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 wear resistance, and, having excellent seizure resistance and crack resistance.SOLUTION: This hot-rolled roll outer layer material contains, in mass%, C: 2.50 to 4.00%, Si: 0.60 to 2.50%, Mn: 0.50 to 4.00%, Cr: 0.50 to 3.50%, Ni: 1.50 to 4.40%, V: 1.00 to 2.50%, Nb: 0.10 to 0.75%, and Mo: 0.30 to 1.50%, with the balance being Fe and unavoidable impurities, wherein the contents of Mn and Ni satisfy the following formula (1), and the contents of Si and V satisfy the following formula (2). 0.20≤Mn / Ni≤0.80 (1) 0.55≤Si / V≤1.85 (2) Here, the symbols in formulas (1) and (2) represent the respective mass% of each corresponding element.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an outer layer material for a hot rolling roll, a method for manufacturing the same, and a composite roll for hot rolling. The outer layer material for a hot rolling roll of the present invention is particularly suitable as an outer layer material for a work roll of a hot rolling mill for steel plates. Further, the composite roll for hot rolling of the present invention is particularly suitable as a work roll of a hot rolling mill for steel plates.

Background Art

[0002] In recent years, with the progress of hot rolling technology for steel plates, the operating environment of rolls has become more severe, and the production volume of steel plates with large rolling loads, such as high-strength steel plates and thin products, has also increased. For this reason, the quality level required for work rolls for rolling has become higher, and high-performance work rolls for rolling are demanded.

[0003] In the latter stands of a hot finishing rolling mill, work rolls having an outer layer made of a Glenn-based cast iron material or a high-alloy Glenn-based cast iron material with excellent accident resistance are used. A rolling accident that may occur in the latter stands of a hot finishing rolling mill is a choking accident.

[0004] A choking accident is a phenomenon in which the end portion of the material to be rolled is folded and caught between the rolls. When a choking accident occurs, the material to be rolled may be seized on the roll surface, and large thermal and mechanical loads may be generated on the roll, which may cause cracks on the roll surface. Such cracks may reach a depth of 1 mm or more. When such cracks occur, it is necessary to grind the roll surface to remove the cracks, which causes problems such as an increase in working costs and a shortening of the roll life. Therefore, even when a choking accident occurs, there is a demand for a hot rolling work roll that is difficult for the steel plate to seize and for cracks to occur and progress, and that has excellent seizure resistance and crack resistance.

[0005] As an outer layer material for hot rolling rolls or for hot rolling rolls, for example, Patent Document 1 proposes an outer layer material for rolling rolls that contains, by mass%, C: more than 3.0% and 4.0% or less, Si: 3.0% or less, Ni: 2.3 to 5.5%, Cr: 1.0 to 2.0%, V: 0.3 to 10.0%, and Ti: 0.01 to 2.0%, with the remainder being Fe and impurity elements, and has graphite and MC-type carbides in its metal structure, with a graphite spheroidization rate of 0.5 or more. Patent Document 1 states that by making the shape of the graphite into fine spheres and uniformly dispersing the graphite in the metal structure, the wear resistance, surface roughness resistance, and accident resistance of the outer layer material are improved.

[0006] Patent Document 2 proposes a composite rolling roll containing, by mass%, C: 3.0-4.5%, Si: greater than 0% and less than or equal to 2.0%, Mn: greater than 0% and less than or equal to 1.5%, Ni: 3.0-5.0%, Cr: 1.4-4.0%, Mo: 0.1-1.5%, and V: greater than 0% and less than or equal to 3.0%, with the remainder being Fe and unavoidable impurity elements, where C, Si, and Cr are 4.0% ≤ C + Si / 3 + Cr / 7.5 ≤ 5.5%, and the metallic structure of the outer layer's circumferential surface subjected to rolling has a cementite area ratio of 40% or more and less than 46%. Patent Document 2 states that because a large amount of hard cementite is present, the composite rolling roll will have excellent wear resistance and surface roughness resistance.

[0007] Patent Document 3 proposes a composite rolling roll in which the metal structure of the circumferential surface subjected to rolling has a cementite area ratio of 40-60% and a graphite area ratio of 0.5-2.0%. Patent Document 3 states that because a large amount of hard cementite crystallizes and the graphite area ratio is further adjusted, the composite rolling roll will have excellent wear resistance, surface roughness resistance, and crack resistance.

[0008] Patent Document 4 proposes a composite rolling roll containing, by mass%, C: 2.2-3.2%, Si: 1.0-3.0%, Mn: 0.3-2.0%, Ni: 3.0-7.0%, Cr: 0.5-2.5%, Mo: 1.0-3.0%, V: 2.5-5.0%, and Nb: greater than 0% but less than or equal to 0.5%, with the remainder being Fe and unavoidable impurities, where Nb and V have a ratio of Nb / V < 0.1, and the ratio of C, Si, Cr, Mo, V, and Nb is 2.1 × C + 1.2 × Si - Cr + 0.5 × Mo + (V + Nb / 2) ≤ 13.0%. Patent Document 4 states that satisfying two relational equations results in a composite rolling roll with excellent crack resistance and wear resistance.

[0009] Patent Document 5 proposes a centrifugal casting composite roll for rolling, which contains, by mass%, C: 1.5~3.5%, Si: 0.3~3.0%, Mn: 0.1~3.0%, Ni: 1.0~6.0%, Cr: 1.5~6.0%, Mo: 0.1~2.5%, V: 2.0~6.0%, Nb: 0.1~3.0%, B: 0.001~0.2%, and N: 0.005~0.070%, with the remainder being Fe and unavoidable impurities, satisfying Ni, Cr, and Mo as 2×Ni + 0.5×Cr + Mo > 10.0, with the outer Shore hardness A of the roll surface being Hs75 ≤ A ≤ Hs85, and the residual stress B of the roll surface being 100 MPa ≤ B ≤ 350 MPa. Patent Document 5 states that by having a predetermined component composition and setting the outer Shore hardness of the roll surface within a predetermined range, a composite roll for rolling with excellent wear resistance and crack resistance can be obtained. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2005-177808 [Patent Document 2] Japanese Patent Publication No. 2018-75638 [Patent Document 3] Japanese Patent Publication No. 2015-193025 [Patent Document 4] Japanese Patent Publication No. 2018-118319 [Patent Document 5] International Publication No. 2020 / 203570 [Overview of the project] [Problems that the invention aims to solve]

[0011] In addition to improving seizure resistance, there has been a growing demand for hot rolling rolls with superior wear resistance in recent years, from the perspective of improving roll efficiency and roll life. However, the outer layer materials or hot rolling rolls proposed in Patent Documents 1 to 5 do not have sufficient wear resistance, and there is room for improvement. The technologies in these patent documents aim to improve wear resistance by generating a large amount of carbides (cementite, MC-type carbides). However, in order to further improve wear resistance while maintaining seizure resistance, a new perspective is needed that is not an extension of conventional technology.

[0012] This invention has been made in view of the above circumstances, and aims to provide an outer layer material for hot rolling rolls that has remarkably excellent wear resistance, as well as good seizure resistance and crack resistance. [Means for solving the problem]

[0013] Hot rolling rolls with a grain cast iron or high-alloy grain cast iron outer layer material prevent the rolled material from seizing up even in the event of a drawing accident, and suppress the occurrence and propagation of cracks, by forming graphite in the metal structure. However, because graphite is soft, a large amount of graphite formation in the metal structure of the outer layer material leads to a problem in which the wear resistance of the roll outer layer material decreases.

[0014] Conventional technologies have attempted to improve wear resistance by generating predetermined amounts of MC-type carbides and / or cementite. However, further improvements in wear resistance were difficult by simply increasing the amount of these carbides, which is an extension of conventional approaches. Furthermore, excessive increases in carbides could lead to a decrease in crack resistance. To solve these problems, the inventors conducted diligent research and obtained the following findings (1) to (3).

[0015] (1) The content of a predetermined element shall be within a predetermined range, and furthermore, the ratio (Mn / Ni) of the contents of Mn and Ni shall be 0.20 to 0.80. (2) When manufacturing the outer layer material for hot rolling rolls, it is necessary that the time from the completion of the component adjustment of the molten metal having the component composition of the outer layer material (molten metal for forming the outer layer material) to the start of pouring the molten metal for forming the outer layer material into the mold is within 3 minutes. (3) It is preferable to have a metal structure in which the area ratio of graphite is 0.5 to 6.0%, the area ratio of cementite is 30 to 55%, and the area ratio of MC type carbide is 0.1 to 5.0%.

[0016] The present invention has been completed through further study based on the above findings, and its gist is as follows. [1] In mass%, C: 2.50 to 4.00%, Si: 0.60 to 2.50%, Mn: 0.50 to 4.00%, Cr: 0.50 to 3.50%, Ni: 1.50 to 4.40%, V: 1.00 to 2.50%, Nb: 0.10 to 0.75%, and Mo: 0.30 to 1.50% are contained, and the balance consists of Fe and inevitable impurities, The outer layer material for hot rolling rolls, in which the contents of Mn and Ni satisfy the following formula (1), and the contents of Si and V satisfy the following formula (2). 0.20 ≤ Mn / Ni ≤ 0.80 ··· (1) 0.55 ≤ Si / V ≤ 1.85 ··· (2) Here, each element symbol in formula (1) and formula (2) indicates the content (mass%) of the corresponding element. [2] The above component composition further contains, in mass%, W: 2.0% or less, Co: 1.00% or less, B: 0.1000% or less, Ti: 0.30% or less, and The roll outer layer material for hot rolling according to [1], containing one or more selected from among those with Zr of 0.30% or less. [3] The roll outer layer material for hot rolling according to [1] or [2], having a metal structure in which the area ratio of graphite is 0.5 to 6.0%, the area ratio of cementite is 30 to 55%, the area ratio of MC type carbide is 0.1 to 5.0%, and the average crystal grain size of martensite and bainite is 2.0 to 20.0 μm. [4] A method for manufacturing the roll outer layer material for hot rolling according to any one of [1] to [3], A method for manufacturing the roll outer layer material for hot rolling, in which after the component adjustment of the molten metal for forming the outer layer material is completed, pouring of the molten metal for forming the outer layer material into the mold is started within 3 minutes. [5] A composite roll for hot rolling having a three-layer structure of an outer layer, an intermediate layer, and an inner layer, or a two-layer structure of an outer layer and an inner layer, The composite roll for hot rolling, in which the outer layer is made of the roll outer layer material for hot rolling according to any one of [1] to [3].

Advantages of the Invention

[0017] According to the present invention, a roll outer layer material for hot rolling that is remarkably excellent in wear resistance and has both good seizure resistance and crack resistance can be obtained.

Brief Description of the Drawings

[0018] [Figure 1] FIG. 1 is a schematic diagram showing the sampling position of a test piece for observing the outer surface of the roll outer layer material (sleeve roll) for hot rolling. [Figure 2] FIG. 2 is a schematic diagram showing the sampling position of a wear test piece from the sleeve roll. [Figure 3] FIG. 3 is a diagram schematically showing the configuration of the testing machine used in the wear test. [Figure 4] FIG. 4 is a diagram schematically showing the configuration of the testing machine used in the seizure test. [[ID=3�]] [Figure 5] FIG. 5 is a schematic diagram showing the sampling position of a test piece (seizure test piece) from the sleeve roll. [Figure 6]Figure 6 is a schematic diagram illustrating the method for evaluating the fused area ratio of a fused test specimen. [Figure 7] Figure 7 is a schematic diagram showing the sampling locations of test specimens (thermal shock test specimens) from the sleeve roll, and a schematic diagram illustrating the method for evaluating crack depth. [Modes for carrying out the invention]

[0019] First, we will explain the reason for limiting the component composition of the hot-rolling roll outer layer material (outer layer of hot-rolling composite roll) of the present invention. Note that, unless otherwise specified, the percentages indicating the content of each component below refer to mass percentages.

[0020] C: 2.50~4.00% Carbon (C) increases the hardness of the matrix (martensite and / or bainite) and combines with elements such as Fe, V, Cr, and Mo to form hard carbides, contributing to improved wear resistance. Furthermore, C crystallizes as graphite in the metal structure when combined with graphitization-promoting elements such as Si and Ni, improving seizure resistance. If the C content is less than 2.50%, the amount of carbides and graphite is insufficient, resulting in reduced wear resistance and seizure resistance. On the other hand, a C content exceeding 4.00% causes coarsening of the carbides, reducing crack resistance. For this reason, the C content is limited to 2.50-4.00%. The C content is preferably 2.70% or more, more preferably 2.80% or more. Also, the C content is preferably 3.80% or less, more preferably 3.60% or less.

[0021] Si: 0.60~2.50% Si acts as a deoxidizing agent and improves the castability of molten metal. Si is also a graphitization-promoting element, causing carbon to crystallize as graphite. To obtain these effects, a Si content of 0.60% or more is required. On the other hand, if the Si content exceeds 2.50%, the matrix structure becomes brittle, reducing wear resistance. Therefore, the Si content is limited to 0.60-2.50%. The Si content is preferably 0.70% or more, more preferably 0.90% or more. Furthermore, the Si content is preferably 2.30% or less, more preferably 2.10% or less.

[0022] Mn: 0.50~4.00% Mn is an element that fixes sulfur as MnS, rendering it harmless, and also partially dissolves into the matrix structure, improving hardenability and strengthening the matrix (solid solution strengthening). To obtain these effects, a Mn content of 0.50% or more is required. On the other hand, if the Mn content exceeds 4.00%, the effect saturates, and the effect commensurate with the content cannot be expected, and in some cases, the material may even become brittle. For this reason, the Mn content is limited to 0.50 to 4.00%. The Mn content is preferably 0.70% or more, more preferably 0.90% or more, and even more preferably more than 1.00%. Furthermore, the Mn content is preferably 3.80% or less, and more preferably 3.60% or less.

[0023] Cr: 0.50~3.50% Cr combines with C to form solid solutions in cementite and / or Cr-based carbides (M7C3, M 23This element forms C6 (and other elements), and further improves the hardenability of the matrix, transforming it into martensite and / or bainite, thereby improving wear resistance. To obtain such effects, a Cr content of 0.50% or more is required. On the other hand, a Cr content exceeding 3.50% is undesirable because it increases the amount of coarse Cr-based carbides, which may reduce crack resistance. Therefore, the Cr content is limited to 0.50-3.50%. The Cr content is preferably 0.70% or more, and more preferably 0.80% or more. Furthermore, the Cr content is preferably 3.30% or less, and more preferably 3.10% or less.

[0024] Ni: 1.50~4.40% Ni is an element that dissolves in the matrix, lowering the transformation temperature of austenite during heat treatment, thereby improving the hardenability of the matrix and transforming it into martensite and / or bainite, thus improving wear resistance. Ni also promotes graphitization, increasing the amount of graphite produced and improving seizure resistance. If the Ni content is less than 1.50%, these effects are insufficient. Therefore, the Ni content should be 1.50% or more. On the other hand, if the Ni content exceeds 4.40%, the transformation temperature of austenite becomes too low, causing austenite to remain after heat treatment, reducing wear resistance. Therefore, the Ni content should be limited to 1.50 to 4.40%. The Ni content is preferably 1.70% or more, and more preferably 1.90% or more. Furthermore, the Ni content is preferably 4.30% or less, and more preferably 4.10% or less.

[0025] V: 1.00~2.50% V is an element that forms extremely hard V-type carbides (MC-type carbides) and improves wear resistance. This effect can be obtained with a V content of 1.00% or more. On the other hand, a V content exceeding 2.50% coarses the MC-type carbides and further reduces the amount of graphite produced, thus decreasing seizure resistance. For this reason, the V content is limited to 1.00 to 2.50%. The V content is preferably 1.10% or more, and more preferably 1.30% or more. Furthermore, the V content is preferably 2.40% or less, and more preferably 2.30% or less.

[0026] Nb: 0.10~0.75% Nb is an element that improves wear resistance by solid-dissolving in MC-type carbides, strengthening them, and increasing their fracture resistance. Nb also has the effect of suppressing segregation of MC-type carbides during centrifugal casting. These effects become significant with an Nb content of 0.10% or more. On the other hand, if the Nb content exceeds 0.75%, coarse MC-type carbides are formed, reducing seizure resistance. Therefore, the Nb content is limited to 0.10-0.75%. The Nb content is preferably 0.15% or more, more preferably 0.20% or more, and preferably 0.70% or less, more preferably 0.65% or less.

[0027] Mo: 0.30~1.50% Mo is an element that combines with C to form solid solutions and / or M2C-type carbides in cementite, thereby improving wear resistance. Furthermore, Mo strengthens hard MC-type carbides formed by the bonding of V, Nb, and C by solid solution. Through these actions, Mo improves the wear resistance of the roll outer layer material. To obtain this effect, a Mo content of 0.30% or more is required. On the other hand, a Mo content exceeding 1.50% generates coarse Mo-based carbides, reducing crack resistance. Therefore, the Mo content is limited to 0.30-1.50%. The Mo content is preferably 0.40% or more, more preferably 0.50% or more. Also, the Mo content is preferably 1.40% or less, more preferably 1.30% or less.

[0028] The remainder of the components other than those listed above may consist of Fe and unavoidable impurities. Examples of unavoidable impurities include Al, S, P, Sb, Mg, Cu, N, O, and Sn. These are introduced from the raw materials or from refractories during the dissolution of the raw materials. Preferably, these unavoidable impurities are present in amounts of Al: 0.1% or less, S: 0.05% or less, P: 0.1% or less, Sb: 0.2% or less, Mg: 0.1% or less, Cu: 0.1% or less, N: 0.1% or less, O: 0.05% or less, and Sn: 0.1% or less.

[0029] In addition to the above component composition, the hot rolling roll outer layer material of the present invention may optionally contain one or more of the following:

[0030] W: 2.0% or less W is an element that dissolves in the matrix, strengthening it and improving wear resistance, and also dissolves in cementite, further improving wear resistance. However, if the W content exceeds 2.0%, not only does the effect saturate, but coarse M2C or M6C carbides are formed, reducing crack resistance. For this reason, when W is included, the W content should be limited to 2.0% or less. The W content is preferably 1.8% or less, and more preferably 1.6% or less. When W is included, in order to obtain the above effects, the W content is preferably 0.1% or more, and more preferably 0.4% or more.

[0031] Co: 1.00% or less Co is an element that dissolves in the matrix, strengthening it and improving wear resistance. However, a Co content exceeding 1.00% can lead to saturation of the effect, making it economically disadvantageous as the effect commensurate with the content can no longer be expected, and the matrix structure may become brittle, reducing wear resistance. For this reason, when Co is included, the Co content should be limited to 1.00% or less. Preferably, the Co content is 0.80% or less. Furthermore, when Co is included, in order to obtain the above-mentioned effect of improving wear resistance, a Co content of 0.05% or more is preferred, and 0.10% or more is more preferred.

[0032] B: 0.1000% or less B is an element that improves the hardenability of the matrix, transforming it into martensite and / or bainite, thereby improving wear resistance. However, a B content exceeding 0.1000% is undesirable because it forms brittle borocarbides, reducing wear resistance. Therefore, when B is included, the B content should be limited to 0.1000% or less. Preferably, the B content is 0.0500% or less. Furthermore, when B is included, to obtain the effect of improving wear resistance, the B content is preferably 0.0010% or more, and more preferably 0.0020% or more.

[0033] Ti: 0.30% or less, Zr: 0.30% or less Ti and Zr are elements that combine with C to form carbides, thereby improving wear resistance. However, if the Ti and Zr content exceeds 0.30% each, it alters the morphology of cementite and reduces wear resistance. Therefore, when Ti and Zr are included, their content should be limited to 0.30% or less each. Preferably, the Ti and Zr content is 0.20% or less each. Furthermore, when Ti and Zr are included, to obtain the effect of improving wear resistance, the Ti and Zr content is preferably 0.01% or more, and more preferably 0.05% or more each.

[0034] Furthermore, in order to obtain a hot-rolling roll outer layer material that has significantly improved wear resistance and also possesses good seizure resistance and crack resistance, the present invention requires that the content of Mn and Ni satisfy the following formula (1), and the content of Si and V satisfy the following formula (2). 0.20 ≤ Mn / Ni ≤ 0.80 ···(1) 0.55 ≤ Si / V ≤ 1.85 ···(2) Here, the element symbols Mn and Ni in equation (1) represent the Mn content (mass%) and Ni content (mass%), respectively. The element symbols Si and V in equation (2) represent the Si content (mass%) and V content (mass%), respectively.

[0035] If the Mn / Ni ratio is less than 0.20, a sufficient improvement in wear resistance cannot be obtained. On the other hand, if the Mn / Ni ratio is greater than 0.80, the amount of graphite produced is insufficient, and the anti-seizure properties decrease. The Mn / Ni ratio is preferably 0.30 or higher, and more preferably 0.35 or higher. Furthermore, the Mn / Ni ratio is preferably 0.70 or lower, and more preferably 0.65 or lower.

[0036] If the Si / V ratio is less than 0.55, the amount of graphite produced will decrease, potentially reducing anti-seizure properties. On the other hand, if the Si / V ratio is greater than 1.85, the amount of MC-type carbides will be insufficient, potentially reducing wear resistance. The Si / V ratio is preferably 0.60 or higher, more preferably 0.70 or higher. Furthermore, the Si / V ratio is preferably 1.70 or lower, more preferably 1.65 or lower.

[0037] The hot-rolling roll outer layer material of the present invention has a metallic structure containing a matrix, graphite, cementite, and MC-type carbides. Graphite is an important element that contributes to seizure resistance by suppressing or mitigating the burning of the rolled material onto the roll surface when a drawing accident occurs, and further contributes to crack resistance by suppressing the propagation of cracks within the roll. In addition, cementite and MC-type carbides are important elements for ensuring wear resistance, but in the present invention, the matrix (martensite and / or bainite) is further strengthened by satisfying the content of Mn and Ni in formula (1), which contributes to a significant improvement in wear resistance.

[0038] In the metal structure of the hot-rolling roll outer layer material of the present invention, the matrix is ​​martensite and / or bainite. To obtain good crack resistance, it is preferable to perform heat treatment (tempering) after the formation of martensite and / or bainite. The area ratio of graphite is preferably 0.5 to 6.0%, more preferably 0.7 to 5.5%. The area ratio of cementite is preferably 30 to 55%, more preferably 35 to 50%. The area ratio of MC-type carbides is preferably 0.1 to 5.0%, more preferably 0.3 to 4.5%. The remainder other than those mentioned above includes one or more of pearlite, retained austenite, M2C-type carbides, and M6C-type carbides. The area ratio of the remainder is preferably 0.1 to 15%, more preferably 1.0 to 10%. Furthermore, the average grain size of the martensite and bainite constituting the matrix is ​​preferably 2.0 to 20.0 μm, and more preferably 3.5 to 18.5 μm. Such an average grain size can be obtained by limiting the time from the completion of the component adjustment of the molten metal for forming the outer layer, as described later, until the start of pouring the molten metal into the mold. That is, by limiting the time from the completion of the component adjustment of the molten metal for forming the outer layer to the start of pouring the molten metal into the mold to within 3 minutes, 0.5 to 6.0% of graphite is generated by area when the molten metal solidifies, and in the subsequent cooling process, martensite and / or bainite with an average grain size of 2.0 to 20.0 μm is generated in the matrix. On the other hand, if the time from the completion of the component adjustment of the molten metal for forming the outer layer to the start of pouring the molten metal into the mold exceeds 3 minutes, the amount of graphite generated when the molten metal solidifies decreases, and the amount of carbon dissolved in the matrix increases. As a result, the average grain size of martensite and / or bainite falls below 2.0–20.0 μm.

[0039] The method for measuring the area ratio of graphite is not particularly limited, but in this invention, the area ratio of graphite was measured by the following method. A test piece for observing the outer surface was taken from the outer layer material of a hot-rolling roll, the observation surface was mirror-polished, and metallographic images of five fields of view were taken at 100x magnification. The area ratio of graphite was measured by image analysis based on the obtained metallographic images, and the average value of the five fields of view was calculated to determine the graphite area ratio of the test piece. More specifically, the area ratio of graphite was measured by the method described in the examples.

[0040] Furthermore, the area ratios of cementite and MC-type carbides were measured using specimens that had their observation surfaces mirror-polished and then etched with 3 vol% nital. For cementite, metallographic images were taken at 100x magnification in five arbitrary fields of view, and for MC-type carbides, metallographic images were taken at 500x magnification in five arbitrary fields of view. The area ratios of each were then calculated by image analysis. The average of these area ratios was then calculated and used as the area ratio of cementite and MC-type carbides in the specimen. The calculation of the area ratios of MC-type carbides and cementite by image analysis was performed using the following method: For MC-type carbides, the area of ​​the white region with an equivalent circle diameter of 10 μm or less was measured by image analysis and divided by the total area of ​​the observation field. For cementite, the area of ​​the white region was measured by image analysis and divided by the total area of ​​the observation field. In the 500x metallographic image, small white areas less than 10 μm in size represent MC-type carbides, while in the 100x metallographic image, the white areas represent cementite.

[0041] The average grain size of martensite and bainite was determined by EBSD measurement after mirror polishing of the observation surface of the outer surface observation specimen. The measurement was performed at an acceleration voltage of 15kV and a step size of 0.1μm, resulting in a 10,000μm step size. 2EBSD measurements were performed in the above regions. Using the obtained data, boundary lines were drawn at locations where the orientation difference between adjacent measurement points was 15° or more. The grain size of the region enclosed by the boundary lines was measured as a single crystal, and the average value was taken as the average grain size of martensite and bainite. In the metal structure of the hot-rolling roll outer layer material of the present invention, the matrix is ​​martensite and / or bainite. That is, if the matrix is ​​martensite, the average grain size refers to the average grain size of martensite. If the matrix is ​​bainite, the average grain size refers to the average grain size of bainite. If the matrix is ​​both martensite and bainite, the average grain size refers to the average grain size of martensite and bainite.

[0042] The outer layer material for hot rolling rolls of the present invention is manufactured by centrifugal casting and can be used as is for ring rolls and sleeve rolls, but it is particularly suitable for use as an outer layer material for hot rolling composite rolls, which are suitable for hot finish rolling. Furthermore, the hot rolling composite roll of the present invention preferably consists of an outer layer 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, instead of an inner layer welded and integrated with the outer layer, an intermediate layer welded and integrated with the outer layer and an inner layer welded and integrated with the intermediate layer may be used. In the present invention, the composition of the inner layer and the intermediate layer is not particularly limited, but it is preferable that the inner layer be made of spheroidal graphite cast iron (ductile cast iron) and the intermediate layer be made of a high-carbon material with C:1.50~3.00 mass%.

[0043] Next, the method for manufacturing the hot-rolling roll outer layer material and the hot-rolling composite roll of the present invention will be described.

[0044] The outer layer material for hot rolling rolls is manufactured by centrifugal casting. After coating the inner surface of the mold with a refractory material mainly composed of zircon or the like to a thickness of 0.5 to 5.0 mm, the mold is rotated and molten metal having the component composition of the hot rolling roll outer layer material described above (molten metal for forming the outer layer material) is poured in to the specified thickness. The centrifugal casting method may use any of the following forms: vertical (rotation axis is vertical), horizontal (rotation axis is horizontal), or inclined (rotation axis is oblique).

[0045] In this invention, the time between the completion of component adjustment of the molten metal for forming the outer layer material and the start of pouring the molten metal into the mold is limited. That is, in this invention, pouring of the molten metal into the mold is started within 3 minutes after the component adjustment of the molten metal for forming the outer layer material is completed. The hot-rolling roll outer layer material of this invention has a limited component composition in order to improve wear resistance, and as a result, it has a component composition that makes it difficult for graphite to form. Therefore, in conventional manufacturing methods, the amount of graphite produced is insufficient, and the anti-seize properties decrease. Therefore, in this invention, it is necessary to start pouring the molten metal into the mold within 3 minutes after the component adjustment of the molten metal for forming the outer layer material is completed. Here, completion of component adjustment of the molten metal for forming the outer layer material refers to the point in time when all the raw materials necessary for component adjustment of the molten metal for forming the outer layer material (including inoculants if inoculation is performed) have been added. By starting the pouring of the molten metal into the mold within 3 minutes after the component adjustment of the molten metal for forming the outer layer material is completed, it is possible to obtain a metal structure in which the area ratio of graphite is 0.5 to 6.0% in the component composition of the hot-rolling roll outer layer material as defined in the present invention. More preferably, the pouring of the molten metal into the mold is started within 1 minute after the component adjustment of the molten metal for forming the outer layer material is completed.

[0046] When forming an intermediate layer, it is preferable to rotate the mold while pouring molten metal for intermediate layer formation (molten metal having the component composition of the intermediate layer) after the outer layer material has solidified, either during its solidification or after it has completely solidified, and then perform centrifugal casting. After the outer layer (or intermediate layer, if present) has completely solidified, the rotation of the mold is stopped and the mold is set upright, and then the inner layer material is cast in a static manner to form a composite roll. As a result, the inner surface of the roll's outer layer material is remelted, and the outer layer and inner layer, or the outer layer and intermediate layer, or the intermediate layer and inner layer are welded together to form a composite roll.

[0047] Furthermore, it is preferable to use spheroidal graphite cast iron or cyanomorphic graphite cast iron (CV cast iron), which have excellent castability and mechanical properties, for the inner layer that is cast by static casting.

[0048] 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 integrally welded together, and components of the outer layer are mixed into the intermediate layer. From the viewpoint of suppressing the amount of outer layer components mixed into the inner layer, it is preferable to reduce the amount of outer layer components mixed into the intermediate layer as much as possible.

[0049] Based on the above, a composite roll for hot rolling can be obtained in which the outer layer is made of the hot rolling roll outer layer material of the present invention, and the roll has a three-layer structure consisting of an outer layer, an intermediate layer, and an inner layer, or a two-layer structure consisting of an outer layer and an inner layer.

[0050] The outer layer material for hot rolling rolls and the composite rolls for hot rolling of the present invention are preferably subjected to heat treatment after casting. The heat treatment is preferably carried out by heating and holding the rolls at 400 to 550°C after removing them from the mold after casting, followed by slow cooling.

[0051] The preferred hardness of the hot-rolling roll outer layer material and hot-rolling composite roll of the present invention is 74 to 88 HS (Shore hardness). A hardness of 74 HS or higher tends to improve wear resistance. A hardness of 88 HS or lower tends to improve crack resistance. A more preferred hardness is 76 HS or higher. A more preferred hardness is 86 HS or lower. Such hardness can be obtained by adjusting the heat treatment temperature. The hardness can be measured by the method described in the examples. [Examples]

[0052] After melting the material in a high-frequency induction furnace and heating it to 1420°C, the molten metal (molten metal for forming the outer layer) having the component composition shown in Table 1, to which the inoculant was added, was supplied to the mold of a horizontal centrifugal casting machine under the casting conditions shown in Table 2 after the addition of the inoculant was complete. This cast a sleeve roll (equivalent to the outer layer material for a hot rolling roll) with an outer diameter of 250 mm, an inner diameter of 150 mm, and a depth of 70 mm. Before supplying the molten metal for forming the outer layer to the mold and casting the sleeve roll, the inner surface of the mold was coated with a zircon-based refractory material to a thickness of 1.0 mm. After casting, the sleeve roll was heated at 410°C for 10 hours and then slowly cooled, and various test pieces were taken. In Table 1, the symbol "-" indicates that the element was intentionally omitted, and this includes not only cases where the element is not present, but also cases where it is inevitably present. Furthermore, the "Time until pouring into the mold begins (minutes)" in Table 2 refers to the time (minutes) from the point when all the raw materials necessary for adjusting the composition of the molten metal for forming the outer layer have been added, that is, in this embodiment, the point when the addition of the inoculant has been completed, until the pouring of the molten metal into the mold begins.

[0053] Test specimens No. 1-15, 40, and 41 shown in Table 1 are examples of the present invention, while test specimens No. 16-38 are comparative examples. Test specimen No. 39 is a conventional example.

[0054] Microstructure observation was performed to evaluate the area ratios of graphite, cementite, and MC-type carbides. The outer surface of the sleeve roll was machined to reduce its outer diameter from 250 mm to 240 mm. Then, as shown in Figure 1, a test specimen 2 for outer surface observation was taken from the outer surface side of the machined sleeve roll 1. The size of the test specimen was 20 mm × 10 mm × 5 mm (20 mm is the depth direction of the sleeve roll, 10 mm is the circumferential direction of the sleeve roll, and 5 mm is the radial direction of the sleeve roll). The 20 mm × 10 mm surface of the test specimen was used as the observation surface (i.e., the observation surface was located 10 mm inward in the thickness direction from the outer surface of the sleeve roll). After mirror polishing the observation surface, metallographic images of five fields of view were taken with an optical microscope at 100x magnification. Based on the obtained metallographic images, the area ratio of graphite was measured by image analysis, and the average value of the five fields of view was calculated to determine the graphite area ratio of the test specimen. The area ratios of cementite and MC-type carbides were measured using the same method as described above. The average grain size of martensite and bainite was determined by EBSD measurement. After mirror polishing the observation surface of specimen 2 for outer surface observation, EBSD was performed at an acceleration voltage of 15 kV and a step size of 0.1 μm, measuring 10,000 μm. 2 EBSD measurements were performed in the above regions. Using the obtained data, boundary lines were drawn at locations where the orientation difference between adjacent measurement points was 15° or more. The grain size of the region enclosed by these boundary lines was measured as a single crystal, and the average value was taken as the average grain size of martensite and bainite. In order to accurately evaluate the grain size of martensite and bainite, when performing EBSD measurements, the measurement software was set to measure only BCC (body-centered cubic) crystal structures.

[0055] Furthermore, Shore hardness (HS) was measured using the following method. For the outer surface observation specimen, the Vickers hardness HV50 was measured using a Vickers hardness tester (test force: 50 kgf (490 N)) in accordance with the provisions of JIS Z 2244 (2020). Then, the converted Shore hardness VHS (HS) was calculated using the conversion formula of JIS B 7731 (2000). There were 10 measurement points for each specimen, and the average value of the 8 points obtained by removing the highest and lowest values ​​was calculated and used as the hardness of the test material.

[0056] A wear test was conducted to evaluate the wear resistance. As shown in Figure 2, a wear test piece 3 (outer diameter 60 mm, wall thickness 10 mm, chamfered) was taken from the outer surface of the sleeve roll 1, which had been machined in the same manner as described above. The wear test was performed using a two-disc sliding rolling method between the wear test piece and a mating piece, as shown in Figure 3. The wear test piece 3 was rotated at 700 rpm while being water-cooled with cooling water 4. Then, a mating piece 6 (material: S45C, outer diameter: 190 mm, width: 15 mm, C1 chamfer), whose outer surface had been heated to 800°C by a high-frequency induction heating coil 5, was brought into contact with the rotating wear test piece 3 with a load of 980 N and rolled at a sliding ratio of 9%. The wear test was conducted for 300 minutes, and the mating piece was replaced with a new one every 50 minutes. A wear test was also conducted in the same manner as described above using a test piece (reference piece) taken from a conventional sleeve roll in the same manner as described above. Then, using the amount of wear obtained from the reference piece as a baseline, the ratio of the amount of wear of the baseline to the amount of wear of each abrasion test piece was calculated as the abrasion ratio (= (abrasion amount of baseline piece) / (abrasion amount of each abrasion test piece)), and the abrasion resistance was evaluated. In this evaluation, an abrasion ratio of 1.10 or higher was judged to be remarkably superior in abrasion resistance.

[0057] To evaluate seizure resistance, a seizure test was conducted using the drop-weight friction thermal shock tester shown in Figure 4. As shown in Figure 5, a 30mm × 25mm × 20mm test piece (seizure test piece) 10 was taken from the outer surface of the sleeve roll 1, which had been machined in the same manner as described above. Note that 30mm is the circumferential direction of the sleeve roll, 25mm is the depth direction of the sleeve roll, and 20mm is the radial direction of the sleeve roll. As shown in Figure 4, the drop-weight friction thermal shock tester works by dropping a weight 8 onto the rack 7 and rotating the pinion 9, rubbing a mating piece 11 (material: mild steel) against the 30mm × 25mm surface of the test piece 10. Seizure resistance was evaluated by the area of ​​the mating piece that seized onto the surface of the test piece 10. Figure 6 schematically shows the evaluation method for seizure resistance (seizure area ratio of seized test pieces). When the mating piece 11 is rubbed against the surface of the test piece 10, the surface temperature of the test piece 10 rises due to frictional heat, and a very thin oxide film is formed on the surface of the test piece, exhibiting a colored appearance called temper color. The area where the mating piece was rubbed is the area where the colored part (temper color part 12) and the part where the mating piece was burned onto (burned part 13) are combined, and the burned area ratio was calculated from these areas using the following formula: Burned area ratio (%) = (Area of ​​burned part 13) / ((Area of ​​temper color part 12) + (Area of ​​burned part 13)) × 100. The burned area ratio obtained when a burn test was performed on a test piece taken in the same manner as above from a conventional sleeve roll was used as a standard, and test pieces with a burned area ratio below the standard were judged to have good burn resistance.

[0058] A thermal shock test was conducted to evaluate crack resistance. As shown in Figure 7, two 40mm × 30mm × 20mm test specimens (thermal shock test specimens) 14 were taken from the outer surface of the sleeve roll 1, which had been machined in the same manner as described above. Note that 40mm is the circumferential direction of the sleeve roll, 30mm is the depth direction of the sleeve roll, and 20mm is the radial direction of the sleeve roll. The thermal shock test was conducted by heating the test specimen at 500°C for 30 minutes, and then immersing the test specimen in 0°C water. After the test specimen temperature had dropped to below 100°C, it was removed from the water and cut in the middle (cut from a 40mm length to a 20mm length). After that, the cut surface was mirror polished, and observed with an optical microscope at a magnification of 50x. The maximum crack depth H observed in each cut test specimen was measured, and the average value was taken as the crack depth of that test specimen. A thermal shock test was conducted in the same manner as described above using a test specimen (reference specimen) taken in the same manner as described above from a conventional sleeve roll. The crack resistance was then evaluated by using the crack depth of the reference specimen as a baseline, and calculating the ratio of the baseline crack depth to the crack depth of each specimen using the crack depth ratio (=(crack depth of baseline specimen) / (crack depth of each specimen)). In this evaluation, a crack depth ratio of 1.00 or higher was considered to indicate good crack resistance.

[0059] Based on the above evaluations of wear resistance, seizure resistance, and crack resistance, an overall evaluation was conducted. Products with outstanding wear resistance (wear ratio ≥ 1.10) and good seizure and crack resistance (seizure area ratio below the standard, crack depth below the standard) were marked with a ○, while all others were marked with a ×. Only products marked with a ○ were considered to have passed.

[0060] [Table 1]

[0061] [Table 2]

[0062] The present invention demonstrates excellent seizure resistance and crack resistance, as well as remarkably superior wear resistance.

[0063] Therefore, according to the present invention, it is possible to manufacture hot rolling roll outer layer materials and hot rolling composite rolls that have remarkably excellent wear resistance, as well as good seizure resistance and crack resistance. As a result, it is also possible to improve the lifespan of rolling rolls and increase the productivity of hot rolling. [Explanation of symbols]

[0064] 1. Sleeve roll (a sleeve roll machined for evaluation purposes) 2. Test specimens for external surface observation 3. Abrasion test specimens 4 Cooling water 5. High-frequency induction heating coil 6 Opponent's piece 7 racks 8 weight 9 pinion 10 Test specimens (seizure test specimens) 11 Opponent's piece 12 Temper Color Section 13. Seized parts 14 Test specimens (thermal shock test specimens) 15 Cracks

Claims

1. In mass percent, C: 2.50-4.00%, Si: 0.60-2.50%, Mn: 0.50-4.00%, Cr: 0.50-3.50%, Ni: 1.50 to 4.40%, V: 1.00-2.50%, Nb: 0.10–0.75%, and The composition has the following components: Mo: contains 0.30-1.50%, with the remainder being Fe and unavoidable impurities. The content of Mn and Ni satisfies the following equation (1), and the content of Si and V satisfies the following equation (2), A roll outer layer material for hot rolling, having a metallic structure with a graphite area ratio of 0.5 to 6.0%, a cementite area ratio of 30 to 55%, an MC-type carbide area ratio of 0.1 to 5.0%, and an average grain size of martensite and bainite of 2.0 to 20.0 μm. 0.20 ≤ Mn / Ni ≤ 0.80 ... (1) 0.55 ≤ Si / V ≤ 1.85 ... (2) Here, the elemental symbols in equations (1) and (2) represent the content (mass%) of the corresponding element.

2. The aforementioned component composition is further expressed in mass%, W: 2.0% or less, Co: 1.00% or less, B: 0.1000% or less, Ti: 0.30% or less, The hot rolling roll outer layer material according to claim 1, comprising one or more selected from Zr: 0.30% or less.

3. A method for manufacturing an outer layer material for hot-rolling rolls according to claim 1 or 2, A method for manufacturing an outer layer material for a hot-rolling roll, comprising: completing the adjustment of the components of the molten metal for forming the outer layer material; and starting to pour the molten metal into a mold within three minutes.

4. A composite roll for hot rolling having a three-layer structure consisting of an outer layer, an intermediate layer, and an inner layer, or a two-layer structure consisting of an outer 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 or 2.

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