阅读说明：本技术 方形钢管及其制造方法以及建筑构造物 (Square steel pipe, method for producing same, and building structure ) 是由 井手信介 松本晃英 松本昌士 冈部能知 于 2020-02-03 设计创作，主要内容包括：本发明提供一种方形钢管及其制造方法、以及使用该方形钢管的建筑构造物。本发明是具有平板部与角部的方形钢管,具有特定的成分组成,从钢管的外表面起在板厚t的1/4t位置处的钢组织中,贝氏体和珠光体的面积率的合计相对于铁素体的面积率的比例为2.0以上20.0以下,并且贝氏体的面积率相对于珠光体的面积率的比例为5.0以上20.0以下,平板部的YS为350MPa以上,TS为520MPa以上,平板部相对于角部的YS之比为0.80以上0.90以下,平板部相对于角部的TS之比为0.90以上1.00以下,平板部的-40℃的夏比吸收能量为100J以上,角部的R为2.3×t以上2.9×t以下。(The invention provides a square steel pipe, a method for manufacturing the same, and a building structure using the same. The present invention is a square steel pipe having flat plate portions and corner portions, having a specific composition, wherein in a steel structure at a position 1/4t of a plate thickness t from an outer surface of the steel pipe, a ratio of a total of an area ratio of bainite and pearlite to an area ratio of ferrite is 2.0 to 20.0, a ratio of an area ratio of bainite to an area ratio of pearlite is 5.0 to 20.0, YS of the flat plate portions is 350MPa or more, TS is 520MPa or more, a ratio of YS of the flat plate portions to the corner portions is 0.80 to 0.90, a ratio of TS of the flat plate portions to the corner portions is 0.90 to 1.00, a Charpy absorption energy at-40 ℃ of the flat plate portions is 100J or more, and R of the corner portions is 2.3 xt to 2.9 xt.)

1. A square steel pipe having a flat plate portion and a corner portion, wherein,

the composition of the components comprises the following components in percentage by mass:

C：0.07～0.20％，

si: the content of the active ingredients is less than 1.0%,

Mn：0.5～2.0％，

p: less than 0.030 percent of the total weight of the composition,

s: less than 0.015% of the total weight of the composition,

Al：0.01～0.06％，

n: the content of the active carbon is less than 0.006%,

the balance being Fe and unavoidable impurities,

in a steel structure at a position 1/4t of a sheet thickness t from an outer surface of a steel pipe, a ratio of a total of an area ratio of bainite and pearlite to an area ratio of ferrite is 2.0 to 20.0 inclusive, and a ratio of the area ratio of bainite to the area ratio of pearlite is 5.0 to 20.0 inclusive,

the YS of the flat plate part is more than 350MPa, the TS is more than 520MPa,

the ratio of YS of the flat plate portion to the corner portion is 0.80 to 0.90, the ratio of TS of the flat plate portion to the corner portion is 0.90 to 1.00,

the plate portion has a Charpy absorption energy of 100J or more at-40 ℃,

r of the corner is 2.3 × t or more and 2.9 × t or less.

2. The square steel pipe according to claim 1, wherein the composition further comprises one or two or more groups selected from the following groups a to C in mass% in addition to the above composition:

group A: is selected from Nb: 0.05% or less, Ti: 0.05% or less, V: 0.10% or less of one or more

Group B: b: less than 0.008%

Group C: is selected from Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01-0.30%, Ca: 0.001-0.010% of one or more than two.

3. A method for manufacturing a square steel pipe according to claim 1 or 2, wherein,

and performing a pipe-making process in which a steel sheet is roll-formed by cold rolling and welded to form a cylindrical end face, and after forming the steel sheet into a cylindrical shape having a ratio of a longitudinal diameter/a transverse diameter of 0.99 to 1.01, the steel sheet is formed into a square shape.

4. A method for manufacturing a square steel pipe according to claim 1 or 2, wherein,

when a square steel pipe is produced by subjecting a steel material to a hot rolling step, a cooling step, a coiling step, and a pipe forming step in this order,

and performing a hot rolling step of heating the steel material to a heating temperature: after 1100 to 1300 ℃, the steel material is taken out from the heating furnace, and before the rough rolling is finished, the number of times of standing for 30 seconds or more is controlled to be one or more and five times or less in a state that the thickness center temperature is 1000 ℃ or more with respect to the heated steel material, and the rough rolling finishing temperature is set as: 1000-800 ℃, finish rolling start temperature: 1000-800 ℃, finish rolling finishing temperature: 900 to 750 ℃,

next, a cooling step is performed, in which cooling is performed once or more with 0.2s or more and less than 3.0s during 10s from the start of cooling, and the average cooling rate is set as the plate thickness center temperature: 4-25 ℃/s, cooling stop temperature: the temperature of the mixture is lower than 580 ℃,

subsequently, the winding temperature was measured: a coiling step of coiling at 580 ℃ or below to produce a steel sheet,

next, a pipe-making step is performed in which the steel sheet after the winding step is formed into a cylindrical end face by cold rolling, roll-formed and welded, and then formed into a cylindrical shape having a ratio of longitudinal diameter/transverse diameter of 0.99 to 1.01, and then formed into a square shape.

5. A building structure in which, in the case of a building,

the building structure uses the square steel pipe according to claim 1 or 2.

Technical Field

The present invention relates to a square steel pipe, a method for manufacturing the same, and a building structure. The square steel pipe having a small difference in strength between the corner portion and the flat plate portion of the present invention is suitable for use as a building structural member.

Background

A square steel pipe (also referred to as a "square column") is usually produced by cold-rolling a hot-rolled steel sheet (hot-rolled steel strip) or a thick plate. The cold-rolling method includes press forming and roll forming. However, in any of these methods, since the corner portions of the rectangular steel pipe are plastically deformed more largely than the flat plate portion of the rectangular steel pipe, the strength of the corner portions is likely to increase, and the strength difference between the corner portions and the flat plate portion becomes large. When the characteristics are significantly different between the corner portion and the flat plate portion, selection of the welding material and the architectural design become very difficult, and it is difficult to use the square steel pipe as the material for architectural construction.

Many examples of such problems have been directly studied, but for example, patent document 1 discloses a technique for a square steel pipe for a building structure. Patent document 1 discloses a cold-rolled square steel pipe obtained by cold-bending a steel sheet, the square steel pipe including C: 0.02 to 0.18% ("%" means "% by mass", the same chemical composition as below), and Si: 0.03 to 0.5%, Mn: 0.7-2.5%, Al: 0.005-0.12% and N: 0.008% or less (not including 0%), and the balance being Fe and unavoidable impurities, wherein the content of the unavoidable impurities is P: 0.02% or less (0% excluded), S: 0.01% or less (excluding 0%), and O: 0.004% or less (not including 0%), and the bending portion is processed to be perpendicular, and the requirements of the following (a) to (C) are satisfied, thereby ensuring the shock resistance.

(A) Yield strength of flat portion of steel pipe: 355MPa or more, tensile strength: the pressure of the mixture is more than 520MPa,

(B) in the microstructure of the flat portion, the area fraction of the bainite structure: more than 40 percent of the total weight of the composition,

(C) vickers hardness Hv of the surface layer portion of the corner of the steel pipe: 350 below, tensile in tensile test: more than 10%, 0 ℃ Charpy absorbed energy vE 0: above 70J.

Patent document 1: japanese patent No. 5385760

A square steel pipe produced by cold rolling by roll forming is formed into a square steel pipe having corners and flat plate portions by roll forming a flat material (for example, a hot rolled material) in a width direction produced by hot rolling and rolling to form a circular steel pipe, and then cold rolling. In such a manufacturing method, the difference in strength between the corner portion and the flat plate portion due to the difference in work hardening is likely to increase. In addition, in hot rolling performed before roll forming, since the material is manufactured by controlling cooling from the surface of the hot rolled material, there is a problem that the strength (hardness) before processing is increased in the vicinity of the surface layer of the hot rolled material where the cooling rate is relatively increased.

However, the technique disclosed in patent document 1 is limited to the temperature control in hot rolling, and does not actively reduce the strength difference between the corner portion and the flat plate portion, so that the hardness of the surface of the steel sheet is not excessively increased. Therefore, it is obvious that the strength of the corner portion of the square steel pipe obtained by cold-bending is relatively higher than that of the flat plate portion even if the characteristics of the corner portion satisfy a certain criterion, for example. In order to suppress an increase in strength of the corner portion, it is effective to reduce plastic deformation of the corner portion. In order to reduce the plastic deformation of the corner portion, it is conceivable to increase the R (roundness) of the corner portion. However, the square steel pipe having a large R at the corner is not preferable because it has a problem in design and a problem in that performance as a building is deteriorated due to generation of a gap when the square steel pipe is combined with other members as a square member.

Disclosure of Invention

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

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

In the present invention, it is first assumed that work hardening is less likely to occur in the vicinity of the surface layer (hereinafter referred to as the vicinity of the outer surface) of a steel pipe in which the work strain (plastic deformation) introduced by cold roll forming is particularly large, thereby reducing the difference in strength between the corner portion and the flat plate portion.

Therefore, the inventors prepared a plurality of samples in which the area ratios of ferrite, bainite, and pearlite were changed as the steel structures of steel sheets and steel pipes, and examined the ease of work hardening. As a result, it has been found that a steel structure which is not easily work hardened can be produced even by cold rolling by setting the ratio of the total amount of bainite and pearlite to ferrite to a constant value or more. This is considered to be because strain concentrates on ferrite having a soft phase and a small work hardening ability, and the work hardening ability of the entire steel structure becomes small.

The present inventors formed a cylindrical round steel pipe having a ratio of longitudinal diameter/transverse diameter of 0.99 to 1.01 once and then formed into a square shape by rolls arranged in the upper and lower sides and the left and right sides in order to suppress work hardening at the corners by utilizing the steel structure of the material (hereinafter, also referred to as a hot rolled material or a steel sheet). This found that a square steel pipe could be obtained without excessively work hardening the corner portions.

Here, the "vertical diameter" refers to an outer diameter in a vertical direction with respect to the tube axis of the circular steel pipe, and the "horizontal diameter" refers to an outer diameter in a horizontal direction with respect to the tube axis of the circular steel pipe.

As a result of the above-described studies, it is considered that, in the present invention, a hot-rolled material (steel sheet) in which the ratio of the total amount of bainite and pearlite to ferrite is set to a specific range with respect to the steel structure in the vicinity of the outer surface of a steel pipe in which the work strain introduced by cold rolling is largest is used, the hot-rolled material is formed into a cylindrical shape having a ratio of longitudinal diameter/transverse diameter of 0.99 to 1.01, and then the material is formed into a square shape by rolls arranged in the upper and lower and left and right sides, thereby manufacturing a square steel pipe having a small difference in strength between the corner portion and the flat plate portion.

In the present invention, "a square steel pipe having a small difference in strength between the corner portion and the flat plate portion" means that the ratio of YS of the flat plate portion to the corner portion is 0.80 to 0.90, and the ratio of TS of the flat plate portion to the corner portion is 0.90 to 1.00.

The present inventors have also repeatedly studied in detail, and as a result, the present invention has been completed. The gist of the present invention is as follows.

[1] A square steel pipe having a flat plate portion and a corner portion, wherein,

the composition of the components comprises the following components in percentage by mass:

C：0.07～0.20％，

si: the content of the active ingredients is less than 1.0%,

Mn：0.5～2.0％，

p: less than 0.030 percent of the total weight of the composition,

s: less than 0.015% of the total weight of the composition,

Al：0.01～0.06％，

n: the content of the active carbon is less than 0.006%,

the balance being Fe and unavoidable impurities,

in a steel structure at a position 1/4t of a sheet thickness t from an outer surface of a steel pipe, a ratio of a total of an area ratio of bainite and pearlite to an area ratio of ferrite is 2.0 to 20.0 inclusive, and a ratio of the area ratio of bainite to the area ratio of pearlite is 5.0 to 20.0 inclusive,

the YS of the flat plate part is more than 350MPa, the TS is more than 520MPa,

the ratio of YS of the flat plate portion to the corner portion is 0.80 to 0.90, the ratio of TS of the flat plate portion to the corner portion is 0.90 to 1.00,

the plate portion has a Charpy absorption energy of 100J or more at-40 ℃,

r of the corner is 2.3 × t or more and 2.9 × t or less.

[2] The square steel pipe according to [1], which comprises one or two or more groups selected from the following groups A to C in mass% in addition to the above composition:

group A: is selected from Nb: 0.05% or less, Ti: 0.05% or less, V: 0.10% or less of one or more

Group B: b: less than 0.008%

Group C: is selected from Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01-0.30%, Ca: 0.001-0.010% of one or more than two.

[3] A method for manufacturing a square steel pipe according to the above item [1] or [2], wherein,

and performing a pipe-making process in which a steel sheet is roll-formed by cold rolling and welded to form a cylindrical end face, and after forming the steel sheet into a cylindrical shape having a ratio of a longitudinal diameter/a transverse diameter of 0.99 to 1.01, the steel sheet is formed into a square shape.

[4] A method for manufacturing a square steel pipe according to the above item [1] or [2], wherein,

when a square steel pipe is produced by subjecting a steel material to a hot rolling step, a cooling step, a coiling step, and a pipe forming step in this order,

and performing a hot rolling step of heating the steel material to a heating temperature: after 1100 to 1300 ℃, the steel material is taken out from the heating furnace, and before the rough rolling is finished, the number of times of standing for 30 seconds or more is controlled to be one or more and five times or less in a state that the thickness center temperature is 1000 ℃ or more with respect to the heated steel material, and the rough rolling finishing temperature is set as: 1000-800 ℃, finish rolling start temperature: 1000-800 ℃, finish rolling finishing temperature: 900 to 750 ℃,

next, a cooling step is performed, in which cooling is performed once or more with 0.2s or more and less than 3.0s during 10s from the start of cooling, and the average cooling rate is set as the plate thickness center temperature: 4-25 ℃/s, cooling stop temperature: the temperature of the mixture is lower than 580 ℃,

subsequently, the winding temperature was measured: a coiling step of coiling at 580 ℃ or below to produce a steel sheet,

next, a pipe-making step is performed in which the steel sheet after the winding step is formed into a cylindrical end face by cold rolling, roll-formed and welded, and then formed into a cylindrical shape having a ratio of longitudinal diameter/transverse diameter of 0.99 to 1.01, and then formed into a square shape.

[5] A building structure in which, in the case of a building,

the building structure uses the square steel pipe according to [1] or [2 ].

According to the present invention, in particular, when square steel pipes are manufactured by cold rolling, square steel pipes having small strength differences between the corner portions and the flat plate portions can be obtained. Since R at the corner of the square steel pipe is controlled to an appropriate size, the square steel pipe can be suitably used as a square steel pipe for a building structural member, for example.

Drawings

Fig. 1 is a schematic view showing an example of an apparatus for manufacturing an electric resistance welded steel pipe.

Fig. 2 is a schematic view showing a process of forming a square steel pipe.

Fig. 3 is a perspective view schematically showing an example of a building structure using the square steel pipe of the present invention.

Fig. 4 is a schematic diagram showing a cross section of a square steel pipe.

Detailed Description

The present invention will be described in detail below.

The square steel pipe of the present invention is as follows. The components comprise: by 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, and the balance of Fe and inevitable impurities. The ratio of the total of the area ratios of bainite and pearlite to the area ratio of ferrite is 2.0 to 20.0, and the ratio of the area ratio of bainite to the area ratio of pearlite is 5.0 to 20.0, for the steel structure from the outer surface of the square steel pipe to the 1/4 depth position of the plate thickness t (hereinafter referred to as the 1/4t position.). In addition, the flat plate portion of the square steel plate has YS of 350MPa or more, TS of 520MPa or more, a ratio of YS of the flat plate portion to the corner portion of 0.80 to 0.90, a ratio of TS of the flat plate portion to the corner portion of 0.90 to 1.00, a Charpy absorption energy at-40 ℃ at a position 1/4t of the flat plate portion of 100J or more, and R of the corner portion of 2.3 × t to 2.9 × t.

First, the reasons for limiting the composition of the components of the present invention will be explained. Further, unless otherwise specified, mass% is represented by% only. In the present invention, the composition of the steel sheet is the same as that of the steel sheet used as the material of the square steel pipe. Therefore, the reasons for limiting the composition of the steel sheet used as the material and the square steel pipe will be described below.

C：0.07～0.20％

C is an element that increases the strength of the steel sheet and the square steel pipe by solid solution strengthening and contributes to the formation of pearlite, which is one of the steel structures of the present invention described later. C needs to be contained by 0.07% or more in order to secure a desired strength and further to secure a desired steel sheet structure. On the other hand, the content of C exceeding 0.20% may cause a martensite structure due to thermal influence during welding of a square steel pipe on site, which may cause weld cracking. Therefore, C is set to 0.07 to 0.20%. C is preferably 0.09% or more, more preferably 0.10% or more. Further, C is preferably 0.18% or less, more preferably 0.17% or less.

Si: 1.0% or less

Si is an element contributing to increase in strength of the steel sheet and the square steel pipe by solid solution strengthening. In order to ensure the desired strength of the steel sheet and the square steel pipe, Si is preferably contained in an amount exceeding 0.01%. However, if Si is contained in excess of 1.0%, the toughness is lowered. Therefore, Si is 1.0% or less. Further, Si is preferably 0.8% or less, more preferably 0.6% or less. More preferably 0.03% or more.

Mn：0.5～2.0％

Mn is an element that increases the strength of the steel sheet and the square steel pipe by solid-solution strengthening, and in order to ensure the desired strength of the steel sheet and the square steel pipe, it is necessary to contain 0.5% or more of Mn. When the Mn content is less than 0.5%, the ferrite transformation start temperature increases, the microstructure excessively coarsens, and the toughness decreases. On the other hand, if Mn is contained in an amount exceeding 2.0%, the hardness of the center segregation portion increases, which may cause cracking during welding of the square steel pipe in the field. Therefore, Mn is set to 0.5 to 2.0%. Mn is preferably 1.8% or less, more preferably 1.6% or less. Mn is preferably 0.6% or more, more preferably 0.7% or more.

P: less than 0.030%

P is an element that segregates in ferrite grain boundaries and has an effect of reducing the toughness of the steel sheet and the square steel pipe. In the present invention, it is preferable that the impurities be reduced as much as possible. However, since an excessive reduction leads to an increase in refining cost, it is preferable to set P to 0.002% or more. Further, the content of P can be allowed to be 0.030%. Therefore, P is set to 0.030% or less. P is preferably 0.025% or less. P is more preferably 0.020% or less.

S: less than 0.015%

S exists as a sulfide in steel, and exists mainly as MnS within the range of the composition of the present invention. MnS thinly extends in the hot rolling process, and adversely affects ductility and toughness of the steel sheet and the square steel pipe. Therefore, in the present invention, it is preferable to reduce MnS as much as possible. However, since an excessive reduction leads to an increase in refining cost, it is preferable to set S to 0.0002% or more. Further, the content of S can be allowed to be 0.015%. Therefore, S is set to 0.015% or less. S is preferably 0.010% or less, more preferably 0.008% or less.

Al：0.01～0.06％

Al functions as a deoxidizer and has an effect of fixing N as AlN. In order to obtain such an effect, Al needs to be contained by 0.01% or more. If the Al content is less than 0.01%, the deoxidation property is insufficient without adding Si, so that oxide inclusions increase and the cleanliness of the steel sheet decreases. On the other hand, Al is contained in an amount exceeding 0.06%, and the amount of solid-solution Al increases, so that when the rectangular steel pipe is welded on the long side (that is, when resistance welding is performed in the longitudinal direction of the steel pipe during the production of the rectangular steel pipe), particularly when welding is performed in the air, the risk of oxide formation at the welded portion increases, and the toughness of the welded portion of the rectangular steel pipe decreases. Therefore, Al is set to 0.01 to 0.06%. Al is preferably 0.02% or more. Further, Al is preferably 0.05% or less.

N: less than 0.006%

N is an element having an action of reducing toughness of the steel plate and the square steel pipe by firmly fixing the movement of the dislocation. In the present invention, N as an impurity is preferably reduced as much as possible, and can be allowed to be 0.006%. Therefore, N is set to 0.006% or less. N is preferably 0.005% or less. In the present invention, although not particularly limited, N is preferably 0.001% or more from the viewpoint of production cost.

The balance being Fe and unavoidable impurities. However, as inevitable impurities, for example, O (oxygen): less than 0.005%.

The above is the basic composition of the present invention. The object characteristics of the present invention can be obtained by the above-described essential elements, but the following elements can be included as necessary.

Is selected from Nb: 0.05% or less, Ti: 0.05% or less, V: less than 0.10%, one or more than two

Nb, Ti, and V are elements that form fine carbides and nitrides in steel and contribute to improvement in strength of steel by precipitation strengthening. In order to obtain such an effect, when Nb, Ti, and V are contained, it is preferable to use Nb: 0.05% or less, Ti: 0.05% or less, V: 0.10% or less, more preferably Nb: 0.04% or less, Ti: 0.04% or less, V: less than 0.08%. When Nb, Ti, and V are contained, it is preferable to set Nb: 0.001% or more, Ti: 0.001% or more, V: 0.001% or more, and more preferably, Nb: 0.003% or more, Ti: 0.003% or more, V: more than 0.003 percent.

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

B: less than 0.008%

B is an element which delays ferrite transformation in the cooling process, promotes the formation of low-temperature transformed ferrite, and has an effect of increasing the strength of the steel sheet and the square steel pipe. The content of B is related to an increase in the yield ratio of the steel sheet, that is, the yield ratio of the square steel pipe. Therefore, in the present invention, if the yield ratio of the square steel pipe is in a range of 90% or less, B can be contained as necessary for the purpose of adjusting the strength. When B is contained, it is preferably 0.008% or less. B is more preferably 0.0015% or less, and still more preferably 0.0008% or less. B is preferably 0.0001% or more, more preferably 0.0003% or more.

Is selected from Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01-0.30%, Ca: 0.001-0.010% of one or more

Cr：0.01～1.0％

Cr is an element that increases the strength of the steel sheet and the square steel pipe by improving the hardenability, and can be contained as needed. In order to obtain such an effect, when Cr is contained, Cr is preferably contained in an amount of 0.01% or more. On the other hand, if Cr is contained in an amount exceeding 1.0%, toughness and weldability may be deteriorated, and therefore, if Cr is contained, it is preferably 1.0% or less. Cr is more preferably 0.02% or more, and still more preferably 0.8% or less.

Mo：0.01～1.0％

Mo is an element that increases the strength of the steel sheet and the square steel pipe by improving the hardenability, and can be contained as needed. In order to obtain such effects, when Mo is contained, Mo is preferably contained in an amount of 0.01% or more. On the other hand, when Mo is contained in an amount exceeding 1.0%, the toughness may be lowered, and therefore, when Mo is contained, it is preferably 1.0% or less. Mo is more preferably 0.02% or more, and still more preferably 0.8% or less.

Cu：0.01～0.50％

Cu is an element that increases the strength of the steel sheet and the square steel pipe by solid solution strengthening, and can be contained as necessary. In order to obtain such effects, Cu is preferably contained in an amount of 0.01% or more. On the other hand, if Cu is contained in an amount exceeding 0.50%, toughness may be lowered, and therefore, in the case of containing Cu, it is preferably 0.50% or less. Cu is more preferably 0.02% or more, and still more preferably 0.4% or less.

Ni：0.01～0.30％

Ni is an element that increases the strength of the steel sheet and the square steel pipe by solid solution strengthening, and can be contained as necessary. In order to obtain such an effect, Ni is preferably contained in an amount of 0.01% or more. On the other hand, if Ni is contained in an amount exceeding 0.30%, the area ratio of ferrite may be easily decreased, and therefore, in the case where Ni is contained, it is preferably 0.30% or less. Ni is more preferably 0.02% or more, and still more preferably 0.2% or less.

Ca：0.001～0.010％

Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that extend thinly in the hot rolling step, and can be contained as needed. In order to obtain such an effect, it is preferable to contain 0.001% or more of Ca. On the other hand, if the Ca content exceeds 0.010%, Ca oxide clusters are formed in the steel, and there is a possibility that toughness is deteriorated. Therefore, when Ca is contained, the content of Ca is preferably 0.001 to 0.010%. Ca is more preferably 0.0015% or more, and still more preferably 0.0050% or less.

Next, the reason for limiting the steel structure of the square steel pipe of the present invention will be described.

The steel structure at the 1/4t position of the square steel pipe of the present invention mainly comprises ferrite, pearlite, and bainite, and the ratio of each structure is such that the ratio ((B + P)/F) of the sum of the area ratios of bainite (B) and pearlite (P) to the area ratio of ferrite (F) is 2.0 to 20.0, and the ratio (B/P) of the area ratio of bainite to the area ratio of pearlite is 5.0 to 20.0.

Ratio of the total area ratio of bainite and pearlite to the area ratio of ferrite: 2.0 to 20.0 inclusive

If the ratio of the total area ratio of bainite and pearlite to the area ratio of ferrite is less than 2.0, bainite and pearlite bearing the strength are insufficient, and the desired strength cannot be obtained. On the other hand, if the ratio of the sum of the area ratios of bainite and pearlite to the area ratio of ferrite exceeds 20.0, strain is easily dispersed in bainite and pearlite and work hardening is easily caused when a square steel pipe is produced by cold roll forming. As a result, a square steel pipe having a small difference in strength between the corner portion and the flat plate portion cannot be obtained.

Ratio of area ratio of bainite to area ratio of pearlite: 5.0 to 20.0 inclusive

When the ratio of the area ratio of bainite to the area ratio of pearlite is less than 5.0, pearlite becomes excessive and toughness decreases. On the other hand, if the ratio of the area ratio of bainite to the area ratio of pearlite exceeds 20.0, strain is easily dispersed in bainite, and work hardening is easily caused. As a result, a square steel pipe having a small difference in strength between the corner portion and the flat plate portion cannot be obtained.

In addition, since the steel structures of the corners and the 1/4t position of the flat plate portion are the same in a square steel pipe manufactured by roll forming a steel sheet (hot rolled steel sheet) as a material, the measurement can be performed at either the 1/4t position of the flat plate portion or the 1/4t position of the corner portion. The steel structure at the position 1/4t of the flat plate portion is defined here.

In the present invention, the above-described effects can be similarly obtained even if the steel structure is present in the range from the 3/16t position to the 5/16t position of the steel pipe. Therefore, the "steel structure at the 1/4t position" in the present invention means that the steel structure is present in any one of the ranges from the 3/16t position to the 5/16t position.

The steel structure was observed by the following method, and the type and area ratio (%) of the structure were determined. The structure observation test piece was collected from a square steel pipe, polished so that the cross section in the rolling direction (L cross section) became the observation surface, and subjected to nital etching. The structure observation was performed by observing and imaging the steel structure with an optical microscope (magnification: 500 times) or a scanning electron microscope (SEM, magnification: 500 times) with the structure at a position 1/4t thick from the surface of the test piece for structure observation (i.e., the outer surface of the square steel pipe) as the center of observation. Here, "t" represents the thickness of the steel plate (plate thickness). The type of the structure was determined from the obtained structure photograph using an image analyzer (image analysis software: Photoshop, product of Adobe corporation), and the area ratio of each structure (ferrite, pearlite, bainite) was calculated. The area ratio of each tissue was observed over five fields and determined as an average of values obtained in each field.

Next, a method for manufacturing a square steel pipe according to the present invention will be described with reference to fig. 1 and 2. Fig. 1 is a schematic view showing an example of an apparatus for manufacturing an electric resistance welded steel pipe. Fig. 2 is a schematic view showing a process of forming a square steel pipe.

The method for manufacturing a square steel pipe according to the present invention is a method for manufacturing a square steel pipe by performing a pipe manufacturing process on a steel sheet. In the pipe-making process of the present invention, a steel sheet is roll-formed by cold rolling and welded to a cylindrical end face. Next, a cylindrical round steel pipe having a ratio of longitudinal diameter/lateral diameter of 0.99 to 1.01 is formed, and then the round steel pipe is further formed into a square shape in a cold state by rollers arranged at the upper and lower sides and the left and right sides, and a square steel pipe having corner portions and flat plate portions is formed.

First, as shown in fig. 1, a steel strip 1 as a material of an electric resistance welded steel pipe is subjected to entry side straightening by, for example, a leveler 2, then subjected to intermediate forming by a cage roll group 3 composed of a plurality of rolls to form an open pipe, and then subjected to finish forming by a fin roll group 4 composed of a plurality of rolls. After the finish forming, the widthwise end portions of the steel strip 1 are resistance-welded by a welder 6 while being pressed by squeeze rollers 5 to form a cylindrical resistance-welded steel pipe 7. In the present invention, the manufacturing equipment of the electric resistance welded steel pipe 7 is not limited to the pipe-making process shown in fig. 1.

Then, as shown in fig. 2, the electric resistance welded steel pipe 7 is formed into a cylindrical shape having a ratio of longitudinal diameter/transverse diameter of 0.99 to 1.01 by holding the cylindrical shape and reducing the diameter while maintaining the cylindrical shape by a shaping roller group (shaping jig) 8 composed of a plurality of rollers. Then, the rectangular steel pipe 10 is formed by forming the rectangular forming roll group (rectangular forming frame) 9 composed of a plurality of rolls into shapes of R1, R2, and R3 in this order. The number of stands of the shaping roller group 8 and the square shaping roller group 9 is not particularly limited.

Here, the reason why the molding is performed in a cylindrical shape having a ratio of a longitudinal diameter/a transverse diameter of 0.99 to 1.01 before the molding in a square shape will be described.

In the present invention, it is important to set the ratio of the longitudinal diameter/the lateral diameter to 0.99 to 1.01 for the following reasons. In general, when a steel pipe is manufactured by roll forming, uneven strain is often applied in the circumferential direction in the process for the purpose of suppressing springback. However, when the molding is finally performed on the premise of being square, it is not necessary to make the cross section of the cylindrical shape, which is the previous stage, a perfect circle. Therefore, even if the steel pipe is cylindrical, the steel pipe is not necessarily perfectly round in the middle of the production of the square steel pipe, and as a result, the obtained square steel pipe cannot reduce the characteristic difference between the flat plate portion and the corner portion. Accordingly, in the present invention, in order to reduce the characteristic difference between the flat plate portion and the corner portion, it is necessary to form the shape into a cylindrical shape having a ratio of a longitudinal diameter/a transverse diameter of 0.99 to 1.01 at a previous stage.

If the molded article is not formed into a cylindrical shape having a ratio of the longitudinal diameter/the transverse diameter of 0.99 to 1.01, the plastic deformation of the corner portion becomes excessively large as compared with the flat plate portion. As a result, the ratio of YS of the flat plate portion to the corner portion is less than 0.80, and the ratio of TS of the flat plate portion to the corner portion is less than 0.90. Since the corner portion is plastically deformed more largely than the flat plate portion, it is needless to say that the ratio of YS of the flat plate portion to the corner portion is 0.90 or less and the ratio of TS of the flat plate portion to the corner portion is 1.00 or less. Therefore, in the present invention, in order to set YS of the flat plate portion to 350MPa or more and 520MPa or more, the ratio of YS of the flat plate portion to the corner portion to 0.80 or more and 0.90 or less, and the ratio of TS of the flat plate portion to the corner portion to 0.90 or more and 1.00 or less, the flat plate portion is formed into a cylindrical shape having a ratio of longitudinal diameter/lateral diameter of 0.99 or more and 1.01 or less.

Further, since the corner portion can be formed in a well-balanced manner in the square formation by forming the cylindrical shape with the ratio of the longitudinal diameter/the lateral diameter of 0.99 to 1.01, R of the corner portion can be set to 2.3 × t to 2.9 × t (where t is a plate thickness). R at the corner portion is 2.3 × t or more and 2.9 × t or less (here, t is a plate thickness), whereby a difference in strength between the corner portion and the flat plate portion can be reduced.

As described above, according to the present invention, since YS of the flat plate portion is 350MPa or more, TS is 520MPa or more, a ratio of YS of the flat plate portion to the corner portion is 0.80 to 0.90, a ratio of TS of the flat plate portion to the corner portion is 0.90 to 1.00, a charpy absorption energy at-40 ℃ of the flat plate portion is 100J or more, and R of the corner portion is 2.3 × t to 2.9 × t, a square steel pipe having a small difference in strength between the corner portion and the flat plate portion can be obtained. Since R at the corner of the square steel pipe is controlled to an appropriate size and the strength difference between the corner and the flat plate portion is small, the square steel pipe can be particularly suitably used as a square steel pipe for a building structural member.

As described above, the square steel pipe of the present invention can be preferably used as a material thereof, which is a steel sheet (hot-rolled steel sheet) obtained by sequentially performing the hot rolling step, the cooling step, and the coiling step described below. In the present invention, the steel sheet may be subjected to the above-described pipe forming step to form a rectangular steel pipe.

An example of a method for producing a steel sheet suitable as a material for a square steel pipe according to the present invention will be described.

A method for producing a steel sheet suitable as a material for a square steel pipe according to the present invention is, for example, a method for producing a steel sheet (hot-rolled steel sheet) by sequentially performing a hot rolling step (hereinafter, referred to as a hot rolling step), a cooling step, and a coiling step on a steel material having the above-described composition under the conditions described below.

For example, when a steel raw material having the above-described composition is heated to a heating temperature: after 1100 to 1300 ℃, the steel material after heating is extracted from the heating furnace and before the rough rolling is finished, the number of times of standing time of 30 seconds or more when the steel material is kept still in a state that the thickness center temperature of the steel material is 1000 ℃ or more is controlled to be one or more than five times, and the rough rolling finishing temperature is set as: 1000-800 ℃, finish rolling start temperature: 1000-800 ℃, finish rolling finishing temperature: hot rolling at 900 to 750 ℃ to produce a hot rolled sheet. Next, the hot-rolled sheet after the hot-rolling step is subjected to a cooling step in which cooling is performed once or more than 10 seconds after the start of cooling, and the average cooling rate of the sheet thickness center temperature of the hot-rolled sheet is: 4-25 ℃/s, cooling stop temperature: below 580 ℃. Next, at the coiling temperature: the hot-rolled sheet after the cooling step is wound at 580 ℃ or lower, and then a cooling step is performed to obtain a steel sheet (hot-rolled steel sheet).

The respective steps will be described in detail below. In the following description of the manufacturing method, unless otherwise specified, the temperature (. degree. C.) is set to the surface temperature of the steel material, the thin slab, the hot rolled plate, or the steel plate, etc. The surface temperature can be measured by a radiation thermometer or the like. Unless otherwise stated, the average cooling rate (. degree.C/s) is set to

A value obtained by ((temperature (. degree. C.) -temperature (. degree. C.)/cooling time (s)) before cooling.

The method for melting the steel material (billet) having the above-described composition is not particularly limited, and the steel material can be melted by a known melting method such as a converter, an electric furnace, or a vacuum melting furnace. The casting method is also not particularly limited, and the steel sheet can be produced to a desired size by a known casting method such as a continuous casting method. Further, there is no problem in applying the ingot-cogging rolling method instead of the continuous casting method. The molten steel may be further subjected to secondary refining such as ladle refining.

Next, the obtained steel material (billet) is subjected to a hot rolling step. In the hot rolling process, a steel material is heated to a heating temperature: 1100-1300 ℃. Then, the heated steel material is subjected to rough rolling. In this case, after the steel material is taken out of the heating furnace, the number of times of standing for 30 seconds or more in a state where the plate thickness center temperature of the steel material is 1000 ℃ or more is controlled so as to be one or more and five or less in the period before the finish of rough rolling, and the rough rolling finish temperature is set as: rough rolling at 1000-800 ℃. Then, the finish rolling start temperature was set as: 1000-800 ℃, finish rolling finishing temperature: finish rolling at 900-750 ℃ to obtain a hot rolled plate.

Further, the temperature distribution in the steel material section is calculated by heat conduction analysis, and the temperature at the center of the thickness of the steel material in the hot rolling step is determined.

Heating temperature: 1100-1300 deg.C

When the heating temperature of the steel material is less than 1100 ℃, the deformation resistance of the material to be rolled becomes too large, and a shortage of load resistance and rolling torque occurs in the roughing mill and the finishing mill, and rolling becomes difficult. On the other hand, if the heating temperature exceeds 1300 ℃, austenite grains are coarsened, and even if the work and recrystallization of austenite grains are repeated in rough rolling and finish rolling, grain refining is difficult, and it is difficult to ensure desired toughness in the hot-rolled steel sheet. Therefore, the heating temperature of the steel material is set to 1100 to 1300 ℃. The heating temperature is preferably 1280 ℃ or lower. The heating temperature is preferably 1150 ℃ or higher.

When there is a margin in the load resistance and rolling torque of each rolling mill, the heating temperature may be selected in a range of 1100 ℃ or lower and not lower than the Ar3 transformation point.

The heated steel material is then rough-rolled to produce a thin slab or the like.

The number of times of standing for 30 seconds or more in a state where the plate thickness center temperature is 1000 ℃ or more: more than one time and less than five times

After the steel material is taken out from the heating furnace, the number of times of standing for 30 seconds or more in a state where the steel material is kept at a plate thickness center temperature of 1000 ℃ or more is set to one or more during a period until rough rolling is completed, thereby promoting the growth of scale and increasing the roughness. This can increase the cooling rate at the position 1/4t from the vicinity of the surface in the subsequent cooling step, and the ratio of the total area ratio of bainite and pearlite to the area ratio of ferrite can be set to 2.0 or more. On the other hand, if the number of times of the leaving time exceeds five times, the scale grows excessively, the ratio of the total area ratio of bainite and pearlite to the area ratio of ferrite exceeds 20.0, and the ratio of the area ratio of bainite to the area ratio of pearlite exceeds 20.0. The number of times of the above-mentioned standing time is preferably two or more. Preferably four times or less. When the number of times of the stand time is set to two or more times, the number of times may be set as appropriate by using a facility in which a plurality of roughing mills are arranged, and performing the stand between the plurality of roughing mills in addition to the first entry side of the roughing mill.

Rough rolling finishing temperature: 1000 to 800 DEG C

The heated steel material is subjected to rough rolling, austenite grains are processed and recrystallized to be refined. When the rough rolling finishing temperature is less than 800 ℃, the load resistance and rolling torque of the rough rolling mill are liable to be insufficient. On the other hand, when the rough rolling finishing temperature exceeds 1000 ℃ and becomes high, austenite grains are coarsened, and the toughness of the steel sheet and the square steel pipe is easily lowered. The rough rolling completion temperature is preferably 820 ℃ or higher, more preferably 840 ℃ or higher. The rough rolling completion temperature is preferably 980 ℃ or lower, and more preferably 950 ℃ or lower.

The rough rolling completion temperature can be achieved by adjusting the heating temperature of the steel material, the cooling conditions in the rough rolling, the residence time between passes of the rough rolling, the thickness of the steel material, and the like. The thickness of the material to be rolled (thickness of a thin slab or the like) at the stage of finishing rough rolling is not particularly limited, and may be a product plate (hot-rolled steel plate) having a desired product thickness by finish rolling. For example, in the production of a square steel pipe for a building structural member, the product thickness is preferably about 12 to 28 mm.

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

Finish rolling start temperature: 1000 to 800 DEG C

In the finish rolling, rolling and recrystallization are repeated to refine austenite (γ) grains. When the finish rolling start temperature (finish rolling entry side temperature) is low, the processing strain introduced by the rolling process is likely to remain, and the γ crystal grains are likely to be refined. When the finish rolling start temperature is less than 800 ℃, the temperature in the vicinity of the surface of the steel sheet in the finish rolling mill becomes not more than the Ar3 transformation point, and the risk of ferrite generation increases. Ferrite grains generated before and during the finish rolling become ferrite grains elongated in the rolling direction by the finish rolling processing after the finish rolling, and cause a reduction in toughness. On the other hand, when the finish rolling start temperature exceeds 1000 ℃ and becomes high, the effect of refining γ crystal grains by the finish rolling is reduced, and the toughness of the steel sheet and the square steel pipe is likely to be lowered. Therefore, the finish rolling start temperature is set to 800 to 1000 ℃. The finish rolling start temperature is preferably 825 to 975 ℃.

Finish rolling finish temperature: 900 to 750 DEG C

If the finish rolling temperature (the finish rolling output side temperature) exceeds 900 ℃ and becomes high, the machining strain added during finish rolling becomes insufficient, the γ crystal grains cannot be refined, and the toughness of the steel sheet and the square steel pipe tends to be lowered. On the other hand, when the finish rolling temperature is less than 750 ℃, ferrite grains elongated in the rolling direction are formed at a temperature in the vicinity of the surface of the steel sheet in the finish rolling mill at or below the Ar3 transformation point, and the ferrite grains become mixed grains. This increases the risk of lowering the toughness. Therefore, the finish rolling finishing temperature is set to 900 to 750 ℃. The finish rolling finishing temperature is preferably 850 ℃ or lower. Preferably above 770 ℃.

Next, the hot-rolled sheet obtained in the hot-rolling step is subjected to a cooling step.

The number of times of cooling release is 0.2s or more and less than 3.0s during 10s from the start of cooling: 1 or more times

In the present invention, 10 seconds (10 seconds period) after the start of cooling of the hot-rolled sheet obtained in the hot-rolling step is used as initial cooling. In the initial cooling in the cooling step, the cooling is performed once or more with 0.2s or more and less than 3.0s of cooling. This is done to suppress the formation of a martensite structure on the front and back surfaces of the steel sheet. In the initial cooling, when cooling is not performed or is performed for less than 0.2s, a martensite structure is generated, and the toughness of the steel plate and the square steel pipe is lowered. In addition, in the initial cooling, when the cold release is 3.0s or more, bainite is insufficient, and a structure mainly composed of ferrite and pearlite is formed, and a desired steel structure cannot be obtained. Therefore, the cooling time of one time in the initial cooling in the cooling step is set to 0.2s or more and less than 3.0 s. The cooling time for one time is preferably 0.4s or more, and preferably 2.0s or less.

In order to obtain the above-described effects, the number of times of cooling in the initial cooling needs to be set to one or more times. The number of times of cooling is appropriately set according to the arrangement of the cooling devices, the cooling stop temperature, and the like. Here, the cooling means natural cooling. The upper limit of the number of times of cooling is not particularly limited, but is preferably 10 times or less from the viewpoint of productivity. When the number of times of cooling is set to a plurality of times, it may be appropriately set, for example, by stopping the injection of water from the nozzles in a section of the water-cooling nozzles described later, and performing intermittent injection or the like.

Average cooling rate of sheet thickness center temperature: 4-25 ℃/s, cooling stop temperature: below 580 deg.C

In the cooling step, the hot rolled sheet obtained by the finish rolling is subjected to cooling at an average cooling rate of 4 to 25 ℃/s at the sheet thickness center temperature from the start of cooling to the stop of cooling (the end of cooling), and at a cooling stop temperature of 580 ℃ or lower. The cooling performed in the cooling step is performed by, for example, water cooling (water cooling) such as water jet cooling, spray cooling, and mist cooling, or gas jet cooling in which a cooling gas is jetted. Further, it is preferable to perform the cooling operation on both surfaces (front and back surfaces) of the hot-rolled sheet (steel sheet) in such a manner that both surfaces are cooled under the same condition.

When the average cooling rate at the center of the thickness of the hot-rolled sheet is less than 4 ℃/s, the ratio of the area fraction of bainite to the area fraction of pearlite is less than 5.0, and the toughness is lowered. On the other hand, when the average cooling rate exceeds 25 ℃/s, the ratio of the area fraction of bainite to the area fraction of pearlite exceeds 20.0, strain is easily dispersed in bainite, and work hardening is easily caused. As a result, a square steel pipe having a small difference in strength between the corner portion and the flat plate portion cannot be obtained. Therefore, the average cooling rate of the center of the thickness of the hot-rolled sheet is set to 4 to 25 ℃/s. The average cooling rate at the center of the thickness of the hot-rolled sheet is preferably 5 ℃/s or more, and preferably 15 ℃/s or less.

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

The thickness was determined by ((temperature (. degree. C.) of the center of the sheet at the start of cooling-temperature (. degree. C.) of the center of the sheet at the stop of cooling)/cooling time (s)).

The temperature distribution in the steel sheet section is calculated by heat conduction analysis, and the temperature at the center of the thickness of the hot-rolled sheet is determined.

When the cooling stop temperature exceeds 580 ℃, the ratio of the area ratio of bainite to the area ratio of pearlite is less than 5.0, and the toughness is lowered. The cooling stop temperature is preferably 560 ℃ or lower.

In order to obtain a desired steel structure at the 1/4t position, the average cooling rate in the temperature range of 750 to 650 ℃ at the surface temperature of the hot-rolled sheet is preferably set to 20 ℃/s or more. When the average cooling rate in this temperature range is less than 20 ℃/s, the ratio of the area ratio of bainite to the area ratio of pearlite tends to be less than 5.0. The average cooling rate in a temperature region of 750 to 650 ℃ at the surface temperature of the hot-rolled sheet is preferably 80 ℃/s or less. If the average cooling rate in this temperature range exceeds 80 ℃/s, the ratio of the area ratio of bainite to the area ratio of pearlite may exceed 20.0. In order to control the amount of pearlite and bainite formed, the cooling step is preferably started immediately after the finish rolling (within 5 seconds).

Subsequently, the cooled hot-rolled sheet is subjected to a winding step to obtain a steel sheet (hot-rolled steel sheet).

Coiling temperature: below 580 deg.C

In the winding step, at a winding temperature: a step of coiling the hot-rolled sheet at 580 ℃ or lower and then cooling the sheet. When the coiling temperature exceeds 580 ℃, ferrite transformation and pearlite transformation are performed after coiling, the proportion of pearlite becomes excessive, and the toughness of the steel sheet and the square steel pipe is lowered. Therefore, the winding temperature is 580 ℃ or lower. The coiling temperature is preferably 550 ℃ or lower. Further, although the coiling temperature is reduced without causing a problem in material quality, if the coiling temperature is less than 400 ℃, particularly in a thick steel sheet having a thickness exceeding 25mm, the coiling deformation resistance is large, and the steel sheet may not be wound in order. Therefore, the winding temperature is preferably 400 ℃ or higher.

Then, the steel sheet (hot-rolled steel sheet) after the coiling step is subjected to the above-described pipe-making step to obtain a square steel pipe.

Next, an example of a building structure using the square steel pipe of the present invention will be described.

Fig. 3 is a perspective view schematically showing a building structure according to an embodiment of the present invention. As shown in fig. 3, a plurality of square steel pipes 11 of the present invention are erected in the building structure of the present embodiment and used as a column material. A plurality of girders 14 made of steel such as H-section steel are installed between the adjacent square steel pipes 11. A plurality of small beams 15 made of steel such as H-shaped steel are bridged between the adjacent large beams 14. The square steel pipes 11 are welded to the spacers 16, and H-shaped steel serving as the girders 14 is welded thereto, whereby the girders 14 made of steel such as H-shaped steel are installed between the adjacent square steel pipes 11. In addition, a stud 17 is provided as necessary for mounting a wall or the like.

Since the square steel pipe 11 of the present invention having a small difference in strength between the corner portion and the flat plate portion is used in the building structure of the present invention, selection of the welding material for welding the square steel pipe 11 and the spacer 16 is facilitated, and a difference in strength between the welding material such as mismatch is unlikely to occur. The occurrence of mismatching is less likely, and thus failures such as breakage of the welded portion can be suppressed. Further, since the angle R of the square steel pipe 11 (R of the corner portion) is controlled to an appropriate size, it is easy to combine the square steel pipe with another structural member having a right-angled cross section. Further, by controlling the angle R of the square steel pipe 11 to an appropriate magnitude, it is possible to withstand a larger external force and improve the shock resistance and the like.

Examples

Hereinafter, the present invention will be described with reference to examples for further understanding. The following examples do not limit the present invention in any way.

< example 1 >

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

A molten steel was produced in a converter, and billets (steel materials: thickness: 250mm) having the composition shown in Table 1 were produced by a continuous casting method. These slabs (steel materials) were heated to a heating temperature under the conditions shown in tables 2-1 and 2-2, and then subjected to a hot rolling step, a cooling step, and a coiling step, followed by cooling to obtain a plate thickness: 16 to 28mm steel plate (hot rolled steel plate). After the finish rolling, the cooling step is started immediately (within 5 seconds). The cooling is performed by water cooling. The cooling during the initial cooling was performed by providing a cooling interval in which water cooling was not performed during the initial cooling for 10 seconds from the start of cooling. Then, the obtained steel sheets were used as a material, and round steel pipes were formed by cold rolling and roll forming under the conditions shown in tables 2-1 and 2-2, and then square steel pipes (400 to 550mm angle) were formed by cold rolling and roll forming.

In the examples of the present invention, test pieces were collected from the obtained square steel pipes, and the structure observation, the tensile test, the charpy impact test, and the measurement of R at the corner were performed. In addition, the tissue observation is observed and measured by the above-described method. The tensile test and charpy impact test were performed by the following methods, and the method of measuring R at the corner was as follows.

(1) Tensile test of square steel pipe

Tensile test pieces according to JIS5 were collected from the flat plate portions and the corner portions of the obtained square steel pipes so that the tensile direction was the longitudinal direction of the pipes. Subsequently, a tensile test was performed according to the regulations of JISZ2241(2011), and the yield strength YS and the tensile strength TS were measured. Using the obtained measurement values, a yield ratio YR (%) defined by (yield strength)/(tensile strength) × 100 (%) was calculated.

(2) Impact test of square steel tube

From the plate thickness 1/4t position of the flat plate portion of the obtained square steel pipe, a V-cut specimen was collected so that the longitudinal direction of the specimen was in the pipe circumferential direction. Next, according to the regulations of JISZ2242(2011), at a test temperature: the Charpy impact test was carried out at-40 ℃ to determine the absorption energy (J). The number of test pieces was 3, and the average value of 3 pieces was set as the value of the impact test result shown in tables 4-1 and 4-2.

(3) Method for measuring R (angle R) of corner

From the obtained square steel pipe, 10 sections perpendicular to the pipe axial direction were arbitrarily cut out, the radii of curvature of the corners located at the four corners of the perpendicular section were measured, and the average value thereof was defined as R of the corner of the section. Specifically, as shown in fig. 4, when the welded portion (seam portion) of the steel pipe is set to 0 °, and the positions of 45 °, 135 °, 225 °, and 315 ° are set to the corner center with reference to the 0 °, the radius of curvature of the corner is a radius of curvature at an intersection of a line (L) forming 45 ° with the adjacent side with the center of the pipe as a starting point and the outside of the corner (the pipe outer surface side of the corner). The radius of curvature of the corner portion is a radius of a sector such that the center angle defined by a line drawn toward a connection point (A, A') between the flat portion and the circular arc portion of the square steel pipe with the center thereof placed on the L is 65 °. In addition, "t" shown in fig. 4 is a plate thickness, and "H" is a length of an outer side. Examples of the method of calculating the radius of curvature include a method of calculating the radius of curvature using a sine theorem based on the measurement result of the distance relationship between three points (an intersection point outside the corner and two points that are connection points between the flat portion and the circular arc portion), and a method of measuring the radius of curvature from a radial gauge that is perfectly matched with the corner in the region of the three points, but the method is not limited to this. In the present embodiment, a radial gauge is used to measure the radius of curvature of the corner. The angle R is an average value at 10 points in a cross section perpendicular to the tube axial direction as described above.

The results obtained are shown in Table 3-1, Table 3-2, Table 4-1 and Table 4-2.

[ Table 1]

[ Table 2-1]

[ tables 2-2]

[ Table 3-1]

[ Table 3-1]

[ tables 3-2]

[ tables 3-2]

[ Table 4-1]

[ tables 4-2]

The characteristics of the present invention were obtained in all the invention examples that are the scope of the present invention (YS of the flat plate portion is 350MPa or more, TS is 520MPa or more, the ratio of YS of the flat plate portion to the corner portion is 0.80 or more and 0.90 or less, the ratio of TS of the flat plate portion to the corner portion is 0.90 or more and 1.00 or less, the Charpy absorption energy at-40 ℃ of the flat plate portion is 100J or more, and R of the corner portion is 2.3 Xt or more and 2.9 Xt or less) (here, t is the plate thickness.). On the other hand, in comparative examples which do not depart from the scope of the present invention, the characteristics of the present invention were not obtained.

Description of reference numerals

1 … steel belt, 2 … leveler, 3 … cage roller group, 4 … fin roller group, 5 … extrusion roller, 6 … welder, 7 … resistance welding steel pipe, 8 … shaping roller group, 9 … square forming roller group, 10 … square steel pipe, 11 … square steel pipe, 14 … girder, 15 … trabecula, 16 … clapboard and 17 … stud.