JP4696893B2 - Method for evaluating local buckling performance of steel pipe, material design method for steel pipe, and method for manufacturing steel pipe - Google Patents

Description

The present invention relates to a method for evaluating local buckling performance of a steel pipe used in a gas / petroleum pipeline, a steel pipe material design method, and a steel pipe manufacturing method .

Construction of gas pipelines and oil pipelines has been promoted as the basis of energy supply. In recent years, gas fields have often been developed far away from consumption areas, especially against the backdrop of increasing demand for natural gas. For this reason, new pipelines in recent years have a tendency to increase the distance, and the tendency to increase the diameter and pressure for mass transportation has increased.
In such a new pipeline, it has been required to use a high-strength steel pipe and to withstand a high internal pressure with a thin pipe thickness even with a large diameter. This is because by reducing the pipe thickness, the welding costs and pipe transportation costs at the site are reduced, and the total cost of construction and operation of the pipeline can be reduced.

  By the way, although a steel pipe can fully utilize the ductility of a material with respect to a tensile load, since a cross-sectional shape is a thin-walled cylinder with respect to a compression load, buckling occurs. And while the uniform elongation is around 10%, the buckling strain due to the compression load is about 1 to 2%, and in the plastic design of the pipeline, the compression local buckling strain may be a dominant factor. high. In particular, in a steel pipe having a thin pipe thickness, the compression local buckling strain tends to be small, and it is important to increase the compression local buckling strain.

Accordingly, the following proposals have been made to increase the buckling performance by increasing the compression local buckling strain.
That is, a tensile test was performed using a tensile test piece collected with the test piece longitudinal direction coinciding with the axial direction of the steel pipe, and in the obtained nominal stress-nominal strain curve, the on-load strain amount was 5% from the yield point. For any of these strains, steel pipes with a positive nominal stress / nominal strain gradient have a significantly larger limit of outer diameter / pipe thickness that causes local buckling than steel pipes with a zero or negative gradient. From the knowledge that local buckling is unlikely to occur, in the nominal stress-nominal strain curve obtained by the axial tensile test, the nominal stress / nominal strain of any strain from the yield point to the on-load strain of 5% The steel pipe has a positive gradient (see Patent Document 1).
JP-A-9-196243

As shown in Patent Document 1 above, conventionally, in order to increase the compression local buckling strain of a steel pipe, the stress strain curve of the steel material has been required to be a so-called continuous hardening type (details will be described later).
In recent years, this concept is common in the pipeline industry, and conversely, such materials with a yielding shelf that is not a continuous hardening type cannot obtain a large compression local buckling strain. It was recognized as unsuitable for steel pipes.

Here, the continuous-curing stress-strain curve is a smooth curve in which the stress increases as the strain increases without yielding a shelf after exceeding the elastic range in the stress-strain curve obtained by tensile testing of the material. (See FIG. 1).
Moreover, the yield shelf type stress-strain curve is a stress-strain curve obtained by a tensile test of a material that yields a yield shelf after a linear region (see FIG. 1). Note that the elastic region indicated by the straight line in the yield shelf type stress-strain curve is the linear region, the region where the strain increases without increasing the stress is the yield region, the smooth curved region after the end of the yield shelf is the strain hardening region, the strain The strain at which the hardening region starts is called strain hardening start strain (see FIG. 2).
Note that, as can be seen from FIG. 2, the strain hardening start strain coincides with the yield shelf end strain. Accordingly, when attention is paid to the yield shelf in this specification, it is sometimes referred to as yield shelf end point strain, and when attention is paid to the strain hardening region, it may be referred to as strain hardening starting strain, which are the same value.

As mentioned above, the compression local buckling strain of a steel pipe having a yield-shelf-type stress-strain curve (yield-shelf model steel pipe) is smaller than that of a steel pipe having a continuous-hardening-type stress-strain curve (steel pipe of a continuous hardening model). This is a general recognition. For this reason, when trying to obtain a steel pipe with high buckling performance as in the construction of a pipeline, the steel pipe of the yield shelf model is automatically excluded based on engineering judgment.
In addition, the steel pipe of the continuous hardening model is obtained by controlling the chemical composition of the steel pipe and the rolling conditions of the steel plate before pipe making, or by subjecting the steel pipe during or after pipe making to heat treatment or processing.

However, even during the production of steel pipes, even if the continuous hardening type is maintained, the material changes due to heat treatment such as a coating process for anticorrosion, and the continuous hardening type cannot be maintained. Sometimes it ends up.
In such a case, it becomes a yield shelf model, and if it is a conventional idea, such a steel pipe is judged to be unsuitable as a steel pipe for a pipeline, for example, because the local buckling performance is low. become.
However, it is not realistic to eliminate such things uniformly. However, in the past, since there was only the idea of eliminating the yield shelf model uniformly, it was not possible to determine what type could be used for the pipeline.

The present invention has been made to solve such a problem, and it is a steel pipe that determines whether a yield shelf model can be applied to an application that requires excellent local buckling performance such as a pipeline. It aims to provide a local buckling performance evaluation method.
It is another object of the present invention to provide a method for designing the material quality of a steel pipe using the technical idea of the local buckling performance evaluation method for the steel pipe.
Furthermore, it aims at providing the manufacturing method of the steel pipe using the material design method of the said steel pipe.

As described above, in the case of the steel pipe of the yield shelf model, the buckling performance of the steel pipe is low, and it has been considered that the steel pipe is inappropriate for application to a pipeline that requires a large deformation performance.
In other words, when a conventional steel pipe evaluation method is illustrated, as shown in FIG. 3A, only a continuous hardening model is used as a criterion, and in the case of a continuous hardening model, there is a possibility of application to a pipeline or the like. In the case of the non-continuous hardening model, that is, the yield shelf model, it was evaluated that there was no possibility of application to a pipeline or the like.
However, if you stick to this idea, if a model that was originally a continuous hardening model changes to a yield shelf model due to heat treatment for coating, etc., it will no longer be in the pipeline. Will not be usable.

  Therefore, the inventor felt the question of distinguishing the local buckling performance of the steel pipe by choosing between the conventional continuous hardening model and the yield shelf model, and as shown in FIG. Even if it meets the predetermined criteria, it exhibits the same local buckling performance as the continuous hardening model, and is based on the idea that there is a possibility of application to pipelines, etc. In addition, the criteria for the yield-shelf model satisfying the possibility of local buckling performance equivalent to that of the continuous hardening model are repeatedly examined, the determination method is found, and the present invention has been completed. Is.

The inventor first examined why the local buckling performance is low in the case of the yield shelf model.
The most important point to consider in the pipeline is the deformation performance against bending deformation. However, there is no theoretical formula for bending buckling strain that indicates deformation performance against bending deformation. Therefore, the inventor has paid attention to the following formula (1), which is a basic formula representing a compression local buckling strain indicating a deformation performance with respect to the compressive force of the steel pipe that receives the compressive force.

In the equation (1), ε _cr is a compression local buckling strain, ν is a Poisson's ratio, t is a tube thickness, and D is a tube diameter. In addition, E _scr indicates the slope of the line connecting the origin and the buckling point (hereinafter referred to as “secant modulus”) in FIG. 4 showing the stress-strain curve of the yield shelf model, and E _Tcr is at the buckling point. The slope of the stress-strain curve (hereinafter referred to as “tangent coefficient”) is shown. In the figure, ε _H represents the strain at the strain hardening start point. However, in FIG. 4, the stress-strain curve in the strain hardening region is drawn with a curve in order to express an arbitrary relationship.
In equation (1), substituting 0.5 as the Poisson's ratio ν for plastic deformation, the following equation (2) is obtained.

The relationship between the compression local buckling strain ε _{cr of the} steel pipe and the pipe diameter pipe thickness ratio (D / t) is shown in the aforementioned equation (2). Accordingly, FIG. 5 shows a graph of (2) with the pipe diameter ratio (D / t) on the horizontal axis and the compression local buckling strain ε _cr on the vertical axis.
As can be seen from FIG. 5, when the D / t of the steel pipe is small (thick-walled steel pipe), the compression local buckling strain ε _cr is large, and the D / t of the steel pipe increases, that is, the compression local buckling with the thinning of the steel pipe. The strain ε _cr decreases. Then, when the compression local buckling strain ε _cr coincides with the strain hardening initiation strain ε _H , the compression local buckling strain ε _cr rapidly decreases, and the subsequent compression local buckling strain ε _cr is almost the same as the yield strain ε _y. It becomes distortion.

From FIG. 5, the reason for the low buckling performance of the yield-shelf model steel pipe is that when the compression local buckling strain ε _cr coincides with the strain hardening initiation strain ε _H , the compression local buckling strain rapidly decreases. It is done. This is because, in the yield shelf region, deformation progresses without increasing the stress, so the buckling waveform grows immediately after the yield strain in steel pipes buckling in the yield shelf region, and the compression local buckling strain is approximately This is because yield strain occurs.

As discussed above, the reason why the deformation performance of the steel pipe of the yield shelf model is low is that the compression local buckling strain of the steel pipe buckling in the yield shelf region is approximately yield strain. From this, it is considered that the yield strain model steel pipe of the yield shelf model is related to the deformation performance of the steel pipe, in other words, the value of the strain hardening initiation strain ε _H in other words, the length of the yield shelf.
That is, a steel pipe having a small strain hardening initiation strain ε _H , that is, a short yield shelf length, is superior in deformation performance to a steel tube having a large strain hardening initiation strain ε _H , that is, a long yield shelf length. it is conceivable that.
Therefore, it is effective to use the value of strain hardening onset strain ε _H as an index to evaluate the deformation performance of the steel pipe of the yield shelf model.

The inventor conducted further studies on an index for evaluating deformation performance in addition to the yield shelf length.
The inventor paid attention to the fact that the compression local buckling strain ε _cr increases as E _Tcr / E _scr increases according to the equation (2). As can be seen from FIG. 4, since E _Tcr is the slope in the stress strain curve, a large slope of the stress strain curve in the vicinity of the yield shelf end point increases the compression local buckling strain ε _cr . I got the knowledge.
From this, it was found that it is effective to use the slope of the stress-strain curve as an index for evaluating the deformation performance of the steel pipe of the yield shelf model.

  As described above, the deformation performance can be evaluated by paying attention to the shape of the stress-strain curve. The stress-strain curve shape of interest here is the length of the yield shelf and the magnitude of the tangential gradient in the strain hardening zone.

The above is a schematic explanation based on the equation (2) that the deformation performance of the steel pipe can be evaluated by the shape of the stress strain curve.
In order to devise a quantitative evaluation method using mathematical formulas, the inventor has devised a mathematical formula that expresses the compression buckling strain of the yield shelf model by modifying the basic formula described above, and further studied.
Hereinafter, this point will be described in detail.

The relationship between stress and strain in the strain hardening region of the stress-strain curve shown in FIG. 4, the slope becomes as shown in FIG. 6 is represented by a straight line of mE, relationship between stress and strain in the strain hardening region, the tangent modulus E _T and secant factor E _S is expressed by the following equation.

When the strain in equation (6) is expressed as a compression local buckling strain ε _cr and substituted into equation (2), the following equation is obtained.

さらに、（8）式を下記（9）式のように変形し、（9）式の右辺第二項を一次近似すると、局部座屈歪ε_crは（10）式のように表される。 When the equation (7) is solved with respect to the compression local buckling strain ε _cr , the compression local buckling strain of the steel pipe in the strain hardening region is expressed as the equation (8).

Further, when the equation (8) is transformed as the following equation (9) and the second term on the right side of the equation (9) is linearly approximated, the local buckling strain ε _cr is expressed as the equation (10).

From the above, the compression local buckling strain ε _cr of the steel pipe of the yield shelf model can be expressed by the following formula (11).

As described above, the compression buckling strain ε _cr of the steel pipe of the yield shelf model is an index of the strain hardening coefficient m representing the slope of the stress strain curve and the length of the yield shelf as shown in Equation (11). Since it can be expressed by the strain hardening initiation strain ε _H , a method for evaluating the local buckling characteristics of the steel pipe using this equation (11) will be specifically described below.

It should be noted that the range of application of the equation (11) for estimating the compression local buckling strain of the yield shelf model is as follows for the pipe diameter ratio D / t by equalizing the compression local buckling strain and the strain hardening onset strain: It can be expressed as That is, when the stress-strain curve characteristic of the yield shelf type model is given, the maximum pipe thickness ratio (D / t) max of the applicable steel pipe is expressed by equation (12). Therefore, the equation (11), which is a local buckling strain estimation equation, cannot be applied to a steel pipe having a D / t larger than (D / t) max.

When a steel pipe is manufactured using the material of the yield shelf model when a pipe diameter D, a pipe thickness t, and a required local buckling strain ε _req are given, the steel pipe satisfies the required local buckling strain ε _req when the pipe is manufactured. In order to be applicable as a steel pipe for lines, the following requirements must be satisfied.
(1) The compression local buckling strain ε _cr of the steel pipe is larger than the required local buckling strain ε _req. (2) The local buckling of the steel pipe does not occur in the yield shelf region, in other words, the local buckling of the steel pipe is not caused. Occurring in the strain hardening region (3) The strain hardening starting strain is larger than the yield strain In other words, when all the conditions (1) to (3) are satisfied, the steel pipe can be applied as a steel pipe for a pipeline. If any of the above conditions (1) to (3) is not satisfied, it can be evaluated that the steel pipe cannot be applied as a steel pipe for pipelines.

FIG. 7 shows the above three conditions as regions on a coordinate plane in which the vertical axis is ε _{y and the} horizontal axis is ε _H.
In the following, the reason why the above three conditions are required will be described, and FIG. 7 showing the conditions will be described.

(1) Conditions under which the compression local buckling strain ε _cr of the steel pipe is larger than the required local buckling strain ε _{req In the} actual design of a steel pipe for a pipeline, the required value of the local buckling strain (the required local buckling strain ε _req ).
Therefore, in order to be able to use the steel pipe as a steel pipe for a pipeline, it is a necessary condition that the compression local buckling strain ε _{cr of the} steel pipe is larger than the required local buckling strain value ε _req . In other words, to evaluate whether the steel pipe can be used as a steel pipe for pipelines, determine whether the compression local buckling strain ε _{cr of the} steel pipe is greater than the value of the required local buckling strain ε _req It is necessary to do.
When expressing that the compression local buckling strain ε _{cr of the} steel pipe is larger than the value of the required local buckling strain ε _req using the equation (11), the following equation (13) is obtained.

When the equation (13) is arranged with respect to ε _y / m, the following equation (14) is obtained, and ε _y / m and ε _H satisfying the inequality sign of the equation (13) are represented by the region below the straight line (a) in FIG. Become. The straight line (a) is expressed by the equation (15) in which the inequality sign in the equation (14) is replaced with an equal sign. The combination of ε _y / m and ε _H on the straight line (a) indicates that ε _cr and ε _req are equal.

In addition, from the viewpoint of safety, ε _cr is required to be larger than ε _req , and thus ε _y / m and ε _H to be selected are values on a straight line positioned below and parallel to the straight line (a). It becomes. In other words, if a combination of ε _y / m and ε _H on the straight line parallel to the straight line (a) is selected, ε _cr becomes larger than ε _req .
However, ε _req cannot exceed the maximum value of ε _cr (maximum compression local buckling strain ε _crmax ). Therefore, a limit value exists parallel to the straight line (a) and below, and this limit value will be described later.

(2) The condition that local buckling of the steel pipe does not occur in the yield shelf region, in other words, the condition for the local buckling of the steel pipe to occur in the strain hardening region. It is a necessary condition that the bending strain ε _cr is not less than the strain hardening initiation strain ε _H. This condition can be expressed as the following equation (16) by replacing the strain hardening starting strain ε _req on the left side of the equation (13) with the strain hardening starting strain ε _H.

When the equation (16) is arranged with respect to ε _y / m, the following equation (17) is obtained, and the values of ε _y / m and ε _H that satisfy the inequality sign of the equation (17) are those below the curve (b) in FIG. It becomes an area. Further, the curve (b) in FIG. 7 is expressed by the equation (18) in which the inequality sign in the equation (17) is replaced with an equal sign. Ε _y / m and ε _H on the straight line (b) indicate that the compression local buckling strain ε _cr and the strain hardening initiation strain ε _req that can be applied to the steel pipe are equal.

Also, the horizontal coordinate (ε _H ) A of the intersection A of the straight line (a) and the curve (b) is a given required local buckling strain ε _req , and the vertical coordinate (ε _y / m) A Is expressed by the following equation (19) by substituting the required local buckling strain ε _req given to the above equation (18).

From the equation (17) and the curve (b) of FIG. 7 showing this as a diagram, it seems that the strain hardening initiation strain ε _H is allowed to increase to any extent. However, the strain hardening initiation strain ε _H defines the length of the yield shelf, and naturally has a maximum value. Therefore, this maximum value is examined.

By arranging the equation (17) for the strain hardening starting strain ε _H , the following equation (20) which is a quadratic equation of ε _H is obtained.

In order for the quadratic equation of equation (20) to have a real root, the discriminant must be positive as shown in equation (21). From this, the relationship between ε _y / m and t / D is expressed as in equation (22). Equation (22) shows the domain of the curve (b) with respect to the vertical axis, and the minimum value for the vertical axis of the curve (b) is equation (23). Equation (23) is the coordinate of the vertical axis of point B of curve (b).

When the relationship of equation (22) holds, the range of solutions that satisfy equation (20) is expressed by equations (24) and (25).

Equation (24) represents that ε _H is a finite value, but equation (25) allows ε _{H to} be infinite. Since ε _H is a finite value, equation (24) is adopted as a solution of equation (20), and equation (25) is rejected. Further, when the minimum value of ε _y / m given by equation (23) is substituted into equation (24), the coordinate of the horizontal axis of point B in curve (b) is obtained as in equation (26).

The coordinate (ε _H ) _{B on} the horizontal axis of the point B represented by the equation (26) indicates the maximum compression local buckling strain ε _crmax . Therefore, as described above, when the straight line (a) is translated downward, the limit value that can be translated downward is when the straight line translated downward passes through point B. Therefore, in the following, this straight line is defined as a straight line (c), and an expression representing the straight line (c) is obtained.
Temporarily, this straight line (c) is expressed as the following formula (27).

Since the straight line (c) passes through the point B, the equation (27) is expressed as the equation (28) by substituting the coordinates of the point B into the equation (27).

(3) The strain hardening initiation strain is greater than the yield strain The condition that the strain hardening initiation strain is greater than the yield strain is given by the following equation (29).

The straight line (d) in FIG. 7 represents ε _H = ε _y , and since it is a necessary condition that the strain hardening initiation strain ε _H is larger than the yield strain ε _y , the solution region is on the right side of the straight line (d). It becomes.

As described above, the solution area is obtained as shown in FIG. Therefore, to evaluate whether a steel pipe with a known pipe diameter D and pipe thickness t gives a compression local buckling strain ε _cr greater than the required local buckling strain ε _req , the yield strain ε _y of the stress strain curve, It is only necessary to determine whether the strain hardening coefficient m and the strain hardening starting strain ε _H are within the region surrounded by the straight lines (a), (c), (d), and the curve (b).
This relationship is expressed by the following two expressions.

In the above description, the yield plane ε _y , strain hardening coefficient m, and strain hardening starting strain ε _H in the steel pipe material are obtained, and these are coordinate planes with the vertical axis ε _y / m and the horizontal axis ε _H Describes the evaluation method of local buckling characteristics when a steel pipe is manufactured with the material depending on whether it is in a specific region defined by the above formulas (30) and (31).
However, the concept shown here can be applied not only to a method for evaluating local buckling characteristics, but also to a material design method for a steel pipe given a pipe diameter D, a pipe thickness t, and a required local buckling strain ε _req . That is, when designing the material of the steel pipe given the pipe diameter D, the pipe thickness t, and the required local buckling strain ε _req , the yield strain ε _y , the strain hardening coefficient m, and the strain hardening starting strain ε _H are ε _{y on} the vertical axis. The yield strain ε _y , the strain hardening coefficient m, and the strain hardening start strain ε _H may be determined so as to be within the specific region described above on the coordinate plane with / m and the horizontal axis ε _H.

  The present invention has been made on the premise of the above examination, and specifically has the following configuration.

(1) A method for evaluating local buckling characteristics of a steel pipe according to the present invention is a method for evaluating local buckling characteristics of a steel pipe given a pipe diameter D, a pipe thickness t, and a required local buckling strain ε _req , and includes stress strain Obtain the stress-strain characteristics of the material with the yield shelf on the characteristics, and yield strain ε _y , strain hardening coefficient m, strain hardening start strain ε _H of the stress-strain curve in the acquired stress-strain characteristics, and the vertical axis is ε _y / m, in the coordinate plane with ε _H as the horizontal axis, it is judged whether it is in the area shown by the following formula. If it is in the area, the steel pipe is changed to a structure that assumes plastic design. The steel pipe is evaluated as having applicability, and when it is not within the region, the steel pipe is evaluated as having no applicability to a structure that is premised on plastic design.

(2) The steel pipe material design method according to the present invention is a steel pipe material design method in which the pipe diameter D, the pipe thickness t, and the required local buckling strain ε _req are given. In determining the stress-strain characteristics of the material having a shelf, the yield strain ε _y , strain hardening coefficient m, strain hardening starting strain ε _H of the stress strain curve of the material to be designed is ε _y / m on the vertical axis, and the horizontal axis Yield strain ε _y , strain hardening coefficient m, strain hardening onset strain ε _H to determine these stress-strain characteristics in the coordinate plane where ε _H is in the region represented by the following formula It is.

(3) Moreover, the manufacturing method of the steel pipe which concerns on this invention uses the material design method of the said steel pipe of said (2), It is characterized by the above-mentioned.

  According to the method for evaluating local buckling characteristics of a steel pipe according to the present invention, since the superiority or inferiority of local buckling performance of the steel pipe can be easily determined, the use of the steel pipe can be easily determined.

[Embodiment 1]
In the present embodiment, when a steel pipe having an outer diameter D = 762.0 mm and a pipe thickness t = 15.24 mm (D / t = 50) is manufactured using nine kinds of materials having the stress-strain characteristics shown in Table 1. Based on the present invention, whether or not the steel pipe can be applied as a steel pipe for an X80 grade line pipe having a local buckling strain requirement value ε _req = 0.5% was evaluated. And whether the evaluation was appropriate was verified by FEM analysis.

Table 1 shows the stress-strain characteristics of nine types of materials related to X80 grade line pipes. Yield strain ε _y of each material is 0.0029 (0.29%), strain hardening onset strain ε _H is 0.003 (0.3%), 0.005 (0.5%) and 0.010 (1.0%). The coefficients of the strain hardening coefficient mE were set to m = 0.015, 0.020, and 0.025. (D / t) max in Table 1 is a value obtained by substituting these values into equation (12). Further, stress strain curves corresponding to P-1 to P-9 are shown in FIGS.

Substitute D = 762.0mm, t = 15.24mm, ε _y = 0.29%, ε _req = 0.5% in the following formula showing the local buckling property evaluation method of the yield shelf model, the vertical axis is ε _y / m, horizontal FIG. 11 shows a region represented by the following expression on the coordinate plane with the axis ε _H.

In FIG. 11, coordinate points (ε _y / m, ε _H ) are plotted for each of the nine types of materials shown in Table 1. Further, in FIG. 11, those within the above formula area are indicated by white circles, and those outside the area are indicated by black circles.
As can be seen from FIG. 11, P-2, P-3, P-5 and P-6 are plotted in the solution region. Therefore, if P-2, P-3, P-5 and P-6 are evaluated as acceptable and a steel pipe can be manufactured under the material design conditions of these four cases, then the compression local buckling strain ε _{cr of the} steel pipe Satisfies the required local buckling strain ε _req .

Next, whether or not the above evaluation was correct was verified by FEM analysis.
Set the outer diameter of the steel pipe for compression buckling analysis by FEM to D = 762.0mm and the pipe thickness to t = 15.24mm (D / t = 50). Table 2 shows the results of the compression buckling analysis.

Table 2 also shows the determination result based on the region of FIG.
As shown in Table 2, the compression local buckling strains ε _cr of these four analytical models by FEM for the four cases P-2, P-3, P-5, and P-6 are 0.58% and 0.82%, respectively. , 0.51% and 0.85%.
As described above, in the four cases P-2, P-3, P-5, and P-6, each buckling strain is larger than the required local buckling strain (0.5%).
As is apparent from Table 2, the results of the four cases P-2, P-3, P-5, and P-6 are determined to be acceptable by the region shown in FIG.
Therefore, the evaluation according to the present invention is consistent with the FEM analysis result, and it was verified that the present invention is effective.

[Embodiment 2]
In this embodiment, when a steel pipe having an outer diameter D = 762.0 mm and a pipe thickness t = 15.6 mm (D / t = 48.8) is manufactured using 10 kinds of materials having the stress-strain characteristics shown in Table 3. Based on the present invention, whether or not the steel pipe can be applied as a steel pipe for an X80 grade line pipe having a local buckling strain requirement value ε _req = 0.5% was evaluated.
Further, when a steel pipe having an outer diameter D = 914.4 mm and a pipe thickness t = 15.2 mm was manufactured using the same material shown in Table 3, the steel pipe had a required value of local buckling strain ε _req = 0.4%. It was evaluated whether it can be applied as a steel pipe for X80 grade line pipe.
Then, whether or not the evaluation is appropriate in any case was verified by FEM analysis.

As shown in Table 3, the yield strain ε _y of the stress-strain curve is 0.17 to 0.31%, and the strain hardening onset strain ε _H is 0.17 to 2.0%. The strain hardening coefficient mE is 0.006 to 0.025. Further, (D / t) max in the table is a value obtained by substituting these values into the equation (12).

FIG. 12 shows the above-described equations (30) and (31) showing the method for evaluating the local buckling characteristics of the yield shelf model, in which D = 762.0 mm, t = 15.6 mm, ε _req = 0.5%, and ε _y shown in Table 3. In the coordinate plane where the vertical axis is ε _y / m and the horizontal axis is ε _H. In FIG. 12, coordinate points (ε _y / m, ε _H ) are plotted for each of the ten types of materials shown in Table 3.
Looking at FIG. 12, Q-1, Q-2 and Q-3 are plotted in the solution region (pass range), and Q-4 to Q-10 are outside the solution region (fail range). It is plotted.

FIG. 13 is a graph showing the method for evaluating the local buckling characteristics of the yield shelf model. In the equations (30) and (31), D = 914.4 mm, t = 15.2 mm, ε _req = 0.4%, and ε _y shown in Table 3 In the coordinate plane where the vertical axis is ε _y / m and the horizontal axis is ε _H. In FIG. 13, coordinate points (ε _y / m, ε _H ) are plotted for each of the ten types of materials shown in Table 3.
As shown in FIG. 13, Q-1, Q-2 and Q-3 are plotted in the solution region (acceptable range), and the solution region of Q-4 to Q-10 is the same as the steel pipe of D = 762.0mm. Is plotted on the outside (failed range).

Next, whether or not the above evaluation was correct was verified by FEM analysis.
Table 4 shows the compression local buckling strains obtained by FEM analysis for the steel pipe of D = 762.0 mm and the steel pipe of D = 914.4 mm. The compression local buckling strain of steel pipe with D = 762.0mm is 0.28 ~ 0.63%, and the steel pipe with D = 914.4mm is 0.28 ~ 0.50%.

  Tables 5 and 6 show the results of checking the determination results based on the diagrams of FIGS. 12 and 13 in comparison with the FEM solution, respectively. The compression local buckling strains shown in Tables 5 and 6 are the values in Table 4 transcribed.

Table 5 shows that when the required buckling strain of a steel pipe having D = 762.0 mm is set to 0.5%, Q-1 to Q-3 pass and other material properties fail.
Table 6 shows that when the required buckling strain of a steel pipe with D = 914.4 mm is set to 0.4%, Q-1 to Q-3 pass and other material properties fail. .
In any case, the determination results based on the diagrams of FIGS. 12 and 13 and the FEM results coincide with each other, and it was verified that the present invention is effective.

In the above description, the yield strain ε _y , the strain hardening coefficient m, and the strain hardening starting strain ε _H in the steel pipe material are acquired, and these are in the coordinate plane where the vertical axis is ε _y / m and the horizontal axis is ε _H. A specific example of a method for evaluating local buckling characteristics when a steel pipe is manufactured from the material depending on whether it is within a specific region defined by the above formulas (30) and (31) has been described.
However, the concept shown here can be applied not only to a method for evaluating local buckling characteristics, but also to a material design method for a steel pipe given a pipe diameter D, a pipe thickness t, and a required local buckling strain ε _req . That is, when designing the material of the steel pipe given the pipe diameter D, the pipe thickness t, and the required local buckling strain ε _req , the yield strain ε _y , the strain hardening coefficient m, and the strain hardening starting strain ε _H are ε _{y on} the vertical axis. The yield strain ε _y , the strain hardening coefficient m, and the strain hardening start strain ε _H may be determined so as to be within the specific region described above on the coordinate plane with / m and the horizontal axis ε _H.

Specifically, when designing a steel pipe material that satisfies the requirements of D = 762.0 mm, t = 15.24 mm, and ε _req = 0.5%, these values are substituted into the above equations (30) and (31). In the coordinate plane where the vertical axis is ε _y / m and the horizontal axis is ε _H , the regions indicated by the equations (30) and (31) are drawn as shown in FIG. Then, the yield strain ε _y , the strain hardening coefficient m, and the strain hardening start strain ε _H are determined so as to be within the solution region shown in FIG. A material having such a strain hardening coefficient m and strain hardening starting strain ε _H satisfies the requirements of D = 762.0 mm, t = 15.24 mm, and ε _req = 0.5%. In this way, the material to be filled with the steel pipe material, that is, the stress-strain characteristic can be easily determined, so that efficient design becomes possible.

  In the above description, the compression local buckling strain has been described. However, since the compression local buckling strain and the bending local buckling strain have a quantitative relationship of about 1: 2, such a quantitative relationship. Can be applied to bending local buckling strain.

It is explanatory drawing of the stress-strain curve of steel materials. It is explanatory drawing of the stress-strain curve of a yield shelf type steel material. It is explanatory drawing explaining the view of this invention. It is explanatory drawing of the stress strain curve of the steel pipe formed with the yield shelf type steel material. It is a graph which shows the buckling distortion of a steel pipe, and the pipe diameter of pipe diameter / pipe thickness. It is the stress-strain curve of the yield shelf model which showed the strain hardening area | region with the straight line. It is the graph which showed the area | region which concerns on the local buckling characteristic evaluation method of this invention. It is the stress-strain curve of the material made into the object of evaluation which concerns on one Embodiment of this invention (the 1). It is the stress-strain curve of the material made into the evaluation object which concerns on one Embodiment of this invention (the 2). It is the stress strain curve of the material made into the evaluation object which concerns on one Embodiment of this invention (the 3). It is the graph which showed the area | region which concerns on the local buckling characteristic evaluation method of one Embodiment of this invention. It is the graph which showed the area | region which concerns on the local buckling characteristic evaluation method of other embodiment of this invention. It is the graph which showed the area | region which concerns on the local buckling characteristic evaluation method of other embodiment of this invention.

Claims (3)

A method for evaluating the local buckling characteristics of a steel pipe given a pipe diameter D, a pipe thickness t and a required local buckling strain ε _req , and acquiring and obtaining the stress strain characteristics of a material having a yield shelf on the stress strain characteristics In the coordinate plane where the yield strain ε _y , strain hardening coefficient m, strain hardening starting strain ε _H of the stress strain curve in the obtained stress strain characteristic is ε _y / m on the vertical axis and ε _{H on} the horizontal axis, It is judged whether it is in the indicated area, and if it is in that area, the steel pipe is evaluated as applicable to a structure that assumes plastic design, and if it is not in that area, the A method for evaluating local buckling characteristics of a steel pipe, characterized in that the steel pipe is evaluated as having no applicability to a structure premised on plastic design.

A steel pipe material design method in which a pipe diameter D, a pipe thickness t, and a required local buckling strain ε _req are given, and in determining the stress strain characteristics of a material having a yield shelf on the stress strain characteristics, In the coordinate plane where the yield strain ε _y , strain hardening coefficient m, strain hardening starting strain ε _H of the stress-strain curve of the material is ε _y / m on the vertical axis and ε _{H on} the horizontal axis. A material design method for a steel pipe, characterized in that a yield strain ε _y , a strain hardening coefficient m, and a strain hardening starting strain ε _H are determined as described in the above.

A method for producing a steel pipe, comprising producing a steel pipe using the material design method for a steel pipe according to claim 2.

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