Outer layer material for hot rolling rolls, method for manufacturing the same, and composite roll for hot rolling.

Abstract

To provide a hot rolling roll outer layer material excellent in seizure resistance and slip resistance, and, a producing method thereof, as well as, a hot rolling composite roll.SOLUTION: In terms of mass%, a composition includes C: 1.1-2.5%, Si: 0.1-3.5%, Mn: 0.1-3.5%, Ni: 0.5-6.5%, Cr: 1.5-6.5%, Mo: 2.5-18.0%, V: 1.5-6.5%, W: 0.5-3.5%, P: 0.01-0.08%, S: 0.001-0.050%, with the balance being Fe and unavoidable impurities. The contents of Cr, Mo, V, and W satisfy formula (1), and the contents of C, Mo, and W satisfy formula (2), an area ratio of all precipitated carbides is 6.0-18.0%, and the area ratio of precipitated M2C type carbides is 1.0-8.0%, the relationship between the area ratio of all precipitated carbides and the area ratio of precipitated M2C type carbides satisfies formula (3).SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a roll outer layer material for hot rolling, a method for manufacturing the same, and a composite roll for hot rolling.

Background Art

[0002] In recent years, the demand for high-quality steel sheets such as high-tensile steel sheets, stainless steel, and electromagnetic steel sheets has been increasing. Along with this, improvements in hot rolling technology and productivity for manufacturing the above-mentioned steel sheets are required. In addition, there is a strong demand for improving the characteristics of hot rolling rolls used in hot rolling equipment for manufacturing steel sheets, such as wear resistance, seizure resistance, and slip resistance.

[0003] As hot rolling rolls, currently, HiCr cast steel rolls introduced with Cr-based M7C3-type carbides, and high-speed steel, which is a type of tool steel, containing carbide-forming elements such as V, Cr, Mo, and W, and high-speed rolls introduced with a large amount of hard carbides such as V-based MC-type carbides, Mo and W-based M2C-type carbides, and Cr-based M7C3-type carbides are used. Here, M described above means a metal element that forms carbides.

[0004] In addition, in the finishing post-stage stand of hot rolling equipment, there is a possibility of a choking accident where the steel sheet becomes multiple and bites into the upper and lower rolls. When a choking accident occurs, a part of the steel sheet adheres to the roll, and the part of the roll where the steel sheet adheres is strongly pressed between the backup roll and the work roll. As a result, drawing cracks may occur in the work roll. In order to reduce the occurrence of cracks due to choking accidents, it is considered important to improve the seizure resistance of the work roll and suppress the seizure of the steel sheet. Therefore, in the finishing post-stage stand, Ni-glen rolls with excellent seizure resistance that crystallize graphite, a solid lubricant, are used.

[0005] On the other hand, since graphite reduces wear resistance, high-speed steel rolls that do not crystallize graphite are sometimes used in the post-finishing stands from the standpoint of rolling efficiency. However, high-speed steel rolls have inferior anti-seizure properties compared to Ni-grain rolls. Therefore, various technologies have been proposed to improve the anti-seizure properties of high-speed steel rolls.

[0006] For example, Patent Document 1 proposes a composite roll for hot rolling characterized by containing C: 0.8~4.0%, Si: 0.2~2.0%, Mn: 0.2~2.0%, Cr: 3.0~15%, V: 3.0~15%, one or two of Mo and W: ≥2%, and Mo + 0.5W: ≥6.1%, or further containing one or more of Ni: 0.2~5%, Co: 0.5~10%, Nb: 0.50~5.0%, and one or more of Al, Ti, and Zr: ≤0.5%, wherein the metallic structure of the outer layer material has carbides accounting for 5~30% by area, and the distribution of each carbide is such that the average gap between adjacent carbides is 20 μm or less. This allows for the distribution of granular carbides in an appropriate amount, with fine particles and small gaps between adjacent carbides, resulting in a composite roll for hot rolling with improved slip resistance and seizure resistance.

[0007] Patent Document 2 proposes a composite roll for hot rolling manufactured by centrifugal casting, characterized by having a chemical composition containing C: 1-3%, Si: 0.4-3%, Mn: 0.3-3%, Ni: 1-5%, Cr: 2-7%, Mo: 3-8%, V: 3-7%, and B: 0.01-0.12%, with the remainder being Fe and unavoidable impurities, and satisfying the relationship expressed by Cr / (Mo+0.5W) <-2 / 3[C-0.2(V+1.19Nb)]+11 / 6 (wherein W=0 and Nb=0 if the optional components W and Nb are not included), and containing 1-15% MC-type carbides, 0.5-20% carbonobides, and 0.5-2.0% Mo-type carbides by area ratio. It is stated that the generated carbonobides improve seizure resistance.

[0008] Patent Document 3 describes a material containing C: 1.3-2.8%, Si: 0.3-1.8%, Mn: 0.3-2.5%, Ni: 0-6.5%, Cr: 1-10%, Mo: 0.9-6%, W: 0-8%, V: 0.5-6%, Nb: 0-3%, and B: 0% or more and less than 0.01%, with the remainder being Fe and unavoidable impurities, and also includes 1000 ≤ 1177-52C+14Si-11Mn An outer layer material for rolling mill rolls has been proposed, characterized by having a chemical composition that satisfies +6.8Cr+1W+0.65Mo+12V+15Nb≦1115 and 5≦Cr+Mo+0.5W+V+1.2Nb≦15 (where C, Si, Mn, Cr, W, Mo, V, and Nb represent the mass percent of each element), and having a structure that contains eutectic carbides and does not contain graphite. It is stated that the lowest solidification exothermic onset temperature among several solidification exothermic onset temperatures determined by differential thermal analysis is preferably 1100°C or lower. This is said to improve lubricity between the rolling material and the roll outer layer and enhance seizure resistance, as the eutectic carbides with low melting points melt due to frictional heat between the rolling material and the outer layer surface when the rolling mill roll encounters a drawing accident. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2004-255457 [Patent Document 2] International Publication No. 2015 / 045984 [Patent Document 3] International Publication No. 2017 / 170570 [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] It is presumed that the anti-seizure properties of high-speed steel rolls are better the smaller the area ratio of the base material that causes metal-to-metal contact, that is, the higher the area ratio of crystallized carbides. On the other hand, if the area ratio of crystallized carbides is high, the surface roughness of the high-speed steel roll decreases, which may make slippage more likely during rolling. The hot rolling rolls described in Patent Documents 1 to 3 are suitable for rolls in finishing rolling stands where drawing accidents occur and seizure is likely to occur even when rolling ordinary steel, and they improve the anti-seizure and anti-slip properties in such finishing rolling stands. However, when the hot rolling rolls described in Patent Documents 1 to 3 are used in rough rolling stands, both anti-seizure properties and anti-slip properties, or one or the other, may not be sufficient. In other words, the hot rolling rolls described in Patent Documents 1 to 3 are mainly used as rolls in finishing rolling stands, and have a higher area ratio of crystallized carbides compared to hot rolling rolls conventionally used in rough rolling stands. Therefore, using the hot rolling rolls described in Patent Documents 1 to 3 in a roughing stand may increase the likelihood of slippage. Furthermore, among the hot rolling rolls manufactured with the component ratios described in Patent Documents 1 to 3, those with a low crystallized carbide area ratio do not have an appropriate generation ratio for each type of crystallized carbide. Therefore, using these in a roughing stand may result in insufficient anti-seize properties. As a result, when using the hot rolling rolls described in Patent Documents 1 to 3 in a roughing stand, both anti-seize properties and anti-slip properties, or either one of them, may be insufficient.

[0011] The present invention was made to solve the above-mentioned problems, and aims to provide a hot-rolling roll outer layer material that has excellent seizure resistance and slip resistance, a method for manufacturing the same, and a hot-rolling composite roll.

[0012] Here, "excellent resistance to seizing" means that the average seized area ratio measured by the drop-weight friction thermal shock test described later is 42.0% or less. The drop-weight friction thermal shock test will be explained in the examples described later.

[0013] Furthermore, "excellent slip resistance" means that the ratio of the friction coefficient in the first test to the friction coefficient in the fifth test (μ5th test / μ1st test), as measured by the hot rolling abrasion test described later, is 0.80 or higher. The hot rolling abrasion test will be explained in the examples described later. [Means for solving the problem]

[0014] To solve the above-mentioned problems, the inventors conducted a detailed investigation of the crystalline carbides, chemical composition, seizure area ratio, and friction coefficient during testing of hot-rolling rolls. As a result, they found that by optimizing the chemical composition so that the amount of crystalline carbides falls within a specific range, and further optimizing the amount of Mo and W-based crystalline M2C-type carbides, it is possible to achieve both improved seizure resistance and slip resistance.

[0015] This invention was completed based on these findings and further investigations. In other words, the gist of this invention is as follows: [1] Contains, by mass%, C: 1.1~2.5%, Si: 0.1~3.5%, Mn: 0.1~3.5%, Ni: 0.5~6.5%, Cr: 1.5~6.5%, Mo: 2.5~18.0%, V: 1.5~6.5%, W: 0.5~3.5%, P: 0.01~0.08%, S: 0.001~0.050%, with the remainder being Fe and unavoidable impurities. Furthermore, the outer layer material for hot rolling rolls satisfies the following equation (1) for the content of Cr, Mo, V, and W, the following equation (2) for the content of C, Mo, and W, the area ratio of total crystallized carbides being 6.0 to 18.0%, the area ratio of crystallized M2C type carbides being 1.0 to 8.0%, and the relationship between the area ratio of total crystallized carbides and the area ratio of crystallized M2C type carbides satisfying the following equation (3). 0.6≦([%Mo]+[%W]) / ([%V]+[%Cr])≦4.0 ···(1) 1.0≦[%C]×([%Mo] / [%W])≦15.0 ···(2) 0.10≦Crystallized M2C type carbide area ratio (%) / total crystallized carbide area ratio (%)≦0.60 (3) In equations (1) and (2), [%C], [%Cr], [%Mo], [%V], and [%W] represent the mass percentage of each element. A method for manufacturing a hot rolling roll outer layer material as described in [2][1], wherein a mold is rotated while pouring molten metal of the outer layer material of the above component composition into the mold, thereby generating centrifugal force in the molten metal poured into the mold, and the molten metal is brought into close contact with the mold by the centrifugal force to centrifugeally cast the hot rolling roll outer layer material, wherein the following formula (4) is satisfied. 0.3℃ / sec ≤ ΔT (casting temperature °C - liquidus temperature °C) / casting time sec ≤ 5.0℃ / sec ···(4) [3] A composite roll for hot rolling having a two-layer structure of an outer layer and an inner layer, or a three-layer structure of an outer layer, an intermediate layer and an inner layer, wherein the outer layer is made of the hot rolling roll outer layer material described in [1]. [Effects of the Invention]

[0016] According to the present invention, it is possible to obtain a hot-rolling roll outer layer material with excellent seizure resistance and slip resistance, a method for manufacturing the same, and a hot-rolling composite roll. As a result, the lifespan of the hot-rolling roll outer layer material and the hot-rolling roll can be improved, and consequently, the productivity of hot-rolled steel sheets manufactured using the hot-rolling roll outer layer material and the hot-rolling roll can be improved. [Brief explanation of the drawing]

[0017] [Figure 1] This diagram illustrates the sampling locations for specimens used in hot rolling abrasion tests from a ring-shaped test material. [Figure 2] This figure shows an example of a hot rolling abrasion testing machine. [Figure 3] This figure shows an example of a drop-weight type frictional thermal shock testing machine.

Mode for Carrying Out the Invention

[0018] Hereinafter, embodiments of the present invention will be described. Note that the following description shows an example of an embodiment of the present invention, and the present invention is not limited to the following embodiments.

[0019] The roll outer layer material for hot rolling of the present invention is manufactured by the centrifugal casting method and can be used as a ring roll or a sleeve roll as it is. Alternatively, the roll outer layer material for hot rolling of the present invention can be applied as an outer layer (outer layer material) of a composite roll for hot rolling suitable for a hot rough rolling stand. In the centrifugal casting method, while pouring the molten metal of the roll outer layer material for hot rolling of the present invention, the mold is rotated to generate a centrifugal force in the molten metal poured into the mold, and the molten metal is adhered to the inner wall surface of the mold by the centrifugal force to centrifugally cast the roll outer layer material for hot rolling.

[0020] Further, the composite roll for hot rolling of the present invention can have a two-layer structure including an outer layer (the roll outer layer material for hot rolling of the present invention) and an inner layer welded and integrated with the outer layer. Alternatively, the composite roll for hot rolling of the present invention may have a three-layer structure in which an intermediate layer is provided between the outer layer and the inner layer. In the composite roll for hot rolling having a three-layer structure, the intermediate layer is welded and integrated with the outer layer, and the inner layer is welded and integrated with the intermediate layer. In the present invention, the composition of the intermediate layer is not particularly limited, but it is preferably a high-carbon material with C: 1.5 to 3.0% by mass.

[0021] (Component Composition of Roll Outer Layer Material for Hot Rolling) First, the reasons for limiting the component composition of the roll outer layer material for hot rolling of the present invention (hereinafter, may be simply referred to as "outer layer material") will be described. Hereinafter, unless otherwise specified, "% by mass" is simply referred to as "%".

[0022] C: 1.1 to 2.5% Carbon (C) combines with elements such as V, Cr, Mo, and W to form hard crystallized carbides, thereby reducing metal-to-metal contact between the rolled material and the matrix structure of the roll material. This contributes to improving the anti-seizure properties of the outer layer material. Examples of hard crystallized carbides include MC-type carbides, M2C-type carbides, and M7C3-type carbides. Furthermore, carbon (C) dissolves into the matrix, increasing its hardness through solid solution strengthening. If the carbon content is less than 1.1%, the amount of crystallized carbides is insufficient, and excellent anti-seizure properties cannot be obtained. On the other hand, if the carbon content exceeds 2.5%, excessive crystallized carbides are produced, resulting in excessively high hardness. This reduces the surface roughness of the outer layer material, lowers the coefficient of friction, and makes it more prone to slipping.

[0023] Therefore, the C content is limited to 1.1% or more and 2.5% or less. The C content is preferably 1.2% or more, more preferably 1.4% or more. Furthermore, the C content is preferably 2.4% or less, more preferably 2.2% or less. The C content is preferably 1.2% or more and 2.4%, and more preferably 1.4% or more and 2.2% or less.

[0024] Si: 0.1~3.5% Si acts as a deoxidizing agent in molten metal, improving its fluidity and preventing casting defects. Below 0.1% Si content, the deoxidizing effect is insufficient. On the other hand, above 3.5% Si content, the deoxidizing effect saturates. Therefore, the Si content is limited to between 0.1% and 3.5%. The Si content is preferably 0.2% or more, more preferably 0.4% or more. Furthermore, the Si content is preferably 3.2% or less, more preferably 3.0% or less. The Si content is preferably between 0.2% and 3.2%, and more preferably between 0.4% and 3.0%.

[0025] Mn: 0.1~3.5% Mn has the effect of fixing and removing sulfur (S), which has adverse effects on the product, as MnS. When the Mn content is less than 0.1%, the effect of fixing S as MnS is not observed. On the other hand, when the Mn content exceeds 3.5%, this effect saturates. Therefore, the Mn content is limited to 0.1% or more and 3.5% or less. The Mn content is preferably 0.2% or more, more preferably 0.4% or more. Furthermore, the Mn content is preferably 3.2% or less, more preferably 3.0% or less. The Mn content is preferably 0.2% or more and 3.2% or less, and more preferably 0.4% or more and 3.0% or less.

[0026] Ni: 0.5~6.5% Ni is an element that improves the hardenability of the matrix and has the effect of improving the hardness of the matrix. If the Ni content is less than 0.5%, the effect of improving the hardness of the matrix is ​​insufficient. On the other hand, if the Ni content exceeds 6.5%, it promotes the retention of austenite, which reduces hardness and increases the charring area, thus reducing charring resistance. For this reason, the Ni content is limited to 0.5% or more and 6.5% or less. The Ni content is preferably 1.0% or more, more preferably 1.5% or more. Furthermore, the Ni content is preferably 5.0% or less, more preferably 4.0% or less. The Ni content is preferably 1.0% or more and 5.0% or less, and more preferably 1.5% or more and 4.0% or less.

[0027] Cr: 1.5~6.5% Cr is a carbide-forming element that combines with C to form M7C3 type carbides. M7C3 type carbides reduce metal-to-metal contact between the roll matrix and the rolled material. Therefore, they have the effect of improving seizure resistance. If the Cr content is less than 1.5%, the amount of M7C3 type carbides is insufficient, and seizure resistance decreases. On the other hand, if the Cr content exceeds 6.5%, coarse M7C3 type carbides are formed, the surface roughness of the outer layer material decreases, the coefficient of friction decreases, and slippage becomes more likely. In addition, the formation of coarse M7C3 type carbides with low toughness makes cracking more likely. Therefore, the Cr content is limited to 1.5% or more and 6.5% or less. The Cr content is preferably 2.5% or more, more preferably 3.0% or more. Furthermore, the Cr content is preferably 6.0% or less, more preferably 5.5% or less. The Cr content is preferably 2.5% to 6.0%, and more preferably 3.0% to 5.5%.

[0028] Mo: 2.5~18.0% Mo is a carbide-forming element that combines with C to form M2C-type carbides. M2C-type carbides are harder crystallized carbides than M7C3-type carbides, crystallizing in a planar and lamellar manner, and further improving seizure resistance. If the Mo content is less than 2.5%, the amount of M2C-type carbides is insufficient, and the effect of improving seizure resistance is inadequate. On the other hand, if the Mo content exceeds 18.0%, coarse, lamellar M2C-type carbides with low toughness are formed, making it easier for cracks caused by M2C-type carbides to occur. Therefore, the Mo content is limited to 2.5% or more and 18.0% or less. The Mo content is preferably 3.0% or more, more preferably 3.5% or more. Also, the Mo content is preferably 16.0% or less, more preferably 15.0% or less. The Mo content is preferably 3.0% or more and 16.0%, and more preferably 3.5% or more and 15.0% or less.

[0029] V: 1.5~6.5% V is a carbide-forming element that combines with C to form MC-type carbides. MC-type carbides have a Vickers hardness of approximately 2800 Hv. Like other crystallized carbides, MC-type carbides reduce metal-to-metal contact between the roll matrix and the rolled material. Therefore, they have the effect of improving seizure resistance. If the V content is less than 1.5%, the amount of MC-type carbides is insufficient, and the effect of improving seizure resistance is inadequate. On the other hand, if the V content exceeds 6.5%, VC (VC stands for vanadium carbide, and is a type of MC-type carbide), which has a lower specific gravity than molten iron, concentrates on the inside of the outer layer material (note that in the case of composite rolls described later, it will be called the "outer layer," and the same applies hereafter) due to the centrifugal force during centrifugal casting, causing segregation. As a result, the accumulation of VC, which has low toughness, on the inside of the outer layer material makes it easier for cracks to occur. Therefore, the V content should be limited to between 1.5% and 6.5%. The V content is preferably 2.0% or more, more preferably 2.5% or more. Furthermore, the V content is preferably 6.0% or less, more preferably 5.5% or less. The V content is preferably 2.0% to 6.0%, and more preferably 2.5% to 5.5%.

[0030] W: 0.5~3.5% W is a carbide-forming element and, like Mo, combines with C to form hard crystallized carbides such as hard M2C-type carbides, thereby improving seizure resistance. If the W content is less than 0.5%, this effect is insufficient, and seizure resistance deteriorates. On the other hand, if the W content exceeds 3.5%, coarse M2C-type carbides are formed, and seizure resistance actually deteriorates. Therefore, the W content is limited to 0.5% or more and 3.5% or less. The W content is preferably 0.6% or more, more preferably 0.8% or more. Furthermore, the W content is preferably 3.2% or less, more preferably 3.0% or less. The W content is preferably 0.6% or more and 3.2% or less, and more preferably 0.8% or more and 3.0% or less.

[0031] P: 0.01~0.08% While phosphorus (P) has been thought to be introduced during the roll manufacturing process and degrade mechanical properties, our inventors have diligently investigated and found that a small amount of P improves seizure resistance. Below 0.01% P content, the improvement in seizure resistance is insufficient. On the other hand, above 0.08% P content, mechanical properties deteriorate. Therefore, the P content is limited to between 0.01% and 0.08%. Preferably, the P content is between 0.02% and 0.06%.

[0032] S: 0.001~0.050% S is usually treated as a harmful element in iron-based alloys and its content is limited to below a certain amount. However, within a predetermined range, MnS has a lubricating effect. Furthermore, as a result of our diligent research, we have revealed that trace amounts of MnS have the effect of improving seizure resistance. On the other hand, if the S content is high, the material becomes brittle. Therefore, the S content is limited to 0.001% to 0.050%. Preferably, the S content is 0.002% to 0.040%.

[0033] Furthermore, the present invention is characterized in that, in addition to the content of C, Cr, Mo, V, and W being within the above-mentioned range, it satisfies the following formulas (1) and (2). 0.6≦([%Mo]+[%W]) / ([%V]+[%Cr])≦4.0 ···(1) 1.0≦[%C]×([%Mo] / [%W])≦15.0 ···(2)

[0034] In equations (1) and (2) described above, [%C], [%Cr], [%Mo], [%V], and [%W] represent the content (mass %) of each element.

[0035] Regarding (([%Mo]+[%W]) / ([%V]+[%Cr])) in equation (1): This parameter represents the ratio of the content of M2C-type carbide-forming elements (Mo, W) to the content of other elements that form crystallized carbides (V, Cr). By adjusting the content of each element to satisfy equation (1), the proportion of crystallized M2C-type carbides can be optimized, improving the seizure resistance and slip resistance of the outer layer material. If the lower limit of the above ratio is less than 0.6, the proportion of crystallized M2C-type carbides will be insufficient, reducing seizure resistance and potentially making it impossible to achieve both seizure resistance and slip resistance. On the other hand, if the upper limit of the above ratio exceeds 4.0, the proportion of M2C-type carbides crystallizing in a planar and lamellar manner may become too high. If such a situation occurs, stress will concentrate on the M2C-type carbides with low toughness, making it easier for cracks caused by the M2C-type carbides to occur. Therefore, the ratio values ​​mentioned above were limited to between 0.6 and 4.0. Preferably, the ratio values ​​mentioned above are between 0.8 and 3.5.

[0036] Regarding ([%C] × ([%Mo] / [%W])) in equation (2): This parameter indicates the amount of M2C-type carbides that crystallize. By adjusting the content of each element to satisfy equation (2), the amount of M2C-type carbides that crystallize can be optimized, thereby improving seizure resistance and slip resistance. If the lower limit of equation (2) is less than 1.0, the amount of M2C-type carbides that crystallize may be insufficient, potentially reducing seizure resistance. On the other hand, if the upper limit of equation (2) exceeds 15.0, the amount of M2C-type carbides that crystallize may be excessive. In that case, cracks caused by M2C-type carbides are more likely to occur. Therefore, the value of equation (2) is limited to between 1.0 and 15.0. Preferably, the value of equation (2) is between 2.5 and 12.5.

[0037] Remainder: Fe and unavoidable impurities The remainder of the mixture, other than the components mentioned above, consists of Fe and unavoidable impurities. Examples of unavoidable impurities include Mg, Zr, REM (rare earth metals), Sn, As, Sb, Bi, Pb, Zn, N, and O. However, within limits that do not impair the effects of the present invention, the mixture may contain 0.03% or less of Mg, Zr, and REM, 0.04% or less of Sn, As, and Sb, and 0.01% or less of Bi, Pb, Zn, N, and O. The REM mentioned above is a collective term for Sc, Y, and a total of 17 lanthanide elements. One or more of these 17 elements may be included as unavoidable impurities, and the REM content refers to the total content of these elements.

[0038] (Structure of the outer layer material for hot-rolling rolls) Next, we will explain the reason for limiting the structure of the hot rolling roll outer layer material of the present invention. From past research results, it is known that in hot rolling roll outer layer materials, the seizure resistance tends to improve as the area ratio of crystallized carbides increases. However, the larger the area ratio of crystallized carbides, the more likely slippage occurs in the hot rolling composite roll having the hot rolling roll outer layer material during rolling. The range of crystallized carbide area ratios that maintains excellent seizure resistance while minimizing slippage has not yet been clarified. Therefore, the inventors of the present invention have made the following discovery as a result of diligent research.

[0039] The hot rolling roll outer layer material of the present invention is characterized by having a component composition within the above-described range, containing 6.0 to 18.0% by area of ​​whole-crystallized carbides, and containing 1.0 to 8.0% by area of ​​crystallized M2C-type carbides. Here, whole-crystallized carbides refer to crystallized MC-type carbides, crystallized M2C-type carbides, and crystallized M7C3-type carbides. Furthermore, the matrix structure of the outer layer material is preferably martensite and / or bainite. In order to achieve an appropriate area ratio and proportion of crystallized carbides, the structure is limited to having a component composition within the above-described range, containing 6.0 to 18.0% by area of ​​whole-crystallized carbides, and containing 1.0 to 8.0% by area of ​​crystallized M2C-type carbides. It has been discovered that by doing so, the anti-seizure and anti-slip properties of the hot rolling roll outer layer material of the present invention are improved. Furthermore, the present invention is characterized in that the area ratio of total crystallized carbides and the area ratio of M2C-type carbides are within the above range, and in addition, the following formula (3) is satisfied. 0.10≦Crystallized M2C type carbide area ratio (%) / total crystallized carbide area ratio (%)≦0.60 (3)

[0040] Equation (3) shows the relationship between the area ratio of whole crystallized carbides and the area ratio of M2C-type carbides, that is, the ratio of the area ratios. If the lower limit of equation (3) is less than 0.10, the amount of M2C-type carbides, or the proportion of M2C-type carbides in the whole crystallized carbides, may be insufficient, potentially reducing the seizure resistance of the outer layer material. On the other hand, if the upper limit of equation (3) exceeds 0.60, the amount of M2C-type carbides, or the proportion of M2C-type carbides in the whole crystallized carbides, may be excessive. In that case, seizure resistance will decrease, and cracks caused by M2C-type carbides will be more likely to occur. For this reason, the above ratio is limited to between 0.10 and 0.60. Preferably, the above ratio is between 0.20 and 0.50.

[0041] Contrary to conventional findings, even when the area ratio of total crystallized carbides is small, if the area ratio of M2C-type crystallized carbides is appropriate, the morphology of the crystallized planar M2C-type carbides contributes to improved seizure resistance. Furthermore, by suppressing the area ratio of total crystallized carbides, a significant decrease in the surface roughness of the outer layer material is prevented, making slip less likely to occur.

[0042] (Method for manufacturing outer layer material for hot-rolling rolls) Next, an embodiment of the method for manufacturing the hot-rolling roll outer layer material of the present invention will be described. When manufacturing the hot-rolling roll outer layer material of the present invention, molten metal having the above-described component composition of the hot-rolling roll outer layer material is poured into a rotating mold, on which the inner surface of the mold is covered with a refractory material mainly composed of zircon to a thickness of 1 to 5 mm, to a predetermined thickness, and centrifugal casting is performed. As described above, the method of manufacturing the hot-rolling roll outer layer material by centrifugal casting corresponds to the method for manufacturing the hot-rolling roll outer layer material of the present invention.

[0043] The hot-rolling composite roll of the present invention, when the outer layer material of the roll is cast by centrifugal casting, has a centrifugal-cast outer layer (hot-rolling roll outer layer material) and an inner layer welded and integrated with the outer layer. An intermediate layer may be placed between the outer layer and the inner layer. That is, the hot-rolling composite roll of the present invention 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 an inner layer welded and integrated with the outer layer. The inner layer is preferably manufactured by static casting.

[0044] For the inner layer, which is cast by static casting, it is preferable to use spheroidal graphite cast iron or cyanomorphic graphite cast iron (CV cast iron), which have excellent castability and mechanical properties. In centrifugal cast rolls, the outer layer and inner layer are welded together, and components of the outer layer material are mixed into the inner layer during this welding process. If carbide-forming elements such as Cr and V contained in the outer layer material are mixed into the inner layer, the inner layer becomes weaker. For this reason, it is preferable to suppress the mixing of outer layer components into the inner layer as much as possible.

[0045] Furthermore, 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 welded together, and components of the outer layer material are mixed into the intermediate layer during the welding process. When the intermediate layer, which contains components of the outer layer material, is welded to the inner layer, the components of the outer layer material mixed into the intermediate layer move into the inner layer. Therefore, in order to suppress the mixing of components of the outer layer material into the inner layer, it is preferable to suppress the mixing of components of the outer layer material into the intermediate layer as much as possible. There are various methods for suppressing the mixing of components of the outer layer material into the intermediate layer, but one example is optimizing the casting temperature and casting timing.

[0046] For the outer layer (outer layer material for hot rolling rolls) constituting the composite roll for hot rolling, it is preferable that the desired microstructure contains 6.0 to 18.0% total crystallized carbides by area and 1.0 to 8.0% crystallized M2C type carbides by area. To achieve this, casting is performed so as to satisfy the following equation (4), which expresses the relationship between ΔT (casting temperature - liquidus temperature) and casting time. Here, the casting temperature is the temperature of the molten metal immediately before pouring into the mold. The casting time is the time required until the entire amount of molten metal is poured into the mold. The mold temperature is preferably 250°C or lower. 0.3℃ / sec ≤ ΔT (casting temperature °C - liquidus temperature °C) / casting time sec ≤ 5.0℃ / sec ···(4)

[0047] If the lower limit of equation (4) is less than 0.3°C / sec, the cooling rate during solidification (sometimes referred to as the casting rate) becomes excessively fast, resulting in smaller particle sizes of crystallized carbides compared to when the cooling rate is slow. As a result, the total amount of crystallized carbides in the outer layer material becomes insufficient. On the other hand, if the upper limit of equation (4) exceeds 5.0°C / sec, the cooling rate during solidification becomes excessively slow, resulting in larger particle sizes of crystallized carbides compared to when the cooling rate is fast. As a result, the total amount of crystallized carbides in the outer layer material becomes excessive. For this reason, the value of equation (4) is limited to between 0.3°C / sec and 5.0°C / sec. Furthermore, the value of equation (4) is preferably 0.4°C / sec or higher, more preferably 0.5°C / sec or higher. Furthermore, the value of equation (4) is preferably 4.5°C / sec or lower, more preferably 4.0°C / sec or lower. The value of formula (4) is preferably 0.4°C / sec or more and 4.5°C / sec or less, and more preferably 0.5°C / sec or more and 4.0°C / sec or less.

[0048] The ΔT (casting temperature °C - liquidus temperature °C) described in equation (4) above will now be explained. ΔT (casting temperature - liquidus temperature) represents the temperature range until solidification begins. By using this parameter, it becomes possible to precisely control the process from casting to the start of solidification. In other words, conventionally, regardless of the thermophysical properties (liquidus temperature) of the material, casting conditions, i.e., casting speed, were managed using "casting temperature / casting time". Therefore, conventionally, it may not have been possible to control the casting temperature and casting time according to each material. In contrast, in this embodiment, the casting conditions, i.e., the cooling rate, are managed using "ΔT (casting temperature - liquidus temperature) / casting time". Therefore, the thermophysical properties (liquidus temperature) of each material can be taken into consideration when managing the cooling rate, and the casting temperature and casting time can be controlled according to each material. The method for determining the liquidus temperature will now be explained. The liquidus temperature was measured using a differential thermogravimetric analyzer (TGD-9800) manufactured by ULVAC, Inc., by cutting a sample (φ5 × 5 mm) from the center of the outer layer's thickness. The measurement temperature was raised from 1000°C to 1530°C, and then cooled from 1530°C to 1000°C. An alumina container was used as the measurement vessel, the heating rate was set to 5°C / min, and the experiment was conducted in an Ar atmosphere. The liquidus temperature of the sample cut from the center of the outer layer's thickness was measured by differential thermogravimetric analysis (DTA measurement). The liquidus temperature was measured for each of the three samples, and the average value was used as the liquidus temperature.

[0049] Furthermore, it is preferable to perform a quenching treatment at least once, in which the outer layer material is heated to 900-1100°C and then cooled by air or blast air cooling. In addition, it is preferable to perform a tempering treatment two or more times, in which the outer layer material is heated and held, and then cooled, so that the tempering parameter P described in formula (5) below is in the range of 13000-18000. At this time, by changing the quenching temperature, tempering parameter, and number of tempering cycles within the range described according to the composition, it becomes easier to obtain the aforementioned microstructure. P = T(log(t) + A) ... (5) Here, T is the tempering temperature (K), t is the tempering time (h), and A is a constant. In this invention, A = 20.

[0050] Based on the above, a composite roll for hot rolling can be obtained having three layers: an outer layer, an intermediate layer, and an inner layer, or two layers: an outer layer and an inner layer.

[0051] (Methods for observing organizations) Next, the method for observing the microstructure of the outer layer material is described below. First, several samples of the outer layer material obtained by the manufacturing method described above were prepared. Each sample was taken from the center of the thickness of the outer layer material obtained by the manufacturing method described above. Each sample was mirror-polished, and the polished surface of the outer layer material was etched with Nital solution. After that, the microstructure of the exposed outer layer material was observed using a digital microscope (magnification 200x). In addition, the microstructure of the exposed outer layer material was photographed, and the area ratio of all crystallized carbides on the polished surface of the outer layer material was measured by image analysis. Then, using an image analysis tool (ImageJ), the photographs of the polished surface at a measurement magnification of 200x were binarized. This is because there is a difference in brightness between the matrix microstructure and the crystallized carbides in the photograph, and by performing binarization, the matrix microstructure and crystallized carbides can be classified and their respective areas can be determined. For each sample, 10 photographs of the polished surface were taken, and for each sample, the average area percentage of total crystallized carbides was calculated, with the entire image (roll structure) being considered as 100%.

[0052] Furthermore, to calculate the area fraction of crystallized M2C-type carbides, the crystal orientations of the crystallized carbides and the matrix were measured using SEM-EBSD on the same sample used for microstructural observation. The images of the crystallized M2C-type carbides were then separated using the differences in each crystal structure. The SEM-EBSD measurement conditions were: acceleration voltage 15kV, magnification 200x, measurement range 600×1100μm, and step size 0.5μm. The crystal orientation of the crystallized M2C-type carbides was analyzed using the Mo2C database in TSL's software. For other elements, Iron(alpha) was used for the matrix structure crystal orientation, and Iron(gamma) was used for the MC-type carbide crystal orientation. The area fraction of only the crystallized M2C-type carbides was measured using an image analysis tool (ImageJ).

[0053] Furthermore, the hardness of the hot-rolling roll outer layer material of the present invention is preferably 44.0 HS to 52.0 HS in Shore hardness at 600°C and 74.0 HS to 86.0 HS in Shore hardness at 20°C. The roll surface temperature during hot rolling is around 600°C, and if the Shore hardness at 600°C is less than 44.0 HS, plastic flow occurs, and the steel material is prone to sticking to the roll surface. On the other hand, if the hardness exceeds 52.0 HS, the roll hardness is too high, making it prone to slippage during rolling. Such hardness can be stably ensured by heat-treating the roll having the components of the present invention so that the tempering parameter P, described later, is within the range of 13000 to 18000. [Examples]

[0054] The present invention will be described in more detail by the following examples. The present invention is not limited to the following examples.

[0055] Table 1 summarizes the chemical composition of each test material in the present invention example and the chemical composition of each test material in the comparative example. Molten metal with the component composition of the hot rolling roll outer layer material shown in Table 1 was melted in a high-frequency melting furnace, and ring-shaped test materials (outer diameter: φ250 mm, width: 65 mm, wall thickness: 55 mm) were prepared by centrifugal casting so as to satisfy the above formula (4). The remainder other than the component composition shown in Table 1 is Fe and unavoidable impurities. In Table 1 and Table 2 described later, the ring-shaped test material is simply referred to as the test material. Next, after cooling, the ring-shaped test material was removed from the mold and hardened at 900 to 1100°C. After that, the ring-shaped test material was heated and held, and then cooled, and this tempering treatment was performed three times so that the tempering parameter P, represented by the above formula (5), was in the range of 13000 to 18000. Subsequently, microstructure observation, measurement of the seizure area ratio using a drop-weight friction thermal shock tester, crack evaluation, and measurement of the friction coefficient using a hot rolling abrasion tester were performed on each of the ring-shaped test materials. The specimens for microstructure observation, drop-weight friction thermal shock testing, and hot rolling abrasion testing were each taken from the center of the ring-shaped test material in the longitudinal direction and from the center in the thickness direction (hereinafter referred to as the thickness center).

[0056] [Table 1]

[0057] The method for measuring the coefficient of friction using a hot rolling abrasion tester was as follows. Figure 1 is a diagram illustrating the sampling position of the test piece for hot rolling abrasion testing from the ring-shaped test material. As shown in Figure 1, a ring-shaped test piece (outer diameter 60 mmφ, width 10 mm, with C1 chamfer) 1 for hot rolling abrasion testing was taken from the center of the ring-shaped test material TM in the longitudinal direction and from the center of the wall thickness for each example of the present invention and each comparative example. C1 chamfer means that the corner of the test piece is cut off at a 45° angle at a position 1 mm inward from the corner of the test piece.

[0058] The hot rolling wear test was performed using a two-disc sliding rolling method with a test piece 1 and a mating piece 4, as shown in Figure 2. The test piece 1 was rotated at 76 rpm while being cooled with cooling water 2. The mating piece 4 (outer diameter 190 mmφ, width 15 mm, C1 chamfer), heated to 1000°C by a high-frequency induction heating coil 3, was brought into contact with the rotating test piece 1 with a load of 686 N, and the test piece 1 and mating piece 4 were rolled under a sliding ratio of 10%. The direction of action of the load described above is indicated by reference numeral 7 in Figure 2. The rotation direction of the test piece 1 is indicated by reference numeral 5 in Figure 2, and the rotation direction of the mating piece 4 is indicated by reference numeral 6 in Figure 2. In the example shown in Figure 2, the rotation direction 5 of the test piece 1 and the rotation direction 6 of the mating piece 4 are opposite to each other, so the test piece 1 rotates clockwise and the mating piece 4 rotates counterclockwise.

[0059] Furthermore, the mating piece 4 was replaced with a new one every hour, and the hot rolling abrasion test was repeated. This was repeated a total of five times, and the hot rolling abrasion test of test piece 1 was performed for a total of five hours. The torque and load were measured during the test, and the coefficient of friction was calculated from the following formula (5). In addition, the ratio of the average value of the coefficient of friction obtained in the first hot rolling abrasion test (the first hour) during the last minute before the end of the test to the average value of the coefficient of friction obtained in the fifth hot rolling abrasion test (the last hour) during the last minute before the end of the test was calculated. For each sample (test piece 1), two sets of the hot rolling abrasion test totaling five hours were performed, and the average value of the above ratio obtained in each hot rolling friction test was calculated. The hot rolling abrasion test was performed twice for each sample. μ = T / P × L ... (5) Here, μ is the coefficient of friction, T is the torque of test specimen 1 (kgf·m), P is the load (kgf), and L is the radius of the test specimen (m).

[0060] The seizure resistance evaluation using the drop-weight friction thermal shock test was performed as follows. Each test specimen was subjected to a seizure test four times using the drop-weight friction thermal shock testing machine shown in Figure 3. Rectangular test specimens (22 × 30 × 23 mm, test surface: 22 × 30 mm) 11 for drop-weight friction thermal shock were taken from the center of the ring-shaped test material in the longitudinal direction and from the center of the wall thickness for each example of the present invention and each comparative example. That is, although the details are not shown, the test specimens for drop-weight friction thermal shock were taken from the ring-shaped test material in the same way as the test specimens for the hot rolling wear test shown in Figure 1. In the drop-weight friction thermal shock testing machine, the weight 8 shown in Figure 3 is dropped onto the rack 9, which pushes the rack 9 downwards and rotates the pinion 10 that meshes with the rack 9. Since the mating piece 12 is attached to the pinion 10, when the pinion 10 rotates, the mating piece 12 rotates (revolves) around the rotational axis of the pinion 10. In this way, the mating material 12 is strongly rubbed against the test piece 11. This seizure test leaves an indentation on the test piece 11, and the mating piece 12 seizes and adheres to part or all of the indentation. The area on the test piece 11 where the mating piece 12 seizes and adheres (hereinafter referred to as the seized area) was measured. In addition, the area of ​​the indentation formed on the test piece 11 (hereinafter referred to as the indented area) (hereinafter referred to as the indentation area) was measured.

[0061] For each ring-shaped test material, the aforementioned seizure test was performed four times, and the seizure area and indentation area were measured for each test. The seizure area ratio (%) was calculated by dividing the seizure area by the indentation area. The average value of the seizure area ratio from the four tests was then calculated. A smaller seizure area ratio indicates better seizure resistance. After the seizure test, the test surface of test piece 11 was observed using a digital microscope to check for the presence or absence of cracks in the indentation area. If a crack occurred in even one of the four tests, it was determined to be cracked. The results of the hot rolling abrasion test and the seizure test are summarized in Table 2.

[0062] [Table 2]

[0063] The results of each test were evaluated as follows: For the seizure area ratio, a ratio of 42.0% or less was considered a "pass" (i.e., excellent seizure resistance), and a ratio exceeding 42.0% was considered a "fail". In addition, for the friction coefficient ratio (μ5th time / μ1st time), a ratio of 0.80 or higher was considered a "pass" (i.e., excellent slip resistance), and a ratio below 0.80 was considered a "fail".

[0064] As is clear from Table 2, the present invention example was found to possess superior seizure resistance and slip resistance compared to the comparative example. Furthermore, in the present invention example, the casting conditions, i.e., the cooling rate, are controlled using "ΔT (casting temperature - liquidus temperature) / casting time". Therefore, the thermophysical properties (liquidus temperature) of each material can be taken into consideration when controlling the cooling rate, and the casting temperature and casting time can be controlled according to each material. Test materials No. 12 and No. 26, which have the same composition, will be explained in comparison. As shown in Table 2, the characteristics of test material No. 26, whose casting conditions do not meet the appropriate values, i.e., equation (4) described above, such as the total crystal carbide area ratio (%) and the ratio of friction coefficients in the hot rolling test, are inferior to those of test material No. 12, whose casting conditions meet the appropriate values.

[0065] Therefore, according to the present invention, it is possible to manufacture hot rolling roll outer layer materials and hot rolling composite rolls that have excellent seizure resistance and slip resistance. As a result, the lifespan of hot rolling roll outer layer materials and hot rolling composite rolls can be improved, and rolling interruptions and time losses due to roll troubles can be reduced. Furthermore, this can improve rolling efficiency and the productivity of hot-rolled steel sheets. [Explanation of Symbols]

[0066] 1. Test specimen for hot rolling abrasion test 2 Cooling water 3. High-frequency induction heating coil 4. Mating piece in hot rolling abrasion test 5. Rotation direction of the test specimen 6. Direction of rotation of the opponent's piece 7. Direction of the line of action of the load 8 weight 9 racks 10 pinion 11. Test specimens for drop-weight frictional thermal shock testing 12. Counterpiece in drop-weight friction thermal shock test TM Ring-shaped Test Material

Claims

1. In mass percent, It contains C: 1.1-2.5%, Si: 0.1-3.5%, Mn: 0.1-3.5%, Ni: 0.5-6.5%, Cr: 1.5-6.5%, Mo: 2.5-18.0%, V: 1.5-6.5%, W: 0.5-3.5%, P: 0.01-0.08%, S: 0.001-0.050%, and The composition consists of Fe and unavoidable impurities. The content of Cr, Mo, V, and W satisfies the following formula (1): The content of C, Mo, and W satisfies the following formula (2): The area ratio of whole crystallized carbides is 6.0 to 18.0%. Crystallization M ２ The area ratio of type C carbides is 1.0 to 8.0%. Total crystallized carbide area ratio and crystallized M ２ A roll outer layer material for hot rolling, wherein the relationship with the area ratio of type C carbides satisfies the following equation (3). 0.6≦([%Mo]+[%W]) / ([%V]+[%Cr])≦4.0...(1) 1.0≦[%C]×([%Mo] / [%W])≦15.0 (2) 0.10≦ Crystallization M ２ C-type carbide area ratio (%) / total crystallized carbide area ratio (%) ≤ 0.60 ・・・(3) In equations (1) and (2), [%C], [%Cr], [%Mo], [%V], and [%W] represent the mass percentage of each element.

2. A method for manufacturing an outer layer material for hot-rolling rolls according to claim 1, A method for manufacturing a hot rolling roll outer layer material, wherein the mold is rotated while pouring molten metal of the outer layer material with the above-mentioned component composition, thereby generating centrifugal force in the molten metal poured into the mold, and the molten metal is brought into close contact with the mold by this centrifugal force to centrifugeally cast the hot rolling roll outer layer material, satisfying the following formula (4). 0.3°C / sec ≤ ΔT (casting temperature °C - liquidus temperature °C) / casting time sec ≤ 5.0°C / sec ... (4)

3. A composite roll for hot rolling having a two-layer structure of an outer layer and an inner layer, or a three-layer structure of an outer layer, an intermediate layer, and an inner layer, A composite roll for hot rolling, wherein the outer layer is made of the hot rolling roll outer layer material described in claim 1.

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