugal casting the poured molten metal for the outer layer material, the centrifugal force on the outer surface of the outer layer material is set to a gravity multiple of 150 to 220 G, and the difference between the pouring temperature of the molten metal for the outer layer material and the liquidus temperature of the molten metal for the outer layer material (hereinafter, this difference will be referred to as "ΔT (unit: °C)") satisfies formula (2), and the vibration acceleration of the mold during centrifugal casting (hereinafter, this vibration acceleration will be referred to as "A (unit: mm / s 2 Centrifugal casting is performed under the condition that the temperature (temperature) satisfies formula (3).

[0059] 30℃≦ΔT≦100℃ (2) 5mm / s 2 ≦A≦40mm / s 2 ···(3) Centrifugal force: 150-220G as a multiple of gravity If the centrifugal force when centrifugal casting the outer layer material for a hot rolling roll is less than 150 G in terms of gravity multiple, not only will the solidification rate decrease, but the fluctuation of the molten metal surface on the inner surface of the casting, between the primary crystals and the remaining liquid phase in the solid-liquid coexistence region, will increase, making lamination segregation more likely to occur. Therefore, the gravity multiple is set to 150 G or more. From the viewpoint of suppressing lamination segregation, the gravity multiple is preferably 155 G or more, and more preferably 160 G or more.

[0060] On the other hand, if the centrifugal force exceeds 220 G in gravity multiple, the mold vibrates significantly during centrifugal casting, increasing the vibration acceleration and resulting in casting defects caused by the vibration. Therefore, the gravity multiple is set to 220 G or less. From the viewpoint of roll quality, the gravity multiple is preferably 210 G or less, and more preferably 200 G or less.

[0061] The gravity multiple (G) is a parameter that indicates how many times the centrifugal force caused by mold rotation is greater than gravity, and is calculated by the following formula (5). G=((D / 2)×ω 2 / g) (5) Here, in equation (5), D is the diameter of the mold, ω is the angular velocity of the mold rotation, and g is the gravitational acceleration.

[0062] Difference between pouring temperature and liquidus temperature of molten outer layer material: 30℃≦ΔT≦100℃ The mechanism of lamination segregation is thought to be mainly due to the shear flow behavior of the primary crystal and the residual liquid phase in the rotational direction in the solid-liquid coexistence region, and although it has not yet been fully elucidated, the inventors have empirically learned that reducing ΔT is effective in suppressing lamination segregation. In other words, lamination segregation can be suppressed by controlling ΔT within the manufacturable range.

[0063] If the value of ΔT is less than 30°C, the molten metal solidifies early in the casting process, which can lead to castability issues. Furthermore, if the value of ΔT is greater than 100°C, it takes a long time for the molten metal to solidify, making lamination segregation more likely to form. For these reasons, in the present invention, the difference between the casting temperature and the liquidus temperature of the molten metal of the outer layer material is controlled so as to satisfy the above formula (2). From the viewpoint of castability, ΔT is preferably 35°C or higher, and more preferably 40°C or higher. Furthermore, ΔT is preferably 95°C or lower, and more preferably 90°C or lower.

[0064] Vibration acceleration of the mold during centrifugal casting: 5mm / s 2 ≦A≦40mm / s 2 The vibration can be measured by the following method.

[0065] 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 3 to 6%. 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.

[0066] 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 due to the small vibration, and 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 6 mm / s 2 More preferably, 8 mm / s or more. 2 That's all.

[0067] On the other hand, if the vibration acceleration is 40 mm / s 2If the vibration acceleration is greater than this, casting defects due to vibration will occur, and the roll quality will worsen. Also, if the vibration acceleration is too large, an external force will be applied to the solid-liquid coexistence region, making lamination segregation more likely to occur. Therefore, the vibration acceleration should be set to 40 mm / s 2 From the viewpoint of roll quality, the vibration acceleration is preferably 38 mm / s 2 or less, and more preferably 36 mm / s 2 The following is the result.

[0068] In the present invention, the above-mentioned effects can be obtained by performing centrifugal casting under the above-mentioned casting conditions, and as a result, the above-mentioned structure having excellent wear resistance and suppressed lamination segregation can be obtained.

[0069] It is desirable to carry out a heat treatment after centrifugal casting. For example, as the heat treatment, it is preferable to carry out two or more times the following: heating the centrifugal cast hot rolling roll outer layer material to a temperature of 900 to 1100°C, followed by quenching by air cooling or air blast cooling, and then tempering by heating and holding the material so that the value of the tempering parameter (P) expressed by formula (6) is in the range of 10000 to 20000, followed by cooling.

[0070] P = T(log(t) + A) (6) In equation (6), T is the tempering temperature (K), t is the tempering time (h), and A is a constant (A=20 in the present invention).

[0071] 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.

[0072] Next, the composite roll for hot rolling and the method for producing the same of the present invention will be described.

[0073] The composite roll for hot rolling 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 outer layer material for hot rolling roll of the present invention having the above-mentioned component composition and structure. This makes it possible to obtain a composite roll that suppresses lamination segregation and has improved wear resistance, as described above.

[0074] The chemical composition and structure of the outer layer material for hot rolling rolls have already been explained, and therefore will not be described here.

[0075] Next, a preferred example of the method for producing the composite roll for hot rolling of the present invention will be described.

[0076] 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.

[0077] Next, an inner layer, or an intermediate layer and an inner layer, are formed inside the outer layer formed by centrifugal casting.

[0078] 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.

[0079] For the inner layer to be statically cast, it is preferable to use spheroidal graphite cast iron or worm-shaped graphite cast iron (CV cast iron), which have 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, they weaken the inner layer. For this reason, it is preferable to minimize the mixing rate of these elements into the outer layer components.

[0080] 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 solidification of the outer layer material or after it has completely solidified, 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 side 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.

[0081] 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.

[0082] 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.

[0083] 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]

[0084] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples.

[0085] 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, and a ring-shaped test piece (outer diameter: φ250 mm, width: 65 mm, thickness: 55 mm) was produced by centrifugal casting. The balance other than the chemical composition shown in Table 1 was Fe and unavoidable impurities.

[0086] Next, centrifugal casting was performed so that the difference (ΔT) between the pouring temperature of the molten metal and the liquidus temperature of the molten metal was set as shown in Table 2, and the centrifugal force applied to the outer periphery of the ring-shaped test piece was set to a gravity multiple as shown in Table 2, and the vibration acceleration (A) of the mold during centrifugal casting was set as shown in Table 2.

[0087] Next, after cooling, the ring-shaped test pieces were taken out and quenched at 900 to 1100°C, and then tempered three times by holding the specimen at this temperature and then cooling it so that the tempering parameter P expressed by the above formula (6) was in the range of 10,000 to 20,000.

[0088] Thereafter, the obtained ring-shaped test pieces were used to carry out microstructural observation and hot rolling wear tests by the methods described below. Hot rolling wear test pieces were taken from the center of the wall thickness.

[0089] 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.

[0090] [Table 1]

[0091] (1) Microstructure observation Using each sample after the heat treatment, the particle size of the eutectic carbide on the roll surface and the area ratio of the eutectic carbide having a particle size of 1.0 μm or more were measured by the above-mentioned measuring method.

[0092] In addition, using each sample after heat treatment, the maximum value of the eutectic carbide area ratio (S max ) and minimum value (S min ) was calculated, and the difference (S) in the eutectic carbide area ratio, expressed by the above formula (1), was calculated for each.

[0093] Furthermore, using each sample after the heat treatment, the diameter of the eutectic cell on the roll surface was determined by the above-mentioned measurement method. In this example, in equation (4) for determining the eutectic cell size, the linear length (Li) was set to 2574 μm. That is, the following equation was used for determining the eutectic cell size (4). Eutectic cell size = ((2574 / N1) + (2574 / N2) + (2574 / N3) + (2574 / N4) + (2574 / N5) + (2574 / N6)) / 6 (4) (2) Hot rolling wear test The hot rolling wear test was performed as follows. Hot rolling wear test specimens (outer diameter: 60 mm, width: 10 mm, with C1 chamfer) were taken from each of the obtained ring-shaped test specimens. The hot rolling wear test was performed using a two-disk sliding rolling method between test specimen 1 and a mating piece 4, as shown in Figure 1 . Test specimen 1 was rotated at 700 rpm while being cooled with cooling water 2. A mating piece 4 (material: S45C, outer diameter: 190 mm, width: 15 mm, with C1 chamfer) heated to 800°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 indicated by the thick arrow in Figure 1 . The rotation direction 5 of test specimen 1 and the rotation direction 6 of mating piece 4 were set so that the tangent at the contact point between test specimen 1 and mating piece 4 was in the same direction. The abrasion test was carried out for 450 minutes, with the mating piece being replaced with a new one every 45 minutes (31,500 rotations of the test piece), and the test was repeated a total of 10 times (total 315,000 rotations of the test piece), and the average amount of wear (g) per test (per 31,500 rotations of the test piece) was calculated.

[0094] The results are shown in Table 2.

[0095] [Table 2]

[0096] Here, the evaluation was made as follows. In Table 2, a wear amount of 0.12 g or less was rated as "pass" (i.e., excellent wear resistance), and a wear amount of more than 0.12 g was rated as "fail." As is clear from Table 2, it was confirmed that the examples of the present invention suppressed lamination segregation and also had excellent wear resistance compared to the comparative examples. Whether or not lamination segregation was suppressed was determined by the difference (S) in the eutectic carbide area ratio in the roll radial direction shown in Table 2, and as described above, a case where the difference (S) was 18.0% or less was determined to be "suppressed of lamination segregation."

[0097] Therefore, according to the present invention, it is possible to manufacture a hot rolling roll outer layer material and a hot rolling composite roll which have excellent wear resistance and which suppress lamination segregation. As a result, the life of the hot rolling roll is improved and time loss due to interruption of rolling 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]

[0098] 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. It has a component composition containing, in mass%, C: 1.3 to 2.8%, Si: 0.1 to 2.5%, Mn: 0.1 to 2.5%, Ni: 0.5 to 6.5%, Cr: 2.5 to 12.5%, Mo: 2.5 to 12.5%, V: 2.5 to 12.5%, W: 0.5 to 7.5%, P: 0.01 to 0.05%, S: 0.001 to 0.030%, and the balance being Fe and unavoidable impurities; The structure on the outer surface of the outer layer material has a eutectic cell diameter of 250 μm or less, and an area ratio of eutectic carbides having a particle size of 1.0 μm or more is 6.0 to 25.0%, 1. A hot rolling roll outer layer material, characterized in that the structure in a region from the outer surface of the outer layer material to 50 mm in the roll radial direction has a difference S in eutectic carbide area ratio expressed by formula (1) of 18.0% or less. S=S ｍａｘ -S ｍｉｎ ・・・(1) Here, S ｍａｘ : The maximum eutectic carbide area ratio (%) among the eutectic carbide area ratios on each observation surface from the outer surface of the outer layer material to 50 mm in the roll radial direction, S ｍｉｎ : The smallest eutectic carbide area ratio (%) among the eutectic carbide area ratios observed on each surface from the outer surface of the outer layer material to 50 mm in the roll radial direction.

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, The centrifugal force on the outer surface of the outer layer material is set to 150 to 220 G in terms of gravity multiple, and the value of ΔT representing the difference between the pouring temperature of the molten metal and the liquidus temperature of the molten metal satisfies formula (2), and wherein A, which represents the vibration acceleration of the mold during centrifugal casting, satisfies formula (3). 30°C ≦ ΔT ≦ 100°C (2) 5mm / s ２ ≦A≦40mm / s ２ ・・・(3)

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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