Cold rolling rolls

Abstract

To provide a cold rolling roller that is obtained by discharge dull work, has improved roughness maintenance properties, and also has improved surface roughening resistance, cracking resistance, and grindability of roll base material.SOLUTION: A cold rolling roller comprises a roll base material and a melted and resolidified layer formed on the surface of the roll base material. The roll base material has a predetermined chemical composition satisfying 3.0≤Mo+W+V+Nb≤17.5. The melted and resolidified layer contains 0.10 to 3.00% of Cu by mass.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a roll for cold rolling. [Background technology]

[0002] In a cold tandem rolling mill consisting of multiple rolling stands, for example, the first rolling stand may use work rolls whose surfaces have been dulled (matt-finished) to improve biting, and the final rolling stand may also use work rolls whose surfaces have been similarly dulled and roughened to prevent coil adhesion during annealing after cold rolling. Also, in temper rolling mills, dulled work rolls may be used to improve the paintability of products (adhesion and clarity of paint film) and to improve the drawability of products by making it easier to retain lubricating oil.

[0003] Electrical discharge texturing is one of the known texturing methods. In this method, a work roll is immersed in an insulating working fluid, a high voltage is applied between the work roll and an opposing electrode (typically composed of copper), and the roll surface is roughened by electrical discharge pulses. The roll surface and electrode are first heated and melted by electrical discharge energy. Simultaneously, the surrounding working fluid rapidly vaporizes and expands, exploding the molten metal. Some of the blown-off metal re-deposits on the roll surface and electrode. Immediately afterward, the surrounding cold working fluid invades, rapidly removing any remaining heat and cooling the roll. This completes one electrical discharge machining cycle, leaving discharge craters on the roll surface. Repeated electrical discharge machining processes result in the accumulation of countless discharge craters on the roll surface, resulting in a dull, roughened roll surface.

[0004] Dulled work rolls are installed in cold rolling mills for use, but as the roll surface roughness gradually decreases with use, strip threading problems such as rolling slippage become more likely to occur. Therefore, it becomes necessary to replace the work rolls periodically or as the roughness decreases. Therefore, work rolls that have been dulled by electrical discharge dulling or the like are required to maintain their roughened surface for a long period of time, i.e., to maintain a high level of roughness.

[0005] In this regard, Patent Document 1 describes a dulling method for a rolling roll in which a voltage is periodically applied between the roll surface and an electrode via an insulator to form predetermined discharge marks on the roll surface, and then, following discharge, sub-zero treatment is performed on at least the discharged portion to form a dull surface with high wear resistance and a long life. Patent Document 1 also teaches that the hardness reduced due to retained austenite, etc., generated in large amounts directly below the dull surface during discharge, can be restored by transforming the retained austenite into martensite through sub-zero treatment, thereby preventing wear of the dull surface and improving its life.

[0006] Patent Document 2 describes a method for treating electric discharge machined metal rolling rolls, which involves using a pair of rolling rolls whose surfaces have been matte-finished by electric discharge machining, and installing the pair of rolling rolls in a mill stand, and running the pair of rolling rolls idly under a predetermined pressure without charging a material to be rolled between them, thereby hardening the surface layers of the rolls. Patent Document 2 also teaches that running the pair of rolling rolls idly causes austenite on the roll surfaces to transform into martensitic, and the work hardening effect makes it possible to harden the roll surfaces to a level that prevents the roll surface layers from falling off, and as a result, the matte-finished rolls can maintain their maximum roughness for a long period of time. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 51-045614 [Patent Document 2] Japanese Patent Application Publication No. 53-026233 Summary of the Invention [Problem to be solved by the invention]

[0008] In addition to the requirement for high roughness maintenance as described above, there is a continuing need for improved properties in the dulled work roll, such as suppression of surface roughness due to corrosion and wear, crack resistance, and excellent grindability, and there is still room for improvement in these respects even in the rolls for rolling according to the prior art.

[0009] Therefore, an object of the present invention is to provide a cold rolling roll obtained by electrical discharge texturing, which has improved roughness maintenance properties, as well as improved surface roughening resistance, crack resistance, and grindability of the roll base material. [Means for solving the problem]

[0010] In order to achieve the above object, the present inventors have conducted investigations focusing on the chemical composition of a roll base material and the components in a molten resolidified layer formed on the surface of the roll base material by electrical discharge texturing. As a result, the present inventors have found that by making the roll base material contain a relatively large amount of specific alloy elements, it is possible to significantly improve the roughness maintenance while maintaining the grindability of the roll base material at a good level, and on the other hand, by controlling the content of Cu derived from the electrode and taken into the molten resolidified layer by electrical discharge texturing within an appropriate range, it is possible to improve both the surface roughening resistance and the crack resistance, and have completed the present invention.

[0011] The present invention, which has achieved the above object, is as follows. (1) A roll base material and a melted and resolidified layer formed on the surface of the roll base material, The roll base material is, in mass%, C: 0.70~2.50%, Si: 0.2 to 2.0% Mn: 0.2 to 1.5%, P: 0.030% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.0200% or less, O: 0.0050% or less, Cr: 2.8 to 11.0%, Mo: 2.0-6.0% V: 1.0-6.0%, Cu: less than 0.10% B: 0.0100% or less, Ni: 0 to 2.0% W: 0-6.0%, Nb: 0 to 2.0%, and The balance is composed of Fe and impurities. It has a chemical composition that satisfies the following formula 1: 3.0≦Mo+W+V+Nb≦17.5...Equation 1 A roll for cold rolling, wherein the melted and resolidified layer contains, by mass %, Cu: 0.10 to 3.00%. Here, the content (mass %) of each element in the roll base material is substituted for each element symbol in the formula 1, and 0 is substituted when the element is not contained. (2) The chemical composition is in mass%: Ni: 0.01 to 2.0% W: 0.01 to 6.0%, and Nb: 0.01 to 2.0% The cold rolling roll according to (1) above, comprising: (3) The roll for cold rolling according to (1) or (2) above, wherein the melted and resolidified layer has a Vickers hardness of 650 Hv or more. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a cold rolling roll obtained by electrical discharge texturing, which has improved roughness maintenance properties, as well as improved surface roughening resistance, crack resistance, and grindability of the roll base material. [Brief explanation of the drawings]

[0013] [Figure 1] 1 shows a cross-sectional schematic diagram of the roll surface of a work roll obtained by electrical discharge texturing. [Figure 2] 1 is a schematic diagram showing a rolling wear test using a two-cylinder rolling wear tester for roll test pieces of Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0014] <Cold rolling rolls> A cold rolling roll according to an embodiment of the present invention comprises a roll base material and a molten and resolidified layer formed on a surface of the roll base material, The roll base material is, in mass%, C: 0.70~2.50%, Si: 0.2 to 2.0% Mn: 0.2 to 1.5%, P: 0.030% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.0200% or less, O: 0.0050% or less, Cr: 2.8 to 11.0%, Mo: 2.0-6.0% V: 1.0-6.0%, Cu: less than 0.10% B: 0.0100% or less, Ni: 0 to 2.0% W: 0-6.0%, Nb: 0 to 2.0%, and The balance is composed of Fe and impurities. It has a chemical composition that satisfies the following formula 1: 3.0≦Mo+W+V+Nb≦17.5...Equation 1 The melted and resolidified layer is characterized by containing, by mass %, Cu: 0.10 to 3.00%. Here, the content (mass %) of each element in the roll base material is substituted for each element symbol in the formula 1, and 0 is substituted when the element is not contained.

[0015] FIG. 1 shows a cross-sectional schematic diagram of the roll surface of a work roll obtained by electrical discharge texturing. Referring to FIG. 1, when a work roll 1, such as a cold rolling roll, is manufactured by electrical discharge texturing, a molten resolidified layer 3 is formed on the surface of the roll base material 2, and this molten resolidified layer 3 constitutes a rough surface portion 4. As mentioned above, a work roll that has been dulled by electrical discharge texturing or the like is required to maintain its rough surface for a long period of time, i.e., to maintain a high level of roughness. However, when the roll surface layer is roughened by electrical discharge texturing, it is generally known that a molten resolidified layer with a lower hardness than the base material is formed on the outermost layer. More specifically, for example, a roll containing alloying elements generally has a two-phase structure including a matrix and carbides formed by alloying elements that are not completely dissolved in the matrix. In contrast, in electrical discharge texturing, the surface layer of a roll having such a structure is instantaneously melted, then cooled and instantaneously solidified. As a result, the carbides that existed separately in the two-phase structure are also instantaneously melted and instantaneously solidified. Therefore, in the molten resolidified layer formed on the outermost surface of the roll, the original two-phase structure is transformed into a single-phase or a structure closer to a single phase. Therefore, it is believed that the alloying element concentration in the molten resolidified layer is relatively higher than that in the original matrix. In addition, in EDM, C from the machining fluid is incorporated into the molten resolidified layer during the melting and solidification processes. Therefore, when the roll surface is roughened by EDM, C and alloying elements are concentrated in the molten resolidified layer formed on the outermost surface of the roll. It has been generally assumed that such concentrated C and alloying elements contribute to a decrease in the Ms point (martensitic transformation temperature) of the roll during the solidification process during EDM. This decrease in the Ms point results in a relatively large amount of austenite remaining, which cannot transform to martensite, i.e., soft retained austenite is more likely to form, resulting in a decrease in the hardness of the final molten resolidified layer. Therefore, if this decrease in hardness of the molten resolidified layer can be suppressed, it is believed that it will be possible to improve the roughness retention of EDM-machined work rolls.

[0016] On the other hand, work rolls are also required to have excellent grindability, but in rolls containing alloying elements, the grindability of the roll base material may be reduced due to the formation of carbides as described above. Also, increasing the hardness to improve the roughness maintenance may result in a reduction in grindability, so it is generally difficult to achieve both the roughness maintenance of the work roll and the grindability of the roll base material.

[0017] Therefore, the present inventors conducted a study focusing on the chemical composition of the roll base material in order to suppress a decrease in the hardness of the molten resolidified layer and improve the roughness maintenance of the work roll while maintaining the grindability of the roll base material at a good level. First, the present inventors investigated the relationship between the components of the roll base material and the hardness and roughness maintenance of the molten resolidified layer. In the case of electrical discharge texturing, the molten resolidified layer formed contains C derived from the machining fluid and Cu derived from the electrode, but the other components are generally equivalent to the components of the roll base material. Therefore, when the roll base material contains alloying elements, the molten resolidified layer also contains alloying elements. However, as mentioned above, it is believed that these alloying elements are incorporated into the molten resolidified layer in a single-phase or nearly single-phase structure without forming carbides. Although it has been believed that these alloying elements contribute to a decrease in the hardness of the molten resolidified layer, the present inventors have found through their recent investigations that when a roll base material contains a relatively large amount of specific alloying elements, more specifically, Mo, W, V, and / or Nb, i.e., when the total content of Mo, W, V, and Nb is set to 3.0% by mass or more in addition to the respective appropriate amounts of Mo, W, V, and Nb, the decrease in hardness of the molten resolidified layer can be suppressed and the roughness retention can be significantly improved. Furthermore, the present inventors have found that by limiting the contents of these alloying elements within appropriate ranges, i.e., by limiting the respective contents of Mo, W, V, and Nb within appropriate ranges and the total content of Mo, W, V, and Nb to 17.5% by mass or less, the formation of coarse carbides can be suppressed, and in connection with this, the grindability of the roll base material can also be maintained at a good level.

[0018] Without intending to be bound by any particular theory, it is believed that the solid solution strengthening and amorphization of the structure by the alloying elements Mo, W, V, and Nb significantly suppress the decrease in hardness of the molten resolidified layer. More specifically, in electrical discharge dulling, the alloying elements Mo, W, V, and Nb are rapidly cooled during melting and resolidification, so most of them are incorporated in a solid solution state into the matrix of the molten resolidified layer without forming carbides. Previously, it was believed that these alloying elements reduced the hardness of the molten resolidified layer by lowering the Ms point of the roll and facilitating the formation of soft retained austenite. However, by incorporating these alloying elements in a sufficiently higher amount than in the prior art, it is believed that specific solid solution strengthening is achieved, thereby significantly suppressing the decrease in hardness of the molten resolidified layer, even considering the negative effects of the lowered Ms point. In addition, the alloying elements Mo, W, V, and Nb have larger atomic radii than Fe, and the atomic radii themselves are different. It is believed that rapid solidification of multiple elements with different atomic radii makes the resulting structure more likely to become amorphous, and that the hardness of the molten resolidified layer can be increased due to the amorphization of the structure.

[0019] Next, in the case of surface roughening by electrical discharge texturing, unlike surface roughening by laser texturing or the like, Cu derived from the electrode is concentrated in the resulting molten resolidified layer. Generally, work rolls may gradually develop surface roughening or cracks may occur on the roll surface due to corrosion, wear, and the like during use. Since Cu has the effect of improving corrosion resistance, the present inventors also investigated the influence of electrode-derived Cu contained in the molten resolidified layer by electrical discharge texturing on roll characteristics. As a result, the present inventors found that by appropriately controlling the Cu content in the molten resolidified layer within a predetermined range, more specifically, within a range of 0.10 to 3.00 mass% by changing the discharge conditions during electrical discharge texturing, it is possible to significantly improve the surface roughening resistance of the roll surface without generating cracks or while sufficiently suppressing the generation of cracks.

[0020] In laser texturing, Cu from the electrode is not incorporated into the molten resolidified layer, and in the prior art, alloying elements such as Mo, W, V, and Nb are not contained, or even if they are contained, they are contained in relatively small amounts, taking into consideration the negative effects due to a decrease in the Ms point. Therefore, by appropriately controlling the Cu content in the molten resolidified layer based on electrical discharge texturing and by incorporating relatively large amounts of alloying elements such as Mo, W, V, and Nb into the roll base material within a predetermined range, it is possible to improve the roughness maintenance, surface roughening resistance, and crack resistance of the obtained cold rolling roll, while also improving the grindability of the roll base material. This fact was not previously known, and has now been discovered for the first time by the present inventors. A cold rolling roll according to an embodiment of the present invention will be described in more detail below.

[0021] [Chemical composition of roll base material] First, the chemical composition of the roll base material will be described. In the following description, the unit of content of each element, "%", means "mass %" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits, unless otherwise specified.

[0022] [C:0.70~2.50%] Carbon (C) is an element necessary for increasing the hardness of the roll surface layer. To fully obtain this effect, the C content is set to 0.70% or more. The C content may be 0.80% or more, 0.90% or more, 1.00% or more, 1.10% or more, or 1.20% or more. On the other hand, if C is contained excessively, coarse carbides may be formed, and the above effect may not be fully obtained. Therefore, the C content is set to 2.50% or less. The C content may be 2.40% or less, 2.30% or less, 2.20% or less, 2.00% or less, or 1.80% or less.

[0023] [Si: 0.2-2.0%] Silicon (Si) is an element that generally deoxidizes steel and improves its hardenability. To fully obtain these effects, the Si content is set to 0.2% or more. The Si content may be 0.3% or more, 0.4% or more, 0.5% or more, 0.6% or more, or 0.7% or more. On the other hand, excessive Si content may cause carbide segregation, resulting in insufficient toughness. Therefore, the Si content is set to 2.0% or less. The Si content may be 1.8% or less, 1.6% or less, 1.4% or less, 1.2% or less, or 1.0% or less.

[0024] [Mn: 0.2-1.5%] Manganese (Mn) is an element that effectively improves hardenability. To fully obtain this effect, the Mn content is set to 0.2% or more. The Mn content may be 0.3% or more, 0.4% or more, 0.5% or more, 0.6% or more, or 0.7% or more. On the other hand, if Mn is contained in an excessive amount, sufficient toughness may not be obtained. Therefore, to effectively improve hardenability and ensure sufficient toughness, the Mn content is set to 1.5% or less. The Mn content may be 1.4% or less, 1.3% or less, 1.2% or less, 1.1% or less, or 1.0% or less.

[0025] [P:0.030% or less] Phosphorus (P) is an unavoidable impurity. That is, the P content is greater than 0%. P may segregate at grain boundaries, reducing the toughness of the steel. Therefore, the P content is set to 0.030% or less. The P content may also be 0.025% or less, 0.022% or less, or 0.020% or less. The P content is preferably as low as possible. However, excessive reduction of the P content significantly increases the refining costs in the steelmaking process. Therefore, considering industrial production, the P content is preferably set to 0.001% or more. The P content may also be 0.002% or more.

[0026] [S:0.020% or less] Sulfur (S) is an unavoidable impurity. That is, the S content is greater than 0%. S may segregate at grain boundaries, reducing the toughness and hot workability of the steel. Therefore, the S content is set to 0.020% or less. The S content may be 0.005% or less, 0.004% or less, or 0.003% or less. The S content is preferably as low as possible. However, excessive reduction of the S content significantly increases the refining costs in the steelmaking process. Therefore, considering industrial production, the S content is preferably set to 0.0001% or more. The S content may be 0.0002% or more.

[0027] [Al:0.050% or less] Aluminum (Al) is an unavoidably contained impurity. That is, the Al content is greater than 0%. Al deoxidizes steel during the molten steel stage. On the other hand, excessive Al content may cause Al nitrides to coarsen, reducing the toughness of the steel. Therefore, the Al content is set to 0.050% or less. The Al content may be 0.040% or less or 0.030% or less. The Al content may be 0.001% or more or 0.002% or more. In this specification, the Al content means the total Al content in the steel.

[0028] [N:0.0200% or less] Nitrogen (N) is an unavoidable impurity. That is, the N content is greater than 0%. N increases the strength of steel through solid solution strengthening. On the other hand, excessive N content may form coarse nitride inclusions, reducing the toughness of the steel. Therefore, the N content is set to 0.0200% or less. The N content may be 0.0150% or less or 0.0100% or less. The N content may be 0.0001% or more or 0.0002% or more.

[0029] [O:0.0050% or less] Oxygen (O) is an unavoidable impurity. That is, the O content is greater than 0%. O forms coarse oxide-based inclusions, which may reduce the toughness of the steel material. Therefore, the O content is set to 0.0050% or less. The O content may be 0.0040% or less, 0.0035% or less, or 0.0030% or less. The O content is preferably as low as possible. However, an extreme reduction in the O content significantly increases the manufacturing cost. Therefore, considering industrial production, the O content is preferably 0.0001% or more or 0.0005% or more. The O content may be 0.0010% or more.

[0030] [Cr: 2.8~11.0%] Chromium (Cr) is an element that improves hardenability and increases the hardness of the roll base material. To fully obtain this effect, the Cr content is set to 2.8% or more. The Cr content may be 3.0% or more, 3.5% or more, 4.0% or more, 4.5% or more, or 5.0% or more. On the other hand, excessive Cr content may cause coarsening of carbides, which may reduce the grindability of the roll base material and the toughness of the cold rolling roll. Therefore, the Cr content is set to 11.0% or less. The Cr content may be 10.0% or less, 9.0% or less, 8.0% or less, 7.0% or less, or 6.0% or less.

[0031] [Mo: 2.0-6.0%] Molybdenum (Mo) is an element that can improve the hardness of the melted and resolidified layer when contained in a relatively large amount. To fully obtain this effect, the Mo content is set to 2.0% or more. The Mo content may be 2.2% or more, 2.5% or more, 2.8% or more, 3.0% or more, or 3.5% or more. On the other hand, excessive Mo content may cause coarsening of carbides, which may reduce the grindability of the roll base material and the toughness of the cold rolling roll. Therefore, the Mo content is set to 6.0% or less. The Mo content may be 5.8% or less, 5.5% or less, 5.0% or less, 4.5% or less, or 4.0% or less.

[0032] [V:1.0~6.0%] Like Mo, vanadium (V) is an element that can improve the hardness of the melted and resolidified layer when contained in a relatively large amount. To fully obtain this effect, the V content is set to 1.0% or more. The V content may be 1.2% or more, 1.5% or more, 1.8% or more, 2.0% or more, or 2.5% or more. On the other hand, excessive V content may cause coarsening of carbides, which may reduce the grindability of the roll base material and the toughness of the cold rolling roll. Therefore, the V content is set to 6.0% or less. The V content may be 5.5% or less, 5.0% or less, 4.5% or less, 4.0% or less, or 3.5% or less.

[0033] [Cu: less than 0.10%] Copper (Cu) is an unavoidable impurity. That is, the Cu content is greater than 0%. Cu may reduce the hot workability of steel. Therefore, the Cu content is less than 0.10%. The Cu content may be 0.09% or less, 0.08% or less, 0.07% or less, 0.06% or less, or 0.05% or less. The Cu content is preferably as low as possible. However, excessive reduction of the Cu content increases manufacturing costs. Therefore, the Cu content is preferably 0.001% or more. The Cu content may be 0.002% or more.

[0034] [B:0.0100% or less] B (boron) is an unavoidably contained impurity. That is, the B content is greater than 0%. B may reduce the toughness of steel. Therefore, the B content is set to 0.0100% or less. The B content may be 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0020% or less, or 0.0010% or less. The B content is preferably as low as possible. However, excessive reduction of the B content increases production costs. Therefore, the B content is preferably set to 0.0001% or more. The B content may be 0.0002% or more.

[0035] The basic chemical composition of the roll base material according to the embodiment of the present invention is as described above. Furthermore, the roll base material may contain one or more of the following elements as necessary.

[0036] [Ni: 0-2.0%] Nickel (Ni) is an element that improves hardenability. The Ni content may be 0%, but to fully obtain this effect, the Ni content is preferably 0.01% or more. The Ni content may be 0.1% or more, 0.2% or more, 0.3% or more, or 0.4% or more. On the other hand, if Ni is contained excessively, retained austenite may be excessively formed, making it impossible to maintain sufficient hardness. Therefore, the Ni content is preferably 2.0% or less. The Ni content may be 1.5% or less, 1.2% or less, 1.0% or less, 0.8% or less, or 0.6% or less.

[0037] [W:0~6.0%] Tungsten (W), like Mo and V, is an element that can improve the hardness of the melted and resolidified layer. Therefore, W can be added in addition to Mo and V depending on the manufacturing convenience of the roll base material. To ensure the effect of adding W, the W content is preferably 0.01% or more or 0.1% or more. The W content may be 0.3% or more, 0.5% or more, 0.8% or more, 1.0% or more, or 1.5% or more. On the other hand, excessive addition of W may cause coarsening of carbides, which may reduce the grindability of the roll base material and the toughness of the cold rolling roll. Therefore, the W content is preferably 6.0% or less. The W content may be 5.0% or less, 4.0% or less, 3.0% or less, 2.5% or less, or 2.0% or less.

[0038] [Nb: 0-2.0%] Niobium (Nb), like Mo and V, is an element that can improve the hardness of the melted and resolidified layer. Therefore, Nb can be added in addition to Mo and V depending on the manufacturing convenience of the roll base material. To ensure the effect of adding Nb, the Nb content is preferably 0.01% or more or 0.1% or more. The Nb content may be 0.2% or more, 0.3% or more, 0.5% or more, 0.8% or more, or 1.0% or more. On the other hand, excessive addition of Nb may cause coarsening of carbides, which may reduce the grindability of the roll base material and the toughness of the cold rolling roll. Therefore, the Nb content is preferably 2.0% or less. The Nb content may be 1.8% or less, 1.7% or less, 1.5% or less, 1.3% or less, or 1.2% or less.

[0039] The balance of the roll base material according to the embodiment of the present invention other than the above elements is composed of Fe and impurities. The impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when the roll base material is industrially manufactured.

[0040] [3.0≦Mo+W+V+Nb≦17.5] The chemical composition of the roll base material according to the embodiment of the present invention is represented by the following formula 1: 3.0≦Mo+W+V+Nb≦17.5...Equation 1 where the content (mass %) of each element in the roll base material is substituted for each element symbol in Equation 1, and 0 is substituted when the element is not contained.

[0041] In addition to controlling the content of each element contained in the roll base material as described above, by further controlling the total content of Mo, W, V, and Nb to 3.0% or more, it is possible to reliably suppress the decrease in hardness of the molten resolidified layer formed on the outermost layer of the roll by electrical discharge texturing due to solid solution strengthening and structural amorphization by these alloying elements as described above. On the other hand, by limiting the total content of Mo, W, V, and Nb to 17.5% or less, it is possible to maintain the grindability of the roll base material and the toughness of the cold rolling roll at a good level. From the viewpoint of further improving the hardness of the molten resolidified layer, the higher the total content of Mo, W, V, and Nb, the more preferable, and may be, for example, 3.5% or more, 4.0% or more, 4.5% or more, 5.0% or more, 5.5% or more, 6.0% or more, 6.5% or more, or 7.0% or more. On the other hand, if the total content of Mo, W, V, and Nb is too high, these alloying elements may form coarse carbides, which may reduce the grindability of the roll base material and the toughness of the cold rolling roll. Therefore, from the viewpoint of suppressing the formation of such coarse carbides and further improving the grindability of the roll base material, the total content of Mo, W, V, and Nb is preferably 17.0% or less, and may be, for example, 16.0% or less, 15.0% or less, 14.0% or less, 13.0% or less, 12.0% or less, 11.0% or less, or 10.0% or less.

[0042] [Cu content in the melted and resolidified layer: 0.10 to 3.00%] A cold rolling roll according to an embodiment of the present invention includes a molten resolidified layer formed on the surface of the roll base material, and the molten resolidified layer contains, by mass %, 0.10 to 3.00% Cu. As described above, in the case of surface roughening by electrical discharge texturing, unlike surface roughening by laser texturing or the like, Cu derived from the electrode is concentrated in the resulting molten resolidified layer. However, in conventional electrical discharge texturing, the amount of Cu concentrated in the molten resolidified layer has not been controlled within a predetermined range to improve the properties of the resulting rolling roll. Since Cu has the effect of improving corrosion resistance, in an embodiment of the present invention, by controlling the Cu content in the molten resolidified layer to 0.10% or more, it is possible to suppress the gradual progression of surface roughening of the roll surface due to corrosion, wear, and the like during use, and as a result, it is possible to significantly improve the surface roughening resistance of the roll surface. From the viewpoint of further improving the surface roughening resistance of the roll surface, the Cu content in the molten resolidified layer is preferably 0.20% or more, and may be, for example, 0.40% or more, 0.60% or more, 0.80% or more, or 1.00% or more. On the other hand, excessive Cu content may cause Cu to segregate at grain boundaries, resulting in the generation of cracks such as microcracks on the roll surface. In contrast, in an embodiment of the present invention, by controlling the Cu content in the molten resolidified layer to 3.00% or less, the grain boundary segregation of Cu can be sufficiently suppressed, thereby improving the crack resistance of the cold rolling roll. From the viewpoint of further improving the crack resistance, the Cu content in the molten resolidified layer is preferably 2.90% or less, and may be, for example, 2.80% or less, 2.60% or less, 2.40% or less, 2.20% or less, 2.00% or less, 1.80% or less, or 1.60% or less.

[0043] The Cu content in the molten resolidified layer is determined by elemental analysis (point analysis) using an EPMA (electron probe microanalyzer) as follows. First, an arbitrary location on the outer periphery of the roll is selected as a reference position, and then a total of eight points are identified by dividing the outer periphery of the roll at equal intervals of one-eighths from the reference position. Next, the Cu content at the center of the thickness direction of the molten resolidified layer from each identified point is measured using the EPMA. Of the total eight measured values, the average of six points excluding the maximum and minimum values ​​is calculated, and the calculated average is determined as the Cu content in the molten resolidified layer. This method makes it possible to appropriately determine the Cu content in the molten resolidified layer and the Vickers hardness of the molten resolidified layer, which will be described below, regardless of the dimensions of the roll.

[0044] [Vickers hardness of melted and resolidified layer: 650Hv or more] According to the roll for cold rolling according to the embodiment of the present invention, an improved hardness of the molten resolidified layer can be achieved, for example, a Vickers hardness of 650 Hv or more can be achieved in the molten resolidified layer. From the viewpoint of improving roughness maintenance, a higher Vickers hardness of the molten resolidified layer is preferable, and may be, for example, 660 Hv or more, 680 Hv or more, 700 Hv or more, 720 Hv or more, 740 Hv or more, or 760 Hv or more. The upper limit is not particularly limited, and the Vickers hardness of the molten resolidified layer may be, for example, 1000 Hv or less, 950 Hv or less, or 900 Hv or less.

[0045] The Vickers hardness of the molten resolidified layer is determined using a micro-Vickers tester in accordance with JIS Z 2244-1:2020 as follows. First, as in the case of the Cu content of the molten resolidified layer, an arbitrary location on the outer periphery of the roll is selected as a reference position, and then a total of eight locations are identified by dividing the outer periphery of the roll at equal intervals of one-eighths from the reference position. Next, the Vickers hardness at the center of the thickness direction of the molten resolidified layer from each identified location is measured using the micro-Vickers tester with a test load of 10 gf. Of the total eight measured values ​​obtained, the average of six points excluding the maximum and minimum values ​​is calculated, and the calculated average is determined as the Vickers hardness of the molten resolidified layer.

[0046] [Roll surface roughness] The cold rolling roll according to the embodiment of the present invention is manufactured by electrical discharge texturing, as is evident from the fact that the molten resolidified layer contains a predetermined amount of Cu. In electrical discharge texturing, repeated electrical discharge machining accumulates countless discharge marks on the roll surface, roughening the roll surface. Therefore, the cold rolling roll according to the embodiment of the present invention has a rough surface portion formed from a molten resolidified layer. The surface roughness of such a rough surface portion on the roll surface is not particularly limited and may be any appropriate value, for example, any appropriate value useful for application as a dull roll. For example, the arithmetic mean roughness Ra of the rough surface portion formed from a molten resolidified layer is not particularly limited, and may be 0.5 μm or more or 1.0 μm or more, and / or 10.0 μm or less or 5.0 μm or less. The arithmetic mean roughness Ra refers to the arithmetic mean roughness Ra specified in JIS B 0601:2013 and is determined in accordance with JIS B 0633:2001.

[0047] [Method for manufacturing cold rolling rolls] The cold rolling roll according to the embodiment of the present invention can be manufactured by any suitable method known to those skilled in the art. Although not particularly limited, for example, the cold rolling roll can be manufactured by a method including a casting process for casting an ingot by ingot casting or the like, a forging process for forming the cast ingot into a roll shape, an annealing process for annealing the formed roll, a rough grinding process for roughly grinding the resulting roll into the desired roll shape, a heat treatment process for heating to a temperature above the A3 point, a quenching process, a tempering process for adjusting the hardness of the roll, a finish grinding process for grinding into the final roll shape, and an electric discharge texturing process for roughening the roll surface machined into the final shape by electric discharge texturing. In the heat treatment process, although not particularly limited, the forged roll is typically heated at a temperature of 900 to 1100°C for several hours, e.g., 3 hours or more. Furthermore, although not particularly limited, the quenching process is preferably carried out by water cooling so that the cooling rate of the roll surface is 150°C / min or more, preferably 300°C / min. In addition, the tempering process is generally preferably carried out in a temperature range of 100 to 600°C. The roll to be subjected to electrical discharge dulling may be a roll manufactured by centrifugal casting or continuous casting overlaying, having a multi-layer structure of an outer layer material and a core material, and including an outer layer material that is not forged. Furthermore, an electrical discharge dulling roll that has been used in rolling and has a reduced roughness can be reused by grinding to remove the molten resolidified layer and then performing electrical discharge dulling again.

[0048] In the electrical discharge texturing process, for example, by using an electrical discharge texturing machine equipped with a conventional Cu electrode, the amount of Cu introduced into the molten resolidified layer and the surface roughness of the roll surface can be controlled within a desired range by appropriately selecting the current value and pulse time to adjust the discharge energy. For example, by increasing the current value and increasing the discharge energy, the amount of Cu introduced into the molten resolidified layer and the surface roughness of the roll surface can be increased. The current value in the electrical discharge texturing process is not particularly limited, but may be appropriately selected within a range of, for example, 3 to 22 A. The pulse time is also not particularly limited, but may be appropriately selected within a range of, for example, 8 to 50 μsec.

[0049] The cold rolling roll according to the embodiment of the present invention is manufactured by electrical discharge texturing, and therefore the roll surface is roughened. Furthermore, by incorporating a relatively large amount of Mo, W, V, and / or Nb into the roll base metal as described above, the grindability of the roll base metal is maintained at a good level, while suppressing a decrease in the hardness of the roughened roll surface, thereby significantly improving the roughness maintenance of the cold rolling roll. In addition, in the cold rolling roll according to the embodiment of the present invention, both the surface roughening resistance and the crack resistance can be improved by controlling the content of Cu introduced into the molten resolidified layer from the electrode by electrical discharge texturing to within a range of 0.10 to 3.00%. Therefore, by using the cold rolling roll according to the embodiment of the present invention, it is possible to significantly improve the roll consumption rate. In addition, the cold rolling roll according to the embodiment of the present invention has high roughness maintenance, etc., and is therefore very useful for use as a dull roll. However, the cold rolling roll according to the embodiment of the present invention is not necessarily limited to use as a so-called dull roll, and can be applied to any cold rolling roll to which electrical discharge texturing is applied.

[0050] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way. [Example]

[0051] In the following examples, roll materials having various chemical compositions shown in Table 1 were manufactured, and roll test pieces measuring φ80 mm x 10 mm were then fabricated. While rotating the circumferential surface of the roll test piece at 3 mm / min, the circumferential surface of the roll test piece was subjected to electrical discharge texturing using a Cu electrode under the conditions of the current (A) and pulse time (μsec) shown in Table 2 below. The roll test pieces other than those in Example 12 were forged steel rolls obtained by casting an ingot using the ingot-making method and then forging the cast ingot. On the other hand, the roll test piece in Example 12 was a cast roll consisting of a single-layer structure with an outer layer material manufactured by the pouring casting method without using a core material for ease of fabrication. Cross-sectional samples were prepared from the surface layers of these roll test pieces, and the Vickers hardness and Cu content of the melted and resolidified layer were measured as shown in Table 2 below. The Vickers hardness and Cu content were measured using the following methods.

[0052] [Vickers hardness of melted and resolidified layer] The Vickers hardness of the molten resolidified layer was determined using a micro-Vickers tester in accordance with JIS Z 2244-1:2020 as follows. First, an arbitrary location on the outer periphery of the roll test piece was selected as a reference position, and then a total of eight locations were identified by dividing the outer periphery of the roll test piece at equal intervals of one-eighths from the reference position. Next, the Vickers hardness at the center of the thickness direction of the molten resolidified layer from each identified location was measured using the micro-Vickers tester with a test load of 10 gf. Of the total eight measured values ​​obtained, the average of six points excluding the maximum and minimum values ​​was calculated, and the calculated average was determined as the Vickers hardness of the molten resolidified layer.

[0053] [Cu content in the melted and resolidified layer] The Cu content in the molten resolidified layer was determined by elemental analysis (point analysis) using EPMA as follows. First, as in the case of the Vickers hardness of the molten resolidified layer, an arbitrary location on the outer periphery of the roll test piece was selected as a reference position, and then a total of eight points were identified by dividing the outer periphery of the roll test piece at equal intervals of one-eighths from the reference position. Next, the Cu content at the center of the molten resolidified layer in the thickness direction from each identified point was measured using EPMA, and the average value of six points excluding the maximum and minimum values ​​was calculated from the total of eight measured values ​​obtained. The calculated average value was determined as the Cu content in the molten resolidified layer.

[0054] [Evaluation of roughness maintenance, surface roughness resistance and crack resistance] Next, the above-mentioned roll test pieces were subjected to evaluation of the roughness maintenance, surface roughening resistance, and crack resistance of the sliding surface using a twin-cylinder rolling wear tester. Figure 2 is a schematic diagram showing a rolling wear test using a twin-cylinder rolling wear tester for roll test pieces of the examples and comparative examples. Using the twin-cylinder rolling wear tester 10 shown in Figure 2, the rolled material test piece 12 to be brought into contact with the roll test piece 11 was made of S45C steel and had dimensions of φ160 mm × 15 mm. The rolling wear test was performed by heating the rolled material test piece 12 to 200°C by high-frequency induction heating using an induction heating coil 13, and then applying a contact load of 700 N, rotating the roll test piece 11 at 2,000 rpm, with a slip ratio of 5%, and rolling up to 20,000 times without lubrication. The slip ratio (%) is defined by the following formula: Slippage ratio (%) = (circumferential speed of rolled material test piece - peripheral speed of roll test piece) / peripheral speed of roll test piece × 100 The roughness retention, surface roughening resistance, and crack resistance were evaluated based on the sliding surface properties of the roll test piece 11 after 20,000 rolling cycles. The roughness retention rate was used as an evaluation index for roughness retention. The roughness retention rate (%) is defined by the following formula. The surface roughness of the sliding surface was evaluated by the arithmetic mean roughness Ra, and roll test pieces with a calculated roughness retention rate of 36% or more were evaluated as having good roughness retention. Roughness retention rate (%) = (surface roughness of sliding surface after test / roughness before test) x 100 The surface roughening resistance and crack resistance were evaluated by visually observing the presence or absence of surface roughening and cracking on the sliding surface of the roll test piece after the test, and roll test pieces that did not have surface roughening or cracking were evaluated as having good surface roughening resistance and cracking resistance.

[0055] [Evaluation of grindability of roll base material] The grindability of the roll specimen's base material was evaluated by separately preparing a roll specimen measuring 80mm x 150mm and conducting a grinding test using a cylindrical grinder. A grinding wheel measuring 350mm x 20mm was used, consisting of alumina abrasive grains bonded with a vitrified bond. The grinding test was performed using wet traverse grinding with a removal allowance of 0.3mm, a cutting depth of 0.005mm per pass, a grinding wheel peripheral speed of 30m / s, a roll specimen peripheral speed of 0.03m / s, and a traverse speed of 500mm / min. After the test, the ground surface of the roll specimen was visually inspected for scratches. Specimens without scratches were deemed to have good roll substrate grindability. The results are shown in Table 2.

[0056] [Table 1]

[0057] [Table 2]

[0058] In this example, when the roughness retention rate was 36% or more and no rough surface, cracks, or scratches occurred, the cold rolling roll was evaluated as having improved roughness retention, as well as improved roughness resistance, crack resistance, and grindability of the roll base material. Referring to Tables 1 and 2, Comparative Examples 1, 2, and 6 had low Mo and V contents, and the total content of Mo, W, V, and Nb was also low, resulting in a decrease in the hardness of the molten resolidified layer and insufficient roughness retention. Comparative Examples 3 and 7 had low current values ​​during electrical discharge texturing, which resulted in a low Cu content in the molten resolidified layer, resulting in roughness and insufficient roughness resistance. Comparative Examples 4 and 8 had high current values ​​during electrical discharge texturing, which resulted in a high Cu content in the molten resolidified layer, which is thought to have caused grain boundary segregation of Cu. As a result, cracks occurred, and sufficient crack resistance was not achieved. In Comparative Example 5, the Mo and V contents were high, and furthermore, the total content of Mo, W, V, and Nb was also high, which is thought to have caused the formed carbides to become coarse. As a result, scratches occurred on the roll base material after grinding, and sufficient grindability of the roll base material could not be achieved. In Comparative Example 9, the total content of Mo, W, V, and Nb was also high, which is thought to have caused the formed carbides to become coarse. As a result, scratches occurred on the roll base material after grinding, and sufficient grindability of the roll base material could not be achieved.

[0059] In contrast, in Examples 1 to 12, by controlling the roll base material within a predetermined chemical composition range and controlling the Cu content introduced into the molten resolidified layer by electrical discharge texturing within a range of 0.10 to 3.00%, the Vickers hardness of the molten resolidified layer was maintained at 650 Hv or more, the roughness retention rate was 36% or more, no roughening or cracking occurred, and further, the occurrence of scratch marks on the roll base material after grinding was suppressed. Therefore, in all Examples, higher roughness retention, surface roughening resistance, cracking resistance, and grindability of the roll base material were achieved compared to Comparative Examples 1 to 9. [Explanation of symbols]

[0060] 1 Work roll 2 Roll base material 3 Melted and resolidified layer 4 Rough surface area 10. Two-cylinder rolling wear tester 11 Roll specimen 12 Rolled material test piece 13 Induction heating coil

Claims

1. A roll base material and a melted and resolidified layer formed on the surface of the roll base material, The roll base material is, in mass%, C: 0.70-2.50%, Si: 0.2-2.0%, Mn: 0.2 to 1.5%, P: 0.030% or less, S: 0.020% or less, Al: 0.050% or less, N: 0.0200% or less, O: 0.0050% or less, Cr: 2.8-11.0%, Mo: 2.0 to 6.0%, V: 1.0-6.0%, Cu: less than 0.10% B: 0.0100% or less, Ni: 0-2.0%, W: 0 to 6.0%, Nb: 0 to 2.0%, and The balance is composed of Fe and impurities. It has a chemical composition that satisfies the following formula 1: 3.0≦Mo+W+V+Nb≦17.5...Formula 1 The roll for cold rolling, wherein the melted and resolidified layer contains, by mass%, Cu: 0.10 to 3.00%. Here, the content (mass %) of each element in the roll base material is substituted for each element symbol in the formula 1, and 0 is substituted when the element is not contained.

2. The chemical composition is, in mass %, Ni: 0.01-2.0%, W: 0.01 to 6.0%, and Nb: 0.01-2.0% The cold rolling roll of claim 1 , comprising:

3. 3. The roll for cold rolling according to claim 1, wherein the melted and resolidified layer has a Vickers hardness of 650 Hv or more.

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