Inclined rolling mill, manufacturing method of seamless billet, and manufacturing method of seamless steel pipe.

Abstract

An inclined rolling apparatus is provided to prevent shape defects in the front and rear unstable portions of a seamless tube blank. An inclined rolling apparatus includes an inclined piercing mill for piercing rolling and an inclined outer diameter mill connected after the inclined piercing mill, wherein the distance LS (mm) between the rolling portion of the piercing roll and the rolling portion of the outer diameter roll satisfies the following equation (1), and the roll spacing DS of the plurality of outer diameter rolls satisfies the following equation (2): 3×(1.2PO+2G)≤LS≤0.8BL…(1) 1.2PO+2G>DS≥DB+2G…(2) In equations (1) and (2), PO is the outer diameter of the mandrel (mm), G is the spacing between the mandrel and the piercing roll (mm), BL is the length of the rod (mm), and DB is the outer diameter of the rod (mm).

Description

Technical Field

[0001] This invention relates to an inclined rolling mill for manufacturing seamless tube blanks, a method for manufacturing seamless tube blanks, and a method for manufacturing seamless steel pipes. Background Technology

[0002] An inclined rolling mill is one of the devices used to pierce solid round billets to obtain hollow seamless tube blanks. This inclined rolling mill is a device that involves tilting and rotating barrel-shaped or conical rolls relative to the rolling line, introducing solid round billets between the rolls, and manufacturing seamless tube blanks by piercing rolling based on the mandrels positioned between the rolls.

[0003] Inclined rolling mills can produce seamless tube blanks of various sizes simply by varying the spacing of the rolls and the shape of the mandrel, making them highly productive and widely used worldwide. Various equipment configurations and rolling methods have been implemented in inclined rolling mills, and improvements in wall thickness and outer diameter dimensional accuracy, as well as piercing rolling of difficult-to-machine materials such as stainless steel, have also been put into practice.

[0004] For example, Patent Document 1 discloses a method in which back pressure is applied to the rear surface of the billet during the rolling of the billet front end in order to improve the bite of the billet into the roll and increase the degree of freedom in the mandrel placement. According to Patent Document 1, by applying back pressure to the rear surface of the billet during the rolling of the billet front end, defects caused by poor bite can be prevented, and seamless tube blanks can be obtained without defects on the inner surface, even when using difficult-to-machine materials that are difficult to pierce and roll.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2016-30275 Summary of the Invention

[0008] The problem that the invention aims to solve

[0009] Regarding piercing rolling in inclined mills, the following effects can be obtained by optimizing the structure of the inclined mill, the roll shape, the pressing mechanism on the rolling inlet side, and the rolling conditions.

[0010] 1. The dimensional accuracy of the manufactured seamless steel pipes has been improved.

[0011] 2. For difficult-to-machine materials such as hard alloy steel, it is also possible to achieve piercing rolling with thin walls and suppress the generation of defects on the inner and outer surfaces.

[0012] However, these methods mainly focus on the stable portion of seamless steel pipes, and the shape of the unstable portion generated on the front and rear ends of the seamless billet after piercing and rolling has not been sufficiently studied.

[0013] The unstable portions at the front and rear ends are cut off and discarded during product manufacturing; therefore, in research to date, shaping the stable portion has become a fundamental objective. In piercing rolling on an inclined mill, completely eliminating the unstable portions at the front and rear ends is theoretically impossible, and they are inevitably generated. These unavoidable unstable portions are not directly related to the product yield. However, the inventors have discovered that controlling the shape of the unstable portions in the hot rolling process following piercing rolling on an inclined mill is extremely important for improving productivity. The hot rolling process following piercing rolling on an inclined mill and the problems arising from the formation of unstable portions at the front and rear ends in the seamless tube blank obtained through piercing rolling will be described below.

[0014] Seamless tube blanks produced by piercing rolling on inclined mills lack sufficient wall thickness, outer diameter, and surface quality for a product. Therefore, after piercing rolling, various steel pipe rolling mills are used to hot-roll seamless steel pipes to adjust the wall thickness, outer diameter, and surface quality. For example, mandrel rolling mills and automatic rolling mills exist as processes for thinning the wall and extending the seamless tube blank. There are also extending rolling mills and Assel rolling mills that use the same inclined mill as piercing rolling to thin the wall and expand the tube. Furthermore, there are also leveling rolling processes that adjust the surface quality of the inner and outer surfaces while slightly thinning the wall. These mills are selected or combined depending on the size and type of seamless steel pipe being manufactured. In the hot rolling process following piercing rolling, which is essential in the manufacture of seamless steel pipe products, the inner surface of the seamless tube blank obtained by piercing rolling is rolled throughout the entire process; therefore, various shapes of inner surface tools corresponding to the rolling mill are inserted into the seamless tube blank. That is, in order to make the seamless tube blank into the product shape, a hot rolling process after piercing rolling is necessary, and in this hot rolling process, an inner surface tool must be inserted into the seamless tube blank.

[0015] An inner surface tool inserted into a seamless tube blank is used to roll the seamless tube blank from its inner surface. Therefore, the inner surface tool is typically configured with a diameter that is the same as or slightly smaller than the inner diameter of the seamless tube blank into which it is inserted. If the shape of the unstable front and rear ends of the pierced and rolled seamless tube blank is, for example, significantly different from an ellipse or a perfect circle, the shorter side of its inner diameter will be shorter than the diameter of the inner surface tool, resulting in poor insertion of the inner surface tool and halting the manufacturing line.

[0016] Not only is poor insertion of the inner surface tool before hot rolling problematic, but also poor shape of the unstable portions at the front and rear ends can induce failures during hot rolling. For example, if the shape of the unstable portions at the front and rear ends differs significantly from an ellipse or a perfect circle, or if the diameter of the unstable portion differs significantly from the diameter of the stable portion on the short and long sides, the unstable portions at the front and rear ends of the seamless billet will be extruded between the rolls and guides, causing hot rolling to stop. If a shape defect occurs in the unstable portion at the front end, it will not make uniform contact with the rolls during biting into the hot rolling mill, causing swirl in the seamless billet. If swirl occurs in the seamless billet, not only will hot rolling stop, but the wall thickness and outer diameter accuracy of the seamless steel pipe product will also decrease. In particular, if a hot rolling stop failure occurs, recovery from the failure requires waiting until the steel pipe cools inside the mill and is then cut and removed. Thus, the recovery time from a hot rolling stop failure is very long, significantly reducing the productivity of the seamless steel pipe product.

[0017] Similarly, in piercing rolling, if a shape defect occurs in the rear unstable section, and the outer diameter of the rear unstable section becomes smaller than the roll spacing and guide spacing on the rolling exit side, the contact pressure between the roll and guide surfaces and the rear unstable section decreases, weakening the forward force of the seamless billet after piercing rolling. Consequently, the seamless billet after piercing rolling does not exit from the rolling exit side, and the transport of the pierced billet to the hot rolling process cannot proceed on the inclined mill, resulting in a line stoppage. Furthermore, if the forward force of the seamless billet after piercing rolling is weakened, the transport to the hot rolling process takes longer, thus also causing a temperature drop in the seamless billet.

[0018] Unsatisfactory shapes of the unstable sections at the front and rear ends are most likely to occur during piercing rolling on inclined mills. Piercing rolling utilizes the plastic deformation of a solid billet to create a hollow tube. Regarding plastic deformation, since the volume is constant, the material is extended in both the axial and circumferential directions of the tube when it becomes hollow. In piercing rolling, the amount of axial and circumferential extension is appropriately controlled by setting suitable piercing rolling conditions or configuring guides, thereby obtaining a seamless tube with suitable wall thickness and outer diameter. However, the unstable sections at the front and rear ends are more difficult to shape control compared to stable sections because of the discontinuous presence of material at the front or rear.

[0019] Furthermore, the piercing rolling principle of the inclined mill makes shape control of the unstable sections at the front and rear ends difficult. Let the roll spacing on the infeed side of the piercing mill, used to bite in solid billets, be designated as spacing 1. Let the roll spacing for the rolling section, where the mandrel positioned between the rolls and the rolls has a suitable wall thickness and outer diameter, be designated as spacing 2. If the roll spacing on the outfeed side, used to discharge the pierced billet from the mill, is designated as spacing 3, then the sizes of these spacings 1, 2, and 3, regardless of the shape of the inclined mill, are: spacing 3 > spacing 1 > spacing 2.

[0020] In other words, the rolling section is where the roll spacing is narrowest and where solid billets are pierced into hollow seamless tube blanks, resulting in the greatest circumferential and axial deformation. Furthermore, due to the characteristics of the inclined mill—where the roll spacing 2 of the rolling section must be larger than the roll spacing 3 of the rolling section compared to the stable section—the shape of the excessively deformed unstable sections at the front and rear ends cannot be adequately controlled using the rolling section. Moreover, shape defects in the unstable sections at the front and rear ends of the seamless tube blank formed by the inclined mill arise from factors such as the lubrication state of the tools, temperature distribution, uneven material composition, and heating temperature distribution. Therefore, it is difficult to prevent shape defects in the unstable sections at the front and rear ends by changing the individual inclined mill unit or rolling conditions.

[0021] Patent Document 1 discloses a method for applying forward pressure to the rear end of the billet during piercing rolling, but the control range is limited to the rolling in side and cannot address the rolling out side. Even assuming the installation of a device to pull out seamless billets that are not discharged, thereby removing the seamless billet from the inside of the inclined mill, the shape defects of the unstable rear portion, which cause poor discharge, are not eliminated, resulting in poor tool insertion and extrusion defects on the inner surface of the hot rolling process. The present invention was made in view of these problems, and its object is to provide an inclined rolling mill that can prevent shape defects of the unstable front and rear ends of the seamless billet, a method for manufacturing a seamless billet using the inclined rolling mill, and a method for manufacturing a seamless steel pipe.

[0022] Methods for solving problems

[0023] The main structure of the present invention that solves the above-mentioned problems is as follows.

[0024] [1] An inclined rolling apparatus includes an inclined piercing mill for performing piercing rolling and an inclined outer diameter mill connected after the inclined piercing mill. The inclined piercing mill has a plurality of piercing rolls arranged on a circumference centered on the rolling line, a mandrel arranged between the plurality of piercing rolls, and a rod for holding the mandrel. The inclined outer diameter mill has a plurality of outer diameter rolls arranged on a circumference centered on the rolling line. The distance LS (mm) between the rolling portion of the piercing rolls and the rolling portion of the outer diameter rolls satisfies the following equation (1), and the roll spacing DS of the plurality of outer diameter rolls satisfies the following equation (2).

[0025] 3×(1.2PO+2G)≤LS≤0.8BL…(1)

[0026] 1.2PO+2G>DS≥DB+2G…(2)

[0027] In equations (1) and (2) above, PO is the outer diameter of the mandrel (mm), G is the distance between the mandrel and the perforated roll (mm), BL is the length of the rod (mm), and DB is the outer diameter of the rod (mm).

[0028] [2] According to the inclined rolling equipment described in [1], the inclined piercing mill has more than three piercing rolls.

[0029] [3] According to the inclined rolling equipment described in [1] or [2], the inclined outer diameter mill has more than three outer diameter rolls.

[0030] [4] The inclined rolling mill according to any one of [1] to [3] further comprises a drive device for changing the roll spacing DS and a sensor for detecting the position of the seamless tube blank.

[0031] [5] The inclined rolling mill according to any one of [1] to [4] further comprises a drive device for driving the outer diameter rolls to rotate.

[0032] [6] A method for manufacturing a seamless tube blank, using any one of the inclined rolling mills described in [1] to [5], wherein the roll spacing DS of the outer diameter rolls is 84% ​​or more and 99% or less of the outer diameter dO2 of the stable portion of the seamless tube blank after piercing rolling.

[0033] [7] A method for manufacturing a seamless tube blank using the inclined rolling mill described in [5], wherein the outer diameter rolling speed of the inclined outer diameter mill is faster than the piercing rolling speed of the inclined piercing mill.

[0034] [8] A method for manufacturing a seamless steel pipe, wherein a steel raw material is pierced and rolled using any one of the inclined rolling equipment described in [1] to [5] to form a seamless tube blank, and the seamless tube blank is hot rolled to manufacture a seamless steel pipe.

[0035] Invention Effects

[0036] By using the inclined rolling apparatus of the present invention, shape defects in the front and rear unstable portions of the seamless billet generated after piercing rolling can be prevented. This eliminates defects in inner surface tool insertion and extrusion during the hot rolling process, thereby improving the productivity and yield of seamless steel pipes. Attached Figure Description

[0037] Figure 1 This is a diagram illustrating the piercing rolls used in an inclined piercing mill.

[0038] Figure 2 This is a schematic diagram illustrating an example of poor shape of the unstable front and rear ends of a seamless tube blank after piercing and rolling.

[0039] Figure 3 This is a schematic diagram illustrating other examples of poor shape of the front and rear unstable portions of a seamless tube blank after piercing and rolling.

[0040] Figure 4 This is a schematic diagram showing an example of the inclined rolling mill 10 of this embodiment.

[0041] Figure 5 This is a diagram illustrating the roll spacing DS in an inclined outer diameter rolling mill with two outer diameter rolls.

[0042] Figure 6 This is a diagram illustrating the roll spacing DS in an inclined outer diameter rolling mill with three outer diameter rolls. Detailed Implementation

[0043] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. Figure 1 This is a diagram illustrating the piercing rolls used in an inclined piercing mill. Figure 1 (a) is a diagram illustrating the shape and cross angle of the piercing rolls, which is viewed from a position where the rolling line and the piercing rolls do not overlap. Figure 1 (b) is a diagram illustrating the tilt angle of the piercing roll, viewed from the position where the rolling line and the piercing roll overlap. In this embodiment, the roll used in the tilted piercing mill is referred to as the piercing roll.

[0044] The piercing rolls used in inclined piercing mills have Figure 1 The cone shape shown in (a-1) and Figure 1 The barrel shape shown in (a-2) is as follows. Figure 1 As shown in (a), the conical piercing rolls are generally configured with an intersection angle β relative to the rolling line. Figure 1 (b) shows a barrel-shaped piercing roll, but both conical and barrel-shaped piercing rolls are generally configured with an inclination angle α relative to the rolling line.

[0045] Depending on the inclination angle α and the rotational speed of the piercing rolls, the workpiece or the seamless billet after piercing rolls advances along the rolling line in the direction of the arrow. The inclination angle α and the crossing angle β are determined by the shape of the inclined piercing mill and the rolling conditions corresponding to the workpiece. The number of piercing rolls also varies depending on the purpose, but generally, it is common to have a pair of rolls facing each other across the rolling line and a pair of guides in their orthogonal direction, or to have no guides and more than two piercing rolls arranged on a circumference centered on the rolling line.

[0046] exist Figure 1In the conical and barrel-shaped piercing rolls shown in (a), the side where the workpiece enters the piercing rolls is designated as the rolling in side. The section where rolling takes place between the mandrel and the piercing rolls is designated as the rolling section. The section from which the pierced seamless billet exits is designated as the rolling out side. Regardless of the type of inclined mill, the piercing rolls on the rolling in side are arranged such that the spacing between the piercing rolls expands towards the rolling in side in order to allow the workpiece to be gripped. The piercing rolls in the rolling section have the narrowest roll spacing, and the piercing mandrel is located near the rolling line of the rolling section. The rolling out side is arranged such that the spacing between the piercing rolls expands towards the rolling out side, configured to allow the pierced seamless billet to exit from the inclined piercing mill.

[0047] Next, the stable and unstable portions of the seamless tube blank after piercing rolling will be explained. Seamless tube blanks manufactured by rolling using a piercing mill will form a stable portion with a stable shape in the central part and an unstable portion with an unstable shape at the front and rear ends.

[0048] Figure 2 This is a schematic diagram illustrating an example of poor shape of the unstable portions at the front and rear ends of a seamless tube blank after piercing and rolling. Figure 2 In the diagram, (a) is the front view, (b) is the top view, (c) is the left view, and (d) is the right view.

[0049] Figure 2 The image shows a seamless tube blank with elliptical unstable sections at both ends. The ellipticization of the unstable sections primarily occurs during piercing rolling using two opposing piercing rolls. This ellipticization occurs when the front and rear ends extend more circumferentially than the stable sections as the tube blank passes through the rolling section and through the rolled-out side.

[0050] Figure 3 This is a schematic diagram illustrating other examples of shape defects in the front and rear unstable sections of a seamless tube blank after piercing and rolling. Figure 3 In the diagram, (a) is the front view, (b) is the top view, (c) is the left view, and (d) is the right view.

[0051] Figure 3 The image shows a seamless tube blank with triangular unstable sections at both ends. This triangularization primarily occurs during piercing rolling using three piercing rolls. There are three points on the rolled side that contact the piercing rolls and those that do not, resulting in triangular unstable sections at both the front and rear ends. Because the workpiece is pierced while rotating in a spiral motion, the vertices of the triangular shapes of the unstable sections are inconsistent at both ends, and their phases are often misaligned. Regardless of the number of piercing rolls, the shape defects of the unstable sections are more pronounced at the rear end than at the front end.

[0052] In inclined piercing mills that perform piercing rolling, it is difficult to eliminate shape defects in the unstable sections at the front and rear ends simply by changing the number, shape, and configuration of the piercing rolls. As mentioned above, if shape defects occur in the unstable sections at the front and rear ends, an operational failure will occur in the next hot rolling process, where the inner surface tool cannot be inserted.

[0053] Next, the cause of the aforementioned operational failure will be explained. Let the minimum inner diameter of the front unstable section be dT, the minimum inner diameter of the rear unstable section be dB, and the diameter of the inner surface tool inserted into the seamless tube blank during the hot rolling process be dN. During the hot rolling process, if the minimum inner diameters dT of the front unstable section and dB of the rear unstable section are smaller than the diameter dN of the inner surface tool, an operational failure will occur where the inner surface tool cannot be inserted into the seamless tube blank. Therefore, to prevent operational failures, it is preferable to manage the minimum inner diameters dT and dB of the unstable sections in a manner that satisfies dN ≤ dT and dN ≤ dB.

[0054] On the other hand, even if dN≤dT and dN≤dB are successfully satisfied, if dT and dB are too large relative to the inner diameter dO1 of the stable section, operational failures such as extrusion into the rolls and guides may occur in the next hot rolling process. Therefore, it is more preferable to manage the minimum inner diameters dT and dB of the unstable sections at both ends in a manner that further satisfies 1.2dO1≥dT and 1.2dO1≥dB.

[0055] The inner perimeter of the unstable parts at the front and rear ends will not be significantly smaller than that of the stable parts. Therefore, if the shape of the unstable parts at the front and rear ends can be corrected to be similar to that of the stable parts, i.e., a perfect circle, then it can be assumed that the minimum inner diameters dT and dB of the unstable parts satisfy dN≤dT, dN≤dB and 1.2dO1≥dT, 1.2dO1≥dB.

[0056] Figure 4 This is a schematic diagram illustrating an example of the inclined rolling mill 10 of this embodiment. Figure 4 In order to show the position of the mandrel 26, only the workpiece 40 is shown in cross section. In order to correct the shape of the unstable front and rear ends of the inclined rolling mill 10 to a shape similar to the shape of the stable part, i.e., a perfect circle, the inclined outer diameter rolling mill 30 is provided after the inclined piercing mill 20 for piercing rolling.

[0057] The inclined piercing mill 20 is an apparatus for piercing and rolling a workpiece 40 made of steel raw material. The inclined piercing mill 20 has two piercing rolls 22 and 24 facing each other around the rolling line 50, a mandrel 26 disposed between the piercing rolls 22 and 24, a rod 28 holding the mandrel 26, and a drive device (not shown) for driving the two piercing rolls 22 and 24. Figure 4 The diagram shows an example of an inclined piercing mill 20 with two piercing rolls 22 and 24 facing each other around the rolling line 50. However, it is not limited to this; more than three piercing rolls can be used on the circumference centered on the rolling line 50. With three piercing rolls, the surface quality of the inner surface of the seamless tube blank after piercing rolling is improved compared to the case with two piercing rolls. Since the outer surface of the workpiece 40 can be held at three points during piercing rolling, the piercing rolling is stable, and the surface quality of the outer surface is also improved. The same effect can be obtained as long as the number of piercing rolls is three or more, so no upper limit is set. However, increasing the number of piercing rolls reduces the roll diameter, which in turn reduces the roll shaft diameter, thus decreasing the load-bearing capacity. Therefore, the number of piercing rolls in an inclined piercing mill is preferably three or four.

[0058] The inclined outer diameter mill 30 has two outer diameter rolls 32 and 34 facing each other with the rolling line 50 as the center. The inclined piercing mill 20 and the inclined outer diameter mill 30 are arranged such that the distance LS (mm) between the rolling portions of the piercing rolls 22 and 24 and the rolling portions of the outer diameter rolls 32 and 34 satisfies the following equation (3). The outer diameter rolls 32 and 34 are arranged such that the roll spacing DS (mm) between the rolling portions of the outer diameter rolls 32 and 34 satisfies the following equation (4).

[0059] 3dO2≤LS≤0.8LH…(3)

[0060] dO2>DS≥dO2-(dO2-DB-2t)…(4)

[0061] In equations (3) and (4) above, dO2 is the outer diameter (mm) of the stable portion of the seamless tube blank obtained by piercing rolling using the inclined piercing mill 20. LH is the length (mm) of the seamless tube blank after piercing rolling. DB is the diameter (mm) of the rod 28. t is the wall thickness (mm) of the seamless tube blank after piercing rolling. The above-mentioned dO2, LH and t are the dimensions of the seamless tube blank in which the workpiece 40 used in the inclined rolling equipment 10 is pierced and rolled using the inclined piercing mill 20 without undergoing outer diameter rolling of the inclined outer diameter mill 30.

[0062] exist Figure 4The diagram shows an example of an inclined outer diameter mill 30 with two outer diameter rolls 32 and 34 facing each other around the rolling line 50. However, it is not limited to this; more than three outer diameter rolls can be used on the circumference centered on the rolling line 50. By having three outer diameter rolls in the inclined outer diameter mill 30, the outer surface of the seamless tube blank can be held at three points during outer diameter rolling, thus stabilizing the outer diameter rolling and improving the surface quality. Using three outer diameter rolls further provides circumferential compression deformation to the seamless tube blank, thereby increasing the correction effect of unstable sections at the front and rear ends. The same effect can be obtained as long as the number of outer diameter rolls is three or more, so no upper limit is required. However, increasing the number of outer diameter rolls reduces the roll diameter, resulting in a smaller roll shaft diameter and thus a decrease in load-bearing capacity. Therefore, the number of outer diameter rolls in the inclined outer diameter mill is preferably three or four.

[0063] The rolling section of the perforated rolls 22 and 24 is the part of the perforated rolls 22 and 24 with the narrowest roll spacing in the direction perpendicular to the rolling line 50. Similarly, the rolling section of the outer diameter rolls 32 and 34 is the part of the outer diameter rolls 32 and 34 with the narrowest roll spacing.

[0064] Figure 5 This is a diagram illustrating the roll spacing DS in an inclined outer diameter rolling mill with two outer diameter rolls 36. Figure 5 (a) is the front view. Figure 5 (b) is a side view. For example... Figure 5 As shown in (b), when two outer diameter rolls 36 are provided, the roll spacing DS of the outer diameter rolls 36 is the shortest distance between each roll at the position (rolling section) where the outer diameter rolls 36 are narrowest.

[0065] Figure 6 This is a diagram illustrating the roll spacing DS in an inclined outer diameter rolling mill with three outer diameter rolls 36. Figure 6 (a) is the front view. Figure 6 (b) is a side view. Figure 6 (c) is Figure 6 (b) is an enlarged view of part A. Figure 6 In (a), to illustrate the relationship between the rolled workpiece 40 and the outer diameter rolls 36, one of the three outer diameter rolls 36 is not shown. Figure 6 As shown in (c), when there are three outer diameter rolls 36, the roll spacing DS of the outer diameter rolls 36 is the diameter of the circle tangent to each outer diameter roll 36 at the position (rolling section) where the outer diameter rolls 36 are narrowest. The roll spacing DS is the same when there are four or more outer diameter rolls 36.

[0066] The shape defects of the unstable portions are formed during the process of the piercing rolls 22 and 24 extending from the rolling section to the roll-out side. Concerned about this, the inventors conceived of providing outer diameter rolls 32 and 34 and utilizing the rolling section of these outer diameter rolls 32 and 34 to correct the shape of the front and rear unstable portions. That is, at the rolling section of the inclined piercing mill 20 where piercing rolling is performed, large circumferential deformation in the tube blank, resulting in shape defects of the front and rear unstable portions caused by piercing rolling, is unavoidable. Therefore, the inventors considered that by reducing the outer diameter of the seamless tube blank and compressing it using the rolling section of the inclined outer diameter mill 30 following piercing rolling, the shape defects of the front and rear unstable portions could be corrected to a good shape.

[0067] To compress and deform the seamless tube blank by reducing its outer diameter using the rolling section of the inclined outer diameter mill 30, the outer diameter rolls 32 and 34 should be configured such that the roll spacing DS of the outer diameter rolls 32 and 34 satisfies the above formula (4). The above formula (4) means that the roll spacing DS of the outer diameter rolls 32 and 34 of the inclined outer diameter mill 30 is smaller than the outer diameter dO2 of the stable part of the seamless tube blank after piercing rolling, and is set to be greater than the outer diameter obtained by subtracting (dO2-DB-2t), which represents the gap between the inner diameter dO1 of the seamless tube blank formed by piercing rolling and the rod 28 penetrating the interior, from the outer diameter dO2 of the stable part of the seamless tube blank. That is, the above formula (4) is the condition for reducing the wall thickness without using the rolling section of the inclined outer diameter mill 30, and is also the condition for reducing the outer diameter of the unstable parts at the front and rear ends to be smaller than the outer diameter dO2 of the stable part of the seamless tube blank.

[0068] As described above, the poor shape of the unstable front and rear ends is caused by the extension of the seamless tube blank in the circumferential direction at the rolling section of the piercing rolls 22 and 24. That is, by satisfying the above equation (4) and reducing the outer diameter without using the rolling section of the outer diameter rolls to reduce the wall thickness, the shape of the unstable front and rear ends can be corrected and the shape can be made appropriate. In order to obtain sufficient shape correction effect of the unstable front and rear ends, it is preferable to set the roll spacing DS of the outer diameter rolls 32 and 34 to a range of 84% or more and 99% or less of dO2. By setting the roll spacing DS to this range, the circumferential wall thickness variation of the seamless tube blank generated during piercing rolling can also be improved.

[0069] After outer diameter rolling, the wall thickness of the billet increases according to the outer diameter rolling (reduction) rate. If the roll spacing DS of the outer diameter rolls is less than 84% of dO2, the thickness increase after outer diameter rolling is large, the thinning amount of the rolling process following outer diameter rolling increases, and the equipment load increases, which is not preferred. When DB is reduced in order to satisfy equation (4), the rigidity of the rod decreases, and the risk of rod breakage during rolling increases. Therefore, the roll spacing DS of the outer diameter rolls is preferably 84% or more of dO2, and 90% or more is preferred from the viewpoint of the increase in wall thickness after outer diameter rolling and rod breakage.

[0070] On the other hand, if the roll spacing DS of the outer diameter rolls exceeds 99% of dO2, the outer diameter rolling rate decreases, and the desired correction effect cannot be obtained, which is therefore undesirable. If the outer diameter rolling rate decreases, the contact with the outer diameter rolls weakens, friction no longer functions, and a speed difference between the roll circumferential speed and the outer surface speed of the tube is generated, which becomes a cause of scratches. Moreover, in the case of driving an outer diameter mill, if the outer diameter rolling rate decreases, the driving force for pushing the tube cannot be obtained, which is also undesirable. Therefore, the roll spacing DS of the outer diameter rolls is preferably 99% or less of dO2, more preferably 95% or less.

[0071] In the inclined rolling mill 10 of this embodiment, the distance LS between the rolling portions of the pierced rolls 22 and 24 and the rolling portion of the outer diameter roll needs to satisfy the above equation (3). Regarding this relationship, the following is also used... Figure 4 To illustrate.

[0072] The above formula (3) is based on the following conditions: the distance LS between the rolling section of the piercing rolls 22 and 24 and the rolling section of the outer diameter rolls 32 and 34 is set to less than 0.8 times the length LH of the seamless tube blank after piercing rolling, and is set to more than 3 times the outer diameter dO2 of the stable section of the seamless tube blank after piercing rolling.

[0073] First, the distance LS between the rolling portions of the piercing rolls 22 and 24 and the rolling portions of the outer diameter rolls 32 and 34 will be set to 0.8 times or less of the length LH of the seamless billet after piercing and rolling. Even if the front-end unstable portion is long, it exists within 20% of the front end of the length LH of the manufactured seamless billet. Therefore, when correcting the shape of the front-end unstable portion, LS is set to 80% or less of LH. Thus, without having a drive device in the inclined outer diameter mill 30 to rotate the outer diameter rolls 32 and 34, the front-end unstable portion can be rolled using the helical rotation of the seamless billet pierced and rolled by the inclined piercing mill 20. By using the inclined outer diameter mill 30 to roll the front-end unstable portion, shape defects of the front-end unstable portion can be corrected.

[0074] Making the distance between the rolling sections of the piercing rolls 22 and 24 and the rolling sections of the outer diameter rolls 32 and 34 zero would cause the devices to interfere with each other, which is theoretically impossible. The size of the inclined piercing mill 20, which performs piercing rolling, increases according to the outer diameter dO2 of the stable portion of the seamless billet to be rolled. The interference between the devices depends on the size of the inclined piercing mill 20 and the inclined outer diameter mill 30. The magnitude related to the interference between the inclined piercing mill 20 and the inclined outer diameter mill 30 can be said to be proportional to the outer diameter dO2 of the stable portion of the seamless billet produced by the inclined piercing mill 20. Therefore, the distance between the rolling sections of the piercing rolls 22 and 24 and the rolling sections of the outer diameter rolls 32 and 34 should also be managed by the outer diameter dO2 of the stable portion of the seamless billet after piercing rolling. Therefore, in order to stably perform piercing rolling and outer diameter rolling while avoiding interference between the inclined piercing mill 20 and the inclined outer diameter mill 30, the distance LS between the rolling section of the piercing rolls 22 and 24 and the rolling section of the outer diameter rolls 32 and 34 needs to be set to 3×dO2 or more.

[0075] In this way, the distance LS between the rolling section of the piercing rolls 22 and 24 and the rolling section of the outer diameter rolls 32 and 34 satisfies the above formula (3) and the roll spacing DS of the outer diameter rolls 32 and 34 satisfies the above formula (4). This can correct the shape of the unstable front and rear ends of the seamless tube blank after piercing and rolling, and can prevent process failures in the hot rolling process of the next process while manufacturing seamless steel pipes.

[0076] Here, the inner diameter dO1 of the stable portion of the seamless tube blank is larger than the maximum outer diameter PO of the mandrel 26 used in the inclined piercing mill 20. The inner diameter of the seamless tube blank is 1.0 to 1.2 times the maximum outer diameter PO of the mandrel 26. Therefore, 1.2PO + 2t is equal to the maximum diameter dO2 of the stable portion in the seamless tube blank after piercing rolling. During piercing rolling, the tube blank wall thickness is formed between the mandrel 26 and the piercing rolls 22 and 24. Therefore, the minimum distance G between the mandrel 26 and the piercing rolls 22 and 24 is equal to the wall thickness t of the formed seamless tube blank.

[0077] If the length of the rod 28 holding the mandrel 26 is set as BL, then BL is equal to the maximum length of the seamless tube blank after piercing and rolling, LH. The outer diameter dO2 and wall thickness t of the stable part of the seamless tube blank after piercing and rolling can be predicted based on these geometric relationships. If these values ​​are used, the above equation (3) can be expressed by the following equation (1), and the above equation (4) can be expressed by the following equation (2).

[0078] 3×(1.2PO+2G)≤LS≤0.8BL…(1)

[0079] 1.2PO+2G>DS≥DB+2G…(2)

[0080] In equations (1) and (2) above, PO is the outer diameter of the mandrel 26 (mm), G is the distance between the mandrel 26 and the piercing rolls 22 and 24 (mm), BL is the length of the rod 28 (mm), and DB is the outer diameter of the rod 28 (mm). The distance G between the mandrel 26 and the piercing rolls 22 and 24 can be calculated by dividing the value obtained by subtracting the outer diameter PO of the mandrel 26 from the outer diameter dO2 of the stabilizing part by 2.

[0081] Similar to equations (3) and (4) above, by having the distance LS between the rolling portions of the piercing rolls 22 and 24 and the rolling portions of the outer diameter rolls 32 and 34 satisfy equation (1) above and the roll spacing DS of the outer diameter rolls 32 and 34 satisfy equation (2) above, the shape of the unstable front and rear ends of the seamless tube blank after piercing rolling can also be corrected, and seamless steel pipes can be manufactured while preventing process failures in the hot rolling process of the next process.

[0082] In the inclined piercing mill 20, regardless of the shape and number of the piercing rolls 22 and 24, shape defects of the unstable front and rear ends will occur, so it can be used regardless of the shape of the inclined piercing mill 20. The inclined outer diameter mill 30 for outer diameter rolling needs to roll the outer diameter of the seamless tube blank after piercing rolling, so at least two outer diameter rolls are required. The roll type of the outer diameter rolls used in the inclined outer diameter mill 30 can be barrel type or conical type. Regarding the configuration of the outer diameter rolls, it can also be changed within the range where the roll spacing DS of the outer diameter rolls satisfies the above formula (2). As for other configurations of the outer diameter rolls, there is an inclination angle α (refer to Figure 1 ) and cross angle β. Regarding the tilt angle α, if the tilt angle of the piercing roll is set as α1 (°), then the tilt angle α2 (°) of the outer diameter roll preferably satisfies the following equation (5).

[0083] α1>α2…(5)

[0084] Thus, by satisfying equation (5) above with the tilt angle α1 of the piercing roll and the tilt angle α2 of the outer diameter roll, the correction effect of the unstable parts at the front and rear ends can be further improved. When the cross angle β1 of the piercing roll is set to a positive value, it is preferable to set the cross angle β2 of the outer diameter roll to a negative value (opposite to the tilt direction of β1 relative to the rolling line). As a result, the circumferential compressive force on the seamless tube blank increases, which can further improve the correction effect of the unstable parts at the front and rear ends. However, if β1 and β2 are increased, it becomes difficult to install the piercing roll and the outer diameter roll toward the rotating shaft. Therefore, β1 and β2 are preferably in the range of -25° or more and 25° or less.

[0085] The inclined outer diameter mill 30 may also include a drive device for changing the roll spacing DS of the outer diameter rolls 32 and 34 and a sensor for detecting the position of the seamless billet. Furthermore, the roll spacing DS of the outer diameter rolls 32 and 34 may be made the same as the diameter DB of the rod 28 of the inclined piercing mill 20 until the distance between the seamless billet after piercing rolling and the outer diameter rolls 32 and 34 becomes below a set distance. When the sensor detects that the distance between the seamless billet and the outer diameter rolls 32 and 34 has become below a set distance, the drive device changes the roll spacing DS of the outer diameter rolls 32 and 34 to a roll spacing DS that satisfies the above formula (2). Thus, the vibration of the rod 28 holding the mandrel 26 during piercing rolling can be suppressed using the outer diameter rolls 32 and 34, thereby improving product dimensional accuracy and production stability.

[0086] The seamless tube blank rotates in a spiral shape via the inclined piercing mill 20. Therefore, the inclined outer diameter mill 30 may not have a drive device to drive the outer diameter rolls 32 and 34 to rotate. As long as the distance LS between the rolling section of the piercing roll and the rolling section of the outer diameter roll satisfies the above equation (1), the shape defect of the unstable front end will be corrected by the rotational force of the inclined piercing mill 20.

[0087] Furthermore, the inclined outer diameter mill 30 may also have a drive device for rotating the outer diameter rolls 32 and 34. By having a drive device for rotating the outer diameter rolls 32 and 34, the following effects (1) to (3) can be obtained.

[0088] (1) Even if a shape defect occurs in the unstable part at the rear end, resulting in a failure to discharge the seamless tube blank from the rolling exit side of the inclined piercing mill 20, the outer diameter rolls 32 and 34 of the inclined outer diameter mill 30 can be driven to pull out the seamless tube blank. As a result, the failure to discharge the seamless tube blank from the exit side of the piercing rolls 22 and 24 can be suppressed.

[0089] (2) By driving the outer diameter rolls 32 and 34 to rotate, the rear unstable part can also be rolled with the outer diameter rolls 32 and 34, just like the front unstable part, so that the shape defects of the rear unstable part can be corrected.

[0090] (3) In the inclined piercing mill 20, a fault may occur in which piercing rolling stops at the rolling inlet or rolling section (without the round billet being fully pierced). In this case, if the front end of the seamless tube billet reaches the outer diameter rolls 32 and 34, the piercing rolling stop fault can be suppressed by driving the outer diameter rolls 32 and 34 to rotate.

[0091] When the inclined outer diameter mill 30 has a drive device, if the circumferential speed of the piercing rolls 22 and 24 is set to V1 (m / min) and the circumferential speed of the outer diameter rolls 32 and 34 is set to V2 (m / min), then V1 and V2 preferably satisfy the following equation (6).

[0092] V1≤V2…(6)

[0093] By satisfying equation (6) above with V1 and V2 (i.e., making the outer diameter rolling speed of the inclined outer diameter mill 30 faster than the piercing rolling speed of the inclined piercing mill 20), piercing rolling and outer diameter rolling can be carried out stably. On the other hand, if V1>V2, the seamless tube blank after piercing rolling will be strongly pressed in by the outer diameter rolls 32 and 34, which may cause adverse conditions on the equipment, and is therefore not preferred. The roll circumferential speed V2 is the circumferential speed of the outer diameter roll. Therefore, if the surface shape of the outer diameter roll is consistent with the circumferential speed V2, the roll diameters of the two or more outer diameter rolls used in the inclined outer diameter mill 30 can also be different. However, if rolls with significantly different outer diameters are used, there will be a difference in the amount of deflection in the direction perpendicular to the load due to the thickness of the roll shaft, and the dimensional accuracy of the tube after outer diameter rolling and the setting accuracy of DS will decrease, which is therefore not preferred. Therefore, when using outer diameter rolls with different diameters, it is preferable to set the roll diameter to be 50% or more of the largest outer diameter roll diameter, and more preferably to set it to be 80% or more of the largest outer diameter roll diameter. The circumferential speed of each outer diameter roll is preferably set within ±10% of the average circumferential speed. This suppresses warping of the tube after outer diameter rolling. When performing the specified outer diameter rolling, the circumferential speed of the outer diameter rolls and the rotational speed of the outer surface of the tube are aligned through friction between the tube and the rolls. Therefore, even when driving the outer diameter rolls to make V2 within the preferred range, it is not necessary to drive all the outer diameter rolls; driving one or more outer diameter rolls is sufficient.

[0094] To transport the seamless tube blank after piercing and rolling to the next process, namely the hot rolling process, and to insert the inner surface tool, the minimum inner diameter of the seamless tube blank on the tool insertion side must be greater than or equal to the diameter of the inserted inner surface tool. The diameter of the inner surface tool inserted into the seamless tube blank is determined based on the inner diameter dO1 of the stable section. That is, the management references for the minimum inner diameters dT and dB of the seamless tube blank are determined relative to the inner diameter dO1 of the stable section. If the inner diameters of the front and rear unstable sections after being straightened by the inclined outer diameter mill 30 are too large relative to the stable section, they will be extruded into the outer diameter rolls, becoming a cause of failure. Therefore, the inner diameters of the front and rear unstable sections after being straightened by the inclined outer diameter mill 30 are preferably determined based on the inner diameter dO1 of the stable section. From the viewpoint of improving the operational stability of the piercing and rolling processes, the range of the inner diameters of the front and rear unstable sections based on the inner diameter dO1 of the stable section has been confirmed. The results confirmed that, to improve the operational stability of the piercing rolling and hot rolling processes, the inner diameter of the front and rear unstable sections after straightening is preferably within the range of 0.9 × dO1 or more and 1.1 × dO1 or less. More preferably, the inner diameter of the front and rear unstable sections is within the range of 0.95 × dO1 or more and 1.05 × dO1 or less. This allows for further improvement in the operational stability of the piercing rolling and hot rolling processes.

[0095] For manufacturing seamless tube blanks using the inclined rolling mill 10 of this embodiment, any material capable of piercing rolling can be used, regardless of its type. Similarly, all piercing rolling temperatures can be applied.

[0096] Example

[0097] Next, embodiments will be described. Seamless tube blanks were manufactured using an inclined rolling apparatus equipped with an inclined piercing mill having two or three piercing rolls and an inclined outer diameter mill having two or three outer diameter rolls. Using carbon steel round billets with an outer diameter of 150 mm and a length of 2500 mm as the rolling material, seamless tube blanks of a comparative example that underwent only piercing rolling after being heated to 1200°C and seamless tube blanks of an inventive example that underwent piercing rolling and subsequent outer diameter rolling after being heated to 1200°C were manufactured.

[0098] First, the shapes of the unstable portions at the front and rear ends of the seamless tube blank, which underwent only piercing rolling in the comparative example, were confirmed. Using a piercing mill with two or three piercing rolls, the piercing rolling conditions were adjusted to ensure good reproducibility of the shapes of both the stable and unstable portions and to obtain the same shape. Seamless tube blanks were manufactured under these piercing rolling conditions. The outer diameter of the stable portion of the manufactured seamless tube blank was 180 mm, the wall thickness was 11 mm, and the length was 7656 mm. Maintaining a mandrel diameter of 128 mm, tapered piercing rolls were used, with the inclination angle set to 9° and the cross angle set to 20°. The inner diameter of the stable portion and the minimum inner diameter of the unstable portions at both ends of the manufactured seamless tube blank were measured, and the ratio of the minimum inner diameter of the unstable portions to the inner diameter of the stable portion was calculated.

[0099] Next, the seamless tube blank, which had undergone piercing rolling without changing the piercing rolling conditions, was rolled to its outer diameter using an inclined outer diameter mill. The inner diameter of the stable portion and the minimum inner diameter of the unstable portions at both ends of the manufactured seamless tube blank were measured, and the ratio of the minimum inner diameter of the unstable portions at both ends to the inner diameter of the stable portion was calculated. The outer diameter PO of the mandrel of the piercing mill, the distance G between the mandrel and the piercing rolls, the outer diameter DB of the bar, and the length BL of the bar are shown in Table 1 below. The distances in Table 1 are obtained by subtracting the outer diameter DB of the bar from the inner diameter dO1 of the seamless tube blank rolled by the piercing mill. The number of rolls in the inclined piercing mill, the number of rolls in the inclined outer diameter mill, the roll spacing DS of the outer diameter rolls, the inclination angle of the outer diameter rolls, the crossing angle of the outer diameter rolls, the distance LS between the rolling section of the piercing rolls and the rolling section of the outer diameter rolls, the presence or absence of a drive device for rotating the outer diameter rolls, the peripheral speed ratio (V2 / V1), and whether the conditions 1.2PO+2G>DS≥DB+2G and DS / dO2×100 are satisfied are shown in Table 2 below. In Table 2, “○” in the column for 1.2PO+2G>DS≥DB+2G indicates that 1.2PO+2G>DS≥DB+2G is satisfied, and “×” indicates that 1.2PO+2G>DS≥DB+2G is not satisfied. On the other hand, although not shown in Table 2, in this embodiment, 3×(1.2PO+2G)≤LS≤0.8BL is satisfied in all examples.

[0100] (DS / dO2)×100 is the ratio (%) of the roll spacing DS of the outer diameter roll to the outer diameter dO2 of the stable portion of the seamless billet after piercing rolling. In this embodiment, in Comparative Examples (No. 1 to 4), the roll spacing DS of the outer diameter roll was controlled to be in the range of 82% or more and 99% or less of dO2 (180 mm), and in Inventive Examples (No. 5 to 19), it was controlled to be in the range of 84% or more and 99% or less. The ratio of the minimum inner diameter of the front and rear unstable portions to the inner diameter of the stable portion in Comparative Examples (No. 1 to 4) and Inventive Examples (No. 5 to 19) is shown in Table 3 below.

[0101] [Table 1]

[0102]

[0103] [Table 2]

[0104]

[0105]

[0106] As shown in Table 3, in the inventive examples (No. 5-19), immediately after piercing rolling, the minimum inner diameter of the unstable front and rear ends is more than 10% smaller than the inner diameter of the stable part, while after outer diameter rolling, the minimum inner diameter of the unstable front and rear ends becomes less than 10% smaller than the inner diameter of the stable part. Therefore, in the inventive examples (No. 5-19), the subsequent hot rolling process can be carried out stably, and seamless steel pipes are successfully manufactured. On the other hand, in the comparative examples (No. 1-4) that do not satisfy the above formula (2), even after outer diameter rolling, the minimum inner diameter of the unstable front and rear ends is more than 10% smaller than the inner diameter of the stable part, and the unstable front and rear ends cannot be corrected to a good shape. In this regard, poor insertion of the inner surface tool occurs in the subsequent hot rolling process, and some of these failures cause production stoppages.

[0107] Explanation of reference numerals in the attached figures

[0108] 10 Inclined Rolling Mill

[0109] 20 Inclined Piercing Mill

[0110] 22-perforated rolls

[0111] 24-perforated rolls

[0112] 26 top

[0113] 28 strokes

[0114] 30 Inclined Outer Diameter Rolling Mill

[0115] 32 outer diameter rolls

[0116] 34 outer diameter rolls

[0117] 36 outer diameter rolls

[0118] 40 rolled pieces

[0119] 50 rolling line.

Claims

1. An inclined rolling apparatus comprising: an inclined piercing mill that performs piercing rolling; and an inclined outer diameter mill that is connected to the inclined piercing mill, wherein the inclined piercing mill has a plurality of piercing rolls provided on a circumference centered on a rolling line, a plug provided between the plurality of piercing rolls, and a rod that holds the plug, the inclined outer diameter mill has a plurality of outer diameter rolls provided on a circumference centered on the rolling line, a distance LS between a rolling portion of the piercing roll and a rolling portion of the outer diameter roll satisfies the following expression (1), the distance LS is in mm, a roll interval DS of the plurality of outer diameter rolls satisfies the following expression (2), the roll interval DS is in mm, 3 x (1.2PO + 2G) ≤ LS ≤ 0.8BL... (1) 1.2PO + 2G > DS ≥ DB + 2G... (2) in the expressions (1) and (2), PO is an outer diameter of the plug, in mm, G is an interval between the plug and the piercing roll, in mm, BL is a length of the rod, in mm, and DB is an outer diameter of the rod, in mm.

2. The inclined rolling apparatus according to claim 1, wherein the inclined piercing mill has three or more piercing rolls.

3. The inclined rolling apparatus according to claim 1 or 2, wherein the inclined outer diameter mill has three or more outer diameter rolls.

4. The inclined rolling apparatus according to claim 1 or 2, wherein the inclined outer diameter mill further has a driving device that changes the roll interval DS and a sensor that detects a position of a seamless pipe blank.

5. The inclined rolling apparatus according to claim 3, wherein the inclined outer diameter mill further has a driving device that changes the roll interval DS and a sensor that detects a position of a seamless pipe blank.

6. The inclined rolling apparatus according to claim 1 or 2, wherein the inclined outer diameter mill further has a driving device that drives the outer diameter rolls to rotate.

7. The inclined rolling apparatus according to claim 3, wherein the inclined outer diameter mill further has a driving device that drives the outer diameter rolls to rotate.

8. The inclined rolling apparatus according to claim 4, wherein the inclined outer diameter mill further has a driving device that drives the outer diameter rolls to rotate.

9. The inclined rolling apparatus according to claim 5, wherein the inclined outer diameter mill further has a driving device that drives the outer diameter rolls to rotate.

10. A method for manufacturing a seamless pipe blank, using the inclined rolling apparatus according to any one of claims 1 to 9, wherein a roll interval DS of the outer diameter roll is 84% or more and 99% or less of an outer diameter dO2 of a stable portion of the seamless pipe blank after the piercing rolling.

11. A method for manufacturing a seamless pipe blank, using the inclined rolling apparatus according to any one of claims 6 to 9, wherein an outer diameter rolling speed of the inclined outer diameter mill is faster than a piercing rolling speed of the inclined piercing mill.

12. A method for manufacturing a seamless steel pipe, wherein a seamless pipe blank is formed by performing piercing rolling on a steel raw material using the inclined rolling apparatus according to any one of claims 1 to 9, and the seamless steel pipe is manufactured by performing hot rolling on the seamless pipe blank. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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