JP2008006475A - Proof stress detection method and stretch bending method using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress a variance in products caused by stretch bending by improving the detection accuracy of proof stress. <P>SOLUTION: The variation ΔF of load in tensile load for very short period of time and the variation ΔL of elongation of a shape 100 for very short period of time are successively measured while loading the tensile load on the shape 100 and the tensile load Max×α at the point of time when the value of ΔF/ΔL reaches the value of a rate α predetermined in accordance with a material with respect to the initial maximum value Max is determined as the proof stress. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for detecting a proof stress, which is a stress when permanent deformation occurs in a metal material, and a tensile bending method using the method.

  When bending a shape made of a metal material, a predetermined product shape may not be obtained due to the occurrence of buckling on the inner peripheral side of bending or the occurrence of springback after unloading. Therefore, if bending is performed while applying a tensile load in the vicinity of the proof stress to the shape material, the bending neutral axis moves to the inner peripheral side and buckling is suppressed, and springback is also suppressed by conventional research. I know it.

  For example, in the tension bending method disclosed in Patent Document 1, first, before mass production, the tension t2 ′ when the desired shape is obtained in the tensile bending process of the profile, and when the profile reaches the yield strength The tension t1 ′ is obtained in advance. Next, during mass production, both ends of the profile are gripped by each clamp (chuck), a tensile load is applied, the distance between the clamps is measured, and the tension t1 at the proof stress at which 0.2% plastic strain occurs is detected. The tension t2 applied during the tensile bending process is obtained by the following formula 1.

t2 / t1 = t2 ′ / t1 ′ (Expression 1)
Then, by performing bending while applying a tensile load to the shape member with this tension t2, even if a springback occurs when unloading after tensile bending, the target shape can be obtained with a certain degree of stability. It is like that.
Japanese Patent Laid-Open No. 11-290962

  However, in the method disclosed in Patent Document 1, the tension t1 when the shape material reaches the proof stress is detected based on the distance between the clamps. Then, if an error occurs in the location where each clamp's profile is gripped, if there is play in the clamp, or if the clamp bites into the profile, an error will be included in the tension t1 at the time of proof stress. The tension t2 applied during processing varies. Therefore, at the time of mass production, there is a problem that the amount of spring back varies from product to product despite the same shape, resulting in product variations.

  Therefore, an object of the present invention is to improve the accuracy of detecting the proof stress and to suppress the product variation due to the tensile bending process.

  The present invention has been made in view of the circumstances as described above, and the proof stress detection method according to the present invention is a method of gripping a metal material with a gripping device and applying a tensile load in a minute time of the tensile load. The load change amount ΔF and the elongation change amount ΔL in a minute time of the metal material were sequentially measured, and the value of ΔF / ΔL reached a value determined in accordance with the material with respect to the initial maximum value. It is determined that the tensile load at the time is the proof stress of the metal material.

  In this case, ΔF / ΔL, which is the ratio of the load change amount ΔF in the minute time of the tensile load and the elongation change amount ΔL in the minute time of the metal material, is used as a judgment criterion for the proof stress. Since the inter-device distance is not used for the proof stress determination, even if an error occurs at a location where each gripping device grips the metal material, the error does not affect the value of ΔF / ΔL. Therefore, it is possible to suppress variations in the proof stress detected by individual metal materials.

  Further, as will be described later, the value of ΔF / ΔL during tension has a characteristic of gradually decreasing with time after showing the initial maximum value, and the same kind of metal material shows the same characteristic. . Therefore, if the ratio of the ΔF / ΔL value to the initial maximum value at the proof stress at which 0.2% plastic strain is generated is examined in advance, the tensile load when the ΔF / ΔL value reaches the ratio value is It can be determined that the metal material has the proof stress, and the proof stress detection accuracy is improved.

  Another proof stress detection method according to the present invention is to grasp a metal material with a grasping device and apply a tensile load, while the load change amount ΔF in the minute time of the tensile load and the elongation of the metal material in the minute time. The amount of change ΔL is sequentially measured, and the tensile load at the time when the value of ΔF / ΔL reaches a preset value corresponding to the material is determined as the yield strength of the metal material.

  In this way, similarly to the above, ΔF / ΔL, which is the ratio of the load change amount ΔF in the minute time of the tensile load and the elongation change amount ΔL in the minute time of the metal material, is used as the judgment criterion of the proof stress. Since the distance between the gripping devices is not used for the proof stress determination, even if an error occurs at the location where each gripping device grips the metal material, the error does not affect the value of ΔF / ΔL. Therefore, it is possible to suppress variations in the proof stress detected by individual metal materials.

  In addition, as will be described later, the value of ΔF / ΔL at the time of tension has a characteristic of increasing once over time and gradually decreasing after showing the initial maximum value, and the same kind of metal material is the same The characteristics of Therefore, if the value of ΔF / ΔL at the proof stress at which 0.2% plastic strain occurs is examined in advance and set as the set value, the tensile load when the value of ΔF / ΔL reaches the set value is It can be determined that the strength of the material, and the accuracy of detecting the strength is improved.

  The tensile bending method of the present invention calculates a value obtained by multiplying the yield strength detected by the above method by a predetermined coefficient determined according to the shape of the metal material and the radius of curvature of the bending. The metal material is bent in a state where the value is maintained as a tensile load.

  In this way, according to the above-described yield strength detection method, the tensile load is determined based on the yield strength of each individual metal material being mass-produced, so that a stable springback amount can always be obtained. Therefore, it is possible to suppress product variations when the metal material is subjected to tensile bending. The springback amount also depends on the shape of the metal material and the radius of curvature of the bending. Therefore, if the value obtained by multiplying the yield strength by a predetermined coefficient determined accordingly is set as the tensile load, the springback amount is reduced. be able to.

  As is clear from the above description, according to the present invention, it is possible to suppress variations in yield strength detected by individual metal materials, and it is also possible to suppress product variations when a metal material is pulled and bent. It becomes.

  Embodiments according to the present invention will be described below with reference to the drawings.

(First embodiment)
FIG. 1 is a block diagram showing a tensile bending apparatus 1 according to the first embodiment of the present invention. As shown in FIG. 1, the tensile bending apparatus 1 includes a pair of chucks 2 (gripping devices) that grip both ends of a shape member 100 made of a metal material, and a pair of tensioning devices that move the chucks 2 in the separating direction. And a cylinder 3. One end of a pair of arms 5 is pivotally supported by each of the tension cylinders 3 by a rotary shaft 4. Each arm 5 extends toward a base 7 fixed at the lower center, and the other end of each arm 5 is pivotally supported on the base 7 so as to be rotatable by a rotary shaft 6. A pair of bending cylinders 8 are connected between the arms 5 and the base 7 via rotating shafts 9 and 10, and the arms 5 can be tilted by expansion and contraction of the bending cylinders 8. Below the profile 100, a mold 11 having an arcuate contact surface with the profile 100 is fixed to the base 7.

  A hydraulic pump 13 for supplying hydraulic oil for reciprocating an internal piston (not shown) is connected to the tension cylinder 3, and a driving motor 14 is connected to the hydraulic pump 13. A load sensor 15 including a pressure sensor is provided in the hydraulic path between the tension cylinder 3 and the hydraulic pump 13. In other words, the load applied to the pulling cylinder 3 can be calculated by detecting the pressure in the pulling cylinder 3.

  The pulling cylinder 3 is provided with a displacement sensor 12 that detects the amount of displacement of the chuck 2 by detecting the amount of movement of an internal piston (not shown). The bending cylinder 8 is connected to a hydraulic pump 16 that supplies hydraulic oil for reciprocating an internal piston (not shown), and a driving motor 17 is connected to the hydraulic pump 16. A controller 18 is connected to the displacement sensor 12, the load sensor 15, and the motors 14 and 17, and the controller 18 drives and controls the motors 14 and 17 based on information received from the displacement sensor 12 and the load sensor 15. It has a configuration.

  Specifically, the controller 18 receives a tensile load signal from the load sensor 15 and applies a displacement amount (from the displacement sensor 12) while applying a tensile load to the profile 100 gripped by the chuck 2 by the tension cylinder 3. (Elongation amount of the shape member 100) is received. Moreover, the controller 18 is configured to sequentially measure the load change amount ΔF in the minute time of the received tensile load and the elongation change amount ΔL in the minute time of the profile 100 during the tension.

  FIG. 2 is a graph showing the relationship between ΔF / ΔL and elapsed time. (The reason why the curve in FIG. 2 does not start from the zero point is that the initial value is omitted.) As shown in FIG. 2, when a tensile load is applied to the profile, ΔF The value of / ΔL once increases with time, shows the initial maximum value Max, and then gradually decreases, and the same kind of profile shows the same characteristics. According to this, as will be described in an experimental example to be described later, the tensile load at the time when the value of ΔF / ΔL reaches the value of the ratio α determined in advance with respect to the initial maximum value Max is 0 for the profile. It can be considered that the yield strength is 2% strain. Note that the ratio α is a value (0 <α <1) determined in advance by experimental verification for each material of the shape, as will be described in an experimental example described later.

  Next, a tensile bending work using the tensile bending apparatus 1 will be described. FIG. 3 is a flowchart for explaining the procedure of tensile bending. FIG. 4 is a drawing for explaining the pulling operation during the proof stress detection. FIG. 5 is a drawing for explaining the tensile bending process. As shown in FIGS. 1 and 3, first, the operator initially sets the ratio α of the initial maximum value Max of ΔF / ΔL serving as a proof stress determination criterion in the controller 18 (step S1). In addition, the operator initially sets in the controller 18 a predetermined coefficient to be multiplied by the detected proof stress, which is experimentally predetermined according to the shape of the shape member 100 and the curvature radius of bending. (Step S2). The coefficient β is set in a range of 0.8 <β <1.2 according to the shape of the profile and the radius of curvature of bending.

  Next, the operator holds the shape member 100 on the chuck 2 and sets it on the tensile bending apparatus 1 (step S3). When the operator starts the controller 18 from this state, the controller 18 commands the motor 14 to drive the hydraulic pump 13 and extend the pulling cylinder 3 as shown in FIG. Then, a tensile load is applied to the shape member 100. At that time, the controller 18 receives the tensile load signal from the load sensor 15 and also receives the displacement amount signal from the displacement sensor 12, and the load change amount ΔF in the minute time of the tensile load and the profile 100. The elongation change amount ΔL in a minute time is sequentially measured (step S4). If the currently calculated ΔF / ΔL value is larger than the previously calculated value, the controller 18 stores the current ΔF / ΔL value in the memory as the initial maximum value Max (step S5). ). On the other hand, if the value of ΔF / ΔL calculated this time is smaller than the value calculated so far, the maximum value up to the previous time is stored in the memory as the initial maximum value Max.

  Next, the controller 18 determines whether or not the value of ΔF / ΔL calculated this time is equal to or less than the value Max · α of the ratio α determined in advance with respect to the initial maximum value Max (step S6). When the value of ΔF / ΔL calculated this time is not less than Max · α, the controller 18 returns to step S4 and repeats the process. On the other hand, when the value of ΔF / ΔL calculated this time is equal to or less than Max · α, the controller 18 determines that the tensile load at that time is the proof stress of the shape member 100, and increases the tensile load in the tension cylinder 3. Is stopped and the proof load is maintained (step S7).

  Next, the controller 18 calculates a value obtained by multiplying the detected proof stress by a coefficient β that is initially set in accordance with the shape of the shape member 100 and the curvature radius of bending (step S8). Then, as shown in FIG. 5, the controller 18 extends the bending cylinder 8 by instructing the motor 17 to drive the hydraulic pump 16 in a state where the value obtained by multiplying the yield strength by the coefficient β is maintained as the tensile load. As a result, the arm 5 is tilted to apply a bending load along the arc surface of the mold 11 to the shape member 100, and tensile bending is performed (step S9).

  According to the configuration described above, the distance between the chucks 2 is not used as a criterion for determining the proof stress, but the value of ΔF / ΔL is used, and an error occurs at the location where each chuck 2 grips the profile 100. However, the error does not affect the value of ΔF / ΔL. If it does so, the dispersion | variation in the yield strength detected by each shape member 100 at the time of mass production will be suppressed, and the stable springback amount will always be obtained. Therefore, it is possible to suppress product variations when the shape member 100 is subjected to tensile bending.

  Further, since the ΔF / ΔL value shows the same characteristics in the same type of shape member 100, the ratio α of the ΔF / ΔL value to the initial maximum value Max at the proof stress at which 0.2% plastic strain occurs is examined in advance. In this case, it can be determined that the tensile load at the time when the value of ΔF / ΔL reaches the value of Max · α is the proof stress of the shape member 100, and the proof stress detection accuracy is improved regardless of the gripping position of the chuck 2. Furthermore, since the springback amount also depends on the shape of the shape member 100 and the radius of curvature of bending, the springback amount can be set by setting a value obtained by multiplying the proof strength by a predetermined coefficient β determined accordingly as the tensile load. Can also be reduced.

(Second Embodiment)
FIG. 6 is a flowchart for explaining the procedure of the tensile bending process of the second embodiment. As shown in FIG. 6, this embodiment is the same as that of 1st Embodiment about step S2-S4 and S7-S9, and step S10 and step S11 differ from 1st Embodiment. The configuration of the tension bending apparatus of the present embodiment is the same as that of FIG. 1 except for the control operation of the controller 18.

  As shown in FIG. 2, when a tensile load is applied to the shape member, the value of ΔF / ΔL once increases with time and shows the initial maximum value Max, and then gradually decreases. And the same kind of profile shows the same characteristics. Therefore, if the value of ΔF / ΔL at the proof stress at which 0.2% plastic strain occurs is experimentally examined in advance and set as the set value T, the tensile force at the time when the value of ΔF / ΔL reaches the set value T is obtained. It can be considered that the load is the yield strength of the profile.

  Next, the tension bending work of 2nd Embodiment is demonstrated. As shown in FIG. 6, first, the operator initially sets a set value T, which is a value of ΔF / ΔL at the time of yield strength, in the controller 18 (step S10). Since steps S2 to S4 are as described above, the description thereof is omitted. Next, the controller 18 determines whether or not the value of ΔF / ΔL calculated this time is equal to or less than a preset setting value T (step S11). When the value of ΔF / ΔL calculated this time is not less than the set value T, the controller 18 returns to step S4 again and repeats the process. On the other hand, when the value of ΔF / ΔL calculated this time is equal to or less than the set value T, the process proceeds to step S7. And step S7-S9 is as above-mentioned, and description is abbreviate | omitted.

  According to the configuration described above, the distance between the chucks 2 is not used as a criterion for determining the proof stress, but the value of ΔF / ΔL is used, and an error occurs at the location where each chuck 2 grips the profile 100. However, the error does not affect the value of ΔF / ΔL. If it does so, a tensile load will be decided based on the yield strength of each shape member 100 at the time of mass production, and a stable amount of springback will always be obtained. Therefore, it is possible to suppress product variations when the shape member 100 is subjected to tensile bending.

  In addition, since ΔF / ΔL shows the same characteristics in the same shape member 100, if the set value T of ΔF / ΔL at the proof stress at which 0.2% plastic strain occurs is examined in advance, ΔF / ΔL It can be determined that the tensile load at the time when reaches the set value T is the proof stress of the shape member 100, and the detection accuracy of the proof stress is improved regardless of the location where the chuck 2 is gripped.

(Experimental example)
Hereinafter, experimental examples verified for the proof stress determination will be described.

  A tensile load was applied to the shape member using the apparatus 1 of FIG. The material of the shape member 100 used is SUS304, and the cross-sectional shape of the shape member 100 has an offset shape as shown in FIG. Each side length M of the profile 100 is the same 40 mm, and the plate thickness is 2 mm.

  FIG. 8 is a graph showing the relationship between the tensile load and the displacement amount of the tension cylinder 3 in the experimental example. (In FIG. 8, the tensile load is generated when the displacement is 0 because the load is applied when the chuck 2 is gripped.) As shown in FIG. The tensile load increases and the slope of the curve gradually decreases.

  FIG. 9 is a graph showing the relationship between ΔF / ΔL and elapsed time based on the data of FIG. In FIG. 9, a moving average process (average of 10 times) is performed to absorb measurement errors. (The reason why the curve in FIG. 9 does not start from the zero point is that the initial value is omitted.) FIG. 9 shows that the value of ΔF / ΔL once increases with time and the initial maximum value. It shows a tendency to gradually decrease after showing.

  Here, if the ratio α used for the proof strength determination described in the first embodiment is 0.1, the value of ΔF / ΔL becomes the value Max · α obtained by multiplying the initial maximum value Max by the ratio α. The elapsed time is 43 s. When this point was described on a stress-strain diagram of the same shape member 100 shown in FIG. 10 by a tensile test, it was found that the point was 0.2% proof stress. (In FIG. 10, the stress is generated when the strain is zero because the load is applied when gripping the chuck.) Therefore, the initial maximum value of ΔF / ΔL at the time of 43 s. It can be seen that the ratio to the value Max may be used as α. It can also be seen that the value of ΔF / ΔL at 43 s may be used as the set value T described in the second embodiment. It can also be seen that the value of ΔF / ΔL at the time of 43 s may be used for the set value T described in the second embodiment. That is, since FIG. 9 and FIG. 10 have a one-to-one relationship, it was confirmed that the proof stress can be estimated from the value of ΔF / ΔL.

  Conversely, if the stress is multiplied by the initial cross-sectional area of the shape material, it becomes a tensile load, and if the strain is multiplied by the initial length of the shape material, it becomes elongation, so ΔF / ΔL and dσ / dε (stress / The same tendency as the strain) is obtained, and it can be said that the ratio α or the set value T with respect to the initial maximum value of ΔF / ΔL used for the proof stress determination can be obtained from the stress-strain diagram.

  In the stress-strain diagram of metallic materials such as aluminum and steel, dσ / dε is Young's modulus E in the elastic region at the initial stage of tension, while dσ / dε is almost the same as the curve of Equation 2 below in the plastic region. I know you will.

σ = Cε ⁿ ... (Formula 2)
By differentiating Equation 1, dσ / dε (stress change / strain change), which is synonymous with ΔF / ΔL, is expressed by the following Equation 3. dσ / dε = Cnε ^(n−1). (Formula 3)
Therefore, as shown in Table 1, by inputting the expected C value, n value, and ε = 0.002 to this Equation 3, dσ / dε at 0.2% proof stress, that is, 0 .DELTA.F / .DELTA.L at 2% yield strength can be estimated.

  Therefore, dσ / dε at the time of plastic strain of 0.2% can be obtained from the n value, the C value, and Equation 3 by setting ε = 0.002. As shown in Table 1, the E, n value, and C value of mild steel, stainless steel, and aluminum are generally known, and the set value T (= dσ / dε) and the ratio α (= (dσ / dε) at the proof stress described above. A rough value of) / E) can be predicted for each material.

  As described above, the proof stress detection method and the tensile bending method using the same according to the present invention suppress the variation in proof stress detected in each metal material, suppress the product variation during the tensile bending process, and the spring back. It has an excellent effect of reducing the amount, and is beneficial when applied to the tensile bending of a profile.

1 is a block diagram illustrating a tensile bending apparatus according to a first embodiment of the present invention. 5 is a graph showing the relationship between ΔF / ΔL and elapsed time. It is a flowchart explaining the procedure of a tension bending process. It is drawing explaining tension | pulling operation | movement during proof stress detection. It is drawing explaining a tension bending process. It is a flowchart explaining the procedure of the tension bending process of 2nd Embodiment. It is sectional drawing of the profile of an experiment example. It is the graph showing the relationship between the tensile load of an experiment example, and the displacement amount of a tension cylinder. 6 is a graph showing the relationship between ΔF / ΔL and elapsed time in an experimental example. It is a stress-strain diagram of an experimental example.

Explanation of symbols

1 Tensile bending machine 2 Chuck (gripping device)
3 Pulling cylinder 8 Bending cylinder 11 Mold 12 Displacement sensor 15 Load sensor 18 Controller 100 Profile (metal material)

Claims (3)

While grasping the metal material with a grasping device and applying a tensile load, the load change amount ΔF in the minute time of the tensile load and the elongation change amount ΔL in the minute time of the metal material are sequentially measured,
A method for detecting a proof stress, characterized in that a tensile load at a time when a value of ΔF / ΔL reaches a value determined in accordance with a material with respect to an initial maximum value is determined as a proof strength of the metal material . While grasping the metal material with a grasping device and applying a tensile load, the load change amount ΔF in the minute time of the tensile load and the elongation change amount ΔL in the minute time of the metal material are sequentially measured,
A yield strength detection method, wherein a tensile load at a time when a value of ΔF / ΔL reaches a preset value corresponding to a material is determined to be a yield strength of the metal material.   A value obtained by multiplying the proof stress detected by the method according to claim 1 or 2 by a predetermined coefficient determined according to the shape of the metal material and the radius of curvature of bending is calculated, and the calculated value A tensile bending method, wherein the metal material is bent while maintaining a tensile load.

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