Cast single roll for hot rolling

Abstract

This invention provides a single cast roll for hot rolling with improved fatigue strength in the neck radius section. [Solution] A single cast roll for hot rolling has a composition of C: 0.8~2.2%, Si: 1.0~3.0%, Mn: 1.5~2.5%, Ni: 0.3~2.0%, Cr: 0.1~2.0%, Mo: 0.1~2.0%, V: 0.1~2.0% by mass, with the remainder being Fe and impurities. Preferably, the maximum diameter of carbides on the microstructure surface at the neck radius is 90 μm or less in terms of equivalent circle diameter. Alternatively, preferably, the area ratio of carbides on the microstructure surface at the neck radius is 0.4% or less.

Description

Technical Field

[0001] The present invention relates to a single-piece casting roll used for hot rolling of steel materials and the like.

Background Art

[0002] Conventionally, for hot rolling of steel materials and the like, casting rolls made of adamite have been used (Patent Documents 1 to 4). Adamite generally has a metal structure in which cementite, which is a kind of carbide, crystallizes between pearlite-based matrices, and is a material having both wear resistance and toughness. Therefore, by adopting a roll structure of a "single-piece roll (or single-layer roll)" in which the barrel part and the shaft part are made of the same material (that is, adamite), a part of the barrel part with excellent wear resistance and a part of the shaft part with excellent toughness can be efficiently produced simultaneously in a single stationary casting process.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0004] Incidentally, during hot rolling, the rolls are subjected to large loads on their rolling surface. At this time, various stresses are generated inside the roll, and repeated stresses concentrate in the "neck radius" of the roll, which will be explained in Figure 1 below. In the case of conventional adamite cast rolls, if the cementite in the metal structure of the neck radius is coarse, this can become the starting point for fatigue fracture. The objective of the present invention is to provide a single cast roll for hot rolling that has improved fatigue strength in the neck radius portion of the roll. [Means for solving the problem]

[0005] The present invention relates to a single cast roll for hot rolling, having a composition of mass% C: 0.8~2.2%, Si: 1.0~3.0%, Mn: 1.5~2.5%, Ni: 0.3~2.0%, Cr: 0.1~2.0%, Mo: 0.1~2.0%, V: 0.1~2.0%, with the remainder being Fe and impurities.

[0006] Furthermore, the present invention is preferably a single cast roll for hot rolling in which the maximum diameter of carbides on the microstructure surface at the neck radius is 90 μm or less in terms of the diameter equivalent to a circle. Alternatively, the present invention is preferably a single cast roll for hot rolling in which the area ratio of carbides on the microstructure surface at the neck radius is 0.4% or less. Alternatively, the present invention is preferably a single cast roll for hot rolling, wherein the hardness of the body is 40HS or higher on a Shore hardness scale. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a single cast roll for hot rolling with improved fatigue strength in the neck radius portion. [Brief explanation of the drawing]

[0008] [Figure 1] This is a schematic cross-sectional view showing an example of the roll structure of a single cast roll for hot rolling. [Figure 2]This figure shows a microstructural photograph of the neck radius portion of a single casting roll for hot rolling according to an example of the present invention, and an example of an image obtained by binarizing the same. [Figure 3] This figure shows a microstructural photograph of the neck radius portion of a single casting roll for hot rolling, a comparative example, and an example of an image obtained by binarizing it. [Figure 4] This figure shows a microstructural photograph of the neck radius portion of a single casting roll for hot rolling according to an example of the present invention, and an example of an image obtained by binarizing the same. [Figure 5] This figure shows a microstructural photograph of the neck radius portion of a single casting roll for hot rolling, a comparative example, and an example of an image obtained by binarizing it. [Modes for carrying out the invention]

[0009] Embodiments of the present invention will be described in detail below, but the present invention is not limited thereto, and various modifications can be made without departing from the technical spirit of the present invention. A description of one embodiment applies to other embodiments unless otherwise specified.

[0010] (1) Roll structure of a single cast roll for hot rolling Figure 1 is a schematic cross-sectional view showing an example of the roll structure of a single cast roll 0 for hot rolling according to the present invention. The shaft portion 2, including the neck portions (journal portions) 3 on both sides, forms the axis of the entire single roll 0 across both ends of the single roll 0. The body portion 1 is formed with a large diameter around the central part of the shaft portion 2 excluding the neck portions 3, and has a rolling surface 4. There is a step between the body portion 1 and the neck portion 3 due to the difference in their diameters, and the corners of this step are rounded (R) to form a neck radius portion 5 (circled by the dotted line).

[0011] (2) Composition of single cast rolls for hot rolling The object of the present invention is to improve the fatigue strength of the neck radius portion of a cast single roll for hot rolling having the roll structure of (1). The inventors have found that in the case of cast single rolls made of adamite, one of the factors contributing to the low fatigue strength of the neck radius portion is the distribution of cementite in the metal structure of that portion. Considering that the entire cast single roll is manufactured in a single casting process such as static casting, the inventors have found that by reviewing the component composition of the entire cast single roll, there is a component composition that is particularly effective in improving the fatigue strength of the neck radius portion. The component composition of the hot-rolling cast single roll of the present invention is described below.

[0012] C: 0.8~2.2% by mass (hereinafter also referred to simply as %) Carbon (C) is an element that causes graphite and cementite to crystallize in the overall metal structure of a cast roll, including the neck radius. If the C content is low, the amount of graphite and cementite crystallized will not be sufficient, making it difficult to ensure sufficient seizure resistance and wear resistance on the rolling surface of the roll. However, since cementite tends to become coarse, if the C content is too high, there is a concern that the fatigue strength will decrease in the neck radius of the roll. Therefore, the C content should be 0.8 to 2.2%. Preferably, it should be 1.0% or more, more preferably 1.1% or more, and even more preferably 1.2% or more. Also, preferably 2.0% or less, more preferably 1.7% or less, and even more preferably 1.5% or less.

[0013] Si: 1.0~3.0% Si is a graphitization-promoting element. In the casting process, it acts as a deoxidizing agent for molten metal and also improves the fluidity of the molten metal, thereby improving castability. However, if the Si content is too high, the roll material becomes brittle. Therefore, the Si content should be 1.0 to 3.0%. Preferably, it should be 1.3% or more, more preferably 1.5% or more, and even more preferably 1.7% or more. Also, preferably 2.7% or less, more preferably 2.5% or less, and even more preferably 2.3% or less.

[0014] Mn: 1.5 - 2.5% Conventionally, Mn mainly served as a deoxidizer for molten metal in the casting process and was contained in adamite casting rolls. However, the Mn in the present invention is actively added while being aware of its effects not only in this casting process but also in the roll products after casting. And in the component composition of the present invention, by adding a relatively large amount of Mn compared to the prior art, supercooling occurs during the crystallization of austenite, causing the austenite to be refined. As a result, it is considered that the cementite is refined. As a result, in the metal structure of the neck fillet portion of the single-cast roll, the cementite is refined, improving the fatigue strength of the neck fillet portion of the roll. However, if the Mn content is too high, the toughness of the roll decreases. Therefore, the Mn content is set to 1.5 - 2.5%. Preferably it is 1.6% or more, more preferably 1.7% or more, still more preferably 1.8% or more, and even more preferably 1.9% or more. Also, preferably it is 2.4% or less, more preferably 2.3% or less, still more preferably 2.2% or less, and even more preferably 2.1% or less.

[0015] Ni: 0.3 - 2.0% Ni is a graphiteization promoting element. However, if the Ni content is too high, austenite is stabilized in the quenched and tempered structure, reducing the base hardness. Therefore, the Ni content is set to 0.3 - 2.0%. Preferably it is 0.5% or more, more preferably 0.6% or more, still more preferably 0.​​​​​Cr is an element that improves the hardness of the matrix and also forms carbides, maintaining the overall hardness of the metal structure. However, if the Cr content is too high, the hardenability improves (i.e., it becomes easier to undergo bainite or martensitic transformation). As a result, if bainite or martensitic transformation occurs during quenching, it can occur at a lower temperature than pearlite transformation, and the transformation stress at this time can cause the roll material to crack. Therefore, the Cr content should be 0.1 to 2.0%. Preferably it is 0.2% or more, more preferably 0.3% or more. Also preferably it is 1.7% or less, more preferably 1.3% or less, even more preferably 0.9% or less, and even more preferably 0.5% or less.

[0017] Mo: 0.1~2.0% Mo is also an element that improves the hardness of the base material and forms carbides, thus maintaining the overall hardness of the metal structure. However, if the Mo content is too high, the improved hardenability may cause the roll material to crack during quenching. Therefore, the Mo content should be 0.1 to 2.0%. Preferably it is 0.2% or more, more preferably 0.3% or more. Also preferably it is 1.7% or less, more preferably 1.3% or less, even more preferably 0.9% or less, and even more preferably 0.5% or less.

[0018] V: 0.1~2.0% In the cast single roll of the present invention, in order to improve the fatigue strength of the neck radius portion, first, as described above, a relatively large amount of Mn is added to refine the cementite in the metal structure of this portion. In addition to this, in the present invention, it is also effective to reduce the amount of cementite "itself" which tends to become coarse. In other words, V plays the role of consuming the carbon (C) that would have originally formed cementite (Fe3C) by forming vanadium carbide (VC) with this carbon. As a result, the vanadium carbide formed in place of cementite can be made into fine crystallized carbides, and together with the effect of refining the cementite, this contributes to improving the fatigue strength of the neck radius. However, if the V content is too high, the crystallization temperature of vanadium carbide rises, and the vanadium carbide tends to become coarser.

[0019] Furthermore, considering that the present invention is a single cast roll, it is conceivable that the carbide distribution in the metal structure of the body (rolling surface) is similar to that of the neck radius section, with a reduced amount of cementite. However, even if this reduction in cementite reduces the wear resistance of the rolling surface, this reduction in wear resistance can be adequately compensated for by the vanadium carbides added in its place. The hardness of the body is 40 HS or higher on the Shore hardness scale, and it can be used to create a single cast roll for hot rolling with a hardness of, for example, 40-60 HS or 40-50 HS. Therefore, the V content should be 0.1 to 2.0%. Preferably, it should be 0.2% or more, more preferably 0.3% or more, and even more preferably 0.4% or more. Also, preferably 1.5% or less, more preferably 1.0% or less, even more preferably 0.7% or less, and even more preferably 0.5% or less.

[0020] Remainder: Fe and impurities The roll material according to the present invention maintains an adamite-based component composition, with the remainder being Fe. P, S, and Al are elements that may be present in the roll material as impurities. Since P, S, and Al can lead to deterioration of the mechanical properties of the roll product, their content should be kept low; for example, their respective contents can be 0.1% or less. Other elements may also be included, as long as they do not inhibit graphitization.

[0021] (3) Metal structure of single cast rolls for hot rolling (3-1) Maximum diameter of carbides on the microstructure at the neck radius In the cast single roll of the present invention, the roll material has a component composition (2) that allows for the refinement of cementite-containing carbides in the metal structure. In this case, it is preferable that the maximum diameter of carbides on the microstructure surface of the neck radius portion of the cast single roll is 90 μm or less in terms of equivalent circular diameter. This maximum diameter of carbides can be a value predicted by extreme value statistics. By doing so, the fatigue strength of the neck radius portion of the cast single roll can be further improved. The maximum diameter of the carbides is preferably 85 μm or less, more preferably 80 μm or less, even more preferably 75 μm or less, and even more preferably 70 μm or less. Furthermore, it is more preferable that the above-mentioned maximum diameter of the carbides is satisfied on both the drive side and the non-drive side of the neck radius.

[0022] Furthermore, there is no lower limit required for the maximum diameter of the carbides mentioned above. However, if consideration is given to ensuring wear resistance on the rolled surface, the maximum diameter of the carbides can be, for example, 30 μm or more, 40 μm or more, or 50 μm or more.

[0023] The maximum diameter of carbides in the neck radius section described above is effectively achieved when the microstructure on the drive side receiving torque satisfies the value of the present invention. Furthermore, by manufacturing the material for the single casting roll by static casting, the value of the present invention is satisfied in the neck radius section on the upper die side (usually the upper die side is often the non-drive side) during casting, where carbides tend to grow relatively easily. This also satisfies the value of the present invention in the neck radius section on the lower die side (drive side), thus allowing the maximum diameter of carbides to be adjusted to the value of the present invention on the microstructure on both neck radius sections.

[0024] (3-2) Area ratio of carbides on the microstructure at the neck radius In the cast single roll of the present invention, the roll material has a component composition (2) that allows for the refinement of cementite-containing carbides in the metal structure. Consequently, the "amount" of cementite, which tends to become coarse, is reduced because the carbon that forms it is consumed by forming the aforementioned VC. However, with this component composition, even if the area ratio of cementite-containing carbides in the microstructure surface at one or both sides of the neck radius of the roll is reduced to 0.4% or less, or even 0.3% or less, the cementite is refined and the fatigue strength is improved. The area ratio of carbides can be calculated using the group of carbides sampled from the microstructure surface at the neck radius when the above extreme value statistical method is performed.

[0025] Furthermore, the lower limit of the area ratio of carbides mentioned above will not be zero percent as long as the component composition of the roll material according to the present invention is satisfied, and an effective area ratio will be achieved. The lower limit of the area ratio effective for carbide refinement achieved by the component composition of (2) of the present invention can be, for example, 0.1% or 0.2%. Furthermore, since the present invention is a single cast roll, the carbide distribution in the neck radius portion is also present on the rolled surface, and the area ratio of carbides on the microstructure of the rolled surface may decrease to, for example, 0.4% or less or 0.3% or less. In that case, there is a concern that the wear resistance will decrease on the rolled surface due to the decrease in cementite, but since the vanadium carbide compensates for this decrease in cementite, the wear resistance of the rolled surface can be maintained.

[0026] (3-3) Area ratio of graphite on the microstructure at the neck radius When graphite crystallizes in the metal structure of the single cast roll of the present invention, the area ratio of graphite on the microstructure surface in the neck radius portion of the roll may be 0.5% or more, or 1.0% or more. Alternatively, the above-mentioned area ratio of graphite may be 10.0% or less, 8.0% or less, 5.0% or less, or 4.0% or less. By including graphite in the neck radius portion, a reduction in the amount of cementite crystallization, which tends to become coarse, can be expected. Furthermore, by regulating the amount of graphite included in the neck radius portion, it can be expected that fracture originating from coarse graphite can be suppressed.

[0027] Furthermore, since the present invention is a single cast roll, the graphite distribution in the neck radius portion is also present on the rolled surface, and the area ratio of graphite in the microstructure of the rolled surface may be, for example, 0.5% or more or 1.0% or more. Similarly, the area ratio of graphite may be, for example, 10.0% or less, 8.0% or less, 5.0% or less, or 4.0% or less. By including graphite in the rolled surface, improved resistance to seizing can be expected. And this does not result in any deficiency in the strength of the rolled surface.

[0028] (4) Method for manufacturing a single cast roll for hot rolling The cast single roll of the present invention can be manufactured by the following method. For example, a molten metal satisfying the component composition of (2) described above is cast by static casting to obtain a roll material in which the body and shaft are integrally formed. Next, this roll material is subjected to various annealing processes such as diffusion annealing and spheroidizing annealing to form a roll material. Then, if necessary, this roll material is subjected to machining and normalization, followed by quenching and tempering, and finally machining to produce the cast single roll of the present invention. The size of the roll can be, for example, a body diameter of 300 to 3000 mm and a body length of 100 to 4000 mm. [Examples]

[0029] The present invention will be described in more detail by the following examples, but the present invention is not limited to these examples.

[0030] <Manufacturing of single cast rolls for hot rolling> Molten metal having the component composition shown in Table 1 was statically cast to obtain a roll material in which the body and shaft were integrated. Next, this roll material was subjected to diffusion annealing at 1000-1100°C, then heated to 700-950°C, spheroidizing annealing was performed, and then quenched. After tempering at 580°C, it was machined into a predetermined shape to produce a single cast roll for hot rolling (Figure 1) with a body diameter of 500-800 mm and a body length of 1000-1500 mm.

[0031] [Table 1]

[0032] <Metal structure of the neck radius section> For each roll, a sample was taken from the neck radius (circled by the dotted line in Figure 1) on the non-driven side (i.e., the upper mold side during stationary casting). Next, the microstructure surface of this sample was mirror-polished and then etched with ammonium persulfate. The etched microstructure surface was then observed with a 50x optical microscope. An example of the microstructure surface of roll 1 (example of the present invention) is shown in Figure 2(A), and an example of the microstructure surface of roll 10 (comparative example) is shown in Figure 3(A). The light-colored areas visible in the matrix are cementite-containing carbides. Ten fields of view (total area 2518 μm × 1681 μm) were observed, with each field of view representing one field of view.

[0033] • Maximum diameter of carbides The optical microscope images of the 10 fields described above were processed, and a binarization process was performed using the boundary between the carbides and the matrix, which is the boundary between the carbides and the matrix due to the corrosion, as the threshold value. This resulted in a binarized image showing the carbides distributed within the matrix on the tissue surface. An example of the binarized image of roll 1 (example of the present invention) is shown in Figure 2(B), and an example of the binarized image of roll 10 (comparative example) is shown in Figure 3(B) (carbides are shown as a distribution of dark colors). If the binarized images in Figures 2(B) and 3(B) contain noise such as scratches that occurred during polishing, processing to remove the noise may be performed on these binarized images. For this series of image processing and analysis, including the calculation of the graphite area ratio described later, we used ImageJ (http: / / imageJ.nih.gov / ij / ), open-source image processing software provided by the National Institutes of Health (NIH).

[0034] By further image processing and analysis of the binarized images described above, 10 fields of view were extracted containing carbides with an equivalent circle diameter of 12 μm or larger that could be identified. These 10 fields of view contain measured data of carbides, which were then used as the sample for the extreme value statistical method. A Gumbel-type extreme value distribution was applied to the data analysis to predict the maximum size of cementite-containing carbides that could be contained in the neck radius (carbide size when the cumulative probability reaches 95%).

[0035] • Area ratio of carbides Using the measured data set of charred material from the 10 fields extracted above, the area ratio of charred material to the total area of ​​the 10 fields was determined from the total area of ​​these charred materials.

[0036] • Graphite area ratio The microstructure surfaces of samples taken from the neck radius of each roll in the same manner as described above were polished with a diamond buff, and these polished microstructure surfaces were observed with a 50x optical microscope. An example of the microstructure surface of roll 1 (example of the present invention) is shown in Figure 4(A), and an example of the microstructure surface of roll 10 (comparative example) is shown in Figure 5(A). The granular dispersions that can be seen as dark in color in the substrate are graphite. Ten fields of view (total area 2518 μm × 1681 μm) were observed, with each field of view being considered as one field of view.

[0037] The optical microscope images of the 10 fields described above were processed, and a binarization process was performed using the boundary between the graphite and the matrix, which is the boundary between the light and dark areas caused by the polishing structure described above, as the threshold value, to obtain a binarized image showing the graphite distributed in the matrix on the structure surface. An example of the binarized image of roll 1 (example of the present invention) is shown in Figure 4(B), and an example of the binarized image of roll 10 (comparative example) is shown in Figure 5(B). (Graphite is shown as a distribution of dark colors). If the binarized images in Figures 4(B) and 5(B) contain noise such as scratches that occurred during polishing, processing to remove the noise may be performed on these binarized images. Then, by further image processing and analysis of the binarized images described above, 10 fields of view were extracted containing graphite with an equivalent circle diameter of 16 μm or larger that could be identified. From the total area of ​​these graphites, the area ratio of graphite to the area of ​​the 10 fields of view was determined.

[0038] <Mechanical properties of the neck radius section> Tensile and compressive fatigue tests were performed on the samples collected as described above. The tests were conducted in accordance with the high-cycle fatigue test of JIS Z 2273 (ASTM E466). The load was then changed in stages for 10 -6 The fatigue limit was defined as the load at which the test specimen did not fracture after a certain number of cycles. The Shore hardness (HS) of the sample was also measured at this time.

[0039] The results are shown in Table 2.

[0040] [Table 2]

[0041] As shown in Table 2, compared to roll 10 of the comparative example, rolls 1-3 of the present invention have finer cementite-containing carbides in their neck radius portion. Furthermore, their fatigue strength is improved (e.g., 170 MPa or higher). In addition, despite the reduction in carbide content, the formation of vanadium carbides maintains sufficient hardness throughout the structure (e.g., Shore hardness of 40-50 HS), and the wear resistance of the rolled surface is also maintained. [Examples]

[0042] <Manufacturing of single cast rolls for hot rolling> Molten metal having the component composition shown in Table 3 was cast by static casting to obtain a roll material in which the body and shaft were integrated. Then, from this roll material, a single cast roll for hot rolling (Figure 1) with a body diameter of 500 to 800 mm and a body length of 1000 to 1500 mm was manufactured in accordance with the procedure of Example 1.

[0043] [Table 3]

[0044] <Metal structure of the neck radius section> For each roll's neck radius (circled by the dotted line in Figure 1), samples were taken from the non-driven side (i.e., the upper mold side during stationary casting). For these samples, the maximum size of cementite-containing carbides that may be contained in the neck radius (carbide size when the cumulative probability is 95%) was predicted in the same manner as in Example 1. Then, the area ratio of carbides and graphite was determined.

[0045] <Mechanical properties of the neck radius section> The samples collected as described above were subjected to tensile and compressive fatigue tests in the same manner as in Example 1, and the fatigue limits were determined. The Shore hardness (HS) of the samples was also measured.

[0046] The results are shown in Table 4.

[0047] [Table 4]

[0048] As shown in Table 4, the rolls 4 and 5 of the present invention, compared to the roll 11 of the comparative example, have finer cementite-containing carbides in their neck radius. As a result, their fatigue strength is improved (e.g., 170 MPa or higher). Furthermore, despite the reduction in carbide content, the formation of vanadium carbides maintains sufficient overall hardness (e.g., Shore hardness of 40-50 HS), and the wear resistance of the rolled surface is also maintained.

[0049] The roll of Comparative Example 12 has an increased Mn content compared to the roll of Comparative Example 11, and the cementite-containing carbides in the metal structure appear to be somewhat finer. However, due to the low V content, the amount of vanadium carbide formation is insufficient, resulting in a large amount of relatively coarse cementite, and thus the improvement in fatigue strength is low. The roll of Comparative Example 13 has an increased V content compared to the roll of Comparative Example 11, and it appears that the amount of vanadium carbide formed in the metal structure has increased. However, due to the low Mn content, the refinement of the cementite-containing carbides is insufficient, making it difficult to obtain an improvement in fatigue strength. Furthermore, graphite was not observed in the rolls of Comparative Examples 11 to 13, likely due to their low Si content. [Explanation of symbols]

[0050] 0 Cast single roll for hot rolling 1 Torso 2. Shaft section 3. Neck section 4. Rolling surface 5. Neck curve section

Claims

1. A single cast roll for hot rolling, characterized by having the following composition by mass%, C: 0.8-2.2%, Si: 1.0-3.0%, Mn: 1.5-2.5%, Ni: 0.3-2.0%, Cr: 0.1-2.0%, Mo: 0.1-2.0%, V: 0.1-2.0%, with the remainder being Fe and impurities.

2. The hot rolling cast single roll according to claim 1, characterized in that the maximum diameter of carbides on the microstructure surface at the neck radius portion is 90 μm or less in terms of the diameter equivalent to a circle.

3. A single cast roll for hot rolling according to claim 1, characterized in that the area ratio of carbides on the microstructure surface at the neck radius is 0.4% or less.

4. A single cast roll for hot rolling according to claim 1, characterized in that the hardness of the body is 40 HS or more on the Shore hardness scale.

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