KR20210135287A - Rectangular steel pipe, manufacturing method thereof, and building structure - Google Patents

Abstract

An object of the present invention is to provide a rectangular steel pipe in which the effect of each part on work hardening is small and surface cracking is suppressed, a method for manufacturing the same, and a building structure using the rectangular steel pipe of the present invention. In the rectangular steel pipe having a flat plate portion and a corner portion, the flat portion has a yield strength of 385 MPa or more, a tensile strength of 520 MPa or more, and a yield ratio of 0.90 or less, and the Vickers hardness of each portion is higher than the Vickers hardness of the outer surface side of each portion The Vickers hardness on the inner surface side is larger, the Vickers hardness on the outer surface side of each part is 280 HV or less, and the difference between the Vickers hardness on the outer surface side of the leg part and the Vickers hardness on the inner surface side of the leg part is 80 HV or less, and the outside of the leg part _{A rectangular steel pipe having a Charpy absorbed energy vE 0} of 70 J or more at 0°C on the surface side.

Description

Rectangular steel pipe, manufacturing method thereof, and building structure

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

BACKGROUND ART In recent years, building structural members used in large-scale buildings such as factories, warehouses, and commercial facilities are being strengthened in order to reduce the construction cost due to weight reduction. In particular, a rectangular steel pipe used as a pillar material for a building is required to have a yield strength of 385 MPa or more and a tensile strength of 520 MPa or more.

For a rectangular steel pipe having such high deformation performance and excellent toughness, specifically, it is necessary that the yield ratio (=yield strength/tensile strength) in the tube axial direction of the flat part be 0.90 or less and the Charpy absorbed energy at 0°C is 70 J or more. have.

A rectangular steel pipe is generally manufactured by using a hot-rolled steel sheet (steel strip) or a thick steel sheet as a raw material, and press bending or roll forming these in cold.

Rectangular steel pipe manufactured by cold press bending forming is manufactured by forming a cross-sectional shape of a thick steel plate into a U-shape or a U-shape by press bending, and joining them by submerged arc welding. In the rectangular steel pipe manufactured by roll forming, a hot-rolled steel sheet is formed into a cylindrical open pipe shape by roll forming, the butt portion thereof is electric-sealed, and then a cylinder is formed by rolls arranged on the top, bottom, left and right. It is manufactured by applying a drawing of several percent in the tube axis direction while remaining in the shape, and then forming it into a square shape.

On the other hand, in the case of a rectangular steel pipe manufactured by cold press bending, work hardening due to bending deformation of the corner portion is remarkable, and the toughness and plastic deformation ability of the corner portion are impaired, so the earthquake resistance strength is deteriorated, It becomes easier to destroy each part as a starting point. In particular, in a high-strength material for a building member containing a hard second phase such as bainite, work hardening becomes remarkable.

Therefore, when manufacturing a high-strength rectangular steel pipe, it is necessary to select a material that reduces the effect of deterioration of toughness due to work hardening of each part during molding, and a manufacturing method that suppresses work hardening of each part. .

Patent Document 1 proposes a rectangular steel pipe characterized in that, in the microstructure of the flat plate portion, the area fraction of the bainite structure is 40% or more.

Patent Document 2 proposes a rectangular steel pipe with a low yield ratio and high toughness characterized in that the entire pipe is subjected to strain relief annealing after pipe forming 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 each part. However, the hardness of the surface layer part of each part is still large, and in order to suppress the fracture|rupture, surface cracking, etc. starting from a leg part as a starting point, a further decrease in hardness is calculated|required.

In addition, since the square steel pipe described in Patent Document 2 requires heat treatment after pipe making, the cost becomes very high compared to the square steel pipe that is cold worked.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a rectangular steel pipe in which the effect of work hardening of each part is small and surface cracking is suppressed, a method for manufacturing the same, and a building structure using the rectangular steel pipe of the present invention .

MEANS TO SOLVE THE PROBLEM The present inventors earnestly examined in order to solve the said subject, and acquired the following recognition.

In the corner-forming stands for the manufacture of rectangular steel pipe by roll forming, the curvature of the portion corresponding to the flat part of the square steel pipe that becomes the final product is reduced by using the forming roll, It is shaping|molding so that it may become a shape. This is performed such that the forming roll pushes in a position corresponding to the center of the flat portion of the square steel pipe of the final product, and the corners form L-shaped corners so as to follow the deformation of the flat portion.

Accordingly, it is possible to form a corner portion having a curvature without directly contacting the roll over the entire corner portion. On the other hand, when the steel pipe material comes into contact with the roll, the flatness of the flat part and the dimensional accuracy such as the curvature of each part are improved, but since it receives a shear force from the roll, work hardening centered on the contact part with the roll occurs. Obvious. Therefore, in order to suppress excessive work hardening of each part, it is necessary to control the contact part of a roll and a leg part so that it may be compatible with dimensional accuracy.

The present inventors have found that, in order to obtain a rectangular steel pipe having small work hardening by bending, a rectangular steel pipe having a larger Vickers hardness on the inner surface side of each part is better than the Vickers hardness on the outer surface side of each part. When forming a rectangular steel pipe from a cylindrical steel pipe into a rectangular steel pipe, the square steel pipe having a Vickers hardness on the inner surface side of each part larger than the Vickers hardness on the outer surface side of each part is square-formed so that the roll does not come into contact with the vicinity of each part during square forming. It is obtained by setting a roll gap and a caliber curvature of a roll, and forming a rectangular steel pipe from a cylindrical steel pipe. This is thought to be because, since 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, work hardening due to bending is small and it is difficult to be affected by the shear force of the roll.

In addition, as a result of examining the hardness of each part by performing molding in which the roll gap of various angular moldings and the caliber curvature of the roll are changed, even when the roll is not in direct contact with each part, the roll is in contact up to the immediate vicinity of each part. It was found that the hardness of the surface of each part increased under the condition. In this point, since shear stress also acts in the circumferential direction by contact with the roll, work hardening occurs in the vicinity of the contact portion with the roll. It was found that the region to which this shear stress acts changes depending on the rigidity of the material to be formed, that is, the 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 recognition, and its characteristics are as follows.

[1] A rectangular steel pipe having a flat plate portion and a corner portion, wherein the flat portion has a yield strength of 385 MPa or more, a tensile strength of 520 MPa or more, and a yield ratio of 0.90 or less,

The Vickers hardness of each part is greater than the Vickers hardness on the inner surface side of each part than the Vickers hardness on the outer surface side of each part, the Vickers hardness on the outer surface side of the leg part is 280 HV or less, and the Vickers hardness on the outer surface side of the leg part and The difference in Vickers hardness on the inner surface side of each part is 80 HV or less,

A rectangular steel pipe in which _{the Charpy absorbed energy vE 0} at 0°C on the outer surface side of each part is 70J or more.

[2] In 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% It contains the following and has a component composition in which the remainder consists of Fe and unavoidable impurities,

In addition, the steel structure at the position of t/4 (t is the tube thickness) from the tube surface contains more than 30% of ferrite and 10% or more of bainite by volume ratio, and the volume ratio of ferrite and bainite is The square steel pipe according to [1], wherein the total amount is 70% or more and 95% or less, and the balance consists of one or two or more types selected from pearlite, martensite, and austenite.

[3] The square steel pipe according to [2], further comprising, in mass%, one or two or more selected from the group consisting of Nb: 0.005 to 0.150%, Ti: 0.005 to 0.150%, and V: 0.005 to 0.150%.

[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 0.30%, Ca: 0.0005 to 0.010%, B: 0.0003 to The square steel pipe according to [2] or [3], which contains one or two or more selected from 0.010%.

[5] The method for manufacturing a rectangular steel pipe according to [1], wherein after forming into a cylindrical shape and then performing square forming into a rectangular shape, the square forming step of performing the square forming step is in the pipe axis direction In a cross section perpendicular to each other, the lengths of adjacent sides are H ₁ (mm) and H ₂ (mm) (H ₁ ≤ H ₂ ), respectively, and a straight line drawn from the center position of _{H 1} and H _{2 toward the inside of the steel pipe.} When the intersection point of the rectangular steel pipe is the central part of the rectangular steel pipe, on a straight line drawn from the central position of _{H 1} toward the inside of the steel pipe, it is offset by _{1/2 (H 2} -H _{1 ) from the center of the rectangular steel pipe in the long side direction.} Let the point be the offset point, and the central angle θ formed by the straight line drawn from the offset point to the center of each part of the prismatic steel pipe and the straight line drawn from the offset point toward the arc part of each part or the flat part of the prismatic steel pipe is expressed by the following formula (1) ) for manufacturing a rectangular steel pipe that satisfies the

step,

H ₁ : side length (short side) (mm)

H ₂ : side length (long side) (mm)

t: pipe thickness (mm)

am.

[6] After heating the steel material having the component composition according to any one of [2] to [4] to a heating temperature: 1100 to 1300 ° C., rough rolling is performed to an end temperature of rough rolling: 850 to 1150 ° C., Finish rolling end temperature: Finish rolling is performed at 750 to 850°C, and the total rolling reduction at 930°C or less in both rough rolling and finish rolling is set to 65% or more, then, at the plate thickness center temperature, 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, the cooling stop temperature is cooled to 450 to 650 ° C. In the angle forming process in which the electric resistance welded steel sheet is formed, the roll-formed steel sheet is electric resistance welded, and then the electric resistance resistance steel pipe is formed into a square steel tube, in a cross section perpendicular to the pipe axis direction, adjacent sides Let the lengths be H ₁ (mm) and H ₂ (mm) (H ₁ ≤ H ₂ ), respectively, and _{the intersection of straight lines drawn from the center of H 1} and H ₂ toward the inside of the steel pipe is the central part of the square steel pipe. When, on _{a straight line drawn from the central 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 rectangular steel pipe is taken as the offset point, and the square shape from the offset point A method for manufacturing a rectangular steel pipe in which a central angle θ formed between a straight line drawn to the center of each part of the steel pipe and a straight line drawn from the offset point toward the arc part of each part or the flat plate part of the rectangular steel pipe satisfies the following formula (1):

step,

H ₁ : side length (short side) (mm)

H ₂ : side length (long side) (mm)

t: pipe thickness (mm)

am.

[7] A building structure using the rectangular steel pipe according to any one of [1] to [4].

According to the rectangular steel pipe of the present invention, it is possible to obtain a rectangular steel pipe in which the influence of the work hardening of each part is small and surface cracking is suppressed. Accordingly, it is possible to greatly contribute to the reduction of the construction cost of large buildings such as factories, warehouses, and commercial facilities. In addition, according to the method for manufacturing a rectangular steel pipe of the present invention, it is possible to manufacture a high-strength rectangular steel pipe in a short period of time due to high productivity compared to cold press bending.

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

(Form for implementing the invention)

The rectangular steel pipe of the present invention has a flat plate part and a leg part, the yield strength (YS) of the plate part is 385 MPa or more, the tensile strength (TS) is 520 MPa or more, and the yield ratio is 0.90 or less, and the Vickers hardness of the leg part is The Vickers hardness on the inner surface side of each part is larger than the Vickers hardness on the outer surface side, the Vickers hardness on the outer surface side of each part is 280 HV or less, and the difference between the Vickers hardness on the outer surface side of each part and the Vickers hardness on the inner surface side of each part is 80 HV _{Below, it is characterized by the Charpy absorbed energy vE 0 at 0} degreeC of the outer surface side of each part being 70 J or more.

The square steel pipe is work-hardened to a greater extent on the side of the flat portion than on the flat portion. In particular, since the outer surface of each part is a tensile stress field in the circumferential direction, it is necessary to secure the toughness of the outer surface of each part in order to suppress the brittle fracture of each part in the final product. That is, it is with the Charpy absorbed energy vE ₀ is at least 70J of 0 ℃ is required in each section outer surface, over at least the flat panel portion yield strength (YS) is 385㎫ and tensile strength (TS) and yield ratio of 0.90 or less is 520㎫ is required

Further, in the present invention, the Vickers hardness on the inner surface side of each part is larger than the Vickers hardness on the outer surface side of each part, the Vickers hardness on the outer surface side of each part is 280 HV or less, and the Vickers hardness on the outer surface side of each part and the inner surface of each part The difference of the Vickers hardness of the side shall be 80 HV or less. In the present invention, by making the Vickers hardness on the inner surface side of each part larger than the Vickers hardness on the outer surface side of each part, and the Vickers hardness on the outer surface side of each part is 280 HV or less, a rectangular steel pipe with small work hardening by bending can be obtained. have. When the Vickers hardness on the outer surface side of each part exceeds 280 HV, since work hardening on the outer surface side advances, the ductility of each part deteriorates remarkably. Moreover, in order to ensure the toughness of the leg part with large surface work hardening by bending deformation, the difference between the Vickers hardness on the side of an outer surface of a leg part and Vickers hardness on the side of a leg part inner surface is made into 80 HV or less. When the difference between the Vickers hardness on the outer surface side of each part and the Vickers hardness on the inner surface side of each part exceeds 80 HV, work hardening on the inner surface side of each part advances, and the residual stress on the inner surface of each part becomes significant. It adversely affects the cracking of plating.

In addition, the Vickers hardness on the outer surface side of each part in this invention is the Vickers hardness in 1±0.2 mm inside from the outer surface of each part, and the Vickers hardness on the side inner surface of each part is 1±0.2 mm inside from the inner surface of each part. Let it say the Vickers hardness in .

The square steel pipe of the present invention, in 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: contains 0.010% or less, the balance has a component composition consisting of Fe and unavoidable impurities, and the steel structure at the position t/4 (t is the tube thickness) from the tube surface is greater than 30% by volume It contains ferrite and 10% or more of bainite, the total volume ratio of ferrite and bainite is 70% or more and 95% or less, and the balance consists of one or two or more types selected from pearlite, martensite, and austenite. it is preferable

In the present invention, the reason for limiting the preferred component composition of the steel material will be described below. In addition, in this specification, unless otherwise indicated, "%" which shows a steel composition is "mass %".

C: 0.04 to 0.50%

C is an element which raises the strength of steel by solid solution strengthening. In addition, C promotes the formation of pearlite, increases hardenability, contributes to the formation of martensite, and contributes to the stabilization of austenite, and thus is an element that also contributes to the formation of a hard phase. It is preferable to contain 0.04% or more of C in order to secure a desired strength. However, when C content exceeds 0.50 %, the ratio of a hard phase will become high, toughness will fall, and weldability also deteriorates. For this reason, it is preferable to make C content into 0.04 % or more and 0.50 % or less. More preferably, the C content is more than C: 0.12% and 0.25% or less.

Si: 2.0% or less

Si is an element which raises the intensity|strength of steel by solid solution strengthening, and can contain it as needed. In order to acquire such an effect, containing of 0.01% or more of Si is preferable. On the other hand, when Si content exceeds 2.0 %, weldability will deteriorate. For this reason, it is preferable to make Si content into 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 contributes to refinement of the structure by lowering the ferrite transformation initiation temperature. It is preferable to contain 0.5% or more of Mn to ensure desired strength and structure. However, when Mn content exceeds 3.0 %, weldability will deteriorate. For this reason, it is preferable to make Mn content into 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

Since P segregates at grain boundaries and causes material inhomogeneity, it is preferable to reduce it as much as possible as an unavoidable impurity. In addition, in the case of containing, the content of 0.10% or less is acceptable. For this reason, it is preferable to make P content into the range of 0.10% or less. More preferably, the P content is 0.03% or less.

S: 0.050% or less

S is normally present as MnS in steel, but MnS is stretched thinly in a hot rolling process, and exerts a bad influence on ductility. For this reason, in this invention, it is preferable to reduce as much as possible. In addition, when containing, content of 0.050 % or less is permissible. For this reason, it is preferable to make S content into 0.050 % or less. More preferably, the S content is 0.015% or less.

Al: 0.005 to 0.10%

Al is an element that acts as a strong deoxidizer. In order to obtain such an effect, it is preferable to contain 0.005% or more of Al. However, when Al content exceeds 0.10 %, while weldability deteriorates, alumina type inclusion increases and surface properties deteriorate. For this reason, it is preferable to make Al content into 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, as an unavoidable impurity, is an element having an action of lowering toughness by firmly fixing the motion of dislocations. In the present invention, it is preferable to reduce N as an impurity as much as possible. In addition, when containing, 0.010 % or less content is permissible. For this reason, it is preferable to make N content into 0.010 % or less. More preferably, the N content is 0.0080% or less.

Although the above components are the basic component compositions of the steel material of the electric resistance resistance steel pipe in the present invention, in addition to these, Nb: 0.005 to 0.150%, Ti: 0.005 to 0.150%, V: 0.005 to 0.150% You may make it contain 1 type or 2 or more types.

One or two or more selected from Nb: 0.005 to 0.150%, Ti: 0.005 to 0.150%, and V: 0.005 to 0.150%

Nb, Ti, and V are elements that form fine carbides and nitrides in steel and contribute to the improvement of strength of steel through precipitation strengthening, and may be contained as necessary. 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 inclusion results in an increase in the yield ratio and a decrease in toughness. For this reason, when Nb, Ti, and V are contained, Nb: 0.005-0.150%, Ti: 0.005-0.150%, V: 0.005-0.150%. Preferably, they are Nb: 0.008-0.10 %, Ti: 0.008-0.10 %, and V: 0.008-0.10 %.

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%, Ca: 0.0005% to 0.010%, B: 0.0003 to 0.010% You may make it contain 1 type(s) or 2 or more types selected.

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 0.010%, B: 0.0003 to 0.010% one or more selected from the group consisting of

Cr, Mo, Cu, and Ni are elements that increase the strength of steel by solid solution strengthening, and all are elements that enhance the hardenability of steel and contribute to stabilization of austenite, so formation of hard martensite and austenite As an element contributing to , it can contain 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 inclusion causes a decrease in toughness and deterioration of weldability. For this reason, 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: 0.01 to 0.30%. Preferably, they are Cr: 0.1-0.5%, Mo: 0.1-0.5%, Cu: 0.1-0.40%, Ni: 0.1-0.20%.

Ca is an element that contributes to the improvement of toughness of steel by spheroidizing sulfides such as MnS that are thinly stretched in the hot rolling process, and may be contained as necessary. In order to acquire such an effect, it is preferable to contain 0.0005% or more of Ca. However, when Ca content exceeds 0.010%, Ca oxide clusters may be formed in steel, and toughness may deteriorate. For this reason, when Ca is contained, Ca content shall be 0.0005 to 0.010 %. Preferably, the Ca content is 0.0010 to 0.0050%.

B is an element contributing to the refinement of the structure by lowering the ferrite transformation start temperature. In order to acquire 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. For this reason, when B is contained, B content shall be 0.0003 to 0.010 %. Preferably, the B content is 0.0005 to 0.0050%.

The remainder other than the above components is Fe and unavoidable impurities.

Next, the reason for limiting the preferred microstructure of the rectangular steel pipe of the present invention will be described.

In the rectangular steel pipe of the present invention, the steel structure at a position of t/4 (t is the pipe thickness) from the pipe surface contains more than 30% of ferrite and 10% or more of bainite by volume, and the ferrite and bay It is preferable that the sum total of the volume fraction of nite is 70% or more and 95% or less, and the balance consists of 1 type(s) or 2 or more types selected from pearlite, martensite, and austenite.

Ferrite is a soft structure, and by mixing it with other hard structures, the yield ratio of the steel pipe material is lowered. In order to acquire such an effect, it is preferable to set it as the volume ratio of more than 30 %.

In addition, bainite is a structure having an intermediate hardness, and increases the strength of steel. Since desired yield strength and tensile strength cannot be obtained with only ferrite, it is preferable to set it as 10% or more by volume.

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

It is preferable that remainder consists of 1 type(s) or 2 or more types chosen from pearlite, martensite, and austenite. Pearlite, martensite, and austenite are hard structures, and in particular, while increasing the tensile strength of steel, mixing with soft ferrite lowers the yield ratio of the steel pipe material. Moreover, in order to acquire such an effect, it is preferable that it is 5 % or more and 30 % or less of volume ratio in total.

Further, since the steel structure of the rectangular steel pipe is uniform in the width direction of the steel pipe, any structure of the leg portion and the flat portion satisfies the scope of the present invention. In addition, the t/4 position from the tube surface allows a range of ±0.2 mm from the t/4 position. In addition, the tube surface may be either the outer surface or the inner surface of a tube.

Next, the manufacturing method of the rectangular steel pipe of this invention is demonstrated.

In the present invention, although not particularly limited, for example, after heating a steel material such as a slab having the above-described chemical composition to a temperature of 1100-1300°C, rough rolling termination temperature: 850-1150°C After the hot rolling process, in which finish rolling is performed, the finish rolling end temperature: 750 to 850 ° C., and the total reduction ratio at 930 ° C. or less in both rough rolling and finish rolling becomes 65% or more. , at the plate thickness center temperature, 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, the cooling stop temperature is cooled to 450 to 650°C and wound, and then allowed to cool. In addition, in the description of the manufacturing method below, unless otherwise stated, the temperature is set as the surface temperature of a steel raw material, a steel plate, etc. This surface temperature can be measured with a radiation thermometer etc. In addition, the average cooling rate is set to ((temperature before cooling - temperature after cooling)/cooling time) unless otherwise stated. In addition, the cooling method is performed by water cooling, such as injection of water from a nozzle, cooling by injection of a cooling gas, etc. Moreover, it is preferable to perform cooling operation on both surfaces of a steel plate so that both surfaces of a hot-rolled steel plate may be cooled under the same conditions. In addition, although there is no designation in particular as for the center temperature of the steel plate in these hot rolling, it computes by the abnormality electrothermal calculation by a difference calculation this time. Specifically, the calculation is performed using material property values such as thermal conductivity, specific heat and density of the steel sheet, and the heat transfer coefficient obtained from the water density of cooling water and the outer surface temperature of the steel sheet as boundary conditions.

A method for producing a steel material having the above-described composition is not particularly limited, melting is performed by a known melting method such as a converter, an electric furnace, and a vacuum melting furnace, and a commonly known casting method such as a continuous casting method , produced to a desired size. The molten steel may be further subjected to secondary refining such as ladle refining. In addition, there is no problem at all even if an ingot casting-slabbing method is applied instead of the continuous casting method.

Heating temperature: 1100-1300℃

When heating temperature is less than 1100 degreeC, the deformation resistance of a to-be-rolled material becomes large, and rolling becomes difficult. On the other hand, when the heating temperature exceeds 1300°C, austenite grains coarsen, and fine austenite grains cannot be obtained in subsequent rolling, making it difficult to secure the desired toughness of the hot-rolled steel sheet, and It becomes difficult to suppress the formation of coarse bainite. For this reason, it is preferable that the heating temperature in a hot rolling process is 1100-1300 degreeC.

Rough rolling end temperature: 850∼1150℃

When the rough rolling end temperature is less than 850° C., the steel sheet temperature becomes below the ferrite transformation start temperature during the subsequent finish rolling, and the risk of ferrite formation increases. The generated ferrite becomes deformed ferrite grains elongated in the rolling direction by subsequent rolling, which causes an increase in the yield ratio. On the other hand, when the rough rolling end temperature exceeds 1150°C, the reduction in the austenite non-recrystallization temperature range is insufficient, and fine austenite grains cannot be obtained, making it difficult to secure the desired toughness of the hot-rolled steel sheet. It becomes difficult to suppress the formation of bainite. For this reason, it is preferable that rough rolling completion|finish temperature is 850-1150 degreeC.

Finish rolling end temperature: 750 to 850°C

When the finish rolling end temperature is less than 750° C., the steel sheet temperature during rolling becomes lower than or equal to the ferrite transformation start temperature, thereby increasing the risk of ferrite formation. The ferrite produced|generated in the above becomes a deformed ferrite grain extended in a rolling direction by subsequent rolling, and becomes a cause of a yield ratio rise. On the other hand, if the finish rolling finish temperature exceeds 850°C, the reduction in the austenite non-recrystallization temperature range is insufficient, and fine austenite grains cannot be obtained, making it difficult to secure the desired toughness of the hot-rolled steel sheet. It becomes difficult to suppress the formation of bainite. For this reason, it is preferable that the finish rolling end temperature is 750-850 degreeC.

Total rolling reduction at 930°C or lower in both rough rolling and finish rolling: 65% or more

In the present invention, by refining subgrains in austenite in hot rolling, ferrite, bainite and residual structure generated in the subsequent cooling and winding process are refined, and a hot-rolled steel sheet having desired strength and toughness is obtained. get In order to refine the sub-grains in austenite in hot rolling, it is necessary to increase the reduction ratio in the austenite non-recrystallization temperature range and introduce sufficient working strain. In order to achieve the above, the total reduction ratio at 930°C or lower in both rough rolling and finish rolling was set to 65% or more. When the total reduction ratio at 930° C. or lower in both rough rolling and finish rolling is less than 65%, sufficient working strain cannot be introduced in hot rolling, and thus a structure having desired toughness cannot be obtained.

Average cooling rate from cooling start to cooling stop: 10 to 30°C/s

When the cooling rate is less than 10°C/s, the frequency of nucleation of ferrite decreases and the ferrite grains become coarse, so that a structure having 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 ratio of ferrite and bainite becomes less than 70%.

Cooling stop temperature: 450-650℃

When the cooling stop 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 ratio 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 coarsen, and the volume ratio of bainite is set to 10% or more because it exceeds the bainite transformation start temperature. Can not.

Next, the pipe making process after hot rolling is demonstrated.

A hot-rolled steel sheet (steel strip) 1 which is a raw material of an electric resistance resistance pipe is manufactured using a manufacturing facility as shown in FIG. 1 . For example, after the hot-rolled steel sheet 1 is subjected to vertical straightening by the leveler 2, it is intermediately formed in a cage roll group 3 consisting of a plurality of rolls to form a cylindrical open tube, and then a pin pass consisting of a plurality of rolls Finish forming (roll forming) is carried out in the roll group 4 . After finish forming, the width end of the steel strip 1 is electrically resistance-welded with a welding machine 6 while pressing with a squeeze roll 5 to obtain an electric resistance welded steel pipe 7 . In addition, in this invention, the manufacturing facility of the electric resistance resistance steel pipe 7 is not limited to the pipe making process as shown in FIG.

Thereafter, the obtained electric resistance resistance steel pipe 7 is subjected to several percent drawing in the pipe axis direction in a cylindrical shape with rolls arranged on the top, bottom, left and right, and then is formed into a square shape to obtain a square steel pipe. Fig. 2 is a schematic diagram showing a forming process of a rectangular steel pipe according to an embodiment of the present invention. As shown in Fig. 2, the electric resistance welded steel pipe 7 is reduced in diameter in a cylindrical shape by a sizing roll group (sizing stands) 8 composed of a plurality of rolls. It is sequentially molded into the same shape as R1, R2, and R3 by a group of rolls (corner forming stand) 9 to form a square steel pipe 10 . The roll of the square-shaped stand is a hole-shaped roll having a caliber curvature, and as it becomes a rear end stand, the caliber curvature radius increases to form a flat plate portion and a corner portion of the square steel pipe. Further, the number of stands of the sizing roll group 8 and the angular forming roll group 9 is not particularly limited.

Next, the manufacturing conditions of the angular shape|mold of this invention are demonstrated.

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

As described above, when the steel pipe material comes into contact with the roll, the flatness of the flat part and the dimensional accuracy such as the curvature of each part are improved. it is clear to do Therefore, in order to suppress excessive work hardening of each part, it is necessary to control the contact part of a roll and a leg part so that it may be compatible with dimensional accuracy.

Therefore, the present inventors set the roll gap and the caliber curvature of the roll during square forming so that the roll does not come into contact with the vicinity of each part during square forming, and formed a square steel pipe from a cylindrical steel pipe. As a result, as shown in FIG. 3 , in the angular forming step, in the cross section perpendicular to the tube axis direction, adjacent side lengths are respectively H ₁ (mm) and H ₂ (mm) (provided that H ₁ ≤ with H ₂ and, H _1, H ₂ are being each side length of the finished product), and the square intersection point to the intersection of a straight line between drawn towards the inner steel pipe from the center position of H ₁ and H _2, when a steel pipe center, H _{On a straight line drawn from the central position of 1 toward the inside of the steel pipe, a point offset by 1/2 (H 2} -H ₁ ) from the central portion of the rectangular steel pipe in the long side direction is taken as the offset point, and each part of the rectangular steel pipe from the offset point The central angle θ between the straight line drawn to the center and the straight line drawn from the offset point toward the arc part of each part or the flat part of the rectangular steel pipe satisfies the following formula (1), so that work hardening due to bending is small, and surface cracks are suppressed can do.

step,

H ₁ : side length (short side) (mm)

H ₂ : side length (long side) (mm)

t: pipe thickness (mm)

am.

In the square forming process, on the rear end stand side, the radius of curvature of the caliber of the roll and the radius of curvature of the flat part of the square steel pipe are approximately equal. , while expanding from the center side of the flat plate portion to the side of each portion, the desired leg size is obtained. In particular, as the ratio t/H of the pipe thickness t and the side length H, which is the thickness of the final product of the rectangular steel pipe, increases, the rigidity of deformation increases. Needs to be. On the other hand, when square forming is performed by directly contacting the perforated roll with each part of the rectangular steel pipe during square forming, work hardening of each part due to the shear force of the perforated roll becomes remarkable.

In order to obtain a desired radius of curvature of each part without causing such excessive work hardening, the area where the hole-shaped roll and the rectangular steel pipe do not contact is passed through the entire stand of the angular forming, and the perimeter corresponding to the thickness at the apex of each part It is necessary to control the distance in the direction. The area is the area on the outer surface side of the steel pipe that satisfies the above (1) with reference to the center of each part of the final shape of the rectangular steel pipe. As a method of controlling the contact position between the perforated roll and the rectangular steel pipe, there are, for example, a method of adjusting the caliber curvature radius of the perforated roll and the gap between the rolls, but is not limited thereto.

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

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 rectangular steel pipes 10 of the present invention are erected and used as a pillar material. A plurality of large beams 11 made of steel materials, such as H-beams, are erected between adjacent rectangular steel pipes 10 . In addition, a plurality of small beams 12 made of steel materials, such as H-beams, are erected between the adjacent beams 11 . By welding the rectangular steel pipe 10 and the diaphragm 13, and welding the H-beam to be the girder 11 thereto, the girder 11 made of steel such as H-beam steel between the rectangular steel pipes 10 adjacent to each other. is hypothesized. In addition, studs 14 are formed as needed for attachment of walls or the like.

Since the building structure of the present invention uses the rectangular steel pipe 10 of the present invention having a small Vickers hardness on the outer surface side of each part, that is, the influence of work hardening is small, the stress in the heat affected zone of each part generated at the time of butt welding It is hard to produce surface cracks etc. by liberation.

Example

Hereinafter, based on an Example, this invention is further demonstrated.

Molten steel having the component composition shown in Table 1 was melted in a converter, and slabs (steel materials) were obtained by a continuous casting method. These were subjected to heating, hot rolling (rough rolling and finish rolling), water cooling, and winding under the conditions shown in Table 1, and then allowed to cool to obtain a hot rolled steel sheet having a predetermined finished sheet thickness. Subsequently, the obtained hot-rolled steel sheet is formed into a cylindrical open tube shape by roll forming, the buttted portion is electric-sealed, and then a few% of drawing is applied in the tube axis direction with the rolls arranged on the top, bottom, left and right, in the cylindrical shape, A cylindrical steel pipe was obtained.

Subsequently, from the obtained cylindrical steel pipe, it passed through two sizing stands, and then passed through four corner forming stands to obtain a rectangular steel pipe in which the curvature of each part was (2.5±0.5) times the plate thickness. At this time, in the angular forming, the gap and caliber curvature of the hole-shaped roll of the angular forming stand were changed to control the contact width in the circumferential direction between the roll and the corner in the vicinity of the corner. The contact width in each square shape stand is calculated using the structural analysis by the finite element method for deformation from a cylindrical steel pipe to a square steel pipe, and the contact width obtained from the caliber curvature condition of the set hole-shaped roll gap is calculated. did. For the contact width, θ was calculated from Equation (1) (the lower limit of allowable θ in Table 2), and a steel pipe was manufactured so as not to contact the range of the molding θ. In addition, the shaping|molding (theta) measured the distance L1 from the position of the center of the flat plate part of a tube to the circumferential direction edge part of a contact part, and computed shaping|molding (theta) from the L1.

A test piece was taken from the obtained square steel pipe, and a structure observation, a tensile test, a Charpy impact test, and a hardness test were performed.

The tissue observation was performed at a position t/4 from the tube surface (outer surface of the steel tube) of the rectangular steel tube flat portion using a scanning electron microscope (SEM). Here, the area ratio obtained by tissue observation was taken as the volume fraction of each tissue. From the obtained SEM image, the area ratio of ferrite, pearlite, bainite, and residual structure was calculated|required. In addition, since it is difficult to discriminate between martensite and austenite on the SEM image, the area ratio of the structure observed as martensite or austenite from the obtained SEM image is measured, and then the volume fraction of austenite measured by the method described later. The value obtained by subtracting ? was taken as the volume fraction of martensite. The observation sample was sampled and polished so that the observation surface became a cross section in the rolling direction at the time of hot rolling, and then was produced by etching in nital. As observation conditions, the magnification was set to 2000, and the observation area was set to 2500 µm ² . Five or more fields of observation were performed, and the average value of the structure|tissue obtained in each field was computed as an area ratio.

Here, ferrite is a product by diffusion transformation, and exhibits a nearly recovered structure due to a low dislocation density. These include polygonal ferrite and quasi-polygonal ferrite. In addition, bainite is a dual-phase structure of lath-shaped ferrite and cementite having a high dislocation density.

The volume fraction of austenite was measured by X-ray diffraction. The measurement sample was prepared by grinding the diffractive surface at a position t/4 from the tube surface of the rectangular steel pipe flat portion, followed by chemical polishing to remove the surface treatment layer. The Kα ray of Mo was used for the measurement, and the volume fraction of austenite was determined from the integrated strengths of the (200), (220), and (311) planes of fcc iron and (200) and (211) planes of bcc iron.

In the tensile test, JIS 5 tensile test specimens and JIS 12B tensile test specimens are respectively taken from the flat plate portion of the rectangular steel pipe so that the tensile direction is parallel to the pipe axis direction, and using these, in accordance with the provisions of JIS Z 2241, yield strength , the tensile strength was measured, and the yield ratio defined by (yield strength)/(tensile strength) was calculated. The number of test pieces was made into three pieces each, and the average value of them was made into a representative value.

The Charpy impact test is performed in accordance with JIS Z 2242 by using a V-notch test piece taken so that the longitudinal direction of the test piece is parallel to the longitudinal direction of the tube at the t/4 position from the pipe surface of each part of the rectangular steel pipe. Temperature: It implemented at 0 degreeC and the absorbed energy (J) was calculated|required. In addition, the number of test pieces was made into three each, and those average values were made into a representative value. The case where an average value became 70J or more was made into (circle), and the case where it became less than 70J was made into x.

The hardness test was performed using a micro Vickers hardness tester at positions 1 mm inside from the outer and inner surfaces of each part of the rectangular steel pipe in a cross section perpendicular to the tube axis direction, in accordance with JIS Z2244:2009. It carried out with the test force of 9.8N. Here, the position within 1 mm from the outer surface and the inner surface of each part refers to a position within the range of 1±0.2 mm from the outer surface side and the inner surface side. At each position, hardness was measured at 5 points each, and their average value was made into a representative value.

In addition, with respect to surface cracks, a welding experiment of a column-through diaphragm welded joint was performed using the obtained rectangular steel pipe. Welding conditions were made into welding wire JISZ3312 GJ59JA1UC3M1T, heat input conditions 40 kJ/cm or less, and an interpass temperature 350 degreeC or less, and it performed by 7 layers and 9 passes. After welding, the presence or absence of crack generation of the steel material surface was determined in the periphery of a welding part.

These results are shown in Table 2.

From Table 2, all of the examples of the present invention were excellent in toughness, and surface cracks did not occur.

From the above, by setting the angle 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 addition, although this embodiment demonstrated the embodiment in which the roll-formed steel sheet is electric resistance-welded to make an electric resistance steel pipe, a seamless steel pipe may be used for shaping|molding into a cylindrical shape.

1: steel rod
2: Leveler
3: Cage roll group
4: pin pass roll group
5: Squeeze Roll
6: welding machine
7: electric resistance steel pipe
8: sizing roll group
9: Angular type roll group
10: square steel pipe
11: Girder
12 : cow beam
13: diaphragm
14 : stud
H ₁ : side length (short side)
H ₂ : side length (long side)
θ: On _{a straight line drawn from the central position of H 1 toward the inside of the steel pipe, a point offset by 1/2 (H 2} -H ₁ ) from the center of the rectangular steel pipe in the long side direction is taken as the offset point, and the rectangular steel pipe from the offset point A central angle determined by a straight line drawn to the center of each part of
t: pipe thickness

Claims (7)

In the rectangular steel pipe having a flat plate portion and a corner portion, the flat portion has a yield strength of 385 MPa or more, a tensile strength of 520 MPa or more, and a yield ratio of 0.90 or less,
The Vickers hardness of each part is greater than the Vickers hardness on the inner surface side of each part than the Vickers hardness on the outer surface side of each part, the Vickers hardness on the outer surface side of the leg part is 280 HV or less, and the Vickers hardness on the outer surface side of the leg part and The difference in Vickers hardness on the inner surface side of each part is 80 HV or less,
A rectangular steel pipe in which _{the Charpy absorbed energy vE 0} at 0°C on the outer surface side of each part is 70J or more. According to claim 1,
In 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%,
It contains N: 0.010% or less, and the balance has a component composition consisting of Fe and unavoidable impurities,
In addition, the steel structure at the position of t/4 (t is the tube thickness) from the tube surface contains more than 30% of ferrite and 10% or more of bainite by volume ratio, and the volume ratio of ferrite and bainite is A square steel pipe whose total is 70% or more and 95% or less, and the balance consists of one or two or more types selected from pearlite, martensite, and austenite. 3. The method of claim 2,
Further, in mass%, Nb: 0.005 to 0.150%,
Ti: 0.005 to 0.150%,
V: A rectangular steel pipe containing one or two or more selected from among 0.005 to 0.150%. 4. The method of claim 2 or 3,
Further, by mass%, 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 0.010%,
B: A rectangular steel pipe containing one or two or more selected from 0.0003 to 0.010%. The method for manufacturing a rectangular steel pipe according to claim 1, wherein the rectangular steel pipe is formed into a cylindrical shape and then subjected to rectangular shape-forming. In a cross section perpendicular to the pipe axis direction, the lengths of adjacent sides are H ₁ (mm) and H ₂ (mm) (H ₁ ≤ H ₂ ), respectively, and the inside of the steel pipe is removed from the center position of _{H 1} and H _{2 .} When the intersection of straight lines drawn toward each other is taken as the central portion of the rectangular steel pipe, on a straight line drawn from the central position of _{H 1 toward the inside of the steel pipe, 1/2 (H 2} -H ₁ ) from the central portion of the rectangular steel pipe in the long side direction The point offset by the offset is the offset point, and the central angle θ formed by the straight line drawn from the offset point to the center of each part of the square steel pipe and the straight line drawn from the offset point toward the arc part of each part or the flat plate part of the square steel pipe is expressed by the following equation (1) A method of manufacturing a prismatic steel pipe that meets

step,
H ₁ : side length (short side) (mm)
H ₂ : side length (long side) (mm)
t: pipe thickness (mm)
am. After heating the steel material having the component composition according to any one of claims 2 to 4 to a heating temperature: 1100 to 1300°C, rough rolling is performed to a rough rolling end temperature: 850 to 1150°C, and finish rolling is performed. Finishing temperature: Finish rolling is performed at 750 to 850°C, and the total reduction ratio at 930°C or less in both rough rolling and finish rolling is 65% or more, and then cooling is started to the plate thickness center temperature. Cooling stop temperature at a cooling rate such that the average cooling rate from to cooling stop is 10 to 30°C/s, cooling to 450 to 650°C, winding, and then allowing to cool, and then forming into a cylindrical shape by roll forming Then, in the angle forming process in which the roll-formed steel sheet is electric resistance welded to make electric resistance steel pipe, and then the electric resistance resistance steel pipe is square-formed into a rectangular steel pipe, in a cross section perpendicular to the pipe axis direction, the lengths of adjacent sides are When H ₁ (mm) and H ₂ (mm) (H ₁ ≤ H ₂ ), respectively, and the intersection point at which straight lines drawn from the central position of _{H 1} and H _{2 toward the inside of the steel pipe intersect with each other,} On _{a straight line drawn from the central position of H 1 toward the inside of the steel pipe, a point offset by 1/2 (H 2} -H ₁ ) from the center of the rectangular steel pipe in the long side direction is taken as the offset point, A method for manufacturing a rectangular steel pipe, wherein a central angle θ formed between a straight line drawn to the center of each part and a straight line drawn from the offset point toward the arc part of each part or the flat part of the rectangular steel pipe satisfies the following formula (1).

step,
H ₁ : side length (short side) (mm)
H ₂ : side length (long side) (mm)
t: pipe thickness (mm)
am. A building structure using the rectangular steel pipe according to any one of claims 1 to 4.

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