JP6874913B2 - Square steel pipe and its manufacturing method and building structure - Google Patents

Description

The present invention particularly relates to a square steel pipe used for a building member of a large building such as a factory, a warehouse, a commercial facility, a method for manufacturing the same, and a building structure.

In recent years, building structural members used in large buildings such as factories, warehouses, and commercial facilities have been increasing in strength in order to reduce construction costs by reducing the weight. In particular, square steel pipes used as column materials for buildings are required to have a yield strength of 385 MPa or more and a tensile strength of 520 MPa or more, and from the viewpoint of earthquake resistance, high plastic deformability and excellent toughness are also required at the same time. It becomes.

A square steel pipe having such high deformation performance and excellent toughness specifically means that the flat plate portion has a yield ratio (= yield strength / tensile strength) in the pipe axial direction of 0.90 or less and Charpy absorption at 0 ° C. The energy needs to be 70J or more.

Square steel pipes are generally manufactured by using hot-rolled steel sheets (steel strips) or thick steel sheets as materials and cold-press bending or roll-forming them.

A square steel pipe manufactured by cold press bending is manufactured by forming a thick steel plate into a square shape or a U shape by press bending and joining them by submerged arc welding. In addition, the square steel pipe manufactured by roll forming is made of hot-rolled steel sheet into a cylindrical open tube shape by roll forming, and after the butt portion is welded by electric stitching, it remains cylindrical by rolls arranged vertically and horizontally. It is manufactured by adding a few percent of rolling in the direction of the pipe axis and then forming it into a square shape.

On the other hand, in the case of a square steel pipe manufactured by cold press bending, work hardening due to bending deformation of the corners is remarkable, and the toughness and plastic deformability of the corners are impaired, so that the seismic strength deteriorates and the corners It becomes easy to destroy from the starting point. In particular, work hardening becomes remarkable in high-strength materials for building members including the hard second phase such as bainite.

Therefore, when manufacturing high-strength square steel pipes, select a material that reduces the effect of deterioration of toughness due to work hardening of the corners during molding, and a manufacturing method that suppresses work hardening of the corners. There is a need to.

Patent Document 1 proposes a square steel pipe characterized in that the surface integral of the bainite structure is 40% or more in the microstructure of the flat plate portion.

Patent Document 2 proposes a square steel pipe having a low yield ratio and high toughness, which is characterized in that all pipes are strain-removed and annealed after being formed by cold forming.

Japanese Patent No. 5385760 Japanese Patent No. 4957671

The square steel pipe described in Patent Document 1 has a Vickers hardness of 350 HV or less at the surface layer of the corner. However, the hardness of the surface layer portion of the corner portion is still large, and further reduction of the hardness is required to suppress fracture and surface cracking starting from the corner portion.

Further, since the square steel pipe described in Patent Document 2 requires heat treatment after the pipe is formed, the cost is very high as compared with the square steel pipe as it is cold-worked.

The present invention has been made in view of the above circumstances, and is a square steel pipe having a small effect on work hardening of corners and suppressing surface cracking, a method for manufacturing the same, and a building structure using the square steel pipe of the present invention. The purpose is to provide.

The present inventors have conducted diligent studies to solve the above problems and obtained the following findings.

In the square forming stand for manufacturing square steel pipes by roll forming, the curvature of the part corresponding to the flat plate part of the square steel pipe that is the final product is reduced by using the forming roll, and the forming is performed so that the cross section changes from a cylindrical shape to a rectangular cross section. Is going. This is done so that the forming roll pushes in the part corresponding to the central part of the flat plate part of the square steel pipe of the final product, and the corner part has an L-shaped corner part so as to follow the deformation of the flat plate part. Form.

Therefore, it is possible to form a curved corner without directly contacting the entire corner with the roll. On the other hand, when the steel pipe material comes into contact with the roll, the dimensional accuracy such as the flatness of the flat plate and the curvature of the corners is improved, but since it receives the shearing force from the roll, it is centered on the contact part with the roll. It is clear that work hardening occurs. Therefore, in order to suppress excessive work hardening of the corners, it is necessary to control the contact portion between the roll and the corners so as to be compatible with dimensional accuracy.

The present inventors have found that in order to obtain a square steel pipe with less work hardening due to bending, a square steel pipe having a larger Vickers hardness on the inner surface side of the corner than a Vickers hardness on the outer surface side of the corner is better. It was. When a square steel pipe having a Vickers hardness on the inner surface side of the corner is larger than the Vickers hardness on the outer surface side of the corner is formed from a cylindrical steel pipe to a square steel pipe, the roll does not come into contact with the vicinity of the corner during square forming. As described above, it is obtained by setting the roll gap during square forming and the caliber curvature of the roll and forming the square steel pipe from the cylindrical steel pipe. It is considered that this is because the change in curvature of the outer surface of the steel pipe is smaller than the change in curvature of the inner surface of the steel pipe, so that work hardening due to bending is small and it is not easily affected by the shearing force of the roll.

In addition, when the roll gap of various square moldings and the caliber curvature of the rolls were changed and the hardness of the corners was investigated, the rolls were rolled to the immediate vicinity of the corners even when the rolls were not in direct contact with the corners. It was found that the hardness of the corner surface increases under the condition that they are in contact with each other. This means that shear stress also acts in the circumferential direction due to contact with the roll, so work hardening occurs in the vicinity of the contact portion with the roll. It was found that the region where this shear stress acts changes depending on the rigidity of the material to be formed, that is, the wall thickness (pipe thickness) t of the steel pipe material and the side length H (H ₁ , H _{2) of the final product.}

The present invention is based on the above findings, and its features are as follows.
[1] In a square steel pipe having a flat plate portion and a square portion, the yield strength of the flat plate portion is 385 MPa or more, the tensile strength is 520 MPa or more, and the yield ratio is 0.90 or less.
The Vickers hardness of the corner is larger than the Vickers hardness of the outer surface of the corner, the Vickers hardness of the inner surface of the corner is 280 HV or less, and the Vickers hardness of the outer surface of the corner is 280 HV or less. The difference between the Vickers hardness on the outer surface side of the corner and the Vickers hardness on the inner surface side of the corner is 80 HV or less.
_{A square steel pipe having a Charpy absorption energy vE 0} of 0 ° C. on the outer surface side of the corner portion of 70 J or more.
[2] In terms of mass%, C: 0.04 to 0.50%, Si: 2.0% or less, Mn: 0.5 to 3.0%, P: 0.10% or less, S: 0.050 % Or less, Al: 0.005 to 0.10%, N: 0.010% or less, and the balance has a component composition consisting of Fe and unavoidable impurities.
Moreover, the steel structure at t / 4 (t is the pipe thickness) from the pipe surface contains ferrite having a volume ratio of more than 30% and bainite having a volume ratio of 10% or more, and the total volume ratio of ferrite and bainite is 70%. The square steel pipe according to [1], which is 95% or more and the balance is one or more selected from pearlite, martensite, and austenite.
[3] Further, in mass%, one selected from Nb: 0.005 to 0.150%, Ti: 0.005 to 0.150%, V: 0.005 to 0.150% or The square steel pipe according to [2], which contains two or more types.
[4] Further, in mass%, Cr: 0.01 to 1.0%, Mo: 0.01% to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0. [2] or [3], which contains one or more selected from .30%, Ca: 0.0005 to 0.010%, and B: 0.0003 to 0.010%. Square steel pipe.
[5] In the method for manufacturing a square steel pipe according to [1], in which the square steel pipe is formed into a cylindrical shape and then squarely formed into a square shape, the square forming step for performing the square forming is In the cross section perpendicular to the pipe axis direction, the adjacent side lengths are H ₁ (mm) and H ₂ (mm) (H ₁ ≤ H ₂ ), respectively, _{and from the center position of H 1} and H ₂ to the inside of the steel pipe. when the intersection headed straight lines drawn intersects the square tube central portion, on a straight line drawn toward the steel tube inside from the center of the H _1, 1/2 from the square tube center portion in the long side direction (H _{The point offset by 2-} H ₁ ) is defined as the offset point, and the straight line drawn from the offset point to the center of the corner of the square steel pipe and the straight line drawn from the offset point toward the arc of the corner or the flat plate of the square steel pipe. A method for manufacturing a square steel pipe in which the central angle θ formed by the above satisfies the following equation (1).

ただし、
Ｈ_１：辺長（短辺）（ｍｍ）
Ｈ_２：辺長（長辺）（ｍｍ）
ｔ：管厚（ｍｍ）
である。
［６］［２］〜［４］のいずれかに記載の成分組成を有する鋼素材を加熱温度：１１００〜１３００℃に加熱した後、粗圧延終了温度：８５０〜１１５０℃とする粗圧延を施し、仕上圧延終了温度：７５０〜８５０℃とする仕上圧延を施し、かつ粗圧延と仕上圧延の両方における９３０℃以下での合計圧下率が６５％以上とし、次いで、板厚中心温度で冷却開始から冷却停止までの平均冷却速度が１０〜３０℃／ｓとなる冷却速度で冷却停止温度：４５０〜６５０℃まで冷却し巻取り、その後放冷し、続いてロール成形により、円筒形状へと成形した後、ロール成形した鋼板を電縫溶接して電縫鋼管とした後、前記電縫鋼管を角形鋼管へと角成形を行う角成形工程において、管軸方向に対して垂直な断面において、隣り合う辺長をそれぞれＨ_１（ｍｍ）およびＨ_２（ｍｍ）（Ｈ_１≦Ｈ_２）とし、Ｈ_１およびＨ_２の中心位置から鋼管内部に向かって引いた直線同士が交わる交点を角形鋼管中央部としたとき、Ｈ_１の中心位置から鋼管内部に向かって引いた直線上において、前記角形鋼管中央部から長辺方向に１／２（Ｈ_２−Ｈ_１）だけオフセットさせた点をオフセット点とし、オフセット点から角形鋼管の角部中央へ引いた直線と、オフセット点から角部の円弧部あるいは角形鋼管の平板部へ向かって引かれる直線とが成す中心角θが下記式（１）を満たす角形鋼管の製造方法。

However,
H ₁ : Side length (short side) (mm)
H ₂ : Side length (long side) (mm)
t: Tube thickness (mm)
Is.
[6] A steel material having the component composition according to any one of [2] to [4] is heated to a heating temperature of 1100 to 1300 ° C., and then rough-rolled to a rough rolling end temperature of 850 to 1150 ° C. Finish rolling is performed so that the finish rolling end temperature is 750 to 850 ° C, and the total reduction rate at 930 ° C or lower in both rough rolling and finish rolling is 65% or more, and then cooling starts at the plate thickness center temperature. Cooling stop temperature: 450 to 650 ° C at a cooling rate at which the average cooling rate until cooling stop is 10 to 30 ° C / s, winding, allowing to cool, and then rolling to form a cylindrical shape. After that, the roll-formed steel plate is electro-stitched and welded to form an electro-sewn steel pipe, and then the electro-sewn steel pipe is square-formed into a square steel pipe. The side lengths are H ₁ (mm) and H ₂ (mm) (H ₁ ≤ H ₂ ), respectively, and the _{intersection of straight lines drawn from the center positions of H 1} and H ₂ toward the inside of the steel pipe is the center of the square steel pipe. Then, on a _{straight line drawn from the center position of H 1} toward the inside of the steel pipe, a point offset by _{1/2 (H 2-} H ₁ ) in the long side direction from the center of the square steel pipe is defined as an offset point. , The central angle θ formed by the straight line drawn from the offset point to the center of the corner of the square steel pipe and the straight line drawn from the offset point toward the arc portion of the corner or the flat plate portion of the square steel pipe satisfies the following equation (1). Manufacturing method of square steel pipe.

ただし、
Ｈ_１：辺長（短辺）（ｍｍ）
Ｈ_２：辺長（長辺）（ｍｍ）
ｔ：管厚（ｍｍ）
である。
［７］［１］〜［４］のいずれかに記載の角形鋼管を用いた建築構造物。

However,
H ₁ : Side length (short side) (mm)
H ₂ : Side length (long side) (mm)
t: Tube thickness (mm)
Is.
[7] A building structure using the square steel pipe according to any one of [1] to [4].

According to the square steel pipe of the present invention, a square steel pipe having a small influence on work hardening of the corner portion and suppressing surface cracking can be obtained. This can greatly contribute to the reduction of construction costs for large buildings such as factories, warehouses, and commercial facilities. Further, according to the method for manufacturing a square steel pipe of the present invention, it is possible to manufacture a high-strength square steel pipe in a short period of time with high productivity as compared with cold press bending.

FIG. 1 is a schematic view showing an example of an electric resistance welded steel pipe manufacturing facility. FIG. 2 is a schematic view showing a molding process of a square steel pipe. FIG. 3 is a schematic view showing a cross section of a square steel pipe perpendicular to the pipe axis direction. FIG. 4 is a perspective view schematically showing an example of a building structure using the square steel pipe of the present invention.

The square steel pipe of the present invention has a flat plate portion and a square portion, and the flat plate portion has a yield strength (YS) of 385 MPa or more, a tensile strength (TS) of 520 MPa or more, and a yield ratio of 0.90 or less. The Vickers hardness of the corner is larger than the Vickers hardness of the outer surface of the corner, the Vickers hardness of the outer surface of the corner is 280 HV or less, and the outer surface of the corner is 280 HV or less. The difference between the Vickers hardness on the side and the Vickers hardness on the inner surface side of the corner is 80 HV or less, and the Charpy absorption energy vE ₀ at 0 ° C. on the outer surface side of the corner is 70 J or more.

Square steel pipes are work-hardened larger at the corners than at the flat plates. In particular, since the outer surface of the corner is a tensile stress field in the circumferential direction, it is necessary to ensure the toughness of the outer surface of the corner in order to suppress brittle fracture of the corner in the final product. _{That is, the Charpy absorption energy vE 0 at} 0 ° C. on the outer surface of the corner is required to be 70 J or more, the yield strength (YS) of the flat plate portion is 385 MPa or more, the tensile strength (TS) is 520 MPa or more, and the yield ratio is 0.90. It is required to be as follows.

Further, in the present invention, the Vickers hardness on the inner surface side of the corner is larger than the Vickers hardness on the outer surface side of the corner, the Vickers hardness on the outer surface side of the corner is 280 HV or less, and the outer surface side of the corner. The difference between the Vickers hardness and the Vickers hardness on the inner surface side of the corner is 80 HV or less. In the present invention, the Vickers hardness on the inner surface side of the corner is larger than the Vickers hardness on the outer surface side of the corner, and the Vickers hardness on the outer surface side of the corner is 280 HV or less, so that work hardening by bending is performed. A small square steel pipe can be obtained. When the Vickers hardness on the outer surface side of the corner exceeds 280 HV, work hardening on the outer surface side is progressing, so that the ductility of the corner is significantly deteriorated. Further, in order to secure the toughness of the corner portion where work hardening of the surface is large due to bending deformation, the difference between the Vickers hardness on the outer surface side of the corner portion and the Vickers hardness on the inner surface side of the corner portion is set to 80 HV or less. When the difference between the Vickers hardness on the outer surface side of the corner and the Vickers hardness on the inner surface side of the corner exceeds 80 HV, work hardening on the inner surface side of the corner progresses and the residual stress on the inner surface of the corner becomes remarkable. It has an adverse effect on cracks in the plating applied in the post-treatment.

The Vickers hardness on the outer surface side of the corner portion in the present invention is the Vickers hardness inside 1 ± 0.2 mm from the outer surface of the corner portion, and the Vickers hardness on the inner surface side of the corner portion is 1 ± ± from the inner surface of the corner portion. Vickers hardness inside 0.2 mm.

The square steel pipe of the present invention has C: 0.04 to 0.50%, Si: 2.0% or less, Mn: 0.5 to 3.0%, P: 0.10% or less, S in mass%. : 0.050% or less, Al: 0.005 to 0.10%, N: 0.010% or less, the balance has a component composition consisting of Fe and unavoidable impurities, and t / 4 (t / 4) from the tube surface. The steel structure at the position (t is the pipe thickness) contains ferrite with a volume ratio of more than 30% and bainite with a volume ratio of 10% or more, and the total volume ratio of ferrite and bainite is 70% or more and 95% or less, and the balance. Is preferably composed of one or more selected from pearlite, martensite and austenite.

The reason for limiting the preferable composition of the steel material in the present invention will be described below. In the present specification, unless otherwise specified, "%" indicating the steel composition is "mass%".

C: 0.04 to 0.50%
C is an element that increases the strength of steel by solid solution strengthening. Further, C is an element that promotes the formation of pearlite, enhances hardenability, contributes to the formation of martensite, and contributes to the stabilization of austenite, and thus contributes to the formation of a hard phase. It is preferable to contain 0.04% or more of C in order to secure the desired strength. However, when the C content exceeds 0.50%, the proportion of the hard phase increases, the toughness decreases, and the weldability also deteriorates. Therefore, the C content is preferably 0.04% or more and 0.50% or less. More preferably, the C content is C: more than 0.12% and 0.25% or less.

Si: 2.0% or less Si is an element that increases the strength of steel by solid solution strengthening, and can be contained as needed. In order to obtain such an effect, the content of Si is preferably 0.01% or more. On the other hand, if the Si content exceeds 2.0%, the weldability deteriorates. Therefore, the Si content is preferably 2.0% or less. More preferably, the Si content is 0.01% or more and 0.5% or less.

Mn: 0.5 to 3.0%
Mn is an element that increases the strength of steel by solid solution strengthening and also contributes to the miniaturization of the structure by lowering the ferrite transformation start temperature. It is preferable to contain Mn of 0.5% or more in order to secure the desired strength and structure. However, if the Mn content exceeds 3.0%, the weldability deteriorates. Therefore, the Mn content is preferably 0.5% or more and 3.0% or less. More preferably, the Mn content is 0.5% or more and 2.0% or less.

P: 0.10% or less P is segregated at the grain boundaries and causes inhomogeneity of the material. Therefore, it is preferable to reduce P as an unavoidable impurity as much as possible. When it is contained, a content of 0.10% or less is acceptable. Therefore, the P content is preferably in the range of 0.10% or less. More preferably, the P content is 0.03% or less.

S: 0.050% or less S usually exists as MnS in steel, but MnS is thinly stretched in the hot rolling process and adversely affects ductility. Therefore, in the present invention, it is preferable to reduce as much as possible. When it is contained, a content of 0.050% or less is acceptable. Therefore, the S content is preferably 0.050% or less. More preferably, the S content is 0.015% or less.

Al: 0.005-0.10%
Al is an element that acts as a potent antacid. In order to obtain such an effect, it is preferable to contain 0.005% or more of Al. However, if the Al content exceeds 0.10%, the weldability deteriorates, and the amount of alumina-based inclusions increases, resulting in deterioration of the surface texture. Therefore, the Al content is preferably 0.005% or more and 0.10% or less. More preferably, the Al content is 0.010% or more and 0.07% or less.

N: 0.010% or less N is an unavoidable impurity and is an element having an action of lowering toughness by firmly fixing the motion of dislocations. In the present invention, it is desirable to reduce N as an impurity as much as possible. When it is contained, a content of 0.010% or less is acceptable. Therefore, the N content is preferably 0.010% or less. More preferably, the N content is 0.0080% or less.

The above components are the basic composition of the steel material of the electric resistance pipe in the present invention, but in addition to these, Nb: 0.005 to 0.150%, Ti: 0.005 to 0.150%, V: One or more selected from 0.005 to 0.150% may be contained.

One or more selected from Nb: 0.005 to 0.150%, Ti: 0.005 to 0.150%, V: 0.005 to 0.150% Nb, Ti, V , Both are elements that form fine carbides and nitrides in steel and contribute to the improvement of steel strength through precipitation strengthening, and can be contained as needed. In order to obtain such an effect, it is preferable to contain Nb: 0.005% or more, Ti: 0.005% or more, and V: 0.005% or more. On the other hand, excessive content leads to an increase in yield ratio and a decrease in toughness. Therefore, when Nb, Ti, and V are contained, Nb: 0.005 to 0.150%, Ti: 0.005 to 0.150%, and V: 0.005 to 0.150%. Preferably, Nb: 0.008 to 0.10%, Ti: 0.008 to 0.10%, V: 0.008 to 0.10%.

In addition to the above, Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.30%, One or more selected from Ca: 0.0005% to 0.010% and B: 0.0003 to 0.010% may be contained.

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: 0.0005 to One or more selected from 0.010%, B: 0.0003 to 0.010% Cr, Mo, Cu, Ni are elements that increase the strength of steel by solid solution strengthening. Further, since all of them are elements that enhance the hardenability of steel and contribute to the stabilization of austenite, they are elements that contribute to the formation of hard martensite and austenite, and can be contained as needed. In order to obtain such an effect, it is preferable to contain Cr: 0.01% or more, Mo: 0.01% or more, Cu: 0.01% or more, and Ni: 0.01% or more. On the other hand, excessive content causes a decrease in toughness and a deterioration in weldability. Therefore, when Cr, Mo, Cu, and Ni are contained, Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: It is set to 0.01 to 0.30%. Preferably, Cr: 0.1 to 0.5%, Mo: 0.1 to 0.5%, Cu: 0.1 to 0.40%, Ni: 0.1 to 0.20%.

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. In order to obtain such an effect, it is preferable to contain 0.0005% 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 set to 0.0005 to 0.010%. Preferably, the Ca content is 0.0010 to 0.0050%.

B is an element that contributes to the miniaturization of the structure by lowering the ferrite transformation start temperature. In order to obtain such an effect, it is preferable to contain 0.0003% or more of B. However, when the B content exceeds 0.010%, the yield ratio increases. Therefore, when B is contained, the B content is set to 0.0003 to 0.010%. Preferably, the B content is 0.0005 to 0.0050%.

The rest other than the above components are Fe and unavoidable impurities.

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

In the square steel pipe of the present invention, the steel structure at a position t / 4 (t is the pipe thickness) from the pipe surface contains ferrite having a volume ratio of more than 30% and bainite having a volume ratio of 10% or more, and the volume ratio of ferrite and bainite. The total is 70% or more and 95% or less, and the balance is preferably one or more selected from pearlite, martensite, and austenite.

Ferrite has a soft structure, and when mixed with other hard structures, the yield ratio of the steel pipe material is lowered. In order to obtain such an effect, the volume ratio is preferably more than 30%.

Bainite is a structure with intermediate hardness and increases the strength of steel. Since the desired yield strength and tensile strength cannot be obtained with ferrite alone, the volume fraction is preferably 10% or more.

Further, if the total volume fraction of ferrite and bainite is less than 70%, the desired yield strength or yield ratio cannot be obtained, and if it exceeds 95%, the desired yield strength or yield ratio cannot be obtained.

The balance preferably comprises one or more selected from pearlite, martensite and austenite. Pearlite, martensite, and austenite are hard structures, and in particular, they increase the tensile strength of steel and when mixed with soft ferrite, the yield ratio of the steel pipe material is lowered. In order to obtain such an effect, the total volume fraction is preferably 5% or more and 30% or less.

Since the steel structure of the square steel pipe is uniform in the width direction of the steel pipe, both the structure of the square portion and the flat plate portion satisfy the scope of the present invention. Further, the t / 4 position from the pipe surface can be within a range of ± 0.2 mm from the t / 4 position. Further, the pipe surface may be either the outer surface or the inner surface of the pipe.

Next, the method for manufacturing the square steel pipe of the present invention will be described.

In the present invention, for example, a steel material such as a slab having the above-mentioned chemical components is heated to a temperature of 1100 to 1300 ° C., and then rough-rolled to a rough rolling end temperature of 850 to 1150 ° C. Finish rolling end temperature: 750 to 850 ° C. After a hot rolling process in which the total rolling reduction at 930 ° C or lower in both rough rolling and finish rolling is 65% or more, the plate thickness center temperature. The average cooling rate from the start of cooling to the stop of cooling is 10 to 30 ° C./s. The cooling stop temperature is cooled to 450 to 650 ° C., rolled up, and then allowed to cool. In the following description of the manufacturing method, the temperature is the surface temperature of a steel material, a steel plate, or the like unless otherwise specified. This surface temperature can be measured with a radiation thermometer or the like. The average cooling rate shall be ((temperature before cooling-temperature after cooling) / cooling time) unless otherwise specified. Further, the cooling method is water cooling such as injection of water from a nozzle, cooling by injection of cooling gas, or the like. It is preferable to perform a cooling operation on both sides of the hot-rolled steel sheet so that both sides of the hot-rolled steel sheet are cooled under the same conditions. The core temperature of the steel sheet in these hot rolling is not specified, but this time, it is calculated by the unsteady heat transfer calculation by the difference calculation. Specifically, the calculation is performed using the physical property values of the material such as the thermal conductivity, specific heat, and density of the steel plate, and the heat transfer coefficient obtained from the water amount density of the cooling water and the outer surface temperature of the steel plate as the boundary condition.

The method for producing the steel material having the above-mentioned composition is not particularly limited, and the steel material is melted by a commonly known melting method such as a converter, an electric furnace, or a vacuum melting furnace, and is melted by a commonly known casting method such as a continuous casting method. , Manufactured to the desired dimensions. The molten steel may be further subjected to secondary refining such as ladle refining. Further, there is no problem even if the ingot-lump rolling method is applied instead of the continuous casting method.

Heating temperature: 1100 to 1300 ° C
When the heating temperature is less than 1100 ° C., the deformation resistance of the material to be rolled becomes large and rolling becomes difficult. On the other hand, when the heating temperature exceeds 1300 ° C., the austenite grains become coarse, fine austenite grains cannot be obtained in the subsequent rolling, it becomes difficult to secure the toughness of the desired hot-rolled steel sheet, and the coarse bainite It becomes difficult to suppress the production. Therefore, the heating temperature in the hot rolling step is preferably 1100 to 1300 ° C.

Rough rolling end temperature: 850 to 1150 ° C
If the rough rolling end temperature is less than 850 ° C., the steel sheet temperature becomes lower than the ferrite transformation start temperature during the subsequent finish rolling, and the risk of ferrite formation increases. The produced ferrite becomes processed ferrite grains elongated in the rolling direction by the subsequent rolling, which causes an increase in the yield ratio. On the other hand, when the rough rolling end temperature exceeds 1150 ° C., the amount of reduction in the austenite unrecrystallized temperature range is insufficient, fine austenite grains cannot be obtained, and it becomes difficult to secure the desired toughness of the hot-rolled steel sheet. In addition, it becomes difficult to suppress the formation of coarse bainite. Therefore, the rough rolling end temperature is preferably 850 to 1150 ° C.

Finish rolling end temperature: 750 to 850 ° C
If the finish rolling end temperature is less than 750 ° C., the steel sheet temperature becomes lower than the ferrite transformation start temperature during rolling, and the risk of ferrite formation increases. The ferrite produced in the above becomes processed ferrite grains elongated in the rolling direction by the subsequent rolling, which causes an increase in the yield ratio. On the other hand, when the finish rolling end temperature exceeds 850 ° C., the amount of reduction in the austenite unrecrystallized temperature range is insufficient, fine austenite grains cannot be obtained, and it becomes difficult to secure the desired toughness of the hot-rolled steel sheet. In addition, it becomes difficult to suppress the formation of coarse bainite. Therefore, the finish rolling end temperature is preferably 750 to 850 ° C.

Total rolling reduction at 930 ° C or lower in both rough rolling and finish rolling: 65% or more In the present invention, ferrite produced in the subsequent cooling and winding steps by refining the subgrain in austenite in hot rolling. , Bainite and the residual structure are refined to obtain a hot-rolled steel sheet with the desired strength and toughness. In order to miniaturize the subgrains in austenite in hot rolling, it is necessary to increase the rolling reduction in the austenite unrecrystallized temperature range and introduce sufficient machining strain. In order to achieve the above, the total rolling reduction at 930 ° C. or lower in both rough rolling and finish rolling was set to 65% or more. When the total rolling reduction at 930 ° C. or lower in both rough rolling and finish rolling is less than 65%, sufficient machining strain cannot be introduced in hot rolling, so that a structure having desired toughness cannot be obtained. ..

Average cooling rate from cooling start to cooling stop: 10 to 30 ° C / s
If the cooling rate is less than 10 ° C./s, the nucleation frequency of ferrite decreases and the ferrite grains become coarse, so that a structure having the desired strength and toughness cannot be obtained. On the other hand, when the cooling rate exceeds 30 ° C./s, a large amount of martensite is generated at the t / 4 position of the steel sheet, and the total volume fraction of ferrite and bainite becomes less than 70%.

Cooling shutdown temperature: 450-650 ° C
When the cooling shutdown temperature is less than 450 ° C., a large amount of martensite is generated at the t / 4 position of the steel sheet, and the total volume fraction of ferrite and bainite is less than 70%. On the other hand, when the cooling stop temperature exceeds 650 ° C., the nucleation frequency of ferrite decreases, the ferrite grains become coarse, and the volume fraction of bainite cannot be 10% or more because it exceeds the bainite transformation start temperature. ..

Next, the pipe making process after hot rolling will be described.

The hot-rolled steel plate (steel strip) 1 which is the material of the electric-sewn steel pipe is manufactured into the electric-sewn steel pipe by using the manufacturing equipment as shown in FIG. For example, after the hot-rolled steel sheet 1 is subjected to side-entry straightening by a leveler 2, it is intermediately formed by a cage roll group 3 composed of a plurality of rolls to form a cylindrical open pipe, and then a finpass roll composed of a plurality of rolls. Finish molding (roll molding) is performed in group 4. After the finish molding, the width end portion of the steel strip 1 is electrically resistance welded by the welding machine 6 while being pressure-welded with the squeeze roll 5 to obtain the electrosewn steel pipe 7. In the present invention, the manufacturing equipment for the electrosewn steel pipe 7 is not limited to the pipe making process as shown in FIG.

Then, the obtained electric resistance sewn steel pipe 7 is drawn in a cylindrical shape by several% in the pipe axis direction by rolls arranged vertically and horizontally, and subsequently formed into a square shape to obtain a square steel pipe. FIG. 2 is a schematic view showing a molding process of a square steel pipe according to an embodiment of the present invention. As shown in FIG. 2, the electrosewn steel pipe 7 is reduced in diameter by a sizing roll group (sizing stand) 8 composed of a plurality of rolls while maintaining a cylindrical shape, and then a square forming roll group (square forming stand) composed of a plurality of rolls. 9 is sequentially formed into shapes such as R1, R2, and R3 to form a square steel pipe 10. The roll of the square forming stand is a hole-shaped roll having a caliber curvature, and the radius of curvature of the caliber increases as the stand becomes a latter stage, forming a flat plate portion and a square portion of a square steel pipe. The number of stands of the sizing roll group 8 and the square forming roll group 9 is not particularly limited.

Next, the manufacturing conditions for square molding of the present invention will be described.

A square steel pipe formed by a method of roll forming, welding, and square forming to obtain a square steel pipe is once formed into a cylindrical shape from a steel plate and then formed into a square shape. In such a manufacturing method, not only bending deformation in the circumferential direction but also distortion in the longitudinal direction due to drawing deformation occurs, and as a result, the neutral axis of bending in the circumferential direction moves to the outer surface side, and the inner surface side The hardness increases.

As described above, when the steel pipe material comes into contact with the roll, the dimensional accuracy such as the flatness of the flat plate and the curvature of the corners is improved, but since it receives the shearing force from the roll, it is centered on the contact part with the roll. It is clear that work hardening occurs. Therefore, in order to suppress excessive work hardening of the corners, it is necessary to control the contact portion between the roll and the corners so as to be compatible with dimensional accuracy.

Therefore, the present inventors set the roll gap during square forming and the caliber curvature of the roll so that the roll does not come into contact with the vicinity of the corner during square forming, and formed the square steel pipe from the cylindrical steel pipe. As a result, as shown in FIG. 3, in the square forming step, in the cross section perpendicular to the pipe axis direction, the adjacent side lengths are H ₁ (mm) and H ₂ (mm), respectively (however, H ₁ ≤ H). ₂ and H ₁ and H ₂ are the side lengths of the final product, respectively.) The _{intersection of the straight lines drawn from the center positions of H 1} and H ₂ toward the inside of the steel pipe is the central part of the square steel pipe. Then, _{on a straight line drawn from the center position of H 1} toward the inside of the steel pipe, a point offset by _{1/2 (H 2-} H ₁ ) in the long side direction from the center of the square steel pipe is defined as an offset point. The central angle θ formed by the straight line drawn from the point to the center of the corner of the square steel pipe and the straight line drawn from the offset point toward the arc portion of the corner or the flat plate portion of the square steel pipe satisfies the following equation (1). , The processing hardening due to bending is small, and surface cracking can be suppressed.

ただし、
Ｈ_１：辺長（短辺）（ｍｍ）
Ｈ_２：辺長（長辺）（ｍｍ）
ｔ：管厚（ｍｍ）
である。

However,
H ₁ : Side length (short side) (mm)
H ₂ : Side length (long side) (mm)
t: Tube thickness (mm)
Is.

In the square forming process, the radius of curvature of the caliber of the roll and the radius of curvature of the flat plate portion of the square steel pipe are almost the same value on the rear stand side, so that the contact width between the hole roll and the square steel pipe in the circumferential direction is large on the rear stand. The desired corner size is obtained while increasing and expanding from the center side of the flat plate portion to the corner portion side. In particular, when the ratio t / H of the pipe thickness t and the side length H, which is the wall thickness of the final product of the square steel pipe, increases, the rigidity of deformation increases, so that the contact width between the hole roll and the square steel pipe in the circumferential direction increases. It is necessary to secure up to the vicinity of the corner. On the other hand, in the case of square forming, when the hole-shaped roll is brought into direct contact with the corner portion of the square steel pipe to perform square forming, the work hardening of the corner portion due to the shearing force of the hole-shaped roll becomes remarkable.
In order to obtain the desired radius of curvature of the corner without causing such excessive work hardening, the region where the hole roll and the square steel pipe do not contact is formed from the apex of the corner through all the square forming stands. It is necessary to control the distance in the circumferential direction corresponding to the wall thickness. The region is a region on the outer surface side of the steel pipe that satisfies the above (1) with reference to the center of the corner of the final shape of the square steel pipe. As a method of controlling the contact position between the hole-shaped roll and the square steel pipe, for example, there is a method of adjusting the radius of curvature of the caliber of the hole-shaped roll and the gap between the rolls, but the method is not limited to this.

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

FIG. 4 is a perspective view schematically showing a building structure according to an embodiment of the present invention. As shown in FIG. 4, in the building structure of the present embodiment, a plurality of square steel pipes 10 of the present invention are erected and used as a pillar material. A plurality of girders 11 made of a steel material such as H-shaped steel are erected between adjacent square steel pipes 10. Further, a plurality of small beams 12 made of a steel material such as H-shaped steel are erected between the adjacent large beams 11. By welding the square steel pipe 10 and the diaphragm 13 and welding the H-shaped steel to be the girder 11, the girder 11 made of a steel material such as H-shaped steel is erected between the adjacent square steel pipes 10. In addition, studs 14 are provided as needed for mounting walls and the like.

Since the building structure of the present invention uses the square steel pipe 10 of the present invention having a small Vickers hardness on the outer surface side of the corner, that is, the influence of work hardening is small, the corner heat-affected zone generated during butt welding is used. Surface cracks due to stress release are unlikely to occur.

Hereinafter, the present invention will be further described based on Examples.

Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab (steel material) by a continuous casting method. These were heated, hot-rolled (coarse-rolled and finish-rolled), water-cooled, and wound under the conditions shown in Table 1, and then allowed to cool to obtain a hot-rolled steel sheet having a predetermined finished plate thickness. The hot-rolled steel sheet obtained in succession is formed into a cylindrical open pipe shape by roll forming, and after the butt portion is welded by electric stitching, the rolls arranged on the top, bottom, left and right keep the shape cylindrical and a few percent in the pipe axis direction. Rolling was added to obtain a cylindrical steel pipe.

続いて、得られた円筒鋼管から、２段のサイジングスタンドを経た後、４段の角成形スタンドを経て角部の曲率が板厚の（２．５±０．５）倍となる角形鋼管を得た。このとき、角成形において、角成形スタンドの孔型ロールのギャップやカリバー曲率を変更して、角部近傍におけるロールと角部の周方向の接触幅を制御した。各角成形スタンドにおける接触幅は、円筒鋼管から角形鋼管への変形に関する有限要素法による構造解析を用いて、設定している孔型ロールのギャップのカリバー曲率の条件から得られる接触幅を算出した。上記接触幅は、式（１）からθを算出し（表２中の許容θ下限）、成形θの範囲を接触しないように鋼管を製造した。なお、成形θは、管の平板部中央の位置から接触部の周方向端部までの距離Ｌ１を測定し、そのＬ１から成形θを算出した。

Subsequently, from the obtained cylindrical steel pipe, a square steel pipe in which the curvature of the corner portion is (2.5 ± 0.5) times the plate thickness is obtained through a two-stage sizing stand and then a four-stage square forming stand. Obtained. At this time, in the square forming, the gap and the caliber curvature of the hole-shaped roll of the square forming stand were changed to control the contact width between the roll and the corner in the circumferential direction in the vicinity of the corner. For the contact width at each square forming stand, the contact width obtained from the condition of the caliber curvature of the gap of the pore-shaped roll to be set was calculated by using the structural analysis by the finite element method regarding the deformation from the cylindrical steel pipe to the square steel pipe. .. For the contact width, θ was calculated from the equation (1) (the lower limit of the allowable θ in Table 2), and the steel pipe was manufactured so as not to contact the range of the forming θ. For the molding θ, the distance L1 from the position of the center of the flat plate portion of the pipe to the circumferential end of the contact portion was measured, and the molding θ was calculated from the L1.

A test piece was collected from the obtained square steel pipe and subjected to a microstructure observation, a tensile test, a Charpy impact test, and a hardness test.

The structure was observed using a scanning electron microscope (SEM) at a position t / 4 from the pipe surface (outer surface of the steel pipe) of the square steel pipe flat plate portion. Here, the area ratio obtained by observing the tissue was used as the volume fraction of each tissue. From the obtained SEM image, the area ratios of ferrite, pearlite, bainite and the residual structure were determined. Since it is difficult to distinguish between martensite and austenite in the SEM image, the area fraction of the tissue observed as martensite or austenite is measured from the obtained SEM image, and the volume fraction of austenite measured by the method described later is subtracted from it. The value was taken as the volume fraction of martensite. The observation sample was prepared by collecting the observation surface so as to have a cross section in the rolling direction during hot rolling, polishing the sample, and then corroding it with nital. As the observation conditions, the magnification was 2000 times and the observation area was 2500 μm ² . Observation was performed in 5 or more visual fields, and the average value of the tissues obtained in each visual field was calculated as the area ratio.

Here, ferrite is a product of diffusion transformation, and exhibits a structure with low dislocation density and almost recovery. This includes polygonal ferrites and pseudopolygonal ferrites. Bainite is a double-phase structure of lath-like ferrite and cementite with high dislocation density.

The volume fraction measurement of austenite was performed by X-ray diffraction. The sample for measurement was prepared by grinding so that the diffraction surface was at a position of t / 4 from the tube surface of the square steel pipe flat plate portion, and then performing chemical polishing to remove the surface processed layer. The Kα ray of Mo was used for the measurement, and the volume fraction of austenite was obtained from the integrated intensities of the (200), (220) and (311) planes of fcc iron and the (200) and (211) planes of bcc iron.

In the tensile test, JIS No. 5 tensile test pieces and JIS No. 12B tensile test pieces were taken from the flat plate portion of the square steel pipe so that the tensile direction was parallel to the pipe axis direction, and these were used to comply with the JIS Z 2241 regulations. The yield strength and the tensile strength were measured, and the yield ratio defined by (yield strength) / (tensile strength) was calculated. The number of test pieces was 3 each, and the average value thereof was used as a representative value.

The Charpy impact test was carried out according to JIS Z 2242 using a V-notch test piece collected so that the longitudinal direction of the test piece was parallel to the longitudinal direction of the pipe at the t / 4 position from the pipe surface at the corner of the square steel pipe. According to this, the test temperature was 0 ° C., and the absorbed energy (J) was determined. The number of test pieces was 3 each, and the average value thereof was used as a representative value. The case where the average value is 70 J or more is evaluated as ◯, and the case where the average value is less than 70 J is evaluated as x.

The hardness test is performed by using a Micro Vickers hardness tester at the position 1 mm inside from the outer surface and inner surface of the corner of the square steel pipe in the cross section perpendicular to the pipe axis direction, as specified in JIS Z2244: 2009. The test force was 9.8 N in accordance with the above. Here, the positions 1 mm inside from the outer surface and the inner surface of the corners refer to the positions within a range of 1 ± 0.2 mm from the outer surface side and the inner surface side. The hardness was measured at 5 points at each position, and the average value thereof was used as a representative value.

For surface cracks, a welding experiment of a column-through diaphragm welded joint was conducted using the obtained square steel pipe. Welding conditions were welding wire JISZ3312 GJ59JA1UC3M1T, heat input condition 40 kJ / cm or less, inter-pass temperature 350 ° C. or less, and 7 layers and 9 passes were used. After welding, it was determined whether or not cracks were generated on the surface of the steel material around the welded portion.

These results are shown in Table 2.

From Table 2, all of the examples of the present invention have excellent toughness and no surface cracks have occurred.

From the above, by setting the square forming conditions within the scope of the present invention, it is possible to provide a square steel pipe having excellent toughness and suppressed surface cracking, which is used for building members of large buildings and the like. In this embodiment, the roll-formed steel plate is electrosewn and welded to form an electrosewn steel pipe, but a seamless steel pipe may be formed into a cylindrical shape.

1 Steel strip 2 Leveler 3 Cage roll group 4 Finpass roll group 5 Squeeze roll 6 Welder 7 Electric sewn steel pipe 8 Sizing roll group 9 Square forming roll group 10 Square steel pipe 11 Large beam 12 Small beam 13 Diaphragm 14 Stud H ₁ Side length ( short side)
H ₂ side length (long side)
On a straight line drawn from the center position of θ H ₁ toward the inside of the steel pipe, the point offset from the center of the square steel pipe by 1/2 (H _2- H ₁ ) in the long side direction is defined as the offset point, and from the offset point. Central angle t pipe thickness determined by the straight line drawn to the center of the corner of the square steel pipe and the line drawn from the offset point toward the connection point between the arc and straight line of the corner.

Claims (5)

In a square steel pipe having a flat plate portion and a corner portion,
The square steel pipe has a mass% of C: 0.04 to 0.50%,
Si: 2.0% or less,
Mn: 0.5-3.0%,
P: 0.10% or less,
S: 0.050% or less,
Al: 0.005 to 0.10%,
N: Contains 0.010% or less, and has a component composition in which the balance is composed of Fe and unavoidable impurities.
Moreover, the steel structure at t / 4 (t is the pipe thickness) from the pipe surface contains ferrite having a volume ratio of more than 30% and bainite having a volume ratio of 10% or more, and the total volume ratio of ferrite and bainite is 70%. More than 95% or less, and the balance consists of one or more selected from pearlite, martensite, and austenite.
The yield strength of the flat plate portion is 385 MPa or more, the tensile strength is 520 MPa or more, and the yield ratio is 0.90 or less.
The Vickers hardness of the corner is larger than the Vickers hardness of the outer surface of the corner, the Vickers hardness of the inner surface of the corner is 280 HV or less, and the Vickers hardness of the outer surface of the corner is 280 HV or less. The difference between the Vickers hardness on the outer surface side of the corner and the Vickers hardness on the inner surface side of the corner is 80 HV or less.
_{A square steel pipe having a Charpy absorption energy vE 0} of 0 ° C. on the outer surface side of the corner portion of 70 J or more. Further, in mass%, Nb: 0.005 to 0.150%,
Ti: 0.005 to 0.150%,
V: The square steel pipe according to claim 1 , which contains one or more selected from 0.005 to 0.150%. Further, in mass%, Cr: 0.01 to 1.0%,
Mo: 0.01% to 1.0%,
Cu: 0.01-0.50%,
Ni: 0.01-0.30%,
Ca: 0.0005 to 0.010%,
B: The square steel pipe according to claim 1 or 2 , which contains one or more selected from 0.0003 to 0.010%. The method for producing a square steel pipe according to any one of claims 1 to 3 , wherein the steel material having the component composition is heated to a heating temperature of 1100 to 1300 ° C, and then rough rolling end temperature: 850 to 1150 ° C. Rough rolling is performed, and finish rolling is performed so that the finish rolling end temperature is 750 to 850 ° C., and the total reduction ratio at 930 ° C. or lower in both rough rolling and finish rolling is 65% or more, and then the plate thickness center. Cooling stop temperature: 450 to 650 ° C at a cooling rate at which the average cooling rate from the start of cooling to the stop of cooling is 10 to 30 ° C / s. In a square forming step in which a roll-formed steel plate is electrosewn and welded to form an electrosewn steel pipe and then the electrosewn steel pipe is squarely formed into a square steel pipe after being formed into a shape, it is perpendicular to the pipe axis direction. In the cross section, the adjacent side lengths are H ₁ (mm) and H ₂ (mm) (H ₁ ≤ H ₂ ), respectively, and the intersections of straight lines drawn from the center positions of _{H 1 and H 2 toward the inside of the steel pipe intersect.} Was defined as the central portion of the square steel pipe, and was offset by _{1/2 (H 2-} H ₁ ) in the long side direction from the central portion of the square steel pipe on a straight line drawn from the center position of _{H 1 toward the inside of the steel pipe.} The central angle θ formed by a straight line drawn from the offset point to the center of the corner of the square steel pipe and a straight line drawn from the offset point toward the arc portion of the corner or the flat plate portion of the square steel pipe is the following equation. A method for manufacturing a square steel pipe that satisfies (1).

However,
H ₁ : Side length (short side) (mm)
H ₂ : Side length (long side) (mm)
t: Tube thickness (mm)
Is. A building structure using the square steel pipe according to any one of claims 1 to 3.

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