Tilt rolling method and method for manufacturing seamless steel pipe

Abstract

To provide an inclined rolling method capable of suppressing an outer surface defect based on a surface defect of a material to be rolled, and a method for manufacturing a seamless steel pipe.SOLUTION: The inclined rolling method for acquiring a hollow element pipe 2A from a material 1A to be rolled satisfies the following expressions (1) and (2). 0.87×D≤G≤0.97×D ... (1), 0.49×G≤L≤0.92×G ... (2). In the expressions (1) and (2), D(mm) denotes the outer diameter of a pipe axial direction perpendicular section of the material to be rolled, G(mm) denotes a roll interval between a pair of rolling rolls for holding the material to be rolled between, and L(mm) denotes a length from the pipe axial direction perpendicular section of a roll interval measuring position to the tip of a plug inserted into the material to be inserted.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a technology of an inclined rolling method performed in the manufacturing process of seamless steel pipes. Specifically, it relates to a technology of a rolling method for suppressing external surface flaws that occur when rolling a billet or a hollow shell having surface defects with a piercer, an elongator, etc., which are inclined rolling mills installed in the seamless steel pipe manufacturing line.

Background Art

[0002] In the manufacture of seamless steel pipes, first, a billet is heated in a rotary furnace and then pierced with a piercer to obtain a shell. Then, the shell is subjected to stretch rolling with a mandrel mill, an elongator, a plug mill, etc. to reduce the shell wall thickness. After the shell with reduced wall thickness is heated in a reheating furnace, it is rolled to a predetermined outer diameter by a hot stretch reducer, a sizer, etc. to manufacture a seamless steel pipe.

[0003] The above-mentioned piercer and elongator are mills that roll a billet or a hollow shell, which is a material to be rolled, with a plug positioned between two barrel-shaped rolls or cone-shaped rolls whose rotating shafts are inclined to each other. Also, a guide shoe is installed to prevent the material to be rolled from swinging during rolling. The material to be rolled is reduced in diameter by biting into the above-mentioned barrel-shaped roll or cone-shaped roll and advances in the rolling direction while being twisted in the circumferential direction. When the material to be rolled advances in the rolling direction, it is rolled by the barrel-shaped roll or cone-shaped roll and the plug. During this rolling, since the material to be rolled is stretched in both the circumferential direction and the rolling direction, it undergoes very complex deformation.

[0004] In seamless steel pipes, external surface defects may be formed, and since these external surface defects cause a decrease in yield, it is desirable to reduce and suppress them. There are various causes for external surface defects in seamless steel pipes. Specifically, external surface defects based on scratches due to adhesion of scale on the surface of tools such as rolls and plugs, external surface defects based on rolling conditions such as ductile fracture during rolling, external surface defects resulting from material deterioration based on inappropriate heating in a heating furnace, etc., and external surface defects caused by defects originating from materials such as initial defects on the surface of the material to be rolled such as billets (hereinafter, also simply referred to as surface defects).

[0005] From the above, technologies for reducing and suppressing external surface defects in seamless steel pipes have been studied. For example, in Patent Document 1, a material containing C: 0.15 mass% or less, Mn: 0.30 to 0.60 mass%, Si: 0.20 to 1.0 mass%, Cr: 8.0 to 10.0 mass%, Mo: 0.85 to 1.10 mass%, P: 0.030 mass% or less, S: 0.030 mass% or less, with the balance being Fe and inevitable impurities, is made into a seamless steel pipe in a mandrel rolling process equipped with various rolling mills. At that time, a method of suppressing external surface defects by controlling the strain rate of the material within a certain range according to the rolling temperature in each rolling mill has been proposed.

[0006] Also, in Patent Document 2, a method of suppressing external surface defects by controlling the contact arc lengths in the rolling direction and circumferential direction of the plain pipe and the plug during inclined rolling within a certain range and suppressing excessive spread of the plain pipe in the circumferential direction to prevent seizure caused by excessive contact of the plain pipe with the guide shoe has been proposed.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, in the prior art disclosed in Patent Documents 1, 2, etc. mentioned above, there is a problem that it is impossible to suppress or reduce surface defects formed by scale built up during heating in a rotary furnace or surface defects of a billet having surface defects formed by pushing in spalled hearth bricks, or outer surface flaws generated by rolling a hollow tube blank.

[0009] In view of the above problems, an object of the present invention is to provide an inclined rolling method and a seamless steel pipe manufacturing method capable of suppressing outer surface flaws based on surface defects of a material to be rolled.

Means for Solving the Problems

[0010] As a result of the inventors investigating the cause of the remaining outer surface flaws due to surface defects of the material to be rolled (billet, tube blank), it was found that these surface defects are subjected to excessive circumferential shear deformation imparted by an inclined rolling mill such as a piercer or an elongator, and the defects extend in the circumferential direction and wall thickness direction of the material to be rolled. Therefore, the inventors considered that by suppressing the strain generated during rolling by a piercer, an elongator, etc., it is possible to reduce or suppress outer surface flaws due to surface defects of the material to be rolled in the seamless steel pipe obtained thereafter. This is because by suppressing the strain during rolling, the surface defects will not extend in the circumferential direction and wall thickness direction of the tube blank.

[0011] The present invention has been made based on the above findings, and the main configuration is as follows. [1] An inclined rolling method for obtaining a hollow tube blank from a material to be rolled, An inclined rolling method that satisfies the following formulas (1) and (2). 0.87×D≦G≦0.97×D ··· Formula (1) 0.49×G≦L≦0.92×G ··· Formula (2) Here, in Formulas (1) and (2), D (mm): The outer diameter of the cross-section perpendicular to the tube axis direction of the material to be rolled, G (mm): The roll gap between a pair of rolling rolls that sandwich the material to be rolled. L (mm): The length from the perpendicular cross-section in the pipe axis direction at the roll gap measurement position to the tip of the plug inserted into the material to be rolled. [2] The strain rate, which is the rate at which the circumferential shear strain is formed on the outer surface of the material to be rolled during inclined rolling, is 0.25 sec -1 or more and 0.41 sec -1 or less, and the inclined rolling method according to the above [1]. [3] A method for manufacturing a seamless steel pipe using the inclined rolling method according to the above [1] or [2].

Advantages of the Invention

[0012] According to the present invention, external surface flaws caused by surface defects of the material to be rolled can be suppressed.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0015] The inclined rolling method of the present invention is an inclined rolling method for obtaining a hollow pipe from a material to be rolled (billet, plain pipe), and is an inclined rolling method that satisfies the following formulas (1) and (2). In the seamless pipe obtained thereafter, it is possible to suppress the formation or increase of external surface defects based on the surface defects of the material to be rolled. 0.87×D≦G≦0.97×D ··· Formula (1) 0.49×G≦L≦0.92×G ··· Formula (2) Here, in Formula (1) and Formula (2), D (mm): Outer diameter of the cross-section perpendicular to the pipe axis of the material to be rolled G (mm): Roll gap between a pair of rolling rolls that sandwich the material to be rolled L (mm): Length from the cross-section perpendicular to the pipe axis at the roll gap measurement position to the tip of the plug inserted into the material to be rolled (hereinafter, also referred to as the plug advancement amount).

[0016] First, before explaining the details of the inclined rolling method of the present invention, an example of a method for manufacturing a seamless pipe to which the inclined rolling method of the present invention can be applied will be described with reference to FIGS. 1 and 2.

[0017] FIG. 1 is a diagram for explaining an example of a method for manufacturing a seamless pipe. As shown in FIG. 1, when manufacturing a seamless pipe, first, the billet 1A is heated in a rotary furnace 100, and then the material to be rolled (billet) 1A is pierced by a piercer 110 to obtain a plain pipe (hollow plain pipe) 2A. Thereafter, the plain pipe 2A is stretch-rolled by a mandrel mill 120 to reduce the wall thickness of the plain pipe. The wall-thickness-reduced plain pipe is heated in a reheating furnace 130 and then subjected to sizing rolling by a hot stretch reducer 140 to produce a seamless pipe 3A having a desired outer diameter.

[0018] FIG. 2 is a diagram for explaining an example of a method for manufacturing a seamless pipe in a mode different from that of FIG. 1. As shown in FIG. 2, when manufacturing a seamless pipe, first, the billet 1B is heated in a rotary furnace 200, and then the billet 1B is pierced by a piercer 210 to obtain a material to be rolled (plain pipe) 2B. After that, the elongator 220 and the plug mill 230 reduce the thickness of the base pipe 2B by stretch rolling the base pipe 2B (raw pipe), and the reel 240 smooths the inner and outer surfaces of the base pipe 2B (raw pipe). After heating the base pipe in the reheating furnace 250, sizing rolling is performed by the sizer 260, and a seamless pipe 3B having a desired outer diameter is manufactured.

[0019] Here, the inclined rolling method of the present invention can be applied to the manufacturing method of seamless pipes. In the following, an example of implementing the inclined rolling method of the present invention using the piercer 110 shown in FIG. 1 and an example of implementing the inclined rolling method of the present invention using the elongator 220 shown in FIG. 2 will be described. However, the inclined rolling method of the present invention is not limited to these examples, and can also be applied to, for example, an inclined rolling method using the piercer 210 shown in FIG. 2 (see "*" in FIGS. 1 and 2).

[0020] FIG. 3 is a diagram for explaining an example of implementing the inclined rolling method of the present invention using the piercer 110. As shown in FIG. 3, the piercer 110 is a mill (inclined rolling mill) having a pair of rolling rolls 11 whose rotation axes are inclined with respect to each other and sandwich the workpiece (billet) 1A, and a plug 12 disposed between the opposing rolling rolls 11 and piercing the workpiece 1A advancing in the rolling direction. Further, the piercer 110 may have a guide shoe (not shown in FIG. 3) disposed between the rolling rolls 11 in the circumferential direction of the workpiece 1A in order to prevent the axis of the workpiece 1A from moving during rolling. The workpiece 1A is reduced in diameter by engaging with the rolling roll 11 which is a barrel-shaped roll or a cone-shaped roll, advances in the rolling direction while being twisted in the circumferential direction, and is rolled by the rolling roll 11 and the plug 12. During rolling, the workpiece 1A undergoes very complex deformation because it is stretched in both the circumferential direction and the rolling direction.

[0021] FIG. 4 is a diagram for explaining an example of implementing the inclined rolling method of the present invention using the elongator 220. As shown in Fig. 4, the elongator 220 is a mill (tilting rolling mill) having a pair of rolling rolls 21 whose rotation axes are inclined to each other and sandwich the material to be rolled (plain pipe) 2B, and a plug 22 disposed between the opposing rolling rolls 21 and inserted into the material to be rolled 1B advancing in the rolling direction. Also in the example shown in Fig. 4, the material to be rolled 2B is reduced in diameter by engaging with the rolling rolls 21 which are barrel-shaped rolls or cone-shaped rolls, and advances in the rolling direction while being twisted in the circumferential direction, and is rolled by the rolling rolls 21 and the plug 22. Similarly, during rolling, the material to be rolled 2B undergoes very complex deformation because it is stretched in both the circumferential direction and the rolling direction.

[0022] In these tilting rollings, in the present invention, with respect to the outer diameter D of the perpendicular cross section in the tube axis direction of the materials to be rolled 1A and 2B, the roll gap G between the rolling rolls 11 and 21, and the plug advancement amount L of the plugs 12 and 22, by controlling within the ranges specified by the following formulas (1) and (2), even if there are surface defects in the materials to be rolled 1A and 2B before rolling, it is possible to suppress the formation of external surface flaws or the increase of external surface flaws in the resulting seamless steel pipe.

[0023] 0.87×D ≦ G ≦ 0.97×D ··· Formula (1) In the present invention, as the relational expression between the outer diameter D (mm) of the materials to be rolled 1A and 2B and the roll gap G (mm), it is made to satisfy 0.87×D ≦ G ≦ 0.97×D. When the roll gap G exceeds 0.97×D, the restraining force of the roll on the material to be rolled becomes weak, and stable rolling cannot be achieved. On the other hand, when the roll gap G is less than 0.87×D, large strains generated during rolling are imparted, and surface defects become deeper and longer in the thickness direction and the circumferential direction. Therefore, in the present invention, it is made to satisfy Formula (1): 0.87×D ≦ G ≦ 0.97×D. Preferably, G is 0.90×D or more. Also preferably, G is 0.95×D or less.

[0024] 0.49×G ≦ L ≦ 0.92×G ··· Formula (2) In the present invention, in addition to the above formula (1), the length L (mm) from the perpendicular cross-section in the pipe axis direction at the roll interval measurement position to the tip of the plugs 12 and 22 inserted into the material to be rolled, that is, the length (plug advancement amount L (mm)) protruding from the perpendicular cross-section in the pipe axis direction at the roll interval measurement position at the tip of the plugs 12 and 22, and the roll interval G (mm) satisfy the relational expression: 0.49×G≦L≦0.92×G. When the plug advancement amount L exceeds 0.92×G, the material to be rolled hits the plug before being bitten by the rolls and rolling becomes impossible. On the other hand, when the plug advancement amount L is less than 0.49×G, large strains generated during rolling are applied, and surface defects become deeper and longer in the thickness direction and circumferential direction. Therefore, in the present invention, the formula (2): 0.49×G≦L≦0.92×G is satisfied. Preferably, L is 0.50×G or more. Also preferably, L is 0.83×G or less.

[0025] Here, the roll interval referred to in the present invention means the distance (shortest distance) between a pair of rolling rolls. That is, it refers to the interval that becomes the minimum when the intervals between a pair of rolling rolls are measured at each position in the rolling direction (pipe axis direction).

[0026] Also, in the length (plug advancement amount) from the perpendicular cross-section in the pipe axis direction at the roll interval measurement position to the tip of the plugs 12 and 22, the roll interval measurement position refers to the position where the interval between a pair of rolling rolls is the minimum. In the present invention, during the rolling process, the plugs 12 and 22 are fixed. Also, the tip of the plugs 12 and 22 is the part that is first inserted into the material to be rolled during rolling.

[0027] The material to be rolled targeted in the present invention is not particularly limited, but it is preferably an outer diameter D of 58 to 360 mm and a pipe length of 250 to 8000 mm. Also, the component composition of the material to be rolled is not particularly limited, but it is preferably a high alloy steel such as SUS420J2.

[0028] In the present invention, the shape and size of the plug are not particularly limited, but it is preferable that the outer diameter E of the plug and the outer diameter D perpendicular to the tube axis direction of the material to be rolled satisfy 0.69×D≦E≦0.78×D. FIG. 5 is a cross-sectional view of the plug. The shape of the plug is not particularly limited, but the plug can be a conical shape with a triangular cross-section, and the outer diameter E of the plug refers to the diameter of the rear end portion of the plug.

[0029] When piercing and rolling the material to be rolled (billet) 1A with the piercer 110, for the above formula (1), G is 0.87×D or more and 0.97×D or less, and preferably 0.90×D or more. Also, G is preferably 0.95×D or less. Also, at this time, for the above formula (2), L is 0.49×G or more and 0.92×G or less, and preferably 0.50×G or more. Also, L is more preferably 0.83×G or less.

[0030] When stretch rolling the material to be rolled (plain tube 2B) with the elongator 220, for the above formula (1), G is 0.87×D or more and 0.97×D or less, preferably 0.91×D or more, and more preferably 0.93×D or more. Also, G is preferably 0.95×D or less, and more preferably 0.94×D or less. Also, at this time, for the above formula (2), L is 0.49×G or more and 0.92×G or less, preferably 0.50×G or more, and more preferably 0.51×G or more. Also, L is preferably 0.89×G or less, and more preferably 0.84×G or less.

[0031] Strain rate during inclined rolling: 0.25 sec -1 Above 0.41 sec -1 Below In the present invention, it is preferable to set the rolling conditions so that the strain rate during inclined rolling satisfies 0.25 sec -1 Above 0.41 sec -1 Below. Here, the strain rate represents the rate at which circumferential shear strain is formed on the outer surface of the material to be rolled during inclined rolling. Regarding the circumferential shear strain, the circumferential shear strain generated on the outer surface of the material to be rolled is calculated by FEM, and the strain rate (sec -1 ) is obtained by strain / inclined rolling time (sec). When the strain rate is less than 0.25 sec -1 , the time for rolling to be completed becomes longer, and at the end of rolling, the temperatures of the obtained seamless pipes 2A and 2B decrease, and bite defects or the like may occur in the next rolling process. Also, when the strain rate exceeds 0.41 sec -1 , surface defects on the billet surface may become deeper and longer due to rolling. Therefore, the strain rate during inclined rolling is preferably 0.25 sec -1 or more and 0.41 sec -1 or less. More preferably, it is 0.27 sec -1 or more and 0.38 sec -1 or less. Further preferably, it is 0.29 sec -1 or more and 0.32 sec -1 or less.

[0032] As described above, according to the inclined rolling method of the present invention, it is possible to suppress the formation of outer surface flaws due to surface defects of the material to be rolled on the seamless pipe. Also, in the present invention, there is provided a method for manufacturing a seamless pipe using this inclined rolling method.

Examples

[0033] Hereinafter, examples of the present invention will be described. The present invention is not limited by the following examples, and can be appropriately modified within a range that conforms to the gist of the present invention, and all of these are included in the technical scope of the present invention. FIG. 6 is a diagram for explaining the V-shaped artificial defect shape imparted to the material to be rolled. A billet (outer diameter D: 58 mm) or a hollow pipe (D: 58 mm × wall thickness t: 19.5 mm) of steel grade SUS420J2 with a V-shaped defect (artificial defect) (depth 2.5 mm, angle 28°) shown in FIG. 6 was used as the material to be rolled. For the billet, it was heated at 1200°C and rolled under the conditions shown in Table 1 using the piercer shown in FIG. 3. For the hollow pipe, it was rolled under the conditions shown in Table 1 using the elongator shown in FIG. 4. The plug outer diameter was 45 mm for the piercer shown in FIG. 3 and 43 mm for the elongator shown in FIG. 4.

[0034] The strain rate represents the rate at which circumferential shear strain is formed in the circumferential direction on the outer surface of the material to be rolled during inclined rolling. Regarding the circumferential shear strain, the circumferential shear strain generated on the outer surface of the material to be rolled was calculated by FEM, and the strain rate (sec -1 ) was calculated from strain / inclined rolling time (sec).

[0035] The defect length and defect depth of the defect (artificial defect) as shown in FIG. 7 were investigated by observing the cross-section perpendicular to the tube axis (circumferential cross-section) at the center in the longitudinal direction of the rolled hollow pipe. Here, the defect length refers to the length from the opening of the flaw (point a, which is the point among points a and b at the outer peripheral surface end that is farther from the tip (innermost part) of the flaw) to the tip (innermost part) of the flaw (the longest length of the flaw in the cross-section perpendicular to the tube axis). Also, as shown in FIG. 7, the defect depth refers to the length of one side in a right triangle having a line parallel to the line from the midpoint of points a and b to the center of the circle in the cross-section perpendicular to the tube axis of the hollow pipe as one side, among the right triangles having a line with the above defect length as the hypotenuse. Both the defect length and defect depth can be measured by observing a sample cut so that the cross-section perpendicular to the tube axis is the cut surface with an optical microscope. Furthermore, the circumferential shear strain imparted by rolling was evaluated by the finite element method. In the finite element method, a hollow plain tube with an outer diameter D of 58 mm and a wall thickness t of 19.5 mm was used as the material to be rolled for the purpose of stabilizing the analysis. In Table 1, for No. 1, 2, 3, 8, 9, 10, 13, 15, and 16 where the billet was used as the material to be rolled, in the analysis by the finite element method, the material to be rolled was analyzed as a hollow plain tube.

[0036] Those with a defect length (artificial defect length) of 3.0 mm or less and a defect depth of 2.0 mm or less were regarded as shallow flaws, and the circumferential shear strain was suppressed to 0.40 or less. The results are shown in Table 1. From Table 1, it was confirmed that in the examples of the present invention, even if a surface defect is formed in the material to be rolled, the outer surface flaw can be suppressed. In the examples of the present invention, by optimizing the rolling conditions, strain occurred in the direction of compression in the thickness direction, and the defect depth of the hollow plain tube after rolling became smaller than the defect depth (2.5 mm) given to the material to be rolled.

[0037]

Table 1

Explanation of Signs

[0038] 1A, 1B billets 2A, 2B plain tubes 3A, 3B seamless pipes 11, 21 rolling rolls 12, 22 plugs 100 rotary furnace 110 piercer 120 mandrel mill 130 reheating furnace 140 hot stretch reducer 200 rotary furnace 210 piercer 220 elongator 230 plug mill 240 reel 250 reheating furnace 260 sizer

Claims

1. An inclined rolling method for manufacturing a seamless steel pipe, which is an inclined rolling method for obtaining a hollow pipe from a material to be rolled, wherein the strain rate, which is the rate at which circumferential shear strain is formed on the outer surface of the material to be rolled during inclined rolling, satisfies 0.25 sec-1 or more and 0.41 sec-1 or less, and satisfies the following formulas (1) and (2): Inclined rolling method. 0.87 × D ≤ G ≤ 0.97 × D... Formula (1) 0.49 × G ≤ L ≤ 0.92 × G... Formula (2) Here, in Formulas (1) and (2), D (mm): Outer diameter of the cross-section perpendicular to the pipe axis of the material to be rolled, G (mm): Roll gap between a pair of rolling rolls that sandwich the material to be rolled, L (mm): Length from the cross-section perpendicular to the pipe axis at the roll gap measurement position to the tip of the plug inserted into the material to be rolled.

2. A method for manufacturing a seamless steel pipe, which manufactures a seamless steel pipe using the inclined rolling method according to Claim 1.

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