Hot rolling roll outer layer material, its manufacturing method, hot rolling composite roll and its manufacturing method

Abstract

To provide a hot-rolling roll outer layer, a method for producing the same, a hot-rolling composite roll and a method for producing the same, featuring excellent wear resistance and fatigue resistance, as well as superior seizing resistance.SOLUTION: A hot-rolling roll outer layer has a specific composition, with a carbide composite dispersed with carbides selected from at least one each of Group A and Group B. Group A: M2C carbide, M6C carbide, MC carbide. Group B: M7C3 carbide, M23C6 carbide.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a roll outer layer material for hot rolling, a manufacturing method thereof, and a hot rolling composite roll and a manufacturing method thereof, and more particularly to a hot rolling roll outer layer material suitable as a work roll for a hot finishing stand for steel plate, a manufacturing method thereof, and a hot rolling composite roll and a manufacturing method thereof. [Background technology]

[0002] In recent years, with the advancement of hot rolling technology for steel sheets, the environment in which rolls are used has become more severe, and the production volume of steel sheets that are subject to a large rolling load, such as high-strength steel sheets and thin-walled products, has also increased. As a result, the quality level required of work rolls for rolling has become higher, and high-performance rolls that are excellent in surface roughening resistance, wear resistance, fatigue resistance, and seizure resistance are required for work rolls for finishing rolling stands in hot rolling facilities.

[0003] The rear stands of hot finishing mills use work rolls with outer layers made of grain-based cast iron or high-alloy grain-based cast iron, which have excellent seizure resistance. One of the rolling problems that can occur in the rear stands of hot finishing mills is drawing. Drawing occurs when the edge of the rolled material folds over and gets caught between the rolls. When drawing occurs, the rolled material seizes onto the roll surface, causing large thermal and mechanical loads on the roll, which can lead to cracks or chipping on the roll surface. These cracks can be more than 1 mm deep, requiring grinding to remove them, which increases work costs and shortens the roll's life. Therefore, there is a demand for hot rolling work rolls that are less likely to develop or propagate cracks even when drawing occurs.

[0004] As an outer layer material for such hot rolling work rolls, for example, Patent Document 1 proposes a rolling roll outer layer material containing, by mass%, more than 3.0% but not more than 4.0% C, 3.0% or less Si, 2.3 to 5.5% Ni, 1.0 to 2.0% Cr, 0.3 to 10.0% V, and 0.01 to 2.0% Ti, with the remainder being Fe and impurity elements, having graphite and MC-based carbides in the metal structure, and a graphite spheroidization rate of 0.5 or more. Patent Document 1 claims that by making the shape of the graphite into fine spheres and dispersing the graphite uniformly in the metal structure, wear resistance, surface roughening resistance, and accident resistance are improved.

[0005] Patent Document 2 proposes a composite roll for rolling, which contains, by mass%, 3.0 to 4.5% C, more than 0% but not more than 2.0% Si, more than 0% but not more than 1.5% Mn, 3.0 to 5.0% Ni, 1.4 to 4.0% Cr, 0.1 to 1.5% Mo, more than 0% but not more than 3.0% V, the balance being Fe and unavoidable impurity elements, in which C, Si, and Cr satisfy 4.0%≦C+Si / 3+Cr / 7.5≦5.5%, and the metal structure of the peripheral surface used for rolling the outer layer has an area ratio of cementite of 40% or more but less than 46%. Patent Document 2 claims that the presence of a large amount of hard cementite results in a composite roll for rolling with excellent wear resistance and surface roughening resistance.

[0006] Patent Document 3 proposes a composite roll for rolling, characterized in that the outer layer contains, by mass, 1-3% C, 0.3-3% Si, 0.1-3% Mn, 0.5-5% Ni, 1-7% Cr, 2.2-8% Mo, 4-7% V, 0.005-0.15% N, and 0.05-0.2% B, with the remainder consisting of Fe and unavoidable impurities, and the intermediate layer contains 0.025-0.15% B by mass, the B content of the intermediate layer being 45-80% of the B content of the outer layer, and the total content of carbide-forming elements in the intermediate layer being 45-90% of the total content of carbide-forming elements in the outer layer. Patent Document 3 lists MnS and carboborides as structures that contribute to galling resistance, and claims that these provide a composite roll for rolling with excellent galling resistance. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-177808 [Patent Document 2] Japanese Patent Application Publication No. 2018-75638 [Patent Document 3] Patent No. 6973416 Summary of the Invention [Problem to be solved by the invention]

[0008] From the viewpoint of manufacturing high-grade steel sheets or improving productivity in hot rolling, the rolling environment in hot rolling is becoming more severe year by year, and higher quality work rolls for hot rolling are required. In particular, there is a demand for hot rolling rolls that have excellent wear resistance and fatigue resistance, as well as excellent seizure resistance so that the steel sheet is less likely to seize when a drawing trouble occurs. However, the composite rolls for rolling manufactured in Patent Documents 1 and 2 produce graphite, so it is difficult to say that they have sufficient wear resistance. In addition, the composite roll for rolling manufactured in Patent Document 3 contains a large amount of B, which may be mixed into the intermediate layer and inner layer, causing embrittlement of the intermediate layer and inner layer.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot rolling roll outer layer material having excellent wear resistance and fatigue resistance, and also excellent seizure resistance, a manufacturing method thereof, and a hot rolling composite roll and a manufacturing method thereof. [Means for solving the problem]

[0010] Hot rolling rolls with grain cast iron or high-alloy grain cast iron outer layer materials contain graphite in their metal structure, making the rolled material less susceptible to seizure even when drawing troubles are encountered, thereby suppressing the occurrence and progression of cracks. Increasing the area ratio of graphite is an effective way to improve seizure resistance, but graphite is softer than carbides and the matrix, which reduces wear resistance. On the other hand, hot rolling rolls with high-speed cast iron or high-alloy white cast iron outer layer materials have a large amount of carbides in their metal structure and have excellent wear resistance, but their fatigue resistance and seizure resistance are inferior to those of grain cast iron. Therefore, the inventors conducted extensive research into roll outer layer materials with excellent wear resistance, fatigue resistance, and seizure resistance. As a result, they found a roll outer layer material that contains one or more carbides selected from M2C-type carbides, M6C-type carbides, and MC-type carbides, and a combination of M7C3-type carbides and M 23 The inventors have made the unprecedented discovery that by forming one or more types of C6 type carbides adjacent to each other, it is possible to obtain an outer layer material for hot rolling rolls that is excellent in wear resistance, fatigue resistance, and seizure resistance.

[0011] The present invention has been completed based on the above findings, and the gist of the present invention is as follows. [1] In mass %, C: 1.5-2.3%, Si: 0.3 to 2.0% Mn: 0.3 to 2.0%, Cr: 3.5 to 7.0%, Mo: 3.0-6.0% V: 3.0-5.0%, Nb: 0.1 to 2.0%, Al: 0.01 to 0.10%, Ni: 0.02 to 2.00% N: 0.050% or less, Contains Or, furthermore, Ti: 0.50% or less, B: 0.090% or less, Co: 1.0% or less, W: 1.5% or less, Zr: 0.50% or less, Contains one or more selected from the following: The balance is Fe and unavoidable impurities, The contents of Cr, Mo, W, V, and Nb have a composition that satisfies the following formulas (1) and (2), The outer layer material for a hot rolling roll is characterized in that a carbide composite having at least one carbide selected from the following Group A and Group B is dispersed therein: 0.85≦%Cr / (%Mo+%W / 2)≦1.15 (1) (%Cr+%Mo+%W) / (%V+%Nb)≧2.2 (2) Here, %Cr, %Mo, %W, %V, and %Nb are the contents (mass%) of each element, and elements that are not contained are set to 0. Group A: M2C type carbide, M6C type carbide, MC type carbide Group B: M7C3 type carbide, M 23 C6 type carbide [2] The outer layer material for a hot rolling roll according to [1], characterized in that the carbide composites have an area ratio of 2.0% or more. [3] A method for producing a hot rolling roll outer layer material, characterized in that a molten metal having the above-mentioned component composition is cast using a non-axisymmetric centrifugal casting mold having a portion that is not point-symmetric about the rotation axis of the centrifugal casting mold in a cross section perpendicular to the rotation axis of the centrifugal casting mold, to form the hot rolling roll outer layer material described in [1] or [2]. [4] A composite roll for hot rolling having two or more layers, an outer layer and an inner layer, characterized in that the outer layer is made of the outer layer material for hot rolling roll according to [1] or [2]. [5] A method for producing a composite roll for hot rolling, characterized in that an outer layer material obtained by the method for producing an outer layer material for a hot rolling roll according to [3] above is used. [Effects of the Invention]

[0012] According to the present invention, it is possible to obtain an outer layer material for a hot rolling roll which is excellent in wear resistance, fatigue resistance and seizure resistance, and this contributes to improving the roll life and rolling quality. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic side view showing an example of a centrifugal casting mold that is symmetrical about the axis of rotation of the centrifugal casting mold. [Figure 2] FIG. 2 is a schematic side view showing an example of a centrifugal casting mold that is asymmetric about the axis of rotation of the centrifugal casting mold. [Figure 3] FIG. 3 is a diagram showing the position where the test piece for EBSD measurement is taken from the ring-shaped test material. [Figure 4] FIG. 4 is a diagram showing a schematic configuration of the testing machine used in the drop weight frictional thermal shock test. [Figure 5] FIG. 5 is a diagram showing the configuration of the testing machine used in the hot abrasion test and a test piece for the hot abrasion test (abrasion test piece). [Figure 6] FIG. 6 is a diagram schematically showing the position at which a test piece for cross-section observation is taken from a test piece for a hot wear test (wear test piece). DETAILED DESCRIPTION OF THE INVENTION

[0014] First, the reasons for limiting the composition of the outer layer (outer layer material) of the composite roll for hot rolling of the present invention will be explained. Note that hereinafter, mass % will be simply referred to as % unless otherwise specified.

[0015] C: 1.5-2.3% Carbon dissolves in the matrix to increase its hardness and bonds with carbide-forming elements to form hard carbides, thereby improving the wear resistance of the roll outer layer material. If the C content is less than 1.5%, the amount of carbide is insufficient, resulting in reduced wear resistance. Therefore, the C content is set to 1.5% or more. The C content is preferably set to 1.6% or more. On the other hand, if the C content exceeds 2.3%, the carbides become coarse and the amount of eutectic carbide increases excessively, promoting the initiation and growth of fatigue cracks and reducing fatigue resistance due to the formation of deep heat cracks. Furthermore, the increased amount of carbide increases residual stress, which may lead to breakage of the roll during roll manufacture or use in rolling. Therefore, the C content is limited to 2.3% or less. Preferably, it is set to 2.2% or less.

[0016] Si: 0.3 to 2.0% Si acts as a deoxidizer and is an element that improves the castability of molten metal. Furthermore, Si dissolves in the matrix and strengthens it. To achieve this effect, a Si content of 0.3% or more is required. The Si content is preferably 0.4% or more. On the other hand, if the Si content exceeds 2.0%, the effect saturates and no effect commensurate with the content can be expected, which is economically disadvantageous. Furthermore, the Si content may embrittle the matrix structure and deteriorate fatigue resistance. Therefore, the Si content is limited to 2.0% or less. Preferably, the Si content is 1.8% or less.

[0017] Mn: 0.3 to 2.0% Mn is an element that fixes S as MnS, rendering it harmless, and also has the effect of improving hardenability by partially dissolving in the matrix structure. Mn also dissolves in the matrix and strengthens it (solid solution strengthening). To achieve this effect, a content of 0.3% or more is required. The Mn content is preferably 0.4% or more. On the other hand, if the content exceeds 2.0%, the effect saturates and no effect commensurate with the content can be expected, and the material may even become embrittled. For this reason, the Mn content is limited to 2.0% or less. Preferably, it is 1.7% or less.

[0018] Cr: 3.5 to 7.0% Cr combines with C to form mainly eutectic carbides (M7C3 type carbides, M 23 Cr is an element that forms C6-type carbides and has the effect of improving wear resistance. To obtain this effect, a Cr content of 3.5% or more is required. The Cr content is preferably 3.8% or more. On the other hand, a Cr content exceeding 7.0% increases the amount of coarse eutectic carbides, thereby reducing fatigue resistance. For this reason, the Cr content is limited to 7.0% or less. Preferably, the Cr content is 6.5% or less.

[0019] Mo: 3.0 to 6.0% Mo is an element that combines with C to form hard carbides (MC carbides and MC carbides) and improves wear resistance. Furthermore, Mo dissolves in hard MC carbides, which are formed by combining V, Nb, and C, strengthening the carbides. It also dissolves in eutectic carbides, increasing the fracture resistance of these carbides. Through these actions, Mo improves the wear resistance of the roll outer layer material. To achieve this effect, a Mo content of 3.0% or more is required. The Mo content is preferably 3.5% or more. On the other hand, a Mo content exceeding 6.0% causes the formation of coarse eutectic carbides and reduces fatigue resistance. Therefore, the Mo content is limited to 6.0% or less. Preferably, it is 5.0% or less.

[0020] V: 3.0 to 5.0% V is an element that provides both wear resistance and fatigue resistance to the roll. V is an element that forms extremely hard carbides (MC type carbides) and improves wear resistance. This effect becomes significant when the V content is 3.0% or more. Therefore, the V content is set to 3.0% or more. The V content is preferably 3.3% or more. On the other hand, a V content exceeding 5.0% coarsens the MC type carbides and reduces seizure resistance. Therefore, the V content is limited to 5.0% or less. Preferably, it is 4.7% or less.

[0021] Nb: 0.1 to 2.0% Nb dissolves in MC carbides to strengthen them and increase their fracture resistance, thereby improving wear resistance. Nb also has the effect of suppressing segregation of MC carbides during centrifugal casting. This effect becomes significant when the Nb content is 0.1% or more. Therefore, the Nb content is set to 0.1% or more. The Nb content is preferably 0.2% or more. On the other hand, if the Nb content exceeds 2.0%, coarse MC carbides are formed, which deteriorates seizure resistance. Therefore, the Nb content is limited to 2.0% or less. Preferably, it is set to 1.8% or less.

[0022] Al: 0.01 to 0.10% Al is an element that acts as a deoxidizer and prevents internal defects such as porosity. This effect becomes significant when the Al content is 0.01% or more. Therefore, the Al content is set to 0.01% or more. The Al content is preferably 0.02% or more. On the other hand, if the Al content exceeds 0.10%, coarse Al-based oxides are formed, which reduces fatigue resistance. Therefore, the Al content is limited to 0.10% or less. Preferably, it is 0.09% or less.

[0023] Ni: 0.02 to 2.00% Ni is an element that dissolves in the matrix, lowers the austenite transformation temperature during heat treatment, and improves the hardenability of the matrix. This effect is significant when the Ni content is 0.02% or more. The Ni content is preferably 0.05% or more. If the Ni content exceeds 2.00%, the austenite transformation temperature becomes too low, making it more likely that austenite will remain after heat treatment. If austenite remains, wear resistance will deteriorate. Therefore, the Ni content is limited to 2.00% or less. From the viewpoint of hardenability, the Ni content is preferably 1.80% or less.

[0024] N: 0.050% or less N is an element that is mixed in from the atmosphere during the raw materials and melting / casting processes, and if it is contained in excess of 0.050%, coarse nitrides are formed, reducing fatigue resistance. Therefore, the N content is limited to 0.050% or less, preferably 0.045% or less.

[0025] The balance is Fe and unavoidable impurities, such as S, P, Cu, Ca, Sb, Zr, and O. These are mixed in from refractories and other materials during the raw materials or melting. In addition to the above components, it may contain one or more of Ti: 0.50% or less, B: 0.090% or less, Co: 1.0% or less, W: 1.5% or less, and Zr: 0.50% or less.

[0026] Ti: 0.50% or less Ti is an element that easily combines with oxygen in the molten metal to form oxides, and these oxides act as nuclei to form fine, uniform carbides in the matrix. This action contributes to improving wear resistance. This effect is significant when the Ti content is 0.50% or less. Therefore, the Ti content is limited to 0.50% or less. Preferably, the Ti content is 0.40% or less. To obtain this effect, the Ti content is preferably 0.01% or more, and more preferably 0.02% or more.

[0027] B: 0.090% or less B is an element that dissolves in the matrix and improves the hardenability of the matrix. This effect is significant when the content is 0.090% or less. If the content exceeds 0.090%, borocarbides are formed, saturating the hardenability improvement effect and reducing fatigue resistance. Furthermore, if B is mixed into the intermediate layer or inner layer, it may embrittle the intermediate layer or inner layer. Therefore, the B content is limited to 0.090% or less. It is preferably 0.080% or less, and more preferably 0.070% or less. From the viewpoint of hardenability, the B content is preferably 0.001% or more, and more preferably 0.002% or more.

[0028] Co: 1.0% or less Co is an element that dissolves in the matrix and increases the hardness of the matrix, thereby improving wear resistance. This effect is significant at a content of 1.0% or less. If the content exceeds 1.0%, the effect saturates and is economically disadvantageous. Therefore, the Co content is limited to 1.0% or less, preferably 0.9% or less. To obtain this effect, the Co content is preferably 0.1% or more, and more preferably 0.2% or more.

[0029] W: 1.5% or less W is an element that dissolves in the matrix, strengthens the matrix, and improves surface roughening resistance. It also forms M2C or M6C carbides, improving wear resistance. However, if the W content exceeds 1.5%, not only does the effect saturate, but coarse M2C or M6C carbides are formed, causing significant fatigue wear and reducing wear resistance. For these reasons, the W content must be 1.5% or less. The W content is preferably 1.2% or less.

[0030] Zr: 0.50% or less Zr is an element that combines with C to form MC carbides, improving wear resistance. However, if the content exceeds 0.50%, not only does the effect saturate, but coarse MC carbides are formed, reducing fatigue resistance. For these reasons, the Zr content must be 0.50% or less. The Zr content is preferably 0.30% or less.

[0031] The composition of the outer layer material for hot rolling rolls of the present invention must satisfy the following formulas (1) and (2): 0.85≦%Cr / (%Mo+%W / 2)≦1.15 (1) (%Cr+%Mo+%W) / (%V+%Nb)≧2.2 (2) Here, %Cr, %Mo, %W, %V, and %Nb are the contents (mass%) of each element, and elements that are not contained are set to 0. By satisfying the above formulas (1) and (2), it is possible to obtain a hot rolling roll outer layer material excellent in wear resistance, fatigue resistance, and seizure resistance. Specifically, if (%Cr / (%Mo+%W / 2)) is less than 0.85, the amount of M2C carbides increases, making it difficult to form M7C3 carbides, and therefore the carbide complex described below does not form; therefore, it must be 0.85 or more, preferably 0.88 or more. On the other hand, if (%Cr / (%Mo+%W / 2)) is more than 1.15, the amount of M7C3 carbides increases, making it difficult to form M2C carbides, and therefore the carbide complex described below does not form; therefore, it must be 1.15 or less, preferably 1.12 or less. Furthermore, if ((%Cr+%Mo+%W) / (%V+%Nb)) is less than 2.2, the amount of MC carbides increases, making it difficult to form M7C3 carbides and M2C carbides, resulting in the amount of carbide composites (described later) falling outside the preferred range. Therefore, it is necessary to make it 2.2 or more. It is preferably 2.3 or more. There is no particular upper limit, but it is preferably 3.8 or less.

[0032] The outer layer material for a hot rolling roll of the present invention is made of at least one carbide selected from Group A (M2C type carbide, M6C type carbide, MC type carbide) and at least one carbide selected from Group B (M7C3 type carbide, M 23 It is necessary to disperse in the matrix structure a carbide complex having at least one carbide selected from M2C type carbide, M6C type carbide, MC type carbide, M7C3 type carbide, M 23 The area of the matrix adjacent to the C6-type carbide is reduced, and the carbide composite is formed in a form that resembles a single coarse carbide, making it possible to improve seizure resistance. When a single coarse carbide is formed to improve seizure resistance, fatigue resistance decreases, but by forming a carbide composite, it is possible to achieve both seizure resistance and fatigue resistance. In particular, when the carbide composite has an area fraction of 2.0% or more, seizure resistance is significantly improved. Therefore, it is preferable that the carbide composite has an area fraction of 2.0% or more.

[0033] In the present invention, the carbide composite is a composite of at least one carbide selected from Group A (M2C type carbide, M6C type carbide, MC type carbide) and Group B (M7C3 type carbide, M 23 It refers to a carbide in which at least one carbide selected from group A (M2C type carbide, M6C type carbide, MC type carbide) and group B (M7C3 type carbide, M 23 The fact that at least one carbide selected from group A (MC type carbide, MC type carbide, MC type carbide) is adjacent to group B (MC type carbide, MC type carbide) means that the two carbides are in contact with each other. However, even if they are not in contact, it is possible that at least one carbide selected from group A (MC type carbide, MC type carbide, MC type carbide) is adjacent to group B (MC type carbide, MC type carbide, MC type carbide). 23 If the shortest distance between at least one carbide selected from the group consisting of MC type carbides, M2C type carbides, M6C type carbides, M7C3 type carbides, M8C type carbides, M9C type carbides, M10C type carbides, M11C type carbides, M12C type carbides, M13C type carbides, M14C type carbides, M15C type carbides, M16C type carbides, M17C type carbides, M18C type carbides, M19C type carbides, M20C type carbides, M21C type carbides, M22C type carbides, M23C type carbides, M24C type carbides, M25C type carbides, M26C type carbides, M27C type carbides, M28C type carbides, M29C type carbides, M30C type carbides, M31C type carbides, M32C type carbides, M33C type carbides, M34C type carbides, M35C type carbides, M36C type carbides, M37C type carbides, M38C type carbides, M39C type carbides, M40C type carbides, M41C type carbides, M42C type carbides, M43C type carbides, M44C type carbides, M45C type carbides, M46C type carbides, M47C type carbides, M48C type carbides, M49 ... 23 There are also C6 type carbides dispersed individually.

[0034] The carbide composite can be determined based on the measurement results of the SEM / EBSD method. Here, the evaluation can be performed by the method described below.

[0035] For the SEM / EBSD measurement, test pieces were taken from five arbitrary locations in the circumferential direction of the outer layer material of a hot rolling roll. After polishing the roll surface side, the sample was scanned at an accelerating voltage of 15 kV, a magnification of 150 times, and a step size of 0.5 μm to obtain a 600 × 600 μm image. 2The EBSD patterns obtained by EBSD measurement were classified into MC type carbides, M2C type carbides, M6C type carbides, M7C3 type carbides, and M 23 The software for the EBSD measurement device (for example, OIM Data Collection by TSL Solutions Co., Ltd.) is used to analyze which of the C6-type carbides best matches the sample. Using the obtained results, carbides belonging to group A (M2C type carbides, M6C type carbides, MC type carbides) are marked in red, and those belonging to group B (M7C3 type carbides, M 23 A structural image is obtained in which carbides belonging to the C6 type carbide are displayed in blue, other phases in black, and boundaries with an orientation difference of 15° or more with adjacent measurement data are displayed in white. The above-mentioned structural image processing is obtained by analyzing data obtained by SEM / EBSD measurement using OIM Analysis manufactured by EDAX. Using this structural image, when a red region and a blue region are adjacent to each other with a white line as the boundary, the combined region of the red region and the blue region is determined to be a carbide composite. The area ratio of the carbide composite is the area C (μm 2 ) and the measurement area of the SEM / EBSD method (600 × 600 μm 2 ) and the average area ratio of five points in the circumferential direction is the area ratio of that test piece.

[0036] Furthermore, since the formation of coarse MC type carbides makes seizure more likely to occur, the size of the MC type carbides is preferably 30 μm or less, more preferably 25 μm or less, in terms of circle equivalent diameter.

[0037] In the outer layer material for hot rolling rolls of the present invention, the matrix (structure other than carbides) preferably comprises 90% or more of bainite and / or tempered martensite in terms of area ratio. If the proportion of bainite and / or tempered martensite is less than 90% in terms of area ratio, the wear resistance and fatigue resistance decrease. Examples of the remaining structure include austenite, ferrite, and pearlite.

[0038] Next, a method for producing the outer layer material for a hot rolling roll and the composite roll for hot rolling according to the present invention will be described.

[0039] When casting a hot rolling roll outer layer material, first, the inner surface of a mold is coated with a refractory material to a thickness of 1 to 5 mm, and then the mold is rotated at a predetermined rotation speed, and a molten metal having the above-mentioned composition of the hot rolling roll outer layer material (simply referred to as outer layer material molten metal) is poured into the mold so as to obtain a predetermined thickness, followed by centrifugal casting. The above-mentioned refractory material may be, for example, a refractory material mainly composed of zircon.

[0040] When forming an intermediate layer, it is preferable to pour molten metal of the intermediate layer composition into the rotating mold during or after the outer layer material has solidified completely, and then perform centrifugal casting. After the outer layer or intermediate layer has completely solidified, it is preferable to stop the rotation of the mold, stand the mold, and then statically cast the inner layer material to form a composite roll. This remelts the inner surface of the roll outer layer material, resulting in a composite roll in which the outer layer and inner layer, or the outer layer and intermediate layer, or the intermediate layer and inner layer are welded together.

[0041] 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 intermediate layer and outer layer are welded together in the same manner, and the molten metal of the outer layer material is mixed into the molten metal of the intermediate layer material in a range of 10 to 35%. From the viewpoint of suppressing the amount of outer layer components mixed into the inner layer, it is important to reduce the amount of outer layer components mixed into the intermediate layer as much as possible, without interfering with the welding of the outer layer and intermediate layer. The rate of mixing of the molten metal of the outer layer material into the molten metal of the intermediate layer material can be calculated by the amount of outer layer material remelted by pouring the molten metal of the intermediate layer material relative to the total amount of the molten metal of the outer layer material ((amount of outer layer material remelted by pouring the molten metal of the intermediate layer material) / (total amount of molten metal of the outer layer material) × 100%).

[0042] As described above, a composite roll for hot rolling having two or more layers, i.e., an outer layer and an inner layer, can be obtained. In particular, it is preferable to obtain a composite roll for hot rolling having a three-layer structure of an outer layer, an intermediate layer, and an inner layer, or a two-layer structure of an outer layer and an inner layer.

[0043] Fig. 1 is a schematic side view showing an example of a conventional centrifugal casting mold that is axisymmetric about its axis of rotation, and Fig. 2 is a schematic side view showing an example of a centrifugal casting mold that is asymmetric about its axis of rotation according to the present invention. In Fig. 1, 1 denotes the axisymmetric centrifugal casting mold, 2a denotes rollers, 3a denotes the rotation axis, and 5a denotes a molten metal feed pipe. In Fig. 2, 4 denotes the asymmetric centrifugal casting mold, 2b denotes rollers, 3b denotes the rotation axis, and 5b denotes a molten metal feed pipe. In conventional centrifugal casting, as shown in Fig. 1, the molten metal for the outer layer material is poured from a molten metal feed pipe 5a into a centrifugal casting mold 1 that is symmetrical about its rotation axis 3a to form the outer layer. In the production of an outer layer material for a hot rolling roll of the present invention, as shown in Fig. 2, it is necessary to pour the molten metal for the outer layer material from a molten metal feed pipe 5b into a non-axisymmetric centrifugal casting mold 4 that is symmetrical about its rotation axis 3b to form the roll outer layer material. When a non-axisymmetric centrifugal casting mold 4 that has portions that are not point-symmetrical about its rotation axis is rotated, the center of gravity of the centrifugal casting mold 4 does not pass through the rotation axis, causing vibrations in the centrifugal casting mold 4. Therefore, by pouring the molten metal of the outer layer material into the mold, vibrations can be applied to the molten metal of the outer layer material during solidification, and as a result of rearrangement of the solid, one or more types of carbides from among MC type carbides, M2C type carbides, and M6C type carbides and M7C3 type carbides or M 23 A carbide composite is formed, which is a region in which one or more types of carbides from among the C6 type carbides are adjacent to each other. Furthermore, the matrix structure becomes finer due to the vibration of the mold, and even if drawing trouble occurs, cracks are less likely to occur and progress, making it possible to obtain an outer layer material for hot rolling rolls with excellent crack resistance. The presence or absence of mold vibration can be measured by measuring the distance between the laser distance meter and the outer surface of the mold using a laser distance meter (displacement meter) or the like, as described below.

[0044] In centrifugal casting, the centrifugal casting mold is rotated by rotating the rollers that come into contact with the mold, so the shape of the parts of the mold that come into contact with the rollers is important for rotating the mold. Therefore, even when a non-axisymmetric centrifugal casting mold 4 is used, the shape of the parts of the mold that come into contact with the rollers 2b must be axisymmetric, and the shape of the parts of the mold that do not come into contact with the rollers 2b must be non-axisymmetric.

[0045] The magnitude (amplitude) of vibration generated in the centrifugal casting mold 4 can be measured by measuring the distance between the laser distance meter and the mold outer surface using a laser distance meter (displacement meter) or the like. To achieve the effects of the present invention, it is necessary to apply vibrations that displace the centrifugal casting mold 4 by 100 μm or more in the direction perpendicular to the rotation axis 3b. The shape of the centrifugal casting mold 4 is determined so that such vibrations occur. To accurately measure the vibration amplitude of the centrifugal casting mold 4 using a laser distance meter or the like, the measurement position of the vibration amplitude must be symmetrical about the rotation axis 3b. For example, in Figure 1 or Figure 2, a laser distance meter is installed on an extension of roller 2a or 2b, and the distance between the laser distance meter and the mold outer surface is measured. The difference between the maximum and minimum values of the measured distance data is taken as the vibration amplitude. Note that the vibration amplitude value varies depending on the rotation speed of the centrifugal casting mold 4, so it is preferable to measure the vibration amplitude while the mold is rotating at the rotation speed used for centrifugal casting. In order to accurately evaluate the amplitude of vibrations occurring in the centrifugal casting mold 4, it is preferable to set the sampling period to 0.1 s or less when measuring the distance between the laser distance meter and the mold outer surface. Furthermore, since the surface roughness of the mold outer surface may affect the value of the vibration amplitude, when measuring the vibration amplitude of a non-axisymmetric mold, it is necessary to machine the mold so that the surface roughness of the mold outer surface is the same as that of an axisymmetric mold.

[0046] The composite roll for hot rolling of the present invention is preferably subjected to heat treatment (quenching and tempering) after casting. The heat treatment is preferably performed by performing a quenching step of heating to 900 to 1100°C and air-cooling or air-blast air-cooling, and a tempering step of further heating and holding at 450 to 570°C and cooling (air-cooling, air-blast air-cooling, furnace cooling) two or more times. Alternatively, after casting, quenching may not be performed, and a tempering step of heating and holding at 400 to 520°C and cooling (air-cooling, air-blast air-cooling, furnace cooling) two or more times.

[0047] The preferred hardness of the surface of the outer layer material in the hot rolling composite roll of the present invention is 75 to 86 HS (Shore hardness). If the hardness is lower than 75 HS, the wear resistance is likely to deteriorate, so 75 HS or more is preferred. More preferably, 76 HS or more. Conversely, if the hardness exceeds 86 HS, it becomes difficult to remove cracks formed on the surface of the hot rolling roll during hot rolling by grinding. A more preferred hardness is 85 HS or less.

[0048] The hot rolling roll outer layer material of the present invention is produced by centrifugal casting and can be used as a ring roll or sleeve roll as is, but is also used as an outer layer material for a hot rolling composite roll suitable for a hot finishing rolling stand. The hot rolling composite roll of the present invention comprises an outer layer and an inner layer welded and integrated with the outer layer. An intermediate layer may be disposed between the outer layer and the inner layer. That is, instead of the inner layer welded and integrated with the outer layer, an intermediate layer welded and integrated with the outer layer and another inner layer welded and integrated with the intermediate layer may be used. In the present invention, the composition of the intermediate layer is not particularly limited, but it is preferably a high-carbon material with 1.5 to 3.0 mass % C. [Example]

[0049] Molten metals for the roll outer layer materials shown in Table 1, Nos. 1 to 27, were melted in a high-frequency induction furnace and centrifugal cast to produce ring-shaped test specimen 6 (outer diameter 250 mm, inner diameter 170 mm, depth 70 mm). A non-axisymmetric centrifugal casting mold was used, with its thickness varying 1 to 5 mm in the circumferential direction. The inner diameter of the mold was 250 mm, and because the mold thickness varied in the circumferential direction, the outer diameter of the mold varied in the circumferential direction. The centrifugal casting mold was fixed to a disk-shaped steel material (outer diameter 350 mm) and connected to a motor via the disk-shaped steel material. The center of the disk-shaped steel material coincided with the axis of rotation. A laser rangefinder was installed approximately 10 mm away from the outer surface of the disk-shaped steel material, and the distance between the disk-shaped steel material and the laser rangefinder was measured at a sampling period of 0.1 s to measure the vibration amplitude of the centrifugal casting mold. The amplitude of vibration of the centrifugal casting mold measured with a laser distance meter before the molten metal for the outer layer material was 202 μm ((maximum value of the distance between the laser distance meter and the centrifugal casting mold measured with the laser distance meter) - (minimum value of the distance between the laser distance meter and the centrifugal casting mold measured with the laser distance meter)). The casting temperature was 1420°C, and the centrifugal force on the outer surface of the ring-shaped test specimen was 120 G (gravity factor). After casting, the specimen was quenched from 1000°C and tempered at 510°C three times. Hardness test specimens, EBSD measurement specimens, drop-weight frictional thermal shock test specimens, and hot abrasion test specimens were collected from the resulting ring-shaped test specimens and subjected to hardness tests, EBSD measurements, drop-weight frictional thermal shock tests, and hot abrasion tests. A reference steel (a conventional comparative steel) with the chemical composition shown in Table 1 was used to prepare ring-shaped test specimens in an axisymmetric centrifugal casting mold. After casting, the specimens were quenched from 950°C and tempered twice at 500°C. The vibration amplitude of the centrifugal casting mold, measured with a laser distance meter before the outer layer material was poured, was 58 μm.

[0050] The hardness test was performed as follows. A 20 × 20 × 10 mm hardness test piece was taken from an arbitrary position on the ring-shaped test material 6 (10 mm is in the radial direction of the ring-shaped test material). The 20 × 20 mm surface of the obtained hardness test piece was used as the hardness measurement surface, and the Vickers hardness HV50 was measured using a Vickers hardness tester (test force: 50 kgf (490 N)) in accordance with the provisions of JIS Z 2244, and the converted Shore hardness VHS (HS) was calculated using the conversion formula of JIS B 7731. Ten measurement points were measured, and the highest and lowest values were removed to calculate the average of the eight points, which was then used as the hardness of the test material.

[0051] The EBSD measurement was carried out as follows. As shown in Figure 3, five 10 x 10 x 5 mm (5 mm in the radial direction of the ring-shaped test material) test pieces 7 for EBSD measurement were taken from a position 10 mm inside from the outer surface of the ring-shaped test material 6. After mirror polishing the 10 x 10 mm surface of the obtained test pieces, they were subjected to EBSD measurement at an accelerating voltage of 15 kV, a magnification of 150, and a step size of 0.5 μm to obtain a 600 x 600 μm 2 Before the EBSD measurement, the MC, M2C, M6C, M7C3, and M 23 The OIM Data Collection by TSL Solutions Co., Ltd. was set up so that C6 type carbides could be measured. The obtained data was image processed using OIM Analysis by EDAX, and carbides belonging to group A (M2C type carbides, M6C type carbides, MC type carbides) were colored red, and carbides belonging to group B (M7C3 type carbides, M 23 A structural image was obtained in which carbides belonging to the C6 type carbide were displayed in blue, other phases in black, and boundaries with an orientation difference of 15° or more with adjacent measurement data displayed in white. Using this structural image, when a red area and a blue area were adjacent to each other with a white line as the boundary, the combined area of the red and blue areas was judged to be a carbide composite, and a 600 × 600 μm area was used. 2 The area C (μm) of the carbide composite within the region 2 The area ratio of the carbide composite was calculated by multiplying the area C by the area measured by the SEM / EBSD method (600 × 600 μm 2The area ratio was calculated using the formula C / (600×600)×100%, and the average area ratio of the five test pieces was taken as the area ratio of the test material.

[0052] The drop-weight frictional thermal shock test was performed as follows. A drop-weight frictional thermal shock test piece 12 (30 × 20 × 20 mm) was taken from the obtained ring-shaped test piece 6. In the drop-weight frictional thermal shock test, as shown in Figure 4, a weight 8 (50 kg) was dropped from a height of 1 m onto a rack 9. The energy generated by the drop-weight frictional thermal shock test was used to rotate a pinion 11 to which a mild steel mating piece 10 was attached, causing the mating piece 10 to rub strongly against the test piece 12. After the test, a photograph of the test piece surface was taken, and the area where the mating piece 10 adhered and the area where the mating piece 10 contacted were measured. The seizure area ratio (= (area where the mating piece 10 adhered) / (area where the mating piece 10 contacted) × 100%) was calculated to evaluate the seizure resistance. A seizure area ratio of less than 45% was rated "Good," and the seizure resistance was deemed acceptable.

[0053] The hot abrasion test was carried out as follows: A hot abrasion test piece 13 (outer diameter 60 mmφ, thickness 10 mm) was taken from the obtained ring-shaped test material 6. As shown in Figure 5, the hot wear test was performed using a two-disk rolling / sliding method between test piece 13 and mating piece 16. Test piece 13 was rotated at 700 rpm while being cooled with cooling water 14. A mating piece 16 (material: S45C, outer diameter: 190 mm, width: 15 mm, C1 chamfer) heated to 800°C by a high-frequency induction heating coil 15 was brought into contact with the rotating test piece 13 under a load of 490 N, and the test pieces were rolled together for 120 minutes at a sliding ratio of 9%. The wear volume of test piece 13 after the test (calculated from outer circumference 13A of the wear test piece before the test and outer circumference 13B of the wear test piece after the test) was measured, and the wear ratio (= (wear volume of the reference piece) / (wear volume of each test piece)) was calculated to evaluate the wear resistance of No. 28 manufactured using conventional technology as the standard. The abrasion resistance was evaluated as "good" when the abrasion ratio was 1.0 or more, and the abrasion resistance was judged to be acceptable.

[0054] The fatigue resistance was evaluated by a hot wear test. A hot wear test piece 13 (outer diameter 60 mm, thickness 10 mm) was taken from the obtained ring-shaped test material 6. The hot wear test was performed using a two-disk rolling and sliding method between the test piece 13 and a mating piece 16, as shown in Figure 5. The test piece 13 was rotated at 10 rpm while being cooled with cooling water 14. A mating piece 16 (material: S45C, outer diameter: 190 mm, width: 15 mm, C1 chamfer) heated to 1000°C by a high-frequency induction heating coil 15 was brought into contact with the rotating test piece 13 under a load of 980 N and rolled for 240 minutes at a sliding ratio of 5%. As shown in Figure 6, cross-sectional observation specimens 17 were taken from three random locations on the test specimen after the test. One of the cut surfaces of each cross-sectional observation specimen was mirror-polished and then observed under an optical microscope to measure the depth of cracks 18 formed on the surface of the hot abrasion test specimen 13 (the contact surface with the mating piece 16). Note that cracks 18 present at the end of the specimen were caused by the specimen shape rather than the specimen material, so if a portion of a crack 18 or the entire crack was contained within a 1 mm range on either side of the specimen, that crack 18 was excluded from the measurement. If the maximum crack depth was less than 1.2 mm, it was rated "Good" and the fatigue resistance was deemed to have passed.

[0055] The overall evaluation was passed when the wear resistance, fatigue resistance and seizure resistance were all rated "good."

[0056] [Table 1]

[0057] [Table 2]

[0058] From Table 2, it can be seen that the example of the present invention has wear resistance equal to or greater than that of Comparative Example No. 28 of the prior art, while also having excellent fatigue resistance and seizure resistance.

[0059] Comparative Example No. 11 had a C content below the range of the present invention, and therefore had a low hardness and did not achieve the desired wear ratio. Comparative Example No. 12 had a Si content below the range of the present invention, and therefore the wear ratio did not reach the desired value. Comparative Example No. 13 had a Mn content below the range of the present invention, and therefore the wear ratio did not reach the desired value. Comparative Example No. 14 had a V content below the range of the present invention, which is thought to have reduced the amount of MC carbides, resulting in a wear ratio that did not reach the desired value. Comparative Example No. 15 had a V content exceeding the range of the present invention, which is thought to have resulted in the formation of coarse MC type carbides, and the seizure area ratio did not reach the desired value. Comparative Example No. 16 had a Cr content below the range of the present invention, so the seizure resistance did not reach the desired value. Also, it is thought that the total amount of carbides was reduced, and the wear ratio did not reach the desired value. Comparative Example No. 17 had a Cr content exceeding the range of the present invention, and therefore the maximum crack depth did not reach the desired value. Comparative Example No. 18 had a Mo content below the range of the present invention, so the seizure resistance did not reach the desired value. Also, the amount of carbides was reduced, so the wear ratio did not reach the desired value. Comparative Example No. 19 had a Mo content exceeding the range of the present invention, and therefore the maximum crack depth and the seizure area ratio did not reach the desired values. Comparative Example No. 20 had a Ni content below the range of the present invention, and therefore had insufficient hardenability and did not achieve the desired wear ratio. Comparative Example No. 21 had a Ni content exceeding the range of the present invention, resulting in a large amount of residual austenite, which reduced hardness and prevented the wear ratio from reaching the desired value. Furthermore, the reduced hardness is thought to have caused slight deformation of the surface of the drop-weight frictional thermal shock test specimen during the drop-weight frictional thermal shock test. As a result, the seizure area ratio also did not reach the desired value. Furthermore, the maximum crack depth also did not reach the desired value. Comparative Example No. 22 had a Nb content below the range of the present invention, and therefore the wear ratio did not reach the desired value, and the maximum crack depth also did not reach the desired value. In the comparative example No. 23, the Nb content exceeded the range of the present invention, so coarse MC type carbides were formed, and the seizure area ratio did not reach the desired value. In addition, the maximum crack depth also did not reach the desired value. Comparative Example No. 24 had a W content exceeding the range of the present invention, so the maximum crack depth and seizure resistance did not reach the desired values. Also, coarse M2C and M6C carbides were formed, so the wear ratio did not reach the desired value. Comparative Example No. 25 had a value of formula (2) below the range of the present invention, and therefore did not achieve the desired seizure resistance. Comparative Example No. 26 had a Ti content exceeding the range of the present invention, so coarse Ti-based carbides were formed, and the seizure resistance did not reach the desired value. Also, the maximum crack depth did not reach the desired value. Comparative Example No. 27 had a value of formula (1) that exceeded the range of the present invention, and therefore the seizure resistance did not reach the desired value. In Comparative Example No. 28, the formulas (1) and (2) were outside the ranges of the present invention, and the seizure resistance did not reach the desired value. Also, the maximum crack depth did not reach the desired value. Therefore, according to the present invention, it is possible to manufacture an outer layer material for a hot rolling roll and a composite roll for hot rolling that are excellent in wear resistance, fatigue resistance, and seizure resistance, which results in a significant improvement in the surface quality of the rolled material and an increase in the roll life. [Explanation of symbols]

[0060] 1. Centrifugal casting mold (axisymmetric) 2a, 2b rollers 3a, 3b Rotation axis 4. Centrifugal casting mold (non-axisymmetric) 5a, 5b molten metal supply pipe 6 Ring-shaped test piece 7. EBSD measurement specimen 8 weight 9 racks 10 Counterpart 11 Pinion 12 Drop weight friction thermal shock test piece 13 Hot wear test piece (test piece) 13A Outer circumference of wear test piece before test 13B Outer circumference of wear test piece after test 14 Cooling water 15 High frequency induction heating coil 16 Counterpart 17 Test piece for cross-section observation 18 Crack

Claims

1. In mass%, C: 1.5-2.3%, Si: 0.3-2.0%, Mn: 0.3-2.0%, Cr: 3.5-7.0%, Mo: 3.0 to 6.0%, V: 3.0-5.0%, Nb: 0.1 to 2.0%, Al: 0.01-0.10%, Ni: 0.02-2.00%, N: 0.050% or less, Contains Or, furthermore, Ti: 0.50% or less, B: 0.090% or less, Co: 1.0% or less, W: 1.5% or less, Zr: 0.50% or less, Contains one or more selected from The balance consists of Fe and unavoidable impurities, The contents of Cr, Mo, W, V, and Nb have a composition that satisfies the following formulas (1) and (2): The outer layer material for a hot rolling roll is characterized in that it has carbides selected from at least one of Group A and Group B below, in which the shortest distance between the carbides is 2 μm or less, and further has dispersed therein carbide composites each having a size of 10 μm or more and 150 μm or less in terms of a circle-equivalent diameter: 0.85≦%Cr / (%Mo+%W / 2)≦1.15... (1) (%Cr+%Mo+%W) / (%V+%Nb)≧2.2... (2) Here, %Cr, %Mo, %W, %V, and %Nb are the contents (mass%) of each element, and elements that are not contained are set to 0. Group A: M ２ C-type carbonization, M ６ C type carbide, MC type carbide Group B: M ７ C ３ Type carbonized material, M ２３ C ６ type carbide

2. 2. The outer layer material for a hot rolling roll according to claim 1, wherein the carbide composites have an area ratio of 2.0% or more.

3. 3. A method for producing a hot rolling roll outer layer material according to claim 1, wherein a molten metal having the above-mentioned component composition is cast using a non-axisymmetric centrifugal casting mold having a portion that is not point-symmetric about the rotation axis of the centrifugal casting mold in a cross section perpendicular to the rotation axis of the centrifugal casting mold, to form the hot rolling roll outer layer material according to claim 1 or 2.

4. 3. A composite roll for hot rolling having two or more layers, an outer layer and an inner layer, characterized in that the outer layer is made of the outer layer material for hot rolling roll according to claim 1 or 2.

5. A method for producing a composite roll for hot rolling, comprising using an outer layer material obtained by the method for producing an outer layer material for a hot rolling roll according to claim 3.

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