WO2020039980A1

WO2020039980A1 - Square steel pipe, manufacturing method thereof, and building structure

Info

Publication number: WO2020039980A1
Application number: PCT/JP2019/031668
Authority: WO
Inventors: 晃英松本; 昌士松本; 井手　信介; 岡部　能知
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2018-08-23
Filing date: 2019-08-09
Publication date: 2020-02-27
Also published as: KR20210032494A; CN112601831B; JPWO2020039980A1; JP6693606B1; CN112601831A; TWI707958B; TW202014531A; KR102498954B1

Abstract

A square steel pipe and a manufacturing method thereof are provided. This square steel pipe has flat sections and corner sections, and the component composition thereof contains, in mass%, 0.04-0.50% C, no more than 2.0% Si, 0.5-3.0% Mn, no more than 0.10% P, no more than 0.05% S, 0.005-0.10% Al, and no more than 0.010% N, the remainder being Fe and unavoidable impurities. The steel material at the position 1/4t of the plate thickness t from the pipe outer surface is, by volume ratio, greater than 30% ferrite and greater than or equal to 10% bainite, the total of the ferrite and bainite is 70-95% of the total steel material at the position 1/4t of the plate thickness t from the pipe outer surface, and the remainder is at least one selected from perlite, martensite and austenite. When the region that is surrounded by the boundary where the orientation difference between adjacent crystals is greater than or equal to 15° is crystal grains, the average circle equivalent diameter of the crystal grains is less than 7.0 μm, and the total of crystal grains with a circle equivalent diameter greater than or equal to 40.0 μm is less than or equal to 30%, by volume ratio, of the total steel material at the position 1/4t, and the yield ratio YRf of the flat sections and the YRc of the corner sections satisfy formula (1). (1) ... YRc-YRf ≤ 0.09

Description

Square steel pipe, method for manufacturing the same, and building structure

{Circle over (1)} The present invention relates to a rectangular steel pipe excellent in strength, deformation performance and toughness, and a method for producing the same, and a building structure using the rectangular steel pipe, which is suitably used particularly for building structural members of large buildings.

In recent years, for example, building structural members used for large buildings (hereinafter, referred to as buildings) such as factories, warehouses, and commercial facilities are increasing in strength in order to reduce construction costs by reducing weight. In particular, in a square steel pipe (square column) having a flat portion and a corner portion used as a column material of a building, mechanical properties such that the flat portion has a yield strength of 385 MPa or more and the flat portion has a tensile strength of 520 MPa or more are required. . At the same time, from the viewpoint of earthquake resistance, it is also required to have high plastic deformability and excellent toughness.

The square steel pipe is generally manufactured by using a hot-rolled steel sheet (hot-rolled steel strip) or a thick steel sheet as a material, and cold-forming this material. As a method of cold forming, there is a method of cold press bending or a method of cold roll forming.

BACKGROUND ART A square steel pipe manufactured by roll forming a material (hereinafter, also referred to as a roll-formed square steel pipe) is formed by cold-rolling a hot-rolled steel sheet into a cylindrical open pipe, and abutting a portion of the pipe with an electric joint. Stitch weld. Thereafter, the cylindrical open pipe (round steel pipe) is drawn down by several percent in the pipe axis direction by rolls arranged on the upper, lower, left and right sides of the open pipe, and is subsequently formed into a square to produce a square steel pipe. . On the other hand, a square steel pipe manufactured by press-bending a material (hereinafter sometimes referred to as a press-formed square steel pipe) is formed by cold bending a thick steel plate to form a square cross section (square shape). ) Or a U-shape (U-shape) and joining them by submerged arc welding.

(4) The method of manufacturing a roll-formed square steel pipe has the advantages of higher productivity and a shorter manufacturing time than the method of manufacturing a press-formed square steel pipe. However, in the case of a press-formed rectangular steel pipe, the flat part is not cold-formed and only the corner is work-hardened, whereas in the case of a roll-formed rectangular steel pipe, the pipe is formed around the entire circumference of the steel pipe, especially when cold-formed into a cylindrical shape. Large working strain is introduced in the axial direction. For this reason, the roll-formed square steel pipe has a problem that the yield ratio in the tube axis direction is high and the toughness is low not only at the corners but also at the flat portions.

Furthermore, in a roll-formed square steel pipe, as the plate thickness increases, the work hardening during roll forming increases, so that the yield ratio increases and the toughness further decreases. Therefore, in particular, when manufacturing a thick-walled roll-formed square steel pipe, it is necessary to select a material that can withstand an increase in yield ratio and a decrease in toughness due to roll-forming.

に対し In response to such demands, for example, Patent Document 1 proposes a square steel pipe having a bainite structure area fraction of 40% or more in the microstructure of a flat plate portion.

Patent Literature 2 proposes a square steel pipe having a steel composition and cleanliness within a predetermined range and having excellent weldability and plastic deformation ability of a cold-worked portion.

Patent Document 3 proposes a square steel pipe having a low yield ratio and a high toughness by subjecting a whole pipe to strain relief annealing after pipe forming by cold forming.

Japanese Patent No. 5385760 Japanese Patent No. 4611250 Japanese Patent No. 4995771

However, the techniques described in

Patent Documents

1 and 2 are based on the premise that a rectangular steel pipe is manufactured by press bending. Therefore, when applying the techniques described in

Patent Literatures

1 and 2 to a roll-formed rectangular steel pipe whose mechanical properties are significantly deteriorated during cold forming, there is a problem that the yield ratio and the toughness cannot be achieved at the same time.

技術 In addition, in the technique described in Patent Document 3, it is necessary to perform heat treatment on the square steel pipe after pipe formation in order to obtain a low yield ratio and high toughness. Therefore, the production cost is significantly higher than that of a square steel pipe that has been cold-worked.

The present invention has been made in view of the above circumstances, and is suitable for building structural members, a square steel pipe excellent in strength, deformation performance and toughness, a method for manufacturing the same, and a building structure using the square steel pipe. The purpose is to provide.

In the present invention, "excellent in strength" means that the yield strength of a flat plate portion of a rectangular steel pipe manufactured by cold roll forming (hereinafter, also referred to as a cold roll-formed rectangular steel pipe) is 385 MPa. As described above, it indicates that the flat plate portion has a tensile strength of 520 MPa or more. Further, “excellent in deformation performance” in the present invention means that the cumulative plastic deformation ratio in the member bending test of the rectangular steel pipe is 28 or more. Further, “excellent in toughness” in the present invention means that the flat portion of the rectangular steel pipe has a Charpy absorbed energy at 0 ° C. of 70 J or more.

The present inventors have conducted intensive studies to solve the above-mentioned problems. As a result, the following findings (1) to (3) were obtained.

(1) In order for the rectangular steel pipe to satisfy the yield strength and tensile strength of the flat plate portion intended in the present invention, the content of C needs to be 0.04% by mass or more. Further, the main structure at a position (a surface layer portion) of 1 / 4t of the plate thickness t from the outer surface of the rectangular steel pipe is a mixed structure of ferrite and bainite, and a region surrounded by a boundary having an orientation difference of 15 ° or more between adjacent crystals is defined as a crystal grain. , The average equivalent circle diameter of the crystal grains needs to be less than 7.0 μm.

(2) In order that the rectangular steel pipe satisfies the deformation performance intended in the present invention, the yield ratio of the flat portion is set to 0.90 or less, and the difference between the yield ratio YRf of the flat portion and the yield ratio YRc of the corner portion. (YRc−YRf) needs to be 0.09 or less. In order to reduce the yield ratio of the flat plate portion to 0.90 or less, the remaining structure at a position １／ t of the plate thickness t from the outer surface of the rectangular steel pipe is selected from hard pearlite, martensite, and austenite. Alternatively, it is necessary to use two or more types.

(3) In order to obtain the toughness of the flat plate portion intended in the present invention in a steel structure in which the rectangular steel pipe satisfies both the above (1) and (2), the average of the crystal grains of the above (1) is required. In addition to making the circle equivalent diameter less than 7.0 μm, the volume fraction of crystal grains having a circle equivalent diameter of 40.0 μm or more needs to be 30% or less.

The present invention has been completed based on these findings, and has the following gist.
[1] A square steel pipe having a flat plate portion and a corner portion,
The component composition is in mass%,
C: 0.04% or more and 0.50% or less,
Si: 2.0% or less,
Mn: 0.5% or more and 3.0% or less,
P: 0.10% or less,
S: 0.05% or less,
Al: 0.005% or more and 0.10% or less,
N: contains 0.010% or less, the balance being Fe and inevitable impurities,
The steel structure at 1 / 4t position of the plate thickness t from the pipe outer surface is as follows:
By volume percentage, ferrite is more than 30%, bainite is 10% or more,
The sum of the ferrite and the bainite is 70% or more and 95% or less with respect to the entire steel structure at a position 1 / 4t of the thickness t from the outer surface of the tube;
The balance consists of one or more selected from pearlite, martensite, and austenite,
When a region surrounded by a boundary where the difference in orientation between adjacent crystals is 15 ° or more is a crystal grain,
The average grain equivalent circle diameter of the crystal grains is less than 7.0 μm, and the total of the crystal grains having a circle equivalent diameter of 40.0 μm or more is 30% or less by volume ratio with respect to the entire steel structure at the ４t position. Yes,
A rectangular steel pipe having a yield ratio YRf of the flat plate portion and a yield ratio YRc of the corner portion satisfying the expression (1).
YRc−YRf ≦ 0.09 (1)
[2] The yield strength of the flat part is 385 MPa or more, the tensile strength of the flat part is 520 MPa or more,
The rectangular steel pipe according to [1], wherein the flat plate portion has a yield ratio of 0.90 or less, and the flat plate portion has a Charpy absorbed energy at 0 ° C. of 70 J or more.
[3] The rectangular steel pipe according to [1] or [2], further comprising one or two groups selected from the following group A and group B in mass% in addition to the component composition.
Group A: Nb: 0.15% or less, Ti: 0.15% or less, V: one or more selected from 0.15% or less, Group B: Cr: 1.0% or less, Mo: 1.0% or less, Cu: 0.5% or less, Ni: 0.3% or less, Ca: 0.010% or less, B: 0.01% or less that's all
[4] The rectangular steel pipe according to any one of [1] to [3], wherein the steel structure has a volume fraction of bainite of 10% or more and less than 40%.
[5] A steel material having the composition described in [1] or [3],
Heating temperature: After heating to 1100 ° C or higher and 1300 ° C or lower,
Rough rolling end temperature: 850 ° C. or more and 1150 ° C. or less, finish rolling end temperature: 750 ° C. or more and 850 ° C. or less, and hot rolling at a total rolling reduction of 930 ° C. or less: 65% or more,
Then, cooling is performed at an average cooling rate of 10 ° C / s or more and 30 ° C / s or less at a sheet thickness center temperature, and a cooling stop temperature of 450 ° C or more and 650 ° C or less,
Then, the rolled hot rolled steel sheet at 450 ° C or higher and 650 ° C or lower,
Next, a method of manufacturing a square steel pipe, wherein the hot-rolled steel sheet is formed into a cylindrical shape by cold roll forming, and then subjected to a pipe forming step of forming a square shape into a square steel pipe.
[6] A building structure in which the rectangular steel pipe according to any one of [1] to [4] is used as a pillar.

According to the present invention, it is possible to provide a square steel pipe excellent in strength, deformation performance and toughness, and a method for producing the same.

1 (a) and 1 (b) are schematic diagrams of a bending test of a square steel pipe performed in the present invention. FIG. 2 is a graph showing the results of performing the bending test shown in FIG. 1A on a roll-formed rectangular steel pipe and a press-formed rectangular steel pipe having a yield strength of a flat plate portion of 385 MPa or more and a tensile strength of 520 MPa or more. is there. FIG. 3 is a perspective view schematically showing an example of a building structure using the rectangular steel pipe of the present invention. FIG. 4 is a schematic diagram showing the sampling positions of a flat plate tensile test piece and a corner tensile test piece implemented in the present invention. FIG. 5 is a diagram showing a detailed sampling position of a corner tensile test piece implemented in the present invention. FIG. 6 is a schematic diagram showing a sampling position of a Charpy test piece implemented in the present invention.

Hereinafter, the present invention will be described in detail.

The present invention is a square steel pipe having a flat portion and a corner portion, and has a component composition in mass% of C: 0.04% or more and 0.50% or less, Si: 2.0% or less, and Mn: 0.1% or less. 5% or more and 3.0% or less, P: 0.10% or less, S: 0.05% or less, Al: 0.005% or more and 0.10% or less, N: 0.010% or less, with the balance being Fe and unavoidable impurities, the steel structure at a position 1 / 4t of the plate thickness t from the tube outer surface has a ferrite content of more than 30% and a bainite content of 10% or more by volume ratio, and the total of the ferrite and the bainite is 70% or more and 95% or less with respect to the entire steel structure at a position 1 / 4t of the plate thickness t from the pipe outer surface, and the balance is one or more selected from pearlite, martensite, and austenite. Enclosed by a boundary where the misorientation of the matching crystal is 15 ° or more When the crystallized area is a crystal grain, the average grain equivalent diameter of the crystal grain is less than 7.0 μm, and the total of the crystal grains having a circle equivalent diameter of 40.0 μm or more is the entire steel structure at the 1 / 4t position. And the difference between the yield ratio YRf of the flat portion and the yield ratio YRc of the corner portion satisfies the expression (1).
YRc−YRf ≦ 0.09 (1)
First, the reason for limiting the yield ratio of the rectangular steel pipe of the present invention will be described.

、 As described above, the press-formed rectangular steel pipe and the roll-formed rectangular steel pipe harden work harder at the corners than at the flat part, regardless of the manufacturing method. Therefore, when the yield ratio of the flat portion is YRf and the yield ratio of the corner portion is YRc, the relationship is YRc ≧ YRf.

Therefore, in the present invention, the relationship between the yield ratio difference (YRc-YRf) between the flat portion and the corner portion of the rectangular steel pipe and the deformation performance was examined. In examining the relationship between the yield ratio difference and the deformation performance, the present invention used the results of the bending test of the square steel pipe shown in FIG. FIG. 1 is a schematic view for explaining a bending test of a rectangular steel pipe 1. FIG. 1 (a) shows a side view of a test body, and FIG. 1 (b) shows an AA shown in FIG. 1 (a). 'Shows a cross-sectional view of the line.

A press-formed square steel pipe and a roll-formed square steel pipe each having a yield strength of the flat plate part of 385 MPa or more and a tensile strength of the flat part of 520 MPa or more are prepared, and as shown in FIG. Specimens were produced by welding the diaphragm 2 through the center position. The test body was pin-supported (rotated and supported) by the support members 3 provided at both ends of the test body so that the movement in the horizontal direction and the vertical direction was fixed. The test piece was repeatedly subjected to a bending test at a position indicated by an arrow shown in FIG. 1A under a load of 45 ° (a diagonal direction of the rectangular cross section shown in FIG. 1B) to determine the cumulative plastic deformation ratio. .

累積 The cumulative plastic deformation ratio is a value obtained by dividing the sum of the plastic rotation angles until the proof stress decreases rapidly due to local buckling or breakage of the specimen by the reference rotation angle corresponding to the total plastic moment. The larger this value is, the more excellent the deformation performance when used as a column material (column member), and the higher the energy absorption capacity at the time of an earthquake.

FIG. 2 is a graph showing the test results. In the graph shown in FIG. 2, the cumulative plastic deformation ratio in the roll-formed rectangular steel pipe and the press-formed rectangular steel pipe having the yield strength of the flat portion of 385 MPa or more and the tensile strength of 520 MPa or more is arranged by the yield ratio difference between the flat portion and the corner portion. did. In FIG. 2, the horizontal axis is “Yield ratio difference (YRc−YRf) between the flat portion and the corner portion in the rectangular steel pipe”, and the vertical axis is “cumulative plastic deformation ratio”. As shown in FIG. 2, as the value of (YRc-YRf) increases, the deformation performance (cumulative plastic deformation ratio) required for the column material decreases. Furthermore, it was found that when the value of (YRc-YRf) was 0.09 or less, the deformation performance (cumulative plastic deformation ratio: 28 or more) required for the column material was stably obtained.

In addition, “cumulative plastic deformation magnification: 28 or more” is a deformation performance required as a column material shown in Reference Document 1 below.
Reference 1: The Architectural Institute of Japan: Retention Strength and Deformation Performance in Building Seismic Design (1990), 1990 In the bending test described above, it is considered that the deformation performance of a corner having a large amount of deformation greatly affects the test result. It is presumed that a rectangular steel pipe having a large value of (YRc-YRf) has a relatively high yield ratio YRc at the corner and a small elongation, and as a result, the deformability has been lowered. In the case of a roll-formed square steel pipe, since the value of (YRc-YRf) is 0.09 or less, the yield ratio YRc of the corner portion is relatively low, and sufficient deformation performance was exhibited in the above bending test. it is conceivable that.

From the above, in order to secure the target characteristics in the present invention, it is necessary that the difference between the yield ratio YRf of the flat portion and the yield ratio YRc of the corner portion satisfies the following expression (1). I understand.
YRc−YRf ≦ 0.09 (1)
In order to satisfy the formula (1), it is important to appropriately control the component composition, the steel structure, and the manufacturing conditions of the obtained roll-formed rectangular steel pipe as described below.

Hereinafter, the rectangular steel pipe of the present invention and the method for producing the same will be described.

理由 In the present invention, the reason for limiting the component composition of the square steel pipe will be described. In this specification, “%” indicating the steel composition is “% by mass” unless otherwise specified.

C: 0.04% or more and 0.50% or less C is an element that increases the strength of steel by solid solution strengthening. C is an element that promotes generation of pearlite, enhances hardenability, contributes to generation of martensite, and contributes to stabilization of austenite, and thus contributes to formation of a hard phase. In order to secure the desired strength and yield ratio in the present invention, it is necessary to contain 0.04% or more of C. However, when the C content exceeds 0.50%, the ratio of the hard phase increases, the toughness decreases, and the weldability also deteriorates. Therefore, the C content is set to 0.04% or more and 0.50% or less. The C content is preferably at least 0.08%, more preferably more than 0.12%, and even more preferably at least 0.14%. Further, the C content is preferably 0.30% or less, more preferably 0.25% or less, and still more preferably 0.22% or less.

Si: 2.0% or less Si is an element that increases the strength of steel by solid solution strengthening, and can be contained as necessary. In order to obtain such effects, it is desirable to contain 0.01% or more of Si. However, when the Si content exceeds 2.0%, an oxide is easily generated in the electric resistance welded portion, and the characteristics of the welded portion deteriorate. Further, the toughness of the base material other than the electric resistance welded portion also decreases. Therefore, the Si content is set to 2.0% or less. The Si content is preferably at least 0.01%, more preferably at least 0.10%. Further, the Si content is preferably 0.5% or less, more preferably 0.4% or less, and even more preferably 0.3% or less.

Mn: 0.5% or more and 3.0% or less Mn is an element that increases the strength of steel by solid solution strengthening. Mn is an element that contributes to the refinement of the structure by lowering the ferrite transformation start temperature. In order to secure the desired strength and structure in the present invention, it is necessary to contain 0.5% or more of Mn. However, when the Mn content exceeds 3.0%, an oxide is easily generated in the electric resistance welded portion, and the characteristics of the welded portion are deteriorated. Further, due to solid solution strengthening and microstructural refinement, the yield strength increases, and a desired yield ratio cannot be obtained. Therefore, the Mn content is set to 0.5% or more and 3.0% or less. The Mn content is preferably at least 0.7%, more preferably at least 0.9%, and even more preferably at least 1.0%. Further, the Mn content is preferably 2.5% or less, more preferably 2.0% or less.

P: 0.10% or less P segregates at the grain boundary and causes inhomogeneity of the material. Therefore, it is preferable to reduce P as an inevitable impurity as much as possible, but the content of 0.10% or less is acceptable. Therefore, the P content is in the range of 0.10% or less. The P content is preferably 0.03% or less, more preferably 0.020% or less, and even more preferably 0.015% or less. Although the lower limit of P is not particularly defined, P is preferably set to 0.002% or more because excessive reduction leads to an increase in smelting cost.

S: 0.05% or less S is usually present as MnS in steel, but MnS is thinly stretched in the hot rolling step and adversely affects ductility. For this reason, in the present invention, it is preferable to reduce S as much as possible, but a content of 0.05% or less is acceptable. Therefore, the S content is set to 0.05% or less. The S content is preferably 0.015% or less, more preferably 0.010% or less, and even more preferably 0.008% or less. Although the lower limit of S is not particularly specified, since excessive reduction leads to a rise in smelting cost, S is preferably set to 0.0002% or more.

Al: 0.005% or more and 0.10% or less Al is an element that acts as a strong deoxidizing agent. In order to obtain such an effect, it is necessary to contain 0.005% or more of Al. However, when the Al content exceeds 0.10%, the weldability deteriorates, and the amount of alumina-based inclusions increases, and the surface properties deteriorate. Also, the toughness of the weld decreases. Therefore, the Al content is set to 0.005% or more and 0.10% or less. The Al content is preferably at least 0.01%, more preferably at least 0.027%. Further, the Al content is preferably 0.07% or less, more preferably 0.04% or less.

N: 0.010% or less N is an unavoidable impurity, and is an element having an effect of reducing toughness by firmly fixing dislocation motion. In the present invention, it is desirable to reduce N as impurities as much as possible, but the N content is acceptable up to 0.010%. Therefore, the N content is set to 0.010% or less. The N content is preferably 0.0080% or less, more preferably 0.0040% or less, and even more preferably 0.0035% or less. In addition, since excessive reduction leads to an increase in smelting cost, the N content is preferably set to 0.0010% or more, more preferably 0.0015% or more.

The balance is Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, it does not refuse to contain O at 0.005% or less.

The above components are the basic component compositions of the steel material of the rectangular steel pipe according to the present invention. Although the properties required in the present invention can be obtained with the above essential elements, the following elements can be contained as necessary.

Nb: at least 0.15%, Ti: at most 0.15%, V: at least one selected from the group consisting of at most 0.15% Nb, Ti and V are all fine carbides in steel , An element that forms nitrides and contributes to the improvement of the strength of steel through precipitation strengthening, and can be contained as necessary. In order to obtain such an effect, when Nb, Ti, and V are contained, it is preferable that Nb: 0.005% or more, Ti: 0.005% or more, and V: 0.005% or more, respectively. On the other hand, an excessive content may cause an increase in the yield ratio and a decrease in toughness. Therefore, when Nb, Ti, and V are contained, it is preferable to set Nb: 0.15% or less, Ti: 0.15% or less, and V: 0.15% or less, respectively. More preferably, Nb: 0.008% to 0.10%, Ti: 0.008% to 0.10%, V: 0.008% to 0.10%. Even more preferably, Nb: 0.010% or more and 0.035% or less, Ti: 0.010% or more and 0.040% or less, and V: 0.010% or more and 0.035% or less.

When two or more selected from Nb, Ti, and V are contained, the total yield (the amount of Nb + Ti + V) may be set to 0.15% or less because the yield ratio may be increased and the toughness may be decreased. Is preferred.

Cr: 1.0% or less, Mo: 1.0% or less, Cu: 0.5% or less, Ni: 0.3% or less, Ca: 0.010% or less, B: 0.010% or less One or more selected Cr: 1.0% or less, Mo: 1.0% or less Cr and Mo are elements that increase the hardenability of steel and increase the strength of steel. Can be contained. In order to obtain the above-described effects, when Cr and Mo are contained, it is preferable that Cr: 0.01% or more and Mo: 0.01% or more, respectively. On the other hand, an excessive content may cause a decrease in toughness and a deterioration in weldability. Therefore, when Cr and Mo are contained, it is preferable to make Cr: 1.0% or less and Mo: 1.0% or less, respectively. For this reason, when Cr and Mo are contained, it is preferable to make Cr: 1.0% or less and Mo: 1.0% or less, respectively. It is preferable that Cr: 0.01% or more and Mo: 0.01% or more. More preferably, Cr: 0.10% to 0.50%, Mo: 0.10% to 0.50%.

Cu: 0.5% or less, Ni: 0.3% or less Cu and Ni are elements that increase the strength of steel by solid solution strengthening, and can be contained as necessary. In order to obtain the above-mentioned effects, when Cu and Ni are contained, it is preferable that Cu: 0.01% or more and Ni: 0.01% or more, respectively. On the other hand, an excessive content may cause a decrease in toughness and a deterioration in weldability. Therefore, when Cu and Ni are contained, it is preferable that Cu: 0.5% or less and Ni: 0.3% or less, respectively. Therefore, when Cu and Ni are contained, it is preferable that Cu: 0.5% or less and Ni: 0.3% or less, respectively. It is preferable that Cu: 0.01% or more and Ni: 0.01% or more. More preferably, Cu: 0.10% to 0.40% and Ni: 0.10% to 0.20%.

Ca: 0.010% or less Ca is an element that contributes to improving the toughness of steel by spheroidizing a sulfide such as MnS that is thinly stretched in the hot rolling step, and can be contained as necessary. In order to obtain such an effect, when Ca is contained, it is preferable to contain 0.0005% or more of Ca. However, when the Ca content exceeds 0.010%, Ca oxide clusters are formed in the steel, and the toughness may deteriorate. For this reason, when Ca is contained, the Ca content is preferably set to 0.010% or less. Note that the Ca content is preferably set to 0.0005% or more. More preferably, the Ca content is 0.0010% or more and 0.0050% or less.

B: 0.010% or less B is an element that contributes to refinement of the structure by lowering the ferrite transformation start temperature. In order to obtain such effects, when B is contained, it is preferable to contain 0.0003% or more of B. However, when the B content exceeds 0.010%, the yield ratio may increase. Therefore, when B is contained, the content is preferably set to 0.010% or less. The B content is preferably 0.0003% or more. More preferably, the B content is 0.0005% or more and 0.0050% or less.

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

In the rectangular steel pipe according to the present invention, the steel structure at a position 1 / 4t of the thickness t from the outer surface of the steel pipe to the steel pipe has a ferrite content of more than 30% and a bainite content of 10% or more by volume ratio, and the total of the ferrite and the bainite. Is 70% or more and 95% or less with respect to the entire steel structure at a position 1 / 4t of the plate thickness t from the outer surface of the tube, and the balance is one or more selected from pearlite, martensite, and austenite. Assuming that a region surrounded by a boundary having an orientation difference of 15 ° or more between adjacent crystals is a crystal grain, the average equivalent circle diameter (average crystal grain size) of the crystal grain is less than 7.0 μm and the equivalent circle diameter. The total of the crystal grains having a crystal grain size of 40.0 μm or more is 30% or less by volume ratio with respect to the entire steel structure at a position １／ t of the thickness t from the outer surface of the tube.

In the present invention, the equivalent circle diameter (crystal grain size) is the diameter of a circle having the same area as the target crystal grain. Further, the steel structure is at a position １／ t of the plate thickness t from the outer surface of the flat plate portion of the rectangular steel pipe except for the electric resistance welded portion. Generally, in a roll-formed square steel pipe made of a hot-rolled steel sheet, the steel structure at the 1 / 4t position of the sheet thickness t from the outer surface of the pipe is the same at both the corner and the flat plate. Therefore, here, the steel structure of the flat plate portion is specified.

Volume ratio of ferrite: more than 30%, volume ratio of bainite: 10% or more, total volume ratio of ferrite and bainite with respect to the entire steel structure: 70% to 95% Ferrite is a soft structure, and has another hard structure. And lowering the yield ratio of the steel. In order to obtain the low yield ratio intended in the present invention by such an effect, the volume ratio of ferrite needs to exceed 30%. The volume fraction of ferrite is preferably at least 40%, more preferably at least 43%, and even more preferably at least 45%. The upper limit is not particularly defined, but the volume ratio of ferrite is preferably less than 75%, more preferably less than 70%, and still more preferably 60% or less in order to secure a desired yield ratio. .

Bainite is a structure having intermediate hardness and increases the strength of steel. Since the yield strength and tensile strength intended in the present invention cannot be obtained only with the above ferrite, the volume ratio of bainite must be 10% or more. The volume fraction of bainite is preferably at least 15%, more preferably at least 20%, and even more preferably at least 25%. Although the upper limit is not particularly defined, the volume ratio of bainite is preferably 55% or less, more preferably 50% or less, and even more preferably 45% or less, in order to secure a desired yield ratio. , Even more preferably less than 40%.

と If the sum of the volume fractions of ferrite and bainite is less than 70%, the yield ratio or Charpy absorbed energy intended in the present invention cannot be obtained. On the other hand, if the sum of the volume fractions of ferrite and bainite exceeds 95%, the yield strength and the yield ratio intended in the present invention cannot be obtained. For this reason, in addition to the above conditions, it is necessary to make the total volume ratio of ferrite and bainite 70% or more and 95% or less. Preferably, it is 75% or more and 93% or less. More preferably, it is 80% or more and 90% or less.

Remainder: One or more selected from pearlite, martensite, and austenite Perlite, martensite, and austenite are hard structures, and in particular, increase the tensile strength of steel and mix it with soft ferrite. The yield ratio of steel decreases. In order to obtain such an effect, it is preferable that pearlite, martensite, and austenite have a total volume ratio of 5% or more and 30% or less. More preferably, it is 7% or more and 25% or less. Still more preferably, it is 10% or more and 20% or less.

Note that the volume fraction of ferrite, bainite, pearlite, martensite, and austenite can be measured by a method described in Examples described later.

Assuming that a region surrounded by a boundary having an orientation difference between adjacent crystals (crystal orientation difference) of 15 ° or more is a crystal grain, the average crystal grain size of the crystal grain is less than 7.0 μm and the crystal grain size is 40.0 μm or more. As described above, in order to obtain the low yield ratio, the yield strength, and the tensile strength that are the objectives of the present invention, the steel structure of the present invention comprises a soft structure and a hard structure. The mixed steel (hereinafter, referred to as “composite structure steel”) is used. However, the composite structure steel has lower toughness than the single structure steel. Therefore, in the present invention, in order to achieve both the above mechanical properties and excellent toughness, when a region surrounded by a boundary having a crystal orientation difference of 15 ° or more is defined as a crystal grain, the average crystal grain size of the crystal grain is defined. . When the average crystal grain size of the crystal grains is 7.0 μm or more, the desired yield strength and toughness cannot be obtained because the ferrite grains are not sufficiently fine. For this reason, by setting the average crystal grain size of the crystal grains to less than 7.0 μm, the yield strength intended in the present invention is obtained and the toughness is secured. The average grain size of the crystal grains is preferably 6.5 μm or less, more preferably 6.0 μm or less.

Generally, the grain size distribution in a single-structure steel or a steel close to a single-structure steel follows a normal logarithmic distribution having one peak and having a large value on a large variable side and a limited variable on a small variable side. However, as in the present invention, it was found that in the crystal grain size distribution in the composite structure steel containing ferrite and bainite, a new peak of bainite appeared on the coarse grain side.

Specifically, in the steel structure of the present invention, that is, in the composite structure steel in which the volume ratio of ferrite is more than 30% and the volume ratio of bainite is 10% or more, a peak of bainite is newly found on the coarse grain side in the crystal grain size distribution. Appears in This indicates that coarse bainite is mixed. The mixture of coarse bainite causes the toughness to be greatly deteriorated. As a result, in the composite structure steel, even if the upper limit of the maximum crystal grain size is specified, the proportion of coarse bainite cannot be suppressed low. Therefore, in order to obtain good toughness, it is necessary to define the upper limit of the ratio of the presence of coarse crystal grains.

Bainite does not grow beyond boundaries with large misorientation (austenite grain boundaries and subboundaries formed by accumulation of dislocations). Therefore, in order to suppress the formation of the coarse bainite, the finish rolling in hot rolling is performed at a temperature as low as possible, a large amount of dislocations are introduced into austenite, the sub-boundary area is increased, and the fine sub-grain structure ( In the following, it is particularly effective to form the structure.

(4) The toughness of the rectangular steel pipe according to the present invention is improved by increasing the total area of the grain boundaries that is resistant to brittle fracture. In the present invention, preliminary experiments have shown that if the volume fraction of coarse crystal grains having a crystal grain size of 40.0 μm or more exceeds 30%, it is not possible to secure a sufficient grain boundary area to obtain necessary toughness. I learned. Therefore, in the present invention, in addition to specifying the upper limit of the average crystal grain size of the crystal grains to be less than 7.0 μm, the volume ratio of crystal grains having a crystal grain size of 40.0 μm or more is set to 30% or less. It is stipulated that The volume fraction of crystal grains having a crystal grain size of 40.0 μm or more is preferably 20% or less, more preferably 15% or less.

差 Note that the crystal orientation difference, the average crystal grain size, and the volume ratio of the crystal grains having a crystal grain size of 40.0 μm or more can be measured by the SEM / EBSD method. Here, it can be measured by the method described in Examples described later.

In the present invention, the above-described effects are similarly obtained even when the above-described steel structure is present within a range of ± 1.0 mm in the thickness direction from the outer surface of the steel pipe to a position １／ t of the thickness t in the thickness direction. Can be Therefore, in the present invention, "the steel structure at a position 1 / 4t of the thickness t from the outer surface of the steel pipe" means "± 1.0 mm in the thickness direction around the 1 / 4t position of the thickness t from the outer surface of the steel pipe. Means that the above-mentioned steel structure is present.

Next, a method for manufacturing a square steel pipe according to an embodiment of the present invention will be described.

The square steel pipe of the present invention is obtained, for example, by heating a steel material having the above-described composition to a heating temperature of 1100 ° C. or more and 1300 ° C. or less, and thereafter, a rough rolling end temperature: 850 ° C. to 1150 ° C., and a finish rolling end temperature: Hot rolling at 750 ° C or higher and 850 ° C or lower and 930 ° C or lower and a total draft of 65% or higher is performed, and then an average cooling rate at a thickness center temperature: 10 ° C / s or higher and 30 ° C / s or lower, Cooling stop temperature: Cooling is performed at 450 ° C or more and 650 ° C or less, then rolled at 450 ° C or more and 650 ° C or less to form a hot-rolled steel sheet, and then formed into a cylindrical shape by cold roll forming. It is obtained by performing a pipe-forming step of forming a square steel pipe by forming it into a square shape.

In the following description of the manufacturing method, “° C.” regarding temperature is the surface temperature of a steel material or a steel plate (hot rolled plate) unless otherwise specified. These surface temperatures can be measured with a radiation thermometer or the like. Further, the temperature at the thickness center of the steel sheet can be obtained by calculating the temperature distribution in the cross section of the steel sheet by heat transfer analysis, and correcting the result by the surface temperature of the steel sheet. The “hot-rolled steel sheet” includes a hot-rolled steel sheet and a hot-rolled steel strip.

において In the present invention, the method of smelting a steel material (steel slab) is not particularly limited, and any known smelting method such as a converter, an electric furnace, and a vacuum melting furnace is suitable. The casting method is also not particularly limited, but is manufactured to a desired size by a known casting method such as a continuous casting method. It should be noted that there is no problem even if the ingot-bulking rolling method is applied instead of the continuous casting method. The molten steel may be further subjected to secondary refining such as ladle refining.

Next, after heating the obtained steel material (steel slab) to a heating temperature of 1100 ° C. or more and 1300 ° C. or less, rough rolling is performed at a rough rolling end temperature of 850 ° C. or more and 1150 ° C. or less, and a finish rolling end temperature is: Finish rolling is performed at 750 ° C. or more and 850 ° C. or less, and a hot rolling step is performed at 930 ° C. or less with a total draft of 65% or more to obtain a hot-rolled sheet.

Heating temperature: 1100 ° C. or more and 1300 ° C. or less If the heating temperature is less than 1100 ° C., the deformation resistance of the material to be rolled becomes large and rolling becomes difficult. On the other hand, when the heating temperature exceeds 1300 ° C., the austenite grains become coarse, and fine austenite grains cannot be obtained in the subsequent rolling (rough rolling, finish rolling), and the average of the steel structure of the square steel pipe intended in the present invention is obtained. It is difficult to secure a crystal grain size. Further, it is difficult to suppress the formation of coarse bainite, and it is difficult to control the volume ratio of crystal grains having a crystal grain size of 40.0 μm or more to a range intended in the present invention. For this reason, the heating temperature in the hot rolling step is 1100 ° C. or more and 1300 ° C. or less. More preferably, it is 1120 ° C or more and 1280 ° C or less.

In the present invention, after the steel slab (slab) is manufactured, it is once cooled to room temperature, and then heated again again. Rolling immediately after performing a slight heat retention, these energy saving processes of direct rolling can be applied without any problem.

Rough rolling end temperature: 850 ° C. or higher and 1150 ° C. or lower If the rough rolling end temperature is lower than 850 ° C., during the subsequent finish rolling, the steel sheet surface temperature becomes lower than the ferrite transformation start temperature, a large amount of ferrite is generated, and bainite is formed. The volume ratio is less than 10%. On the other hand, when the rough rolling end temperature exceeds 1150 ° C., the amount of reduction in the austenite non-recrystallization temperature region is insufficient, and fine austenite grains cannot be obtained. As a result, it becomes difficult to secure the average crystal grain size of the steel structure of the rectangular steel pipe intended in the present invention. Further, it becomes difficult to suppress the generation of coarse bainite. For this reason, the rough rolling end temperature is set to 850 ° C or higher and 1150 ° C or lower. It is more preferably 860 ° C or more and 1000 ° C or less. It is even more preferably 870 ° C or more and 980 ° C or less.

Finish rolling finish temperature: 750 ° C. or more and 850 ° C. or less When the finish rolling finish temperature is less than 750 ° C., the surface temperature of the steel sheet becomes lower than the ferrite transformation start temperature during finish rolling, a large amount of ferrite is formed, and the volume fraction of bainite is increased. Is less than 10%. On the other hand, when the finish rolling end temperature exceeds 850 ° C., the amount of reduction in the austenite non-recrystallization temperature region is insufficient, and fine austenite grains cannot be obtained. As a result, it becomes difficult to secure the average crystal grain size of the steel structure of the rectangular steel pipe intended in the present invention. Further, it becomes difficult to suppress the generation of coarse bainite. For this reason, the finish rolling end temperature is 750 ° C. or more and 850 ° C. or less. It is more preferably 770 ° C or higher and 830 ° C or lower. It is even more preferably 780 ° C or higher and 820 ° C or lower.

In the present invention, the ferrite, bainite and residual structure generated in the subsequent cooling step and winding step are refined by making the sub-grains in austenite fine in the hot rolling step. Thus, a steel structure of a square steel pipe having the strength and toughness desired in the present invention can be obtained. In order to refine the subgrains in austenite in the hot rolling step, it is necessary to increase the rolling reduction in the austenite non-recrystallization temperature range and to introduce a sufficient working strain. In order to achieve this, in the present invention, the total rolling reduction from 930 ° C. to the finish rolling end temperature is 65% or more.

If the total rolling reduction up to the finish rolling temperature of 930 ° C. or less is less than 65%, sufficient working strain cannot be introduced in the hot rolling step, so that the structure having the crystal grain size intended in the present invention is not obtained. I can't get it. The total rolling reduction from 930 ° C. to the finish rolling end temperature is more preferably 70% or more, and even more preferably 71% or more. There is no particular upper limit, but if it exceeds 80%, the effect of improving the toughness against an increase in the rolling reduction decreases, and only the equipment load increases. For this reason, the total rolling reduction from 930 ° C. to the finish rolling end temperature is preferably 80% or less. It is more preferably at most 75%, even more preferably at most 74%.

The reason why the temperature is set to 930 ° C. or lower is that if the temperature exceeds 930 ° C., austenite is recrystallized in the rolling step, dislocations introduced by rolling disappear, and fine austenite cannot be obtained.

The above-mentioned total rolling reduction refers to the total rolling reduction of each rolling pass in a temperature range of 930 ° C or lower and up to the finish rolling end temperature.

When the slab is hot-rolled, hot rolling may be performed so that the total reduction from 930 ° C. to the finish rolling end temperature is 65% or more in both the rough rolling and the finish rolling. Alternatively, hot rolling may be performed such that the total reduction from 930 ° C. or less to the finish rolling end temperature is 65% or more only by finish rolling. In the latter case, when the total rolling reduction to 930 ° C. or less to the finish rolling end temperature cannot be made 65% or more only by finish rolling, the slab is cooled during the rough rolling to reduce the temperature to 930 ° C. or less. The total rolling reduction from 930 ° C. to the finish rolling end temperature in both the rough rolling and the finish rolling is 65% or more.

上限 In the present invention, the upper limit of the finished plate thickness is not particularly specified, but from the viewpoint of securing the required draft and controlling the steel plate temperature, the finished plate thickness is preferably more than 20 mm and 32 mm or less.

後 After the hot rolling step, the hot rolled sheet is subjected to a cooling step. In the cooling step, cooling is performed at an average cooling rate up to the cooling stop temperature: 10 ° C./s to 30 ° C./s, and at a cooling stop temperature: 450 ° C. to 650 ° C.

Average cooling rate from the start of cooling to the stop of cooling (end of cooling): 10 ° C./s or more and 30 ° C./s or less At the center temperature of the thickness of the hot-rolled sheet, the average cooling rate in the temperature range from the start of cooling to the stop of cooling described later If it is less than 10 ° C./s, the frequency of ferrite nucleation decreases and the ferrite grains become coarse, so that the average crystal grain size cannot be less than 7.0 μm. In addition, it is difficult to control the crystal grain size targeted in the present invention to a range of a volume ratio of 40.0 μm or more. On the other hand, when the average cooling rate exceeds 30 ° C./s, a large amount of martensite is generated at a position １／ t of the plate thickness t from the outer surface of the steel structure of the obtained square steel pipe, and the volume fraction of ferrite and bainite is reduced. The total is less than 70%. The average cooling rate is preferably at least 15 ° C./s, more preferably at least 17 ° C./s. It is preferably at most 25 ° C / s, more preferably at most 23 ° C / s.

In the present invention, from the viewpoint of suppressing ferrite formation on the steel sheet surface before cooling, it is preferable to start cooling immediately after finishing rolling.

Cooling stop temperature: 450 ° C. or more and 650 ° C. or less At the center temperature of the thickness of the hot-rolled sheet, when the cooling stop temperature is less than 450 ° C., a large amount is obtained at a position １／ t of the thickness t from the outer surface of the steel structure of the obtained square steel pipe. In some cases, the total volume fraction of ferrite and bainite may be less than 70%. Further, the volume ratio of ferrite may be 30% or less. On the other hand, when the cooling stop temperature exceeds 650 ° C., the nucleation frequency of ferrite decreases, the ferrite grains become coarse, and the bainite volume ratio cannot be increased to 10% or more because the temperature exceeds the bainite transformation start temperature. . The cooling stop temperature is preferably 480 ° C or higher, and more preferably 490 ° C or higher. Preferably it is 620 ° C or lower, more preferably 600 ° C or lower.

In the present invention, unless otherwise specified, the average cooling rate is a value obtained by ((thickness center temperature of hot rolled sheet before cooling−thickness center temperature of hot rolled sheet after cooling) / cooling time). (Cooling rate). Examples of the cooling method include water cooling such as injection of water from a nozzle and cooling by injection of a cooling gas. In the present invention, it is preferable to perform a cooling operation (treatment) on both sides of the hot rolled sheet so that both sides of the hot rolled sheet are cooled under the same conditions.

After the cooling step, a winding step of winding the hot rolled sheet and then allowing it to cool is performed.
In the winding step, winding is performed at a winding temperature of 450 ° C. or more and 650 ° C. or less from the viewpoint of the steel sheet structure. If the winding temperature is lower than 450 ° C., a large amount of martensite is generated, and the total volume ratio of ferrite and bainite may be lower than 70%. Further, the volume ratio of ferrite may be 30% or less. If the winding temperature exceeds 650 ° C., the frequency of ferrite nucleation decreases, the ferrite grains become coarse, and the bainite volume ratio cannot be increased to 10% or more because the temperature exceeds the bainite transformation start temperature in some cases. The winding temperature is more preferably 480 ° C or more and 620 ° C or less, and even more preferably 490-590 ° C.

後に After the winding process, a tube forming process is performed. In the pipe forming process, a hot rolled steel sheet is roll-formed into a cylindrical open pipe (round steel pipe), and the butted portions are welded by electric resistance welding. Thereafter, the roll is placed vertically, horizontally, and horizontally with respect to the round steel pipe, and while the cylindrical shape is kept, several percent of the drawing is applied in the pipe axis direction to form a square steel pipe.

The rectangular steel pipe in the present invention is not limited to a rectangular steel pipe having all equal side lengths (the value of (long side length / short side length) is 1.0). Also included are rectangular steel pipes having a value of (length) exceeding 1.0. However, if the value of (long side length / short side length) of the rectangular steel pipe exceeds 2.5, local buckling is likely to occur on the long side, and the compressive strength in the pipe axis direction decreases. Therefore, the value of (long side length / short side length) of the rectangular steel pipe is preferably set to 1.0 or more and 2.5 or less. The value of (long side length / short side length) is more preferably 1.0 or more and 2.0 or less.

Thus, the square steel pipe of the present invention is manufactured. According to the present invention, the yield strength of the flat portion is 385 MPa or more, the tensile strength of the flat portion is 520 MPa or more, the yield ratio of the flat portion is 0.90 or less, and the Charpy absorbed energy at 0 ° C. of the flat portion is 70 J or more. A square steel pipe can be obtained. This makes it possible to produce a high-strength roll-formed square steel pipe with high productivity and short delivery time (short term) as compared with cold press bending. The roll-formed rectangular steel pipe can be suitably used particularly for building members of large buildings such as factories, warehouses, commercial facilities, and the like, and can greatly contribute to reduction of construction costs.

Therefore, the present invention can be suitably used particularly for thick-walled rectangular steel pipes. Here, “thick” means that the thickness of the flat plate portion of the rectangular steel pipe is more than 20 mm.

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

FIG. 3 schematically shows an example of a building structure using the above-described square steel pipe of the present invention. As shown in FIG. 3, in the building structure of the present embodiment, a plurality of square steel pipes 1 of the present invention are erected and used as pillars. A plurality of girders 4 made of a steel material such as an H-section steel are provided between adjacent rectangular steel pipes 1. Further, a plurality of small beams 5 made of a steel material such as an H-section steel are provided between the adjacent large beams 4. The H-shaped steel to be the square steel pipe 1 and the girder 4 is welded and joined via a through-diaphragm 6 so that the girder 4 made of a steel material such as an H-shaped steel is erected between the adjacent square steel pipes 1. Further, a stud 7 is provided as needed for mounting a wall or the like.

角 Since the rectangular steel pipe 1 of the present invention is excellent in strength, deformation performance and toughness, even when used in a large building, the deformation performance of the entire structure can be sufficiently ensured. Therefore, the building structure of the present invention exhibits more excellent seismic performance than a building structure using a conventional rectangular steel pipe.

Hereinafter, the present invention will be described in more detail with reference to examples. Note that the present invention is not limited to the following embodiments.

溶 Molten steel having the component composition shown in Table 1 was smelted in a converter and made into a slab (steel material: wall thickness 250 mm) by a continuous casting method. The obtained slab was subjected to a hot rolling step, a cooling step, and a winding step under the conditions shown in Table 2 to obtain a hot-rolled steel sheet for a rectangular steel pipe.

After the winding step, the following tube forming step was performed.

Some of the hot-rolled steel sheets for square steel pipes were formed into cylindrical round steel pipes by roll forming, and the butted portions were subjected to ERW welding. After that, several percent of drawing is performed in the axial direction of the pipe by means of rolls arranged at the top, bottom, left and right of the round steel pipe to form a square shape, and a roll-formed square steel pipe having a side length (mm) and a plate thickness (mm) shown in Table 2 I got

For the remaining hot-rolled steel sheet for rectangular steel pipes, the cross-sectional shape was formed into a square shape or a U-shape by press bending, and these were joined by submerged arc welding, and the side length (mm) and sheet thickness shown in Table 2 were obtained. (Mm) was obtained.

試験 A test piece was sampled from the obtained square steel pipe (roll-formed square steel pipe, press-formed square steel pipe), and the following structure observation, tensile test, and Charpy impact test were performed.

(Tissue observation)
The test piece for structure observation was taken from the side (the 3 o'clock or 9 o'clock side when the welded portion was in the 12 o'clock direction) next to the side including the welded portion of the rectangular steel pipe. The test piece for structure observation was collected from the flat plate portion serving as the adjacent side portion such that the observation surface was a cross section in the tube axis direction at the time of hot rolling and a position １／ t of the plate thickness t from the tube outer surface, After polishing, it was produced by nital corrosion.

Microstructure observation was performed using an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) to observe the structure at the 1 / 4t position of the plate thickness t from the outer surface of the flat portion of the rectangular steel tube. Then, an image was taken. From the obtained optical microscope image and SEM image, the area ratio of ferrite, pearlite, bainite and the remaining structure was determined.
The area ratio of each tissue was observed in five or more visual fields using a test piece collected from one representative flat plate portion, and calculated as an average value obtained in each visual field. Here, the area ratio obtained by observing the tissue was defined as the volume ratio of each tissue.

フェライト Here, ferrite is a product by diffusion transformation, and exhibits a structure with low dislocation density and almost recovered. This includes polygonal ferrite and pseudopolygonal ferrite. Bainite has a double phase structure of lath-like ferrite and cementite having a high dislocation density.

It is difficult to distinguish between martensite and austenite in the optical microscope image and the SEM image. For this reason, the area ratio of the structure observed as martensite or austenite was measured from the obtained SEM image, and the value obtained by subtracting the volume ratio of austenite measured by the method described later was defined as the volume ratio of martensite.

The measurement of the volume fraction of austenite was performed by X-ray diffraction. The test piece for structure observation was prepared by grinding the diffraction surface from the outer surface of the flat plate portion of the steel tube to a position of 1 / 4t of the plate thickness t, and then performing chemical polishing to remove the surface processed layer. The Kα line of Mo was used for the measurement, and the volume fraction of austenite was determined from the integrated intensity of the (200), (220), and (311) faces of fcc iron and the (200) and (211) faces of bcc iron.

The average equivalent circle diameter (average crystal grain diameter) and the volume fraction of crystal grains having an equivalent circle diameter (crystal grain diameter) of 40.0 μm or more were measured using the SEM / EBSD method. The crystal grain size was obtained by determining the azimuth difference between adjacent crystal grains, and setting a boundary where the azimuth difference was 15 ° or more as a crystal grain boundary. The arithmetic mean of the particle size was determined from the obtained crystal grain boundaries, and the result was defined as the average crystal particle size. The measurement area was 500 μm × 500 μm, and the measurement step size was 0.5 μm. In the crystal grain size analysis, those having a crystal grain size of 2.0 μm or less were excluded from the analysis as measurement noise, and the obtained area ratio was assumed to be equal to the volume ratio.

(Tensile test)
FIG. 4 is a schematic diagram showing the sampling positions of the tensile test piece in the flat plate portion and the tensile test piece in the corner portion. FIG. 5 is a schematic diagram showing a detailed sampling position of a tensile test piece at a corner.

(4) In the tensile test, as shown in FIG. 4, JIS No. 5 tensile test pieces and JIS 12B tensile test pieces were sampled from the flat plate portion and the corner portion of the rectangular steel pipe so that the tensile direction became parallel to the pipe axis direction. Using these materials, the yield strength YS and the tensile strength TS were measured in accordance with the provisions of JIS {Z} 2241, and the yield ratio defined by (yield strength) / (tensile strength) was calculated. In addition, the tensile test piece of a flat plate part is a flat plate in the side part other than the side part including the welded part of a square steel pipe (side part at the 3 o'clock, 6 o'clock, or 9 o'clock side when the welded part is 12 o'clock direction). The sample was taken from the position at the center of the width of the part (see FIG. 4). The corner tensile test pieces were taken from the corners of the square steel pipe at 45 ° corners (see FIG. 5). The number of test pieces was two each, and their average values were calculated to obtain YS, TS, and yield ratio.

[Charpy impact test]
FIG. 6 is a schematic diagram showing a sampling position of a Charpy test piece.

As shown in FIG. 6, the Charpy impact test was conducted at a position 1 / 4t of the plate thickness t from the outer surface of the square steel pipe so that the longitudinal direction of the test piece was parallel to the pipe axis direction. A V-notch standard test piece conforming to the standard was used. A Charpy impact test was conducted at a test temperature of 0 ° C. in accordance with JIS Z 2242 to determine the absorbed energy (J). The number of test pieces was three, and the average value was calculated to determine the absorbed energy (J).

Table 3 shows the results obtained.

鋼 In Table 3, steel No. 1, 5, 13, 14, 17, 19, 22, 27 to 34, 36 to 46, and 48 are examples of the present invention, and steel Nos. 2 to 4, 6 to 12, 15, 16, 18, 20, 21, 23 to 26, 35, 47, and 49 are comparative examples.

In the square steel pipes of the present invention, the steel structure includes ferrite with a volume ratio of more than 30% and bainite of 10% or more, the total volume ratio of ferrite and bainite is 70% or more and 95% or less, and the balance is When a region composed of one or more selected from pearlite, martensite, and austenite and surrounded by a boundary having a misorientation of 15 ° or more is defined as a crystal grain, the average equivalent circle diameter of the crystal grain is less than 7.0 μm. And the volume fraction of crystal grains having an equivalent circle diameter of 40.0 μm or more was 30% or less. Further, the yield strength in the flat part is 385 MPa or more, the tensile strength in the flat part is 520 MPa or more, the yield ratio in the flat part is 0.90 or less, and the Charpy absorbed energy at 0 ° C. in the flat part is 70 J or more. The difference in yield ratio between the parts was 0.09 or less.

On the other hand, No. of the comparative example. In all of 2, 6, 18, and 20, the difference in the yield ratio between the flat plate portion and the corner portion was more than 0.09 because the square steel pipe was formed by press bending.

No. of the comparative example. In No. 3, since the C content was lower than the range of the present invention, the yield strength and the tensile strength of the flat plate portion were out of the range of the present invention.

No. of the comparative example. No. 4, since the Mn content was below the range of the present invention, the crystal grains were coarsened, and the average crystal grain size and the volume fraction of the crystal grains having a crystal grain size of 40.0 μm or more were out of the range of the present invention. Was. As a result, the yield strength, tensile strength and Charpy absorbed energy at 0 ° C. of the flat portion did not reach the desired values.

No. of the comparative example. In No. 7, the slab heating temperature exceeded the range of the present invention, the crystal grains became coarse, and the average crystal grain size and the volume ratio of the crystal grains having a crystal grain size of 40.0 μm or more were out of the range of the present invention. . As a result, the tensile strength of the flat plate portion and the Charpy absorbed energy at 0 ° C. did not reach desired values.

No. of the comparative example. In No. 8, the total rolling reduction at 930 ° C. or lower was lower than the range of the present invention, the formation of coarse bainite could not be suppressed, and the volume ratio of crystal grains having a crystal grain size of 40.0 μm or more was out of the range of the present invention. It became. As a result, the Charpy absorbed energy at 0 ° C. of the flat portion did not reach a desired value.

No. of the comparative example. In No. 9, the finish rolling end temperature was lower than the range of the present invention, a large amount of ferrite was generated during hot rolling, and the volume ratio of bainite was less than 10%. As a result, the yield strength and tensile strength of the flat part did not reach desired values.

No. of the comparative example. 10, since the finish rolling end temperature exceeded the range of the present invention, the total rolling reduction at 930 ° C. or lower was lower than the range of the present invention, the formation of coarse bainite could not be suppressed, and the crystal grain size was 40.0 μm or more. The volume fraction of crystal grains was out of the range of the present invention. As a result, the Charpy absorbed energy of the flat plate portion at 0 ° C. did not reach a desired value.

No. of the comparative example. In No. 11, since the average cooling rate was lower than the range of the present invention, the crystal grains were coarsened, and the average crystal grain size and the volume ratio of the crystal grains having a crystal grain size of 40.0 μm or more were out of the range of the present invention. As a result, the yield strength, tensile strength and Charpy absorbed energy at 0 ° C. of the flat portion did not reach the desired values.

No. of the comparative example. In No. 12, since the average cooling rate exceeded the range of the present invention, the volume ratio of ferrite was out of the range of the present invention. As a result, the yield ratio of the flat portion did not reach the desired value.

No. of the comparative example. In No. 15, the volume ratio of bainite was out of the range of the present invention because the cooling stop temperature exceeded the range of the present invention. As a result, the yield strength and tensile strength of the flat part did not reach desired values.

No. of the comparative example. In No. 16, since the cooling stop temperature and the winding temperature were below the range of the present invention, the total volume fraction of ferrite and bainite was out of the range of the present invention. As a result, the yield ratio of the flat portion did not reach the desired value.

No. of the comparative example. In No. 21, since the C content exceeded the range of the present invention, the total volume ratio of ferrite and bainite was out of the range of the present invention. As a result, the Charpy absorbed energy of the flat plate portion at 0 ° C. did not reach a desired value.

No. of the comparative example. In No. 23, since the content of Si exceeded the range of the present invention, the yield strength was excessively increased by solid solution strengthening without finer structure. As a result, the Charpy absorbed energy of the flat plate portion at 0 ° C. did not reach a desired value.

No. of the comparative example. In No. 24, since the Mn content exceeded the range of the present invention, the yield strength was excessively increased by solid solution strengthening. As a result, the yield ratio of the flat portion did not reach the desired value.

No. of the comparative example. In No. 25, it is considered that the grain boundary strength was lowered because the content of P exceeded the range of the present invention. As a result, the Charpy absorbed energy of the flat plate portion at 0 ° C. did not reach a desired value.

No. of the comparative example. In No. 26, since the S content exceeded the range of the present invention, it is considered that coarse inclusions serving as starting points of destruction such as MnS were generated. As a result, the Charpy absorbed energy of the flat plate portion at 0 ° C. did not reach a desired value.

No. of the comparative example. In No. 35, since the C content was lower than the range of the present invention, the yield strength and the tensile strength of the flat plate portion were out of the range of the present invention. Further, the formation of pearlite, which is a hard phase, was suppressed, and the total of the volume fractions of ferrite and bainite was out of the range of the present invention. As a result, the yield ratio of the flat portion did not reach the desired value.

No. of the comparative example. In No. 47, since the cooling stop temperature and the winding temperature were below the range of the present invention, the volume ratio of ferrite was out of the range of the present invention, and the yield ratio of the flat plate portion did not reach the desired value.

No. of the comparative example. In No. 49, since the cooling rate was lower than the range of the present invention, the average crystal grain size was out of the range of the present invention, and the Charpy absorption energy at 0 ° C. of the flat plate portion did not reach a desired value.

DESCRIPTION OF SYMBOLS 1 Square steel pipe 2 Through diaphragm 3 Supporting material 4 Large beam 5 Small beam 6 Diaphragm 7 Stud

Claims

A square steel pipe having a flat portion and a corner portion,
The component composition is in mass%,
C: 0.04% or more and 0.50% or less,
Si: 2.0% or less,
Mn: 0.5% or more and 3.0% or less,
P: 0.10% or less,
S: 0.05% or less,
Al: 0.005% or more and 0.10% or less,
N: 0.010% or less, the balance being Fe and unavoidable impurities,
The steel structure at 1 / 4t position of the plate thickness t from the pipe outer surface is as follows:
By volume percentage, ferrite is more than 30%, bainite is 10% or more,
The sum of the ferrite and the bainite is 70% or more and 95% or less with respect to the entire steel structure at a position 1 / 4t of the thickness t from the outer surface of the tube;
The balance consists of one or more selected from pearlite, martensite, and austenite,
When a region surrounded by a boundary where the difference in orientation between adjacent crystals is 15 ° or more is a crystal grain,
The average equivalent circle diameter of the crystal grains is less than 7.0 μm,
And the total of the crystal grains having an equivalent circle diameter of 40.0 μm or more is 30% or less by volume ratio with respect to the entire steel structure at the 1 / 4t position,
A rectangular steel pipe having a yield ratio YRf of the flat plate portion and a yield ratio YRc of the corner portion satisfying the expression (1).
YRc−YRf ≦ 0.09 (1)
2. The flat plate part according to claim 1, wherein the yield strength of the flat part is 385 MPa or more, the tensile strength of the flat part is 520 MPa or more, the yield ratio of the flat part is 0.90 or less, and the Charpy absorbed energy at 0 ° C. of the flat part is 70 J or more. Square steel pipe.
The square steel pipe according to claim 1 or 2, further comprising one or two groups selected from the following group A and group B in mass% in addition to the component composition.
Group A: Nb: 0.15% or less, Ti: 0.15% or less, V: one or more selected from 0.15% or less, Group B: Cr: 1.0% or less, Mo: 1.0% or less, Cu: 0.5% or less, Ni: 0.3% or less, Ca: 0.010% or less, B: 0.01% or less that's all
The rectangular steel pipe according to any one of claims 1 to 3, wherein the steel structure has a bainite content of 10% or more and less than 40% by volume.
A steel material having the component composition according to claim 1 or 3,
Heating temperature: After heating to 1100 ° C or higher and 1300 ° C or lower,
Rough rolling end temperature: 850 ° C. or more and 1150 ° C. or less, finish rolling temperature: 750 ° C. or more and 850 ° C. or less, and hot rolling at a total rolling reduction of 930 ° C. or less: 65% or more,
Then, cooling is performed at an average cooling rate of 10 ° C / s or more and 30 ° C / s or less at a sheet thickness center temperature, and a cooling stop temperature of 450 ° C or more and 650 ° C or less,
Next, the rolled hot rolled steel sheet at 450 ° C or higher and 650 ° C or lower,
Then, after forming the hot-rolled steel sheet into a cylindrical shape by cold roll forming, performing a pipe-forming step of forming the steel sheet into a square shape to obtain a square steel pipe.
(5) An architectural structure in which the rectangular steel pipe according to any one of (1) to (4) is used as a pillar.