Roll outer layer material for hot rolling and composite roll for hot rolling

Abstract

To provide a roll outer layer for hot rolling and a composite roll for hot rolling which demonstrate superior wear resistance.SOLUTION: A roll outer layer for hot rolling has a specific composition, and includes a structure containing 5.0-20.0% fine carbide with a particle size of 1 μm or less by area ratio, wherein the fine carbide has an average particle size of 0.50-0.80 μm in circle-equivalent diameter, the Shore hardness is 75.0 HS or more and 85.0 HS or less at 20°C, and the Shore hardness is 47.0 HS or more at 600°C. 3.0≤([%V]+[%Cr]+[%Mo]+[%W]) / ([%Si]+[%Mn]+[%Ni])≤12.0 (1) and 20.0≤[%C]×([%V]+[%Cr]+[%Mo]+[%W])≤65.0 (2), where, [%C], [%Si], [%Mn], [%Ni], [%Cr], [%Mo], [%V], and [%W] each denote the content of the corresponding element (mass%).SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a hot rolling roll outer layer material having excellent wear resistance and a hot rolling composite roll, and particularly to a hot rolling roll outer layer material and a hot rolling composite roll suitable for rough rolling of steel plates.

Background Art

[0002] In recent years, the demand for high-quality steel plates has been increasing, and accordingly, there has been a demand for improving the hot rolling technology of steel plates. Therefore, there has been a strong demand for improving the characteristics of hot rolling rolls used in hot rolling equipment, particularly improving wear resistance. To improve wear resistance, high-speed steel, which is a kind of tool steel, contains carbide-forming elements such as V, Cr, Mo, and W, and V-based MC carbides, Mo, W-based M 2 C carbides, Cr-based M 7 C 3 High-speed rolls with a large amount of hard carbides such as carbides have been developed.

[0003] As an outer layer material of such a high-speed roll, for example, Patent Document 1 proposes a rolling roll characterized by being an Fe-based alloy containing C: 1.0 to 2.6%, Cr: 4.0 to 10.0%, Mo: 5.0 to 10.0%, W: less than 5.0%, V: 3.0 to 8.0%, and satisfying 12.0% ≤ 2Mo + W ≤ 20.0%, 2Mo / W ≥ 3.0, 0.2% ≤ C - 0.24V ≤ 0.7%. By this, MC, M 4 C 3 、M 2 C、M 6 C carbides are generated in an optimal range, and it is said that a rolling roll excellent in wear resistance is obtained.

[0004] In addition, Patent Document 2 proposes an outer layer material for a composite roll for rolling, which contains, by mass%, C: 0.7 to 3.6%, Si: 0.2 to 2.5%, Mn: 0.2 to 2.0%, Cr: 2.0 to 10%, Mo: 0.2 to 10%, V: 2.0 to 10%, B: 0.001 to 0.50%, Al: 0.001 to 0.50%, Ti: 0.001 to 0.50%, Zr: 0.001 to 0.50%, Cu: 0.001 to 0.50%, Mg: 0.001 to 0.50%, Ca: 0.001 to 0.50%, with the balance being Fe and inevitable impurities, and further contains one or more of Ni: 0.1 to 10%, W: 0.2 to 10%, Nb: 0.2 to 10%, Co: 0.2 to 10%. By obtaining a microstructure in which MC carbides are crystallized finely, uniformly, and spherically, it is said to be an outer layer material for a composite roll for rolling with excellent wear resistance.

[0005] Patent Document 3 proposes a composite roll for hot rolling, which has a chemical composition containing, by mass basis, C: 1 to 3%, Si: 0.4 to 3%, Mn: 0.3 to 3%, Ni: 1 to 5%, Cr: 2 to 7%, Mo: 3 to 8%, V: 3 to 7%, and B: 0.01 to 0.12%, with the balance being Fe and inevitable impurities, and satisfies the relationship represented by the following formula (1): Cr / (Mo + 0.5W) < -2 / 3[C - 0.2(V + 1.19Nb)] + 11 / 6 (wherein when not containing optional components W and Nb, W = 0 and Nb = 0), and contains, by area ratio, 1 to 15% of MC carbides, 0.5 to 20% of carbon borides, and 0.5 to 20% of Mo-based carbides. It is said to be a composite roll for hot rolling with excellent wear resistance due to the MC carbides.

[0006] Patent Document 4 discloses an Fe alloy which, on a mass basis, contains C: 1.50 to 2.70%, Si: 0.3 to 3%, Mn: 0.1 to 3%, Ni: 0.1 to 2.5%, Cr: 4.0 to 7.0%, Mo: 4.1 to 8.0%, V: 5.0 to 10.0%, W: 0 to 0.4%, Nb: 0.1 to 3.0%, N: 0.005 to 0.15%, B: 0 to 0.05%, further contains at least one selected from the group consisting of Co: 0.1 to 5%, Zr: 0.01 to 0.5%, Ti: 0.05 to 0.5% and Al: 0.001 to 0.5%, with the balance being substantially Fe and inevitable impurities, the ratio V / Nb of the V content (mass%) to the Nb content (mass%) being 1 to 20.0, and C-bal represented by the following formula: C-bal = C% - 0.2×V% - 0.06×Cr% - 0.063×Mo% - 0.033×W% - 0.13×Nb% [where C%, V%, Cr%, Mo%, W% and Nb% are the contents (mass%) of C, V, Cr, Mo, W and Nb respectively] being 0 to 0.28, and proposes a centrifugally cast composite roll for rolling with excellent wear resistance.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, as the demand for high-quality steel plates increases and the hot rolling technology of steel plates improves, the properties required for hot rolling rolls are becoming increasingly strict, and in particular, higher wear resistance is required. Therefore, in the conventional hot rolling rolls described in Patent Documents 1 to 4, there are cases where the wear resistance is not sufficient.

[0009] Therefore, an object of the present invention is to provide a hot rolling roll outer layer material and a hot rolling composite roll excellent in wear resistance, which solve the above problems.

[0010] Here, excellent wear resistance means that the wear amount measured by the following hot rolling rolling wear test is 0.46 g or less. (1) Take a hot rolling rolling wear test piece (outer diameter 60 mmφ, width 10 mm, with C1 chamfer). (2) Conduct the wear test in a two-disk sliding rolling manner between the test piece and the mating piece. (3) Rotate the test piece at 700 rpm while cooling it with cooling water, and bring the mating piece (outer diameter 190 mmφ, width 15 mm, with C1 chamfer) heated to 800 °C by a high-frequency induction heating coil into contact with the rotating test piece at a load of 686 N and roll it. (4) Conduct the wear test for 135 minutes, and update the mating piece with a new one every 45 minutes (31,500 rotations of the test piece) for a total of 3 times (94,500 rotations of the test piece). (5) Measure the mass change of the test piece before and after the test as the wear amount.

Means for Solving the Problems

[0011] The present inventors investigated in detail the relationship between the base, carbide, hardness, wear amount, and chemical composition (component composition) of hot rolling rolls. As a result, it was found that the wear resistance can be improved by optimizing the chemical composition and heat treatment conditions so that the size and area ratio of the carbide are within a specific range.

[0012] The present invention has been completed based on the above findings, and the gist thereof is as follows. [1] By mass%, C: 0.9 to 2.6%, Si: 0.15 - 2.50%, Mn: 0.15 - 2.60%, Ni: 0.2 - 8.0%, Cr: 1.2 - 12.0%, Mo: 2.5 - 10.0%, V: 2.0 - 8.5%, W: 0.1 - 6.0%, P: 0.01 - 0.05%, S: 0.001 - 0.011%, and 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), with the balance being Fe and inevitable impurities, having a component composition of a structure containing 5.0 - 20.0% of fine carbides with a particle size of 1 μm or less in area ratio, the average particle size of the fine carbides with a particle size of 1 μm or less is 0.50 - 0.80 μm in equivalent circle diameter, the Shore hardness at 20°C is 75.0 HS or more and 85.0 HS or less, and the Shore hardness at 600°C is 47.0 HS or more, an outer layer material for a hot rolling roll. 3.0 ≤ ([%V] + [%Cr] + [%Mo] + [%W]) / ([%Si] + [%Mn] + [%Ni]) ≤ 12.0 ··· (1) 20.0 ≤ [%C] × ([%V] + [%Cr] + [%Mo] + [%W]) ≤ 65.0 ··· (2) Here, [%C], [%Si], [%Mn], [%Ni], [%Cr], [%Mo], [%V], [%W] are the contents (mass %) of each element. [2] A composite roll for hot rolling having an outer layer and an inner layer, wherein the outer layer, in mass %, C: 0.9 - 2.6%, Si: 0.15 - 2.50%, Mn: 0.15 - 2.60%, Ni: 0.2 - 8.0%, Cr: 1.2 - 12.0%, Mo: 2.5 - 10.0%, V: 2.0 to 8.5%, W: 0.1 to 6.0%, P: 0.01 to 0.05%, S: containing 0.001 to 0.011%, 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), having a component composition consisting of the balance being Fe and unavoidable impurities, having a structure containing, by area ratio, 5.0 to 20.0% of fine carbides having a particle size of 1 μm or less, The average particle size of the fine carbides having a particle size of 1 μm or less is 0.50 to 0.80 μm in terms of equivalent circle diameter, The Shore hardness at 20 °C is 75.0 HS or more and 85.0 HS or less, A hot rolling composite roll having a Shore hardness at 600 °C of 47.0 HS or more. 3.0 ≤ ([%V] + [%Cr] + [%Mo] + [%W]) / ([%Si] + [%Mn] + [%Ni]) ≤ 12.0 ··· (1) 20.0 ≤ [%C] × ([%V] + [%Cr] + [%Mo] + [%W]) ≤ 65.0 ··· (2) Here, [%C], [%Si], [%Mn], [%Ni], [%Cr], [%Mo], [%V], and [%W] are the contents (mass %) of the respective elements.

Advantages of the Invention

[0013] According to the present invention, it is possible to provide a roll outer layer material for hot rolling and a hot rolling composite roll having excellent wear resistance. As a result, the life of the hot rolling roll is improved, and accordingly, the productivity of the hot-rolled steel sheet is also improved.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0015] The outer layer material for hot rolling rolls of the present invention contains, by mass%, C: 0.9 to 2.6%, Si: 0.15 to 2.50%, Mn: 0.15 to 2.60%, Ni: 0.2 to 8.0%, Cr: 1.2 to 12.0%, Mo: 2.5 to 10.0%, V: 2.0 to 8.5%, W: 0.1 to 6.0%, P: 0.01 to 0.05%, S: 0.001 to 0.011%. The contents of Si, Mn, Ni, Cr, Mo, V, and W satisfy the following formula (1), and the contents of C, Cr, Mo, V, and W satisfy the following formula (2). The balance is composed of Fe and inevitable impurities, and it has a structure containing, by area ratio, fine carbides with a particle size of 1 μm or less: 5.0 to 20.0%. The average particle size of the fine carbides with a particle size of 1 μm or less is 0.50 to 0.80 μm in terms of the equivalent circle diameter, the Shore hardness at 20 °C is 75.0 HS or more and 85.0 HS or less, and the Shore hardness at 600 °C is 47.0 HS or more. 3.0 ≤ ([%V] + [%Cr] + [%Mo] + [%W]) / ([%Si] + [%Mn] + [%Ni]) ≤ 12.0 ···(1) 20.0 ≤ [%C] × ([%V] + [%Cr] + [%Mo] + [%W]) ≤ 65.0 ···(2) Here, [%C], [%Si], [%Mn], [%Ni], [%Cr], [%Mo], [%V], and [%W] are the contents (mass%) of the respective elements.

[0016] Further, the composite roll for hot rolling of the present invention has two layers of an outer layer and an inner layer, or three layers of an outer layer, an intermediate layer, and an inner layer, and the outer layer can have the same configuration as the outer layer material for hot rolling rolls described above. That is, the hot rolling composite roll of the present invention is a hot rolling composite roll having an outer layer and an inner layer, wherein the outer layer contains, by mass%, C: 0.9 to 2.6%, Si: 0.15 to 2.50%, Mn: 0.15 to 2.60%, Ni: 0.2 to 8.0%, Cr: 1.2 to 12.0%, Mo: 2.5 to 10.0%, V: 2.0 to 8.5%, W: 0.1 to 6.0%, P: 0.01 to 0.05%, S: 0.001 to 0.011%, the contents of Si, Mn, Ni, Cr, Mo, V, and W satisfy the above formula (1), the contents of C, Cr, Mo, and V satisfy the above formula (2), the balance is composed of Fe and inevitable impurities, and has a structure containing, by area ratio, fine carbides having a particle size of 1 μm or less: 5.0 to 20.0%, the average particle size of the fine carbides having a particle size of 1 μm or less is 0.50 to 0.80 μm in terms of equivalent circle diameter, the Shore hardness at 20°C is 75.0 HS or more and 85.0 HS or less, and the Shore hardness at 600°C is 47.0 HS or more.

[0017] Hereinafter, first, the reasons for limiting the component composition of the outer layer material of the hot rolling roll of the present invention will be described. Hereinafter, mass% is simply denoted as % unless otherwise specified.

[0018] C: 0.9 to 2.6% C combines with V, Cr, Mo, W, etc. to form hard carbides, contributing to the improvement of wear resistance. It also dissolves in the matrix to increase hardness. If the C content is less than 0.9%, the amount of carbides is insufficient, and excellent wear resistance cannot be obtained. On the other hand, if the C content exceeds 2.6%, excessive carbides are generated, and the resistance to surface roughness and crack resistance decrease. Therefore, the C content is set to 0.9 to 2.6%. The C content is preferably 1.3% or more, more preferably 1.5% or more. Also, the C content is preferably 2.3% or less, more preferably 2.0% or less.

[0019] Si: 0.15 to 2.50% Si acts as a deoxidizer in the molten metal, improves the fluidity of the molten metal, and can prevent casting defects. If the Si content is less than 0.15%, the deoxidation effect is insufficient. On the other hand, when the Si content exceeds 2.50%, the effect saturates. Therefore, the Si content is set to 0.15 - 2.50%. The Si content is preferably 0.30% or more, more preferably 0.60% or more. Also, the Si content is preferably 2.00% or less, more preferably 1.60% or less.

[0020] Mn: 0.15 - 2.60% Mn exerts a deoxidation effect on the molten metal and an effect of fixing S that has an adverse effect as MnS. If the Mn content is less than 0.15%, the addition effect is insufficient. On the other hand, when the Mn content exceeds 2.60%, the effect saturates. Therefore, the Mn content is set to 0.15 - 2.60%. The Mn content is preferably 0.25% or more, more preferably 0.35% or more. Also, the Mn content is preferably 2.00% or less, more preferably 1.60% or less.

[0021] Ni: 0.2 - 8.0% Ni improves the hardenability of the matrix and has the effect of improving the hardness of the matrix. If the Ni content is less than 0.2%, the effect is insufficient. On the other hand, when the Ni content exceeds 8.0%, austenite tends to remain and the hardness decreases. Therefore, the Ni content is set to 0.2 - 8.0%. The Ni content is preferably 0.8% or more, more preferably 1.4% or more. Also, the Ni content is preferably 5.0% or less, more preferably 3.5% or less.

[0022] Cr: 1.2 - 12.0% Cr is a carbide - forming element and combines with C to form M 7 C 3 carbide. M 7 C 3 carbide is a hard carbide, so it has the effect of improving wear resistance. If the Cr content is less than 1.2%, M 7 C 3The amount of carbide is insufficient, and the wear resistance decreases. On the other hand, when the Cr content exceeds 12.0%, coarse M 7 C 3 carbides are formed, and instead, the wear resistance deteriorates. Therefore, the Cr content is set to 1.2 to 12.0%. The Cr content is preferably 2.5% or more, more preferably 4.0% or more. Also, the Cr content is preferably 8.0% or less, more preferably 6.5% or less.

[0023] Mo: 2.5 to 10.0% Mo is a carbide-forming element and combines with C to form M 2 C carbides. Since M 2 C carbides are hard carbides, they have the effect of improving wear resistance. When the Mo content is less than 2.5%, those effects are insufficient. On the other hand, when the Mo content exceeds 10.0%, coarse M 2 C carbides are formed and the toughness decreases. Therefore, the Mo content is set to 2.5 to 10.0%. The Mo content is preferably 3.5% or more, more preferably 4.5% or more. Also, the Mo content is preferably 8.5% or less, more preferably 7.0% or less.

[0024] V: 2.0 to 8.5% V is a carbide-forming element and combines with C to form MC carbides. MC carbides have a Vickers hardness Hv of about 2800 and are one of the hardest carbides. When the V content is less than 2.0%, the precipitation amount of MC carbides is insufficient and the wear resistance deteriorates. On the other hand, when the V content exceeds 8.5%, VC carbides lighter in specific gravity than the molten iron are concentrated inside the outer layer due to the centrifugal force during centrifugal casting, causing segregation. Therefore, the V content is set to 2.0 to 8.5%. The V content is preferably 3.0% or more, more preferably 4.0% or more. Also, the V content is preferably 7.5% or less, more preferably 7.0% or less.

[0025] W: 0.1 to 6.0% W is a carbide-forming element and combines with C to form hard M 2It produces hard carbides such as C, increases the hardness of the outer layer, and has the effect of improving wear resistance. When the W content is less than 0.1%, the effect is insufficient and the wear resistance deteriorates. On the other hand, when the W content exceeds 6.0%, coarse M 2 C carbides are produced and the wear resistance deteriorates instead. Therefore, the W content is 0.1 to 6.0%. The W content is preferably 0.5% or more, more preferably 1.0% or more. Also, the W content is preferably 5.0% or less, more preferably 4.0% or less.

[0026] P: 0.01 - 0.05% P has been considered to be mixed in during the manufacturing process and reduce mechanical properties. However, as a result of the inventors' intensive studies, it has been clarified that the inclusion of a small amount of P has the effect of improving hardness and wear resistance. When the P content is less than 0.01%, the effect is not sufficient, while when the P content exceeds 0.05%, the mechanical properties deteriorate. Therefore, the P content is 0.01 to 0.05%. The P content is preferably 0.02% or more. Also, the P content is preferably 0.04% or less.

[0027] S: 0.001 - 0.011% S is usually treated as a harmful element in iron-based alloys and is limited to a content of a certain amount or less. Within that range, MnS has the effect of a lubricant. On the other hand, when the content is high, the material becomes brittle. Therefore, the S content is 0.001 to 0.011%. The S content is preferably 0.002% or more. Also, the S content is preferably 0.006% or less.

[0028] In addition, in the present invention, the contents of C, Si, Mn, Ni, Cr, Mo, V, and W are within the above ranges, and in addition, the following formula (1) and the following formula (2) are satisfied. 3.0 ≦ ([%V] + [%Cr] + [%Mo] + [%W]) / ([%Si] + [%Mn] + [%Ni]) ≦ 12.0 ···(1) 20.0 ≦ [%C] × ([%V] + [%Cr] + [%Mo] + [%W]) ≦ 65.0 ···(2) Here, [%C], [%Si], [%Mn], [%Ni], [%Cr], [%Mo], [%V], [%W] are the contents (mass %) of the respective elements.

[0029] (1) Regarding ([%V] + [%Cr] + [%Mo] + [%W]) / ([%Si] + [%Mn] + [%Ni]) in the formula, this parameter indicates the ratio of carbide - forming elements (V, Cr, Mo, W) to carbide - non - forming elements (Si, Mn, Ni). By adjusting it to satisfy the above - mentioned (1) formula, the formation amount and size of carbides are optimized, and the wear resistance is greatly improved. When the value of ([%V] + [%Cr] + [%Mo] + [%W]) / ([%Si] + [%Mn] + [%Ni]) is less than 3.0, the formation amount of carbides is insufficient, and sufficient wear resistance cannot be obtained. On the other hand, when the value of ([%V] + [%Cr] + [%Mo] + [%W]) / ([%Si] + [%Mn] + [%Ni]) exceeds 12.0, the carbides become coarse, and instead, the wear resistance decreases. Therefore, the value of ([%V] + [%Cr] + [%Mo] + [%W]) / ([%Si] + [%Mn] + [%Ni]) shall be 3.0 or more and 12.0 or less. ([%V] + [%Cr] + [%Mo] + [%W]) / ([%Si] + [%Mn] + [%Ni]) is preferably 4.0 or more, and more preferably 5.0 or more. Also, ([%V] + [%Cr] + [%Mo] + [%W]) / ([%Si] + [%Mn] + [%Ni]) is preferably 9.0 or less, and more preferably 7.0 or less.

[0030] (2) Regarding [%C] × ([%V] + [%Cr] + [%Mo] + [%W]) in the formula, this parameter indicates the relationship between carbon and carbide - forming elements (V, Cr, Mo, W). By adjusting it to satisfy the above - mentioned (2) formula, the C content in the matrix is optimized, the hardness is improved, and thereby the wear resistance is improved. When the value of [%C] × ([%V] + [%Cr] + [%Mo] + [%W]) is less than 20.0 or exceeds 65.0, the hardness in the matrix is insufficient, and the wear resistance decreases. Therefore, the value of [%C]×([%V]+[%Cr]+[%Mo]+[%W]) shall be 20.0 or more and 65.0 or less. The value of [%C]×([%V]+[%Cr]+[%Mo]+[%W]) is preferably 25.0 or more, more preferably 30.0 or more. Also, the value of [%C]×([%V]+[%Cr]+[%Mo]+[%W]) is preferably 50.0 or less, more preferably 40.0 or less.

[0031] Balance: Fe and inevitable impurities The balance other than the above-described components consists of Fe and inevitable impurities.

[0032] Next, the reason for limiting the structure of the outer layer material for hot rolling rolls of the present invention will be described.

[0033] The outer layer material for hot rolling rolls of the present invention has a component composition within the above-described range, and has a structure containing, by area ratio, 5.0 to 20.0% of fine carbides having a particle size of 1 μm or less, and the average particle size of the fine carbides having a particle size of 1 μm or less is 0.50 to 0.80 μm in terms of equivalent circle diameter. The fine carbide is an MC carbide precipitated during heat treatment. Also, since the precipitated carbide present in the matrix has a particle size of 1 μm or less, the fine carbide in the present invention is a carbide having a particle size of 1 μm or less. Although fine carbides have been considered not to have a significant effect on wear resistance, as a result of intensive studies by the present inventors, it has been discovered that wear resistance is greatly improved by forming fine carbides in the matrix. It is considered that by forming fine carbides of an appropriate size in the matrix, the hardness of the matrix at high temperatures is maintained, and accordingly, the wear resistance is improved. Here, the matrix is preferably martensite or bainite.

[0034] When the average particle size of the fine carbide is less than 0.50 μm in terms of the equivalent circle diameter, since the particle size is small, the fine carbide hardly contributes to the wear resistance. On the other hand, when it exceeds 0.80 μm in terms of the equivalent circle diameter, since the particle size is large, chipping from the base is likely to occur, and conversely, the wear resistance deteriorates. Therefore, the average particle size of the fine carbide is set to 0.50 to 0.80 μm in terms of the equivalent circle diameter. Preferably, it is 0.55 μm or more, and more preferably, it is 0.60 μm or more. Also, preferably, it is 0.75 μm or less, and more preferably, it is 0.70 μm or less.

[0035] When the fine carbide is less than 5.0% in terms of the area ratio with respect to the entire outer layer material, since there are too few carbides responsible for the wear resistance, the wear resistance deteriorates. On the other hand, when the fine carbide exceeds 20.0% in terms of the area ratio with respect to the entire outer layer material, the chipping from the base increases, and furthermore, the base hardness softens due to the decrease in the carbon concentration in the base, and conversely, the wear resistance deteriorates. Therefore, the fine carbide is set to 5.0 to 20.0% in terms of the area ratio. Preferably, the fine carbide is 6.0% or more in terms of the area ratio, and more preferably, it is 7.0% or more. Also, preferably, it is 18.0% or less, and more preferably, it is 17.0% or less. Also, in order to make the average particle size of the fine carbide having a particle size of 1 μm or less be 0.50 to 0.80 μm in terms of the equivalent circle diameter and make the fine carbide exist at an area ratio of 5.0 to 20.0%, the heat treatment conditions described later may be controlled.

[0036] In the structure (base structure), in addition to the carbide, it may have 80.0 to 95.0% of martensite or bainite.

[0037] The method for observing the structure is as follows. First, after mirror polishing the obtained outer layer material, the structure is observed by SEM. Utilizing the difference in brightness between the images of the base structure and the carbide, after performing binarization processing, the area ratio and the equivalent circle diameter of the carbide are calculated using an image analysis tool (ImageJ). Regarding the values of the area ratio and the equivalent circle diameter of the carbide, five images at a magnification of 1500 times are taken for each sample, and the average value (number average value) is calculated.

[0038] In addition, the hardness of the outer layer material of the hot rolling roll of the present invention is 75.0 HS or more and 85.0 HS or less in Shore hardness at 20°C, and 47.0 HS or more in Shore hardness at 600°C. If the Shore hardness at 20°C is less than 75.0 HS, the wear resistance deteriorates. On the other hand, if the Shore hardness at 20°C exceeds 85.0 HS, it becomes difficult to grind and remove the cracks formed on the surface of the hot rolling roll during hot rolling. Therefore, the hardness of the outer layer material of the hot rolling roll of the present invention is 75.0 HS or more and 85.0 HS or less in Shore hardness at 20°C. Preferably, it is 75.5 HS or more in Shore hardness at 20°C, and more preferably 76.5 HS or more. Also preferably, it is 84.5 HS or less in Shore hardness at 20°C, and more preferably 82.5 HS or less. In addition, the roll surface temperature during hot rolling is around about 600°C. When the Shore hardness at 600°C (600°C Shore hardness) is 47.0 HS or more, the wear resistance during hot rolling is improved. Therefore, the hardness of the outer layer material of the hot rolling roll of the present invention is 47.0 HS or more in Shore hardness at 600°C. The upper limit of the Shore hardness at 600°C is not particularly limited, but when the high-temperature hardness is high, slip is likely to occur during rolling, so it is preferably 60.0 HS or less. To stably ensure such hardness, it can be obtained by adjusting the tempering parameter P described later within the range of 10,000 to 20,000 according to the composition.

[0039] Regarding the Shore hardness at 20°C and the Shore hardness at 600°C, first, measure the Vickers hardness HV at 20°C and 600°C five times each with a Vickers hardness tester (test force: 1 kgf), and calculate the average value. First, for the Vickers hardness measurement at 20°C, an experiment is conducted using a testing machine, Nikon QM-2 (simultaneous heating type for indenter and specimen), with a diamond indenter, in an argon gas atmosphere, and a load holding time of 10 seconds. Then, after heating to 600°C at a heating rate of 20°C / min, the Vickers hardness at 600°C is measured under the same test conditions as at 20°C. In addition, for the Vickers hardness measurement at 600°C, it complies with JIS Z2252 "High-temperature Vickers hardness test method". These obtained Vickers hardness values are converted to Shore hardness using the calculation formula of JIS B 7731.

[0040] Next, a preferred manufacturing method for the hot rolling roll outer layer material and the hot rolling composite roll of the present invention will be described.

[0041] In the present invention, the manufacturing method of the roll outer layer material is not particularly limited, and methods such as centrifugal casting and continuous overlay casting are preferred. However, from the perspective of manufacturing cost, the centrifugal casting method is more preferred. When adopting the centrifugal casting method, first, a molten metal of the above-described hot rolling roll outer layer material composition (simply referred to as the outer layer material molten metal) is poured into a rotating mold coated with a refractory material mainly composed of zircon or the like with a thickness of 1 to 5 mm on the inner surface so as to have a predetermined wall thickness, and centrifugally cast.

[0042] The hot rolling composite roll of the present invention, when the roll outer layer material is cast by the centrifugal casting method, has a centrifugally cast outer layer and an inner layer welded and integrated with the outer layer. The hot rolling composite roll of the present invention may be composed of this outer layer and an inner layer welded and integrated with the outer layer. In addition, an intermediate layer may be provided between the outer layer and the inner layer. That is, instead of the inner layer welded and integrated with the outer layer, it may be an intermediate layer welded and integrated with the outer layer and an inner layer welded and integrated with the intermediate layer. In addition, it is preferable that the inner layer is manufactured by the static casting method.

[0043] For the stationary-cast inner layer, it is preferable to use spherical graphite cast iron, vermicular graphite cast iron (CV cast iron), etc., which are excellent in castability and mechanical properties. In the centrifugally cast roll, the outer layer and the inner layer are integrally welded, and the components of the outer layer material are mixed into the inner layer. When carbide-forming elements such as Cr and V contained in the outer layer material are mixed into the inner layer, the inner layer is weakened. Therefore, it is preferable to suppress the mixing rate into the outer layer components as much as possible.

[0044] Also, 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 the outer layer are integrally welded, and the 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 suppress the mixing rate of the outer layer material into the intermediate layer as much as possible.

[0045] The hot-rolling composite roll of the present invention is preferably heat-treated after casting. As the target structure in the outer layer (hot-rolling roll outer layer material) constituting the hot-rolling composite roll, the average particle size of the fine carbides with a particle size of 1 μm or less is 0.50 to 0.80 μm in terms of the equivalent circle diameter, and in order to have a structure in which the fine carbides are present at an area ratio of 5.0 to 20.0%, the heat treatment is a quenching treatment of heating to 900 to 1100 °C and air-cooling or blast air-cooling, and further, a tempering treatment of heating and holding and then cooling is performed two or more times so that the tempering parameter P described in the following formula (3) is in the range of 10000 to 20000. At this time, by changing the quenching temperature, tempering parameter, and tempering number within the described range according to the components, it is possible to obtain the above-described structure. P = T(log(t) + A) ···(3) Here, T is the tempering temperature (K), t is the tempering time (h), and A is a constant. (In the present invention, A = 20 is used) From the above, it is possible to obtain a hot-rolling composite roll having three layers of an outer layer, an intermediate layer, and an inner layer, or two layers of an outer layer and an inner layer.

Example

[0046] The present invention will be described in more detail by the following examples, but the present invention is not limited to these examples.

[0047] Using the chemical compositions of the roll outer layer materials for hot rolling shown in Table 1 (the balance is Fe and inevitable impurities), each test material of the present invention examples Nos. 1 to 8 and each test material of the comparative examples Nos. 9 to 21 were heated and melted up to 1450 to 1550 °C, and cast into a Y-shaped keel block mold (rectangular parallelepiped part: thickness 35 mm, width 230 mm, height 120 mm). After cooling, the ingot was taken out, quenched at 900 to 1100 °C, and then subjected to a tempering treatment of heating and holding followed by cooling three times so that the tempering parameter P was within the range of 10000 to 20000. Thereafter, microstructure observation, hardness measurement, and hot rolling wear test were carried out. The test pieces were taken from the center of the wall thickness.

[0048]

Table 1

[0049] For each sample cut out from the ingots of the present invention examples and comparative examples, the Vickers hardness HV at 20 °C and the Vickers hardness HV at 600 °C were each measured at 5 points with a Vickers hardness tester (test force: 1 kgf), and the average value was calculated. Regarding the measurement of the Vickers hardness at 20 °C, using a testing machine of Nikon QM-2 (simultaneous heating type of indenter and test piece), the indenter used diamond, the test atmosphere was an argon gas atmosphere, and the load holding time was 10 sec for the experiment. Thereafter, the temperature was raised to 600 °C at a rate of 20 °C / min, and then the Vickers hardness at 600 °C was measured under the same test conditions as at 20 °C. In the measurement of the Vickers hardness at 600 °C, it conformed to JIS Z2252 "High-temperature Vickers hardness test method". The obtained Vickers hardness was converted to shore hardness by the calculation formula of JIS B 7731.

[0050] The hot rolling wear test method was as follows. From the ingots of each inventive example and each comparative example obtained, hot rolling wear test pieces (outer diameter 60 mmφ, width 10 mm, with C1 chamfer) were taken. As shown in Fig. 1, the wear test was carried out in a two-disk sliding rolling manner between the test piece and the mating piece. The test piece 1 was rotated at 700 rpm while being water-cooled with cooling water 2, and the mating piece (outer diameter 190 mmφ, width 15 mm, with C1 chamfer) 4 heated to 800 °C by a high-frequency induction heating coil 3 was brought into contact with the rotating test piece 1 and rolled while applying a load of 686 N. The wear test was carried out for 135 minutes, and the mating piece was renewed with a new one every 45 minutes (31,500 rotations of the test piece), and a total of 3 tests (94,500 rotations of the test piece) were carried out. The mass change of the test piece before and after the test (before starting the first test and after the end of the third test), that is, the wear amount, was measured.

[0051] For each heat-treated sample, after mirror polishing, microstructure observation was carried out by SEM. Using the difference in brightness between the images of the matrix structure and carbides, after binarization processing, the area ratio and equivalent circle diameter of fine carbides with a particle size of 1 μm or less were calculated using an image analysis tool (ImageJ). For the area ratio and equivalent circle diameter values of fine carbides with a particle size of 1 μm or less, 5 images of each sample were taken at a magnification of 1500 times, and their average value (number average value) was calculated.

[0052] The results obtained are shown in Table 2.

[0053]

Table 2

[0054] As is clear from Table 2, it can be confirmed that the inventive examples have excellent wear resistance compared to the comparative examples. In addition, a case where the wear amount is 0.46 g or less was regarded as qualified, and a case where the value is greater than 0.46 g was regarded as unqualified. In the inventive examples, by forming fine carbides of appropriate size in the matrix, the hardness of the matrix at high temperature was maintained at an appropriate hardness, and accordingly, the wear resistance was improved.

[0055] Figure 2 is a diagram showing the relationship between the wear amount by the hot rolling wear test shown in Table 2 and the average particle size of fine carbides of 1 μm or less. As shown in Figure 2, it was found that by setting the average particle size of the carbides (fine carbides of 1 μm or less) in the range of 0.50 to 0.80 μm, the wear amount can be suppressed to 0.46 g or less.

[0056] In addition, since the outer layer material for hot rolling rolls of the present invention example is excellent in wear resistance, a hot rolling composite roll 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 with this outer layer material for hot rolling rolls as the outer layer was also found to be excellent in wear resistance.

[0057] Therefore, according to the present invention, it becomes possible to manufacture an outer layer material for hot rolling rolls and a composite roll excellent in wear resistance. As a result, the effect that the life of the hot rolling roll is improved and the productivity of the hot rolled steel sheet is improved accordingly can also be obtained.

Explanation of symbols

[0058] 1 Specimen 2 Cooling water 3 High-frequency induction heating coil 4 Opposite piece

Claims

1. By mass percentage, C: 0.9 to 2.6%, Si: 0.15 to 2.50%, Mn: 0.15 to 2.60%, Ni: 0.2 to 8.0%, Cr: 1.2 to 12.0%, Mo: 2.5 to 10.0%, V: 2.0 to 8.5%, W: 0.1 to 6.0%, P: 0.01 to 0.05%, S: 0.001 to 0.011%, and 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), the balance consists of Fe and unavoidable impurities, and has a component composition, by area ratio, has a structure containing 5.0 to 20.0% of fine carbides having a particle size of 1 μm or less, the average particle size of the fine carbides having a particle size of 1 μm or less is 0.50 to 0.80 μm in terms of equivalent circle diameter, the Shore hardness at 20°C is 75.0 HS or more and 85.0 HS or less, and the Shore hardness at 600°C is 47.0 HS or more, an outer layer material for a hot rolling roll. 3.0 ≤ ([%V] + [%Cr] + [%Mo] + [%W]) / ([%Si] + [%Mn] + [%Ni]) ≤ 9.0... (1) 20.0 ≤ [%C] × ([%V] + [%Cr] + [%Mo] + [%W]) ≤ 65.0... (2) Here, [%C], [%Si], [%Mn], [%Ni], [%Cr], [%Mo], [%V], and [%W] are the contents (mass %) of the respective elements.

2. A composite roll for hot rolling having an outer layer and an inner layer, wherein the outer layer, by mass percentage, C: 0.9 to 2.6%, Si: 0.15 to 2.50%, Mn: 0.15 to 2.60%, Ni: 0.2 to 8.0%, Cr: 1.2 to 12.0%, Mo: 2.5 to 10.0%, V: 2.0 to 8.5%, W: 0.1 to 6.0%, P: 0.01 to 0.05%, S: 0.001 to 0.011%, and 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), the balance consists of Fe and unavoidable impurities, and has a component composition, by area ratio, has a structure containing 5.0 to 20.0% of fine carbides having a particle size of 1 μm or less, the average particle size of the fine carbides having a particle size of 1 μm or less is 0.50 to 0.80 μm in terms of equivalent circle diameter, the Shore hardness at 20°C is 75.0 HS or more and 85.0 HS or less, and the Shore hardness at 600°C is 47.0 HS or more, a composite roll for hot rolling. 3.0 ≤ ([%V] + [%Cr] + [%Mo] + [%W]) / ([%Si] + [%Mn] + [%Ni]) ≤ 9.0... (1) 20.0 ≤ [%C] × ([%V] + [%Cr] + [%Mo] + [%W]) ≤ 65.0... (2) Here, [%C], [%Si], [%Mn], [%Ni], [%Cr], [%Mo], [%V], and [%W] are the contents (mass %) of the respective elements.

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