JP7314862B2 - Rectangular steel pipe, manufacturing method thereof, and building structure - Google Patents

Description

TECHNICAL FIELD The present invention relates to a square steel pipe and a method for manufacturing the same. The square steel pipe of the present invention having excellent plastic deformability is suitably used as a building structural member, and the present invention relates to a building structure using this square steel pipe.

A square steel pipe (also referred to as a "square column") is usually made from a hot-rolled steel sheet (hot-rolled steel strip) or a thick plate, and is manufactured by cold forming. Cold forming methods include press forming and roll forming. Roll columns, which are square steel pipes manufactured by roll forming, are widely used as building structural members because of their high productivity and high cost performance. However, since the roll column is plastically strained at the time of manufacture, it is inferior in plastic deformability as compared with a four-sided box column used for building structural members manufactured only by welding, for example. Furthermore, the corners of the square steel pipe are subjected to greater strain than the flat plate portion of the square steel pipe, which causes a further deterioration in plastic deformability as a member (building structural member).

Although there are not many examples in which direct studies have been conducted on this problem, there is, for example, the technique of Patent Document 1 as a square steel pipe for building structures.

Patent Document 1 discloses a square steel pipe obtained by cold-bending a steel plate, and the square steel pipe contains C: 0.02 to 0.18% (meaning "% by mass", the same applies to chemical compositions hereinafter), Si: 0.03 to 0.5%, Mn: 0.7 to 2.5%, Al: 0.005 to 0.12%, and N: 0.008% or less (excluding 0%), and the balance is consists of Fe and unavoidable impurities, of which P: 0.02% or less (not including 0%), S: 0.01% or less (not including 0%), and O: 0.004% or less (not including 0%) are respectively suppressed, the bent portion is in a state as it is bent at a right angle, and the following requirements (A) to (C) are satisfied, thereby ensuring earthquake resistance.
(A) Yield strength at the flat part of the steel pipe: 355 MPa or more, tensile strength: 520 MPa or more,
(B) In the microstructure of the flat portion, the area fraction of the bainite structure is 40% or more.
(C) The surface layer at the corner of the steel pipe has a Vickers hardness Hv of 350 or less, an elongation in a tensile test of 10% or more, and a Charpy absorbed energy vE ₀ at 0° C. of 70 J or more.

Japanese Patent No. 5385760

As described above, the square steel pipe is formed by cold roll forming a material flat in the width direction (for example, hot rolled material) produced by hot rolling. Due to such a manufacturing method, there is a problem that the plastic deformability tends to decrease due to work hardening.

However, the technique disclosed in Patent Document 1 is limited to preventing excessive increase in hardness of the surface of the steel sheet by temperature control in hot rolling in the method for manufacturing square steel pipes, and does not suppress deterioration of plastic deformability in cold forming. Therefore, it was unclear whether the obtained square steel pipe had sufficient plastic deformability as a building structural member.

In order to suppress the deterioration of the plastic deformability, it is effective to reduce the plastic strain caused by roll forming. However, plastic strain due to roll forming always occurs when forming square steel pipes. Therefore, in the roll column, it has been difficult to achieve both realization of a desired shape and suppression of a decrease in plastic deformability.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a rectangular steel pipe having excellent plastic deformability and a method for manufacturing the same. Further, the square steel pipe of the present invention is suitable for building structural members, and it is an object of the present invention to provide a building structure using this square steel pipe.

In the present invention, "excellent plastic deformability" means that the cumulative plastic deformation ratio η in the three-point bending test of the square steel pipe is (40/α+15) or more with respect to the equivalent width-thickness ratio α. The 3-point bending test can be performed by the method described in Examples below.

The present inventors have made intensive studies to solve the above problems, and as a result, have obtained the following knowledge.

First, the present inventors paid attention to properly controlling the plastic strain of the flat plate portion and the corner portion of a square steel pipe (roll column) manufactured by roll forming. It has been found that by controlling the ratio X _R of the X-ray half width of the corner portion to the X-ray half width of the flat plate portion within an appropriate range in relation to the wall thickness t (mm) and side length H (mm) of the square steel pipe, the plastic strain can be appropriately controlled, and as a result, a square steel pipe having excellent plastic deformability can be obtained.

Furthermore, the inventors of the present invention have found that a rectangular steel pipe having excellent plastic deformability as described above can be obtained by appropriately controlling the outer peripheral length when the steel plate is formed into a cylindrical shape and the outer peripheral length when the steel plate is formed into a square tube when roll forming is performed.

The inventors of the present invention have completed the present invention through more detailed studies. The gist of the present invention is as follows.
[1] A square steel pipe having a flat plate portion and corner portions,
The square steel pipe has a yield strength of 385 to 535 MPa and a tensile strength of 520 to 670 MPa in the pipe axis direction,
A square steel pipe, wherein the ratio X _R of the X-ray half width of the corner portion to the X-ray half width of the flat plate portion is (−1.16×t/H+1.10) or more and (−1.16×t/H+1.20) or less (where t (mm) is the wall thickness of the square steel pipe, and H (mm) is the side length of the square steel pipe).
[2] The component composition is mass%,
C: 0.07 to 0.20%,
Si: 1.0% or less,
Mn: 0.6-1.5%,
P: 0.03% or less,
S: 0.015% or less,
Al: 0.01-0.06%,
N: 0.006% or less, Nb: 0.002 to 0.05%,
Ti: 0.002 to 0.05%,
V: 0.002-0.10%
The square steel pipe according to [1], containing one or more selected from among and the balance being Fe and unavoidable impurities.
[3] In addition to the above component composition, in mass%,
B: The square steel pipe according to [2], containing 0.008% or less.
[4] In addition to the above component composition, in mass%,
Cr: 1.0% or less,
Mo: 1.0% or less,
Cu: 0.50% or less,
Ni: 0.30% or less,
Ca: The square steel pipe according to [2] or [3], containing one or more selected from 0.010% or less.
[5] A method for manufacturing a square steel pipe according to any one of [1] to [4],
A steel plate having a tensile strength of 510 to 660 MPa is cold roll-formed into a cylindrical shape, and the end faces are welded,
A method for producing a square steel pipe, wherein the wall thickness of the square steel pipe is t (mm), and the side length of the square steel pipe is H (mm).
[6] A building structure in which the square steel pipe according to any one of [1] to [4] is used as a pillar material.

ADVANTAGE OF THE INVENTION According to this invention, the rectangular steel pipe excellent in plastic deformability, its manufacturing method, and a building structure can be provided. In particular, when manufacturing a square steel pipe by cold roll forming, a square steel pipe having excellent plastic deformability can be obtained. Since this square steel pipe has high earthquake resistance and the like, it can be suitably used, for example, as a square steel pipe for building structural members.

FIG. 1 is a plan view of a cross section perpendicular to the axial direction of the square steel pipe of the present invention. FIG. 2 is a graph showing the relationship between the ratio _XR of the X-ray half-value width of the corner portion to the X-ray half-value width of the flat portion and the cumulative plastic deformation ratio η in the three-point bending test of the rectangular steel pipe in a square steel pipe having a wall thickness of t and a side length of H. FIG. 3 is a graph in which the results shown in FIG. 2 are arranged using t/H and _XR . FIG. 4 is a schematic diagram showing an example of manufacturing equipment suitable for roll forming in the pipemaking process. FIG. 5 is a schematic diagram illustrating the process of forming a square steel pipe. FIG. 6 is a perspective view showing an example of a building structure using the square steel pipe of the present invention.

The present invention will be described in detail below.

An embodiment of the square steel pipe of the present invention will be described.

The square steel pipe of the present invention has a flat plate portion and a corner portion, and has a yield strength of 385 to 535 MPa and a tensile strength of 520 to 670 MPa in the axial direction of the square _steel pipe. Here, t is the wall thickness (mm) of the square steel pipe, and H is the side length (mm) of the square steel pipe. The wall thickness t and the side length (one side length) H correspond to t and H shown in the sectional view of the square steel pipe in FIG.

First, the reason why the X-ray half width is specified in the present invention will be explained.

Ratio X _R of the X-ray half-value width of the corner portion to the X-ray half-value width of the flat plate portion: (−1.16 × t / H + 1.10) or more (-1.16 × t / H + 1.20) or less X-ray half width (hereinafter sometimes referred to as “half width”) corresponds to microscopic strain introduced by cold working, and is effective as a parameter representing plastic deformability. Normally, the corner portion has larger microscopic strain than the flat plate portion, so that the half-value width becomes larger.

In the present invention, the relationship between the half-value width of the flat portion and the corner portion of the square steel pipe and the plastic deformability was investigated.

The plastic deformability of a square steel pipe changes depending on the wall thickness t, the side length H, and the yield strength. Therefore, in the present invention, various square steel pipes (thickness t, side length H, predetermined yield strength) were manufactured in which the ratio _XR of the X-ray half width of the corner portion to the X-ray half width of the flat plate portion of the square steel pipe was changed by changing the manufacturing conditions of the square steel pipe, and the plastic deformability thereof was investigated. As described above, the square steel pipe of the present invention has a yield strength of 385 to 535 MPa, but the yield strength was set to 450 MPa here.

The graphs of FIGS. 2 and 3 show the relationship between the wall thickness t, the side length H, the yield strength of the square steel pipe, the ratio _XR of the X-ray half width of the corner portion to the flat portion, and the cumulative plastic deformation ratio η, respectively. Incidentally, the cumulative plastic deformation ratio η is a value obtained by a three-point bending test of a rectangular steel pipe described in Examples below.

FIG. 2 shows the relationship between the ratio X _R of the X-ray half-value width of the corner portion to the X-ray half-value width of the flat portion and the cumulative plastic deformation ratio η in square steel pipes with various wall thicknesses t (mm) and side lengths H (mm). The symbols shown in FIG. 2 are "●" for a square steel pipe with t/H of 0.08, "▴" for a square steel pipe with t/H of 0.05, and "▪" for a square steel pipe with t/H of 0.01. In addition, the dotted line in the figure indicates a line where the cumulative plastic deformation ratio η is (40/α+15) with respect to the equivalent width-thickness ratio α. When the cumulative plastic deformation ratio η is greater than this (that is, (40/α+15) or more), the plastic deformability is judged to be sufficient, and when it is less than this, it is judged to be insufficient.

Square steel pipes are less prone to buckling and have _better plastic deformability as the wall thickness t (t/H) to the side length H increases.

As shown in FIG. 2, in a square steel pipe with the same t/H, if the ratio _XR of the X-ray half-value width of the corner portion to the X-ray half-value width of the flat portion becomes significantly large, the plastic deformability decreases. On the other hand, if the ratio X _R of the X-ray half-value width of the corner to the X-ray half-value width of the flat plate portion is small, the strain for forming the corner becomes insufficient, and the R (radius) of the corner increases.

The graph in FIG. 3 summarizes the results obtained in FIG. 2 using t/H and _XR . As for the symbols shown in FIG. 3, “×” indicates insufficient plastic deformability, and “◯” indicates sufficient plastic deformability. From the graph in FIG. 3, it was found that by controlling the X _R to be (−1.16×t/H+1.10) or more and (−1.16×t/H+1.20) or less, square steel pipes having excellent plastic deformability, which is the object of the present invention, can be obtained.

Here, as an example, a yield strength of 450 MPa is shown, but it was found that the same tendency was obtained in the yield strength range of 385 to 535 MPa for square steel pipes targeted by the present invention.

Therefore, in the square steel pipe of the present invention, the ratio X _R of the X-ray half width of the corner portion to the X-ray half width of the flat portion is limited to (−1.16×t/H+1.10) or more and (−1.16×t/H+1.20) or less. When the ratio X _R of the X-ray half-value width of the corner to the X-ray half-value width of the flat plate portion is less than (−1.16×t/H+1.10), the R (radius) of the corner increases and the cross-sectional area decreases, thereby lowering the plastic deformability. On the other hand, when the ratio X _R of the X-ray half width of the corner to the X-ray half width of the flat plate exceeds (−1.16×t/H+1.20), the microscopic strain difference between the flat plate and the corner increases, and the plastic deformability decreases. The ratio X _R of the X-ray half-value width of the corner portion to the X-ray half-value width of the flat portion is more preferably (−1.16×t/H+1.12) or more and (−1.16×t/H+1.18) or less.

The side length H of the square steel pipe is preferably 200 to 600 mm. The wall thickness t of the square steel pipe is preferably 6 to 28 mm.

Further, the square steel pipe of the present invention has a yield strength of 385 to 535 MPa and a tensile strength of 520 to 670 MPa in the pipe axial direction. The yield strength and tensile strength indicate the yield strength and tensile strength in the direction of the tube axis of the flat plate portion on the side of the square steel pipe other than the side including the welded portion (the side on the 3 o'clock or 9 o'clock side when the welded portion is set to the 12 o'clock direction).

As described above, the square steel pipe of the present invention is excellent in plastic deformability and is particularly suitable as a square steel pipe for building structural members. The yield strength (YS) in the direction of the tube axis is set to 385 to 535 MPa, and the tensile strength (TS) is set to 520 to 670 MPa in order to increase the degree of freedom in design by increasing the span and height of buildings (building structures) to which general roll columns (for example, YS 295 MPa class) are applied and increasing the effective indoor area. Preferably, the yield strength is 420-530 MPa and the tensile strength is 540-650 MPa. The yield strength and tensile strength of the square steel pipe can be measured by the methods described in Examples of the present invention described later.

In the present invention, the component composition of the square steel pipe is not limited.
As described above, the square steel pipe of the present invention can be preferably applied to members of building structures. I will explain why. Hereinafter, unless otherwise specified, "% by mass" is simply described as "%".

C: 0.07-0.20%
C is an element that increases the strength of the square steel pipe by solid-solution strengthening and formation of a hard layer. In order to obtain the desired strength, the C content is preferably 0.07% or more. On the other hand, if the C content exceeds 0.20%, there is a concern that a martensite structure is generated due to the heat effect during welding of square steel pipes, which may cause weld cracks. Therefore, C is preferably 0.07 to 0.20%. C is more preferably 0.09% or more. Also, C is more preferably 0.18% or less.

Si: 1.0% or less Si is an element that contributes to increasing the strength of square steel pipes through solid-solution strengthening. In order to ensure the desired strength of the square steel pipe, it is desirable that the Si content exceeds 0.01%. However, if the Si content exceeds 1.0%, the toughness is lowered. Therefore, Si is preferably 1.0% or less. Incidentally, Si is more preferably 0.8% or less.

Mn: 0.6-1.5%
Mn is an element that increases the strength of square steel pipes by solid-solution strengthening and microstructure refinement. If the Mn content is less than 0.6%, the temperature at which ferrite transformation starts will rise, the structure will become excessively coarse, and the strength and toughness will decrease. On the other hand, if the Mn content exceeds 1.5%, the hardness of the central segregation portion increases, which may cause cracks during welding of square steel pipes. Therefore, Mn is preferably 0.6 to 1.5%. Mn is more preferably 1.3% or less. Mn is more preferably 0.7% or more.

P: 0.03% or less P is an element that segregates at ferrite grain boundaries and has the effect of lowering the toughness of square steel pipes. In the present invention, it is desirable to reduce the amount of impurities as much as possible, preferably 0.03% or less. However, excessive reduction causes a rise in refining costs, so P is preferably 0.002% or more. The content of P is more preferably 0.025% or less.

S: 0.015% or less S exists as a sulfide in steel, and mainly exists as MnS within the range of the chemical composition of the present invention. MnS is drawn thin in the hot rolling process and adversely affects the ductility and toughness of square steel pipes. Therefore, in the present invention, it is desirable to reduce MnS as much as possible, preferably 0.015% or less. However, excessive reduction causes a rise in refining costs, so S is preferably 0.0002% or more. The S content is more preferably 0.010% or less.

Al: 0.01-0.06%
Al is an element that acts as a deoxidizing agent and has an action of fixing N as AlN. In order to obtain such an effect, the content of Al is preferably 0.01% or more. If Al is less than 0.01%, oxide-based inclusions increase and cleanliness deteriorates. On the other hand, if the Al content exceeds 0.06%, the amount of solid solution Al increases, and during longitudinal welding of square steel pipes (that is, during electric resistance welding in the longitudinal direction of steel pipes in the manufacture of square steel pipes), especially in the case of welding in the atmosphere, the risk of forming oxides at the welds increases, and the toughness of the square steel pipe welds decreases. Therefore, Al is preferably 0.01 to 0.06%. Al is more preferably 0.02% or more. Also, Al is more preferably 0.05% or less.

N: 0.006% or less N is an element that has the effect of lowering the toughness of the square steel pipe by firmly fixing the movement of dislocations. In the present invention, it is desirable to reduce N as an impurity as much as possible, preferably 0.006% or less. N is more preferably 0.005% or less. Although not specified in the present invention, N is preferably 0.001% or more from the viewpoint of manufacturing cost.

One or more selected from Nb: 0.002 to 0.05%, Ti: 0.002 to 0.05%, and V: 0.002 to 0.10% Nb, Ti, and V are all elements that form fine carbides and nitrides in steel and contribute to improving the strength of steel through precipitation strengthening. In order to obtain such an effect, Nb, Ti, and V are preferably Nb: 0.05% or less, Ti: 0.05% or less, and V: 0.10% or less, respectively. More preferably, Nb: 0.04% or less, Ti: 0.04% or less, and V: 0.08% or less. Moreover, Nb, Ti, and V are preferably Nb: 0.002% or more, Ti: 0.002% or more, and V: 0.002% or more, respectively. More preferably, Nb: 0.003% or more, Ti: 0.003% or more, and V: 0.003% or more.

When two or more selected from Nb, Ti, and V are contained, the total content is preferably 0.2% or less, preferably 0.005% or more.

It is preferable to use the above elements as a basic component composition. This basic component composition provides the properties aimed at in the present invention. In the present invention, in addition to the above elements, the following elements may be contained as necessary for the purpose of further improving strength and the like.

B: 0.008% or less (including 0%)
B is an element that delays the ferrite transformation in the cooling process, promotes the formation of low-temperature transformed ferrite, and increases the strength of the square steel pipe. When B is contained, it is preferably 0.008% or less. B is more preferably 0.0015% or less, still more preferably 0.0008% or less. B is preferably 0.0001% or more, more preferably 0.0003% or more.

One or more selected from Cr: 1.0% or less, Mo: 1.0% or less, Cu: 0.50% or less, Ni: 0.30% or less, Ca: 0.010% or less Cr: 1.0% or less (including 0%)
If the Cr content exceeds 1.0%, the toughness and weldability may be lowered. More preferably, it is 0.8% or less. Moreover, Cr is an element that increases the strength of the square steel pipe by increasing the hardenability, and can be contained as necessary. When Cr is contained in order to obtain such effects, it is preferable to contain 0.01% or more of Cr. Cr is more preferably 0.02% or more.

Mo: 1.0% or less (including 0%)
If the Mo content exceeds 1.0%, the toughness may be lowered, so when Mo is included, it is preferably 1.0% or less. More preferably, it is 0.8% or less. Mo is an element that increases the strength of the square steel pipe by increasing the hardenability, and can be contained as necessary. When Mo is contained in order to obtain such an effect, it is preferable to contain 0.01% or more of Mo. Mo is more preferably 0.02% or more.

Cu: 0.50% or less (including 0%)
If the Cu content exceeds 0.50%, the toughness may be lowered. More preferably, it is 0.4% or less. Cu is an element that increases the strength of the square steel pipe by solid-solution strengthening, and can be contained as necessary. When Cu is contained in order to obtain such an effect, it is preferable to contain 0.01% or more of Cu. Cu is more preferably 0.02% or more.

Ni: 0.30% or less (including 0%)
If the Ni content exceeds 0.30%, the area ratio of ferrite tends to decrease, so when Ni is included, it is preferably 0.30% or less. More preferably, it is 0.2% or less. Ni is an element that increases the strength of the square steel pipe by solid-solution strengthening, and can be contained as necessary. When Ni is contained in order to obtain such an effect, it is preferable to contain 0.01% or more of Ni. Ni is more preferably 0.02% or more.

Ca: 0.010% or less (including 0%)
If the Ca content exceeds 0.010%, Ca oxide clusters are formed in the steel, possibly deteriorating toughness. Therefore, when Ca is contained, the Ca content is preferably 0.010% or less. More preferably, the Ca content is 0.0050% or less. Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly drawn in the hot rolling process, and can be contained as necessary. When Ca is contained in order to obtain such effects, it is preferable to contain 0.001% or more of Ca. Ca is more preferably 0.0015% or more.

The balance other than the above elements is Fe and unavoidable impurities. However, as long as the effects of the present invention are not impaired, it is permissible to contain, for example, O (oxygen): 0.005% or less as an unavoidable impurity.

Next, one embodiment of the method for manufacturing a square steel pipe according to the present invention will be described with reference to FIGS. 4 and 5. FIG. FIG. 4 shows manufacturing equipment for electric resistance welded steel pipes as an example of manufacturing equipment suitable for roll forming in the pipemaking process. FIG. 5 shows an example of the process of forming a square steel pipe.

A method for manufacturing a square steel pipe according to the present invention is to make a square steel pipe by subjecting a steel plate to a pipe-making process. In the pipe-making process of the present invention, a steel plate is cold-roll-formed into a cylindrical shape, and the end faces are welded to form a cylindrical round steel pipe having an outer circumference of (4×H−3.0×t) or more and (4×H−1.5×t) or less. get a tube Here, t (mm) is the wall thickness of the obtained square steel pipe, and H (mm) is the side length of the obtained square steel pipe. Further, the tensile strength of the steel plate is 510-660 MPa.

First, as shown in FIG. 4, a steel strip 1, which is a raw material for an electric resistance welded steel pipe, is subjected to entry-side straightening, for example, by a leveler 2, and then intermediately formed into an open pipe by a cage roll group 3 consisting of a plurality of rolls. After that, it is finish formed by a fin pass roll group 4 consisting of a plurality of rolls. After finish forming, the width end portions of the steel strip 1 are electric resistance welded by a welding machine 6 while pressure welding is performed by squeeze rolls 5 to form a cylindrical electric resistance welded steel pipe 7 . The steel strip 1 may be, for example, a carbon steel hot-rolled steel plate. Further, in the present invention, the equipment for manufacturing the electric resistance welded steel pipe 7 is not limited to the pipe-making process shown in FIG.

Thereafter, as shown in FIG. 5, the electric resistance welded steel pipe 7 is reduced in diameter while remaining cylindrical by a sizing roll group (sizing stand) 8 consisting of a plurality of rolls, and the outer circumference is made into a cylindrical round steel pipe having a length of (4×H−3.0×t) or more and (4×H−1.5×t) or less. After that, it is sequentially formed into shapes such as R1, R2, and R3 by a group of corner forming rolls (corner forming stand) 9 consisting of a plurality of rolls, and the outer circumference becomes a square steel pipe 10 having a length of (4 x H - 5.2 x t) or more and (4 x H - 3.4 x t) or less. The number of stands of the sizing roll group 8 and the angle forming roll group 9 is not particularly limited.

Here, the welded steel pipe is formed into a cylindrical shape having an outer circumference of (4 × H - 3.0 × t) or more and (4 × H - 1.5 × t) or less, and then the outer circumference is (4 × H - 5.2 × t) or more (4 × H - 3.4 × t) or less.

When obtaining a rectangular steel pipe, forming it into a rectangular tubular shape after once forming it into a cylindrical shape has the effect of reducing the difference in microscopic strain between the corner portion and the flat plate portion. However, if the outer circumference is too long when formed into a cylindrical shape, microscopic strain will increase when formed into a square tube later, and the plastic deformability of the resulting square steel pipe will be reduced. On the other hand, if the outer circumference is too short when formed into a cylindrical shape, the angle R (R of the corner portion) becomes large when formed into a rectangular tube later, and the cross-sectional area decreases, resulting in a decrease in the plastic deformability of the obtained square steel pipe.

In addition, if the outer circumference of the square tube is too long, microscopic strain at the corners increases, and the plastic deformability of the obtained square steel pipe decreases. On the other hand, if the outer circumference is too short when formed into a square tube shape, the angle R (R of the corner portion) becomes large, and the cross-sectional area decreases, resulting in a decrease in the plastic deformability of the obtained square steel pipe.

As described above, the plastic deformability varies depending on the wall thickness (t) and side length (H) of the square steel pipe, so the permissible pipe-making strain also varies depending on the wall thickness (t) and side length (H) of the square steel pipe. For this reason, in the pipe-making process of the present invention, when forming into a cylindrical shape, the length is limited to (4×H−3.0×t) or more and (4×H−1.5×t) or less at the outer circumference, and when forming into a rectangular tube shape, the outer circumference is limited to (4×H−5.2×t) or more and (4×H−3.4×t) or less. As a result, a square steel pipe having excellent plastic deformability can be obtained.
Preferably, when molded into a cylindrical shape, the outer circumference is (4 × H - 2.9 × t) or more and (4 × H - 1.6 × t) or less, and when molded into a square tube shape, the outer circumference is (4 × H - 5.1 × t) or more and (4 × H - 3.5 × t) or less.

In addition, in the manufacturing method of the rectangular steel pipe of the present invention, the manufacturing method of the raw material of the rectangular steel pipe is not particularly limited.

For example, a steel plate (hot-rolled steel plate, hot-rolled steel strip) having a tensile strength of 510 to 660 MPa, which is suitable as a material for the square steel pipe of the present invention, is preferably produced by the following method.

Molten steel is melted using a melting method such as a converter, an electric furnace, a vacuum melting furnace, etc., and a slab (steel material) having the above-described chemical composition is formed by a casting method such as a continuous casting method. The thickness of the steel material is preferably 200 to 350 mm from the viewpoint of securing the rolling reduction in the hot rolling process.

After that, the obtained slab is heated to 1100 to 1250° C. and subjected to a hot rolling process (hot rolling process), a cooling process and a coiling process in this order under the following conditions to obtain a steel sheet (hot rolled steel sheet). You may stand to cool after a winding process.

In the hot rolling process, the heated slab is subjected to rough rolling and finish rolling to form a hot rolled sheet. At this time, it is preferable to set conditions such as rough rolling end temperature: 1000 to 850° C., finish rolling start temperature: 1000 to 800° C., finish rolling end temperature: 900 to 750° C., and finish rolling reduction ratio: 40 to 70%.

In the cooling step, the hot-rolled sheet is cooled. At this time, it is preferable to set the average cooling rate at the plate thickness center temperature: 4 to 25°C/s, and the cooling stop temperature: 350°C or higher and 650°C or lower.

In the winding step, it is preferable to wind under conditions of 350° C. or more and 580° C. or less. It should be noted that the plate thickness of the steel plate, which is the raw material for the square steel pipe, is preferably 6 to 28 mm.
After that, the obtained steel plate is subjected to the above-described pipe-making process to obtain a rectangular steel pipe.

Next, one embodiment of a building structure using the square steel pipe of the present invention will be described with reference to FIG. FIG. 6 shows an example of a building structure using the square steel pipe of the present invention as a member of the building structure.

As shown in FIG. 6, in the building structure of this embodiment, a plurality of rectangular steel pipes 11 of the present invention are erected and used as pillars. A plurality of girders 12 made of steel such as H-shaped steel are installed between the adjacent square steel pipes 11 . A plurality of small beams 13 made of steel such as H-shaped steel are installed between adjacent large beams 12 . The square steel pipes 11 and the H-section steel forming the girders 12 are welded together via through-diaphragms 14, so that the large girders 12 made of a steel material such as H-section steel are constructed between the adjacent square steel pipes 11. Further, studs 15 are provided as necessary for attachment to a wall or the like.

As described above, according to the present invention, it is possible to obtain a square steel pipe excellent in plastic deformability, which has a yield strength in the pipe axial direction of 385 to 535 MPa, a tensile strength of 520 to 670 MPa, and a ratio X _R of the X-ray half width of the corner portion to the X-ray half width of the flat portion of not less than (−1.16×t/H+1.10) and not more than (−1.16×t/H+1.20). In addition, according to the present invention, it is possible to achieve both the realization of a desired shape and the suppression of a decrease in plastic deformability, even in the case of a roll column, which was difficult with the conventional technology.

In the square steel pipe of the present invention, the difference in microscopic strain between the flat plate portion and the corner portion introduced by cold working is appropriately controlled, and the square steel pipe has excellent plastic deformability. In addition, since the building structure of the present invention exhibits excellent earthquake resistance performance compared to building structures using conventional square steel pipes, it can be suitably used as a square steel pipe for building structural members.

EXAMPLES Hereinafter, the present invention will be described using examples for further understanding. In addition, the present invention is not limited to the following examples.

The square steel pipe of the present invention will be described.

Molten steel was melted in a converter and made into slabs (steel material: thickness 250 mm) having the composition shown in Table 1 by continuous casting. These slabs (steel materials) were heated to 1200° C., subjected to a hot rolling process, a cooling process, and a coiling process, and then allowed to cool to form steel sheets (hot rolled steel sheets) having a thickness of 16 to 28 mm. Here, the hot rolling process is controlled so that the rough rolling end temperature: 1000 to 900 ° C., the finish rolling start temperature: 1000 to 850 ° C., the finish rolling end temperature: 900 to 750 ° C., and the finish rolling reduction ratio: 50 to 60%. The winding temperature was controlled to be 0°C or higher and 580°C or lower. The obtained steel sheets were subjected to a tensile test by the method shown below to measure the tensile strength.

After that, using the obtained steel plate as a raw material, a pipe-making process by cold roll forming was performed under the conditions shown in Table 2 to obtain a square steel pipe of predetermined dimensions (side length H: 400 to 550 mm, wall thickness t: 6 to 25 mm). In the pipe-making process, the outer circumference when the welded steel pipe was once formed into a cylindrical shape (cylindrical outer circumference) and the outer circumference when it was subsequently formed into a square tube (square tube outer circumference) were measured.

For the obtained square steel pipe, the X-ray half-value width of the corner portion and the flat portion was measured by X-ray diffraction, and then a test piece was taken from each of the square steel pipe and the plastic deformability was evaluated by a bending test. The evaluation methods for the X-ray half width and plastic deformability were as follows. Also, a tensile test piece was taken from the square steel pipe and subjected to a tensile test by the method shown below to measure the tensile strength and yield strength.

(1) Tensile test of steel plate In the tensile test of the steel plate, a tensile test piece of JIS No. 5 was collected so that the tensile direction was parallel to the rolling direction of the steel plate, and the tensile strength (TS) was measured according to the provisions of JIS Z 2241.

(2) X-Ray Diffraction Using the obtained square steel pipe, the X-ray half width of the flat plate portion and the corner portion was measured by the cos α method. The measurement direction was the longitudinal direction of the square steel pipe. The measurement positions of the flat plate portion were the surfaces on both sides of the surface having the welded portion (that is, the sides at 3 o'clock and 9 o'clock when the welded portion was set at 12 o'clock), and the average value was adopted. Four corners were measured and the average value was adopted.

(3) Bending test A through-diaphragm was welded to the central position of the obtained square steel pipe in the longitudinal direction to prepare a test piece. A three-point bending test was performed in which both ends of the test piece were pin-supported (rotational support) with supporting materials so that the horizontal and vertical movements of the test piece were fixed, and the central portion was repeatedly loaded with alternating positive and negative amplitudes. Using the elastic member rotation angle .theta.p corresponding to the total plastic moment Mp, each amount of deformation is given twice with gradually increasing amplitudes of 2.theta.p, 4.theta.p, 6.theta.p, . . . The loading was terminated when the yield strength suddenly decreased or when the yield strength decreased by 10% from the maximum yield strength. Good plastic deformability was determined when the cumulative plastic deformation ratio η up to the final point was (40/α+15) or more with respect to the equivalent width-thickness ratio α.

Here, the "equivalent width-thickness ratio α" is (σ _y /E) × (H/t) ² , σ _y is yield strength, and E is Young's modulus (=206 GPa). In addition, the amount of deformation was determined by measuring the displacement of both ends and the center of the specimen using a displacement meter, and calculating the rotation angle of the specimen from the measured values. “Cumulative plastic deformation ratio η” is a value obtained by dividing the cumulative value of the plastic deformation component of the rotation angle of the test piece by θp.

(4) Tensile test of square steel pipe In the tensile test of square steel pipe, JIS No. 5 tensile test pieces and JIS No. 12B tensile test pieces were taken from the flat plate portion of the square steel pipe so that the tensile direction was parallel to the pipe axial direction. Using these, a tensile test was performed in accordance with JIS Z 2241 to measure yield strength (YS) and tensile strength (TS).

The tensile test piece was taken from the width center of the flat plate portion on a side of the square steel pipe other than the side including the welded portion (the 3 o'clock or 9 o'clock side when the welded portion is set at 12 o'clock). In addition, the number of test pieces was set to 2 each, and YS and TS were obtained by calculating the average value thereof.

The results obtained are shown in Tables 2 and 3.

In the square steel pipes of the invention examples within the range of the present invention, the yield strength in the pipe axial direction was 385 to 535 MPa, and the tensile _strength was 520 to 670 MPa. On the other hand, in the square steel pipes of the comparative examples outside the scope of the present invention, the ratio X _R of the X-ray half-value width of the corner portion to the X-ray half-value width of the flat portion did not satisfy the predetermined range, and excellent plastic deformability could not be obtained.

1 Steel strip 2 Leveler 3 Cage roll group 4 Fin pass roll group 5 Squeeze roll 6 Welding machine 7 Electric resistance welded steel pipe 8 Sizing roll group 9 Corner forming roll group 10 Square steel pipe 11 Square steel pipe 12 Large beam 13 Small beam 14 Diaphragm 15 Stud

Claims (5)

A square steel pipe having a flat plate portion and a corner portion,
The square steel pipe has a yield strength of 385 to 535 MPa and a tensile strength of 520 to 670 MPa in the pipe axis direction,
The ratio X _R of the X-ray half width of the corner portion to the X-ray half width of the flat plate portion is (−1.16×t/H+1.10) or more and (−1.16×t/H+1.20) or less (where t (mm) is the wall thickness of the square steel pipe, and H (mm) is the side length of the square steel pipe);
The component composition is in mass %,
C: 0.07 to 0.20%,
Si: 1.0% or less,
Mn: 0.6-1.5%,
P: 0.03% or less,
S: 0.015% or less,
Al: 0.01-0.06%,
N: 0.006% or less
and additionally
Nb: 0.002 to 0.05%,
Ti: 0.002 to 0.05%,
V: 0.002-0.10%
A square steel pipe containing one or more selected from among and the balance being Fe and unavoidable impurities . In addition to the component composition, in mass%,
B: The square steel pipe according to claim 1 , containing 0.008% or less. In addition to the component composition, in mass%,
Cr: 1.0% or less,
Mo: 1.0% or less,
Cu: 0.50% or less,
Ni: 0.30% or less,
3. The square steel pipe according to claim 1 or 2 , containing one or more selected from Ca: 0.010% or less. A method for manufacturing a square steel pipe according to any one of claims 1 to 3 ,
A steel plate having a tensile strength of 510 to 660 MPa is cold roll-formed into a cylindrical shape, and the end faces are welded,
A method for producing a square steel pipe, wherein the wall thickness of the square steel pipe is t (mm), and the side length of the square steel pipe is H (mm). A building structure in which the square steel pipe according to any one of claims 1 to 3 is used as a pillar material.

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