KR20210114041A - Rectangular steel pipe, manufacturing method thereof, and building structure - Google Patents

Abstract

A rectangular steel pipe, a manufacturing method thereof, and a building structure using the rectangular steel pipe are provided. The present invention is a rectangular steel pipe having a flat plate portion and a corner portion, has a specific component composition, and in the steel structure at a depth of 1/4 of the plate thickness t from the outer surface of the steel pipe, ferrite is an area ratio of 55% or more and 80% or less, the average aspect ratio of the hard phase is 0.1 to 0.8, the YS of the flat part is 350 MPa or more, and the TS is 520 MPa or more, and the ratio of YS of the flat part to each part is 0.80 or more and 0.90 or less, and the TS of the flat part to each part is The ratio is 0.90 or more and 1.00 or less, the Charpy absorbed energy at -40°C of the flat part is 100 J or more, and R of each part is (2.3×t) or more and (2.9×t) or less.

Description

Rectangular steel pipe, manufacturing method thereof, and building structure

The present invention relates to a rectangular steel pipe, a method for manufacturing the same, and a building structure. The rectangular steel pipe having a small difference in strength between each part and the flat part of the present invention is preferably used as a building structural member.

A rectangular steel pipe (also referred to as a "square column") is usually manufactured by cold forming using a hot rolled steel sheet (hot rolled steel strip) or a thick sheet as a raw material. As a method of cold forming, there exist press forming and roll forming. However, in any of these methods, since a greater plastic deformation is applied to the corners of the rectangular steel pipe compared to the flat portions of the rectangular steel pipe, the strength of the respective portions is likely to increase, and there is a problem that the strength difference between the corners and the flat portion becomes large. When the characteristics are greatly different between the corner portion and the flat portion, it becomes very difficult to select a welding material or to design a building, so that it becomes difficult to use a square steel pipe as a material for building structures.

Although there are not many examples in which direct examination was performed about such a problem, there exists the description of patent document 1 as a square steel pipe for building structures, for example. In Patent Document 1, a rectangular steel pipe obtained by cold bending a steel sheet, wherein the steel pipe has a C: 0.02 to 0.18% (“%” means “mass%” and is the same for the following chemical components), Si: 0.03 to 0.5%, Mn: 0.7 to 2.5%, Al: 0.005 to 0.12%, and N: 0.008% or less (0% is not included.) , among the unavoidable impurities, P: 0.02% or less (0% is not included), S: 0.01% or less (does not include 0%), and O: 0.004% or less (does not include 0%). A cold-formed rectangular steel pipe is disclosed, which is restrained from each other, the bent portion is in a state of being processed at a right angle, and which ensures seismic resistance by satisfying the requirements of (A) to (C) below.

(A) Yield strength in the flat part of the steel pipe: 355 MPa or more, tensile strength: 520 MPa or more,

(B) in the microstructure of the flat part, the area fraction of the bainite structure: 40% or more,

(C) Vickers hardness Hv: 350 or less, elongation in tensile test: 10% or more, Charpy absorbed energy vE0: 70 J or more at 0 degreeC of the surface layer part in each part of a steel pipe.

Japanese Patent No. 5385760

A rectangular steel pipe manufactured by cold roll forming is formed by rolling a flat material (hot rolled material) in the width direction produced by hot rolling into an annular steel pipe, and then forming a rectangular steel pipe having corners and flat parts. do. On such a manufacturing method, the difference in strength between each part and a flat part tends to become large by the difference in work hardening. Furthermore, in the hot rolling performed before roll forming, since the material is manufactured by controlling cooling from the surface of the hot rolled material, in the vicinity of the surface layer of the hot rolled material where the cooling rate becomes relatively large, the strength (hardness) before processing There was a problem with growing

However, in the technique disclosed in Patent Document 1 described above, it is limited to preventing the hardness of the surface of the steel sheet from increasing excessively by temperature control in hot rolling, and actively reducing the difference in strength between each portion and the flat portion is no. Therefore, it was clear that the strength of each part of the rectangular steel pipe obtained by cold bending was relatively higher than that of the flat part, even if the characteristics of each part satisfies predetermined standards. In order to suppress an increase in the strength of each part, it is effective to reduce the plastic deformation of each part. In order to make the plastic deformation of each part small, it is conceivable to increase R (round part) of each part. However, a rectangular steel pipe having a large R of each part is not preferable because, as a rectangular member, when it is combined with other members, there is a problem in design, and there is a problem that leads to a decrease in the performance as a building due to the occurrence of a gap.

The present invention has been made in view of these circumstances, and an object of the present invention is to provide a rectangular steel pipe having a small difference in strength between each part and a flat plate, and a method for manufacturing the same, and a building structure using the rectangular steel pipe.

MEANS TO SOLVE THE PROBLEM The present inventors acquired the following knowledge, as a result of earnestly examining in order to solve the said subject.

In the present invention, in the vicinity of the surface layer of the steel pipe (hereinafter referred to as the vicinity of the outer surface), where the working strain (plastic strain) introduced in cold roll forming is particularly large (hereinafter referred to as the vicinity of the outer surface), work hardening is made difficult to occur, so that the strength of each part and the flat plate part The idea was to make the car smaller.

Therefore, the present inventors prepared a plurality of samples in which the area ratio of ferrite and the aspect ratio of hard phases other than ferrite (hereinafter referred to as hard phases) were changed as the steel structure of the rectangular steel pipe, and the easiness of work hardening was investigated. . Here, although the hard phase includes bainite, pearlite, martensite, retained austenite, etc., it is not specifically limited.

As a result, in a rectangular steel pipe having a strength of YS of 350 MPa or more and TS of 520 MPa or more in the flat portion, by setting the ratio of ferrite to a certain level or higher and the average aspect ratio of the hard phase being 0.1 to 0.8, a steel that is difficult to work harden found that tissue could be manufactured. It is thought that this is because the work hardenability of the steel structure as a whole becomes small because the work hardenability of ferrite is small, and the strain becomes easy to concentrate on the ferrite.

In addition, in order to suppress work hardening of each part by utilizing the steel structure of the raw material (hot-rolled material), the present inventors, when manufacturing a square steel pipe, first use a cylindrical annular steel pipe whose vertical diameter/lateral diameter ratio is 0.99 or more and 1.01 or less. After shaping|molding, it shape|molds into each shape whose R of each part is 2.3xt (t is plate thickness) or more and 2.9xt or less by the rolls arrange|positioned up and down and left and right. In this way, it was found that a rectangular steel pipe could be obtained without excessive work hardening of the corners. Here, "vertical diameter" refers to the outer diameter of the annular steel pipe in a vertical direction with respect to the tube axis, and "transverse diameter" refers to the outer diameter in a horizontal direction with respect to the tube axis of the annular steel pipe.

From the above, in the present invention, the ratio of ferrite to the steel structure near the outer surface of the rectangular steel pipe in which the working strain introduced during cold roll forming is greatest is set to a certain level or more, and the average aspect ratio of the hard phase is 0.1 to 0.8. In addition, it is thought that a rectangular steel pipe with a small difference in strength between each part and a flat part can be manufactured by forming the hot-rolled material into a cylindrical shape of 0.99 or more and 1.01 or less at the ratio of longitudinal diameter to 1.01, and then forming it into angular shape with rolls disposed on the top and bottom and left and right. did.

Further, in the present invention, "a square steel pipe with a small difference in strength between each part and flat part" means that the ratio of YS of the flat part to each part is 0.80 or more and 0.90 or less, and the ratio of the TS of the flat part to each part is 0.90 or more and 1.00 or less.

MEANS TO SOLVE THE PROBLEM The present inventors repeated further detailed examination, and came to complete this invention. The gist of the present invention is as follows.

[1] A rectangular steel pipe having a flat plate portion and a corner portion, comprising:

The component composition is in mass %,

C: 0.07 to 0.20%;

Si: 1.0% or less;

Mn: 0.5~2.0%;

P: 0.030% or less;

S: 0.015% or less;

Al: 0.01~0.06%;

N: 0.006% or less

contains, and the balance consists of Fe and unavoidable impurities,

In the steel structure at a depth position of 1/4 of the plate thickness t from the outer surface of the steel pipe, ferrite is 55% or more and 80% or less in area ratio, and the average aspect ratio of the hard phase is 0.1 to 0.8,

The flat portion, YS is 350 MPa or more, TS is 520 MPa or more,

The ratio of YS of the flat part to each part is 0.80 or more and 0.90 or less, and the ratio of TS of the flat part to each part is 0.90 or more and 1.00 or less,

The Charpy absorbed energy at -40 ° C of the flat portion is 100 J or more,

R of each part is (2.3 × t) or more (2.9 × t) or less

prismatic steel pipe.

[2] The rectangular steel pipe according to [1], further comprising, in mass%, one or two or more groups selected from the following groups A to C, in addition to the above component composition.

Group A: Nb: 0.05% or less, Ti: 0.05% or less, V: 0.10% or less 1 type or 2 or more types selected from

Group B: B: 0.008% or less

Group C: Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.30%, Ca: 0.001 to 0.010% One or more selected from among

[3] The rectangular steel pipe according to [1] or [2], wherein the steel structure further has an average equivalent circle diameter of the hard phase of 20 µm or less.

[4] The method for manufacturing a rectangular steel pipe according to any one of [1] to [3], comprising:

A method for producing a rectangular steel pipe in which a steel sheet is rolled in cold, a cylindrical cross-section is welded, and then formed into a cylindrical shape of 0.99 or more and 1.01 or less at a ratio of vertical diameter to horizontal diameter, followed by a pipe making step of forming a square shape .

[5] The method for manufacturing a rectangular steel pipe according to any one of [1] to [3], comprising:

When a square steel pipe is manufactured by performing a hot rolling process, a cooling process, a winding process, and a pipe making process on a steel material in this order,

After heating the steel material to a heating temperature: 1100 ~ 1300 ℃,

Rough rolling time at a plate thickness center temperature of 1000 ℃ or more: 200 sec or more and 400 sec or more, Rough rolling end temperature: 1000 ~ 800 ℃,

A hot-rolling step of starting temperature of finish rolling: 1000 to 800° C. and finishing temperature of finish rolling: 900 to 750° C. is performed to obtain a hot-rolled sheet;

Next, the hot-rolled sheet was subjected to cooling at least once for 0.2 s or more and less than 3.0 s in the initial cooling step for 10 s from the start of cooling, and the average cooling rate at the plate thickness center temperature: 4 to 25°C/s performing a cooling process that

Next, a winding process of winding the hot-rolled sheet at a winding temperature: 580 ° C. or less is performed to obtain a steel sheet,

Next, the steel sheet is roll-formed in cold, and the cylindrical cross section is welded to form a cylindrical shape of 0.99 or more and 1.01 or less at the ratio of vertical diameter to horizontal diameter, and then a rectangular steel pipe manufacturing process is performed. .

[6] The method for manufacturing a rectangular steel pipe according to [5], wherein the cooling stop temperature of the cooling step is 580°C or less.

[7] A building structure using the rectangular steel pipe according to any one of [1] to [3].

According to the present invention, it is possible to obtain a rectangular steel pipe having a small difference in strength between each part and the flat plate part. This rectangular steel pipe can be suitably used, for example, as a rectangular steel pipe for building structural members because R of each part is controlled to an appropriate size.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram which shows an example of the manufacturing facility of an electric resistance resistance steel pipe.
Fig. 2 is a schematic diagram showing the forming process of each steel pipe.
3 is a perspective view schematically showing an example of a building structure using the rectangular steel pipe of the present invention.
4 is a schematic diagram showing a cross section of each steel pipe.

Hereinafter, the present invention will be described in detail.

The rectangular steel pipe of the present invention is as follows. Component composition, in mass%, C: 0.07 to 0.20%, Si: 1.0% or less, Mn: 0.5 to 2.0%, P: 0.030% or less, S: 0.015% or less, Al: 0.01 to 0.06%, N: 0.006 % or less, and the balance consists of Fe and unavoidable impurities. In the steel structure at a depth position of 1/4 of the plate thickness t from the outer surface of this rectangular steel pipe (hereinafter referred to as a 1/4t position), ferrite is 55% or more and 80% or less in area ratio, and the average of the hard phase The aspect ratio is 0.1 to 0.8. In addition, the flat plate portion of the square steel sheet has YS of 350 MPa or more and TS of 520 MPa or more, the ratio of YS of the flat part to each part is 0.80 or more and 0.90 or less, and the ratio of TS of the flat part to each part is 0.90 or more and 1.00 or less, The Charpy absorbed energy at -40°C at the plate thickness 1/4t position of the flat plate is 100 J or more, and R of each part is (2.3×t) or more and (2.9×t) or less.

First, the component composition of the present invention will be described. Incidentally, unless otherwise specified, mass % is simply expressed as %. In the present invention, the component composition of the rectangular steel pipe and the steel sheet used for the raw material of the rectangular steel pipe is the same. Therefore, in the following, explanation will be made as a reason for limiting the component composition of the square steel pipe and the steel sheet used for the raw material.

C: 0.07 to 0.20%

C increases the strength of the steel sheet and the rectangular steel pipe by solid solution strengthening. On the other hand, C is an element that decreases the amount of ferrite produced by increasing the amount of hard phase produced. In order to secure a desired strength and a desired steel structure of a steel sheet and a rectangular steel pipe, the content of C is required to be 0.07% or more. On the other hand, the content of C exceeding 0.20% makes it difficult to ensure a desired amount of ferrite. For this reason, C is set to 0.07 to 0.20%. C is preferably 0.09% or more, and more preferably 0.10% or more. Moreover, C becomes like this. Preferably it is 0.18 % or less, More preferably, it is 0.17 % or less.

Si: 1.0% or less

Si is an element contributing to an increase in strength of a steel sheet and a rectangular steel pipe by solid solution strengthening. In order to ensure the strength of a desired steel plate and a square steel pipe, it is preferable to contain Si in excess of 0.01 %. However, when Si is contained exceeding 1.0 %, toughness will fall. For this reason, Si is made into 1.0 % or less. Moreover, Si becomes like this. Preferably it is 0.8 % or less, More preferably, it is 0.6 % or less. More preferably, it is 0.03% or more.

Mn: 0.5~2.0%

Mn is an element that increases the strength of a steel sheet and a rectangular steel pipe through solid solution strengthening, and in order to secure the desired strength of a steel sheet and a rectangular steel pipe, it is required to contain 0.5% or more. When the Mn content is less than 0.5%, an increase in the ferrite transformation initiation temperature is caused, whereby the hard phase tends to be excessively coarsened. On the other hand, when Mn is contained in an amount exceeding 2.0%, the hardness of the central segregation portion increases, which may cause cracks during in-situ welding of the rectangular steel pipe. For this reason, Mn is set to 0.5 to 2.0%. Mn becomes like this. Preferably it is 1.8 % or less, More preferably, it is 1.6 % or less. Mn becomes like this. Preferably it is 0.6 % or more, More preferably, it is 0.7 % or more.

P: 0.030% or less

P is an element having an action of segregating at the ferrite grain boundary and reducing the toughness of the steel sheet and the rectangular steel pipe. In this invention, it is preferable to reduce as much as possible as an impurity. However, excessive reduction of P results in higher refining costs, so it is preferable to set it as 0.002% or more. In addition, the content of P is permissible up to 0.030%. For this reason, P is made into 0.030 % or less. P is preferably 0.025% or less, and more preferably 0.020% or less.

S: 0.015% or less

S exists as a sulfide in steel, and exists mainly as MnS if it is the range of the component composition of this invention. MnS is thinly stretched in the hot rolling process, and adversely affects the ductility and toughness of the steel sheet and the rectangular steel pipe. For this reason, in this invention, it is preferable to reduce MnS as much as possible. However, since excessive reduction of S results in higher refining costs, S is preferably set to 0.0002% or more. In addition, the content of S can be allowed up to 0.015%. For this reason, S is made 0.015% or less. S is preferably 0.010% or less, and more preferably 0.008% or less.

Al: 0.01~0.06%

Al is an element which acts as a deoxidizer and has an effect|action which fixes N as AlN. In order to obtain such an effect, it is necessary to contain 0.01% or more of Al. If Al is less than 0.01%, deoxidation power is insufficient in the case of no Si addition, oxide-based inclusions increase, and the cleanliness of the steel sheet decreases. On the other hand, the content of Al exceeding 0.06% increases the amount of dissolved Al, and in the case of longitudinal welding of a rectangular steel pipe (that is, electric resistance welding in the longitudinal direction of the steel pipe in the manufacture of a rectangular steel pipe), especially in the case of welding in the atmosphere Therefore, the risk of forming oxides in the welded portion increases, and the toughness of the welded portion of the rectangular steel pipe decreases. For this reason, Al is set to 0.01 to 0.06%. Al is preferably 0.02% or more. Moreover, Al becomes like this. Preferably it is 0.05 % or less.

N: 0.006% or less

N is an element having an action of lowering the toughness of a steel sheet and a rectangular steel pipe by firmly fixing the motion of dislocations. In the present invention, it is preferable to reduce N as an impurity as much as possible, and up to 0.006% is acceptable. For this reason, N is made into 0.006 % or less. N is preferably 0.005% or less. Although not specifically prescribed|regulated in this invention, it is preferable to make N into 0.001 % or more from a viewpoint of manufacturing cost.

The remainder is Fe and unavoidable impurities. However, in the range which does not impair the effect of this invention, it is permissible to contain O (oxygen):0.005 % or less as an unavoidable impurity, for example.

The above is the basic component composition of the present invention. Although the properties aimed at in the present invention are obtained with the above essential elements, the following elements may be contained as necessary.

One or two or more selected from Nb: 0.05% or less, Ti: 0.05% or less, V: 0.10% or less

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. For this reason, in this invention, you may contain for the purpose of adjusting intensity|strength. In order to obtain such an effect, in the case of containing Nb, Ti, and V, it is preferable that Nb: 0.05% or less, Ti: 0.05% or less, V: 0.10% or less, respectively, Nb: 0.04% or less, Ti : 0.04 % or less, V: It is more preferable to set it as 0.08 % or less. When containing Nb, Ti, and V, it is preferable to set it as Nb: 0.001 % or more, Ti: 0.001 % or more, V: 0.001 % or more, respectively, Nb: 0.003 % or more, Ti: 0.003 % or more, V: respectively. It is more preferable to set it as 0.003 % or more.

Moreover, when it contains 2 or more types selected from Nb, Ti, and V, it is preferable to set it as 0.2 % or less in total, and it is preferable to set it as 0.005 % or more.

B: 0.008% or less

B is an element having an action of delaying the ferrite transformation in the cooling process, promoting the formation of low-temperature transformation ferrite, and increasing the strength of the steel sheet and the rectangular steel pipe. The inclusion of B leads to an increase in the yield ratio of the steel sheet, that is, the yield ratio of the rectangular steel pipe. For this reason, in the present invention, as long as the yield ratio of the rectangular steel pipe is in the range of 90% or less, B may be contained as needed for the purpose of adjusting the strength. When B is contained, it is preferable to set it as B: 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.

Cr: 0.01 ~ 1.0 %, Mo : 0.01 ~ 1.0 %, Cu : 0.01 ~ 0.50 %, Ni : 0.01 ~ 0.30 %, Ca : 0.001 ~ 0.010 % One or more selected from among

Cr: 0.01~1.0%

Cr is an element that increases the strength of a steel sheet and a rectangular steel pipe by improving hardenability, and may be contained as necessary. In the case of containing Cr in order to obtain such an effect, it is preferable to contain 0.01% or more of Cr. On the other hand, since there exists a possibility that toughness and weldability may fall when it contains exceeding 1.0 %, when it contains Cr, it is preferable to set it as 1.0 % or less. Cr is more preferably 0.02% or more, and more preferably 0.8% or less.

Mo: 0.01~1.0%

Mo is an element which increases the strength of a steel plate and a square steel pipe by improving hardenability, and can be contained as needed. When Mo is contained in order to obtain such an effect, it is preferable to contain 0.01% or more of Mo. On the other hand, when Mo is contained in excess of 1.0%, there is a fear that toughness is reduced. Mo is more preferably 0.02% or more, and more preferably 0.8% or less.

Cu: 0.01~0.50%

Cu is an element that increases the strength of a steel sheet and a rectangular steel pipe by solid solution strengthening, and may be contained as necessary. In the case of containing Cu in order to obtain such an effect, it is preferable to contain 0.01% or more of Cu. On the other hand, since there exists a possibility that toughness may fall when Cu is contained exceeding 0.50 %, when Cu is contained, it is preferable to set it as 0.50 % or less. Cu is more preferably 0.02 % or more, and more preferably 0.4 % or less.

Ni: 0.01~0.30%

Ni is an element that increases the strength of a steel sheet and a rectangular steel pipe by solid solution strengthening, and may 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. On the other hand, when Ni is contained in an amount exceeding 0.30%, there is a fear that the area ratio of ferrite is likely to decrease. Therefore, when Ni is contained, it is preferably 0.30% or less. Ni is more preferably 0.02% or more, and more preferably 0.2% or less.

Ca: 0.001 to 0.010%

Ca is an element which contributes to the toughness improvement of steel by spheroidizing sulfides, such as MnS, extended thinly in a hot rolling process, and can contain it as needed. In order to obtain such an effect, when Ca is contained, it is preferable to contain 0.001% or more of Ca. However, when Ca content exceeds 0.010 %, Ca oxide clusters may be formed in steel, and there exists a possibility that toughness may deteriorate. For this reason, when Ca is contained, it is preferable that Ca content shall be 0.001 to 0.010 %. Ca is more preferably 0.0015% or more, and more preferably 0.0050% or less.

Next, the steel structure of the rectangular steel pipe of the present invention will be described.

In the steel structure at a position 1/4t from the outer surface of the rectangular steel pipe of the present invention, ferrite is 55% or more and 80% or less in area ratio, and the average aspect ratio of the hard phase is 0.1 to 0.8. The steel structure can further set the average equivalent round diameter of the hard phase to 20 µm or less.

Ferrite: 55% or more and 80% or less by area ratio

In the rectangular steel pipe of the present invention, in order to secure a desired strength, the steel structure at a position 1/4t from the outer surface of the steel pipe is made of ferrite and other hard phases. Here, the hard phase includes phases other than ferrite, ie, bainite, pearlite, martensite, retained austenite, and the like. This hard phase is 20 to 45% of the total area ratio of each phase.

When the ferrite content is less than 55% in terms of area ratio, the proportion of bainite becomes excessive in the range of the component composition of the present invention, and the strain tends to be dispersed in the hard phase, so that it is easy to work harden. As a result, a rectangular steel pipe with a small difference in strength between each part and a flat part cannot be obtained. On the other hand, when ferrite exceeds 80% in area ratio, desired strength cannot be obtained. Ferrite is preferably 60% or more, and preferably 75% or less.

Average aspect ratio of hard phase: 0.1 ~ 0.8

If the average aspect ratio of the hard phase is less than 0.1, the origin of cracks tends to occur, so that toughness decreases. On the other hand, when the average aspect-ratio of the hard phase exceeds 0.8, the hard phase tends to be dispersed in strain, so that it is easy to work harden. As a result, when a rectangular steel pipe is manufactured using a steel plate, a rectangular steel pipe with a small difference in strength between each part and a flat part cannot be obtained. More preferably, it is 0.2 or more, More preferably, it is 0.7 or less. In the present invention, as will be described later, the aspect ratio of the hard phase is an average value of the aspect ratio of particles having a nanohardness of 3.0 GPa or more in structures other than ferrite.

Average equivalent round diameter of hard phase: 20 µm or less (suitable conditions)

Since toughness will fall when the average equivalent circle diameter of a hard phase exceeds 20 micrometers, it is preferable to set it as 20 micrometers or less. More preferably, it is 15 micrometers or less. In the present invention, as will be described later, the average equivalent diameter of the hard phase is an average value of the equivalent diameters of particles having a nanohardness of 3.0 GPa or more in structures other than ferrite.

In general, in a rectangular steel pipe manufactured by roll forming using a steel sheet (hot rolled steel sheet) as a raw material, since the steel structure at the 1/4t position is the same for both the corner part and the flat plate part, the flat part 1/4t position or the leg part It may be measured anywhere at the 1/4t position of . Here, the steel structure at the 1/4t position of the flat plate is prescribed.

In this invention, even if the above-mentioned steel structure exists in the range of 3/16t position - 5/16t position of a steel pipe, the above-mentioned effect is similarly acquired. Therefore, in the present invention, "steel structure at 1/4t position" means that the above-mentioned steel structure exists anywhere in the range of the 3/16t position to the 5/16t position.

The above-mentioned steel structure is observed by the following method, and the type of structure and area ratio (%) are calculated|required. A test piece for tissue observation is prepared by taking a rectangular steel pipe, grinding it so that the cross section in the rolling direction (L section) becomes an observation face, and performing nital corrosion. The tissue observation was performed with an optical microscope (magnification: 500 times) or a scanning electron microscope with the tissue at a plate thickness of 1/4 t from the surface of the tissue observation specimen (that is, the outer surface of the rectangular steel pipe) as the center of observation. (SEM, magnification: 500 times) The steel structure is observed and imaged. The measurement area was set to 500 µm × 500 µm. Here, "t" represents the thickness (plate thickness) of the steel pipe. From the obtained structure photograph, the type of structure is specified using an image analysis apparatus (image analysis software: Photoshop, manufactured by Adobe), and the area ratio of ferrite is calculated. The area ratio of the structure was observed in 5 or more fields of view, and was calculated as the average value of the values obtained in each field of view.

In addition, the average aspect-ratio of a hard phase was calculated|required as follows. First, the nanohardness was calculated|required by the nanoindentation method about structures other than ferrite from the structure photograph obtained above. For particles having a nanohardness of 3.0 GPa or more, a value calculated by (average of length in plate thickness direction/average of length in rolling direction) of all the particles was obtained, and the average aspect ratio was determined.

In addition, the average equivalent circle diameter of a hard phase was measured using the SEM/EBSD method. The average equivalent circle diameter calculated|required the orientation difference of the adjacent crystal grain, and measured the boundary whose orientation difference is 15 degrees or more as a crystal grain boundary. The arithmetic mean of the particle diameters was calculated|required from the obtained crystal grain boundary, and it was set as the average round equivalent diameter. The measurement area was 500 µm × 500 µm, and the measurement step size was 0.5 µm. In addition, in the crystal grain size analysis, the thing with a crystal grain diameter of 2.0 micrometers or less was made into measurement noise, and the thing of nano-hardness less than 3.0 GPa was excluded from an analysis target as a non-hard phase.

Next, the manufacturing method of the rectangular steel pipe of this invention is demonstrated using FIG. 1, FIG. BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram which shows an example of the manufacturing facility of an electric resistance resistance steel pipe. Fig. 2 is a schematic diagram showing the forming process of each steel pipe.

The method for manufacturing a rectangular steel pipe of the present invention is to perform a pipe making process on a steel sheet to obtain a rectangular steel pipe. In the pipe making process of this invention, the steel plate is roll-formed by cold, and the cylindrical cross section is welded. Next, after forming into a cylindrical annular steel pipe of 0.99 or more and 1.01 or less at the ratio of vertical diameter and horizontal diameter, the annular steel pipe is further cold formed into square shapes by rolls arranged on the top and bottom and left and right, molded into steel pipe.

First, as shown in Fig. 1, the steel strip 1, which is the raw material of the electric resistance welded steel pipe, is in the middle in the cage roll group 3 consisting of a plurality of rolls after performing vertical correction by the leveler 2, for example. After shaping|molding to become an open tube, it is finish-molded by the pin pass roll group 4 which consists of a plurality of rolls. After finish forming, the width end of the steel strip 1 is electrically resistance-welded with a welding machine 6 while pressing with a squeeze roll 5 to obtain a cylindrical electric resistance welded steel pipe 7 . In addition, in this invention, the manufacturing facility of the electric resistance resistance steel pipe 7 is not limited to the pipe making process like FIG.

Thereafter, as shown in Fig. 2, the electric resistance resistance steel pipe 7 is reduced in diameter while being in a cylindrical shape by a sizing roll group (sizing stand) 8 composed of a plurality of rolls, and has a cylindrical shape of 0.99 or more and 1.01 or less at a ratio of vertical diameter/lateral diameter. becomes Then, it is sequentially molded into the same shape as R1, R2, R3 by a square forming roll group (a square forming stand) 9 composed of a plurality of rolls to form a square steel pipe 10 . In addition, the number of stands in particular of the sizing roll group 8 and the square forming roll group 9 is not restrict|limited.

Here, the reason for shaping|molding into the cylindrical shape of 0.99 or more and 1.01 or less by the ratio of a vertical diameter/horizontal diameter before shaping|molding into a square shape is demonstrated here.

In the present invention, for the following reasons, it is important to set the ratio of the vertical diameter to the horizontal diameter to be 0.99 or more and 1.01 or less. In general, when a steel pipe is manufactured by roll forming, in the process, non-uniform deformation in the circumferential direction is often applied for the purpose of suppressing springback. However, in the case where it is assumed that forming into a square shape is the last, the cross section of the cylindrical shape, which is the shear layer, does not necessarily need to be a perfect circle. Therefore, even if it has a cylindrical shape, it is not necessarily a perfect circle at a stage in the course of manufacturing a square steel pipe, and as a result, the obtained square steel pipe cannot reduce the characteristic difference between the flat part and each part. In this regard, in the present invention, in order to reduce the characteristic difference between the flat plate portion and the leg portion, it is essential to form the shape of the shear layer into a cylindrical shape with a vertical diameter/lateral diameter ratio of 0.99 or more and 1.01 or less.

If it is not molded into a cylindrical shape of 0.99 or more and 1.01 or less at the ratio of the above-mentioned vertical diameter/lateral diameter, the plastic deformation of each part becomes excessively large compared to the flat part. As a result, the ratio of YS of the flat portion with respect to each portion is less than 0.80, and the ratio of TS of the flat portion with respect to each portion is less than 0.90. Moreover, since the plastic deformation of each part becomes large compared with a flat part, it is natural that the ratio of YS of the flat part to each part will be 0.90 or less, and the ratio of TS of the flat part to each part will be 1.00 or less. Therefore, the YS of the flat part intended in the present invention is 350 MPa or more and the TS is 520 MPa or more, the YS ratio of the flat part to each part is 0.80 or more and 0.90 or less, and the ratio of the TS of the flat part to each part is 0.90 or more and 1.00 or less In order to achieve this, before angular molding, it is molded into a cylindrical shape with a ratio of vertical diameter/lateral diameter of 0.99 or more and 1.01 or less.

In addition, by molding into a cylindrical shape of 0.99 or more and 1.01 or less at the above-mentioned ratio of vertical diameter/horizontal diameter, each part can be uniformly molded at the time of square molding, so that R of each part is (2.3 × t) or more and (2.9 × t) or less (here, t is the plate thickness). When R of each part becomes (2.3 x t) or more and (2.9 x t) or less (here, t is plate thickness), the difference in strength between each part and flat part can be made small.

As described above, according to the present invention, YS of the flat part is 350 MPa or more and TS is 520 MPa or more, the ratio of YS of the flat part to each part is 0.80 or more and 0.90 or less, and the ratio of TS of the flat part to each part is 0.90 or more 1.00 Hereinafter, since the Charpy absorbed energy at -40°C of the flat portion is 100 J or more and the R of each portion is (2.3 × t) or more and (2.9 × t) or less, a rectangular steel pipe with a small difference in strength between the flat portion and the flat portion can be obtained. This rectangular steel pipe can be particularly preferably used as a rectangular steel pipe for building structural members because R of each part is controlled to an appropriate size, and the difference in strength between each part and a flat part is small.

In addition, as described above, as the raw material for the square steel pipe of the present invention, a steel sheet (hot rolled steel sheet) obtained by performing the hot rolling process, cooling process, and winding process described below in this order can be preferably used. In the present invention, the steel sheet may be subjected to the above-described pipe making step to obtain a rectangular steel pipe.

An example of a method for manufacturing a steel sheet suitable as a raw material for the rectangular steel pipe of the present invention will be described.

A method for producing a steel sheet suitable as a raw material for a rectangular steel pipe of the present invention is, for example, a hot rolling process (hereinafter referred to as a hot rolling process), cooling a steel raw material having the above-described component composition, under the conditions described below. The process and the winding process can be performed in this order, and it can be set as a steel plate (hot-rolled steel plate).

For example, after heating a steel material having the above component composition to a heating temperature: 1100 to 1300 ° C, rough rolling time at a plate thickness center temperature of 1000 ° C or more: 200 seconds or more and 400 seconds or less, rough The rolling end temperature: 1000 to 800°C, the finish rolling start temperature: 1000 to 800°C, and the finish rolling end temperature: 900 to 750°C are subjected to a hot rolling process to obtain a hot-rolled sheet. Next, the hot-rolled sheet after the hot-rolling step is allowed to cool at least once for 0.2 s or more and less than 3.0 s in the initial cooling period for 10 s from the start of cooling, and the average cooling rate at the plate thickness center temperature of the hot-rolled sheet: 4 to 25 °C/s, cooling stop temperature: A cooling step of 580 °C or less is performed. Next, the hot-rolled sheet after a cooling process is wound up at winding temperature: 580 degrees C or less, and the winding-up process of standing to cool after that is implemented, and a steel plate (hot-rolled steel sheet) is obtained.

Below, each process is demonstrated in detail. In addition, in the description of the manufacturing method below, the temperature (°C) is the surface temperature of a steel raw material, a sheet bar, a hot-rolled sheet, or a steel sheet, unless otherwise specified. These surface temperatures can be measured with a radiation thermometer or the like. The average cooling rate (°C/s) is, unless otherwise specified,

((temperature before cooling - temperature after cooling)/cooling time)

to the value obtained by .

In the present invention, the melting method of the steel material (steel slab) having the above-described component composition is not particularly limited, and can be melted using a known melting method such as a converter, an electric furnace, and a vacuum melting furnace. The casting method is also not specifically limited, It can manufacture to a desired dimension by well-known casting methods, such as a continuous casting method. In addition, there is no problem even if the ingot-ingot rolling method is applied instead of the continuous casting method. In addition to molten steel, you may perform secondary refining, such as briquette refining.

Next, the obtained steel raw material (steel slab) is subjected to a hot rolling process. In the hot rolling process, the steel material is heated to a heating temperature of 1100 to 1300°C. Then, for the heated steel material extracted from the heating furnace, the rough rolling time when the plate thickness center temperature of the steel material is 1000 ° C. or more: 200 seconds or more and 400 seconds or less After controlling, the rough rolling end temperature: 1000 ~ 800 ° C. After rough rolling, the finish rolling is performed at a finish rolling start temperature: 1000 to 800°C and finish rolling end temperature: 900 to 750°C to obtain a hot-rolled sheet.

In addition, the temperature of the plate|board thickness center of the steel raw material in a hot rolling process is calculated|required by calculating the temperature distribution in the steel raw material cross section by electrothermal analysis.

Heating temperature: 1100~1300℃

If the heating temperature of the steel material is less than 1100°C, the deformation resistance of the material to be rolled becomes excessively large, and the load-bearing mill and the finish rolling mill lack load-bearing and rolling torque, making rolling difficult. On the other hand, when heating temperature exceeds 1300 degreeC, austenite crystal grains will coarsen, and even if it repeats processing and recrystallization of austenite grains in rough rolling and finish rolling, it becomes difficult to refine|refine it. Moreover, it may become difficult to ensure the desired toughness in a hot-rolled steel sheet. For this reason, the heating temperature of a steel raw material shall be 1100-1300 degreeC. Heating temperature becomes like this. Preferably it is 1280 degrees C or less. The heating temperature is preferably 1150°C or higher.

In addition, when the load-bearing capacity and rolling torque of each rolling mill have a margin, you may select as a heating temperature the temperature in the range of 1100 degrees C or less Ar3 transformation point or more.

The heated steel raw material is then subjected to rough rolling to obtain a sheet bar or the like.

Rough rolling time at 1000 ℃ or higher at the plate thickness center temperature: 200 seconds or more and 400 seconds or more

After the steel material is extracted from the heating furnace, the rough rolling time at 1000°C or higher at the plate thickness center temperature of the steel material is set to within 400 seconds, whereby the vicinity of the surface of the rolled material is preferentially cooled. Thereby, coarsening of the austenite grain size at the 1/4t position from the surface of the rolled material is suppressed, and subsequent ferrite transformation is promoted. As a result, the ratio of ferrite in the steel structure can be 55% or more in terms of area ratio. When the rough rolling time exceeds 400 seconds, austenite is granulated, and a desired amount of ferrite cannot be secured, and a square steel pipe with a small difference in strength between each part and a flat part cannot be obtained. On the other hand, if the rough rolling time is less than 200 seconds, the austenite grain size becomes too fine, the proportion of ferrite in the steel structure exceeds 80% in terms of area ratio, and the strength is insufficient. Rough-rolling time becomes like this. Preferably it is 220 second or more, More preferably, it is 250 second or more. Rough-rolling time becomes like this. Preferably it is 380 second or less, More preferably, it is 350 second or less.

Rough rolling end temperature: 1000 ~ 800 ℃

Austenite grains are processed and recrystallized by rough rolling, and the heated steel material is refined. If the rough rolling end temperature is less than 800°C, the load carrying capacity of the roughing mill and insufficient rolling torque are likely to occur. On the other hand, when the rough rolling end temperature exceeds 1000°C and becomes high, the austenite grains coarsen and the toughness of the rectangular steel pipe is liable to decrease. The rough rolling end temperature is preferably 820°C or higher, more preferably 840°C or higher. The rough rolling end temperature is preferably 980°C or lower, and more preferably 950°C or lower.

In addition, this rough rolling completion|finish temperature can be achieved by adjusting the heating temperature of a steel raw material, cooling conditions during said rough rolling, retention between passes of rough rolling, steel raw material thickness, etc. The thickness of the material to be rolled (thickness of a sheet bar, etc.) in the stage where rough rolling is completed does not need to be particularly limited, and may be a product sheet (hot rolled steel sheet) having a desired product thickness in finish rolling. For example, in manufacture of the rectangular steel pipe for building structural members, about 12-28 mm of product thickness is preferable.

After rough rolling, the to-be-rolled material is finish-rolled by, for example, a tandem rolling mill, and becomes a hot-rolled sheet.

Finish rolling start temperature: 1000 ~ 800℃

In finish rolling, rolling processing and recrystallization are repeated, and refinement|miniaturization of austenite (gamma) grain advances. When the finish rolling start temperature (finish rolling entry temperature) is lowered, the processing strain introduced by the rolling processing tends to remain, and it is easy to achieve miniaturization of the γ grains. When the finish rolling start temperature is less than 800 DEG C, the temperature near the surface of the steel sheet in the finish rolling mill becomes equal to or lower than the Ar3 transformation point, and the risk of ferrite formation increases. The generated ferrite becomes ferrite grains elongated in the rolling direction by the subsequent finish rolling, which causes a decrease in workability. On the other hand, when the finish rolling start temperature exceeds 1000°C and becomes high, the effect of refining the γ grains by the finish rolling described above is reduced, and the toughness of the rectangular steel pipe tends to decrease. For this reason, the finish rolling start temperature shall be 800-1000 degreeC. The finish rolling start temperature is preferably 825 to 975°C.

Finish rolling end temperature: 900 ~ 750 ℃

When the finish rolling end temperature (finish rolling exit temperature) exceeds 900° C. and becomes high temperature, the processing strain added at the time of finish rolling is insufficient, the refinement of the γ grain is not achieved, and the toughness of the rectangular steel pipe is lowered. easy to lower On the other hand, when the finish rolling end temperature is less than 750°C, the temperature near the surface of the steel sheet in the finish rolling mill is below the Ar3 transformation point, and ferrite grains extending in the rolling direction are formed, and the ferrite grains are mixed. Thereby, the risk that toughness will fall increases. For this reason, the finish-rolling completion temperature shall be 900-750 degreeC. The finish rolling end temperature is preferably 850°C or less. Preferably it is 770 degreeC or more.

After finishing rolling, a cooling process is performed on the hot-rolled sheet.

Number of times of standing to cool for 0.2 s or more and less than 3.0 s in initial cooling for 10 s from the start of cooling: 1 time or more

In this invention, let 10 second (for 10 s) be an initial stage cooling process after starting cooling of the hot-rolled sheet obtained in a hot rolling process. In the initial cooling process of a cooling process, it forms and cools a stand-to-cool process for 0.2 s or more and less than 3.0 s once or more. This is performed in order to suppress the production|generation of a martensite structure in the front and back surfaces of a steel plate. In the initial cooling step, when no cooling step is provided or the cooling step is less than 0.2 s, the steel structure of the front and back surfaces of the steel sheet becomes a martensitic structure, and the toughness of the rectangular steel pipe decreases. Moreover, in an initial stage cooling process, when a stand-to-cool process is 3.0 s or more, the average aspect-ratio of a hard phase becomes less than 0.1, and toughness is insufficient. For this reason, the time of one cooling process performed during the initial stage cooling process of a cooling process shall be 0.2 s or more and less than 3.0 s. The time of one cooling process becomes like this. Preferably it is 0.4 s or more, Preferably it is 2.0 s or less.

In order to acquire the said effect, the frequency|count of the standing-to-cool process performed during an initial stage cooling process requires 1 or more times. In addition, what is necessary is just to set the number of times of a standing-to-cool process suitably according to the arrangement|sequence of a cooling installation, cooling stop temperature, etc. Here, standing to cool is set as natural cooling. Although the upper limit of the number of times of a standing-to-cool process is not specifically limited, From a viewpoint of productivity, Preferably it is set as 10 or less. What is necessary is just to set suitably by setting it as intermittent injection, etc. by stopping the injection of the water from the nozzle in some sections of the nozzle for water cooling mentioned later, for example, when making the number of times of standing cooling into multiple times.

Average cooling rate at plate thickness center temperature: 4 ~ 25 ℃/s, cooling stop temperature: 580 ℃ or less

In the cooling step, in the hot-rolled sheet obtained in the finish rolling, the average cooling rate at the plate thickness center temperature from the start of cooling to the stop of cooling (finish of cooling) is 4 to 25°C/s, and the cooling stop temperature is 580°C or less. carry out Cooling performed in a cooling process is implemented, for example by water cooling (water cooling), such as water column cooling, spray cooling, mist cooling, which injects water from a nozzle, gas jet cooling etc. which inject a cooling gas. Moreover, it is preferable to perform cooling operation on both surfaces of a hot-rolled sheet (steel sheet) so that both surfaces (front and back) of a hot-rolled sheet may be cooled under the same conditions.

When the average cooling rate at the center of the sheet thickness of the hot-rolled sheet is less than 4°C/s, the generation frequency of ferrite grains decreases, the ferrite grains become coarse, and toughness decreases. On the other hand, when the average cooling rate exceeds 25°C/s, the generation of ferrite is suppressed and becomes less than 55%, and the average aspect ratio of the hard phase exceeds 0.8, making it difficult to work harden, and a steel structure cannot be obtained. For this reason, the average cooling rate at the center of the thickness of the hot-rolled sheet is 4 to 25°C/s. The average cooling rate at the center of the thickness of the hot-rolled sheet is preferably 5°C/s or more, and preferably 15°C/s or less.

Here, the average cooling rate at the center of the plate thickness of the hot-rolled sheet is,

(((Temperature of plate thickness center at the start of cooling (℃) - Temperature of plate thickness center when cooling is stopped (℃))/Cooling time (s))

is saved with The temperature at the center of the sheet thickness of the hot-rolled sheet can be obtained by calculating the temperature distribution in the cross section of the steel sheet by means of an electrothermal analysis.

When cooling stop temperature exceeds 580 degreeC, the average equivalent circle diameter of the hard phase of a steel plate exceeds 20 micrometers, and there exists a possibility that toughness may fall. The cooling stop temperature is more preferably 560°C or less.

In addition, in order to obtain the steel structure at the desired 1/4t position, it is preferable that the average cooling rate in a temperature range of 750°C to 650°C at the surface temperature of the hot-rolled sheet is 20°C/s or more. If the average cooling rate in this temperature range is less than 20°C/s, the average equivalent circle diameter of the hard phase may exceed 20 µm. The average cooling rate in a temperature range of 750°C to 650°C as the surface temperature of the hot-rolled sheet is preferably 80°C/s or less. When the average cooling rate in this temperature range exceeds 80°C/s, the average aspect ratio of the hard phase may exceed 0.8. Moreover, in order to set the average aspect-ratio of a hard phase to 0.8 or less, it is preferable to start a cooling process immediately (within 5 seconds) from the end of finish rolling.

After completion of cooling, the hot-rolled sheet is subjected to a winding process to obtain a steel sheet (hot-rolled steel sheet).

Coiling temperature: below 580℃

In a winding process, a hot-rolled sheet is wound up at winding temperature: 580 degrees C or less, and it stands to cool after that. When the coiling temperature exceeds 580°C, ferrite transformation and pearlite transformation proceed after winding, the proportion of pearlite becomes excessive, and the toughness of the rectangular steel pipe decreases. For this reason, a coiling temperature shall be 580 degrees C or less. A coiling temperature becomes like this. Preferably it is 550 degrees C or less. In addition, even if the coiling temperature is lowered, there is no material problem, but when the coiling temperature is less than 400 ° C, especially in thick steel plates with a plate thickness of more than 25 mm, the coiling deformation resistance becomes large, and the coiling may not be neatly wound. . For this reason, it is preferable to make coiling temperature into 400 degreeC or more.

Thereafter, the steel sheet (hot rolled steel sheet) after the winding step is subjected to the above-described pipe making step to obtain a rectangular steel pipe.

Next, an example of a building structure using the square steel pipe of this invention is demonstrated.

It is a perspective view which shows typically the building structure which concerns on embodiment of this invention. As shown in FIG. 3 , in the building structure of the present embodiment, a plurality of rectangular steel pipes 11 of the present invention are erected, and are used as a main material. A plurality of masses 14 made of steel materials, such as H-shaped steel, are erected between adjacent rectangular steel pipes 11 . Moreover, between the adjacent masses 14, two or more small quantities 15 which consist of steel materials, such as H-shaped steel, are erected. By welding the rectangular steel pipe 11 and the diaphragm 16, and welding the H-shaped steel that becomes the bulk 14 thereto, the bulk 14 made of steel materials such as H-shaped steel between the adjacent rectangular steel pipes 11 ) is hypothesized. In addition, for mounting on a wall or the like, a cross section 17 is formed as needed.

Since the building structure of the present invention uses the rectangular steel pipe 11 of the present invention with a small difference in strength between each part and the flat part, it is easy to select a welding material for welding the square steel pipe 11 and the diaphragm 16, and under It is hard to produce a strength difference with welding materials, such as a match. Troubles, such as a fracture|rupture in a welding part, can be suppressed by being hard to produce undermatch. Moreover, since each R (R of each part) of the rectangular steel pipe 11 is controlled to an appropriate size, it is easy to combine with other structural members whose cross section is a right angle. Moreover, when each R of the square steel pipe 11 is controlled to an appropriate magnitude|size, it can withstand a larger external force, and earthquake resistance etc. improve.

Example

Hereinafter, the present invention will be further described in detail based on Examples. In addition, the present invention is not limited to the following examples.

Molten steel was melted in a converter, and a slab (steel material: thickness 250 mm) having the composition shown in Table 1 was obtained by a continuous casting method. These slabs (steel raw materials) are heated to the heating temperature shown in Table 2, then subjected to a hot rolling process, a cooling process, and a winding process under the conditions shown in Table 2, followed by standing to cool, a steel plate having a plate thickness of 16 to 28 mm (Hot rolled steel sheet). In addition, after finishing the finish rolling, the cooling process was started immediately (within 5 seconds). Cooling was performed by water cooling. The standing-to-cool process in an initial stage cooling process was performed by forming the stand-to-cool section in which water cooling is not performed during the initial stage cooling process which is 10 s from a cooling start. Then, using the obtained hot-rolled steel sheet as a raw material, a round steel pipe was formed by roll forming in cold under the conditions shown in Table 2, and then a rectangular steel pipe (400 to 550 mm in width) was obtained by roll forming in cold.

In the Examples of the present invention, test pieces were taken from the obtained rectangular steel pipe, and tissue observation, tensile test, Charpy impact test, and R of each part were measured, respectively. In addition, tissue observation was observed and measured by the said method. In addition, the test method of a tensile test, a Charpy impact test, and the measuring method of R of each part were carried out as follows.

(1) Tensile test of square steel pipe

JIS 5 tensile test specimens were taken from the flat plate portion and the corner portion of the obtained square steel pipe so that the tensile direction was the tube longitudinal direction. Next, based on the regulation of JIS Z 2241 (2011), a tensile test was performed, and the yield strength YS and the tensile strength TS were measured. Yield ratio YR (%) defined by (yield strength)/(tensile strength) x 100 (%) was computed using the obtained measured value.

(2) Square steel pipe impact test

A V-notch test piece was taken from the position of the plate thickness of 1/4t of the obtained square steel pipe so that the longitudinal direction of the test piece became the pipe circumferential direction. Next, based on the regulation of JIS Z 2242 (2011), the Charpy impact test was performed at test temperature: -40 degreeC, and the absorbed energy (J) was calculated|required. In addition, the number of test pieces was made into three each, and the average value of each was made into the value of the impact test result shown in Table 3-2.

(3) Measurement method of R (each R) of each part

From the obtained rectangular steel pipe, 10 points of the cross section perpendicular to the pipe axis direction were arbitrarily cut out, the curvature radii of the corners at the four corners of the vertical cross section were measured, and the average value was taken as R of the corners. Specifically, as shown in Fig. 4, the welded portion (core) of the steel pipe is set to 0°, and the positions of 45°, 135°, 225°, and 315° are set as the center of each part, respectively, based on this 0°. In this case, the radius of curvature of each part refers to the radius of curvature at the intersection of the line L that forms 45° with the adjacent side from the center of the pipe as a starting point and the outside of the leg (the pipe outer surface side of each part). The radius of curvature of each part is the radius of the sector in which the central angle determined by the line drawn toward the connection point (A, A') of the flat part and the arc part of the rectangular steel pipe is 65°, centered on the L above. . In addition, "t" shown in FIG. 4 is a plate|board thickness, and "H" points out the length of the side of an external shape. As a method of calculating the radius of curvature, for example, a method of calculating the radius of curvature by using the sine theorem from the measurement result of the distance relationship of three points (the intersection of the outside of each part, and the two points that are the connection points of the flat part and the arc part) , a method of measuring a radius of curvature from a radial gauge that matches well with a corner portion within the region of the three points, and the like, but is not limited thereto. In this example, a radial gauge was used to measure the radius of curvature of each part. In addition, each R was made into the average value of 10 cross-sections perpendicular|vertical with respect to the tube axis direction as mentioned above.

The obtained results are shown in Table 3-1 and Table 3-2.

[Table 1]

[Table 2]

[Table 3-1]

[Table 3-2]

In the invention examples that were within the scope of the present invention, all of the characteristics of the present invention (YS of the flat part is 350 MPa or more, TS is 520 MPa or more, the ratio of YS of the flat part to each part is 0.80 or more and 0.90 or less, and the ratio of TS of the flat part to each part is 0.90. It was possible to obtain 1.00 or less, the Charpy absorbed energy at -40°C of the flat part was 100 J or more, and the R of each part was (2.3 × t) or more (2.9 × t or less) (where t is the plate thickness). , in the comparative example outside the scope of the present invention, the characteristics of the present invention could not be obtained.

1: steel rod
2: Leveler
3: Cage roll group
4: pin pass roll group
5: Squeeze Roll
6: welding machine
7: electric resistance steel pipe
8: sizing roll group
9: Angular type roll group
10: square steel pipe
11: square steel pipe
14: Bulk
15: small amount
16: diaphragm
17: considered

Claims (7)

A prismatic steel pipe having a flat plate portion and a corner portion, comprising:
The component composition is in mass %,
C: 0.07 to 0.20%;
Si: 1.0% or less;
Mn: 0.5~2.0%;
P: 0.030% or less;
S: 0.015% or less;
Al: 0.01~0.06%;
N: 0.006% or less
contains, and the balance consists of Fe and unavoidable impurities,
In the steel structure at a depth of 1/4 of the plate thickness t from the outer surface of the steel pipe, ferrite is 55% or more and 80% or less in area ratio, and the average aspect ratio of the hard phase is 0.1 to 0.8,
The flat portion, YS is 350 MPa or more, TS is 520 MPa or more,
The ratio of YS of the flat part to each part is 0.80 or more and 0.90 or less, and the ratio of TS of the flat part to each part is 0.90 or more and 1.00 or less,
Charpy absorbed energy at -40 ℃ of the flat portion is 100 J or more,
R of each part is (2.3 × t) or more (2.9 × t) or less
prismatic steel pipe. The method of claim 1,
A prismatic steel pipe containing one or two or more groups selected from the following groups A to C, in mass%, in addition to the above component composition.
Group A: Nb: 0.05% or less, Ti: 0.05% or less, V: 0.10% or less 1 type or 2 or more types selected from
Group B: B: 0.008% or less
Group C: Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.30%, Ca: 1 or 2 or more selected from among 0.001 to 0.010% 3. The method according to claim 1 or 2,
The steel structure further includes a rectangular steel pipe having an average equivalent round diameter of the hard phase of 20 µm or less. A method for manufacturing a rectangular steel pipe according to any one of claims 1 to 3, comprising:
A method for producing a rectangular steel pipe in which a steel sheet is formed into a cylindrical shape by cold roll forming, a cylindrical cross-section is welded, and then formed into a cylindrical shape of 0.99 or more and 1.01 or less at a ratio of vertical diameter to horizontal diameter, followed by forming a rectangular steel pipe. A method for manufacturing a rectangular steel pipe according to any one of claims 1 to 3, comprising:
When the steel material is subjected to a hot rolling process, a cooling process, a winding process, and a pipe making process in this order to manufacture a square steel pipe,
After heating the steel material to a heating temperature: 1100 ~ 1300 ℃,
Rough rolling time at a plate thickness center temperature of 1000 ℃ or higher: 200 sec or more and 400 sec or more, Rough rolling end temperature: 1000 ~ 800 ℃,
A hot-rolling step of starting temperature of finish rolling: 1000 to 800°C and finishing temperature of finishing rolling: 900 to 750°C is performed to obtain a hot-rolled sheet;
Next, the hot-rolled sheet is allowed to cool at least once for 0.2 s or more and less than 3.0 s in the initial cooling step for 10 s from the start of cooling, and the average cooling rate at the plate thickness center temperature: 4 to 25°C/s performing a cooling process that
Next, a winding process of winding the hot-rolled sheet at a winding temperature: 580 ° C. or less is performed to obtain a steel sheet,
Next, the steel sheet is roll-formed in cold, and the cylindrical cross section is welded to form a cylindrical shape of 0.99 or more and 1.01 or less at the ratio of vertical diameter/horizontal diameter, and then a rectangular steel pipe manufacturing process is performed. . 6. The method of claim 5,
A method for manufacturing a rectangular steel pipe, wherein the cooling stop temperature of the cooling step is 580°C or less. A building structure using the rectangular steel pipe according to any one of claims 1 to 3.

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