Square steel pipe

Abstract

To provide a square steel pipe by deriving conditions of under-matching welding for avoiding a preceding yield of a seam welding part for base metal, even under the strictest conditions that a surface where the seam welding part is formed is parallel to a direction in which a column is subjected to shear force.SOLUTION: A square steel pipe has a corner round rectangular cross-section which includes four side parts and four angle parts, where a seam welding part extended in a length direction of the square steel pipe is formed on at least one side part, and, for a plate thickness t[mm] and an outer diameter D[mm] of the square steel pipe, and a reinforcement height e[mm] and a welding line width w[mm] of the seam welding part, a tensile strength σu,WM[N / mm2] of the seam welding part and a tensile strength σu,BM[N / mm2] of a base metal of the side part satisfy a formula (i) and a formula (ii).SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to square steel pipes.

Background Art

[0002] When welding steel members, in order not to prematurely fracture the welded portion with respect to the base material, generally, overmatching is performed in which the tensile strength of the weld metal is made greater than the tensile strength of the base material. For example, for a base material of tensile strength grade 490 N / mm 2 , a weld metal material and welding conditions exceeding 490 N / mm 2 are selected. However, in the welding of high-strength weld metal materials selected by overmatching, preheating is required to prevent welding cracks and ensure the strength and toughness of the weld metal, or the upper limits of the heat input and interpass temperature are strict, resulting in an increase in the labor involved in welding work, an increase in cost, and a decrease in production efficiency.

[0003] Problems due to overmatching of the weld metal as described above can also occur in the seam welds of cold-formed square steel pipes used for columns in building steel structures. In recent years, the steel grades of square steel pipes have become diversified, and square steel pipes of multiple steel grades are manufactured on the same production line. In such a case, if the weld metal material and welding conditions are set so as to achieve overmatching for each steel grade, it is necessary to replace the weld metal material and change the welding conditions of the seam weld each time the steel grade of the manufactured square steel pipe changes, which is a factor in reducing productivity.

[0004] On the other hand, for example, Patent Document 1 discloses a technique that enables under-matching welding, that is, welding in which the tensile strength of the weld metal is smaller than the tensile strength of the base material, for the welded portion between a square steel pipe column and a through diaphragm.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] Since the seam weld portion of the cold-formed square steel pipe is subjected to a shearing force in the direction along the welding line direction, in the case of under-matching welding, the welded metal material of the seam weld portion may shear yield and plastically deform prior to the base material. When such plastic deformation occurs, the assumption of plane retention of the cross-section of the square steel pipe is no longer valid, and large deformation occurs with a bending moment smaller than the bending strength assumed based on the plane retention assumption at the time of design, resulting in excessive interlayer deformation. Since under-matching welding is applied in Patent Document 1 to a welded portion that receives a tensile force in a direction perpendicular to the welding line direction, the technique of under-matching welding described in Patent Document 1 cannot be directly applied to the seam weld portion of the square steel pipe as described above.

[0007] Therefore, an object of the present invention is to derive the conditions for under-matching welding so that the seam weld portion does not yield prior to the base material even under the most severe conditions where the surface on which the seam weld portion is formed and the direction in which the column receives a shearing force are parallel with respect to the square steel pipe.

Means for Solving the Problems

[0008] [1] A square steel pipe having a square-cornered rectangular cross-section including four side portions and four corner portions, wherein at least one of the side portions is formed with a seam weld portion extending in the longitudinal direction of the square steel pipe, and with respect to the plate thickness t [mm] and outer diameter D [mm] of the square steel pipe, and the reinforcement height e [mm] and weld line width w [mm] of the seam weld portion, the tensile strength σ u,WM [N / mm 2 of the seam weld portion and the tensile strength σ u,BM [N / mm 2 of the base material of the side portion satisfy the expressions (i) and (ii). A square steel pipe. TIFF2025104311000002.tif22170[2] A square steel pipe having a cross section of a rounded rectangular shape including four side portions and four corner portions, wherein at least one of the side portions is formed with a seam weld portion extending in the longitudinal direction of the square steel pipe, and with respect to the plate thickness t [mm] and the outer diameter D [mm] of the square steel pipe, the tensile strength σ u,WM [N / mm 2 of the seam weld portion and the tensile strength σ u,BM [N / mm 2 of the base material of the side portion satisfy the formulas (iii) and (iv). A square steel pipe. TIFF2025104311000003.tif17170[3] The square steel pipe according to [1] or [2], wherein the seam weld portion is formed only on one of the side portions. [4] The square steel pipe according to [1] or [2], wherein the seam weld portion is formed on two of the side portions facing each other.

Advantages of the Invention

[0009] According to the above configuration, since the ratio of the maximum shear stress to the shear strength of the weld metal material of the seam weld portion of the square steel pipe does not exceed the ratio of the maximum shear stress to the shear strength of the base material of the side portion, it is possible to realize under-matching welding in which the seam weld portion does not yield prior to the base material.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6A

Figure 6B

Figure 7A

Figure 7B

Figure 7C

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Embodiments for Carrying Out the Invention

[0011] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the present specification and drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant descriptions are omitted.

[0012] Figs. 1 and 2 are diagrams showing examples of square steel pipes according to embodiments of the present invention. In the illustrated examples, the square steel pipe 1 has a rounded rectangular cross-section including four side portions 11 and four corner portions 13, and a seam weld portion 15 extending in the length direction of the square steel pipe 1 (X direction in the figure) is formed in at least one side portion 11. More specifically, the seam weld portion 15 is formed only in one side portion 11 as in the example of Fig. 1, or the seam weld portions 15 are formed in two side portions 11 facing each other as in the example of Fig. 2. Here, the side portion 11 is a straight portion without curvature in the cross-section of the square steel pipe 1 (Y-Z cross-section in the figure), and the corner portion 13 is a corner portion with curvature in the cross-section of the square steel pipe 1.

[0013] The square steel pipe 1 as described above is manufactured, for example, by joining the ends of cold-formed members with the seam weld portion 15. More specifically, the square steel pipe 1 in the example of Fig. 1 is manufactured by joining the ends of a member formed in a C-shaped cross-section with the seam weld portion 15. The square steel pipe 1 in the example of Fig. 2 is manufactured by joining the ends of two members formed in a U-shaped cross-section with the seam weld portion 15.

[0014] As already described, when the seam weld portion 15 in the square steel pipe 1 is subjected to under-matching welding, it is necessary to prevent the weld metal material of the seam weld portion 15 from shearing and yielding prior to the base materials of the side portion 11 and the corner portion 13.

[0015] Fig. 3 is a diagram showing the shear stress generated in the side portion and the seam weld portion of the cross-section of the square steel pipe. When the plate thickness of the square steel pipe is t [mm] and the outer diameter is D [mm], the shear stress τ at the position of the distance y [mm] from the center of the side portion with respect to the external force Q is expressed as in the following formula (1). Note that the outer diameter D of the square steel pipe is the distance between the outer surfaces of the side portions facing each other.

[0016]

Equation

[0017] Furthermore, assuming that the seam weld is located at the center of the side, when the excess height of the seam weld is e [mm] and the weld line width is w [mm], the shear stress τ at the center of the seam weld WM , and the shear stress τ on the base metal side near the end of the seam weld BM are expressed by the following formulas (2) and (3), respectively.

Equation

[0018] The above shear stress τ WM , τ BM are the maximum shear stresses acting on the weld metal material and the base metal of the side of the seam weld, respectively. Therefore, the condition for the weld metal material not to shear yield prior to the base metal is that the shear strength τ of the weld metal material u,WM and the shear strength τ of the base metal of the side u,BM and the shear stress τ WM , τ BM are expressed as in the following formula (4), Formula (4) is transformed as in formula (5). Furthermore, from the relationship between the shear strength τ u,WM , τ u,BM and the tensile strength σ u,WM , σ u,BM , the above condition can be expressed using the wall thickness t [mm] and outer diameter D [mm] of the square steel pipe, the excess height e [mm] and weld line width w [mm] of the seam weld, and the respective tensile strengths σ of the weld metal material and the base metal of the side u,WM , σ u,BM [N / mm 2 .

[0019]

Equation

[0020] Note that the fact that the seam weld is under-matching welding is expressed by the following formula (8) in which the tensile strength σ of the base metal u,BM is greater than or equal to the tensile strength σ of the weld metal material u,WM .

[0021]

Number

[0022] Here, the tensile strength of the seam weld is generally confirmed by a joint tensile test including the seam weld. When the seam weld is under-matched, since the seam weld breaks, the tensile strength of the joint tensile test including the seam weld is determined by the tensile strength of the weld metal of the seam weld. Therefore, as the tensile strength σ of the weld metal material in Equation (7), instead of the tensile strength of the weld metal of the seam weld, the tensile strength of the joint tensile test of the seam weld may be used. Alternatively, as the tensile strength of the seam weld, the tensile strength of the weld metal of the seam weld confirmed by a tensile test using a test piece taken so as to include only the weld metal part of the seam weld between punctuation marks may be used. u,WM In the above Equation (7), under general conditions, when the weld line width w of the seam weld is sufficiently small with respect to the outer diameter D of the square steel pipe (w≪D), and in the case of a general welding method such as GMAW (Gas Metal Arc Welding) or SAW (Submerged Arc Welding), the thickness of the seam weld including the reinforcement height e is about 1.15 to 1.30 times the plate thickness t of the square steel pipe (e + t≒1.15t to 1.30t). Utilizing this, it can also be expressed as the following Equation (9).

[0023]

[0024]

Number

[0025] However, since stress concentration due to shape discontinuity occurs more as the reinforcement of the seam weld is larger, in practice, it is desirable to make the seam weld under-matched within the range of the following Equation (10) obtained regressively from the FEA described later with respect to Equation (9).

[0026]

Number

[0027] Here, the smaller the reinforcement of the seam weld, the smaller the cross-section of the weld and the greater the average stress, while the stress concentration due to shape discontinuity is alleviated. That is, the stress relaxation due to the reinforcement and the stress concentration due to the shape discontinuity of the reinforcement act to cancel each other out. Therefore, within the range of Equation (9), the influence of the size of the reinforcement on the condition of under-matching welding for preventing the welded metal material from shear yielding prior to the base material can be ignored. Therefore, in the subsequent discussion, the approximate median value of e + t ≒ 1.23t within the range of e + t ≒ 1.15t to 1.30t, which is the premise of Equation (9), is targeted.

[0028] Hereinafter, the results of verification using FEA for the under-matching welding conditions shown above will be described. In FEA, the shear strength and deformation of the square steel pipe under bending shear, and the strains of the base material at the sides and corners as well as the seam weld were calculated. Figure 4 is a diagram showing the boundary conditions in the analysis. In the analysis, reverse symmetric bending shear with a forced displacement of the interlayer deformation angle of 1 / 20 was reproduced in a model of 1 / 2 of the story height considering the symmetry conditions, with a constant compressive axial force P applied. Figure 5 is a diagram showing the direction of the external force acting on the cross-section of the square steel pipe in the analysis. The loading angle θ indicating the direction of the external force was set to 90 degrees when the loading direction is parallel to the side including the seam weld, 0 degrees when the loading direction is perpendicular to the side including the seam weld, and 45 degrees or 135 degrees when the loading direction coincides with the diagonal direction of the cross-section. Figures 6A and 6B are diagrams showing the models in the analysis. In each case of the analysis, a 1 / 2 model of the cross-section was used considering the symmetry of the cross-section of the square steel pipe. Commonly in each case, the reinforcement heights on the outer surface side and the inner surface side of the seam weld were set to 5.05 mm on the outer surface side and 3.2 mm on the inner surface side. In this case, the thickness of the seam weld including the reinforcement height (36 + 5.05 + 3.2 = 44.25 mm) is approximately 1.23 times the plate thickness (36 mm) of the square steel pipe. The cross-sectional dimensions of the square steel pipe, story height L, axial force ratio, loading angle θ, and the ratio of the tensile strength of the welding material / base material σ u,WM / σ u,BM(Hereinafter, this is also called the matching rate. If it is less than 1, it will be an undermatching weld.) is shown in Table 1 below. For the cross-sectional dimensions of the square steel pipe, for example, "□600×36" means that the outer diameter D is 600 mm and the plate thickness t is 36 mm. In the case of "□600×36", the diameter-thickness ratio (D / t) of the square steel pipe is 16.6, and in the case of "□600×22", the diameter-thickness ratio (D / t) of the square steel pipe is 27.2. The bending radius R (see Figure 5) of the outside corner was 3.5 times the plate thickness in all cases (126 mm when the plate thickness t is 36 mm, and 77 mm when it is 22 mm). The floor height L (see Figure 4) was 3 m or 6 m. The axial force ratio is the ratio of the axial force acting on the square steel pipe to the yield strength of the cross section. In No. 1 to No. 23, the axial force ratio is set to 0 because the axial force acting on the square steel pipe is not taken into account. In Nos. 24 to 31, the axial force ratio was set to 0.3 or 0.5.

[0029] [Table 1]

[0030] Figures 7A, 7B and 7C are contour diagrams of the equivalent plastic strain calculated by the analysis. Among the cases with a loading angle θ = 90°, the equivalent plastic strains of cases No. 10 (matching ratio 1.00), No. 11 (matching ratio 0.90) and No. 17 (matching ratio 0.60) are shown. As shown in the graph in Figure 8, which compares cases No. 2, 8, 13 and 32 with the same matching ratio (0.80), the case of a loading angle θ = 90° has the largest ratio of equivalent plastic strain in the seam weld to the base material of the square steel pipe, and it is considered that the effect of undermatching welding is most prominent. Below, the results of cases No. 10 to No. 17 with a loading angle θ = 90° (shown as SW-0.60 to SW-1.0 in the graph using the matching ratio) will be further examined.

[0031] Fig. 9 is a graph showing the relationship between the bending moment at the end of the square steel pipe and the inter-story deformation calculated by analysis. The bending moment on the vertical axis is the fully plastic moment M p The horizontal axis of the inter-story deformation is the total plastic moment Mp The elastic deformation (bending + shear) δ at that time p is normalized respectively. Also, the yield moment M y and the corresponding elastic deformation δ y are also shown in the graph. In the relationship between the bending moment and the interlayer deformation, the results of each case almost overlapped, and no influence of the matching ratio was observed.

[0032] Figure 10 is a graph showing the equivalent plastic strain of the seam weld calculated by analysis. The vertical axis of the graph is the equivalent plastic strain of the seam weld, and the horizontal axis is the interlayer deformation normalized in the same way as in Figure 9. Regarding the equivalent plastic strain of the seam weld, it was shown that the smaller the matching ratio, that is, the larger the degree of undermatching welding, the larger it becomes. Therefore, as shown in Figure 11, the relationship between the ratio of the maximum equivalent plastic strain of the seam weld to the maximum equivalent plastic strain of the base material part (hereinafter also referred to as the maximum equivalent plastic strain ratio) and the interlayer deformation was graphed. In the case where the matching ratio is 0.85 or less, the ratio on the vertical axis exceeds 1 in the region where the interlayer deformation δ / δ p on the horizontal axis is less than 1, indicating that the plasticization of the seam weld precedes. Furthermore, as shown in Figure 12, when the maximum value of the maximum equivalent plastic strain ratio is plotted against the matching ratio (σ u,WM / σ u,BM ) and approximated by a quadratic polynomial in the range where the matching ratio (σ u,WM / σ u,BM ) is 0.6 or more and 1 or less, the following equation is obtained. Here, y represents the maximum equivalent plastic strain ratio on the vertical axis.

[0033]

Equation

[0034] Furthermore, when the matching ratio (σ u,WM / σ u,BM ) when the equation (11) becomes y = 1 is obtained, it is 0.8876. Therefore, σ u,WM / σ u,BMIt can be seen that the maximum value of the maximum equivalent plastic strain ratio does not exceed 1 in the range of ≦0.887. As described above, this result shows that, for the conditions for the welded metal material not to shear yield prior to the base material in the under-matching welding shown in equations (7) to (9), a slightly higher strength of the welded metal material is required. Specifically, when the second-order infinitesimal term is ignored in the above equation (7) with w≪D and e + t = 1.23t is substituted, the following equation (12) is obtained.

[0035]

Equation

[0036] From the coefficients of equations (10) and (12), the increase ratio of the strength of the welded metal material in the seam weld considering the stress concentration in the shape of the weld reinforcement is 0.887 / 0.813 ≒ 1.09. By using the following (13) considering this increase ratio with respect to equation (7), it is possible to more reliably prevent the plasticization of the seam weld from occurring first.

[0037]

Equation

[0038] Figure 13 is a graph obtained by additionally plotting the results of Cases No. 18 to No. 31 with the loading angle θ = 90° on the graph of Figure 12 where the results of Cases No. 10 to No. 17 are plotted as described above. In Figure 13, the area where the results of Cases No. 18 to No. 31 are plotted is enlarged and shown. In the results of the illustrated Cases No. 18 to No. 31, the matching ratio (σ u,WM / σ u,BM) The maximum equivalent plastic strain ratio of the vertical axis with respect to all cases is smaller than that of Cases No. 10 to No. 17, and the maximum equivalent plastic strain ratio does not exceed 1 in any case. From this result, it can be seen that in Cases No. 18 to No. 31, the same conditions as those of Cases No. 10 to No. 17 examined above can be applied as conditions on the safer side. The reasons why the maximum equivalent plastic strain ratio is smaller in Cases No. 18 to No. 31 are that when the story height is high, that is, when the column length of the square steel pipe is long, when the diameter-thickness ratio of the cross-section of the square steel pipe is large, and when the absolute value of the axial force ratio applied to the square steel pipe is large, the ratio of the plastic strain of the seam welded part to the base metal part is relaxed, and it is considered that the allowable range of under-matching welding becomes larger.

[0039] More specifically, in Cases No. 18 to No. 20, the story height L is 6 m, and in Cases No. 10 to No. 17, the story height L is 3 m. Therefore, it can be said that the above conditions can be applied at least when the story height L is 6 m or less. Also, in Cases No. 21 to 23, the cross-sectional dimensions of the square steel pipe are “□600×22” (diameter-thickness ratio 27.2), and in Cases No. 10 to No. 17, the cross-sectional dimensions of the square steel pipe are “□600×36” (diameter-thickness ratio 16.6). Therefore, it can be said that the above conditions can be applied at least when the diameter-thickness ratio of the cross-section of the square steel pipe is 16 or more. The upper limit of the diameter-thickness ratio of the cross-section of the square steel pipe is not particularly limited, but it is, for example, 67 or less. In Cases No. 24 to No. 31, considering the axial force applied to the square steel pipe, the axial force ratio was set to 0.3 or 0.5. However, since safer results were obtained than in Cases No. 10 to No. 17 where the axial force was not considered, it can be seen that when considering the axial force applied to the square steel pipe, more specifically, the above conditions can be applied at least when the axial force ratio is 0.5 or less.

Explanation of symbols

[0040] 1…Square steel pipe, 11…Side part, 13…Corner part, 15…Seam welded part.

Claims

Claim 1 A square steel pipe having a rounded rectangular cross-section including four side portions and four corner portions, wherein at least one of said side portions is formed with a seam weld extending in the longitudinal direction of said square steel pipe. For the plate thickness t [mm] and outer diameter D [mm] of the square steel pipe, and the buildup height e [mm] and weld line width w [mm] of the seam welded portion, the tensile strength σ ｕ，ＷＭ [N / mm ２ ] and the tensile strength σ ｕ，ＢＭ [N / mm ２ ] of the base material of the side portion satisfy Expressions (i) and (ii). A square steel pipe Claim 2 A square steel pipe having a rounded rectangular cross-section including four side portions and four corner portions, wherein at least one of said side portions is formed with a seam weld extending in the longitudinal direction of said square steel pipe. For the plate thickness t [mm] and the outer diameter D [mm] of the square steel pipe, the tensile strength σ of the seam welded part ｕ，ＷＭ [N / mm ２ and the tensile strength σ of the base material of the side part ｕ，ＢＭ [N / mm ２ satisfy the square steel pipe of Formula (iii) and Formula (iv). Claim 3 The square steel pipe according to claim 1 or claim 2, wherein said seam weld is formed on only one of said side portions. Claim 4 The square steel pipe according to claim 1 or claim 2, wherein said seam weld is formed on two of said side portions facing each other.

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