JP6436564B2 - Bending metal strip manufacturing method - Google Patents

Description

  The present invention relates to a method of manufacturing a bent metal strip, and in particular, may occur in a member surface layer portion when hot bending a metal strip having a face-centered cubic / FCC structure. It relates to technology to prevent cracking.

  Metal strips with a face-centered cubic (hereinafter referred to as “FCC”) structure, for example, metal tubes made of austenitic alloys, are thick, high temperature and pressure resistant, such as steam tubes for power generation boilers. As a member, it is used today in industrial facilities such as power plants, various plants and factories.

  These metal strips (the target member of the present invention is not limited to a pipe, but will be described below as an example of a pipe as a typical mode) are standardized and pre-shaped, such as elbows and bends. While pipes are used, pipes made by bending straight pipes (called “bend pipes”) according to the construction object can be flexibly adapted to the demands for various curvatures and pipe shapes. It is used.

  In order to manufacture such a bent tube, in general, a part of a straight metal tube to be processed is heated in an annular shape by an induction heating coil or the like, and at the front end side of the metal tube induction heating coil (tube tip) by a clamp arm. Side) and push the metal tube toward the induction heating coil. The clamp arm pivots around a support shaft and regulates the path of the metal tube in an arc shape. By pushing the metal tube while gripping the metal tube with such a clamp arm, the clamp arm is bent to the part heated by the induction heating coil. A moment is applied, and the metal tube can be continuously plastically deformed to draw an arc.

  Moreover, there is the following patent document as a technique for disclosing a technique related to the bending process of such a metal strip.

JP 2014-34725 A JP 2002-178042 A JP2013-158830A

  By the way, the FCC structural material tends to cause fine cracks (cracks) in the member surface layer portion during hot bending, and proposals such as those in the above-mentioned patent document have been conventionally made to prevent this.

  For example, in the invention described in Patent Document 1, an austenitic alloy member containing components such as Ni, Cr and W has a maximum hardness HV0.1 (max) in a region from the outer surface to a depth of 5 mm, and an average crystal of the member If the particle size d (μm) is controlled so as to satisfy the formula [HV0.1 ≦ (−1/6) × d + 300], it is assumed that cracks can be prevented from occurring during hot bending.

  However, the members satisfying the above conditions, that is, the average crystal grain size d = 500 μm, the hardness is 180 HV, and [180 ≦ (−1/6) × 500 + 300 = 216.7] When a member was prepared and subjected to a bending test, it was confirmed that cracks were observed in the surface layer of the member, and the invention described in Patent Document 1 did not necessarily prevent the occurrence of cracks.

  The invention described in Patent Document 3 is a bending process after heating a superalloy member made of a Ni-based alloy or a Fe-Ni-based alloy in a heating furnace at a temperature of 1175 ° C. or higher and lower than 1250 ° C. (work hardening layer disappearing step). By carrying out, the grain boundary cracking of the member surface layer is prevented. However, when the present inventors performed hot bending after performing the same heat treatment (heating to 1200 ° C.), it was not possible to prevent cracking of the member surface layer portion. Moreover, in the invention described in this document (the same is true of the invention described in Patent Document 2), a separate process called heat treatment is required before bending, and there is a difficulty in increasing the number of processes.

  Thus, it cannot be said that the conventional crack prevention technology in the FCC structural material is sufficient, and it is desired to provide a new technology that can prevent cracks more completely and simply.

  Accordingly, an object of the present invention is to make it possible to easily and sufficiently prevent the occurrence of cracks in a member surface layer portion during hot bending of a metal strip having an FCC structure.

  In order to solve the above-mentioned problems and achieve the object, a method for producing a bent metal strip according to the present invention heats a part of a metal strip having a face-centered cubic lattice structure in an annular shape and heats the metal strip. The clamp is formed by gripping the metal strip by a clamp arm that can grip a position in the vicinity of the section and can turn around a support shaft that is spaced from the gripping section by a certain distance, and propelling the metal strip in the direction of the axis of the metal At least a part of the metal strip is applied by applying a bending moment to the heating portion of the metal strip by turning the gripping part by the arm and guiding the metal strip so that at least a part of the metal strip is curved in an arc. A method for producing a bent metal strip that is continuously plastically deformed into a curved state, wherein an allowable value corresponding to the crystal grain size of the metal strip is determined based on a relationship between a predetermined crystal grain size and an allowable tensile strain. When tensile deformation is calculated and the metal strip is plastically deformed, a tensile strain reduction process is performed to make the tensile strain of the surface layer portion of the strip on the outer periphery side of the metal strip smaller than the calculated allowable tensile strain. To do.

  In the present invention, a part of the metal strip to be processed is continuously plastically deformed by applying a bending moment while it is heated in an annular shape. In the processing, the relationship between the crystal grain size and the allowable tensile strain (later Based on the crack susceptibility map described in the description of the embodiment, the allowable tensile strain of the metal strip to be processed is calculated, and the tensile strain generated during bending is smaller than the allowable tensile strain. Perform the tensile strain reduction process when processing.

  As a specific aspect of the tensile strain reduction treatment, for example, a compressive force in the axial direction is applied to the metal strip by applying a force in a direction in which the clamp arm is pulled back during the propulsion of the metal strip. If such a compressive force is applied during processing, the tensile strain accompanying the processing can be reduced, and this can be suppressed within the allowable tensile strain. In addition, bending by simply applying a bending moment to the strip without applying compressive force is called "simple bending", while bending while applying compressive force in the direction of the material axis. Is referred to as “compression bending”.

  The method for applying a compressive force to the metal strip is not particularly limited. For example, an external force (a force for pulling back the clamp arm) may be applied to the clamp arm so as to rotate the clamp arm in the opposite direction to the rotation of the clamp arm by propelling the metal strip, and the rotation axis of the clamp arm ( A reverse rotational force may be applied to the support shaft. Also, braking may be applied to the rotating shaft, or other methods may be used.

  Further, as a tensile strain reduction treatment, the bending radius of the metal strip may be increased instead of applying the compressive force or in addition to applying the compressive force. If the bending radius is increased, the tensile strain generated in the metal strip is reduced, so that the treatment can reduce the tensile strain and keep it within the allowable tensile strain.

  The relationship between the crystal grain size and the allowable tensile strain is, for example, the following formula as described later with reference to the drawings in the description of the embodiment, and the tensile strain generated by bending is calculated from the following formula. Make it smaller. In the formula, x represents the crystal grain size (average grain size), and y represents the allowable tensile strain.

y = 2.59 × 10 ⁵ × x ^-2.95

  The metal strip having a face-centered cubic (referred to as “FCC”) structure referred to in the present invention is, in other words, a metal strip made of an austenitic alloy. The metal strips are included. The metal strip is typically a tubular member (metal tube / hollow member), but is not limited thereto, and rod-like members (solid members) having various cross-sectional shapes are also referred to in the present invention. Included in strip material.

  According to the method for producing a bent metal strip according to the present invention, it is possible to more easily and satisfactorily prevent a crack from occurring in a member surface layer portion during hot bending of a metal strip having an FCC structure.

  Other objects, features, and advantages of the present invention will become apparent from the following description of embodiments of the present invention described with reference to the drawings. In addition, in each figure, the same code | symbol shows the same or an equivalent part.

FIG. 1 is a diagram showing the relationship between crystal grain size and allowable tensile strain. FIG. 2 is a diagram showing the relationship between the circumferential position of the metal tube and the tensile strain. FIG. 3 is a cross-sectional view showing the circumferential position of the metal tube. FIG. 4 is a plan view schematically showing a manufacturing method (initial state) of a bent metal strip according to an embodiment of the present invention. FIG. 5 is a plan view schematically showing a method for manufacturing a bent metal strip according to the embodiment (a state during bending).

  FIG. 1 is a diagram showing the relationship between the crystal grain size and the allowable tensile strain (referred to as “crack susceptibility map”), and it relates to a plurality of metal tubes made of a material having an FCC structure bent by the inventor. In addition to measuring the crystal grain size and tensile strain generated during processing, it was determined whether or not cracking occurred during bending, and these data were plotted with the crystal grain size on the horizontal axis and the tensile strain on the vertical axis. It is a plot.

  As can be seen from FIG. 1, there is a correlation between the crystal grain size and the presence or absence of cracks, and cracks are generated as the crystal grain size increases even if the tensile strain is small. The curve in FIG. 1 shows the boundary of whether or not cracking occurs, and in the region A where the tensile strain is smaller than this curve (especially when the crystal grain size x is in the range of 100 μm to 400 μm), cracking occurs during processing. In the region B where the tensile strain is larger than this curve, cracks are generated during processing. The curve is expressed by the following formula 1 where the crystal grain size of the metal tube is “x” and the tensile strain is “y”. The crystal grain size x is an average grain size measured by a method according to JIS G 0551 “Steel—Microscopic Test Method for Crystal Grain Size”.

y = 2.59 × 10 ⁵ × x ^−2.95 (Formula 1)

  Therefore, in the method for manufacturing a bent metal strip according to an embodiment of the present invention (metal tube bending method), the crystal grain size x of the metal tube to be processed is measured in advance, and the allowable tensile strain is calculated according to the above equation 1. When y is calculated and bending is performed so that the tensile strain generated by the processing is smaller than the allowable tensile strain y, a tensile strain reduction process (a specific method will be described later) is performed.

  On the other hand, when bending a metal tube, the magnitude of tensile strain differs depending on the position in the circumferential direction. FIG. 3 shows a cross section of the bent metal tube. As shown in FIG. 3, the upper end of the metal tube is 0 ° and the right end is 90 ° in the clockwise direction (circumferential position). When the lower end is defined as 180 ° and the left end is defined as 270 °, the tensile strain at each circumferential position is maximum at the 90 ° position (right end) on the bending outer periphery side as shown in FIG. Minimum at 270 ° position (left end). The metal tube is bent horizontally in the left direction (see FIG. 5 described later), and as shown in FIG. 3, the right side of the metal tube is bent and the wall thickness is reduced.

  FIG. 2 shows the result of bending a metal tube having a crystal grain size of 350 μm by four methods (relationship between the circumferential position and tensile strain). That is, (1) simple bending with 3DR, (2) simple bending with 4DR, (3) compression bending with 3DR, and (4) compression bending with 3DR while applying a larger compressive force than (3). Note that “3DR” means bending with a bending radius R (see FIGS. 4 to 5) that is three times the nominal pipe diameter D of the metal pipe. “4DR” indicates that the metal pipe is similarly bent at a bending radius R four times the nominal pipe diameter D of the metal pipe.

  As can be seen from FIG. 2, a large tensile strain is generated on the bending outer periphery side (45 °, 90 °, and 135 ° positions), and the tensile strain is maximum at the right end (90 ° position). In addition, the tensile strain of each processing example in FIG. 1 indicates a value on the bending outer peripheral side (90 ° circumferential position in FIG. 3) of the metal tube having the maximum magnitude. As is clear from comparison between (1) 3DR bending and (2) 4DR bending, the tensile strain decreases as the bending radius increases. Furthermore, as can be seen by comparing (3) 3DR bending with a small compressive force and (4) 3DR bending with a large compressive force, applying a large compressive force can further reduce the tensile strain. In FIG. 2, a negative tensile strain (bending inner peripheral side / for example, 225 ° position, 270 ° position, etc.) indicates that compressive strain is generated.

  As for cracks, no cracks were observed in the region A where the tensile strain was less than 0.8% (circumferential position where the tensile strain was less than 0.8% for each metal tube in (1) to (4)). Region B where the tensile strain is greater than 0.8%, that is, 45 °, 90 ° and 135 ° of metal pipes ((1) and (2) in FIG. 2) subjected to 3DR and 4DR bending without applying a compressive force. Cracks were observed at the 90 ° position of the metal tube ((3) in FIG. 2) subjected to 3DR bending while applying a small compressive force. Further, in the metal pipe (3 (4) in FIG. 2) subjected to 3DR bending while applying a larger compressive force, no crack was observed at any circumferential position.

  Therefore, in the manufacturing method (bending method) of the present embodiment, the tensile strain on the bending outer peripheral side (in this example, the right end that is the circumferential position of 90 °) where the tensile strain is maximum is expressed as the crack sensitivity map (FIG. 1). ) To reduce the tensile strain to a value smaller than the allowable tensile strain y. As this tensile strain reduction processing, in this embodiment, a compressive force is applied in the material axis direction of the metal tube during processing. As described above, the method for applying the compressive force is not particularly limited. An example of the specific method will be described below.

  4 to 5 show a bending apparatus for carrying out a method for manufacturing a bent metal strip (a method for bending a metal pipe) according to an embodiment of the present invention. As shown in these drawings, this apparatus includes an induction heating coil 12 (hereinafter sometimes simply referred to as “coil”) for heating a part of a metal tube 11 to be processed in an annular shape, and a metal tube toward the coil 12. 11 has a propulsion mechanism 14 that propels 11 and a front clamp 33 that grips the front portion of the metal tube 11, and a bending moment is imparted to the metal tube 11 by rotating about the support shaft 32 as the metal tube 11 is propelled. And a compression mechanism 21 that generates a retracting force that is a force in a direction opposite to the propulsion of the metal tube 11 and applies a compressive force to the metal tube 11.

  Like FIG. 3, FIGS. 4 and 5 show the front, rear, left and right directions, and this embodiment will be described based on these directions. Further, in the figure, reference numeral C indicates a center line of the metal tube, reference numeral D indicates a diameter of the metal tube, and reference symbol R indicates a bending radius of the metal tube.

  The propulsion mechanism 14 that propels the metal tube 11 includes a rear clamp 15 that holds the rear portion of the metal tube 11 and a propulsion drive unit 16 that applies a forward propulsive force to the metal tube 11 through the rear clamp 15. The propulsion drive unit 16 is constituted by, for example, a hydraulic cylinder.

  On the other hand, the compression mechanism 21 that applies a compression force to the metal tube 11 is fixed to the lower end portion of the clamp arm 31 and rotates together with the clamp arm 31 around the support shaft 32 of the clamp arm 31 (see arrow P4). The chain 23 meshes with the sprocket 24, and the compression drive unit 22 that generates a pulling back force P5 that pulls the chain 23 in the direction opposite to the propulsion direction of the metal tube 11 (rearward). The compression drive unit 22 may be constituted by, for example, a hydraulic cylinder.

  Note that the propulsion mechanism 14 and the compression mechanism 21 are not limited to specific structures, and the propulsion mechanism 14 in the present invention is capable of propelling the metal tube 11, and the compression mechanism 21 is a metal tube. Any structure can be used as long as it can apply a compressive force to 11 and is not limited to the illustrated structure.

  The induction heating coil 12 that heats the metal tube 11 is disposed behind the clamp arm 31 and is driven by a drive unit (not shown) including a power source that supplies power to the coil 12. Further, a cooling mechanism capable of injecting cooling water so that the metal pipe 11 can be cooled immediately after bending (the cooling mechanism itself is not shown, but the cooling water injected from the cooling mechanism is indicated by reference numeral 10 in FIG. 5). Is integrally formed with the coil 12, and at the time of processing, the cooling water 10 is sprayed toward the surface of the metal tube 11 immediately in front of the coil 12 to cool the heated and bent metal tube 11. Reference numeral 13 in the drawing denotes a guide roller for guiding the metal tube 11.

  In processing, the rear portion of the metal tube 11 is held by the rear clamp 15 and the front portion of the metal tube 11 is held by the front clamp 33, and the metal tube 11 is pushed forward by the hydraulic cylinder 16 via the rear clamp 15 (arrow). P1).

  When the metal tube 11 is propelled forward (see arrow P2), the clamp arm 31 receives the propulsive force P2 and rotates horizontally around the support shaft 32 (see arrow P3) to grip the metal tube 11. The front clamp 33 is pivoted to draw an arc around the support shaft 32. Along with this, a bending moment is applied to the metal tube 11 held by the front clamp 33, the portions heated by the coil 12 are bent one after another, and the metal tube 11 is continuously plastically deformed in an arc shape (FIG. 5). reference).

  On the other hand, the rotation of the clamp arm 31 accompanying the propulsion of the metal tube 11 causes the sprocket 24 to rotate and the chain 23 to be taken up. The compression driving unit 22 applies a force (retracting force backward) P5 against this. Is applied to the clamp arm 31 via the chain 23 and the sprocket 24. Thereby, the compressive force of a material axial direction is provided with respect to the metal pipe 11, the tensile strain which generate | occur | produces in the metal pipe 11 during a bending process is reduced, and generation | occurrence | production of a crack is prevented. In addition, by applying the compression force, in addition to preventing the occurrence of cracking, it is also possible to suppress the thickness reduction (thinning of the wall thickness) on the bending outer periphery side of the metal tube 11.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to these, It is clear to those skilled in the art that a various change can be made within the range as described in a claim.

  For example, in the above-described embodiment, the compressive force is applied to the metal tube 11 as the tensile strain reduction process. However, the bending radius R may be increased instead of or together with this. Specifically, the position of the front clamp 33 shown in FIG. 4 can be changed in the left-right direction along the clamp arm 31 (see arrow S) (the propulsion mechanism 14, the coil 12, and the guide roller 13 are also included). The position of the metal tube 11 to be machined can be changed at a position farther from the support shaft 32 by moving the front clamp 33 to the right along the clamp arm 31. By gripping, the bending radius R can be increased.

C Metal tube center line D Metal tube diameter R Bending radius 11 Metal strip (metal tube)
DESCRIPTION OF SYMBOLS 12 Induction heating coil 13 Guide roller 14 Propulsion mechanism 15 Back clamp 16 Propulsion drive part 21 Compression mechanism 22 Compression drive part 23 Chain 24 Sprocket 31 Clamp arm 32 Spindle 33 Front clamp

Claims (3)

While heating a part of the metal strip having a face-centered cubic lattice structure in an annular shape,
The metal strip can be gripped by a clamp arm that can grip a position in the vicinity of the heating portion of the metal strip and can pivot around a support shaft that is spaced from the gripping portion by a certain distance,
The metal strip is heated by propelling the metal strip in the direction of the axis of the shaft and rotating the gripping portion by the clamp arm so that at least a part of the metal strip is curved in an arc. A bending metal strip is produced by applying a bending moment to plastically deforming at least a part of the metal strip continuously into a curved state,
It is a relational expression between the crystal grain size x and the allowable tensile strain y when the crystal grain size is x and the allowable tensile strain is y.
y = 2.59 × 10 ⁵ × x ^-2.95
Based on the above, calculate the allowable tensile strain corresponding to the crystal grain size of the metal strip,
When the metal strip is plastically deformed, a tensile strain reduction treatment for making the tensile strain of the surface layer portion of the strip on the outer periphery side of the metal strip smaller than the calculated allowable tensile strain is performed. Bending metal strip manufacturing method. The bending according to claim 1, wherein the tensile strain reduction process is a process of applying a compressive force in a material axial direction to the metal strip by applying a force in a direction in which the clamp arm is pulled back during the propulsion of the metal strip. Manufacturing method of metal strip. The method for producing a bent metal strip according to claim 1, wherein the tensile strain reducing process is a process of increasing a bending radius of the metal strip.

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