JP6813140B1 - Square steel pipe and its manufacturing method, and building structures - Google Patents

Abstract

Provided are a square steel pipe, a method for manufacturing the same, and a building structure using the square steel pipe. The present invention is a square steel pipe having a flat plate portion and a square portion, which has a specific component composition, and the steel structure at a depth of 1/4 of the plate thickness t from the outer surface of the steel pipe has ferrite as an area ratio. 55% or more and 80% or less, the average aspect ratio of the hard phase is 0.1 to 0.8, and the flat plate portion has a YS of 350 MPa or more and a TS of 520 MPa or more, and the YS of the flat plate portion with respect to the corner portion. The ratio 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 of -40 ° C of the flat plate portion is 100 J or more, and the corner portion. R is (2.3 × t) or more and (2.9 × t) or less.

Description

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 suitably used as a building structural member.

Square steel pipes (also referred to as "square columns") are usually manufactured by cold forming a hot-rolled steel plate (hot-rolled steel strip) or thick plate as a material. Cold forming methods include press forming and roll forming. However, in any of these methods, since a large plastic strain is applied to the corner portion of the square steel pipe as compared with the flat plate portion of the square steel pipe, the strength of the corner portion tends to increase, and the strength difference between the corner portion and the flat plate portion. There is a problem that becomes large. If the characteristics of the corner and the flat plate are significantly different, it will be very difficult to select the welding material and design the building, and it will be difficult to use the square steel pipe as the material for the building structure.

Although there are not many examples of direct studies on such problems, for example, there is a technique of Patent Document 1 as a square steel pipe for a building structure. Patent Document 1 describes a square steel pipe obtained by cold bending a steel plate, and the steel pipe has a C: 0.02 to 0.18% (“%” means “mass%”. The same applies to 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%. Each contains the following (not including 0%), the balance is composed of Fe and unavoidable impurities, and among the unavoidable impurities, P: 0.02% or less (not including 0%), S: 0. It is suppressed to 01% or less (not including 0%) and O: 0.004% or less (not including 0%), respectively, and the bent portion is in a state of being processed at a right angle and described below. Cold-formed square steel pipes that ensure earthquake resistance by satisfying the requirements (A) to (C) are disclosed.
(A) Yield strength at a flat portion of a steel pipe: 355 MPa or more, tensile strength: 520 MPa or more.
(B) In the microstructure of the flat portion, the surface integral of the bainite structure: 40% or more.
(C) The surface layer portion at the corner of the steel pipe has a Vickers hardness Hv: 350 or less, elongation in a tensile test: 10% or more, and Charpy absorption energy vE 0: 70J or more at 0 ° C.

Japanese Patent No. 5385760

Square steel pipes manufactured by cold roll forming are made from a material (hot rolled material) that is flat in the width direction, which is made by hot rolling, into round steel pipes by roll forming, and then the corners and flat plates. It is formed into a square steel pipe having a portion. Due to such a manufacturing method, the difference in strength between the corner portion and the flat plate portion tends to be large due to the difference in work hardening. Furthermore, in hot rolling performed before roll forming, the material is built in by cooling control from the surface of the hot rolled material, so processing is performed near the surface layer of the hot rolled material where the cooling rate is relatively high. There was a problem that the previous strength (hardness) increased.

However, in the technique disclosed in Patent Document 1 described above, the temperature control in hot rolling is limited to preventing the surface hardness of the steel sheet from increasing excessively, and the corners and the flat plate are positively used. It does not reduce the difference in strength of the parts. Therefore, in the square steel pipe obtained by cold bending, the strength of the corner portion is relatively higher than the strength of the flat plate portion even if the characteristics of the corner portion satisfy a predetermined standard. Was clear. In order to suppress the increase in the strength of the corners, it is effective to reduce the plastic strain of the corners. In order to reduce the plastic strain of the corners, it is conceivable to increase the R (roundness) of the corners. However, a square steel pipe having a large R at a corner is not preferable because when it is combined with another member as a square member, there is a problem in design and a problem that the performance as a building is deteriorated due to the occurrence of a gap or the like.

The present invention has been made in view of such circumstances, and an object of the present invention 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. ..

As a result of diligent studies to solve the above problems, the present inventors have obtained the following findings.

In the present invention, the corners are made less likely to occur in the vicinity of the surface layer of the steel pipe (hereinafter, referred to as the vicinity of the outer surface) where the processing strain (plastic strain) introduced by cold roll forming is particularly large. The idea was to reduce the difference in strength between the part and the flat plate part.

Therefore, the present inventors prepared a plurality of samples in which the area ratio of ferrite and the aspect ratio of a hard phase other than ferrite (hereinafter referred to as a hard phase) were changed as the steel structure of a square steel pipe, and were work-hardened. I checked the ease. Here, the hard phase includes, but is not limited to, bainite, pearlite, martensite, retained austenite, and the like.

As a result, in a square steel pipe having a strength of 350 MPa or more in YS and 520 MPa or more in TS of the flat plate portion, the ratio of ferrite is set to a certain level or higher, and the average aspect ratio of the hard phase is set to 0.1 to 0.8. , Found that it is possible to create a steel structure that is difficult to work harden. It is considered that this is because the work hardening ability of the ferrite is small and the strain is easily concentrated on the ferrite, so that the work hardening ability of the steel structure as a whole becomes small.

Further, in order to utilize the steel structure of the material (hot rolled material) to suppress work hardening of the corners, the present inventors once manufacture a square steel pipe, the vertical diameter / horizontal diameter ratio. After forming into a cylindrical round steel pipe of 0.99 or more and 1.01 or less, the corner R is 2.3 × t (t is the plate thickness) or more by the rolls arranged vertically and horizontally. It is molded into a square shape of 9 × t or less. As a result, it has been found that a square steel pipe can be obtained without excessive work hardening of the corners. Here, the "vertical diameter" refers to the outer diameter in the vertical direction with respect to the pipe axis of the round steel pipe, and the "horizontal diameter" refers to the outer diameter in the horizontal direction with respect to the pipe axis of the round steel pipe.

Based on the above, in the present invention, the ratio of ferrite is set to a certain value or more and the average aspect ratio of the hard phase is set to a certain value or more for the steel structure near the outer surface of the square steel pipe having the largest processing strain introduced by cold roll forming. It shall be 0.1 to 0.8. Further, the hot-rolled material is formed into a cylindrical shape having a vertical diameter / horizontal diameter ratio of 0.99 or more and 1.01 or less, and then formed into a square shape with rolls arranged vertically and horizontally to form a corner portion. It was thought that a square steel pipe with a small difference in strength between the flat plate and the flat plate could be manufactured.

In the present invention, the "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 or more and 0.90 or less, and the TS of the flat plate portion with respect to the corner portion. It shows that the ratio is 0.90 or more and 1.00 or less.

The present inventors have repeated more detailed studies and have completed the present invention. The gist of the present invention is as follows.
[1] A square steel pipe having a flat plate portion and a corner portion.
Ingredient composition is 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: Contains 0.006% or less, 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 has an area ratio of 55% or more and 80% or less, and a hard phase average aspect ratio is 0.1 to 0.8.
The flat plate portion has a YS of 350 MPa or more and a TS of 520 MPa 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, and 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 ° C. of the flat plate portion is 100 J or more.
A square steel pipe having an R of the corner portion of (2.3 × t) or more and (2.9 × t) or less.
[2] The square steel pipe according to [1], which further contains one group or two or more groups selected from the following groups A to C in mass% in addition to the component composition.
Group A: Nb: 0.05% or less, Ti: 0.05% or less, V: 0.10% or less, one or more selected from Group B: B: 0.008% or less C Group: 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. One or more selected from 001 to 0.010% [3] The steel structure further described in [1] or [2], wherein the average circle equivalent diameter of the hard phase is 20 μm or less. Square steel pipe.
[4] The method for manufacturing a square steel pipe according to any one of [1] to [3].
A pipe making process in which a steel plate is coldly roll-formed to form a cylindrical end face, welded, formed into a cylindrical shape with a vertical / horizontal diameter ratio of 0.99 or more and 1.01 or less, and then formed into a square shape. A method of manufacturing a square steel pipe.
[5] The method for manufacturing a square steel pipe according to any one of [1] to [3].
When a square steel pipe is manufactured by subjecting a steel material to a hot rolling process, a cooling process, a winding process, and a pipe making process in this order.
After heating the steel material to a heating temperature: 1100 to 1300 ° C.
Rough rolling time when the center temperature of the plate thickness is 1000 ° C or higher: 200 seconds or more and 400 seconds or less, rough rolling end temperature: 1000 to 800 ° C,
A hot rolling process of finishing rolling start temperature: 1000 to 800 ° C. and finish rolling end temperature: 900 to 750 ° C. is performed to obtain a hot-rolled plate.
Next, the hot-rolled plate has one or more cooling releases of 0.2 s or more and less than 3.0 s in the initial cooling step from the start of cooling to 10 s, and the average cooling rate at the plate thickness center temperature: 4 to 25 ° C. A cooling process of / s is performed, and
Next, the hot-rolled plate is subjected to a winding step of winding at a winding temperature of 580 ° C. or lower to obtain a steel sheet.
Next, the steel plate is cold-rolled to form a cylindrical end face, and the end face is welded to form a cylindrical shape having a vertical diameter / horizontal diameter ratio of 0.99 or more and 1.01 or less, and then formed into a square shape. A method for manufacturing a square steel pipe to be subjected to a pipe making process.
[6] The method for manufacturing a square steel pipe according to [5], wherein the cooling stop temperature in the cooling step is 580 ° C. or lower.
[7] A building structure using the square steel pipe according to any one of [1] to [3].

According to the present invention, it is possible to obtain a square steel pipe having a small difference in strength between a square portion and a flat plate portion. Since the R of the corner portion is controlled to an appropriate size, this square steel pipe can be suitably used as, for example, a square steel pipe for building structural members.

FIG. 1 is a schematic view showing an example of an electric resistance pipe manufacturing facility. FIG. 2 is a schematic view showing a molding process of 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 view showing a cross section of a square steel pipe.

Hereinafter, the present invention will be described in detail.

The square steel pipe of the present invention is as follows. The composition of the components is C: 0.07 to 0.20%, Si: 1.0% or less, Mn: 0.5 to 2.0%, P: 0.030% or less, S: 0. It contains 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 the 1/4 depth position (hereinafter referred to as 1 / 4t position) of the plate thickness t from the outer surface of this square steel pipe, the area ratio of ferrite is 55% or more and 80% or less, and the hard phase The average aspect ratio is 0.1 to 0.8. The flat plate portion of the square steel plate has a YS of 350 MPa or more and a TS of 520 MPa 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, and the ratio of TS of the flat plate portion to the corner portion is It is 0.90 or more and 1.00 or less, the Charpy absorption energy at −40 ° C. at the plate thickness 1/4 t position of the flat plate portion is 100 J or more, and the R of the corner portion is (2.3 × t) or more (2. 9 × t) or less.

First, the component composition of the present invention will be described. Unless otherwise specified, mass% is simply expressed as%. In the present invention, the composition of the square steel pipe and the steel plate used as the material of the square steel pipe are the same. Therefore, in the following, the reason for limiting the component composition of the square steel pipe and the steel plate used as the material will be described.

C: 0.07 to 0.20%
C increases the strength of the steel plate and the square steel pipe by solid solution strengthening. On the other hand, C is an element that reduces the amount of ferrite produced by increasing the amount of hard phase produced. In order to secure the desired strength and the steel structure of the desired steel plate and square steel pipe, C needs to be contained in an amount of 0.07% or more. On the other hand, if the content of C exceeds 0.20%, it becomes difficult to secure a desired ferrite amount. Therefore, C is set to 0.07 to 0.20%. C is preferably 0.09% or more, and more preferably 0.10% or more. Further, C is preferably 0.18% or less, and more preferably 0.17% or less.

Si: 1.0% or less Si is an element that contributes to increasing the strength of steel sheets and square steel pipes by solid solution strengthening. In order to secure the desired strength of the steel plate and the square steel pipe, it is desirable that Si is contained in an amount of more than 0.01%. However, if Si is contained in excess of 1.0%, the toughness decreases. Therefore, Si is set to 1.0% or less. Si is preferably 0.8% or less, more preferably 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 the steel plate and the square steel pipe through solid solution strengthening, and needs to be contained in an amount of 0.5% or more in order to secure the desired strength of the steel plate and the square steel pipe. If the Mn content is less than 0.5%, the ferrite transformation start temperature is increased, and the hard phase tends to be excessively coarsened accordingly. On the other hand, if Mn is contained in excess of 2.0%, the hardness of the central segregated portion increases, and there is a concern that it may cause cracks during welding of the square steel pipe at the site. 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: 0.030% or less P is an element that segregates into ferrite grain boundaries and has the effect of reducing the toughness of steel sheets and square steel pipes. In the present invention, it is desirable to reduce impurities as much as possible. However, excessive reduction of P causes an increase in refining cost, so it is preferably 0.002% or more. The content of P is acceptable up to 0.030%. Therefore, P is set to 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 mainly exists as MnS within the range of the component composition of the present invention. MnS is thinly stretched in the hot rolling process and adversely affects the ductility and toughness of steel sheets and square steel pipes. Therefore, in the present invention, it is desirable to reduce MnS as much as possible. However, excessive reduction of S causes an increase in refining cost, so S is preferably 0.0002% or more. The content of S is acceptable up to 0.015%. Therefore, S is set to 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 that acts as an antacid and also has an action of fixing N as AlN. In order to obtain such an effect, the content of Al of 0.01% or more is required. If Al is less than 0.01%, the deoxidizing power is insufficient when Si is not added, oxide-based inclusions increase, and the cleanliness of the steel sheet decreases. On the other hand, if the content of Al exceeds 0.06%, the amount of solid-dissolved Al increases, and during longitudinal welding of square steel pipes (that is, during electric welding in the longitudinal direction of steel pipes in the production of square steel pipes), especially in the atmosphere. In the case of welding, the risk of forming oxides in the welded portion increases, and the toughness of the square steel pipe welded portion 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: 0.006% or less N is an element that has the effect of reducing the toughness of steel sheets and square steel pipes by firmly fixing the motion of dislocations. In the present invention, it is desirable to reduce N as an impurity as much as possible, and up to 0.006% is acceptable. Therefore, N is set to 0.006% or less. N is preferably 0.005% or less. Although not particularly specified in the present invention, N is preferably 0.001% or more from the viewpoint of manufacturing cost.

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

The above is the basic composition of the present invention. Although the above-mentioned essential elements can obtain the properties desired in the present invention, the following elements can be contained as needed.

One or more selected from Nb: 0.05% or less, Ti: 0.05% or less, V: 0.10% or less Nb, Ti, V are all fine carbides in steel. , It is an element that forms nitride and contributes to the improvement of steel strength through precipitation strengthening. Therefore, in the present invention, it may be contained for the purpose of adjusting the strength. In order to obtain such an effect, when Nb, Ti, and V are contained, it is preferable that Nb: 0.05% or less, Ti: 0.05% or less, and V: 0.10% or less, respectively. , Nb: 0.04% or less, Ti: 0.04% or less, V: 0.08% or less, more preferably. When Nb, Ti, and V are contained, Nb: 0.001% or more, Ti: 0.001% or more, V: 0.001% or more are preferable, and Nb: 0.003% or more, respectively. It is more preferable that Ti: 0.003% or more and V: 0.003% or more.

When two or more kinds selected from Nb, Ti, and V are contained, the total content is preferably 0.2% or less, and preferably 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 transformed ferrite, and increasing the strength of the steel plate and the square steel pipe. The content of B leads 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, B can be contained as necessary for the purpose of adjusting the strength as long as the yield ratio of the square steel pipe is within the range of 90% or less. When B is contained, it is preferably 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, and more preferably 0.0003% or more.

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 One or more selected from 0.010% Cr: 0.01-1.0%
Cr is an element that increases the strength of steel sheets and square steel pipes by enhancing hardenability, and can be contained as necessary. When Cr is contained in order to obtain such an effect, it is preferable to contain Cr of 0.01% or more. On the other hand, if it is contained in excess of 1.0%, the toughness and weldability may be lowered. Therefore, when Cr is contained, it is preferably 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 that increases the strength of steel plates and square steel pipes by enhancing hardenability, and can be contained as necessary. When Mo is contained in order to obtain such an effect, it is preferable to contain 0.01% or more of Mo. On the other hand, if Mo is contained in excess of 1.0%, the toughness may be lowered. Therefore, when Mo is contained, it is preferably 1.0% or less. 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 steel sheets and square steel pipes by solid solution strengthening, and can be contained as needed. When Cu is contained in order to obtain such an effect, it is preferable to contain 0.01% or more of Cu. On the other hand, if Cu is contained in excess of 0.50%, the toughness may be lowered. Therefore, when Cu is contained, it is preferably 0.50% or less. Cu is more preferably 0.02% or more, and more preferably 0.4% or less.

Ni: 0.01 to 0.30%
Ni is an element that increases the strength of steel sheets and square steel pipes by solid solution strengthening, and can be contained as needed. 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, if Ni is contained in excess of 0.30%, the area ratio of ferrite may easily 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 that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly stretched in the hot rolling process, and can be contained as needed. When Ca is contained in order to obtain such an effect, it is preferable to contain 0.001% or more of Ca. However, if the Ca content exceeds 0.010%, Ca oxide clusters may be formed in the steel and the toughness may deteriorate. Therefore, when Ca is contained, the Ca content is preferably 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 square steel pipe of the present invention will be described.

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

Ferrite: 55% or more and 80% or less in area ratio In order to secure the desired strength of the square steel pipe of the present invention, the steel structure at 1/4 t position from the outer surface of the steel pipe is composed of ferrite and other hard phases. Here, the hard phase includes phases other than ferrite, that is, bainite, pearlite, martensite, retained austenite, and the like. This hard phase has a total area ratio of 20 to 45% for each phase.

When the area ratio of ferrite is less than 55%, the proportion of bainite becomes excessive in the range of the above-mentioned component composition of the present invention, and strain is easily dispersed in the hard phase, so that work hardening is easily performed. As a result, it is not possible to obtain a square steel pipe having a small difference in strength between the corner portion and the flat plate portion. On the other hand, if the area ratio of ferrite exceeds 80%, the desired strength cannot be obtained. Ferrite is preferably 60% or more, 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, crack origins are likely to occur, resulting in a decrease in toughness. On the other hand, when the average aspect ratio of the hard phase exceeds 0.8, strain is easily dispersed in the hard phase, so that work hardening is likely to occur. As a result, when manufacturing a square steel pipe using a steel plate, it is not possible to obtain a square steel pipe having a small difference in strength between the corner portion and the flat plate portion. It is more preferably 0.2 or more, and more preferably 0.7 or less. In the present invention, as will be described later, the aspect ratio of the hard phase is the average value of the aspect ratios of grains having a nanohardness of 3.0 GPa or more in a structure other than ferrite.

Average circle equivalent diameter of hard phase: 20 μm or less (optimal conditions)
If the average circle-equivalent diameter of the hard phase exceeds 20 μm, the toughness decreases, so it is preferably 20 μm or less. More preferably, it is 15 μm or less. In the present invention, as will be described later, the average circle-equivalent diameter of the hard phase is the average value of the circle-equivalent diameter of grains having a nanohardness of 3.0 GPa or more in a structure other than ferrite.

In general, a square steel pipe manufactured by roll forming using a steel plate (hot-rolled steel plate) has the same steel structure at the 1 / 4t position in both the corner portion and the flat plate portion, so that the flat plate portion is located at the 1 / 4t position. Alternatively, measurement may be performed at either the 1 / 4t position of the corner portion. Here, the steel structure at the 1 / 4t position of the flat plate portion is defined.

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

The above steel structure is observed by the following method, and the type of structure and area ratio (%) are determined. The test piece for structure observation is collected from a square steel pipe, polished so that the cross section in the rolling direction (L cross section) becomes the observation surface, and subjected to nital corrosion to prepare the test piece. For structure observation, the structure at a position of 1/4 t in thickness from the surface of the test piece for structure observation (that is, the outer surface of the square steel tube) is the center of observation, and an optical microscope (magnification: 500 times) or a scanning electron microscope (SEM) is used. , Magnification: 500 times) to observe and image the steel structure. The measurement area was 500 μm × 500 μm. Here, "t" indicates the thickness (plate thickness) of the steel pipe. From the obtained tissue photograph, an image analysis device (image analysis software: Photoshop, manufactured by Adobe) is used to identify the type of structure and calculate the area ratio of ferrite. The area ratio of the tissue was determined by observing in 5 or more visual fields and as an average value of the values obtained in each visual field.

The average aspect ratio of the hard phase was determined as follows. First, the nanohardness of structures other than ferrite was determined by the nanoindentation method from the microstructure photographs obtained above. For grains having a nanohardness of 3.0 GPa or more, the value calculated by (average length in the plate thickness direction / average length in the rolling direction) for all the grains was obtained and used as the average aspect ratio.

The average circle equivalent diameter of the hard phase was measured by using the SEM / EBSD method. For the average circle equivalent diameter, the orientation difference between adjacent crystal grains was determined, and the boundary where the orientation difference was 15 ° or more was measured as the crystal grain boundary. The arithmetic mean of the particle size was obtained from the obtained grain boundaries and used as the average circle-equivalent diameter. The measurement area was 500 μm × 500 μm, and the measurement step size was 0.5 μm. In the crystal grain size analysis, those having a crystal grain size of 2.0 μm or less were regarded as measurement noise, and those having a nanohardness of less than 3.0 GPa were excluded from the analysis as non-hard phases.

Next, the method for manufacturing the square steel pipe of the present invention will be described with reference to FIGS. 1 and 2. FIG. 1 is a schematic view showing an example of an electric resistance pipe manufacturing facility. FIG. 2 is a schematic view showing a molding process of a square steel pipe.

In the method for manufacturing a square steel pipe of the present invention, a steel plate is subjected to a pipe making process to obtain a square steel pipe. In the pipe making process of the present invention, a steel plate is coldly rolled and a cylindrical end face is welded. Next, after forming into a cylindrical round steel pipe with a vertical diameter / horizontal diameter ratio of 0.99 or more and 1.01 or less, the round steel pipe is further coldly formed into a square shape by rolls arranged vertically and horizontally. It is formed into a square steel pipe having a square portion and a flat plate portion.

First, as shown in FIG. 1, the steel strip 1, which is the material of the electrosewn steel pipe, is subjected to, for example, the entry side correction by the leveler 2, and then intermediately formed by the cage roll group 3 composed of a plurality of rolls to form an open pipe. After that, the fin pass roll group 4 composed of a plurality of rolls is finished and molded. After the finish molding, the width end of the steel strip 1 is electrically resistance welded by the welding machine 6 while being pressure-welded with the squeeze roll 5 to form a cylindrical electric resistance welded steel pipe 7. Further, in the present invention, the manufacturing equipment of the electric resistance pipe 7 is not limited to the pipe making process as shown in FIG.

After that, as shown in FIG. 2, the electrosewn steel pipe 7 is reduced in diameter while remaining cylindrical by a sizing roll group (sizing stand) 8 composed of a plurality of rolls, and the ratio of vertical diameter / horizontal diameter is 0.99 or more. It has a cylindrical shape of 01 or less. After that, it is sequentially formed into a shape such as R1, R2, and R3 by a square forming roll group (square forming stand) 9 composed of a plurality of rolls to form a square steel pipe 10. The number of stands of the sizing roll group 8 and the square forming roll group 9 is not particularly limited.

Here, the reason for forming into a cylindrical shape having a vertical diameter / horizontal diameter ratio of 0.99 or more and 1.01 or less before forming into a square shape will be described.

In the present invention, it is important that the ratio of vertical diameter / horizontal diameter is 0.99 or more and 1.01 or less for the following reasons. In general, when a steel pipe is manufactured by roll forming, a non-uniform strain is often applied in the circumferential direction for the purpose of suppressing springback in the process. However, on the premise that the shape is finally formed into a square shape, the cross section of the cylindrical shape, which is the previous step, does not necessarily have to be a perfect circle. Therefore, even if it is cylindrical, it is not necessarily a perfect circle in the middle of manufacturing a square steel pipe, and as a result, the obtained square steel pipe cannot reduce the characteristic difference between the flat plate portion and the square portion. From this, in the present invention, in order to reduce the characteristic difference between the flat plate portion and the corner portion, the shape is formed into a cylindrical shape having a vertical diameter / horizontal diameter ratio of 0.99 or more and 1.01 or less in the previous step. Is essential.

Unless the shape is formed into a cylindrical shape having a vertical diameter / horizontal diameter ratio of 0.99 or more and 1.01 or less, the plastic strain of the corner portion becomes too 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 plastic strain of the corner portion is larger than that of the flat plate portion, 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. Of course. Therefore, the YS of the flat plate portion, which is the object of the present invention, is 350 MPa or more, the TS is 520 MPa or more, the ratio of the YS of the flat plate portion to the corner portion is 0.80 or more and 0.90 or less, and the TS of the flat plate portion with respect to the corner portion. In order to make the ratio 0.90 or more and 1.00 or less, the vertical diameter / horizontal diameter ratio is formed into a cylindrical shape of 0.99 or more and 1.01 or less before square molding.

Further, by molding into a cylindrical shape having a vertical diameter / horizontal diameter ratio of 0.99 or more and 1.01 or less, the corners can be uniformly molded at the time of square molding, so that the R of the corners is (2). It can be .3 × t) or more (2.9 × t) or less (where t is the plate thickness). By making the R of the corner portion equal to or greater than (2.3 × t) (2.9 × t) or less (where t is the plate thickness), the difference in strength between the corner portion and the flat plate portion is reduced. Can be done.

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

As described above, as the material of the square steel pipe of the present invention, a steel plate (hot-rolled steel plate) obtained by performing the hot-rolling step, the cooling step, and the winding step described below in this order is preferably used. Can be done. In the present invention, the steel sheet may be subjected to the above-mentioned pipe making process to form a square steel pipe.

An example of a method for manufacturing a steel plate suitable as a material for a square steel pipe of the present invention will be described.

A method for producing a steel sheet suitable as a material for a square steel pipe of the present invention is, for example, a hot rolling step (hereinafter referred to as a hot rolling step) on a steel material having the above-mentioned composition under the conditions described below. A steel sheet (hot-rolled steel sheet) can be obtained by performing a cooling step and a winding step in this order.

For example, after heating a steel material having the above-mentioned component composition to a heating temperature of 1100 to 1300 ° C., rough rolling time at a plate thickness center temperature of 1000 ° C. or higher: 200 seconds or more and 400 seconds or less, rough rolling A hot rolling process is performed in which the end temperature is 1000 to 800 ° C., the finish rolling start temperature is 1000 to 800 ° C., and the finish rolling end temperature is 900 to 750 ° C. to obtain a hot-rolled plate. Next, the hot-rolled plate after the hot-rolling step has one or more cooling releases of 0.2 s or more and less than 3.0 s in the initial cooling for 10 s from the start of cooling, and the average of the hot-rolled plates at the center temperature of the plate thickness. A cooling step is performed in which the cooling rate is 4 to 25 ° C./s and the cooling stop temperature is 580 ° C. or lower. Next, the hot-rolled sheet after the cooling step is wound at a winding temperature of 580 ° C. or lower, and then a winding step of allowing to cool is performed to obtain a steel sheet (hot-rolled steel sheet).

Each step will be described in detail below. In the following description of the manufacturing method, the temperature (° C.) is the surface temperature of a steel material, a sheet bar, a hot-rolled plate, a steel plate, or the like unless otherwise specified. These surface temperatures can be measured with a radiation thermometer or the like. The average cooling rate (° C./s) shall be ((temperature before cooling-temperature after cooling) / cooling time) unless otherwise specified.
The value obtained in.

In the present invention, the melting method of the steel material (steel slab) having the above-mentioned composition is not particularly limited, and melting is performed using a known melting method such as a converter, an electric furnace, or a vacuum melting furnace. Can be done. The casting method is not particularly limited, and it can be manufactured to a desired size by a known casting method such as a continuous casting method. It should be noted that there is no problem even if the ingot-lump rolling method is applied 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 (steel slab) is subjected to a hot spreading process. In the hot rolling process, the steel material is heated to a heating temperature of 1100 to 1300 ° C. After that, on the heated steel material extracted from the heating furnace, the rough rolling time at a plate thickness center temperature of 1000 ° C. or higher: 200 seconds or more and 400 seconds or less is controlled, and then the rough rolling end temperature: 1000 to After rough rolling at 800 ° C., finish rolling at a finish rolling start temperature of 1000 to 800 ° C. and a finish rolling end temperature of 900 to 750 ° C. is performed to obtain a hot-rolled plate.

The temperature at the center of the thickness of the steel material in the heat spreading process is obtained by calculating the temperature distribution in the cross section of the steel material by heat transfer analysis.

Heating temperature: 1100 to 1300 ° C
If the heating temperature of the steel material is less than 1100 ° C., the deformation resistance of the material to be rolled becomes too large, and the rough rolling mill and the finishing rolling mill lack the load capacity and rolling torque, which makes rolling difficult. On the other hand, when the heating temperature exceeds 1300 ° C., the austenite crystal grains become coarse, and even if the processing and recrystallization of the austenite grains are repeated in rough rolling and finish rolling, it becomes difficult to make the austenite grains finer. In addition, it may be difficult to secure the desired toughness of the hot-rolled steel sheet. Therefore, the heating temperature of the steel material is set to 1100 to 1300 ° C. The heating temperature is preferably 1280 ° C. or lower. The heating temperature is preferably 1150 ° C. or higher.

If there is a margin in the load capacity and rolling torque of each rolling mill, a temperature in the range of 1100 ° C. or lower and Ar3 transformation point or higher may be selected as the heating temperature.

The heated steel material is then roughly rolled to form a seat bar or the like.

Rough rolling time at 1000 ° C or higher at the center temperature of the plate thickness: 200 seconds or more and 400 seconds or less After extracting the steel material from the heating furnace, the rough rolling time at the center temperature of the plate thickness of the steel material at 1000 ° C or higher is within 400 seconds. By doing so, the vicinity of the surface of the material to be rolled is preferentially cooled. As a result, coarsening of the austenite particle size at a position 1/4 t from the surface of the material to be rolled 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. If the rough rolling time exceeds 400 seconds, the austenite is coarse-grained, the desired amount of ferrite cannot be secured, and a square steel pipe having a small difference in strength between the square portion and the flat plate portion cannot be obtained. On the other hand, if the rough rolling time is less than 200 seconds, the austenite particle size becomes too fine, the proportion of ferrite in the steel structure exceeds 80% in terms of area ratio, and the strength is insufficient. The rough rolling time is preferably 220 seconds or more, and more preferably 250 seconds or more. The rough rolling time is preferably 380 seconds or less, and more preferably 350 seconds or less.

Rough rolling end temperature: 1000-800 ° C
Austenite grains are processed and recrystallized in the heated steel material by rough rolling to make it finer. If the rough rolling end temperature is less than 800 ° C., the load capacity and rolling torque of the rough rolling mill are likely to be insufficient. On the other hand, when the rough rolling end temperature exceeds 1000 ° C. and becomes high, the austenite grains become coarse and the toughness of the square steel pipe tends 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, more preferably 950 ° C. or lower.

The rough rolling end temperature can be achieved by adjusting the heating temperature of the steel material, the cooling conditions during the rough rolling, the retention between the rough rolling passes, the thickness of the steel material, and the like. The thickness of the material to be rolled (thickness of the sheet bar, etc.) at the stage when the rough rolling is completed is not particularly limited, and a product plate (hot-rolled steel plate) having a desired product thickness for finish rolling shall be used. I wish I could. For example, in the production of square steel pipes for building structural members, the product thickness is preferably about 12 to 28 mm.

After rough rolling, the material to be rolled is finished rolled by, for example, a tandem rolling mill to be a hot-rolled plate.

Finish rolling start temperature: 1000-800 ° C
In finish rolling, rolling and recrystallization are repeated, and miniaturization of austenite (γ) grains progresses. When the finish rolling start temperature (finish rolling inlet side temperature) becomes low, the machining strain introduced by the rolling process tends to remain, and it is easy to achieve miniaturization of γ grains. If the finish rolling start temperature is less than 800 ° 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 produced ferrite becomes ferrite grains elongated in the rolling direction by the subsequent finish rolling process, 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 miniaturizing γ grains by the finish rolling described above is reduced, and the toughness of the square steel pipe is likely to be lowered. Therefore, the finish rolling start temperature is set to 800 to 1000 ° C. The finish rolling start temperature is preferably 825 to 975 ° C.

Finish rolling end temperature: 900-750 ° C
When the finish rolling end temperature (finish rolling output side temperature) exceeds 900 ° C., the processing strain applied during finish rolling becomes insufficient, γ grain miniaturization is not achieved, and the toughness of the square steel pipe decreases. It will be easier. 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 becomes equal to or lower than the Ar3 transformation point, ferrite grains elongated in the rolling direction are formed, and the ferrite grains are mixed. This increases the risk of reduced toughness. Therefore, the finish rolling end temperature is set to 900 to 750 ° C. The finish rolling end temperature is preferably 850 ° C. or lower. It is preferably 770 ° C. or higher.

After the finish rolling is completed, the hot-rolled plate is subjected to a cooling process.

Number of times of cooling of 0.2 s or more and less than 3.0 s in the initial cooling for 10 s from the start of cooling: 1 time or more In the present invention, 10 seconds (10 s) after the start of cooling of the hot-rolled plate obtained in the hot-rolling step. (Between) is the initial cooling step. In the initial cooling step of the cooling step, a cooling step of 0.2 s or more and less than 3.0 s is provided at least once to cool. This is done to suppress the formation of martensite structure on the front and back surfaces of the steel sheet. In the initial cooling step, if the cooling step is not provided or the cooling step is less than 0.2 s, the steel structure on the front and back surfaces of the steel sheet becomes a martensite structure, and the toughness of the square steel pipe is lowered. Further, in the initial cooling step, when the cooling step is 3.0 s or more, the average aspect ratio of the hard phase is less than 0.1, and the toughness is insufficient. Therefore, the time of one cooling step performed during the initial cooling step of the cooling step is set to 0.2 s or more and less than 3.0 s. The time of one cooling step is preferably 0.4 s or more, preferably 2.0 s or less.

In order to obtain the above effect, the number of cooling steps performed during the initial cooling step needs to be one or more. The number of cooling steps may be appropriately set depending on the arrangement of cooling equipment, the cooling stop temperature, and the like. Here, the cooling is naturally cooled. The upper limit of the number of cooling steps is not particularly limited, but is preferably 10 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 by, for example, intermittent injection by stopping the injection of water from the nozzle in a part section of the water cooling nozzle described later.

Plate thickness Average cooling rate at center temperature: 4 to 25 ° C / s, cooling stop temperature: 580 ° C or less In the cooling process, the hot-rolled plate obtained by finish rolling is cooled from the start to the stop (cooling end). Cooling is performed so that the average cooling rate at the center temperature of the plate thickness is 4 to 25 ° C./s and the cooling stop temperature is 580 ° C. or less. The cooling performed in the cooling step is performed by, for example, water cooling (water cooling) such as water column cooling, spray cooling, mist cooling, etc., in which water is injected from a nozzle, gas jet cooling in which cooling gas is injected, or the like. It is preferable to perform a cooling operation on both sides of the hot-rolled plate (steel plate) so that both sides (front and back surfaces) of the hot-rolled plate are cooled under the same conditions.

When the average cooling rate at the center of the thickness of the hot-rolled plate is less than 4 ° C./s, the frequency of forming ferrite grains decreases, the ferrite crystal grains become coarse, and the toughness decreases. On the other hand, when the average cooling rate exceeds 25 ° C./s, the formation of ferrite is suppressed to less than 55%, and the average aspect ratio of the hard phase exceeds 0.8 to obtain a steel structure that is difficult to work harden. I can't. Therefore, the average cooling rate at the center of the thickness of the hot-rolled plate is set to 4 to 25 ° C./s. The average cooling rate at the center of the thickness of the hot-rolled plate is preferably 5 ° C./s or more, and preferably 15 ° C./s or less.

Here, the average cooling rate at the center of the thickness of the hot-rolled plate is
((Temperature at the center of plate thickness at the start of cooling (° C.)-Temperature at the center of plate thickness at the start of cooling (° C.)) / Cooling time (s))
Is required by. The temperature at the center of the thickness of the hot-rolled sheet can be obtained by calculating the temperature distribution in the cross section of the steel sheet by heat transfer analysis.

If the cooling stop temperature exceeds 580 ° C., the average circle equivalent diameter of the hard phase of the steel sheet exceeds 20 μm, and the toughness may decrease. The cooling stop temperature is more preferably 560 ° C. or lower.

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

After the cooling is completed, the hot-rolled plate is subjected to a winding process to obtain a steel plate (hot-rolled steel plate).

Winding temperature: 580 ° C. or less In the winding process, the hot rolled plate is wound at a winding temperature: 580 ° C. or less, and then allowed to cool. When the winding temperature exceeds 580 ° C., ferrite transformation and pearlite transformation proceed after winding, the proportion of pearlite becomes excessive, and the toughness of the square steel pipe decreases. Therefore, the winding temperature is set to 580 ° C. or lower. The take-up temperature is preferably 550 ° C. or lower. Although there is no problem with the material even if the take-up temperature is lowered, when the take-up temperature is less than 400 ° C., the take-up deformation resistance becomes large especially in a thick steel sheet having a plate thickness of more than 25 mm. It may not be able to be wound cleanly. Therefore, the winding temperature is preferably 400 ° C. or higher.

Then, the steel sheet (hot-rolled steel sheet) after the winding process is subjected to the above-mentioned pipe making process 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, in the building structure of the present embodiment, a plurality of square steel pipes 11 of the present invention are erected and used as pillars. A plurality of girders 14 made of steel such as H-shaped steel are erected between adjacent square steel pipes 11. Further, a plurality of small beams 15 made of a steel material such as H-shaped steel are erected between the adjacent large beams 14. By welding the square steel pipe 11 and the diaphragm 16 and welding the H-shaped steel to be the girder 14, the girder 14 made of a steel material such as H-shaped steel is erected between the adjacent square steel pipes 11. In addition, studs 17 are provided as needed for mounting walls and the like.

Since the building structure of the present invention uses the square steel pipe 11 of the present invention in which the strength difference between the square portion and the flat plate portion is small, it becomes easy to select a welding material for welding the square steel pipe 11 and the diaphragm 16, and undermatch or the like. There is little difference in strength from the welding material of. Since undermatch is unlikely to occur, troubles such as breakage at the weld can be suppressed. Further, since the corner R (R of the corner portion) of the square steel pipe 11 is controlled to an appropriate size, it is easy to combine with other structural members having a right-angled cross section. Further, since the angle R of the square steel pipe 11 is controlled to an appropriate size, it can withstand a larger external force and improves earthquake resistance and the like.

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

The molten steel was melted in a converter to obtain a slab (steel material: wall thickness 250 mm) having the composition shown in Table 1 by a continuous casting method. The slabs (steel materials) are heated to the heating temperatures shown in Table 2, then subjected to a hot rolling step, a cooling step, and a winding step under the conditions shown in Table 2, and then allowed to cool. A steel plate of 16 to 28 mm (hot-rolled steel plate) was used. The cooling process was started immediately (within 5 seconds) after the finish rolling was completed. Cooling was done by water cooling. The cooling step during the initial cooling step was performed by providing a cooling section in which water cooling was not performed during the initial cooling step, which was 10 seconds after the start of cooling. Then, using the obtained hot-rolled steel sheet as a material, a round steel pipe was formed by cold roll forming under the conditions shown in Table 2, and then a square steel pipe (400 to 550 mm square) was formed by cold roll forming.

In the examples of the present invention, test pieces were collected from the obtained square steel pipe, and microstructure observation, tensile test, Charpy impact test, and measurement of corner radius were carried out, respectively. The tissue was observed and measured by the above method. The test method of the tensile test, the Charpy impact test, and the method of measuring the radius of the corner are as follows.

(1) Tensile test of square steel pipe From the flat plate portion and the corner portion of the obtained square steel pipe, JIS No. 5 tensile test pieces were collected so that the tensile direction was the longitudinal direction of the pipe. Next, a tensile test was carried out in accordance with the provisions of JIS Z 2241 (2011), and the yield strength YS and the tensile strength TS were measured. Using the obtained measured values, the yield ratio YR (%) defined by (yield strength) / (tensile strength) × 100 (%) was calculated.

(2) Impact Test of Square Steel Pipe A V-notch test piece was taken from the plate thickness 1/4 t position of the flat plate portion of the obtained square steel pipe so that the longitudinal direction of the test piece was the pipe circumferential direction. Then, in accordance with the provisions of JIS Z 2242 (2011), a Charpy impact test was carried out at a test temperature of −40 ° C. to determine the absorbed energy (J). The number of test pieces was 3 each, and the average value of each 3 pieces was taken as the value of the impact test result shown in Table 3-2.

(3) Method for measuring R (angle R) of corners From the obtained square steel pipe, 10 cross sections perpendicular to the pipe axis direction are arbitrarily cut out, and the radii of curvature of the corners at the four corners of the vertical cross section are obtained. The measurement was performed, and the average value was taken as the radius of the corner. Specifically, as shown in FIG. 4, the welded portion (seam portion) of the steel pipe is set to 0 °, and the positions of 45 °, 135 °, 225 °, and 315 ° are set at the center of each corner with reference to this 0 °. If, the radius of curvature of the corner is the curvature at the intersection of the line (L) that starts at the center of the pipe and forms 45 ° with the adjacent sides and the outside of the corner (the outer surface side of the pipe). Refers to the radius. The radius of curvature of the corner is centered on L, and the central angle determined by the line drawn toward the connection point (A, A') between the flat part and the arc part of the square steel pipe is 65 °. Use a fan-shaped radius. In addition, "t" shown in FIG. 4 is a plate thickness, and "H" indicates the length of the side of the outer shape. As a method of calculating the radius of curvature, for example, the radius of curvature is calculated using the law of sines from the measurement results of the distance relationship of three points (the intersection on the outside of the corner and the two points that are the connection points between the flat portion and the arc portion). There are a method of calculating and a method of measuring the radius of curvature from a radial gauge that matches well with the corners in the three points, but the present invention is not limited to this. In this example, a radial gauge was used to measure the radius of curvature of the corner. The angle R is an average value of 10 cross sections perpendicular to the pipe axis direction as described above.

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

In the examples of the invention that were within the scope of the present invention, the characteristics of the present invention (YS of the flat plate portion is 350 MPa or more, TS is 520 MPa 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, angle. The ratio of TS of the flat plate portion to the portion is 0.90 or more and 1.00 or less, the Charpy absorption energy of -40 ° C of the flat plate portion is 100 J or more, and the R of the corner portion is (2.3 × t) or more (2.9 ×). (T or less) (where t is the plate thickness) was obtained. On the other hand, in the comparative example outside the scope of the present invention, the characteristics of the present invention could not be obtained.

1 Steel strip 2 Leveler 3 Cage roll group 4 Finpass roll group 5 Squeeze roll 6 Welder 7 Electric resistance steel pipe 8 Sizing roll group 9 Square forming roll group 10 Square steel pipe 11 Square steel pipe 14 Large beam 15 Small beam 16 Diaphragm 17 Stud

Claims (7)

A square steel pipe having a flat plate and corners,
Ingredient composition is 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: Contains 0.006% or less, 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 has an area ratio of 55% or more and 80% or less, and the average aspect ratio of the hard phase is 0.1 to 0.8. ,
The flat plate portion has a YS of 350 MPa or more and a TS of 520 MPa 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, and 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 ° C. of the flat plate portion is 100 J or more.
A square steel pipe having an R of the corner portion of (2.3 × t) or more and (2.9 × t) or less. The square steel pipe according to claim 1, further containing one group or two or more groups selected from the following groups A to C in mass% in addition to the component composition.
Group A: Nb: 0.05% or less, Ti: 0.05% or less, V: 0.10% or less, one or more selected from Group B: B: 0.008% or less C Group: 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. One or more selected from 001 to 0.010% The square steel pipe according to claim 1 or 2, wherein the steel structure further has an average circle-equivalent diameter of the hard phase of 20 μm or less. The method for manufacturing a square steel pipe according to any one of claims 1 to 3.
A pipe making process in which a steel plate is coldly roll-formed to form a cylindrical end face, welded, formed into a cylindrical shape with a vertical / horizontal diameter ratio of 0.99 or more and 1.01 or less, and then formed into a square shape. A method of manufacturing a square steel pipe. The method for manufacturing a square steel pipe according to any one of claims 1 to 3.
When a square steel pipe is manufactured by subjecting a steel material to a hot rolling process, a cooling process, a winding process, and a pipe making process in this order.
After heating the steel material to a heating temperature: 1100 to 1300 ° C.
Rough rolling time when the center temperature of the plate thickness is 1000 ° C or higher: 200 seconds or more and 400 seconds or less, rough rolling end temperature: 1000 to 800 ° C,
A hot rolling process of finishing rolling start temperature: 1000 to 800 ° C. and finish rolling end temperature: 900 to 750 ° C. is performed to obtain a hot-rolled plate.
Next, the hot-rolled plate has one or more cooling releases of 0.2 s or more and less than 3.0 s in the initial cooling step from the start of cooling to 10 s, and the average cooling rate at the plate thickness center temperature: 4 to 25 ° C. A cooling process of / s is performed, and
Next, the hot-rolled plate is subjected to a winding step of winding at a winding temperature of 580 ° C. or lower to obtain a steel sheet.
Next, the steel plate is cold-rolled to form a cylindrical end face, and the end face is welded to form a cylindrical shape having a vertical diameter / horizontal diameter ratio of 0.99 or more and 1.01 or less, and then formed into a square shape. A method for manufacturing a square steel pipe to be subjected to a pipe making process. The method for manufacturing a square steel pipe according to claim 5, wherein the cooling stop temperature in the cooling step is 580 ° C. or lower. A building structure using the square steel pipe according to any one of claims 1 to 3.

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