Centrifugal cast composite rolls for rolling mills

Abstract

The steel sheet has an outer layer and an inner layer, and the outer layer contains, by mass%, 1.8 to 2.3% C, 0.4 to 1.5% Si, 0.1 to 1.0% Mn, 0.3 to 2.5% Ni, 4.0 to 6.0% Cr, 4.5 to 7.0% Mo, 4.0 to 6.1% V, 0.1 to 1.0% W, 0.1 to 0.5% Nb, and 0.005 to 0.07% N, with the balance being Fe. and unavoidable impurities, wherein in the composition, C-bal (C%-0.2×V%-0.06×Cr%-0.063×Mo%-0.033×W%-0.13×Nb%) is 0.28 or more, a primary crystal MC judgment value D (2.0×C%+V%+1.1×Nb%) is 10.5 or less, and a matrix hardness at a discard diameter is 700 HV or more.

Description

[Technical Field]

[0001] The present invention relates to a centrifugally cast composite roll for rolling which is excellent in wear resistance and surface roughening resistance. [Background technology]

[0002] High-speed steel roll materials widely used for the outer layer of composite rolls for rolling mills contain several percent of alloying elements such as Cr, Mo, W, and V, and have high hardness due to the crystallization or precipitation of high-hardness carbides (for example, MC carbide, which is a V-based carbide, and MC and MC carbides, which are Mo and W-based carbides), and exhibit excellent wear resistance because the reduction in matrix hardness at high temperatures is suppressed by Mo, W, etc. However, simply adding a large amount of alloying elements may cause the hard carbides to crystallize unevenly, resulting in uneven wear patterns of the roll and a risk of reducing wear resistance and surface roughening resistance.

[0003] In order to solve such problems, Patent Document 1 discloses a steel sheet containing, in mass %, C: 0.7 to 3.6%, Si: 0.2 to 2.5%, Mn: 0.2 to 2.0%, Cr: 2.0 to 10%, Mo: 0.2 to 10%, V: 2.0 to 10%, B: 0.001 to 0.50%, Al: 0.001 to 0.50%, Ti: 0.001 to 0.50%, Zr: 0.001 to 0.50%, Patent Document 1 discloses an outer layer material for a rolling composite roll containing 0.001-0.50% Cu, 0.001-0.50% Mg, and 0.001-0.50% Ca, the balance being Fe and unavoidable impurities, and further containing, by mass%, at least one of 0.1-10% Ni, 0.2-10% W, 0.2-10% Nb, and 0.2-10% Co. Patent Document 1 states that by forming oxides such as MgO and CaO in the outer layer material, hard MC carbides are made spheroidal, refined, and uniform, and that the matrix structure is strengthened by adding Cu, which enables significant improvements in surface roughening resistance and wear resistance, thereby extending the life of the rolling roll.

[0004] Patent Document 2 discloses a centrifugally cast composite roll having an inner layer made of ductile cast iron formed inside an outer layer made of high-speed steel, in which the Shore hardness of the outer layer over the entire effective diameter T (=newly produced diameter - discarded diameter) is 80 Hs or more, and the composition of the ductile cast iron in the inner layer is, by mass ratio, C: 2.5 to 4.0%, Si: 1.5 to 3.5%, Mn: 0.1 to 1.0%, P: 0.1% or less, S: 0.1% or less, Ni: 0.1 to 3.0%, Mg: 0.01 to 0.1%, Bi: 0.0005 to 0.05%, and Sn: 0.01 to 0.2%, with the balance being Fe and unavoidable impurity elements. Patent Document 2 describes that the centrifugally cast composite roll has a high Shore hardness of 80 Hs or more throughout the outer layer, and that the toughness of the inner layer (shaft core portion) is greatly improved.

[0005] Patent Document 3 discloses a steel sheet containing, in mass %, C: 2.0 to 3.0%, Si: 0.2 to 1.0%, Mn: 0.2 to 1.0%, Cr: 4.0 to 7.0%, Mo: 3.0 to 6.5%, V: 5.0 to 7.5%, Nb: 0.5 to 3.0%, Ni: 0.05 to 3.0%, Co: 0.2 to 5.0%, and W: 0.5 to 5.0%, and the contents of C, Cr, Mo, V, Nb, Ni, and W are 0.05≦(%C−%V×0 Patent Document 3 discloses a hot rolling roll outer layer material which satisfies the formula (0.177-%Nb×0.129-%Cr×0.099-%Mo×0.063-%W×0.033)+(%Ni)≦4.0, with the balance being Fe and unavoidable impurities, in which 85% or more of the base structure is a tempered martensite and / or bainite structure, and the minor axis of the tempered martensite or bainite is 0.5 to 3.0 μm. Patent Document 3 states that this outer layer material ensures wear resistance and reduces pit-like defects on the roll surface, making it possible to provide a hot rolling composite roll with excellent surface roughening resistance.

[0006] Patent Document 4 discloses an outer layer material containing, by mass, 1.50-2.70% C, 0.3-3% Si, 0.1-3% Mn, 0.1-2.5% Ni, 4.0-7.0% Cr, 4.1-8.0% Mo, 5.0-10.0% V, 0-0.4% W, 0.1-3.0% Nb, 0.005-0.15% N, and 0-0.05% B, with the balance being essentially Fe and unavoidable impurities, in which the ratio of V to Nb (V / Nb) is 1-20.0 and C-bal is 0-0.28. Patent Document 4 states that this outer layer material has satisfactory wear resistance and surface roughening resistance.

[0007] However, with recent advances in rolling technology and the trend toward higher quality and more sophisticated rolled steel sheets, the environment in which rolls are used has become increasingly severe, and further improvements in wear resistance have become desirable. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-161331 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-270991 [Patent Document 3] International Publication No. 2018 / 047444 Brochure [Patent Document 4] Japanese Patent Application Publication No. 2020-22989 Summary of the Invention [Problem to be solved by the invention]

[0009] SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a centrifugally cast composite roll for rolling which has improved wear resistance so as to withstand increasingly severe operating environments while maintaining good surface roughening resistance. [Means for solving the problem]

[0010] As a result of intensive research in view of the above object, the present inventors have found that, in the chemical composition of an outer layer material containing carbide-forming elements such as C, V, Cr, Mo, W, and Nb in predetermined proportions, by balancing the content of C with the content of carbide-forming elements such as V, Cr, Mo, W, and Nb within an optimum range, it is possible to suppress primary MC carbides that cause structural segregation during centrifugal casting, make the rolled surface uniform, and increase the matrix hardness, thereby obtaining a centrifugally cast composite roll for rolling that is excellent in both surface roughening resistance and wear resistance, and have arrived at the present invention.

[0011] That is, the centrifugally cast composite roll for rolling of the present invention comprises an outer layer and an inner layer, and the outer layer is formed from an Fe alloy having a composition containing, by mass%, C: 1.8 to 2.3%, Si: 0.4 to 1.5%, Mn: 0.1 to 1.0%, Ni: 0.3 to 2.5%, Cr: 4.0 to 6.0%, Mo: 4.5 to 7.0%, V: 4.0 to 6.1%, W: 0.1 to 1.0%, Nb: 0.1 to 0.5%, and N: 0.005 to 0.07%, with the balance being Fe and unavoidable impurities, In the composition, C-bal expressed by the formula C-bal=C%-0.2×V%-0.06×Cr%-0.063×Mo%-0.033×W%-0.13×Nb% (where C%, V%, Cr%, Mo%, W% and Nb% represent the contents (mass%) of C, V, Cr, Mo, W and Nb, respectively), is 0.28 or more, and the primary crystal MC judgment value D expressed by the formula D=2.0×C%+V%+1.1×Nb% is 10.5 or less, The matrix hardness at the discard diameter is 700 HV or more.

[0012] The outer layer preferably further contains 0.05% by mass or less of B.

[0013] The outer layer preferably further contains 0.5 mass % or less of Ti.

[0014] In a preferred embodiment of the present invention, the outer layer has a composition, in mass %, of C: 2.0-2.2%, Si: 0.5-1.4%, Mn: 0.2-0.8%, Ni: 0.4-2.0%, Cr: 4.2-5.8%, Mo: 4.8-6.5%, V: 4.5-5.9%, W: 0.2-0.8%, Nb: 0.15-0.4%, and N: 0.01-0.06%, with the remainder consisting of Fe and unavoidable impurities, wherein the C-bal is 0.28-0.55, and the primary crystal MC judgment value D is 7.7-10.3. [Effects of the Invention]

[0015] The outer layer of the centrifugally cast composite roll for rolling of the present invention suppresses primary MC carbides that cause structural segregation during centrifugal casting, and has high matrix hardness, so that it maintains surface roughening resistance while further improving wear resistance, which can contribute to further extending the life of the rolling roll and reducing the cost of rolling operations. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an example of a centrifugally cast composite roll for rolling. [Figure 2] 1 is a photomicrograph (magnification: 400 times) showing the state in which a diamond indenter is pressed against the microstructure of the outer layer of the centrifugally cast composite roll for rolling of Example 1. [Figure 3] 1 is a graph showing the correlation between the primary crystallization MC judgment value D and the difference (=Tγ−TMC) between the crystallization temperature Tγ of the outer layer matrix and the crystallization temperature TMC of the MC carbide. DETAILED DESCRIPTION OF THE INVENTION

[0017] Although the embodiments of the present invention will be described in detail below, the present invention is not limited thereto, and various modifications may be made within the scope of the technical concept of the present invention. The description of one embodiment also applies to other embodiments unless otherwise specified. When simply written as "%", it means "% by mass" unless otherwise specified.

[0018] [1] Centrifugal cast composite roll for rolling mill Fig. 1 shows a centrifugally cast composite roll 10 for rolling, which comprises an outer layer 1 formed by a centrifugal casting method and an inner layer 2 integrally welded to the outer layer 1. The inner layer 2 made of ductile cast iron has a body core 21 welded to the outer layer 1 and shaft portions 22 and 23 extending integrally from both ends of the body core 21.

[0019] The centrifugally cast composite roll 10 for rolling is used again after the surface of the outer layer 1, damaged by use in rolling, is restored to a normal surface without damage by grinding or the like. The centrifugally cast composite roll 10 for rolling is used by repeating rolling and grinding in this manner until the roll diameter changes from the initial diameter at delivery to the discard diameter. Therefore, in order to maintain rolling quality, it is extremely important that the outer layer 1 has sufficient hardness from the initial diameter to the discard diameter, so that good rolling can be performed. However, even if the initial diameter has a sufficient hardness, for example, Vickers hardness of 700 HV or more, it is difficult to obtain sufficient hardness at the deep radial portion, especially at the discard diameter, because it is difficult to ensure a sufficient cooling rate during quenching during roll manufacturing. Therefore, it is important for the centrifugally cast composite roll 10 for rolling to maintain sufficient hardness not only at the initial diameter but also at the discard diameter, thereby maintaining excellent wear resistance and surface roughening resistance and enabling good rolling.

[0020] (A) Outer layer The outer layer of the centrifugally cast composite roll for rolling of the present invention is made of an Fe alloy having a composition containing, by mass%, 1.8 to 2.3% C, 0.4 to 1.5% Si, 0.1 to 1.0% Mn, 0.3 to 2.5% Ni, 4.0 to 6.0% Cr, 4.5 to 7.0% Mo, 4.0 to 6.1% V, 0.1 to 1.0% W, 0.1 to 0.5% Nb, and 0.005 to 0.07% N, with the balance being Fe and unavoidable impurities; C-bal expressed by the formula C-bal=C%-0.2×V%-0.06×Cr%-0.063×Mo%-0.033×W%-0.13×Nb% (where C%, V%, Cr%, Mo%, W%, and Nb% represent the contents (mass%) of C, V, Cr, Mo, W, and Nb, respectively.) is 0.28 or more, The primary crystal MC judgment value D, expressed by the formula D=2.0×C%+V%+1.1×Nb%, is 10.5 or less.

[0021] The outer layer may further contain 0.05% by mass or less of B and 0.5% by mass or less of Ti. Furthermore, the outer layer may contain at least one element selected from the group consisting of 5% by mass or less of Co, 0.5% by mass or less of Zr, and 0.5% by mass or less of Al.

[0022] (1) Essential elements (a) C: 1.8~2.3% by mass Carbon combines with V, Cr, Mo, Nb, and W to form hard carbides, contributing to improved wear resistance of the outer layer. If the carbon content is less than 1.8 mass%, the amount of hard carbides crystallized is small, and the carbon content in the matrix is ​​too low to ensure sufficient matrix hardness. On the other hand, if the carbon content exceeds 2.3 mass%, the austenite phase crystallization temperature decreases, the difference with the MC carbide crystallization temperature decreases, and the primary MC carbides tend to crystallize. When the primary MC carbides crystallize, the MC carbides crystallized by centrifugal separation concentrate on the outer surface if they are heavier than the liquid phase, or on the inner surface if they are lighter, resulting in a non-uniform structure. Therefore, the carbon content is set to 1.8 to 2.3 mass% so that the MC carbides crystallize after the austenite phase crystallizes.

[0023] The lower limit of the C content is preferably 2.0 mass %, and the upper limit of the C content is preferably 2.2 mass %.

[0024] (b) Si: 0.4~1.5% by mass Si deoxidizes the molten metal to reduce oxide defects, dissolves in the matrix to improve seizure resistance, and improves the fluidity of the molten metal to prevent casting defects. If the Si content is less than 0.4% by mass, the deoxidizing effect of the molten metal is insufficient, the fluidity of the molten metal is insufficient, and the defect occurrence rate is high. On the other hand, if the Si content increases, the alloy matrix becomes embrittled and the toughness of the outer layer decreases. Therefore, the Si content is set to 1.5% by mass or less. The lower limit of the Si content is preferably 0.5% by mass, and the upper limit of the Si content is preferably 1.4% by mass.

[0025] (c) Mn: 0.1~1.0% by mass In addition to the deoxidizing effect on the molten metal, Mn also has the effect of fixing S as MnS. MnS has a lubricating effect and is effective in preventing seizure of rolled materials, so it is preferable to contain a desired amount of MnS. If the Mn content is less than 0.1% by mass, the effect of adding it is insufficient. On the other hand, if the Mn content is too high, no further effect can be obtained. Therefore, the Mn content is set to 1.0% by mass or less. The lower limit of the Mn content is preferably 0.2% by mass, more preferably 0.3% by mass. The upper limit of the Mn content is preferably 0.8% by mass.

[0026] (d) Ni: 0.3~2.5% by mass Ni has the effect of improving the hardenability of the matrix of the outer layer, so when Ni is added to a large composite roll, the generation of pearlite during cooling can be prevented and the hardness of the outer layer can be improved. The effect of adding Ni is almost negligible when it is less than 0.3% by mass, but when it exceeds 2.5% by mass, austenite becomes too stabilized, making it difficult to improve hardness. The lower limit of the Ni content is preferably 0.4% by mass, more preferably 0.5% by mass. The upper limit of the Ni content is preferably 2.0% by mass, more preferably 1.5% by mass.

[0027] (e) Cr:4.0~6.0% by mass Cr is an element that effectively maintains the hardness of the matrix by converting it to bainite or martensite, thereby maintaining the wear resistance of the outer layer. If the Cr content is less than 4.0% by mass, this effect is insufficient, and if the Cr content exceeds 6.0% by mass, the toughness of the matrix structure decreases. The lower limit of the Cr content is preferably 4.2% by mass, more preferably 4.4% by mass. The upper limit of the Cr content is preferably 5.8% by mass, more preferably 5.6% by mass.

[0028] (f) Mo: 4.5~7.0% by mass Mo combines with C to form hard carbides (MC, MC), increasing the hardness of the outer layer and improving the hardenability of the matrix. Mo also forms tough and hard MC carbides together with V or Nb, improving wear resistance. If the Mo content is less than 4.5% by mass, these effects are insufficient. On the other hand, if the Mo content exceeds 7.0% by mass, the toughness of the outer layer decreases. The lower limit of the Mo content is preferably 4.8% by mass. The upper limit of the Mo content is preferably 6.5% by mass, more preferably 6.0% by mass.

[0029] (g) V:4.0~6.1% by mass V is an element that combines with C to form hard MC carbides. MC carbides have a Vickers hardness of 2500 to 3000 HV and are the hardest of all carbides. If the V content is less than 4.0% by mass, the amount of MC carbides precipitates insufficiently, resulting in reduced wear resistance of the outer layer. On the other hand, if the V content is too high, the MC carbides, which have a lower specific gravity than the molten iron, are concentrated toward the inside of the outer layer due to centrifugal force during centrifugal casting. This not only results in significant radial segregation of the MC carbides, but also in coarsening of the MC carbides, resulting in a coarse alloy structure and increased surface roughness during rolling. On the other hand, if the V content exceeds 6.1% by mass, the crystallization temperature of the MC carbides increases, increasing the tendency for primary MC carbides to crystallize. The lower limit of the V content is preferably 4.5% by mass, more preferably 5.0% by mass. The upper limit of the V content is preferably 5.9% by mass, more preferably 5.7% by mass.

[0030] (h) W:0.1~1.0% by mass W combines with C to form hard carbides such as M6C, contributing to improved wear resistance of the outer layer. It also dissolves in MC carbides, increasing their specific gravity and reducing segregation. However, if the W content exceeds 1.0 mass%, the amount of M6C carbides increases, resulting in a heterogeneous structure and rough surface. When W is contained, the W content should be 0.1 mass% or more to achieve the above effects. The W content is preferably 0.2 mass% or more. The upper limit of the W content is preferably 0.8 mass%, more preferably 0.6 mass%.

[0031] (i) Nb: 0.1~0.5% by mass Like V, Nb also combines with C to form hard MC carbides. Adding a small amount of Nb in combination with V and Mo strengthens the MC carbides by dissolving in them, improving the wear resistance of the outer layer. Because Nb has a larger atomic weight than V, dissolving in V-based MC carbides increases the specific gravity of the V-based MC carbides, which have a lower specific gravity than the molten iron. This reduces the segregation of MC carbides due to centrifugal force during centrifugal casting. Nb content less than 0.1% by mass contributes little to the crystallization of MC carbides, failing to reduce the segregation of MC carbides. On the other hand, too much Nb increases the crystallization of MC carbides, which are primarily composed of Nb and have a higher specific gravity than the molten iron, leading to their concentration and segregation on the surface due to centrifugal force. Furthermore, Nb has a stronger effect than V in raising the crystallization temperature of MC carbides, so a Nb content greater than 0.5% by mass increases the tendency for primary MC carbides to crystallize. The lower limit of the Nb content is preferably 0.15 mass %, and the upper limit of the Nb content is preferably 0.4 mass %, more preferably 0.3 mass %, and most preferably 0.25 mass %.

[0032] (j) N:0.005~0.07% by mass N combines with V, Nb, and other trace elements to form nitrides, which act as carbide solidification nuclei and refine the carbides. These nitrides, in particular, act as effective nuclei when MC carbides solidify from the liquid phase, significantly affecting the temperature at which MC carbides form and their distribution, which contribute to wear resistance and surface roughening resistance. When the N content is less than 0.005 mass%, the nitrides that form MC carbide nuclei are insufficient, preventing the homogeneous distribution of fine MC carbides. When MC carbides are distributed in a heterogeneous state, not only does wear resistance become insufficient, but damage differentials occur between areas with few and many MC carbides, resulting in insufficient surface roughening resistance. On the other hand, when the N content exceeds 0.07 mass%, the number of MC carbide nuclei in the liquid phase increases, facilitating their crystallization. Therefore, MC carbides crystallize first as single granular crystals at a temperature higher than the temperature at which austenite crystallizes in the iron matrix. If MC carbides crystallize as a granular solid phase when austenite, which accounts for the majority by volume, is still in the liquid phase, segregation of the granular MC carbides is likely to occur before austenite crystallizes. The segregation results in the formation of hard MC carbides non-uniformly, resulting in overall deterioration of wear resistance and insufficient surface roughening resistance. The upper limit of the N content is preferably 0.06% by mass, more preferably 0.05% by mass. To obtain a sufficient carbide refinement effect, the lower limit of the N content is preferably 0.01% by mass, more preferably 0.02% by mass.

[0033] (k) C-bal: 0.28 or more C-bal, which indicates the amount of carbon in the matrix, is calculated using the following formula (1): C-bal=C%-0.2×V%-0.06×Cr%-0.063×Mo%-0.033×W%-0.13×Nb%...(1) [wherein C%, V%, Cr%, Mo%, W% and Nb% represent the contents (mass%) of C, V, Cr, Mo, W and Nb, respectively.]

[0034] C-bal is an index that indicates the balance between the content of carbide-forming elements and the content of carbon, which combines with these elements to form carbides. Higher C-bal increases the carbon content in the iron matrix, enabling improved matrix hardness. Furthermore, concerns about the crystallization of low-hardness carbides such as cementite due to higher C-bal can be alleviated by limiting the contents of carbide-forming elements such as Cr, Mo, and V within the above ranges. Therefore, optimizing the balance between the carbon content and the contents of carbide-forming elements such as Cr, Mo, and V ensures that C-bal is 0.28 or higher. This increases the hardness of the outer layer throughout the entire radial direction, thereby achieving a matrix hardness of 700 HV or higher at the discard diameter.

[0035] As long as the content of the carbide-forming elements is within the above range, the upper limit of C-bal is not limited, but is preferably 0.55, more preferably 0.50, and most preferably 0.45. The lower limit of C-bal is preferably 0.29.

[0036] (l) Primary crystal MC judgment value D: 10.5 or less The primary MC determination value D is an index showing the amount of MC carbide, and is expressed by the following formula (2): D=2.0×C%+V%+1.1×Nb% (2) It is shown as follows.

[0037] Both V and Nb are elements that form hard MC carbides. If D exceeds 10.5, primary MC carbides tend to crystallize, and the MC carbides tend to move due to centrifugal force during centrifugal casting, causing segregation. The upper limit of D is preferably 10.3. The lower limit of D is not limited as long as the contents of C, V, and Nb are within the above ranges, but is preferably 7.7, and more preferably 8.5.

[0038] (2) Arbitrary elements The outer layer may further contain at least one of 0.05% by mass or less of B and 0.5% by mass or less of Ti.

[0039] (a) B: 0.05% by mass or less B, which may be contained as an impurity in a proportion of 0.001% by mass or more, dissolves in carbides and has the effect of lowering the melting point of the carbides. This portion becomes the final solidification portion during the solidification process and, depending on the casting method, is likely to remain as a shrinkage cavity defect. This effect becomes significant when B exceeds 0.05% by mass, which may impose restrictions on roll materials and casting methods. The upper limit of the B content is more preferably 0.04% by mass, and even more preferably 0.03% by mass.

[0040] (b) Ti: 0.5% by mass or less Ti combines with C and N to form hard granular compounds such as TiC, TiN, or TiCN. These act as nuclei for MC carbides, which have the effect of uniformly dispersing the MC carbides and contribute to improving the wear resistance and surface roughness resistance of the outer layer. Therefore, for example, 0.001% by mass or more of Ti can be contained. However, if the Ti content exceeds 0.5% by mass, the viscosity of the molten metal increases, making casting defects more likely to occur. The upper limit of the Ti content is more preferably 0.3% by mass, and even more preferably 0.2% by mass.

[0041] The outer layer may further contain at least one selected from the group consisting of 5 mass % or less of Co, 0.5 mass % or less of Zr, and 0.5 mass % or less of Al.

[0042] (c) Co: 5% by mass or less Co dissolves in the matrix to increase the hot hardness of the matrix and improve the abrasion resistance and surface roughness resistance of the outer layer, so it can be contained in an amount of 0.1% by mass or more. However, excessive Co content does not further improve the above effects and increases raw material costs, so the Co content is preferably 5% by mass or less. The Co content is more preferably 3% by mass or less, even more preferably 2% by mass or less, and most preferably 0.5% by mass or less.

[0043] (d) Zr: 0.5% by mass or less Like V and Nb, Zr combines with C to form MC carbides, improving the wear resistance of the outer layer. Zr also forms oxides that act as crystal nuclei in the molten metal, refining the solidification structure. Furthermore, Zr increases the specific gravity of MC carbides, effectively preventing segregation. Therefore, Zr can be contained in an amount of 0.01% by mass or more, preferably 0.02% by mass or more. However, excessive Zr content is undesirable because it forms inclusions. Therefore, the Zr content is preferably 0.5% by mass or less, more preferably 0.3% by mass or less.

[0044] (e) Al: 0.5% by mass or less Al has a high affinity for oxygen and therefore acts as a deoxidizer. Furthermore, Al bonds with N and O, forming oxynitrides that are suspended in the molten metal and act as nuclei, causing fine MC carbides to uniformly crystallize. Therefore, the Al content is preferably 0.001% by mass or more, more preferably 0.01% by mass or more. However, excessive Al content makes the outer layer brittle, so the Al content is preferably 0.5% by mass or less, more preferably 0.2% by mass or less.

[0045] (f) Elements of Groups 2 and 3 of the Periodic Table: 0.1% by mass or less in total Elements of Groups 2 and 3 of the periodic table include Mg, Ca, Sr, Ba, La, Ce, etc. These elements have a high affinity with O (oxygen) and S (sulfur) and have deoxidizing and desulfurizing properties, which purify the molten metal and prevent the occurrence of casting defects. However, considering the O and S contents of ordinary molten metals, adding the above elements in a total amount exceeding 0.1% by mass is meaningless. The upper limit of the total content of the above elements is more preferably 0.05% by mass, and even more preferably 0.03% by mass.

[0046] (3) Inevitable impurities The remainder of the outer layer composition consists of Fe and unavoidable impurities. Of the unavoidable impurities, P and S cause deterioration of mechanical properties, so it is preferable to reduce their contents as much as possible. Specifically, the P content is preferably 0.1% by mass or less, and the S content is preferably 0.05% by mass or less. Furthermore, the O content is preferably 0.05% by mass or less. Other unavoidable impurities include Cu, Sb, Te, etc., but to ensure excellent wear resistance and surface roughening resistance of the outer layer, Cu is preferably 0.5% by mass or less, and the total of Sb and Te is preferably 0.1% by mass or less.

[0047] (4) Organization The outer layer structure contains MC carbides precipitated in the matrix and eutectic carbides formed at the matrix grain boundaries. MC carbides are granular and very hard, and have a significant effect on wear resistance. Eutectic carbides include M2C, M6C, and M7C3, where the metal M is primarily at least one of Fe, Cr, Mo, V, Nb, and W. Eutectic carbides, which act to support the matrix, improve surface roughening resistance by preventing plastic flow of the matrix due to rolling load.

[0048] The proportions of MC carbide and eutectic carbide vary depending on the composition of the outer layer, but the area ratio of MC carbide is preferably 6 to 18%, and the area ratio of eutectic carbide is preferably 2 to 10%. By containing MC carbide at an area ratio of 6 to 18%, the outer layer has excellent wear resistance. The area ratio of MC carbide is more preferably 7 to 14%. Furthermore, the area ratio of eutectic carbide is more preferably 3% or more, and even more preferably 3.5% or more. It is preferable that graphite is not present in the outer layer structure.

[0049] (B) Inner layer (1) Composition The inner layer of the centrifugally cast composite roll for rolling of the present invention preferably has a graphite cast iron composition, for example, containing, by mass, 2.4 to 3.6% C, 1.5 to 3.5% Si, 0.1 to 2% Mn, 0.1 to 2% Ni, less than 0.7% Cr, less than 0.5% Mo, less than 1% V, and 0.01 to 0.1% Mg, with the balance being substantially Fe and unavoidable impurities.

[0050] Of the above-mentioned unavoidable impurities, P, S, and N cause deterioration of mechanical properties, so it is preferable to reduce their contents as much as possible. Specifically, the P content is preferably 0.1% by mass or less, the S content is preferably 0.05% by mass or less, and the N content is preferably 0.07% by mass or less. Furthermore, B inhibits graphitization of the inner layer, so it is preferable that its content be less than 0.05% by mass. Other unavoidable impurities include, particularly when the outer layer contains elements such as Zr, Co, Ti, and Al, as well as elements such as Ca, Ba, Cu, Sb, Te, Ce, and rare earth metal elements. The total content of these elements is preferably 0.7% by mass or less.

[0051] (2) Organization The inner layer 2 of the centrifugally cast composite roll for rolling of the present invention is preferably made of graphite cast iron in which graphite is crystallized. Graphite cast iron is softer and has greater deformability than white cast iron, which does not contain graphite. Graphite cast iron is classified according to the shape of the graphite, such as spheroidal, flake, and block. In particular, spheroidal graphite cast iron in which spheroidal graphite is crystallized is more preferable as a roll inner layer material because of its high toughness.

[0052] The area ratio of graphite in graphite cast iron is preferably 2 to 12%. If the area ratio of graphite is less than 2%, the cementite content is high, resulting in insufficient elongation of the material, making it unable to withstand the thermal and mechanical loads during rolling and increasing the risk of roll breakage. On the other hand, since the upper limit of the C content in the inner layer is 3.6 mass%, the upper limit of the area ratio of graphite is preferably 12%.

[0053] (C) Middle layer Although the centrifugally cast composite roll for rolling comprising an outer layer 1 and an inner layer 2 has been described with reference to Fig. 1, an intermediate layer may be provided as a buffer layer between the outer layer 1 and the inner layer 2. The intermediate layer preferably has an intermediate composition between the outer layer 1 and the inner layer 2, and its thickness is preferably 10 to 30 mm.

[0054] (D) Roll size The size of the centrifugally cast composite roll for rolling of the present invention is not particularly limited, but preferred examples are an outer diameter of the outer layer of 200 to 1300 mm, a roll barrel length of 500 to 6000 mm, and a thickness of the outer layer used for rolling [(initial diameter - discarded diameter) / 2] of 25 to 200 mm.

[0055] [2] Manufacturing method of centrifugally cast composite rolls for rolling Centrifugal cast composite rolls are integrally cast by (1) forming the outer layer by centrifugal casting, (2) stopping the rotation of the centrifugal casting machine after the outer layer has solidified, and (3) pouring molten ductile cast iron (spheroidal graphite cast iron) for the inner layer into the inner diameter of the outer layer by static casting.

[0056] When an intermediate layer is provided, the molten metal for the outer layer is poured by centrifugal casting, and then the molten metal for the intermediate layer is poured by centrifugal casting while the inner surface of the outer layer is solidifying or after solidification. After the intermediate layer has solidified, the rotation of the centrifugal casting machine is stopped, and the molten metal of ductile cast iron for the inner layer is poured into the inner diameter portion of the intermediate layer by static casting.

[0057] After casting, the casting surface of the body is removed, and then the body is subjected to heat treatments of quenching and tempering to achieve the desired hardness.

[0058] The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

[0059] Examples 1 to 5 and Comparative Examples 1 to 8 (1) Fabrication of centrifugal cast composite rolls for rolling The outer layer was centrifugal cast using each molten metal having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities), and then the inner layer was integrally formed by static casting of graphite cast iron (ductile cast iron) containing, by mass %, approximately 3.4% C, 2.4% Si, 0.5% Mn, 0.5% Ni, 0.1% Cr, 0.03% Mo, 0.03% V, and 0.05% Mg, with the balance being essentially Fe and unavoidable impurities, at the inner diameter portion of the outer layer, to produce a centrifugally cast composite roll.

[0060] To evaluate the uniformity of the MC carbides in the structure of each outer layer, the crystallization temperature T γ and the crystallization temperature of MC carbide, T MC was measured by differential thermal analysis (DTA), and the austenite phase crystallization temperature T γ and the crystallization temperature T of MC carbide MC The difference between these was found.

[0061] Each of the composite rolls obtained was quenched by heating to 1060°C and then rapidly cooled, and then tempered three times at 500-550°C. After that, cutting was performed to obtain composite rolls with an outer diameter of 760 mm, an outer layer thickness of 50 mm, and a body length of 2000 mm (Fig. 1).

[0062] [Table 1-1]

[0063] [Table 1-2] Note: (1) The balance in each chemical composition is Fe and unavoidable impurities, of which P≦0.1 mass%, S≦0.05 mass%, Cu≦0.5 mass%, and (Sb+Te)≦0.1 mass%. (2) The units are ppm. (3) D represents the primary crystal MC judgment value. (4) "-" indicates that B and Ti are each less than 0.001 mass %, O is 50 ppm or less, and N is 250 ppm or less.

[0064] (2) Measurement of the area ratio of MC carbides and eutectic carbides The microstructure of the outer layer of each centrifugally cast composite roll for rolling at the initial diameter was observed under a microscope (magnification 400x), and the area ratios of MC carbides and eutectic carbides at the matrix grain boundaries were measured. First, the mirror-polished surface was etched with Murakami's reagent to determine the area ratio of region A, which was then revealed. This was taken as the area ratio of eutectic carbides. Next, the surface etched with Murakami's reagent was further etched by electrolytic corrosion to determine the area ratio of region B, and the value obtained by subtracting the area ratio of region A from the area ratio of region B was taken as the area ratio of MC carbides. If the region for which the area ratio of each carbide was determined contained a defective region other than carbides, such as scratches after mirror polishing, this defective region was excluded. The results are shown in Table 2.

[0065] [Table 2]

[0066] As is clear from Table 2, in Examples 1 to 5, the area ratio of MC carbides was in the range of 6 to 18%, and the area ratio of eutectic carbides at the matrix grain boundaries was in the range of 2 to 10%. In contrast, the comparative examples also had the above-mentioned ranges of carbide area ratios, but showed lower values ​​for the area ratio of eutectic carbides.

[0067] (3) Measurement of matrix hardness of the outer layer In order to investigate the distribution of matrix hardness in the thickness range from the initial diameter (outer diameter: 760 mm) before use to the discard diameter (outer diameter: 660 mm) at the end of use for the outer layer (thickness: 50 mm) of each centrifugally cast composite roll for rolling, the matrix hardness of the outer layer was measured at depths of 5 mm, 10 mm, 20 mm, 30 mm, 40 mm, and 50 mm from the surface of the initial diameter by the following procedure. The 50 mm depth position corresponds to the discard diameter.

[0068] First, the measurement surface at each depth position of the outer layer was corroded with picric acid to make it easier to distinguish between the matrix and the crystallized carbides. Then, while observing the microstructure of the measurement surface under a microscope (magnification 400x), the diamond indenter of a micro Vickers hardness tester (DMH-2, manufactured by Matsuzawa Seiki) was pressed against the surface so as to make an indentation in the region of the matrix only (approximately 40 μm or more in equivalent circle diameter) without any crystallized carbides. The matrix hardness was measured using a measurement load of 200 gf and a loading time of 20 seconds. Figure 2 shows a micrograph (magnification 400x) of the outer layer surface of Example 1 (5 mm deep from the initial diameter) after the Vickers hardness measurement. In Figure 2, the crystallized carbides 120 are present as amorphous islands with low contrast, and the indentation 100 is located within the matrix 110. In the microstructure of the outer layer surface shown in Figure 2, the area fraction of MC carbides was 9.8%, and the area fraction of eutectic carbides at the matrix grain boundaries was 5.1%.

[0069] (4) Measurement of the crystallization temperature of the austenite phase and MC carbides Crystallization temperature T of the austenite phase in the outer layer of each centrifugally cast composite roll for rolling mills γ and the crystallization temperature of MC carbide, T MC was measured by differential thermal analysis (DTA).

[0070] The matrix hardness at each depth of the outer layer of each centrifugally cast composite roll for rolling was measured at the austenite phase crystallization temperature T γ and the crystallization temperature of MC carbide, T MC The results are shown in Table 3.

[0071] [Table 3] Notes: (1) Discard diameter. (2) T γ represents the crystallization temperature of the austenite phase. (3) T MC represents the crystallization temperature of MC carbide. (4) No measurements.

[0072] As can be seen from Table 3, the matrix hardness of the outer layer of Examples 1 to 5 was not only high but also had a small radial distribution, and maintained 700 HV or more even when the roll reached the discard diameter. γ is the crystallization temperature of MC carbide, T MC Since the temperature was 9°C or more higher than the above, and particularly 13°C or more higher in Examples 1 and 3 to 5, the MC carbides crystallized after the austenite phase had fully crystallized during the solidification process during centrifugal casting, which suppressed the formation of primary MC carbides and resulted in a structure in which the MC carbides were uniformly distributed.When the composite rolls of Examples 1 to 5 were used in the hot rolling of steel sheets, it was confirmed that they were superior in wear resistance and surface roughening resistance to conventional high-speed steel rolls.

[0073] In contrast, the centrifugally cast composite rolls for rolling of Comparative Examples 1 to 6 had low matrix hardness in the outer layer, and the matrix hardness at the time of reaching the discard diameter was less than 700 HV. Therefore, the composite rolls of Comparative Examples 1 to 6 had lower wear resistance than the composite rolls of Examples 1 to 5.

[0074] The composite rolls of Comparative Examples 7 and 8 had sufficient wear resistance, but their surface roughening resistance was poor compared to the composite rolls of Examples 1 to 5. This is because the primary crystal MC judgment value D exceeded 10.5 and T γ -T MC is a negative value, it is considered that primary MC carbides were crystallized and structural segregation occurred.

[0075] FIG. 3 shows the relationship between the primary crystal MC judgment value D and T in the composite rolls of Examples 1 to 5 and Comparative Examples 1 to 8. γ -T MC The correlation with the primary crystal MC judgment value D is shown. γ -T MC When the primary MC determination value D exceeds 10.5, the T γ -T MC It can be seen that the value becomes negative, and the tendency for primary MC to crystallize becomes stronger.

Claims

1. A centrifugally cast composite roll for rolling having an outer layer and an inner layer, the outer layer is made of an Fe alloy having a composition containing, in mass%, 1.8 to 2.3% C, 0.4 to 1.5% Si, 0.1 to 1.0% Mn, 0.3 to 2.5% Ni, 4.0 to 6.0% Cr, 4.5 to 7.0% Mo, 4.0 to 6.1% V, 0.1 to 1.0% W, 0.1 to 0.5% Nb, and 0.005 to 0.07% N, with the balance being Fe and unavoidable impurities; In the composition, C-bal expressed by the formula C-bal = C% - 0.2 x V% - 0.06 x Cr% - 0.063 x Mo% - 0.033 x W% - 0.13 x Nb% (where C%, V%, Cr%, Mo%, W% and Nb% represent the contents (mass%) of C, V, Cr, Mo, W and Nb, respectively) is 0.28 or more, and the primary crystal MC judgment value D expressed by the formula D = 2.0 x C% + V% + 1.1 x Nb% is 10.5 or less, A centrifugally cast composite roll for rolling, characterized in that the matrix hardness at the discard diameter is 700 HV or more.

2. 2. The centrifugally cast composite roll for rolling according to claim 1, wherein the outer layer further contains 0.05% by mass or less of B.

3. 2. The centrifugally cast composite roll for rolling according to claim 1, wherein the outer layer further contains 0.5% by mass or less of Ti.

4. 2. The centrifugally cast composite roll for rolling according to claim 1, characterized in that the composition of the outer layer contains, in mass%, C: 2.0 to 2.2%, Si: 0.5 to 1.4%, Mn: 0.2 to 0.8%, Ni: 0.4 to 2.0%, Cr: 4.2 to 5.8%, Mo: 4.8 to 6.5%, V: 4.5 to 5.9%, W: 0.2 to 0.8%, Nb: 0.15 to 0.4%, and N: 0.01 to 0.06%, with the balance being Fe and unavoidable impurities, the C-bal is 0.28 to 0.55, and the primary crystal MC judgment value D is 7.7 to 10.3.

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