METHOD FOR PRODUCING HIGH Cr SEAMLESS STEEL TUBE AND ROLLING METHOD FOR THE SAME

Abstract

To provide a method for producing high Cr seamless steel tube and a rolling method for the same, each enabling suppression of rolling defect due to ductile fracture occurring on the external surface of steel raw material, even when the steel raw material is a difficult-to-work steel raw material containing Cr of 9% or more and 17% or less.SOLUTION: A method for producing high Cr seamless steel tube has: a step in which a steel slab containing Cr of 9% or more and 17% or less by mass is heated in a heating furnace; a step in which the steel slab is bored by a piercer mill, thereby forming a tube stock; and a step in which the tube stock is molded and rolled to a tube body shape by a rolling mill. The molding and rolling step includes stretching and molding of the tube stock by an elongator included in the rolling mill, and when stretching and molding, a stationery shoe of which the curvature radius of a circular arc part is an outlet side tube stock dimension, which is a tube stock diameter on an outlet side, or more and the outlet tube stock dimension×1.022 or less is applied as left and right guide shoes of the elongator, and the relationship of 0.87≤(E / H)≤0.94 is satisfied. Herein, E represents a rolling mill roll interval, H represents a guide shoe interval.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a seamless steel pipe with a high Cr joint and a rolling method.

Background Art

[0002] The seamless steel pipe widely used for oil well pipes and the like is manufactured using a steel slab called a billet as a raw material. Specifically, the steel slab (for example, a round billet) as the raw material is heated in a heating furnace, and then a plug is pressed against the raw material by a piercer mill (tilt rolling mill) to form a raw pipe. Subsequently, a plug is similarly inserted into the hole of the raw pipe, and the raw pipe is formed and rolled into a pipe shape by a rolling mill composed of an elongator, a plug mill, a reel, or a mandrel mill that performs pipe expansion, stretching, and pipe polishing. Further, the obtained pipe body passes through a reheating furnace and is drawn to a predetermined outer diameter and wall thickness by a multi-stage drawing rolling mill called a sizing mill to be made into a product.

[0003] When using a high Cr material with inferior hot workability compared to general steel as the raw material for seamless steel pipes, various techniques have been proposed to address the problem of rolling defects.

[0004] Patent Document 1 proposes a technique for manufacturing a high Cr seamless steel pipe without causing multiple rolling defects by heating a steel pipe raw material containing 9% by mass or more of Cr to a temperature of (Ac 4 transformation point + 10°C) or higher, piercing by rolling using a tilt rolling method, and stretching to obtain a seamless steel pipe of a predetermined size.

[0005] Further, Patent Document 2 proposes a technique capable of suppressing rolling defects in the piercing rolling process of high alloy steel when manufacturing a high alloy seamless steel pipe.

[0006] Patent Document 3 proposes a manufacturing technique for suppressing internal surface defects caused by drawing rolling mills such as a sizing mill or a stretch reducer by manufacturing a seamless steel pipe made of duplex stainless steel within a range defined by a predetermined formula for the outer diameter working ratio Rd (%) and the wall thickness reduction amount ΔT (mm).

[0007] Patent Document 4 proposes a technique for producing a seamless steel pipe with excellent inner surface quality by coating at least a part of the surface of a piercing rolling tool with a plastic lubricant containing a Li-containing compound in an amount of 0.01% or more and less than 20% in terms of mass% based on the total amount of the lubricant and performing piercing rolling with the tool. 2 O 3 Patent Document 5 proposes a technique for producing a seamless steel pipe with excellent inner surface quality by performing piercing rolling with a plug having a through-hole extending on the central axis of a piercing rolling tool and passing through the central part of a billet to be pierced.

[0008] Patent Document 6 is not related to rolling defects, but is related to suppressing out-of-roundness defects when expanding a high-Cr seamless steel pipe with an elongator, and describes general processes for producing a high-Cr seamless steel pipe.

[0009]

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Summary of the Invention

Problems to be Solved by the Invention

[0011] Incidentally, when manufacturing a seamless steel pipe using a high - material containing 9% or more and 17% or less of Cr by mass (hereinafter, unless otherwise specified, the notation of % of chemical components indicates mass %), there is a problem that rolling defects due to ductile fracture occur on the outer surface of the product. However, it is difficult to address this problem with the technologies of Patent Documents 1 to 5 as described below.

[0012] That is, the technology described in Patent Document 1 involves rolling with a heating temperature of (Ac 4 transformation point + 10°C) or higher. However, for steel grades containing 17% of Cr, the Ac 4 transformation point is often 1300°C or higher, and heating at a temperature exceeding 1300°C is required. However, generally, the higher the temperature of the heating furnace, the greater the damage to the refractory, and in operations exceeding 1300°C, restrictions on the maximum temperature may be necessary. Also, when manufacturing thin - wall materials with a wall thickness of 10 mm or less from a steel material containing 9% of Cr and a steel material containing 13% of Cr according to the technology of Patent Document 1, rolling defects on the outer surface of the product occur due to ductile fracture accompanying an increase in the processing load during rolling.

[0013] Moreover, the technology described in Patent Document 2 contributes to suppressing rolling defects in the piercing - rolling process, but it has no effect on rolling defects with different generation factors occurring in processes other than piercing - rolling (for example, the stretch - rolling process), and cannot address rolling defects caused by ductile fracture occurring on the outer surface of a high - Cr seamless steel pipe containing 9% or more and 17% or less of Cr.

[0014] Furthermore, the technologies described in Patent Documents 3 to 5 are for suppressing internal surface defects of seamless steel pipes, and still cannot suppress the occurrence of rolling defects on the outer surface of the product in a high - Cr seamless steel pipe containing 9% or more and 17% or less of Cr.

[0015] Note that Patent Document 6 is a technology for suppressing out - of - roundness defects when expanding a high - Cr seamless steel pipe with an elongator, and does not consider suppressing rolling defects caused by ductile fracture occurring on the outer surface.

[0016] Therefore, the present invention provides a method for manufacturing a high-Cr seamless steel pipe and a rolling method that can suppress rolling defects caused by ductile fracture occurring on the outer surface even in a difficult-to-machine steel material containing 9% or more and 17% or less of Cr.

Means for Solving the Problems

[0017] To solve the above problems, the present invention provides the following means [1] to [5].

[0018] [1] A method for manufacturing a high-Cr seamless steel pipe containing 9% or more and 17% or less of Cr by mass, a step of heating a steel slab containing 9% or more and 17% or less of Cr by mass in a heating furnace; then, a step of piercing the steel slab with a piercer mill to form a raw pipe; a step of shape-rolling the raw pipe into a pipe shape by a rolling mill, having, wherein the shape-rolling step includes stretch-rolling the raw pipe by an elongator included in the rolling mill, and during the stretch-rolling, as the left and right guide shoes of the elongator, a fixed shoe having a radius of curvature of its arc portion of not less than the outer diameter of the raw pipe on the outlet side (outlet raw pipe dimension) and not more than 1.022 times the outlet raw pipe dimension is applied, and the following (1) is satisfied. A method for manufacturing a high-Cr seamless steel pipe. 0.87 ≦ (E / H) ≦ 0.94 (1) However, E is the rolling roll interval, and H is the guide shoe interval.

[0019] [2] The method for manufacturing a high-Cr seamless steel pipe according to [1], wherein when stretch-rolling the raw pipe by the elongator, the pipe expansion rate is 1.10% or less.

[0020] [3] The method for manufacturing a high-Cr seamless steel pipe according to [1] or [2], wherein the heating temperature during the heating step is from the AC 4 transformation point - 60°C or higher and 1350°C or lower.

[0021] [4] A method for rolling a high-Cr seamless pipe containing 9% or more and 17% or less of Cr by mass, comprising heating a steel slab containing 9% or more and 17% or less of Cr by mass in a heating furnace, piercing the steel slab with a piercer mill, and then subjecting it to stretch rolling with an elongator. As the left and right guide shoes of the elongator, a fixed shoe having a radius of curvature of its arc portion of not less than the outlet tube diameter of the outlet side, which is the outlet side tube size, and not more than 1.022 times the outlet side tube size, is applied, and stretch rolling is performed so as to satisfy the following (1). A method for rolling a high-Cr seamless pipe. 0.87 ≦ (E / H) ≦ 0.94 (1) However, E is the rolling roll interval and H is the guide shoe interval.

[0022] [5] When the raw tube is stretch-rolled by the elongator, the expansion ratio is set to 1.10% or less. The method for rolling a high-Cr seamless pipe according to [4].

Advantages of the Invention

[0023] According to the present invention, even for a difficult-to-process steel material containing 9% or more and 17% or less of Cr, it is possible to suppress rolling defects caused by ductile fracture occurring on the outer surface, and it becomes possible to stably manufacture high-quality seamless pipes without joints.

Brief Description of the Drawings

[0024]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0025] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic diagram for explaining an example of a seamless steel pipe process by the Mannesmann plug mill method used in one embodiment.

[0026] In the present embodiment, a steel slab (for example, a round billet) containing 9% or more and 17% or less of Cr is used as the raw material 1, and the raw material 1 is heated in a heating furnace 2. Next, a plug 4 is pressed against the raw material 1 (steel slab) by a piercer mill (tilting rolling mill) 3 to form a hollow tube 5. Then, the plug 4 is similarly inserted into the hole of the hollow tube 5, and the hollow tube 5 is subjected to tube expansion, stretching, and tube polishing by a rolling mill composed of an elongator 6, a plug mill 7, a reel 8, or a mandrel mill (not shown) to be formed and rolled into a tube shape. The obtained tube 9 is reheated in a reheating furnace 10 and then reduced in diameter and wall thickness by a sizing mill 11, which is a multi-stage drawing rolling mill, until they reach predetermined dimensions to obtain a seamless steel pipe (product).

[0027] In the present embodiment, in the above manufacturing process, attention is paid to the conditions of elongation rolling (elongator rolling) in the elongator 6 when forming and rolling into a tube shape by a rolling mill.

[0028] In elongator rolling, as shown in FIG. 2, the hollow tube 5, which is the material to be rolled, is stretched while being reduced in wall thickness and expanded by the upper and lower barrel-shaped rolling rolls 61, 62 (61 is the upper roll and 62 is the lower roll) of the elongator 6 and a plug (plug for restraining the inner surface of the tube) 63 between the rolling rolls. On the other hand, the outer diameter is determined by being restrained by the left and right guide shoes (not shown in FIG. 2). In the figure, AA is the advanced amount of the plug from the roll center.

[0029] Figure 3 is a cross-sectional view taken along the line X-X of FIG. 2 showing the inside of the roll bite of the elongator. FIG. 3(a) shows the case where drive roller shoes (hereinafter referred to as DRS) are applied as the left and right guide shoes. During the elongation rolling, the hollow raw tube 5 spreads in the diagonal direction of the roll bite surrounded by the upper and lower rolling rolls 61 and 62 and the left and right guide shoes 64 and 65 configured as DRS. For this reason, a guide plate 66 is arranged to prevent the circumferential portion of the tube from protruding outward due to the spread of the raw tube 5 in the diagonal direction. Further, FIG. 3(b) shows the case where fixed shoes are applied as the guide shoes. During the elongation rolling, the hollow raw tube 5 similarly spreads in the diagonal direction of the roll bite surrounded by the upper and lower rolling rolls 61 and 62 and the left and right guide shoes 67 and 68 configured as fixed shoes. Note that FIG. 3(b) shows the roll interval E and the guide shoe interval H together.

[0030] Figure 4 is a view showing the guide shoes 67 and 68 configured as fixed shoes. (a) is a cross-sectional view in the radial direction of the raw tube diameter, and (b) is a side view in the axial direction of the raw tube. In the figure, R is the radius of curvature. As the fixed shoe, those having different radius of curvature R and axial length L of the arc portion can be selected according to the outer diameter of the raw tube on the inlet side and the outer diameter of the raw tube on the outlet side of the elongator.

[0031] In elongation rolling, from the viewpoint of tool cost, usually DRS is used as the guide shoe. For example, in Patent Document 6, DRS is used as the guide shoe in the elongation rolling of a steel material containing 5% or more of Cr.

[0032] By the way, in the rolling of difficult-to-machine materials containing a large amount of Cr, if a bulging portion (see FIG. 3(a)) occurs in the roll bite during rolling, when the bulging is springback, the deformation load of the raw tube 5 increases, and rolling defects due to ductile fracture occur on the outer surface.

[0033] It can be said that if the radius of curvature of the bulging portion in the roll bite during elongation rolling is small, the deformation load of the raw tube increases accordingly, and rolling defects due to ductile fracture are likely to occur.

[0034] When finishing to the target dimensions, the outer diameter of the raw tube is roughly determined by the guide shoe interval H. Also, the wall thickness is roughly determined by the gap between the rolling roll / plug. The guide shoe interval H is uniquely determined by the target outer diameter. On the other hand, for the wall thickness, multiple equipment arrangements for obtaining the same wall thickness are conceivable.

[0035] Therefore, the inventors conducted experiments on the premise of obtaining a seamless steel pipe with the same wall thickness using a difficult-to-machine steel material containing 9% or more and 17% or less of Cr, and examined the correlation between the rolling defects on the outer surface generated during elongator rolling and various parameters. As a result, there is a correlation between the type of guide shoe, the guide shoe shape, and the ratio (E / H) of the rolling roll interval (E) to the guide shoe interval (H) and the number of rolling defects. As the left and right guide shoes, a fixed shoe with a radius of curvature of the arc portion being equal to or greater than the outer diameter of the outgoing raw tube, which is the outgoing raw tube dimension, and equal to or less than 1.022 times the outgoing raw tube dimension, is applied, and by satisfying the following (1), it was found that rolling defects can be suppressed. 0.87 ≦ (E / H) ≦ 0.94 (1)

[0036] The experiments at this time will be described below. Round billets of Cr steel (17% Cr steel) containing 17% of Cr heated at 1290°C in a rotary heating furnace were pierced with a piercer mill, and a hollow raw tube with an outer diameter of 194 to 265 mm was obtained. This hollow raw tube was elongation-rolled with an elongator at an expansion ratio such that the outer diameter became 199 to 279 mm, and the relationship between the ratio (E / H) of the roll interval E to the guide shoe interval H within the roll bite, as well as the type and shape of the guide shoe and the number of rolling defects on the outer surface, was grasped. For the determination of defects, in the case of steel grades with a low frequency of defects, there are methods using ultrasonic flaw detection (UT) and methods of determination based on the frequency of defect occurrence per unit length. For the 17% Cr steel used in this experiment, the rolling defects were targeted at convex-shaped defects having a depth of 0.1 mm or more, and when the number of rolling defects was 1.0 piece / m or less, it was marked as ○, and when it was more than 1.0 piece / m, it was marked as ×.

[0037] The results are shown in Fig. 5. Fig. 5 is a diagram showing these relationships with the ratio of the radius of curvature R of the fixed shoe to the outer diameter on the exit side of the elongator on the horizontal axis and E / H on the vertical axis. From the results of Fig. 5, it can be seen that when performing elongation rolling with an elongator, by applying fixed shoes with a radius of curvature R of the left and right guide shoes being equal to or greater than the exit-side bare tube dimension, which is the outer diameter on the exit side, and equal to or less than the exit-side bare tube dimension × 1.022, and satisfying the above formula (1), rolling defects of high-Cr seamless steel pipes can be suppressed.

[0038] Next, the reasons for limiting the type of guide shoe, the shape of the guide shoe, and the ratio (E / H) of the roll gap (E) to the guide shoe gap in this way will be explained.

[0039] First, when the ratio (E / H) of the roll gap to the guide shoe gap becomes small, the flattening amount of the bare tube during elongator rolling tends to increase. And when E / H is less than 0.87, the flattening amount of the bare tube during elongator rolling becomes too large, the radius of curvature of the bulging part in the roll bite becomes small, and rolling defects due to ductile fracture are likely to occur on the outer surface. On the other hand, when the ratio (E / H) of the roll gap to the shoe gap becomes large, the processing amount tends to be insufficient. And when E / H exceeds 0.94, troubles such as biting failure are likely to occur due to insufficient processing amount. Also, when using DRS as the left and right guide shoes and when using fixed shoes with the radius of curvature R of the arc part being larger than the exit-side bare tube dimension × 1.022, the bulging suppression effect in the roll bite during elongator rolling becomes small, the radius of curvature of the bulging part in the roll bite becomes small, and rolling defects due to ductile fracture are likely to occur on the outer surface. On the other hand, when applying fixed shoes with the radius of curvature R of the arc part being less than the exit-side bare tube dimension as the left and right guide shoes, the bare tube contacts the end of the fixed shoe and problems such as seizure defects occur.

[0040] As described above, in the present invention, a steel slab containing 9% or more and 17% or less of Cr is heated in a heating furnace, and then the steel slab is pierced with a piercer mill to form a raw tube, and the raw tube is roll-formed into a tube shape by a forming machine. In the manufacturing method of a high-Cr seamless steel pipe in which the roll-forming process includes elongator rolling, the conditions of the elongator rolling are defined as described above, and other manufacturing conditions are not particularly limited.

[0041] However, from the viewpoint of reducing the processing load during rolling, it is preferable to restrict the tube expansion ratio during elongator rolling to 1.10% or less, which is more stringent than the tube expansion ratio described in Patent Document 6. Further, as the heating temperature of the heating furnace, a temperature of not less than the AC 4 transformation point - 60°C or more and 1350°C or less is applicable, but considering the influence of the refractory load of the heating furnace, about 1270 to 1310°C, for example, about 1300°C is preferable.

[0042] Further, the present invention can be applied when using a difficult-to-process material containing 9% or more and 17% or less of Cr as a raw material, and the components other than Cr are not particularly limited.

Examples

[0043] Hereinafter, examples will be described. A round billet of 17% Cr steel heated to 1290°C in a rotary heating furnace was pierced with a piercer mill to obtain a hollow raw tube having an outer diameter of 235 mm. This hollow raw tube was roll-expanded with an elongator to an outer diameter of 249 mm. At this time, the E / H inside the roll bite and the types and shapes of the guide shoes were changed. Regarding the E / H inside the roll bite, it was controlled by the balance between the roll interval E and the advancement amount of the plug 63 from the roll center. For example, when the roll interval E was increased, the wall thickness of the outlet raw tube changed, so the advancement amount of the plug 63 was appropriately adjusted so that the wall thickness of the outlet raw tube became constant.

[0044] Regarding the steel pipe obtained by the tube expanding rolling as described above, the number of rolling defects was evaluated. The rolling defects were targeted at bulge-shaped defects having a depth of 0.1 mm or more, and when the number of rolling defects was 1.0 or less per meter, it was marked as ○, and when it was more than 1.0 per meter, it was marked as ×. The results are shown in Table 1.

[0045]

Table 1

[0046] In Table 1, Nos. 1 to 4 are comparative examples where the E / H inside the roll bite and the types and shapes of the guide shoes do not satisfy the present invention, and Nos. 5 to 7 are examples of the present invention where the E / H inside the roll bite and the types and shapes of the guide shoes satisfy the present invention. As shown in Table 1, the evaluations of the number of rolling defects for the comparative examples Nos. 1 to 4 were ×, but the evaluations of the number of rolling defects for the examples of the present invention Nos. 5 to 7 were ○, showing good results.

Explanation of Signs

[0047] 1 Material (steel casting) 2 Heating furnace 3 Piercer mill 4 Plug 5 Plain pipe 6 Elongator 7 Plug mill 8 Reeler 9 Pipe body 10 Reheating furnace 11 Sizing mill 61, 62 Rolling rolls 63 Plug 64, 65 Guide shoes (drive roller shoes) 66 Guide plate 67, 68 Guide shoes (fixed shoes)

Claims

1. A method for manufacturing a high-Cr seamless steel pipe containing 9% or more and 17% or less of Cr by mass%, comprising: heating a steel slab containing 9% or more and 17% or less of Cr by mass% in a heating furnace; then, piercing the steel slab with a piercer mill to form a raw pipe; and shape-rolling the raw pipe into a pipe shape with a rolling mill, wherein the shape-rolling step includes stretch-rolling the raw pipe with an elongator included in the rolling mill, and during the stretch-rolling, as the left and right guide shoes of the elongator, a fixed shoe having a radius of curvature of its arc portion of not less than the outlet raw pipe dimension which is the outlet raw pipe diameter and not more than 1.022 times the outlet raw pipe dimension is applied, and the following (1) is satisfied. A method for manufacturing a high-Cr seamless steel pipe. 0.87 ≤ (E / H) ≤ 0.94 (1) However, E: rolling roll interval, H: guide shoe interval.

2. The method for manufacturing a high-Cr seamless steel pipe according to claim 1, wherein when stretch-rolling the raw pipe with the elongator, the pipe expansion rate is 1.10% or less.

3. Set the heating temperature during the step of heating to AC ４ The method for manufacturing a high-Cr seamless pipe according to claim 1 or claim 2, wherein the heating temperature during the heating step is set to be not lower than the transformation point - 60°C and not higher than 1350°C.

4. A rolling method for a high-Cr seamless steel pipe containing 9% or more and 17% or less of Cr by mass%, comprising heating a steel slab containing 9% or more and 17% or less of Cr by mass% in a heating furnace, then piercing the steel slab with a piercer mill, and then when stretch-rolling with an elongator, as the left and right guide shoes of the elongator, a fixed shoe having a radius of curvature of its arc portion of not less than the outlet raw pipe dimension which is the outlet raw pipe diameter and not more than 1.022 times the outlet raw pipe dimension is applied, and stretch-rolling is performed so as to satisfy the following (1). A rolling method for a high-Cr seamless steel pipe. 0.87 ≤ (E / H) ≤ 0.94 (1) However, E: rolling roll interval, H: guide shoe interval.

5. The rolling method for a high-Cr seamless steel pipe according to claim 4, wherein when stretch-rolling the raw pipe with the elongator, the pipe expansion rate is 1.10% or less.

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