KR102400687B1 - Manufacturing method of each steel pipe and each steel pipe - Google Patents

Abstract

An object of the present invention is to provide a method for conveniently manufacturing a square steel pipe having excellent dimensional accuracy of the radius of curvature of each part, and a square steel pipe. A square steel pipe characterized in that a Vickers hardness in each part of the square steel pipe satisfies a predetermined equation, and a radius of curvature of the respective part satisfies a predetermined formula.

Description

Manufacturing method of each steel pipe and each steel pipe

The present invention relates to a technique for obtaining a square steel pipe having excellent dimensional accuracy by controlling each part of the steel pipe to a target value in a method for manufacturing a square steel pipe from a steel pipe.

Conventionally, square steel pipes for construction have been manufactured by a method (BCP method) in which a thick steel sheet is press-formed into a square shape with a press machine and then welded. On the other hand, in recent years, instead of the low productivity BCP method, in order to reduce the cost, a square steel pipe is produced by roll forming, then welding, and then square forming to obtain a square steel tube (BCR method). Attempts to manufacture were made. In addition, since the dimensions of each steel pipe are determined according to each floor of the building, in order to unify the building materials into the BCR (“BCR” is a registered trademark of the Japan Steel Federation) for the recently increasing high-rise buildings, Thickening is required.

For each steel pipe, a predetermined value is required for each size for the radius of curvature (corner R) of each part. In addition, from the viewpoint of earthquake resistance and prevention of local buckling, a square steel pipe with high dimensional accuracy is required. When a steel pipe is manufactured from a steel pipe by the roll forming method, the steel pipe is passed through a group of rolls of multiple stages, the roll is pressed against the outer surface side of the steel pipe from all directions to straighten the cylindrical part, and the square steel pipe is molded into a square or rectangular cross-sectional shape. are getting However, if the molding conditions are not set properly, the problem of a large radius of curvature of the corners of the four corners of the steel pipe, the problem of dimensional defects such that the straight part included in the side of each steel pipe becomes uneven, and the surplus There was a problem of embrittlement of each part due to work hardening.

In response to the problem of dimensional accuracy of each steel pipe, in Patent Document 1, the roll caliber is made smaller (concave to convex) as the thickness/outer diameter ratio increases, and the flat portion is formed to be bent within a certain range. A method for manufacturing a square steel pipe with high dimensional accuracy is disclosed.

In addition, in Patent Document 2, it is said that a rectangular steel pipe having a shape according to the use can be manufactured by controlling the set press-fitting rate determined by the outer diameter, thickness, and maximum caliber height of the forming roll of a predetermined element pipe. there is.

Japanese Patent Laid-Open No. 4-224023 Japanese Patent Publication No. 3197661

Like Patent Documents 1 and 2, in order to improve the dimensional accuracy of a cross-sectional shape, it is effective to control a roll caliber or a set press-fit ratio. However, since the roll caliber and the set press-fit ratio have an effect on the deformation of the straight part included in the edge, the effect on the radius of curvature of each part is small. In particular, in the case of a thick rectangular steel pipe, since the rigidity of the cross section increases, the corners become more and more difficult to bend and the radius of curvature of the corners exceeds the target value. In order to obtain the target radius of curvature, it is necessary to increase the press-fitting amount of the roll with respect to the straight portion included in the side of each steel pipe. However, when the press-fitting amount of the roll is increased, since the linear portion included in the edge is largely pushed as the center, the straight portion included in the edge becomes concave as a result, resulting in poor dimensionality.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a method and a steel tube capable of easily manufacturing a square steel pipe having excellent dimensional accuracy of the radius of curvature of each part.

MEANS TO SOLVE THE PROBLEM The present inventors earnestly examined about the various factors which affect the curvature radius of the leg part in a square steel pipe. As a result, the following was found.

(1) When forming a steel pipe, which is a cylindrical element pipe, into a square steel pipe, as described above, by passing the steel pipe through a plurality of roll groups, the square shape is gradually performed from a cylindrical shape to a square shape. Here, in the angle forming, bending restoration of a straight part included in the side, bending of the leg part, and shrinkage deformation in the circumferential direction occur. In particular, around the corners, the rolls hardly come into contact, and the corner forming is completed. That is, in the angle forming, the corners are projected by free deformation, so that the corners are formed from the steel pipe.

(2) The corner part is formed from the surplus of the circumferential shrinkage in square shaping with respect to the circumferential length of the steel pipe immediately before square shaping. In the present invention, when the ratio of the circumferential length of the steel pipe immediately before square forming to the circumferential length of the steel pipe immediately after square forming falls within a specific range, a square steel pipe having excellent dimensional accuracy of the radius of curvature of each part can be obtained.

(3) The ratio of the circumferential length of the steel pipe immediately before angular forming to the perimeter of the steel pipe immediately after angular forming can be expressed by the relational expression between the thickness of each steel pipe and the distance between the outer surfaces of opposite sides. In addition, by controlling the gap of the sizing stand just before square forming, the ratio of the circumferential length of the steel pipe immediately before square shaping to the circumferential length of the steel pipe immediately after square shaping can be controlled within a specific range.

This invention is based on the said knowledge, The characteristic is as follows.

[1] Vickers hardness at each part of each steel pipe satisfies the following formula (2), and

A square steel pipe in which the radius of curvature of each part satisfies the following formula (3).

10 ≤ HV _I － HV _O ≤ 80 … (2)

R _max − R _min ≤ 0.25 × t … (3)

In addition, in formulas (2) and (3),

HV _O : Vickers hardness (HV) at a position in the range of 1 ± 0.2 mm from the outer surface side of the steel pipe in each part of each steel pipe

HV _I : Vickers hardness (HV) in the range of 1 ± 0.2 mm from the inner side of the steel pipe in each part of each steel pipe

R _max : The maximum value of the radius of curvature of each part in an arbitrary vertical section with respect to the axial direction of the steel pipe (mm)

R _min : Minimum value of the radius of curvature of each part in an arbitrary vertical section with respect to the axial direction of the steel pipe (mm)

t: thickness (mm)

am.

[2] The square steel pipe according to [1], wherein the Vickers hardness at each part of the square steel pipe satisfies the following formula (4).

290 × t/H - 3.2 ≤ HV _I - HV _O ≤ 579 × t/H + 33.7 … (4)

[3] The square steel pipe according to [1] or [2], wherein the thickness is 25 to 30 mm.

[4] A steel sheet as a raw material is roll-formed, and then the roll-formed steel sheet is electric resistance welded to make an electric resistance welded steel pipe, and then the electric resistance welded steel pipe is formed into a plurality of sizing stands, followed by angular forming with a plurality of angular forming stands As a method of manufacturing each steel pipe,

A method for manufacturing a square steel pipe in which the gap of a sizing stand immediately before square forming is controlled so that the following formula (1) is satisfied.

C _IN /C _OUT ≥ 0.50 × t/H + 0.99 … (One)

In addition, in formula (1),

C _IN : Circumference length of the steel pipe at the entrance of the first angular forming stand (mm)

C _OUT : Circumference length of the steel pipe at the exit side of the angular forming stand at the final stage (mm)

t: thickness (mm)

H: Distance between the outer surfaces of opposite sides (mm)

am.

[5] The method for manufacturing a square steel pipe according to [4], wherein the thickness is 25 to 30 mm.

ADVANTAGE OF THE INVENTION According to this invention, a square steel pipe with high dimensional accuracy can be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram which shows an example of the manufacturing facility of an electric resistance resistance steel pipe.
Fig. 2 is a schematic diagram showing the forming process of each steel pipe.
3 is a schematic diagram showing a cross section of each steel pipe.
Fig. 4 shows the ratio of the circumferential length C _IN of the steel pipe at the entrance of the first-stage square forming stand to the circumferential length C _OUT of the steel pipe at the exit side of the square forming stand at the last stage, and the thickness (t) of the opposite side; It is a graph which shows the relationship of the ratio of the distance (H) between outer surfaces.

The manufacturing method of the square steel pipe of this invention is demonstrated based on drawing.

First, FIG. 1 is a schematic diagram showing an example of a manufacturing facility for an electric resistance resistance steel pipe. The steel strip 1, which is the raw material of the electric resistance welded steel pipe, is subjected to vertical straightening by, for example, a leveler 2, and is intermediately formed by a cage roll group 3 consisting of a plurality of rolls to form an open tube, and is then made into a plurality of rolls. It is finish-molded by the formed pin pass roll group 4 . After finish forming, the width end of the steel strip 1 is electrically resistance-welded with a welding machine 6 to obtain an electric resistance welded steel pipe 7 , while pressing with a squeeze roll 5 . In addition, in this invention, the manufacturing facility of the electric resistance resistance steel pipe 7 is not limited to the pipe making process like FIG.

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 (multi-stage sizing stand) 8 composed of a plurality of rolls, and then a angular forming roll group (multi-stage sizing stand) consisting of a plurality of rolls. By the square forming stand) (9), it is sequentially molded into the same shape as R1, R2, R3 to form a square steel pipe (10). In addition, the number of stands in particular of the sizing roll group 8 and the square forming roll group 9 is not restrict|limited. Moreover, as for the caliber curvature of the sizing roll group 8 or the square forming roll group 9, 1 condition is preferable.

3 is a cross-sectional view showing a cross section of each steel pipe 10 perpendicular to the pipe axis direction. As shown in FIG. 3 , when the positions of 45°, 135°, 225°, and 315° are respectively taken as the center of the welded part (seam) of the steel pipe as 0°, the radius of curvature of each part is shown in FIG. As shown in , it refers to the radius of curvature at the intersection of the line L forming 45° with the adjacent side from the center of the tube as the starting point and the outside of each part. The radius of curvature of each part is determined by a line drawn toward the connection point (A, A') of the flat part (the straight part of the side in the distance between the outer surfaces of the opposing sides) and the circular arc part, centered on the L. Let it be the radius of the sector where the central angle is 65°. In addition, as a method of calculating the radius of curvature, for example, the radius of curvature is calculated using the sine theorem from the measurement result of the distance relationship of three points (the intersection of the outer side of each part, and the two points that are the connection points of the flat part and the arc part) There are, but is not limited to, a method of measuring a radius of curvature from a radial gauge that matches well with a corner portion within the region of the three points.

For each steel pipe obtained by the BCR method, the radius of curvature of each part is defined as (2.5 ± 0.5) x t (t: thickness). That is, the difference between the maximum value R _max and the minimum value R _min of the radius of curvature of each part in a cross section perpendicular to the tube axis direction is allowed up to a thickness equivalent to the maximum.

However, when the difference between the maximum value R _max and the minimum value R _min of the radius of curvature of each part is a value of about thickness, a large dimensional error occurs. Then, since maintenance is required at the time of attaching a backing metal, a bad effect arises in workability. Moreover, when a corner|angular part with an extremely small radius of curvature exists, the flat part adjacent to a corner|angular part becomes long. As a result, sufficient dimensional accuracy cannot be obtained, and it becomes a cause of occurrence of local buckling.

As a result of intensive studies by the present inventors, if the difference between the maximum value R _max and the minimum value R _min of the radius of curvature in an arbitrary vertical section is 25% or less of the thickness, it is found that the workability of the weld joint and the internal buckling property are not affected. paid

Next, the present inventors intensively studied a method for manufacturing a steel pipe in which the difference between the maximum value R _max and the minimum value R _min of the radius of curvature in an arbitrary vertical section satisfies 25% or less of the thickness.

As described above, especially in the case of a thick rectangular steel pipe, since the rigidity of the cross section increases, the corners become more and more difficult to bend and deform, and the radius of curvature of the corners exceeds the target value. It is thought that the increase in the stiffness of this cross-section is caused by an increase in the thickness t or a decrease in the distance H between the outer surfaces of opposite sides.

When a steel pipe, which is a cylindrical element pipe, is formed into a square steel pipe, as described above, the steel pipe is passed through a plurality of roll groups to gradually form a cylindrical shape into a square shape. In such an angle forming, bending restoration of a straight part included in the side, bending of the leg part, and shrinkage deformation in the circumferential direction occur. The present inventors noted that in the angular forming, the angular forming was completed with little roll contact, particularly around the corners.

That is, in the angle forming, the corner portion is formed by being projected by free deformation. The present inventors consider that each part is formed from the surplus of the circumferential shrinkage in square forming with respect to the circumferential length of the steel pipe immediately before square forming, and the circumferential length of the steel pipe immediately before square forming and the circumference of the steel pipe immediately after square forming It was decided to examine the relationship of the length and the relationship between the thickness (t) and the distance (H) between the outer surfaces of the opposite sides.

First, for each steel pipe obtained by the BCR method, a steel pipe in which the difference between the maximum value R _max and the minimum value R _min of the radius of curvature in an arbitrary section satisfies 25% or less of the thickness is passed (○), and the difference exceeds 25% was evaluated as fail (×). Next, for each steel pipe evaluated, the perimeter of the steel pipe immediately before angular forming (the perimeter of the steel pipe at the entrance of the first angular forming stand, hereinafter referred to as “C _IN ”) and the angular forming The ratio of the perimeter of the steel pipe immediately after (the perimeter of the steel pipe at the exit side of the angular forming stand at the end, hereinafter referred to as “C _OUT ”), and the thickness (t) and the distance between the outer surfaces of the opposite sides (H) The relationship between the ratios was reviewed.

4 shows the results. As shown in FIG. 4 , it was found that, when C _IN /C _OUT satisfies the following formula (1), a square steel pipe having excellent dimensional accuracy of the radius of curvature of each part can be easily obtained.

C _IN /C _OUT ≥ 0.50 × t/H + 0.99 … (One)

In addition, in formula (1),

C _IN : Circumference length of the steel pipe at the entrance of the first angular forming stand (mm)

C _OUT : Circumference length of the steel pipe at the exit side of the angular forming stand at the final stage (mm)

t: thickness (mm)

H: Distance between the outer surfaces of opposite sides (mm)

am.

The present inventors studied a method of controlling C _IN /C _OUT that satisfies the formula (1). As a result, it was found that by controlling the gap of the sizing stand just before square forming, a square steel pipe satisfying Equation (1) could be obtained.

In the present invention, with respect to the gap between the concave portions of the caliber roll, the sizing immediately before angular forming is such that the difference between the gap of the sizing stand immediately before angular forming and the gap of the angular forming stand is 70 t/H to 180 t/H (mm). It is desirable to adjust the gap of the stand. If it is less than 70 t/H, it is impossible to obtain a sufficient length for C _IN to satisfy Equation (1), and if it exceeds 180 t/H, the extrusion amount of the square forming stand becomes large, so that problems such as scratches on the outer surface occur.

In addition, the position for measuring the circumferential length C _IN of the steel pipe at the entrance of the first angular forming stand may be measured, for example, by measuring an intermediate position between the sizing stand immediately before the angular forming and the first angular forming stand. In addition, as for the position to measure the circumferential length C _OUT of the steel pipe at the exit side of the square forming stand at the final stage, it is sufficient to measure the circumferential length of the steel pipe at a position 1 m behind the roll of the final square forming stand. In addition, the measuring method of the circumferential length includes, but is not limited to, a method of winding a tape measure around a steel pipe.

Next, each steel pipe obtained by the manufacturing method of the present invention will be described.

The steel pipe of the present invention is characterized in that the Vickers hardness at each part of the steel pipe satisfies the following formula (2), and the radius of curvature of the respective part satisfies the following formula (3).

10 ≤ HV _I － HV _O ≤ 80 … (2)

R _max − R _min ≤ 0.25 × t … (3)

In addition, in formulas (2) and (3),

HV _O : Vickers hardness (HV) at a position in the range of 1 ± 0.2 mm from the outer surface side of the steel pipe in each part of each steel pipe

HV _I : Vickers hardness (HV) in the range of 1 ± 0.2 mm from the inner side of the steel pipe in each part of each steel pipe

R _max : The maximum value of the radius of curvature of each part in an arbitrary vertical section with respect to the axial direction of the steel pipe (mm)

R _min : Minimum value of the radius of curvature of each part in an arbitrary vertical section with respect to the axial direction of the steel pipe (mm)

t: thickness (mm)

am.

A square steel pipe to be formed by the BCR method is formed into a cylindrical shape once from a steel plate, and then is formed into a square shape. In such a BCR method, not only bending deformation in the circumferential direction but also distortion in the longitudinal direction due to shrinkage deformation occurs. 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. When the difference of the Vickers hardness between the outer surface side of the steel pipe and the inner surface side of the steel pipe is less than 10 HV, since work hardening on the outer surface side is progressing, the ductility of each part is remarkably deteriorated. When the difference in Vickers hardness is more than 80 HV, the degree of workability on the inner surface side of each part advances, and since the residual stress on the inner surface of each part becomes remarkable, it exerts a bad influence on the crack etc. of plating performed by post-processing. Preferably, the difference of Vickers hardness is 30-60 HV.

Further, as described above, in the steel pipe of the present invention, the difference between the maximum value R _max and the minimum value R _min of the radius of curvature in an arbitrary vertical cross section satisfies 25% or less of the thickness. That is, the square steel pipe of the present invention is characterized in that the radius of curvature of each part satisfies the above formula (3). By satisfying the above formula (3), there is no influence on the workability of the weld joint or the local buckling property.

Moreover, in this invention, it is preferable that the Vickers hardness in each part of a square steel pipe satisfy|fills following formula (4).

290 × t/H - 3.2 ≤ HV _I - HV _O ≤ 579 × t/H + 33.7 … (4)

As described above, in the BCR method, not only bending deformation in the circumferential direction, but also distortion in the longitudinal direction due to shrinkage deformation occurs. 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 get bigger At this time, when the thickness of each steel pipe increases, the rigidity increases, and the distortion required for shaping|molding increases. In addition, as the ratio t/H of the distance between the thickness of each steel pipe and the outer surface of the opposite side increases, the forming distortion due to the shrinkage deformation increases, and the hardness of the entire thickness of the steel pipe increases. Therefore, in a square steel pipe with a large t/H, work hardening of each part becomes more remarkable. For this reason, the present inventors considered that there is a relationship between the hardness of each part and t/H of a square steel pipe. As a result of the present inventors earnestly examining, by satisfying said Formula (4), the influence (ductility deterioration and the toe crack of a weld part) of work hardening of each part can be suppressed. When the difference in Vickers hardness between the outer surface side of the steel pipe and the inner surface side of the steel pipe is less than 290 × t/H - 3.2 HV, since work hardening on the outer surface side is in progress, the ductility of each part is significantly deteriorated. When the difference in Vickers hardness is more than 579 × t/H + 33.7 HV, the degree of workability on the inner surface side of each part advances, and the residual stress on the inner surface of each part becomes significant, so it adversely affects the cracking of plating performed by post-treatment .

In addition, as shown in FIG. 3, each part in the square steel pipe of this invention is centered on the line L which forms 45 degrees with the adjacent side with the center of a pipe|tube as a starting point, and a flat part and an arc part It refers to the range of the radius of the sector where the central angle determined by the line drawn toward the connection point (A, A') is 65°.

In this invention, it is preferable that plate|board thickness t is 25-30 mm.

The component composition of the steel pipe in the present invention is not particularly limited, but in terms of mass%, C: 0.04 to 0.50%, Si: 2.0% or less, Mn: 0.3 to 3.0%, P: 0.10% or less, S: 0.050% Hereinafter, Al: 0.005 to 0.10%, N: 0.010% or less is contained, and it is preferable that it is a component composition which consists of remainder Fe and an unavoidable impurity. Below, the reason for limitation of each component is described.

C: 0.04 to 0.50%

C is an element contributing to the formation of pearlite, which is one of the second phases, while increasing the strength of the steel sheet by solid solution strengthening. C is an element contributing to the formation of a hard phase as well as contributing to the formation of martensite by increasing quenching properties and contributing to the stabilization of austenite. Therefore, in order to ensure desired tensile properties, toughness, and desired steel sheet structure, it is preferable to contain 0.04% or more. On the other hand, when it contains more than 0.50%, the ratio of the hard phase increases and toughness decreases, and a martensitic structure is generated during welding of square steel pipes (for example, when welding square steel pipes) to prevent weld cracking. There is a possibility that it may become a cause. For this reason, it is preferable that it is the range of 0.04 to 0.50 %, and, as for C, 0.07 to 0.20 % is more preferable. More preferably, it is more than 0.12 % and 0.25 % or less.

Si: 2.0% or less

Si is an element that contributes to an increase in the strength of the steel sheet by solid solution strengthening, and may be contained as necessary in order to secure a desired strength of the steel sheet. In order to acquire such an effect, it is preferable to contain 0.01 % or more of Si. On the other hand, when Si content exceeds 2.0 %, weldability will deteriorate. For this reason, it is preferable to set it as 2.0 % or less, and, as for Si content, it is more preferable to set it as 0.5 % or less. Moreover, when it contains 0.4% or more, it becomes easy to form the payalite called red scale on the surface of a steel plate, and the external appearance property of the surface falls in many cases. For this reason, when it contains, it is more preferable to set it as less than 0.4 %. Moreover, especially when Si is not added, Si is an unavoidable impurity, and the level is less than 0.01 %.

Mn: 0.3 to 3.0%

Mn is an element that increases the strength of a steel sheet through solid solution strengthening, and is an element that contributes to the refinement of the structure by lowering the ferrite transformation initiation temperature. When the content is less than 0.3%, the ferrite transformation initiation temperature rises, and the structure tends to become excessively coarse. Moreover, in order to ensure the desired steel plate strength and structure, it is preferable to contain 0.3% or more of Mn. However, when Mn content exceeds 3.0 %, weldability will deteriorate. For this reason, it is preferable to make Mn content into 0.3 to 3.0 %. Moreover, when it contains exceeding 2.0 %, the hardness of a center segregation part rises, and there exists a possibility that it may become a cause of the crack at the time of welding on-site of a square steel pipe. For this reason, it is more preferable that Mn is 0.3 to 2.0 %. Most preferably, it is 0.5 to 2.0%.

P: 0.10% or less

P is an element having the action of segregating at the ferrite grain boundary to reduce toughness, and in the present invention, it is preferable to reduce it as much as possible as an impurity. However, since excessive reduction causes a sharp increase in refining cost, it is preferable to set it as 0.002% or more. In addition, up to 0.10 % is allowable. For this reason, it is preferable that P is 0.10 % or less. P is more preferably 0.03% or less, still more preferably 0.025% or less.

S: 0.050% or less

S exists as a sulfide in steel, and exists mainly as MnS if it is the composition range of this invention. Since MnS is thinly stretched in the hot rolling process and adversely affects ductility and toughness, in the present invention, it is preferable to reduce MnS as much as possible. However, since excessive reduction causes a sharp increase in the refining cost, S is preferably set to 0.0002% or more. In addition, up to 0.050 % is allowable. For this reason, it is preferable that S is 0.050 % or less. S is more preferably 0.015%, still more preferably 0.010% or less.

Al: 0.005 to 0.10%

Al is an element which acts as a deoxidizer and has an effect|action which fixes N as AlN. In order to acquire such an effect, it is required to contain 0.005 % or more. If it is less than 0.005 %, deoxidation power is insufficient in the case of no addition of Si, oxide-type inclusions increase, and the cleanliness of a steel plate falls. On the other hand, when it contains more than 0.10%, the amount of dissolved Al increases, so that oxides are formed in the weld zone during long welding of square steel pipes (welding in the production of square steel pipes), especially in the case of welding in the atmosphere. The risk increases, the toughness of the welded square steel pipe decreases, the alumina-based inclusions increase, and the surface properties deteriorate. For this reason, it is preferable that Al is 0.005 to 0.10 %. Al is more preferably 0.01 to 0.06%.

N: 0.010% or less

N is an element having an action of lowering toughness by firmly fixing the movement of dislocations. In this invention, it is preferable to reduce N as an impurity as much as possible, and it is permissible up to 0.010 %. For this reason, it is preferable that N is 0.010 % or less. N is more preferably 0.0080 % or less, still more preferably 0.006 % or less, and most preferably 0.005 % or less.

The above components are the basic component compositions of the steel material of the electric resistance resistance steel pipe in the present invention. In the present invention, in addition to the above components, 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% may be contained.

Nb: 0.005 to 0.150%, Ti: 0.005 to 0.150%, V: 0.005 to 0.150% one or more selected from the group consisting of

Nb, Ti, and V are all 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 needed. In order to obtain such an effect, it is necessary to contain Nb: 0.005 % or more, Ti: 0.005 % or more, and V: 0.005 % or more. On the other hand, excessive content causes an increase in the yield ratio and a decrease in toughness. For this reason, it is preferable that content of Nb, Ti, and V sets it as Nb:0.005 to 0.150%, Ti:0.005 to 0.150%, and to set it as V:0.005 to 0.150%. More preferably, they are Nb:0.008-0.10%, Ti:0.008-0.10%, and V:0.008-0.10%.

In the present invention, in addition to the above components, Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.30%, Ca: 0.001 to 0.010%, B: 0.0005 to You may make it contain 1 type(s) or 2 or more types selected from 0.010 %.

Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.30%

Cr, Mo, Cu, and Ni are elements that increase the strength of steel by solid solution strengthening, and all are elements that enhance hardenability of steel and contribute to stabilization of austenite, so hard martensite and austenite As an element contributing to the formation of , it can be contained as needed. In order to obtain such an effect, it is necessary 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 deterioration of weldability. For this reason, it is preferable that content of Cr, Mo, Cu, and Ni sets it as Cr: 0.01-1.0%, Mo: 0.01-1.0%, Cu: 0.01-0.50%, and Ni: 0.01-0.30%. More preferably, they are Cr:0.1-1.0%, Mo:0.1-1.0%, Cu:0.1-0.50%, and Ni:0.1-0.30%.

Ca: 0.001 to 0.010%

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

B: 0.0005 to 0.010%

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 required to contain 0.0005% or more of B. However, when B content exceeds 0.010 %, a yield ratio will rise. For this reason, it is preferable to make B content into 0.0005 to 0.010 %. More preferably, the B content is 0.0005 to 0.0050%.

The balance consists of Fe and unavoidable impurities.

Example

A hot-rolled steel sheet having the composition shown in Table 1 is continuously formed into an open tube with an elliptical cross section by a cage roll group and a pin pass roll group, and then the end surface facing the open tube is heated to a melting point or higher by high-frequency induction heating or high-frequency resistance heating, , it was press-contacted with a squeeze roll to obtain an electric resistance welded steel pipe. The obtained electric resistance welded steel pipe was formed into a cylindrical shape by a group of sizing rolls of 2 stands, and then square forming was performed with a group of square forming rolls of 4 stands to obtain various BCR295 rectangular steel pipes as shown in Table 2. In addition, with respect to the circumferential length C _IN of the steel pipe at the entrance to the first angular forming stand, the intermediate position between the sizing stand and the first angular forming stand immediately before angular forming was measured with a tape measure, and the steel pipe circumferential length C _IN . Regarding the circumferential length C _OUT of the steel pipe at the exit side of the square forming stand at the final stage, a position 1 m from just below the roll of the fourth stand of the square forming roll group was measured with a tape measure, and the steel pipe circumferential length C _OUT was measured.

In addition, when the circumferential lengths C _IN and C _OUT of the steel pipe are obtained, the difference between the gap between the gap of the sizing stand immediately before angular forming and the recess of the caliber roll of the first angular forming stand is the thickness (t) of the product A coefficient G (mm) divided by the ratio t/H of the distance (H) between the outer surfaces of opposite sides was calculated.

For each steel pipe, 10 cross-sections perpendicular to the pipe axis direction were arbitrarily cut out, and the radius of curvature of each part at the four corners of the vertical cross-section was measured. A radial gauge was used for the measurement of the radius of curvature of each part, and specifically, as shown in FIG. 3 , the distance at the intersection of the outside of each part was measured as the radius of curvature. As a result of measurement at 10 points of an arbitrary vertical cross section, if the difference between the maximum value R _max and the minimum value R _min of the radius of curvature was 25% or less of the thickness, it was evaluated as ○ in all the 10 cross sections. On the other hand, if the difference between the maximum value R _max and the minimum value R _min was more than 25% of the thickness even at one of the 10 cross sections, it was evaluated as x.

Further, for each of the various steel pipes, the Vickers hardness of each part on the inner surface side of the steel pipe and the Vickers hardness of each part on the outer surface side of the steel pipe were measured, and the difference was determined. Specifically, in a micro Vickers hardness test (JIS Z2244:2009), the position 1 mm inside from each part was made into the test force of 9.8N.

A result is shown in Table 2.

From the result of Table 2, all of the invention examples are excellent in the dimensional accuracy of each part.

1: steel strip
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: Angled roll group
10: each steel pipe
R1 ~ R3 : (steel pipe) shape

Claims (14)

In terms of mass% as a component composition, C: 0.04 to 0.50%, Si: 2.0% or less, Mn: 0.3 to 3.0%, P: 0.10% or less, S: 0.050% or less, Al: 0.005 to 0.10%, N: 0.010% The Vickers hardness in each part of each steel pipe containing the following, the remainder Fe and unavoidable impurities, satisfies the following formula (2),
The radius of curvature of each part satisfies the following formula (3),
A square steel pipe in which the Vickers hardness at each part of the square steel pipe satisfies the following formula (4).
10 ≤ HV _I － HV _O ≤ 80 … (2)
R _max − R _min ≤ 0.25 × t … (3)
290 × t/H - 3.2 ≤ HV _I - HV _O ≤ 579 × t/H + 33.7 … (4)
In addition, in Formula (2), Formula (3), Formula (4),
HV _O : Vickers hardness (HV) at a position in the range of 1 ± 0.2 mm from the outer surface side of the steel pipe in each part of each steel pipe
HV _I : Vickers hardness (HV) in the range of 1 ± 0.2 mm from the inner side of the steel pipe in each part of each steel pipe
R _max : The maximum value of the radius of curvature of each part in an arbitrary vertical section with respect to the axial direction of the steel pipe (mm)
R _min : Minimum value of the radius of curvature of each part in an arbitrary vertical section with respect to the axial direction of the steel pipe (mm)
t: thickness (mm)
H: Distance between the outer surfaces of opposite sides (mm)
am. delete The method of claim 1,
Each steel pipe having a thickness of 25 to 30 mm. The method of claim 1,
Further, a square steel pipe containing, 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. The method of claim 3,
Further, a square steel pipe containing, 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%. The method of claim 1,
In addition, 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: 0.001 to 0.010%, B: 0.0005 to 0.010% Each steel pipe containing 1 type or 2 or more types. 4. The method of claim 3,
In addition, 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: 0.001 to 0.010%, B: 0.0005 to 0.010% Each steel pipe containing 1 type or 2 or more types. 5. The method of claim 4,
In addition, 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: 0.001 to 0.010%, B: 0.0005 to 0.010% Each steel pipe containing 1 type or 2 or more types. 6. The method of claim 5,
In addition, 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: 0.001 to 0.010%, B: 0.0005 to 0.010% Each steel pipe containing 1 type or 2 or more types. A steel sheet as a raw material is roll-formed, and then the roll-formed steel sheet is electric resistance welded to make an electric resistance steel pipe, and then the electric resistance resistance steel pipe is formed with a plurality of sizing stands, and then squarely formed with a plurality of angle forming stands to form a square steel pipe As a method of manufacturing,
The component composition of each steel pipe is, in mass%, C: 0.04 to 0.50%, Si: 2.0% or less, Mn: 0.3 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 consists of Fe and unavoidable impurities, the following formula (1) is satisfied, and the difference between the gap of the sizing stand immediately before the square forming and the gap of the square forming stand is 70 t/ A method for manufacturing a square steel pipe in which the gap of a sizing stand just before square forming is controlled so that it becomes H to 180 t/H (mm).
C _IN /C _OUT ≥ 0.50 × t/H + 0.99 … (One)
In addition,
C _IN : Circumference length of the steel pipe at the entrance of the first angular forming stand (mm)
C _OUT : Circumference length of the steel pipe at the exit side of the angular forming stand at the final stage (mm)
t: thickness (mm)
H: Distance between the outer surfaces of opposite sides (mm)
am. 11. The method of claim 10,
A method for manufacturing a square steel pipe having a thickness of 25 to 30 mm. 12. The method according to claim 10 or 11,
Further, a method for manufacturing a steel pipe comprising, in mass%, 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%. 12. The method according to claim 10 or 11,
In addition, 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: 0.001 to 0.010%, B: 0.0005 to 0.010% A method for manufacturing a square steel pipe containing one or two or more types. 13. The method of claim 12,
In addition, 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: 0.001 to 0.010%, B: 0.0005 to 0.010% A method for manufacturing a square steel pipe containing one or two or more types.

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