JP6649187B2 - Method for estimating tensile properties - Google Patents

Description

  The present invention relates to a method for estimating tensile properties.

  For example, in a steel pipe for a building, since the load acting on the steel pipe is mostly in the pipe axis direction, the strength of the steel pipe is specified by a tensile test in the pipe axis direction. Specifically, for example, the yield stress, tensile strength, and yield ratio in the tube axis direction are specified. In general, in such a tensile test, a full-thickness test piece cut out from one location in the circumferential direction of the steel pipe and having a length in the pipe axis direction or a round bar test piece cut out from the outside at a position 1/4 of the plate thickness is used. Is done. Further, in steel pipes for line pipes, not only characteristics in the circumferential direction of the pipe, which have been conventionally defined, but also specifications in the axial direction of the pipe are required. Therefore, it is required to estimate the tensile properties of the steel pipe in a direction (pipe axis direction) orthogonal to the bending direction from the steel plate to the steel pipe.

  Patent Literature 1 discloses a method for evaluating the influence of the degree of work and the annealing treatment on the mechanical properties in bending of a steel sheet. Patent Literature 2 discloses a method for estimating tensile properties in a direction orthogonal to a bending direction after bending a steel sheet.

Japanese Patent No. 3721517 JP 2014-222160 A

  In the influence evaluation method of Patent Document 1, the Bauschinger effect accompanying bending is not taken into consideration, and there is room for improvement in the estimation accuracy of tensile properties. In addition, this impact assessment method needs to collect a lot of actual product data, which requires much time and cost.

  In the estimation method of Patent Document 2, the Bauschinger effect is considered, but the difference between the bending direction from the steel plate to the steel pipe and the pipe axis direction of the steel pipe is not considered. Further, as the estimation result, only the estimation accuracy of the yield stress is mentioned, and the estimation accuracy of the tensile strength and the yield ratio is not mentioned. Therefore, in the estimation method of Patent Literature 2, the estimation accuracy of the tensile strength and the yield ratio is unknown.

  The present invention provides a method for accurately estimating, from the tensile properties of a steel sheet, the tensile properties of a steel pipe in a direction (pipe axis direction) perpendicular to the processing direction from the steel sheet to the steel pipe when the steel sheet is bent into a steel pipe. Make it an issue.

  The method for estimating tensile properties of the present invention is a method for estimating tensile properties of the steel pipe in a direction perpendicular to a bending direction from the steel sheet to the steel pipe when bending the steel sheet into a steel pipe, Yield stress before working, which is the tensile property before working, tensile strength before working, and yield ratio before working are obtained by actual measurement, and are tensile properties after working in a direction orthogonal to the working direction from the steel sheet to the steel pipe. The post-working yield stress, the post-working tensile strength, and the post-working yield ratio are obtained by actual measurement, and the bending from the steel sheet to the steel pipe is reproduced on the analysis using the pre-working tensile properties, Calculate the equivalent plastic strain and the back stress at the evaluation points by analysis, define the Bauschinger effect as a function of the back stress, and estimate the post-working yield stress as a function of the Bauschinger effect and the pre-working yield ratio. Addition Estimate as post-yield stress, estimate the post-working tensile strength as an estimated post-working tensile strength as a function of the pre-working yield ratio, divide the estimated post-working yield stress by the estimated post-working tensile strength, Estimating the post-machining yield ratio as an estimated post-machining yield ratio.

  According to this method, it is possible to accurately estimate the tensile properties of a steel pipe in a direction (pipe axis direction) orthogonal to the direction in which the steel sheet is formed into a steel pipe by bending the steel sheet. This highly accurate estimation is realized by considering the kinematic hardening rule. Back stress is used to consider the kinematic hardening law. Since the back stress is an element corresponding to the amount of displacement of the yield surface in the kinematic hardening law, by using the Bauschinger effect amount specified as a function of the back stress for estimation, the bending of the steel sheet based on the kinematic hardening law It is possible to consider changes in the tensile properties in the pipe axis direction of the steel pipe obtained by processing. Furthermore, the inventor of the present application has found through numerous trials that the estimation accuracy is improved by estimating not only the Bauschinger effect amount but also the estimated post-processing yield stress as a function including the pre-processing yield ratio, The estimation accuracy is further improved. Further, the Bauschinger effect amount may be defined as a function of the back stress and the yield stress before processing.

For a plurality of corresponding post-working yield stresses and the estimated post-working yield stress, constants a, b, and c that minimize the following first evaluation function were determined, and the constants a, b, and c were substituted. Based on the following formula (1) for calculating the post-working yield stress, the estimated post-working yield stress may be calculated from the pre-working tensile properties.

  According to this method, a specific calculation formula (1) of the estimated post-working yield stress including the yield ratio before working, the Bauschinger effect amount, and the work hardening amount can be defined. When the calculation formula (1) includes these elements, even when the bending direction of the steel sheet is different from the tensile test direction (tube axis direction) of the steel pipe obtained by the bending, the Bauschinger effect and work hardening are obtained. It is possible to estimate the yield stress in the pipe axis direction of a steel pipe, which varies in a complicated manner, from the tensile properties of the steel sheet with high accuracy.

For a plurality of the corresponding post-working tensile strengths and the estimated post-working tensile strengths, constants s, t, and u for minimizing the following second evaluation function were determined, and the constants s, t, and u were substituted. The estimated tensile strength after processing may be calculated from the tensile properties before processing based on the following formula (2) for calculating the estimated tensile strength after processing.

  According to this method, it is possible to define a specific calculation formula (2) of the estimated post-working tensile strength including the yield ratio before working and the equivalent plastic strain. When the calculation formula (2) includes these elements, similarly to the above, the tensile strength in the pipe axis direction of the steel pipe that changes in a complicated manner can be estimated with high accuracy from the tensile properties of the steel sheet.

  According to the present invention, it is possible to consider a change in tensile properties in a direction (pipe axis direction) orthogonal to a bending direction from a steel sheet to a steel pipe based on the kinematic hardening law. Can be estimated.

The perspective view of the steel plate used by embodiment of this invention. The perspective view of the steel pipe obtained by bending the steel plate of FIG. 5 is a flowchart illustrating a method for estimating tensile properties according to the embodiment of the present invention. FIG. 4 is a stress-strain diagram showing the amount of work hardening. 4 is a graph showing an approximate expression of an estimated post-working yield stress with respect to plastic strain. 5 is a graph showing a difference between a post-working yield stress and an estimated post-working yield stress. 5 is a graph showing a difference between the tensile strength after processing and the estimated tensile strength after processing.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

  When bending is performed from the state of the steel sheet 10 as shown in FIG. 1 to the state of the steel pipe 20 as shown in FIG. 2, the tensile properties change. In particular, the tensile properties in the direction (tube axis direction, that is, the Z-axis direction in FIGS. 1 and 2) perpendicular to the bending direction from the steel plate 10 to the steel pipe 20 are complicatedly changed by work hardening. In the present embodiment, the tensile property of the steel pipe 20 in the pipe axis direction is estimated with high accuracy by using the Bauschinger effect amount in consideration of the work hardening law (movement hardening law) from the tensile properties of the steel sheet 10. .

  Here, the tensile properties include the yield stress, the tensile strength, and the yield ratio. In addition, in the setting of analysis coordinates described below and later, the bending direction (the width direction of the steel plate 10 and the circumferential direction of the steel pipe 20) is the X direction, the thickness direction is the Y direction, and the tensile test direction (the longitudinal direction and the The rolling direction and the pipe axis direction of the steel pipe 20) are defined as the Z direction.

  The method for estimating the tensile properties of the steel pipe 20 in the pipe axis direction will be described with reference to the flowchart shown in FIG.

When the estimation is started (Step S3-1), the tensile properties (tensile properties before working) of the steel sheet 10 are obtained by actual measurement (Step S3-2). The thickness of the steel plate 10 used is about 10 to 100 mm, which is a thickness that can be bent. As the tensile properties of the steel plate 10 to be acquired, for example, specifications indicated by a steel material maker or the like may be used. Table 1 below, the yield stress (before processing yield stress) YS _p is the tensile properties of the obtained five different steel A~E in this embodiment, the tensile strength (tensile before processing strength) TS _p And the yield ratio (yield ratio before processing) YR _p are shown. Here, the unprocessed yield ratio YR _p is determined as a percentage obtained by dividing the unprocessed yield stress YS _p in unprocessed tensile strength _{TS p (YR p = YS p} / TS p × 100). In the present embodiment, the steel plate A~E having high and low two kinds of unprocessed yield ratio YR _p is prepared. Specifically, before processing yield ratio of the steel sheet A through C YR _p is relatively low, the steel plate D, unprocessed yield ratio YR _p of E is relatively high.

After acquiring the tensile properties of the steel sheet 10 including the contents shown in Table 1, the steel sheet 10 is bent into a steel pipe 20, and the tensile properties in the pipe axis direction of the steel pipe 20 (post-processing tensile properties) are obtained by actual measurement. (Step S3-3). The bending from the steel plate 10 to the steel pipe 20 is performed by, for example, press bend, roll bend, UO bending, or the like. In the present embodiment, the steel pipe 20 is a circular pipe, but the shape is not particularly limited, and may be, for example, an elliptical pipe or a square pipe. Table 2 below shows the post-working tensile properties of the steel pipes obtained by bending the steel plates A to E in Table 1. Specifically, the yield stress (after processing yield stress) YS _o in the tube axis direction of the steel pipe 20, the tensile strength (tensile after working strength) and TS _o are shown. The processing after the yield ratio YR _o is not listed in Table 2, obtained by the percentage obtained by dividing the post-processing yield stress YS _o by machining after the tensile strength _{TS o (YR o = YS o} / TS o × 100). Since the bending process from the steel plate 10 to the steel tube 20 varies, Table 2 shows test pieces taken from four or six places of each of the steel pipes obtained by bending each of the steel plates A to E. Are obtained.

  After acquiring the post-processing tensile properties of the steel pipe 20 including the contents shown in Table 2, the bending from the steel plate 10 to the steel pipe 20 is reproduced on an analysis (step S3-4). In this analysis, it is preferable to simulate actual pipe making conditions as much as possible with respect to the tool shape, the tool size, the bending method, the stroke, the press pitch, and the like. This analysis is a 2D analysis using a plane strain element or a 3D analysis using a solid element, and the analysis uses the tensile properties before processing (see Table 1) acquired in step S3-2. In this analysis, a general kinematic hardening rule prepared in general-purpose finite element analysis software is used as a work hardening rule accompanying bending. When the analysis is completed, a setting is made so as to output the back stress in addition to the stress and the plastic strain, and at the completion of the analysis, the equivalent plastic strain ε at the evaluation point and the back stress α are obtained (step S3-5). Here, the back stress is an element indicating the movement of the yield surface.

  Generally, in the tensile test, a round bar test piece cut out from the outside of the steel pipe 10 at a position of 1/4 of the plate thickness is used. Therefore, the evaluation point in the analysis is set at the nearest node or element center of the round bar sampling position which is the actual evaluation target. In addition, the equivalent plastic strain ε of the evaluation point means how much processing was applied to this round bar sampling position, so it is sometimes called the degree of processing, and in the following description, the two descriptions are not distinguished. use.

When the analysis is completed, the Bauschinger effect amount B is calculated based on the following equation (3) (step S3-6). The calculation of the Bauschinger effect size B, in the present embodiment, the back stress α and processed before yield stress YS _p is used. In the following equation (3), the X, Y, and Z components of the back stress α correspond to α _x , α _y , and α _z , respectively.

Next, the work hardening amount H is calculated based on the following equation (4) (step S3-7). Here, the equivalent stress of the evaluation point (e.g. Mises stress) and? F, and define by subtracting the unprocessed yield stress YS _p from the equivalent stress? F value and work hardening amount H. As shown in FIG. 4, the same solution can be obtained even if the work hardening amount H is derived from the stress-strain diagram obtained by the analysis and the equivalent plastic strain ε at the evaluation point as shown in the figure.

After calculating the Bauschinger effect amount B and work hardening amount H, the constant a that minimizes the first evaluation function f ₁ as shown in formula (5), b, calculates the c (step S3-8). The first evaluation function f ₁ is defined as the sum of the squares of the difference between the estimated post-process yield stress YS _e and post-process yield stress YS _o. The estimated post-working yield stress YS _e is calculated by using the pre-working yield stress YS _p as a function of the Bauschinger effect amount B, the work hardening amount H, and the pre-working yield ratio YR _p as in the following equation (5). Is defined. Constant a that minimizes the first evaluation function f _1, b, the calculation of c, it is sufficient to use a method of least squares.

Similarly, based on the following equation (6), the constant s that minimizes the second evaluation function _{f 2,} t, and calculates the u (step S3-8). Second evaluation function f ₂ is defined as the sum of the squares of the difference between the estimated post-process the tensile strength TS _e and processing after the tensile strength TS _o. Tensile After estimating machining strength TS _e uses strength TS _p tensile before processing, are defined as the following equation (6) as a function of the equivalent plastic strain ε and unprocessed yield ratio YR _p. Constant s that minimizes the second evaluation function f _2, t, the calculation of u, it is sufficient to use a method of least squares.

Table 3 below shows the equivalent plastic strain ε, back stress α (α _x , α _y , α _z ), Bauschinger effect amount B, work hardening amount H, estimated yield stress YS _e obtained in this manner. , and the estimated post-process the tensile strength TS _e is shown. At this time, the estimated post-process yield ratio YR _e is determined as a percentage obtained by dividing the estimated post-process yield stress YS _e estimated machining after the tensile strength _{TS e (YR e = YS e} / TS e × 100). In the present embodiment, the constants a, b, c, s, t, and u are calculated by the least square method, and the values are a = 0.2980, b = 0.6464, and c = 0.6600. , S = 1.0209, t = 0.0293, and u = 0.0355.

Further, as shown in FIG. 5, in the present embodiment, based on the data of five points before processing yield ratio YR _p is lower steel sheet 10 shown in Table 3 (A5, B1, B4, C2, C5), any calculates the approximate expression indicating the value of the estimated post-process yield stress YS _e for equivalent plastic strain epsilon. In the present embodiment, the approximate expression (YS _e = -13385ε ² + 2822.5ε + 376.37) is calculated using the quadratic function. However, the approximate expression is not limited to the quadratic function, and any function can be used. . Similarly, although not described in detail, based on the data (D4, E2, E6) of three points of the steel sheet 10 having a high yield ratio YR _p before processing shown in Table 3, the estimated yield after processing for any equivalent plastic strain ε and we calculate an approximate expression indicating the value of the stress YS _e. In this way, by expressing the relationship between the equivalent plastic strain ε and the estimated yield stress YS _e by an approximate expression using a plurality of data of the same steel type, the estimated yield after machining at any equivalent plastic strain ε is obtained. The stress YS _e can be estimated. Furthermore, we estimate the estimation processing after the tensile strength TS _e at any equivalent plastic strain ε in the same manner.

  FIGS. 6 and 7 show the results of estimating the tensile properties of all the steel pipe 20 data (A1 to E6) shown in Table 2 using the above-described approximate expression.

As shown in FIG. 6, the error between the estimated post-processing yield stress YS _e and the post-processing yield stress YS _o is within ± 20 MPa (within the broken line in FIG. 6). Further, as shown in FIG. 7, the error of the intensity TS _e tensile after estimation processing, the strength TS _o tensile after processing is within ± 20 MPa (in the broken line in FIG. 7). Therefore, the estimation error of the estimated post-process yield stress YS _e, compared to the error -30 to + 20 MPa disclosed error ± 50 MPa disclosed in Patent Document 1 and Patent Document 2, the present embodiment the high estimation accuracy Have. Further, the estimation error of the tensile after estimating machining strength TS _e, the present embodiment as compared to the error ± 50 MPa disclosed in Patent Document 1 has high estimation accuracy. In Patent Document 2, the estimation error of the tension after the estimated machining strength TS _e is not disclosed.

As described above, according to the method of the present embodiment, the tensile properties of the steel pipe 20 in the direction (tube axis direction) orthogonal to the processing direction of the steel sheet 10 into the steel pipe 20 can be estimated with high accuracy. This highly accurate estimation is realized by considering the kinematic hardening rule. The back stress α is used for consideration of the kinematic hardening rule. Since the back stress α is an element corresponding to the amount of movement of the yield surface in the kinematic hardening law, by using the Bauschinger effect amount B defined as a function of the back stress α for estimation, based on the kinematic hardening law, It is possible to consider changes in the tensile properties in the pipe axis direction of the steel pipe 20 obtained by bending the steel plate 10. Furthermore, the inventor of the present application has found that the estimation accuracy is improved by estimating the post-machining yield stress YS _e as a function including not only the Bauschinger effect amount B but also the yield ratio YR _p before machining through numerous trials. Are found, and the estimation accuracy is further improved.

Further, in this embodiment, defines a front yield ratio YR _p machining, the Bauschinger effect size B, specific calculation equation of the estimated post-process yield stress YS _e including a work hardening amount H (5) . When the calculation formula (5) includes these elements, even when the bending direction of the steel plate 10 is different from the tensile test direction (tube axis direction) of the steel pipe 20 obtained by the bending, the Bauschinger effect and the processing are obtained. The yield stress in the pipe axis direction of the steel pipe 20, which changes in a complicated manner due to the hardening, can be estimated with high accuracy from the tensile properties of the steel sheet 10.

Further, in this embodiment, it defines a front yield ratio YR _p processed, specific calculation formula for equivalent plastic strain ε and the estimated post-process the tensile strength TS _e containing a (6). When the calculation formula (6) includes these elements, the tensile strength in the tube axis direction of the steel pipe 20 that changes in a complicated manner can be estimated from the tensile properties of the steel plate 10 with high accuracy, similarly to the above.

10 steel plate 20 steel pipe

Claims (4)

A method for estimating the tensile properties of the steel pipe in a direction orthogonal to a bending direction from the steel sheet to the steel pipe when bending the steel sheet into a steel pipe,
The pre-working yield stress, which is the pre-working tensile property of the steel sheet, the pre-working tensile strength, and the pre-working yield ratio are obtained by actual measurement,
The post-processing yield stress, which is the post-processing tensile property in a direction orthogonal to the processing direction from the steel sheet to the steel pipe, the post-processing tensile strength, and the post-processing yield ratio are obtained by actual measurement,
The bending process from the steel plate to the steel pipe is reproduced on the analysis using the tensile property before the processing, and the equivalent plastic strain and the back stress at the evaluation point are calculated by the analysis,
The Bauschinger effect amount is defined as a function of the back stress,
Estimating the post-working yield stress as a post-working yield stress as a function of the Bauschinger effect amount and the pre-working yield ratio,
Estimating the post-working tensile strength as a function of the pre-working yield ratio as an estimated post-working tensile strength,
A method for estimating tensile properties, comprising: dividing the estimated post-working yield stress by the estimated post-working tensile strength, and estimating the post-working yield ratio as the estimated post-working yield ratio.   The method for estimating tensile properties according to claim 1, wherein the Bauschinger effect amount is a function of the back stress and the yield stress before processing. For a plurality of corresponding post-working yield stresses and the estimated post-working yield stress, determine constants a, b, and c that minimize the following first evaluation function:
3. The estimated post-working yield stress is calculated from the pre-working tensile characteristics based on the following calculation formula of the estimated post-working yield stress into which the constants a, b, and c are substituted. Method for estimating tensile properties of steel.

For a plurality of corresponding post-working tensile strengths and the estimated post-working tensile strengths, determine constants s, t, u that minimize the following second evaluation function;
4. The estimated tensile strength after machining is calculated from the tensile properties before machining based on the following equation for calculating the estimated tensile strength after machining into which the constants s, t, and u are substituted. The method for estimating tensile properties according to any one of the above.

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