METHOD FOR BEND STRAIGHTENING

Abstract

Method for bend straightening in which a steel pipe having a bend is placed in a state where the steel pipe is convex upward, the steel pipe is pressed from above by a press unit at a target load value, whereby the steel pipe is straightened, the method including: (a) measuring a bottom-dead-center load value Pk when the press unit is at a bottom dead center in the pressing; (b) measuring an amount of change 46 in amount of bend between an amount of bend of the steel pipe before the pressing in step (a) and an amount of bend of the steel pipe after the pressing in the step (a); (c) repeating step (a) and step (b) a plurality of times to create a relational expression Pk=(Δδ); and (d) determining a target load value for next pressing from the relational expression Pk =(Δδ).

Claims

1. A method for bend straightening in which a pipe body having a bend is placed in a state where the pipe body is convex upward, the pipe body is pressed from above by a press unit at a target load value, whereby the pipe body is straightened, the method comprising: measuring a bottom-dead-center load value when the press unit is at a bottom dead center in the pressing; measuring an amount of change in amount of bend between an amount of bend of the pipe body before the pressing and an amount of bend of the pipe body after the pressing; repeating measuring the bottom-dead-center load value and measuring the amount of change a plurality of times to create a relation between the bottom-dead-center load value and the amount of change in the amount of bend; and determining a target load value for next pressing from the relation.

2. The method for bend straightening according to claim 1, further comprising: measuring the bottom-dead-center load value when the press unit is at the bottom dead center in a case where the pressing is performed at the target load value determined in the step (d); measuring an amount of change in amount of bend between an amount of bend of the pipe body before the pressing performed at the target load value and an amount of bend of the pipe body after the pressing performed at the target load value; and updating the relation using the bottom-dead-center load value measured when the pressing is performed at the target load value and the amount of change in the amount of bend measured.

3. The method for bend straightening according to claim 1, further comprising measuring a residual amount of bend of the pipe body after the pressing, wherein when the target load value is determined, the target load value is determined from the relation and the residual amount of bend.

4. The method for bend straightening according to claim 3, wherein when the residual amount is measured, after the pressing, the pipe body is rotated about an axis direction, a rotation amplitude value of the pipe body is measured, and from the rotation amplitude value, the residual amount of bend is measured.

5. The method for bend straightening according to claim 1, wherein measuring the bottom-dead-center load value, measuring the amount of change, repeating measuring the bottom-dead-center load value and measuring the amount of change, and determining the target load value are repeated until an amount of bend of the pipe body after the pressing is equal to or less than a threshold value.

6. The method for bend straightening according to claim 1, wherein the pipe body is a steel pipe.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0018] FIG. 1 is a diagram used for describing a press straightener that performs press straightening on a steel pipe.

[0019] FIG. 2 is a diagram used for describing an amount of bend of a steel pipe.

[0020] FIG. 3 is a diagram used for describing the rotation amplitude value measured by the displacement gauge.

[0021] FIG. 4 is a graph used for describing a relational expression Pk=f(Δδ).

[0022] FIG. 5 is a flowchart illustrating a procedure of a method for bend straightening.

[0023] FIG. 6 is a table showing experimental results obtained by performing the method for bend straightening using the relational expression Pk=f(Δδ).

[0024] FIG. 7 is a graph used for describing a conventional method for bend straightening.

[0025] FIG. 8 is a diagram illustrating a relation between load and amount of bend in pressing operation with various amounts of press stroke.

DESCRIPTION OF EMBODIMENTS

[0026] A method for bend straightening to be described below is a method for straightening a steel pipe being a pipe body by pressing a bend portion that occurs in the steel pipe using a press straightener.

[0027] FIG. 1 is a diagram used for describing a press straightener 1 that performs press straightening on a steel pipe 100.

[0028] The steel pipe 100 that is to be subject to the press straightening by the press straightener 1 has, for example, a curve-like bend. The bend of the steel pipe 100 occurs in, for example, hot or cold working in a producing process of the steel pipe 100.

[0029] The press straightener 1 includes a two-point support 2. The two-point support 2 supports the steel pipe 100 at two points, with its axis direction set to be a horizontal direction. The steel pipe 100 is placed on the two-point support 2 to be convex upward.

[0030] The press straightener 1 includes a press unit 3. The press unit 3 includes a press cylinder 31 and a press plate 32. The press unit 3 presses the steel pipe 100 placed on the two-point support 2 to apply a load to the steel pipe 100.

[0031] The press cylinder 31 includes a rod not illustrated, which has a rod end provided with the press plate 32. The press cylinder 31 is coupled to a hydraulic unit 33 and expands and contracts the rod via working of the hydraulic unit 33 to perform ascending and descending operation of the press plate 32 and the pressing control on the steel pipe 100.

[0032] The press plate 32 ascends and descends via the expansion and contraction of the press cylinder 31. By the rod of the press cylinder 31 expanding, the press plate 32 presses the steel pipe 100 placed on the two-point support 2 from above, applying the load to the steel pipe 100.

[0033] The press cylinder 31 is provided with a load cell 34. The load cell 34 measures a load value P with which the press plate 32 presses the steel pipe 100. During the pressing by the press plate 32, the load cell 34 measures the load value P as appropriate and outputs a value of the measurement to a controller 10 described below.

[0034] The press straightener 1 is provided with a displacement gauge 4. The displacement gauge 4 measures an amount of bend δ of the steel pipe 100 being pressed by the press unit 3.

[0035] FIG. 2 is a diagram used for describing the amount of bend δ of the steel pipe 100. The amount of bend δ is an amount of displacement from a reference level that is set at support points of the steel pipe 100 provided by the two-point support 2.

[0036] The displacement gauge 4 measures an amount of bend δ of the steel pipe 100 by the press unit 3 with respect to the axis direction of the steel pipe 100. The displacement gauge 4 outputs a value of the measurement to the controller 10 described below. The displacement gauge 4 may be of a contact type that comes in contact with the steel pipe 100 to measure the amount of bend δ or may be of a noncontact type that measures the amount of bend δ without contact with the steel pipe 100.

[0037] The press straightener 1 includes a pair of rotating rollers 51 and the displacement gauge 4. The pair of rotating rollers 51 rotates while sandwiching one end of the steel pipe 100 to hold the steel pipe 100. The press straightener 1 causes the press unit 3 to press the steel pipe 100, lifts up the steel pipe 100 from the two-point support 2 after unloading, and causes the pair rotating rollers 51 to rotate the steel pipe 100.

[0038] The displacement gauge 4 measures a rotation amplitude value S of the steel pipe 100 rotated by the pair rotating rollers 51. The rotation amplitude value S is measured by the displacement gauge 4, and half of the rotation amplitude value can be measured as a residual amount of bend δa of the steel pipe 100 after the pressing. The displacement gauge 4 is, for example, a dial gauge.

[0039] FIG. 3 is a diagram used for describing the rotation amplitude value S measured by the displacement gauge 4.

[0040] When the pressing by the press unit 3 starts, the steel pipe 100 deforms elastically, and as the pressing further continues, the steel pipe 100 deforms plastically. When the steel pipe 100 is unloaded being bent in the plastic deformation region, the steel pipe 100 maintains its bent state. In the state where the steel pipe 100 is deformed plastically after the unloading, the amount of bend δ from the reference level described above is the residual amount of bend δa.

[0041] The displacement gauge 4 measures the rotation amplitude value S of the steel pipe 100 rotated by the pair rotating rollers 51 when the steel pipe 100 is bent downward. In other words, the residual amount of bend δa of the steel pipe 100 after the pressing is obtained by halving the rotation amplitude value S measured by the displacement gauge 4.

[0042] Return to FIG. 1. The press straightener 1 includes the controller 10. The controller 10 is, for example, a control circuit using a PC or the like. The controller 10 drives and controls the hydraulic unit 33 to press the steel pipe 100. The controller 10 drives and controls the pair of rotating rollers 51 to rotate the steel pipe 100 about a rotation axis Ax.

[0043] The controller 10 acquires results of the measurement from the load cell 34 and the displacement gauge 4.

[0044] The controller 10 uses the value of the measurement by the load cell 34 to perform feedback control on the hydraulic unit 33 and perform the pressing control on the press unit 3. In the pressing, the controller 10 measures a bottom-dead-center load value Pk when the press unit 3 is at its bottom dead center. When the press unit 3 is at its bottom dead center refers to when the rod (not illustrated) of the press cylinder 31 is at its bottom dead center.

[0045] From the value of the measurement by the displacement gauge 4, the controller 10 measures an amount of change M between the amount of bend δ of the steel pipe 100 before the pressing and the amount of bend δ of the steel pipe 100 after the pressing. Here, after the pressing means a state where the load of the pressing is removed, and no load is applied to the steel pipe 100 by the press unit 3.

[0046] In addition, the controller 10 measures the residual amount of bend δa of the steel pipe 100 from the rotation amplitude value S measured by the displacement gauge 4, as described with reference to FIG. 3.

[0047] The controller 10 collects pluralities of bottom-dead-center load values Pk and amounts of change M to create Pk=f(Δδ), a relational expression of Pk-M.

[0048] FIG. 4 is a graph used for describing the relational expression Pk=f(Δδ). In FIG. 4, black dots each indicate a bottom-dead-center load value Pk and an amount of change Δδ measured in one press. FIG. 4 illustrates data in a case where carbon steel pipes each of which has an outer diameter of 34 mm and a wall thickness of 2.6 mm are supported at a press span of 1500 mm and pressed by the press unit 3. The press span is a distance between the two support points with which the two-point support 2 supports the steel pipe 100.

[0049] In FIG. 4, for example, in a case where the steel pipe 100 is pressed with a bottom-dead-center load value Pk of 2.9 [kN], the amount of change M in the amount of bend δ of the steel pipe 100 before and after the pressing is about 0.4 [mm]. In this case, the steel pipe 100 is deformed plastically.

[0050] In contrast, for example, in a case where the steel pipe 100 is pressed with a bottom-dead-center load value Pk of 2.7 [kN], the amount of change M in the amount of bend δ of the steel pipe 100 before and after the pressing is about 0 [mm]. In this case, the steel pipe 100 is deformed elastically.

[0051] After collecting the pluralities of bottom-dead-center load values Pk and amounts of change M measured, the controller 10 creates Pk=f(Δδ), the relational expression of Pk-Δδ. The relational expression Pk=f(Δδ) is a regression formula that is acquired from a plurality of data items, as illustrated as a solid line in FIG. 4. The relational expression Pk=f(Δδ) is expressed by Formula (1) shown below.

Pk=a.Math.Δδ.sup.4+b.Math.Δδ.sup.3+c.Math.Δδ.sup.2+d.Math.Δδ+e   (1)

[0052] where a=−0.0269, b=0.174, c=−0.416, d=0.467, e=2.77. Note that Formula (1) is updated whenever a bottom-dead-center load value Pk and an amount of change M are measured.

[0053] From the relational expression Pk=f(Δδ) shown by Formula (1) and the residual amount of bend δa of the steel pipe 100, the controller 10 determines a load value for the next pressing (hereinafter, referred to as target load value Pa). Using the relational expression Pk=f(Δδ) makes it possible to determine a load value necessary to perform the bend straightening by deforming the steel pipe 100 plastically by the residual amount of bend δa. For example, in a case where the residual amount of bend δa of the steel pipe 100 is about 1.8 mm, a load value necessary to deform the steel pipe 100 plastically by about 1.8 mm is about 3.0 kN (see FIG. 4). The controller 10 sets a target load value Pa at about 3.0 kN to press the steel pipe 100.

[0054] When performing the pressing at the target load value Pa, the controller 10 performs feedback control on the hydraulic unit 33 to press the steel pipe 100 with reference to the result of the measurement by the load cell 34. The controller 10 then repeats the pressing of the steel pipe 100 until the residual amount of bend δa becomes not more than a threshold value, so as to straighten the steel pipe 100. The threshold value is preferably 1 mm, more preferably 0.5 mm.

[0055] A method for bend straightening performed by the press straightener 1 will be described below in detail. FIG. 5 is a flowchart illustrating a procedure of the method for bend straightening.

[0056] First, the controller 10 causes the press unit 3 to press the steel pipe 100 (S1). Specifically, the controller 10 operates the hydraulic unit 33 to expand the rod of the press cylinder 31. The controller 10 then uses the press plate 32 to press the steel pipe 100 placed on the two-point support 2. At this time, the controller 10 controls the hydraulic unit 33 such that the load value P becomes the target load value Pa while acquiring the result of the measurement by the load cell 34.

[0057] In a case where step δ1 is performed first in the production or the relational expression Pk=f(Δδ) described above is not created with high accuracy because the database is built insufficiently, that is, in a case where the target load value Pa is not determined strictly from the relational expression Pk=f(Δδ), the controller 10 may start the operation based on a material strength level of the material and a simple elastic bending deformation prediction formula. Alternatively, the pressing may be performed according to an empirical rule of a worker or a load value determined by another method.

[0058] Next, the controller 10 measures the bottom-dead-center load value Pk (S2). Thereafter, the controller 10 performs unloading (S3) and measures the amount of change M (S4). The amount of change M is measured as the difference between the amount of bend δ of the steel pipe 100 before the pressing and the amount of bend δ of the steel pipe 100 after the pressing, as described above.

[0059] In step S5, from the bottom-dead-center load value Pk and the amount of change Δδ, the controller 10 creates the relational expression Pk=f(Δδ). At this time, in a case where the relational expression Pk=f(Δδ) has not been created, the controller 10 repeats step δ1 to step S4 a plurality of times to collect pluralities of bottom-dead-center load values Pk and amounts of change M, and then create the relational expression Pk=f(Δδ). In contrast, in a case where the relational expression Pk=f(Δδ) has already been created, the controller 10 uses the bottom-dead-center load value Pk measured in step S2 and the amount of change M measured in step S4 to update the relational expression Pk=f(Δδ).

[0060] Subsequently, the controller 10 controls the rotation of the pair of rotating rollers 51 to rotate the steel pipe 100 (S6). The controller 10 causes the displacement gauge 4 to measure the rotation amplitude value S (S7). The controller 10 determines the residual amount of bend δa of the steel pipe 100 from the measured rotation amplitude value S (S8).

[0061] The controller 10 determines whether the residual amount of bend δa of the steel pipe 100 after the straightening is not more than the threshold value (e.g., 1 mm or 0.5 mm) (S9). When the residual amount of bend δa is not more than the threshold value (S9: YES), the controller 10 determines that the steel pipe 100 is straight. The method for bend straightening is thus finished.

[0062] When the residual amount of bend δa is more than the threshold value (S9: NO), the controller 10 determines that the steel pipe 100 is not straight. The controller 10 then determines the target load value Pa from the relational expression Pk=f(Δδ) and the residual amount of bend δa (S10). Specifically, the controller 10 substitutes the residual amount of bend δa for Δδ in Formula (1) to calculate the load value Pk. The controller 10 determines the calculated load value Pk as the target load value Pa.

[0063] The controller 10 returns to step δ1 to press the steel pipe 100 with the determined target load value Pa. In a case where the steel pipe 100 is convex downward before being pressed in step S1, the steel pipe 100 is rotated to be convex upward.

[0064] By repeating the process described above, the bend of the steel pipe 100 is straightened, and the steel pipe 100 becomes straight. Since the relational expression Pk=f(Δδ) is updated whenever necessary, a reliability of the target load value Pa determined from the relational expression Pk=f(Δδ) increases as the pressing for the bend straightening is repeated. As a result, an accuracy of the bend straightening increases.

[0065] FIG. 6 is a table showing experimental results obtained by performing the method for bend straightening using the relational expression Pk=f(Δδ). In FIG. 6, “Path” means the process for performing the pressing by the press unit 3. “Path No” of “1” means that the steel pipe 100 is pressed once, and “Path No” of “2” means that the steel pipe 100 is pressed twice. “Actual load value” is a load value P measured by the load cell 34 when the controller 10 controls the hydraulic unit 33 to perform the pressing with the “Target load value Pa”.

[0066] In the method for bend straightening of the present embodiment, the pressing control is performed using only the target load value Pa without measuring the amount of bend of the steel pipe 100 being pressed. Furthermore, as shown in FIG. 6, the actual load value substantially matches the target load value Pa. In other words, in the present embodiment, the pressing control is performed accurately. As a result, as read from FIG. 6, the amount of bend δ of the steel pipe 100 is 0.5 mm or less at fourth pressing. As seen from the above, the method for bend straightening of the present embodiment provides results of high accuracy.

[0067] As described above, in conventional practice, the bend straightening is performed while observing a displacement of a steel pipe being pressed in the plastic deformation region using the inclination λ2 of an unload curve created in the plastic deformation region, as illustrated in FIG. 7; in contrast, in the present embodiment, the relation expressed by Formula (1) shown above is created in (the region where the load increases with the inclination λ1), which is seemingly observed as the elastic deformation region immediately after starting the press loading illustrated in FIG. 7. Then, in the region, the bend straightening is performed at the target load value Pa determined from the relation expressed by Formula (1). Using the load value as a control parameter dispenses with the observation of the displacement of the steel pipe being pressed in the bend straightening as in conventional practice, and the pressing control can be performed with high accuracy even when an amount of bend displacement is small. In particular, the present embodiment is effective when a quality level of 1 mm to 0.5 mm or less per length of 1 m in terms of the bend displacement is required.

REFERENCE SIGNS LIST

[0068] 1 press straightener [0069] 2 two-point support [0070] 3 press unit [0071] 4 displacement gauge [0072] 10 controller [0073] 31 press cylinder [0074] 32 press plate [0075] 33 hydraulic unit [0076] 34 load cell [0077] 51 rotating roller [0078] 100 steel pipe