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

Abstract

To provide a roll outer layer material for hot rolling and a composite roll for hot rolling, both having excellent wear resistance, slip resistance and roughening resistance.SOLUTION: A roll outer layer material for hot rolling has a component composition containing, by mass, 1.1 to 2.6% C, 0.15 to 2.50% Si, 0.15 to 2.50% Mn, 0.1 to 6.0% Ni, 1.5 to 10.0% Cr, 3.5 to 12.5% Mo, 2.5 to 7.5% V, 0.1 to 6.0% W, 0.01 to 0.04% P and 0.001 to 0.015% S, the content of Si, Mn, Ni, Cr, Mo, V and W satisfying a specific formula, the content of C, Cr, Mo, V and W satisfying a specific formula, and the balance Fe with inevitable impurities, in which a carbide having a particle diameter of 1.0 μm or larger is present with an area ratio of 7.5 to 18.0%, a eutectic cell size is 65 to 100 μm, and a Shore hardness at 600°C is ≥44.0 HS and ≤52.0 HS.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 that are excellent in wear resistance, slip resistance, and surface roughening resistance, and more particularly to a hot rolling roll outer layer material and a hot rolling composite roll that are suitable for rough rolling of steel plates. [Background technology]

[0002] In recent years, the demand for high-quality steel sheets has increased, necessitating the need for improvements in hot-rolling technology. Therefore, there is a strong demand for improvements in the properties of hot-rolling rolls used in hot-rolling equipment, specifically, improvements in wear resistance, seizure resistance, slip resistance, and surface roughening resistance. To improve wear resistance, HiCr cast steel rolls, which incorporate Cr-based M7C3 carbides into their structure, and high-speed steel rolls, which are based on high-speed steel (a type of tool steel) and contain carbide-forming elements such as V, Cr, Mo, and W, and incorporate large amounts of hard carbides such as V-based MC carbides, Mo- and W-based MC carbides, and Cr-based MC carbides (M represents the metallic element that forms the carbides), are being used. However, when a large amount of carbide is introduced to improve wear resistance, the roll surface roughness decreases due to high hardness and good wear resistance, and the coefficient of friction during rolling decreases, making slippage more likely to occur and also making the surface more prone to roughening due to the shedding of coarse carbide particles that occur when a large amount of carbide is introduced.

[0003] Various techniques have been disclosed to solve such problems. For example, Patent Document 1 proposes a composite roll for hot rolling, which contains one or more of C: 0.8 to 4.0%, Si: 0.2 to 2.0%, Mn: 0.2 to 2.0%, Cr: 3.0 to 15%, V: 3.0 to 15%, one or two of Mo and W: ≧2%, and Mo+0.5W: ≧6.1%, or further contains one or more of Ni: 0.2 to 5%, Co: 0.5 to 10%, Nb: 0.50 to 5.0%, and one or more of Al, Ti, and Zr: ≦0.5%, and the metal structure of the outer layer material has carbides with an area ratio of 5 to 30%, and the distribution of each carbide is such that the average gap between adjacent carbides is 20 μm or less. This allows the appropriate amount of granular carbide to be finely dispersed with small gaps between adjacent carbides, resulting in a composite roll for hot rolling with improved slip resistance and seizure resistance.

[0004] Patent Document 2 also describes a hot rolling roll having improved wear resistance, surface roughening resistance, and seizure resistance, which contains C: 1.0 to 2.6%, Si: 1.2% or less, Mn: 1.2% or less, Ni: 3.0% or less, Cr: 1.5 to 6.0%, Mo and W (as Mo+0.5W) 1.5 to 5.0%, V: 6.0 to 12.0%, Co: 5.0% or less, and at least one of Ti: 2.0% or less and Nb: 2.0% or less, with the balance being Fe and unavoidable impurities. MC type carbides having an area ratio Sr (%) of carbides with an equivalent particle size √Sc of 1 μm or more of 20% or less are dispersed so that the average intergranular spacing Lc (μm) is 30 μm or less.

[0005] Patent Document 3 discloses a steel sheet containing, by mass %, C: 0.90 to 1.40%, Si: 0.50 to 1.50%, Mn: 0.50 to 1.50%, Ni: 0.5 to 2.0%, Cr: 9.0 to 16.0%, Mo: 1.00 to 3.00%, Al: 0.010 to 0.030%, and at least one of V: ​​0.05 to 0.50%, Ti: 0.05 to 0.50%, and Nb: 0.02 to 0.20%, and the steel sheet contains, by mass %, 0.90 to 1.40%, Si: 0.50 to 1.50%, Mn: 0.50 to 1.50%, Ni: 0.5 to 2.0%, Cr: 9.0 to 16.0%, Mo: 1.00 to 3.00%, and ... the steel sheet contains, by mass %, 0.90 to 1.40%, Si: 0.50 to 1.50%, Mn: 0.50 to 1.50%, Ni: 0.5 to 2.0%, Cr: 9.0 to 16.0%, Mo: 1.0 A work roll for a roughing mill stand in a hot rolling mill has been proposed, characterized in that it has an outer shell made of cast steel having a molten metal composition that satisfies (1) the following formulas: (1) 8≦Cr / C≦14, (2) 3.0≦12.3C+0.55Cr−15.2≦7.0, and (3) 26.0≦15.5C+Cr, with the balance being Fe and unavoidable impurities, and a core material made of ductile cast iron cast inside the outer shell, which are integrated via an intermediate layer. This results in a work roll for a roughing mill stand in a hot rolling mill that is both highly resistant to wear and slip, achieved by adding appropriate amounts of C and Cr and controlling the amount of M7C3 carbide. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-255457 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-28453 [Patent Document 3] Japanese Patent Publication No. 2020-63485 Summary of the Invention [Problem to be solved by the invention]

[0007] However, with the increasing demand for high-quality steel sheets and the improvement of hot rolling technology for steel sheets, the properties required of hot rolling rolls are becoming increasingly strict, and in particular, there is a strong demand for improved wear resistance. If rolls are designed to meet the wear resistance requirement, they will become more prone to slippage and surface roughening, resulting in an increased frequency of roll trouble. The conventional hot rolling rolls described in Patent Documents 1 to 3 are insufficient in any of wear resistance, slip resistance, and surface roughening resistance.

[0008] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an outer layer material for a hot rolling roll and a composite roll for hot rolling which have excellent wear resistance, slip resistance and surface roughening resistance and which solve the above problems. [Means for solving the problem]

[0009] The inventors have conducted detailed investigations into the relationship between the base of a hot rolling roll, carbides in the structure, hardness, wear amount, friction coefficient, chemical components (composition), and the structure of the roll material, and the scale adhering to the roll. As a result, they have found that wear resistance, slip resistance, and surface roughening resistance can be improved by optimizing the chemical components, casting method, and heat treatment conditions so that the amount and type of carbides, hardness, and eutectic cell size fall within specific ranges. The present invention was completed based on these findings and further investigations. That is, the gist of the present invention is as follows. [1] In mass %, C: 1.1-2.6%, Si: 0.15 to 2.50% Mn: 0.15 to 2.50% Ni: 0.1 to 6.0% Cr: 1.5 to 10.0%, Mo: 3.5-12.5% V: 2.5-7.5%, W: 0.1 to 6.0%, P: 0.01-0.04%, S: Contains 0.001 to 0.015% The contents of Si, Mn, Ni, Cr, Mo, V, and W satisfy the following formula (1), the contents of C, Cr, Mo, V, and W satisfy the following formula (2), and the balance is Fe and unavoidable impurities, A roll outer layer material for hot rolling, characterized in that carbides having a particle size of 1.0 μm or more are present in an area ratio of 7.5 to 18.0%, the eutectic cell size is 65 to 100 μm, and the Shore hardness at 600°C is 44.0 HS or more and 52.0 HS or less. 50.0≦([%Cr]×[%Mo]×[%V]×[%W]) / ([%Si]×[%Mn]×[%Ni])≦150.0 (1) 0.90≦[%C]×((0.177×[%V]) / (0.099×[%Cr]+0.063×[%Mo]+0.033×[%W]))≦3.00 (2) In the formulas (1) and (2), [%C], [%Si], [%Mn], [%Ni], [%Cr], [%Mo], [%V], and [%W] are the contents (mass%) of each element. [2] A composite roll for hot rolling having two layers of an outer layer and an inner layer, or three layers of an outer layer, an intermediate layer and an inner layer, wherein the outer layer is, in mass%, C: 1.1-2.6%, Si: 0.15 to 2.50% Mn: 0.15 to 2.50% Ni: 0.1 to 6.0% Cr: 1.5 to 10.0%, Mo: 3.5-12.5% V: 2.5-7.5%, W: 0.1 to 6.0%, P: 0.01-0.04%, S: Contains 0.001 to 0.015% The contents of Si, Mn, Ni, Cr, Mo, V, and W satisfy the following formula (1), the contents of C, Cr, Mo, V, and W satisfy the following formula (2), and the balance is Fe and unavoidable impurities, A composite roll for hot rolling characterized in that carbides having a particle size of 1.0 μm or more are present in an area ratio of 7.5 to 18.0%, the eutectic cell size is 65 to 100 μm, and the Shore hardness at 600°C is 44.0 HS or more and 52.0 HS or less. 50.0≦([%Cr]×[%Mo]×[%V]×[%W]) / ([%Si]×[%Mn]×[%Ni])≦150.0 (1) 0.90≦[%C]×((0.177×[%V]) / (0.099×[%Cr]+0.063×[%Mo]+0.033×[%W]))≦3.00 (2) In the formulas (1) and (2), [%C], [%Si], [%Mn], [%Ni], [%Cr], [%Mo], [%V], and [%W] are the contents (mass%) of each element. [Effects of the Invention]

[0010] The present invention provides a hot rolling roll outer layer material and a hot rolling composite roll that are excellent in wear resistance, slip resistance, and surface roughening resistance, thereby improving the life of the hot rolling roll and the productivity of hot rolled steel sheets. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is an explanatory diagram schematically illustrating the configuration of a testing machine used in a hot rolling wear test. DETAILED DESCRIPTION OF THE INVENTION

[0012] The reasons for limiting the composition of the outer layer material of the hot rolling roll (hereinafter simply referred to as the outer layer) of the present invention will be explained below. Note that hereinafter, mass % will be simply referred to as % unless otherwise specified.

[0013] C: 1.1 to 2.6% Carbon combines with V, Cr, Mo, W, etc. to form hard carbides, contributing to improved wear resistance. It also improves matrix hardness through solid solution strengthening. If the carbon content is less than 1.1%, the amount of carbides is insufficient, making it impossible to obtain excellent wear resistance. Furthermore, the hardness decreases, causing plastic flow, which increases the coefficient of friction due to surface roughening caused by plastic flow. On the other hand, if the carbon content exceeds 2.6%, excessive carbides are formed, resulting in reduced surface roughening resistance. Furthermore, if the carbon content exceeds 2.6%, the hardness increases, causing a decrease in the coefficient of friction and slippage. Therefore, the carbon content is limited to 1.1% or more and 2.6% or less. The carbon content is preferably 1.3% or more, more preferably 1.5% or more. The carbon content is preferably 2.3% or less, more preferably 2.0% or less.

[0014] Si: 0.15 to 2.50% 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 0.15%, the deoxidizing effect is insufficient, and if the Si content exceeds 2.50%, the deoxidizing effect saturates. Therefore, the Si content is limited to 0.15% or more and 2.50% or less. The Si content is preferably 0.25% or more, and more preferably 0.60% or more. Furthermore, the Si content is preferably 2.00% or less, and more preferably 1.50% or less.

[0015] Mn: 0.15 to 2.50% Mn has the effect of deoxidizing the molten metal and fixing S, which has a negative effect on the product, as MnS. If the Mn content is less than 0.15%, the effect of fixing S as MnS is insufficient. On the other hand, if the Mn content exceeds 2.50%, the effect saturates. Therefore, the Mn content is limited to 0.15% or more and 2.50% or less. The Mn content is preferably 0.25% or more, and more preferably 0.35% or more. Furthermore, the Mn content is preferably 2.00% or less, and more preferably 1.50% or less.

[0016] Ni: 0.1 to 6.0% Ni has the effect of improving the hardenability of the matrix and improving the hardness of the matrix. If the Ni content is less than 0.1%, the effect of improving the matrix hardness is insufficient. On the other hand, if the Ni content exceeds 6.0%, austenite tends to remain, reducing the hardness. Therefore, the Ni content is limited to 0.1% or more and 6.0% 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.

[0017] Cr: 1.5 to 10.0% Cr is a carbide-forming element and combines with C to form M7C3 carbide. M7C3 carbide is a hard carbide and therefore has the effect of improving wear resistance. If the Cr content is less than 1.5%, the amount of M7C3 carbide is insufficient, resulting in reduced wear resistance. On the other hand, if the Cr content exceeds 10.0%, coarse M7C3 carbides are formed, which actually worsens wear resistance. Therefore, the Cr content is limited to 1.5% or more and 10.0% or less. The Cr content is preferably 2.5% or more, more preferably 3.5% or more. Furthermore, the Cr content is preferably 8.0% or less, more preferably 6.0% or less.

[0018] Mo: 3.5 to 12.5% Mo is a carbide-forming element and combines with C to form M2C carbides. M2C carbides are harder than M7C3 carbides, and therefore have the effect of further improving wear resistance. If the Mo content is less than 3.5%, the amount of M2C 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 carbides are formed, which deteriorates wear resistance and reduces toughness. Therefore, the Mo content is limited to 3.5% or more and 12.5% ​​or less. The Mo content is preferably 4.0% or more, more preferably 4.5% or more. The Mo content is also preferably 10.5% or less, more preferably 8.0% or less.

[0019] V: 2.5 to 7.5% V is a carbide-forming element and combines with C to form MC carbides. MC 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 carbides is insufficient, resulting in insufficient wear resistance. On the other hand, if the V content exceeds 7.5%, VC carbides, which have a lower specific gravity than the molten iron, concentrate inside the outer layer due to centrifugal force during centrifugal casting, causing segregation. Therefore, the V content is limited to 2.5% or more and 7.5% or less. The V content is preferably 3.0% or more, more preferably 4.0% or more. The V content is also preferably 7.0% or less, more preferably 6.5% or less.

[0020] W: 0.1 to 6.0% W is a carbide-forming element and combines with C to form hard carbides such as hard M2C, which increases the hardness of the outer layer and improves wear resistance. If the W content is less than 0.1%, this effect is insufficient, and wear resistance deteriorates. On the other hand, if the W content exceeds 6.0%, coarse M2C carbides are formed, which actually deteriorates wear resistance. Therefore, the W content is limited to 0.1% or more and 6.0% or less. The W content is preferably 0.5% or more, and more preferably 1.0% or more. Furthermore, the W content is preferably 5.0% or less, and more preferably 4.5% or less.

[0021] P: 0.01 to 0.04% It has been thought that P is mixed in during the roll manufacturing process and deteriorates the mechanical properties. However, 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 0.04%, the mechanical properties deteriorate. Therefore, the P content is limited to 0.01% or more and 0.04% or less. The P content is preferably 0.02% or more. Furthermore, the P content is preferably 0.03% or less.

[0022] S: 0.001 to 0.015% 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 acts as a lubricant. On the other hand, a high content makes the material brittle. Therefore, the S content is limited to 0.001% or more and 0.015% or less. The S content is preferably 0.002% or more. Furthermore, the S content is preferably 0.010% or less.

[0023] The present invention is also characterized in that the contents of C, Si, Mn, Ni, Cr, Mo, V, and W are within the above ranges, and further satisfy the following formulas (1) and (2): 50.0≦([%Cr]×[%Mo]×[%V]×[%W]) / ([%Si]×[%Mn]×[%Ni])≦150.0 (1) 0.90≦[%C]×((0.177×[%V]) / (0.099×[%Cr]+0.063×[%Mo]+0.033×[%W]))≦3.00 (2) In the formulas (1) and (2), [%C], [%Si], [%Mn], [%Ni], [%Cr], [%Mo], [%V], and [%W] are the contents (mass%) of each element.

[0024] In equation (1), ([%Cr] × [%Mo] × [%V] × [%W]) / ([%Si] × [%Mn] × [%Ni]) represents the ratio of the product of the contents of each carbide-forming element (V, Cr, Mo, W) to the product of the contents of each non-carbide-forming element (Si, Mn, Ni). By adjusting the value of equation (1), the amount of carbide formed and the eutectic cell size are optimized, resulting in improved wear resistance, slip resistance, and surface roughness resistance. If the value of ([%Cr] × [%Mo] × [%V] × [%W]) / ([%Si] × [%Mn] × [%Ni]) is less than 50.0, the amount of carbide formed is insufficient, resulting in insufficient wear resistance. Furthermore, the eutectic cell size becomes too large, making it difficult to maintain a high coefficient of friction during rolling, resulting in insufficient slip resistance. On the other hand, if it exceeds 150, a large amount of coarse carbides are generated, making it impossible to obtain sufficient wear resistance and surface roughening resistance. Furthermore, the eutectic cell size becomes smaller, which increases the friction coefficient during rolling and makes the steel more susceptible to seizure. Therefore, the value of ([%Cr] × [%Mo] × [%V] × [%W]) / ([%Si] × [%Mn] × [%Ni]) is limited to 50 or more and 150 or less. It is more preferably 70 or more and 130 or less.

[0025] In equation (2), [%C] × ((0.177 × [%V]) / (0.099 × [%Cr] + 0.063 × [%Mo] + 0.033 × [%W])) represents the ratio of the amount of MC carbide to (the amount of MC carbide + the amount of MC carbide). By adjusting the amount of carbide to satisfy equation (2), the ratio of each carbide is optimized, improving wear resistance and surface roughness resistance and enabling the friction coefficient during rolling to be set to a value that prevents seizure and slippage. If the value of [%C] × ((0.177 × [%V]) / (0.099 × [%Cr] + 0.063 × [%Mo] + 0.033 × [%W])) is less than 0.90, the amount of hard MC carbide is low, and sufficient wear resistance cannot be obtained. Furthermore, the amount of MC carbide decreases, making it impossible to maintain a high coefficient of friction during rolling and resulting in insufficient slip resistance. On the other hand, if the ratio exceeds 3.00, the proportion of MC carbide increases. This is a fine granular carbide, and the number of protrusions on the roll surface increases, increasing the coefficient of friction during rolling and making seizure more likely to occur. Therefore, the value of [%C] × ((0.177 × [%V]) / (0.099 × [%Cr] + 0.063 × [%Mo] + 0.033 × [%W])) is limited to 0.90 or more and 3.00 or less. More preferably, it is 1.20 or more and 2.50 or less.

[0026] Remainder: Fe and unavoidable impurities The balance other than the above components consists of Fe and unavoidable impurities.

[0027] Next, the reasons for limiting the structure of the outer layer material for hot rolling rolls of the present invention will be explained.

[0028] The outer layer material for hot rolling rolls of the present invention is characterized by having a chemical composition within the above-mentioned ranges, an area ratio of 7.5 to 18.0% of carbides with a particle size of 1.0 μm or more, and a eutectic cell size of 65 to 100 μm. The carbides are MC carbides, M2C carbides, and M7C3 carbides crystallized during solidification. Furthermore, since the precipitated carbides present in the matrix are less than 1.0 μm, the carbides are limited to carbides of 1.0 μm or more. Here, the matrix is ​​preferably martensite or bainite. To achieve an appropriate ratio of each carbide, we have discovered that by limiting the structure to one with a composition within the above-mentioned ranges, an area ratio of 7.5 to 18.0% of carbides with a particle size of 1 μm or more, and a eutectic cell size of 65 to 100 μm, wear resistance, slip resistance, and surface roughening resistance are improved. The eutectic cell size is the average value of the eutectic cell size measured by the method described below. Although various studies have been conducted on hot rolling rolls, the relationship between hot rolling rolls and scale has received little attention. As a result of extensive research, the inventors discovered that scale adheres to the base of the roll but not to the carbides, resulting in unevenness on the roll surface due to the presence or absence of scale. It was previously believed that it was impossible to simultaneously satisfy all of the following requirements: wear resistance, slip resistance, and surface roughness resistance. However, by optimizing the carbide area ratio and the ratio of each carbide type to improve wear resistance, adjusting the eutectic cell size, and controlling the unevenness spacing on the roll surface, it is possible to maintain a high friction coefficient and improve slip resistance. Furthermore, it is believed that scale peeling, one of the causes of surface roughness, occurs when the scale on the base inside the eutectic cell is subjected to stress in the circumferential direction of the roll due to roll rotation, causing the scale to peel off. Therefore, by controlling the eutectic cell size within an appropriate range, it is believed that scale peeling can be suppressed and surface roughness resistance can be improved.

[0029] If the eutectic cell size is less than 65 μm, the surface irregularities are too great, resulting in a high coefficient of friction and seizure. On the other hand, if it exceeds 100 μm, the surface irregularities are too small, resulting in an insufficient coefficient of friction and slippage. Therefore, the eutectic cell size is limited to 65 μm or more and 100 μm or less. The eutectic cell size is preferably 70 μm or more, more preferably 75 μm or more. Furthermore, the eutectic cell size is preferably 95 μm or less, more preferably 90 μm or less.

[0030] The structure (matrix structure) may contain martensite or bainite in an area ratio of 82.0 to 92.5% in addition to carbides.

[0031] The method for observing the tissue is as follows. First, the obtained outer layer material was mirror-polished and then etched with nital solution, after which the structure was observed using a digital microscope. Images were taken in a field of view where more than 200 eutectic cells could be seen. Images were then binarized using an image analysis tool (ImageJ) at 200x magnification. Because there was a difference in brightness between the matrix and carbides in the photographs, binarization enabled classification of the matrix and carbides and calculation of their areas. Five images were taken for each sample, and the average carbide area ratio was calculated. The diameter of the carbides was measured from the digital microscope images taken at 200x magnification, and this diameter was used as the carbide grain size. Furthermore, if the carbide shape was elliptical, a perpendicular bisector was drawn across the longest line within the carbide, and the length between the two points where this perpendicular bisector intersected with the grain boundary of the carbide was used as the carbide grain size.

[0032] The outer layer material obtained was then mirror-polished and etched with nital solution, after which the structure was observed using a digital microscope at a magnification of 100x. Three fields of view were photographed for each sample, and a total of six lines were drawn on each resulting image. As shown in equation (3) below, the eutectic cell size was calculated by dividing the number of intersections between the lines and carbides with a grain size exceeding 5.0 μm by the length of the lines. The eutectic cell size for each sample was obtained from the average value of the three fields of view. The eutectic cell size was determined using the above method, taking advantage of the fact that carbides with a grain size exceeding 5.0 μm exist at the boundaries of eutectic cells. ((L1 / N1)+(L2 / N2)+(L3 / N3)+(L4 / N4)+(L5 / N5)+(L6 / N6)) / 6 ···(3) Here, Li is the length of the ith straight line, and Ni is the number of intersections between the ith straight line and carbides with a grain size exceeding 5.0 μm.

[0033] The outer layer material for hot rolling rolls of the present invention has a Shore hardness of 44.0 HS or more and 52.0 HS or less at 600° C. The Shore hardness at 20° C. is preferably 74.0 HS or more and 86.0 HS or less.

[0034] Furthermore, the roll surface temperature during hot rolling is approximately 600°C, and if the Shore hardness at 600°C is less than 44.0HS, plastic flow occurs, and the steel material is likely to seize onto the roll surface. On the other hand, if the hardness exceeds 52.0HS, the roll hardness is too high, making slippage more likely to occur during rolling. Such hardness can be reliably ensured by heat treating a roll having the composition of the present invention so that the tempering parameter P, described below, is in the range of 10,000 to 20,000.

[0035] Regarding the Shore hardness at 20°C and at 600°C, first, Vickers hardness HV is measured at five points each at 20°C and 600°C using a Vickers hardness tester (test force: 1 kgf), and the average value is calculated.

[0036] First, to measure Vickers hardness at 20°C, a Nikon QM-2 testing machine (simultaneous heating of indenter and test piece) is used, with a diamond indenter, and the test atmosphere is argon gas, with a load holding time of 10 seconds. After that, the temperature is raised to 600°C at a rate of 20°C / min, and the Vickers hardness at 600°C is measured under the same test conditions as at 20°C. Note that the Vickers hardness measurement at 600°C complies with JIS Z2252 "High-Temperature Vickers Hardness Test Method." These obtained Vickers hardness values ​​are converted into Shore hardness values ​​using the calculation formula of JIS B 7731.

[0037] Next, a preferred 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.

[0038] In the present invention, the method for producing the roll outer layer material is not particularly limited, and centrifugal casting, continuous build-up casting, etc. are preferred, but from the viewpoint of production costs, centrifugal casting is more preferred. When centrifugal casting is adopted, first, a molten metal having the above-mentioned composition of the outer layer material for hot rolling rolls (simply referred to as molten metal for outer layer material) is poured into a rotating mold whose inner surface is covered 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.

[0039] When the roll outer layer material is cast by centrifugal casting, 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. An intermediate layer may be disposed between the outer layer and the inner layer. That is, the composite roll for hot rolling may have three layers: an intermediate layer welded and integrated with the outer layer, and an inner layer welded and integrated with the intermediate layer, instead of the inner layer welded and integrated with the outer layer. The inner layer is preferably produced by static casting.

[0040] For the statically cast inner layer, it is preferable to use spheroidal graphite cast iron, worm-shaped graphite cast iron (CV cast iron), or the like, which has excellent castability and mechanical properties. In a centrifugal cast roll, the outer layer and 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.

[0041] Furthermore, when an intermediate layer is formed, it is preferable to use graphite steel, high carbon steel, hypoeutectic cast iron, etc. as the intermediate layer material. The intermediate layer and the outer layer are welded together, and components of the outer layer material are mixed into the intermediate layer. In order to suppress the mixing rate of the outer layer material components into the inner layer, it is preferable to minimize the mixing rate of the outer layer material into the intermediate layer.

[0042] To obtain the target structure in the outer layer (hot rolling roll outer layer material) constituting the hot rolling composite roll, in which carbides with a particle size of 1.0 μm or more are present in an area ratio of 7.5 to 18.0% and the eutectic cell size is 65 to 100 μm, the mold temperature is set to 250°C or less and casting is performed at a casting temperature of 1350 to 1550°C. Furthermore, the heat treatment is preferably a quenching treatment in which the material is heated to 900 to 1100°C and then air-cooled or air-blast cooled, and a tempering treatment in which the material is heated and held at that temperature and then cooled two or more times so that the tempering parameter P defined in the following formula (4) is in the range of 10,000 to 20,000. The quenching temperature, tempering parameter, and number of tempering treatments can be changed within the ranges specified depending on the components to obtain the aforementioned structure. P = T(log(t) + A) (4) where T is the tempering temperature (K), t is the tempering time (h), and A is a constant (A=20 in the present invention).

[0043] As a result, a composite roll for hot rolling having three layers, namely, an outer layer, an intermediate layer, and an inner layer, or two layers, namely, an outer layer and an inner layer, can be obtained. [Example]

[0044] The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

[0045] The test materials Nos. 1 to 6 of the present invention and Nos. 7 to 16 of the comparative examples, each having the chemical composition of the outer layer material for hot rolling rolls shown in Table 1 (the remainder being Fe and unavoidable impurities), were heated to 1450 to 1550°C, melted, and cast into a Y-shaped keel block mold (rectangular parallelepiped portion: thickness 35 mm, width 230 mm, height 120 mm). After cooling, the ingots were removed and quenched at 900 to 1100°C. Then, tempering treatment was performed three times, in which the temperature was heated and held and then cooled, so that the tempering parameter P was in the range of 10,000 to 20,000. Then, microstructure observation, hardness measurement, and hot rolling wear test were performed. Note that in these examples, the test specimens for microstructure observation, hardness measurement, and hot rolling wear test were taken from the center of the wall thickness, but they may be taken from any position within the outer layer material.

[0046] [Table 1]

[0047] Each sample cut out from the ingots of the present invention and comparative examples was measured for Vickers hardness HV at 20°C and 600°C at five points using a Vickers hardness tester (test force: 1 kgf), and the average value was calculated.

[0048] For Vickers hardness measurements at 20°C, a Nikon QM-2 testing machine (simultaneous heating of indenter and test piece) was used, with a diamond indenter. The test was conducted in an argon gas atmosphere with a load holding time of 10 seconds. The temperature was then raised to 600°C at a rate of 20°C / min, and the Vickers hardness was measured at 600°C under the same test conditions as at 20°C. The Vickers hardness measurements at 600°C were conducted in accordance with JIS Z2252, "High-Temperature Vickers Hardness Test Method." The obtained Vickers hardness was converted to Shore hardness using the formula in JIS B 7731.

[0049] The hot rolling wear test was performed as follows. Hot rolling wear test specimens (outer diameter 60 mm, width 10 mm, C1 chamfered) were collected from the resulting ingots of each of the invention examples and comparative examples. The wear test was performed using a two-disk sliding rolling method between test specimen 1 and a mating piece 4, as shown in FIG. 1 . Test specimen 1 was rotated at 700 rpm while being cooled with cooling water 2, and a mating piece 4 (outer diameter 190 mm, width 15 mm, C1 chamfered) heated to 800°C by a high-frequency induction heating coil 3 was rolled against the rotating test specimen 1 under a load of 686 N in load direction 7. The direction of rotation 5 of test specimen 1 and the direction of rotation 6 of mating piece 4 were determined so that the tangent at the contact point between test specimen 1 and mating piece 4 was the same direction. The wear test was carried out for 135 minutes, with the mating piece replaced with a new one every 45 minutes (31,500 rotations of the test piece) for a total of three tests (94,500 rotations of the test piece), and the mass loss of the test piece before and after the test, i.e., the amount of wear, was measured.

[0050] The friction coefficient was measured using a hot rolling wear tester as follows. As in the hot rolling wear test, hot rolling wear test specimens (outer diameter 60 mm, width 10 mm, C1 chamfered) were collected from the resulting ingots of each of the invention examples and comparative examples. The 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 76 rpm while being cooled with cooling water 2. A mating piece 4 (outer diameter 190 mm, width 15 mm, C1 chamfered) heated to 1000°C by a high-frequency induction heating coil 3 was rolled against the rotating test specimen 1 under a load of 686 N in load direction 7. The rotation direction 5 of test specimen 1 and the rotation direction 6 of mating piece 4 were the directions in which the tangents at the contact points between test specimen 1 and mating piece 4 were aligned. The friction coefficient measurement test was performed for 5 hours, with the mating piece replaced with a new one every hour, for a total of 5 tests. The torque and load during the test were measured, and the friction coefficient was calculated using the following formula (5). μ = T / P × L (5) where μ is the friction coefficient, T is the torque (kgf m), P is the load (kgf), and L is the radius of the test piece (m).

[0051] Since the above test is based on the assumption of continuous hot rolling operation, the mating piece is heated to 1000°C and the friction coefficient and seizure state are evaluated after 300 minutes.

[0052] After heat treatment, each sample was mirror-polished and etched with nital solution. The microstructure was then observed using a digital microscope. Images were taken in a field of view that allowed for at least 200 eutectic cells to be identified. Images were then binarized using an image analysis tool (ImageJ) at 200x magnification. Because the brightness of the matrix and carbides differed in the photographs, binarization allowed for classification and area determination. Five images were taken for each sample, and the average carbide area ratio was calculated. The diameter of the carbides was measured from the digital microscope images taken at 200x magnification, and this was used as the carbide grain size. In addition, for carbide shapes such as ellipses, a perpendicular bisector was drawn across the longest line within the carbide, and the distance between the two points where this perpendicular bisector intersected with the grain boundary of the carbide was used as the carbide grain size.

[0053] To calculate the eutectic cell size, the outer layer material of each heat-treated sample was mirror-polished and then etched with nital solution, after which the structure was observed at 100x magnification using a digital microscope. Three fields of view were photographed for each sample, and a total of six lines were drawn on each resulting image. As shown in equation (3) below, the eutectic cell size was calculated by dividing the number of intersections between the lines and carbides with a grain size exceeding 5.0 μm by the length of the lines. The eutectic cell size for each sample was obtained from the average value of the three fields of view. ((2574 / N1)+(2574 / N2)+(2574 / N3)+(2574 / N4)+(2574 / N5)+(2574 / N6)) / 6 ···(3) Here, the straight line length is 2574 μm, and Ni is the number of intersections between the i-th line and carbides with a grain size exceeding 5.0 μm.

[0054] The results obtained are shown in Table 2.

[0055] [Table 2]

[0056] In Table 2, a wear amount of 0.46 g or less was considered pass (meaning good wear resistance), and a value greater than 0.46 g was considered fail. The coefficient of friction was also considered pass in the range of 0.12 to 0.30, while values ​​less than 0.12 or greater than 0.30 were considered fail. When the coefficient of friction was less than 0.12, the low coefficient of friction during the test meant insufficient slip resistance, so a friction coefficient of 0.12 or greater was considered excellent in slip resistance. Furthermore, when neither seizure nor surface roughening occurred, the surface roughening resistance was considered excellent. As is clear from Table 2, the comparative examples were not excellent in either or both of the wear resistance and slip resistance, while the examples of the present invention were confirmed to have excellent wear resistance and slip resistance. Furthermore, when the coefficient of friction was greater than 0.30, seizure was likely caused by a high coefficient of friction during the test. By optimizing the carbide area ratio and the ratio of each carbide type to improve wear resistance, and by adjusting the eutectic cell size and controlling the irregularity intervals on the roll surface, it became possible to maintain a high coefficient of friction and simultaneously improve slip resistance. Furthermore, the test specimen surfaces of the inventive examples showed good surface roughening resistance without the occurrence of seizure or surface roughening.

[0057] Therefore, according to the present invention, it is possible to manufacture a hot rolling roll outer layer material and a composite roll that are excellent in wear resistance, slip resistance, and surface roughening resistance. 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]

[0058] 1: Test piece 2: Cooling water 3: High frequency induction heating coil 4: Opposite side 5: Rotation direction of test piece 6: Rotation direction of the mating piece 7: Load direction

Claims

1. In mass%, C: 1.1-2.6%, Si: 0.15-2.50%, Mn: 0.15-2.50%, Ni: 0.1 to 6.0%, Cr: 1.5-10.0%, Mo: 3.5-12.5%, V: 2.5-7.5%, W: 0.1-6.0%, P: 0.01-0.04%, S: 0.001 to 0.015%; The contents of Si, Mn, Ni, Cr, Mo, V, and W satisfy the following formula (1), the contents of C, Cr, Mo, V, and W satisfy the following formula (2), and the balance is Fe and unavoidable impurities, A roll outer layer material for hot rolling, characterized in that carbides having a particle size of 1.0 μm or more are present in an area ratio of 7.5 to 18.0%, the eutectic cell size is 65 to 100 μm, and the Shore hardness at 600°C is 44.0 HS or more and 52.0 HS or less. 50.0≦([%Cr]×[%Mo]×[%V]×[%W]) / ([%Si]×[%Mn]×[%Ni])≦150.0...(1) 0.90≦[%C]×((0.177×[%V]) / (0.099×[%Cr]+0.063×[%Mo]+0.033×[%W]))≦3.00 (2) In the formulas (1) and (2), [%C], [%Si], [%Mn], [%Ni], [%Cr], [%Mo], [%V], and [%W] represent the contents (mass%) of each element.

2. 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, wherein the outer layer comprises, in mass %, C: 1.1-2.6%, Si: 0.15-2.50%, Mn: 0.15-2.50%, Ni: 0.1 to 6.0%, Cr: 1.5-10.0%, Mo: 3.5-12.5%, V: 2.5-7.5%, W: 0.1-6.0%, P: 0.01-0.04%, S: 0.001 to 0.015%; The contents of Si, Mn, Ni, Cr, Mo, V, and W satisfy the following formula (1), the contents of C, Cr, Mo, V, and W satisfy the following formula (2), and the balance is Fe and unavoidable impurities, A composite roll for hot rolling, characterized in that carbides having a particle size of 1.0 μm or more are present in an area ratio of 7.5 to 18.0%, the eutectic cell size is 65 to 100 μm, and the Shore hardness at 600°C is 44.0 HS or more and 52.0 HS or less. 50.0≦([%Cr]×[%Mo]×[%V]×[%W]) / ([%Si]×[%Mn]×[%Ni])≦150.0...(1) 0.90≦[%C]×((0.177×[%V]) / (0.099×[%Cr]+0.063×[%Mo]+0.033×[%W]))≦3.00 (2) In the formulas (1) and (2), [%C], [%Si], [%Mn], [%Ni], [%Cr], [%Mo], [%V], and [%W] represent the contents (mass%) of each element.

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