TWI707958B - Angular steel pipe and its manufacturing method and building structure - Google Patents

Abstract

The present invention provides an angle steel pipe and a manufacturing method thereof. The present invention is an angular steel pipe with a flat part and a corner part, and the component composition contains 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 by mass% , S: 0.05% or less, Al: 0.005% or more, 0.10% or less, N: 0.010% or less, the remaining part is composed of Fe and inevitable impurities, from the outside of the tube to the steel structure at the position of 1/4t of the plate thickness t , The volume ratio of ferrite is more than 30%, and the amount of bristled iron is more than 10%. The total of ferrite and bristled iron is relative to the entire steel structure from the outside of the tube to the position of 1/4t of the plate thickness t. 70% or more and 95% or less, the remaining part is composed of one or more types selected from perlite, matian scattered iron, austenitic iron, and the area surrounded by the azimuth difference of adjacent crystals at a boundary of 15° or more When defined as crystal grains, the average circle equivalent diameter of the crystal grains is less than 7.0 μm, and the total equivalent circle diameter of crystal grains of 40.0 μm or more is 30 relative to the total steel structure at the position of 1/4t. % Or less, the yield ratio YRf of the flat part and the yield ratio YRc of the corner part satisfy the formula (1).

Description

Angular steel pipe and its manufacturing method and building structure

The present invention relates to an angle steel pipe with excellent strength, deformation performance and toughness, especially suitable for use in large-scale building construction components, and a manufacturing method thereof, and a building structure using the angle steel pipe.

In recent years, building structural members used in large buildings such as factories, warehouses, and commercial facilities (hereinafter referred to as buildings) have been increasing in strength due to reduction in construction costs with lighter weights. Particularly, in angular steel pipes (corner columns) with flat plates and corners used as pillars of buildings, mechanical properties such as the yield strength of the flat part of 385 MPa or more and the tensile strength of the flat part of 520 MPa or more are sought. At the same time, from the viewpoint of shock resistance, it is also sought to have high plastic deformation performance and excellent toughness.

In general, angled steel pipes are manufactured by cold-forming hot-rolled steel plates (hot-rolled steel strips) or thick steel plates as materials. As a method of cold forming, there is a method of cold pressing and bending or a method of cold forming.

An angular steel pipe (hereinafter, also referred to as a roll-formed angular steel pipe) made of roll forming materials is cold rolled to form a cylindrical open pipe from a hot-rolled steel plate, and its butt joints are electrically seam welded. Then, with rollers placed on the upper, lower, left, and right sides of the open pipe, a few% of the hole diameter is added to the cylindrical open pipe (round steel pipe) in the pipe axis direction to form an angled shape to form an angled steel pipe. On the other hand, the angled steel pipe (hereinafter also referred to as the press formed angled steel pipe) made of press-bent material is cold-press-bent-formed to form a cross-sectional shape of a zigzag (square shape). Or U-shaped (U-shaped), these are manufactured by submerged arc welding.

Comparing the manufacturing method of roll forming angle steel pipe with the manufacturing method of press forming angle steel pipe, it has the advantages of high productivity and can be manufactured in a short period of time. However, for the stamped and formed angle steel pipe, cold forming is not added to the flat part, and only the corner part is worked hardened. When the angle steel pipe is roll-formed, especially when cold formed into a cylindrical shape, the entire circumference of the steel pipe is in the direction of the pipe axis. Introduce larger processing strain. Therefore, not only the corners of the roll-formed angle steel pipes are flat, but also the production ratio in the pipe axis direction is high and the toughness is low.

Furthermore, the larger the thickness of the roll forming angle steel pipe, the greater the work hardening during roll forming, the higher the yield ratio and the lower the toughness. Therefore, it is necessary to select materials that can withstand the increase in output ratio and the decrease in toughness caused by roll forming, especially when manufacturing thick meat roll forming angle steel pipes.

For such a requirement, for example, Patent Document 1 proposes a microstructure in the flat portion, and an angular steel pipe with the area fraction of the bristled iron structure set to 40% or more.

Patent Document 2 proposes an angular steel pipe with excellent weldability and plastic deformation ability of the cold-worked part within the specified range of steel composition and cleanliness.

Patent Document 3 proposes an angled steel pipe with a low yield ratio and high toughness by performing annealing to obtain the strain of the entire pipe after making the pipe by cold forming. Prior art literature Patent literature

Patent Document 1: Japanese Patent No. 5385760 Patent Document 2: Japanese Patent No. 4611250 Patent Document 3: Japanese Patent No. 4957671

Problems to be solved by the invention

However, the techniques described in Patent Documents 1 and 2 presuppose the manufacture of angled steel pipes formed by press bending. Therefore, when the technology described in Patent Documents 1 and 2 is applied to the roll-formed angle steel pipe whose mechanical properties are greatly deteriorated during cold forming, there is a problem that the yield ratio and toughness cannot be achieved at the same time.

In addition, in the technique described in Patent Document 3, in order to obtain a low yield ratio and high toughness, it is necessary to perform a heat treatment on the angle steel pipe after pipe manufacturing. Therefore, the manufacturing cost becomes very high compared with the angle steel pipe which is directly cold worked.

The present invention was completed in view of the above-mentioned matters, and aims to provide an angled steel pipe with excellent strength, deformation performance and toughness, and a method for manufacturing the same, and a building structure using the angled steel pipe.

Furthermore, the term "excellent strength" in the present invention means that the flat plate portion of the angle steel pipe manufactured by cold roll forming (hereinafter also referred to as cold roll formed angle steel pipe) has a yield strength of 385 MPa or more. The tensile strength of the part is 520MPa or more. In addition, the term "excellent in deformation performance" in the present invention means that the cumulative plastic deformation magnification in the bending test of the angled steel pipe member is 28 or more. In addition, the term "excellent toughness" in the present invention means that the Charpy absorption energy at 0°C of the flat part of the angle steel pipe is 70 J or more. Means to solve the problem

The inventors of the present invention made diligent studies to solve the above-mentioned problems. As a result, the following findings (1) to (3) were obtained.

(1) In order to satisfy the yield strength and tensile strength of the flat plate portion of the angled steel pipe, the content of C must be 0.04% by mass or more. Furthermore, the main structure from the outer surface of the angled steel pipe to the position (surface part) of the 1/4t of the plate thickness t is defined as a mixed structure of ferrite and metabrimed iron, which will be determined by the orientation difference of adjacent crystals When the region surrounded by a boundary of 15° or more is defined as crystal grains, it is necessary to set the average circle equivalent diameter of the crystal grains to be less than 7.0 μm.

(2) In order to satisfy the deformability for the purpose of the present invention, it is necessary to set the yield ratio of the flat part to 0.90 or less, and the yield ratio of the flat part YRf to the corner part yield ratio YRc (YRc-YRf ) Is set at 0.09 or less. In order to set the output ratio of the flat part to 0.90 or less, it is necessary to select the residual structure from the outside of the angled steel pipe at the position of 1/4t of the plate thickness t to be selected from hard perlite, Matian loose iron, and Austin One or more of iron.

(3) In order to obtain the toughness of the flat plate part for the purpose of the present invention, the angle steel pipe meets the steel structure of both (1) and (2) above, in addition to the need for the average circle equivalent diameter of the crystal grains in (1) above It is set to be less than 7.0μm, and the volume ratio of crystal grains with an equivalent circle diameter of 40.0μm or more is set to 30% or less.

。 [2]如[1]所記載之角形鋼管，其中，平板部之降伏強度為385MPa以上，平板部之拉伸強度為520MPa以上，平板部之產量比為0.90以下，平板部在0℃之夏比吸收能量為70J以上。 [3]如[1]或[2]所記載之角形鋼管，其係除了前述成分組成之外，進一步以質量%含有選自下述A群及B群當中之1群或2群， 記 A群：選自Nb：0.15%以下、Ti：0.15%以下、V：0.15%以下當中之1種或2種以上 B群：選自Cr：1.0%以下、Mo：1.0%以下、Cu：0.5%以下、Ni：0.3%以下、Ca：0.010%以下、B：0.010%以下當中之1種或2種以上。 [4]如[1]～[3]中任一項所記載之角形鋼管，其中，前述鋼組織以體積率係變軔鐵為10%以上且未滿40%。 [5]一種角形鋼管之製造方法，其係將具有如[1]或[3]所記載之成分組成的鋼素材加熱至加熱溫度：1100℃以上1300℃以下後，實施 粗軋延結束溫度：850℃以上1150℃以下、精軋延結束溫度：750℃以上850℃以下、且於930℃以下之合計壓下率：65%以上的熱軋延， 其次，以板厚中心溫度且平均冷卻速度：10℃/s以上30℃/s以下、冷卻停止溫度：450℃以上650℃以下來實施冷卻， 其次，以450℃以上650℃以下捲繞而成為熱軋鋼板， 其次，藉由冷輥成形，實施將前述熱軋鋼板成形成圓筒狀後，再成形成角形狀，而成為角形之鋼管的造管步驟。 [6]一種建築構造物，其係將如[1]～[4]中任一項所記載之角形鋼管作為柱材使用。 發明效果The present invention was completed based on these findings, and is composed of the following gist. [1] An angled steel pipe, which is an angled steel pipe having a flat part and a corner part, characterized in that the component composition contains C: 0.04% or more and 0.50% or less, Si: 2.0% or less, and Mn: 0.5% or more 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 remainder is composed of Fe and inevitable impurities, from the outside of the tube to the thickness of the plate t The steel structure at the position of 1/4t is more than 30% ferrite and 10% or more of ferrite in volume ratio. The total of the ferrite and the ferrite is relative to 1 of the thickness t from the outside of the tube to The total steel structure at the /4t position is 70% or more and 95% or less, and the remaining part is composed of one or more types selected from perlite, Asada iron, and austenitic iron. The orientation difference of adjacent crystals When the area surrounded by a boundary of 15° or more is defined as crystal grains, the average circle equivalent diameter of the crystal grains is less than 7.0 μm, and the total equivalent circle diameter of 40.0 μm or more of the crystal grains is relative to the 1/4t position The total steel structure of the steel is 30% or less in volume ratio, and the yield ratio YRf of the flat part and the yield ratio YRc of the corner part satisfy the formula (1)

. [2] The angle steel pipe as described in [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 output ratio of the flat part is 0.90 or less, and the flat part is at 0℃ in summer The specific absorption energy is 70J or more. [3] The angled steel pipe as described in [1] or [2], which, in addition to the aforementioned component composition, further contains 1 or 2 groups selected from the following group A and group B by mass%, denoted as A Group: selected from one or more of Nb: 0.15% or less, Ti: 0.15% or less, V: 0.15% or more. B group: selected from Cr: 1.0% or less, Mo: 1.0% or less, Cu: 0.5% One or more of Ni: 0.3% or less, Ca: 0.010% or less, and B: 0.010% or less. [4] The angled steel pipe according to any one of [1] to [3], wherein the steel structure is changed to a volume ratio of bremsstrahlung of 10% or more and less than 40%. [5] A method for manufacturing an angle steel pipe, which is to heat a steel material with the composition described in [1] or [3] to a heating temperature: 1100°C or more and 1300°C or less, and then implement the rough rolling end temperature: 850°C or more and 1150°C or less, finishing temperature of finishing rolling: 750°C or more, 850°C or less, and 930°C or less Total reduction ratio: 65% or more of hot rolling, and secondly, based on the thickness center temperature and average cooling rate : 10°C/s or more and 30°C/s or less, cooling stop temperature: 450°C or more and 650°C or less to perform cooling, secondly, the hot-rolled steel sheet is wound at 450°C or more and 650°C or less, and secondly, it is formed by cold roll forming , Implement the pipe-making step of forming the aforementioned hot-rolled steel sheet into a cylindrical shape, and then forming an angular shape to form an angular steel pipe. [6] A building structure using the angle steel pipe described in any one of [1] to [4] as a column material. Invention effect

According to the present invention, it is possible to provide an angled steel pipe with excellent strength, deformation performance and toughness and a manufacturing method thereof.

Hereinafter, the present invention will be described in detail.

首先，針對限定本發明之角形鋼管的產量比之理由進行說明。The present invention is an angular steel pipe with a flat part and a corner part. The component composition includes 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, and P: 0.10% or less by mass% , S: 0.05% or less, Al: 0.005% or more, 0.10% or less, N: 0.010% or less, the remaining part is composed of Fe and inevitable impurities, from the outside of the tube to the steel structure at the position of 1/4t of the plate thickness t , The volume ratio of ferrite is more than 30%, and the variable bristled iron is more than 10%. The total of the ferrite and the variable bristled iron is relative to the whole steel structure from the outside of the tube to the position of 1/4t of the plate thickness t , Is more than 70% and less than 95%, the remaining part is composed of one or two types selected from perlite, sand iron, austenitic iron, and the azimuth difference of adjacent crystals is surrounded by a boundary of 15° or more When the region is defined as crystal grains, the average equivalent circle diameter of the crystal grains is less than 7.0μm, and the total equivalent circle diameter of the crystal grains is 40.0μm or more relative to the total steel structure at the position of 1/4t, by volume The rate is 30% or less, and the yield ratio YRf of the flat plate portion and the yield ratio YRc of the corner portion satisfy the formula (1).

First, the reason for limiting the output ratio of the angle steel pipe of the present invention will be explained.

As described above, even if the press-formed angle steel pipe and the roll-formed angle steel pipe are produced by either method, the corner parts are more work hardened than the flat part. Therefore, when the yield ratio of the flat part is YRf and the yield ratio of the corner part is YRc, the relationship becomes YRc≧YRf.

Therefore, in the present invention, the relationship between the yield ratio difference (YRc-YRf) of the flat part and the corner part of the angle steel pipe and the deformation performance was investigated. To investigate the relationship between the yield ratio difference and the deformation performance, the present invention is the result of a bending test using the angle steel pipe shown in FIG. 1. Fig. 1 is a schematic view for explaining the bending test of the angle steel pipe 1. Fig. 1(a) shows a side view of the test body, and Fig. 1(b) shows a cross-sectional view taken along the line A-A' shown in Fig. 1(a).

Prepare press-formed angle steel pipes and roll-formed angle steel pipes with the yield strength of the flat part of 385 MPa or more and the tensile strength of the flat part of 520 MPa or more, as shown in Figure 1(a), respectively make the angle steel pipe 1 through the longitudinal direction The test body passing through the partition plate 2 is welded at the center. The test body was carried out by fixing the movement in the horizontal and vertical directions, and the needle support (rotation support) was provided by the support materials 3 provided at both ends of the test body. The test body was subjected to repeated bending tests at the position of the arrow shown in Fig. 1(a) under a load of 45° (diagonal direction of the quadrangular section shown in Fig. 1(b)) to obtain the cumulative plastic deformation ratio.

Furthermore, the so-called cumulative plastic deformation magnification is the value obtained by subtracting the sum of the plastic rotation angles from the partial buckling or rupture of the test body until the endurance is rapidly reduced by the reference rotation angle corresponding to the full plastic moment. It means that the larger the value, the better the deformation performance when used as a column (column member), and the higher the energy absorption capacity during earthquakes.

Figure 2 shows a graph of the test results. In the graph shown in Figure 2, the yield strength of the flat part is 385 MPa or more, the tensile strength is 520 MPa or more, and the cumulative plastic deformation ratio of the roll forming angle steel pipe is the ratio of the output of the flat part to the corner part. Poor finishing. In Figure 2, the horizontal axis is "the yield ratio difference (YRc-YRf) between the flat portion and the corner of the angle steel pipe", and the vertical axis is the "cumulative plastic deformation magnification". As shown in Figure 2, it is understood that when the value of (YRc-YRf) is increased, the deformation performance (cumulative plastic deformation ratio) required as a column material is reduced. Furthermore, if the value of (YRc-YRf) is 0.09 or less, it is possible to stably obtain the required deformation performance as a column material (cumulative plastic deformation ratio: 28 or more).

Note that "cumulative plastic deformation ratio: 28 or more" is the necessary deformation performance for the column shown in Reference 1 below. Reference 1: The Architectural Society of Japan: Retaining endurance and deformation performance in seismic design of buildings (1990), 1990 In the above-mentioned bending test, it is believed that the deformation performance of the corner with larger deformation has a greater influence on the test result. The angled steel pipe with a larger value of (YRc-YRf) has a higher relative yield than YRc and a smaller elongation. As a result, it is estimated that the deformation performance is lowered. When the angle steel pipe is roll-formed, it is considered that since the value of (YRc-YRf) becomes 0.09 or less, the yield of the corner portion is relatively lower than that of YRc, and the above-mentioned bending test shows sufficient deformation performance.

為了滿足此(1)式，重要的是將所得之輥成形角形鋼管的成分組成、鋼組織及製造條件如後述般適當控制。From the above, it is understood that in order to ensure the characteristics targeted by the present invention, the difference between the yield ratio YRf of the flat part and the yield ratio YRc of the corner part must satisfy the following formula (1).

In order to satisfy this formula (1), it is important to appropriately control the component composition, steel structure, and manufacturing conditions of the resulting roll formed angle steel pipe as described later.

The angled steel pipe of the present invention and its manufacturing method will be described below.

In the present invention, the reason for limiting the component composition of the angle steel pipe will be explained. In this manual, unless otherwise specified, the "%" of the steel composition is the "mass%".

C: Above 0.04% and below 0.50% C is an element that improves the strength of steel by solid solution strengthening. In addition, C promotes the formation of perlite, improves the hardenability, and contributes to the formation of Asada iron. Since it contributes to the stabilization of austenitic iron, it is also an element that contributes to the formation of the hard phase. In order to ensure the strength and yield ratio for the purpose of the present invention, it is necessary to contain more than 0.04% 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 at 0.04% or more and 0.50% or less. The C content is preferably 0.08% or more, more preferably more than 0.12%, and still more preferably 0.14% or more. Moreover, 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 enhances the strength of steel by solid solution strengthening, and can be contained if necessary. In order to obtain such an effect, it is desirable to contain 0.01% or more of Si. However, when the Si content exceeds 2.0%, it becomes easy to generate oxides in the electro-seam welded part, which reduces the characteristics of the welded part. In addition, the toughness of the base material part other than the electric seam welding part is also reduced. Therefore, the Si content is set to 2.0% or less. The Si content is preferably 0.01% or more, more preferably 0.10% or more. Furthermore, the Si content is preferably 0.5% or less, more preferably 0.4% or less, and still more preferably 0.3% or less.

Mn: 0.5% above 3.0% Mn is an element that improves the strength of steel through solid solution strengthening. In addition, Mn is an element that contributes to the refinement of the structure by lowering the ferrite conversion start temperature. In order to ensure the strength and structure for the purpose of the present invention, it is necessary to contain 0.5% or more of Mn. However, when the Mn content exceeds 3.0%, it becomes easy to generate oxides in the electric seam welded portion, and the characteristics of the welded portion are reduced. In addition, in order to strengthen the solid solution and refine the structure, the yield strength is increased, and the desired yield ratio cannot be obtained. Therefore, the Mn content is determined to be 0.5% or more and 3.0% or less. The Mn content is preferably 0.7% or more, more preferably 0.9% or more, and still more preferably 1.0% or more. In addition, the Mn content is preferably 2.5% or less, more preferably 2.0% or less.

P: 0.10% or less P segregation in grain boundaries causes material inhomogeneity. It is preferable to reduce as an inevitable impurity as much as possible, but the content can be allowed to 0.10% or less. Therefore, the P content is set within the range of 0.10% or less. The P content is preferably 0.03% or less, more preferably 0.020% or less, and still more preferably 0.015% or less. In particular, although the lower limit of P is not specified, it is preferable to set P to be 0.002% or more because excessive reduction causes an increase in melting costs.

S: 0.05% or less Although S is generally present as MnS in steel, MnS is thinned and stretched in the hot rolling step, which has an adverse effect on ductility. Therefore, in the present invention, although it is preferable to reduce S as much as possible, the content can be allowed to below 0.05%. 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 still more preferably 0.008% or less. In particular, although the lower limit of S is not specified, it is preferable to set S to be 0.0002% or more because excessive reduction causes an increase in melting costs.

Al: 0.005% or more and 0.10% or less Al is an element that acts as a powerful deacidification agent. In order to obtain such effects, it is necessary to contain 0.005% or more of Al. However, when the Al content exceeds 0.10%, the weldability deteriorates, and the alumina-based inclusions increase and the surface properties deteriorate. In addition, the toughness of the welded portion is also reduced. Therefore, the Al content is set to 0.005% or more and 0.10% or less. The Al content is preferably 0.01% or more, more preferably 0.027% or more. In addition, the Al content is preferably 0.07% or less, and more preferably 0.04% or less.

N: 0.010% or less N is an inevitable impurity, which is an element that has the effect of reducing toughness by firmly fixing the movement of the index. In the present invention, although N as an impurity is expected to be reduced as much as possible, the allowable N content is 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 still more preferably 0.0035% or less. Furthermore, since excessive reduction causes an increase in the smelting cost, the N content is preferably set to 0.0010% or more, more preferably set to 0.0015% or more.

The remaining part is Fe and unavoidable impurities. However, in the range that does not impair the effects of the present invention, the content of O less than 0.005% will not be rejected.

The above-mentioned composition is the basic composition of the steel material of the angle steel pipe of the present invention. Although the above-mentioned essential elements are obtained in the present invention as the objective characteristics, the following elements may be contained if necessary.

One or two or more selected from Nb: 0.15% or less, Ti: 0.15% or less, V: 0.15% or less Nb, Ti, and V are all elements that form fine carbides and nitrides in steel and contribute to the improvement of the strength of steel through precipitation strengthening, and can be contained if necessary. In order to obtain such effects, when Nb, Ti, and V are contained, it is preferable to set Nb: 0.005% or more, Ti: 0.005% or more, and V: 0.005% or more, respectively. On the other hand, 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% or more and 0.10% or less, Ti: 0.008% or more and 0.10% or less, and V: 0.008% or more and 0.10% or less. 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.

Furthermore, when two or more selected from among Nb, Ti, and V are contained, since there is a risk of increasing the yield ratio and decreasing the toughness, it is preferable to set the total amount (the amount of Nb+Ti+V) to 0.15 %the following.

Selected from one or more of 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 Cr: 1.0% or less, Mo: 1.0% or less Cr and Mo are elements that improve the hardenability of steel and increase the strength of steel, and can be contained if necessary. In order to obtain the aforementioned effects, when Cr and Mo are contained, it is preferable to set Cr: 0.01% or more and Mo: 0.01% or more, respectively. On the other hand, excessive inclusion may cause a decrease in toughness and deterioration of weldability. Therefore, when Cr and Mo are contained, it is preferable to set Cr: 1.0% or less and Mo: 1.0% or less, respectively. Therefore, when Cr and Mo are contained, it is preferable to set Cr: 1.0% or less and Mo: 1.0% or less, respectively. Still, it is preferable to set Cr: 0.01% or more and Mo: 0.01% or more. More preferably, Cr: 0.10% or more and 0.50% or less, and Mo: 0.10% or more and 0.50% or less.

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 if necessary. In order to obtain the aforementioned effects, when Cu and Ni are contained, it is preferable to set Cu: 0.01% or more and Ni: 0.01% or more, respectively. On the other hand, excessive inclusion may cause a decrease in toughness and deterioration of weldability. Therefore, when Cu and Ni are contained, it is preferable to set Cu: 0.5% or less and Ni: 0.3% or less, respectively. Therefore, when Cu and Ni are contained, it is preferable to set Cu: 0.5% or less and Ni: 0.3% or less, respectively. Still, it is preferable to set Cu: 0.01% or more and Ni: 0.01% or more. More preferably, Cu: 0.10% or more and 0.40% or less, and Ni: 0.10% or more and 0.20% or less.

Ca: 0.010% or less Ca is an element that contributes to the improvement of the toughness of steel by spheroidizing the sulfide such as MnS, which is thinned and stretched in the hot rolling step, and can be contained if necessary. In order to obtain such effects, when Ca is contained, it is preferable to contain Ca at 0.0005% or more. However, when the Ca content exceeds 0.010%, Ca oxide clusters are formed in the steel, and the toughness may deteriorate. Therefore, when Ca is contained, the content of Ca is preferably set to 0.010% or less. Still, 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: Below 0.010% B is an element that contributes to the refinement of the structure by lowering the start temperature of ferrite conversion. In order to obtain such an effect, when B is contained, it is preferable to contain B at 0.0003% or more. However, when the B content exceeds 0.010%, the yield ratio may increase. Therefore, when B is contained, it is preferably set to 0.010% or less. Still, the B content is preferably set to 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 angle steel pipe of the present invention will be explained.

In the angled steel pipe of the present invention, the steel structure from the outer surface of the steel pipe at the position of 1/4t of the plate thickness t is more than 30% ferrite in volume ratio, and more than 10% bramble iron. The ferrite and The total amount of this variable bristled iron is 70% to 95% of the total steel structure at the position of 1/4t of the plate thickness t from the outside of the tube, and the remainder is made of 1 selected from perlite, Asada scattered iron, and Austenitic iron. One or two or more types. When the area surrounded by the azimuth difference of adjacent crystals with a boundary of 15° or more is regarded as crystal grains, the average circle equivalent diameter (average crystal grain size) of the crystal grains is less than 7.0μm, and the circle equivalent diameter (crystal The total size of the crystal grains of 40.0 μm or more is 30% or less in volume ratio with respect to the entire steel structure at the position of 1/4t of the plate thickness t from the outside of the tube.

In the present invention, the so-called equivalent circle diameter (crystal grain size) is defined as the diameter of a circle equal to the area of the target crystal grain. In addition, the steel structure is determined to be 1/4t of the plate thickness t from the outside of the flat part of the angled steel pipe except the electric seam welding part. Generally speaking, in roll-formed angle steel pipes using hot-rolled steel sheets as the material, the steel structures of the corners and flat portions at 1/4t of the plate thickness t from the outside of the pipe are the same. Therefore, the steel structure of the flat part is specified here.

The volume ratio of ferrite: more than 30%, the volume ratio of ferrite: 10% or more, the total volume ratio of ferrite and ferrite relative to the total steel structure: 70% or more and 95% or less Ferrite is a soft structure. By mixing with other hard structures, it reduces the yield ratio of steel. With such an effect, in order to obtain the low yield ratio for the purpose of the present invention, the volume ratio of ferrite needs to exceed 30%. The volume ratio of ferrite is preferably 40% or more, more preferably 43% or more, and still more preferably 45% or more. Although the upper limit is not particularly specified, in order to ensure the desired yield ratio, the volume ratio of ferrite is preferably less than 75%, more preferably less than 70%, and still more preferably 60% or less.

It changes the bramble iron into a structure with intermediate hardness and improves the strength of steel. Since the above-mentioned ferrite alone cannot obtain the yield strength and tensile strength which are the objects of the present invention, the volume ratio of the modified bremsstrahlung must be set to 10% or more. The volume ratio of bramble iron is preferably 15% or more, more preferably 20% or more, and still more preferably 25% or more. Although the upper limit is not specifically specified, in order to ensure the desired yield ratio, the volume ratio of bremsstrahlung iron is preferably 55% or less, more preferably 50% or less, still more preferably 45% or less, and even better Is less than 40%.

However, when the total volume ratio of ferrite and metabrimed iron is less than 70%, the yield ratio or Charpy absorbed energy targeted by the present invention cannot be obtained. On the other hand, when the total volume ratio of ferrite and metabrimed iron exceeds 95%, the yield strength and yield ratio targeted by the present invention cannot be obtained. Therefore, in addition to the above-mentioned conditions, it is necessary to set the total volume ratio of ferrite and bramble iron to 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.

Residual part: selected from one or more of perlite, Madian loose iron, and austenitic iron Perlite, Asada loose iron and austenitic iron are hard structures, especially to increase the tensile strength of steel, and by mixing with soft ferrite, reduce the yield ratio of steel. In order to obtain such an effect, it is preferable that the total volume ratio of perlite, Asada iron, and austenitic iron is 5% or more and 30% or less. More preferably, it is 7% to 25%. More preferably, it is 10% or more and 20% or less.

In addition, the volume ratio of ferrite, metabrimed iron, perlite, Asada iron, and austenitic iron can be measured by the method described in the following Examples.

When the azimuth difference (crystal azimuth difference) of adjacent crystals is defined as a crystal grain with a boundary of 15° or more, the average crystal grain size of the crystal grains: less than 7.0μm, and the crystal grain size is 40.0μm or more The total volume rate of crystal grains: 30% or less As described above, in order to obtain the low yield ratio, yield strength, and tensile strength for the purpose of the present invention, the steel structure of the present invention is defined as a steel with a mixed soft structure and a hard structure (hereinafter referred to as "composite structure steel"). However, the composite structure steel has poor toughness compared with the single structure steel. Therefore, in the present invention, in order to have the above-mentioned mechanical properties and excellent toughness, the average crystal grain size of the crystal grains is defined when the area surrounded by the boundary of the crystal orientation difference of 15° or more is defined as the crystal grains. When the average crystal grain size of the crystal grains is 7.0 μm or more, since the ferrite grains are not very fine, the desired yield strength and toughness cannot be obtained. Therefore, by setting the average crystal grain size of the crystal grains to be less than 7.0 μm, the yield strength which is the object of the present invention is obtained and the toughness is ensured. The average crystal grain size of the crystal grains is preferably set to be 6.5 μm or less, more preferably set to be 6.0 μm or less.

Generally speaking, the grain size distribution of single-structure steel or steel close to single-structure steel is based on a regular logarithmic distribution that has one peak and spreads greatly on the side of the larger variable and the side with the smaller variable. However, according to the present invention, it is understood that in the crystal grain size distribution of the composite structure steel containing ferrite and metabrimed iron, the peak of metabrimed iron newly appears on the coarse grain side.

Specifically, in the steel structure of the present invention, that is, the composite structure steel in which the volume ratio of ferrite exceeds 30% and the volume ratio of metabrimbrane iron exceeds 10%, the crystal grain size distribution is new on the coarse grain side. The peak of the variable bremsstrahlung appears. This system means mixed in the coarse change of bramble iron. Mixed in the coarse change bramble iron becomes the cause of the sharp deterioration of toughness. As a result, in the composite structure steel, even if the upper limit of the maximum crystal grain size is specified, it is impossible to suppress the proportion of coarse bramble iron that is present at a low level. Therefore, in order to obtain good toughness, it is also necessary to specify the upper limit of the proportion of coarse crystal grains.

Transformed bramble iron will not grow if it exceeds the boundary with larger azimuth difference (austenitic iron grain boundary or secondary grain boundary formed by accumulation of transposition). Therefore, to suppress the formation of the above-mentioned coarse deformed bramble iron, the finish rolling during hot rolling will be carried out as low as possible, and a large amount of transposition is introduced into the austenitic iron to increase the area of the secondary grain boundary and form the fine secondary. The crystal grain structure (hereinafter, also referred to as "refining") is particularly effective.

The toughness of the angled steel pipe of the present invention is improved by increasing the total area of grain boundaries that are resistant to brittle failure. In the present invention, it is newly discovered that through preliminary experiments, when the volume ratio of coarse crystal grains with a crystal grain size of 40.0 μm or more exceeds 30%, a sufficient grain boundary area cannot be ensured in order to obtain the necessary toughness. Therefore, in the present invention, in addition to the above-mentioned upper limit of the average crystal grain size of the crystal grains being less than 7.0 μm, the volume ratio of crystal grains having a crystal grain size of 40.0 μm or more is also regulated to 30% or less . The volume ratio of crystal grains having a crystal grain size of 40.0 μm or more is preferably set to 20% or less, more preferably set to 15% or less.

Furthermore, the crystal orientation difference, the average crystal grain size, and the volume ratio of crystal grains with 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 the following Examples.

In the present invention, from the outer surface of the steel pipe to the position of 1/4t of the plate thickness t as the center, within the range of ±1.0 mm in the plate thickness direction, the above-mentioned effects can be obtained even if the steel structure described above exists. Therefore, the “steel structure from the outside of the steel pipe to the position of 1/4t of the plate thickness t” in the present invention means that the center is from the outside of the steel pipe to the position of 1/4t of the plate thickness t. In any of the range of ±1.0 mm in the plate thickness direction, the above-mentioned steel structure exists.

Next, a method of manufacturing the angle steel pipe in one embodiment of the present invention will be explained.

The angle steel pipe of the present invention is implemented by, for example, heating the steel material with the above-mentioned composition to a heating temperature: 1100°C or more and 1300°C or less, and then performing rough rolling finishing temperature: 850°C or more and 1150°C or less, and finish rolling Rolling end temperature: 750°C or higher, 850°C or lower, and 930°C or lower total reduction ratio: 65% or higher of hot rolling, followed by plate thickness center temperature and average cooling rate: 10°C/s or more and 30°C/ s or less, cooling stop temperature: cooling is performed at 450°C or higher and 650°C or lower, followed by winding at 450°C or higher and 650°C or lower to form a hot-rolled steel sheet, and secondly, the aforementioned hot-rolled steel sheet is formed into a circle by cold roll forming After the cylindrical shape, it is then formed into an angular shape to obtain an angular steel pipe.

Still, in the description of the manufacturing method below, the "°C" for temperature means the surface temperature of the steel material or steel plate (hot-rolled plate) unless otherwise specified. These surface temperatures can be measured with radiation thermometers. In addition, the temperature at the center of the thickness of the steel plate can be obtained by calculating the temperature distribution in the cross section of the steel plate by heat transfer analysis, and correcting the result by the surface temperature of the steel plate. In addition, the "hot rolled steel sheet" includes hot rolled steel sheet and hot rolled steel strip.

In the present invention, the melting method of the steel material (steel blank) is not particularly limited, and it is suitable for any of the known melting methods such as a converter, an electric furnace, and a vacuum melting furnace. Although the casting method is not particularly limited, it is manufactured to a desired size by a known casting method such as a continuous casting method. Still, even if the continuous casting method is replaced, there is no problem in applying the block-slicing rolling method. The molten steel can be further subjected to secondary refining such as barrel refining.

Next, heat the obtained steel material (steel billet) to a heating temperature: 1100°C or more and 1300°C or less, and then perform rough rolling end temperature: set the rough rolling to 850°C or more and 1150°C or less, and implement the finish rolling end temperature : Set to finish rolling at 750°C or higher and 850°C or lower, and implement a hot rolling step at 930°C or lower: 65% or higher to form a hot-rolled sheet.

Heating temperature: 1100℃ above 1300℃ When the heating temperature is less than 1100°C, the deformation resistance of the rolled material increases and rolling becomes difficult. On the other hand, when the heating temperature exceeds 1300°C, the austenitic iron grains become coarse, and after rolling (rough rolling, finish rolling), fine austenitic iron grains cannot be obtained, which is the object of the present invention. The average crystal grain size of the steel structure of the square steel pipe becomes difficult. In addition, it is difficult to suppress the production of coarse bramble iron, and it is difficult to control the volume ratio of crystal grains having a crystal grain size of 40.0 μm or more within the range targeted by the present invention. Therefore, the heating temperature in the hot rolling step is set at 1100°C or more and 1300°C or less. More preferably, it is 1120°C or more and 1280°C or less.

Still, in the present invention, in addition to the conventional method of once cooling to room temperature and then reheating after manufacturing the steel blank (blank), the warm piece is directly charged into the heating furnace without cooling to room temperature, or a slight heat preservation is performed The energy-saving process of direct-feed rolling such as rolling immediately afterwards can also be applied without problems.

Finishing temperature of rough rolling: 850℃ above 1150℃ When the rough rolling end temperature is less than 850°C, the surface temperature of the steel sheet during the subsequent finish rolling is lower than the ferrite conversion start temperature, a large amount of ferrite is produced, and the volume ratio of bramble iron becomes less than 10%. On the other hand, when the rough rolling finishing temperature exceeds 1150°C, the reduction in the non-recrystallization temperature range of austenitic iron is insufficient, and fine austenitic iron particles cannot be obtained. As a result, it becomes difficult to secure the average crystal grain size of the steel structure of the angle steel pipe which is the object of the present invention. In addition, it becomes difficult to suppress the formation of coarse bristles. Therefore, the rough rolling end temperature is set at 850°C or more and 1150°C or less. More preferably, it is 860°C or more and 1000°C or less. More preferably, it is 870°C or more and 980°C or less.

Finish rolling end temperature: 750℃ above 850℃ When the finishing temperature of the finish rolling is less than 750°C, the surface temperature of the steel sheet during the finish rolling is below the ferrite conversion start temperature, a large amount of ferrite is produced, and the volume ratio of the bramble iron becomes less than 10%. On the other hand, when the finishing temperature of finish rolling exceeds 850°C, the reduction in the non-recrystallization temperature region of austenitic iron is insufficient, and fine austenitic iron particles cannot be obtained. As a result, it becomes difficult to secure the average crystal grain size of the steel structure of the angle steel pipe which is the object of the present invention. In addition, it becomes difficult to suppress the formation of coarse bristles. Therefore, the finishing temperature of finish rolling is set at 750°C or more and 850°C or less. More preferably, it is 770°C or more and 830°C or less. More preferably, it is 780°C or more and 820°C or less.

Total reduction rate below 930℃: 65% or more In the present invention, the secondary grains in the austenitic iron are refined in the hot rolling step, followed by the refining of the ferrite generated in the cooling step and the winding step, the modified bremsstrahlung iron, and the remaining part of the structure to obtain the structure in the present invention. The steel structure of the angle steel pipe for the purpose of strength and toughness. In the hot rolling step, in order to refine the secondary grains of the austenitic iron, it is necessary to increase the reduction rate in the non-recrystallization temperature range of the austenitic iron and to introduce sufficient processing strain. In order to achieve this, in the present invention, the total reduction ratio to the finish rolling end temperature of 930°C or lower is set to 65% or more.

When the total reduction ratio at the finishing temperature of the finish rolling below 930°C is less than 65%, the hot rolling step cannot introduce sufficient processing strain, so the structure with the crystal grain size targeted by the present invention cannot be obtained. . The total rolling reduction to the finishing temperature of the finish rolling below 930°C is more preferably 70% or more, and still more preferably 71% or more. Although the upper limit is not specifically defined, when it exceeds 80%, the effect of improving toughness relative to the increase in the reduction ratio is reduced, and only the equipment load is increased. Therefore, it is preferable that the total reduction ratio to the finish rolling end temperature of 930° C. or lower is 80% or lower. More preferably, it is 75% or less, and still more preferably 74% or less.

However, it is set below 930°C because when it exceeds 930°C, the austenitic iron is recrystallized in the rolling step, and it is not possible to obtain the refined austenitic iron in which the index introduced by the rolling disappears.

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

In addition, at the time of hot rolling and billeting, both the rough rolling and the finish rolling mentioned above can be hot rolling with the total rolling reduction to the finishing temperature of 930°C or lower at 65% or more. Alternatively, it may be hot rolling in which the total reduction ratio of the finishing temperature of the finishing rolling to 930°C or lower is 65% or more in the finishing rolling. In the latter case, when the total reduction ratio of the finishing temperature of the finish rolling to below 930°C cannot be set at 65% or more in the latter, the billet is cooled during the rough rolling and the temperature is lowered to 930°C or lower. The total reduction ratio of both rough rolling and finish rolling to the finishing temperature of finish rolling below 930°C is set at 65% or more.

In the present invention, the upper limit of the fine plate thickness is not specifically defined, but from the viewpoint of ensuring the necessary reduction ratio or the temperature management of the steel plate, the fine plate thickness is preferably set to be more than 20 mm and 32 mm or less.

After the hot rolling step, a cooling step is performed on the hot rolled sheet. In the cooling step, the average cooling rate to the cooling stop temperature: 10°C/s or more and 30°C/s or less, and the cooling stop temperature: 450°C or more and 650°C or less for cooling.

Average cooling rate from cooling start to cooling stop (cooling end): 10℃/s or more and 30℃/s or less When the average cooling rate of the thickness center temperature of the hot-rolled sheet in the temperature range from the start of cooling to the stop of the cooling described below is less than 10°C/s, the frequency of ferrite nucleation decreases and the ferrite grains become coarse Therefore, the average crystal grain size cannot be determined to be less than 7.0 μm. In addition, it is difficult to control the range of the volume ratio in which the crystal grain size targeted by the present invention is 40.0 μm or more. On the other hand, when the average cooling rate exceeds 30°C/s, a large amount of Asada scattered iron, ferrite, and meta-brimed iron are generated from the outer surface of the steel structure of the obtained angular steel pipe to the position of 1/4t of the plate thickness t. The sum of the volume ratios becomes less than 70%. The average cooling rate is preferably 15°C/s or more, more preferably 17°C/s or more. It is preferably 25°C/s or less, more preferably 23°C/s or less.

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

Cooling stop temperature: 450℃ above 650℃ When the thickness center temperature of the hot-rolled steel sheet and the cooling stop temperature are less than 450°C, the steel structure of the angle steel tube obtained reaches the position of 1/4t of the sheet thickness t, and a large amount of Asada scattered iron is generated, with iron oxide The sum of the volume ratios of the body and the variable bremsstrahlung iron becomes less than 70%. In addition, the volume ratio of ferrite may become 30% or less. On the other hand, when the cooling stop temperature exceeds 650°C, the frequency of ferrite nucleation is reduced, the ferrite grains become coarser, and the temperature at which the transformation of brambled iron is exceeded, so the volume ratio of brambled iron cannot be set to 10 %the above. The cooling stop temperature is preferably 480°C or higher, more preferably 490°C or higher. It is preferably 620°C or lower, more preferably 600°C or lower.

In the present invention, unless otherwise specified, the average cooling rate is determined as ((the thickness center temperature of the hot rolled sheet before cooling-the thickness center temperature of the hot rolled sheet after cooling)/cooling time) The value (cooling rate). The cooling method includes water cooling by spraying water from a nozzle or cooling by spraying cooling gas. In the present invention, it is preferable that both sides of the hot-rolled sheet are cooled in the same condition, and the cooling operation (treatment) is performed on both sides of the hot-rolled sheet.

After the cooling step, the hot-rolled sheet is coiled, and then the coiling step of cooling is performed. In the winding step, the winding temperature is 450°C or more and 650°C or less from the viewpoint of the steel sheet structure. When the winding temperature is less than 450°C, a large amount of Asada scattered iron is produced, and the total volume ratio of ferrite and metabrimed iron may be less than 70%. In addition, the volume ratio of ferrite may become 30% or less. When the winding temperature exceeds 650℃, the frequency of ferrite nucleation is reduced, and the ferrite grains become coarser and exceed the transformation start temperature of bramble iron. Therefore, the volume rate of bramble iron cannot be set to 10% or more. Case. The winding temperature is more preferably 480°C or more and 620°C or less, and still more preferably 490 to 590°C.

After the winding step, the tube making step is implemented. In the pipe making step, the hot-rolled steel sheet is formed into a cylindrical open pipe (round steel pipe) by roll forming, and the butt joint is welded by electric seam welding. Then, for the round steel pipe, by arranging the up, down, left, and right rollers, a few% of the hole diameter is directly added to the pipe axis in a cylindrical shape to form an angular shape to obtain an angular steel pipe.

Furthermore, in the angle steel pipe of the present invention, it is not limited to angle steel pipes with all sides of the same length (the value of (long side length/short side length) is 1.0), but also includes the (long side length/short side length) Angular steel pipe with a value exceeding 1.0. However, when the value of (long side length/short side length) of the angled steel pipe exceeds 2.5, it becomes easy to produce local buckling on the long side, reducing the compressive strength in the pipe axis direction. Therefore, the value of (long side length/short side length) of the angle-shaped steel pipe is preferably 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.

From the above, the angled steel pipe of the present invention is manufactured. According to the present invention, a square steel pipe with a yield strength of a flat part of 385 MPa or more, a tensile strength of a flat part of 520 MPa or more, a yield ratio of the flat part of 0.90 or less, and a Charpy absorption energy of 70 J or more of the flat part at 0°C is obtained. As a result, compared with cold press bending forming, it becomes possible to produce high-productivity, short-term (short-term), and high-strength roll forming angle steel pipes. This roll-formed angle steel pipe can be particularly suitable for use in construction components of large buildings such as factories, warehouses, and commercial facilities, so it can greatly contribute to the reduction of construction costs.

Therefore, the present invention can be particularly suitable for use in thick-fried angular steel pipes. Still, the so-called "thick meat" here means that the plate thickness of the flat part of the angle steel pipe exceeds 20mm.

Next, a description will be given of a building structure using the angular steel pipe in one embodiment of the present invention.

Fig. 3 schematically shows an example of a building structure using the aforementioned angular steel pipe of the present invention. As shown in Fig. 3, the building structure of the present embodiment is constructed by erecting a plurality of angle steel pipes 1 of the present invention and used as a column. Between the angled steel pipes 1 adjacent to each other, a plurality of beams 4 made of steel such as H-shaped steel are erected. In addition, between the adjacent beams 4, a plurality of small beams 5 made of steel such as H-shaped steel are erected. The H-shaped steel formed by the angle steel tube 1 and the beam 4 is welded and joined through the penetrating partition 6, and a beam 4 made of steel such as H-shaped steel is erected between the angle steel tubes 1 adjacent to each other. In addition, due to the installation of walls, etc., a spacer 7 can be provided if necessary.

Since the angled steel pipe 1 of the present invention has excellent strength, deformation performance and toughness, even if it is used in a large building, it can fully ensure the deformation performance of the entire structure. Therefore, the building structure of the present invention exhibits superior seismic performance compared with building structures using conventional angled steel pipes. Example

Hereinafter, the present invention will be described in further detail based on examples. However, the present invention is not limited to the following examples.

The molten steel having the composition shown in Table 1 was smelted in a converter, and an embryo (steel material: meat thickness 250 mm) was produced by a continuous casting method. The obtained blank was subjected to a hot rolling step, a cooling step, and a winding step under the conditions shown in Table 2 to form a hot-rolled steel sheet for an angle steel pipe.

After the winding step, proceed to the pipe making step shown below.

For a part of the hot-rolled steel plate for the angled steel pipe, it is formed into a cylindrical round steel pipe by roll forming, and the butt joint is welded by electric seam welding. Then, through the rolls placed on the upper, lower, left and right sides of the round steel pipe, a few% of the hole diameter is added to the pipe axis to form an angular shape, and the side length (mm) and plate thickness (mm) shown in Table 2 are obtained. Roll forming angle steel pipe.

Regarding the remaining hot-rolled steel sheet for the angled steel pipe, the cross-sectional shape is defined as a mouth-shaped or a U-shaped by press-bending forming, and these are joined by submerged arc welding to obtain the side length (mm) shown in Table 2 ), plate thickness (mm) stamping forming angle steel pipe.

Test pieces were taken from the obtained angular steel pipes (roll-formed angular steel pipes, press-formed angular steel pipes), and the following structure observation, tensile test, and Charpy impact test were carried out.

[Organization Observation] The test piece for structure observation was taken from the side adjacent to the side containing the welded part of the angled steel pipe (the side at the 3 o'clock or 9 o'clock side when the welded part is set in the 12 o'clock direction). The test piece for structure observation is taken from the flat plate part adjacent to this side, and the observation surface becomes a cross section in the tube axis direction from the hot rolling delay time, and the outside of the tube is taken to a position of 1/4t of the plate thickness t. After grinding, it is produced by etching with nitrate (Nital).

The structure observation system uses an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) to observe the structure from the outside of the flat part of the angle steel pipe to the position of 1/4t of the plate thickness t Shoot. From the obtained optical microscope image and SEM image, the area ratio of the ferrite, perlite, metabrimed iron, and remaining part of the structure was calculated. The area ratio of each tissue was observed using a test piece taken from a representative flat plate section at 5 or more fields of view, and calculated as the average value of the values obtained in each field of view. Here, the area ratio obtained by tissue observation is defined as the volume ratio of each tissue.

Here, the product of the ferrite system through diffusion transformation shows a structure with low translocation density and almost recovery. Polygonal ferrite and pseudo-polygonal ferrite are included here. In addition, the brambled iron is a composite structure of lath-shaped ferrite and carburized with high index density.

Still, in the optical microscope image and the SEM image, it is difficult to distinguish between Asada Scattered Iron and Austenitic Iron. Therefore, from the obtained SEM image, as Asada loose iron or austenitic iron, the area ratio of the observed structure is measured, and the value obtained by removing the volume ratio of the austenitic iron measured by the method described later is determined as Asada Volume rate of scattered iron.

The volume rate of austenitic iron is measured by X-ray diffraction. The test piece for structure observation is made by grinding the diffraction surface from the outer surface of the flat part of the steel pipe to the position of 1/4t of the plate thickness t, then chemically polishing and removing the surface processing layer. The Kα line of Mo was used in the measurement, and the volume ratio of austenitic iron was obtained from the integrated intensities of the (200), (220), and (311) planes of fcc iron and the (200) and (211) planes of bcc iron.

In addition, the average circle-equivalent diameter (average crystal grain size) and the volume ratio of crystal grains with a circle-equivalent diameter (crystal grain size) of 40.0 μm or more are measured using the SEM/EBSD method. The crystal grain size is determined by determining the azimuth difference between adjacent crystal grains, and determining the boundary with an azimuth difference of 15° or more as the crystal grain boundary. The arithmetic average of the particle size obtained from the obtained crystal grain boundaries is defined as the average crystal particle size. The measurement area is defined as 500 μm×500 μm, and the measurement step is defined as 0.5 μm. In the crystal particle size analysis, those with a crystal particle size of 2.0 μm or less are excluded from the analysis target as measurement noise, and the resulting area ratio is determined to be equal to the volume ratio.

[Stretching test] Fig. 4 is a schematic diagram showing the locations of the tensile test piece in the flat part and the tensile test piece in the corner part. Fig. 5 is a schematic diagram showing the detailed locations of the tensile test pieces at the corners.

The tensile test is as shown in FIG. 4, and JIS No. 5 tensile test specimens and JIS No. 12B tensile test specimens are taken from the flat part and the corner of the angle steel pipe so that the tensile direction becomes parallel to the pipe axis direction. Using these, implement in accordance with JIS Z 2241, measure yield strength YS and tensile strength TS, and calculate yield ratio defined by (yield strength)/(tensile strength). Still, the tensile test piece of the flat part is from the side part other than the side part of the welded part including the angled steel pipe (the welded part is set to the side of the 3 o'clock, 6 o'clock, or 9 o'clock side when the welded part is set in the 12 o'clock direction) Take the position of the center of the width of the flat plate (refer to Figure 4). The corner tensile test piece was taken from the corner of the angle steel pipe at a position of 45° (refer to Fig. 5). The number of test pieces is set to 2 pieces each, and the average value of these pieces is calculated to obtain YS, TS, and yield ratio.

[Charpy Impact Test] Fig. 6 is a schematic diagram showing the location of the Charpy test piece.

The Charpy impact test is shown in Figure 6. From the outside of the angled steel pipe to the position of 1/4t of the plate thickness t, the test piece is used in such a way that the longitudinal direction of the test piece becomes parallel to the pipe axis direction according to JIS Z 2242. V-shaped groove standard test piece. According to JIS Z 2242, the Charpy impact test is carried out at the test temperature: 0°C to obtain the absorbed energy (J). Still, the number of test pieces is set to 3 pieces each, and the average value of these pieces is calculated to obtain the absorbed energy (J).

The results obtained are shown in Table 3.

In Table 3, steel Nos. 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.

The angle-shaped steel pipe of the example of the present invention contains ferrite with a volume ratio of more than 30% in any steel structure, 10% or more of ferrite, and the total volume ratio of ferrite and ferrite is 70% If the remaining part is less than 95% of the above, the remaining part is made up of one or two or more types selected from perlite, Asada iron, and austenitic iron, and the area surrounded by a boundary with an azimuth difference of 15° or more is defined as crystal grains, The average circle equivalent diameter of the crystal grains is less than 7.0 μm, and the volume ratio of crystal grains with an equivalent circle diameter of 40.0 μm or more is 30% or less. Furthermore, the yield strength at the flat part is 385 MPa or more, the tensile strength at the flat part is 520 MPa or more, the yield ratio at 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. The difference in the corner yield ratio is 0.09 or less.

On the other hand, in Comparative Example Nos. 2, 6, 18, and 20, since any one of them is formed by press-bending to form an angular steel pipe, the difference in the yield ratio between the flat portion and the corner portion exceeds 0.09.

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

In Comparative Example No. 4, since the content of Mn is below the range of the present invention, the crystal grains are coarsened, and the average crystal grain size and the volume ratio of crystal grains with a crystal grain size of 40.0 μm or more are outside 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 expected values.

The heating temperature of the No. 7 embryo of the comparative example exceeds the range of the present invention, so the crystal grains are coarsened, and the average crystal grain size and the volume ratio of crystal grains with a crystal grain size of 40.0 μm or more are outside the range of the present invention. As a result, the tensile strength of the flat portion and the Charpy absorbed energy at 0°C did not reach the expected values.

Comparative Example No. 8 has a total reduction ratio below 930°C that is lower than the range of the present invention, and cannot suppress the production of coarsely deformed bramble iron. The volume ratio of crystal grains with a crystal grain size of 40.0 μm or more is outside the range of the present invention. . As a result, the Charpy absorption energy of the flat plate portion at 0°C did not reach the expected value.

No. 9 of the comparative example has a finishing temperature of finish rolling which is lower than the range of the present invention, a large amount of ferrite is generated during the hot rolling, and the volume ratio of the bristled iron becomes less than 10%. As a result, the yield strength and tensile strength of the flat portion did not reach the desired values.

No. 10 of the comparative example, because the finishing temperature of the finish rolling exceeds the range of the present invention, the total reduction at 930°C or less is lower than the range of the present invention, and the production of coarse bristled iron cannot be suppressed, and the crystal grain size is 40.0μm or more The volume ratio of the crystal grains is outside the scope of the present invention. As a result, the Charpy absorption energy of the flat plate portion at 0°C did not reach the expected value.

In Comparative Example No. 11, since the average cooling rate is lower than the range of the present invention, the crystal grains are coarsened, and the average crystal grain size and the volume ratio of crystal grains with a crystal grain size of 40.0 μm or more are outside 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 expected values.

In Comparative Example No. 12, since the average cooling rate exceeds the range of the present invention, the volume ratio of the ferrite is outside the range of the present invention. As a result, the yield ratio of the flat portion did not reach the desired value.

In No. 15 of the comparative example, the cooling stop temperature exceeds the range of the present invention, and the volume ratio of the bramble iron is outside the range of the present invention. As a result, the yield strength and tensile strength of the flat portion did not reach the desired values.

In Comparative Example No. 16, since the cooling stop temperature and the winding temperature are lower than the range of the present invention, the sum of the volume ratios of ferrite and metabrimed iron is outside the range of the present invention. As a result, the yield ratio of the flat portion did not reach the desired value.

In Comparative Example No. 21, since the content of C exceeds the range of the present invention, the sum of the volume ratios of ferrite and metabrimed iron is outside the range of the present invention. As a result, the Charpy absorption energy of the flat plate portion at 0°C did not reach the expected value.

In Comparative Example No. 23, since the content of Si exceeds the range of the present invention, the structure is not refined, and the yield strength is excessively increased due to solid solution strengthening. As a result, the Charpy absorption energy of the flat plate portion at 0°C did not reach the expected value.

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

In Comparative Example No. 25, it is considered that the P content exceeds the range of the present invention, thereby reducing the grain boundary strength. As a result, the Charpy absorption energy of the flat plate portion at 0°C did not reach the expected value.

In Comparative Example No. 26, it is considered that since the content of S exceeds the range of the present invention, coarse inclusions that become the starting point of destruction of MnS and the like are generated. As a result, the Charpy absorption energy of the flat plate portion at 0°C did not reach the expected value.

In No. 35 of the comparative example, since the content of C is lower than the range of the present invention, the yield strength and tensile strength of the flat plate portion are outside the range of the present invention. In addition, the formation of perlite of the hard phase is suppressed, and the total volume ratio of ferrite and variable bremsstrahlung iron is outside the scope of the present invention. As a result, the yield ratio of the flat portion did not reach the desired value.

In Comparative Example No. 47, since the cooling stop temperature and the winding temperature are lower than the range of the present invention, the volume ratio of the ferrite is outside the range of the present invention, and the yield ratio of the flat portion does not reach the desired value.

In Comparative Example No. 49, since the cooling rate is lower than the range of the present invention, the average crystal grain size is outside the range of the present invention, and the Charpy absorption energy at 0°C of the flat plate portion does not reach the desired value.

1: Angle steel pipe 2: Through the partition 3: Support material 4: beam 5: Trabecular 6: partition 7: Column

[Fig. 1] Fig. 1(a) and Fig. 1(b) are schematic diagrams of the bending test of the angle steel pipe implemented in the present invention. [Fig. 2] Fig. 2 is a graph showing the results of the bending test shown in Fig. 1(a) for roll-formed angle steel pipes and press-formed angle steel pipes having yield strength of 385 MPa or higher and tensile strength of 520 MPa or higher in the flat portion. [Fig. 3] Fig. 3 schematically shows a perspective view of an example of a building structure using the angled steel pipe of the present invention. [Fig. 4] Fig. 4 shows a schematic diagram of the locations of the flat tensile test pieces and corner tensile test pieces implemented in the present invention. [Fig. 5] Fig. 5 is a diagram showing the detailed picking position of the corner tensile test piece implemented in the present invention. [Fig. 6] Fig. 6 shows a schematic view of the collection position of the Charpy test piece implemented in the present invention.

Claims (7)

An angle steel pipe, which is an angle steel pipe with a flat part and a corner part, characterized in that the component composition contains 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 remainder is composed of Fe and inevitable impurities, from the outside of the tube to 1/th of the plate thickness t The steel structure at the 4t position is more than 30% of ferrite and 10% or more of ferrite in terms of volume ratio. The total of the ferrite and the ferrite is 1/4t from the outside of the tube to the plate thickness t The total steel structure at the position is 70% or more and 95% or less. The remaining part is composed of one or more types selected from perlite, Asada iron, and Austen iron. The orientation difference of adjacent crystals is 15 When the area surrounded by the boundary above ° is defined as crystal grains, the average circle equivalent diameter of the crystal grains is less than 7.0μm, and the total equivalent circle diameter of 40.0μm or more of the crystal grains is relative to the steel at the position of 1/4t For the whole organization, the volume ratio shall be less than 30%, The yield ratio YRf of the flat plate portion and the yield ratio YRc of the corner portion satisfy the formula (1) YRc-YRf≦0.09. . . (1). Such as the angle steel pipe of claim 1, wherein the yield strength of the flat part is 385MPa or more, the tensile strength of the flat part is 520MPa or more, the output ratio of the flat part is 0.90 or less, and the Charpy absorbed energy of the flat part at 0°C is 70J the above. For example, the angle-shaped steel pipe of claim 1 or 2, which, in addition to the aforementioned component composition, further contains 1 or 2 groups selected from the following group A and group B by mass%, denoted as group A: selected from Nb: 0.15 % Or less, Ti: 0.15% or less, V: 0.15% or less, one or more than two types B group: selected from 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 of them. Such as the angled steel pipe of claim 1 or 2, wherein the aforementioned steel structure is changed to a volume ratio of bremsstrahlung of 10% or more and less than 40%. Such as the angled steel pipe of claim 3, in which the aforementioned steel structure is changed to 10% or more and less than 40% of the bremsstrahlii in the volume ratio. A method for manufacturing angled steel pipes, which is to heat a steel material having the composition described in claim 1 or 3 to a heating temperature: 1100°C or more and 1300°C or less, and then perform rough rolling finish temperature: 850°C or more and 1150°C Below, finishing temperature of finishing rolling: 750℃ above 850℃, and below 930℃ Total reduction ratio: 65% or above hot rolling, secondly, based on plate thickness center temperature and average cooling rate: 10℃/s Above 30°C/s or less, cooling stop temperature: 450°C or more and 650°C or less for cooling, secondly, coiling at 450°C or more and 650°C or less to become a hot-rolled steel sheet, and secondly, by cold roll forming, the heat After the rolled steel sheet is formed into a cylindrical shape, it is then formed into an angular shape to form an angular steel pipe. A building structure that uses the angle-shaped steel pipe of any one of Claims 1 to 5 as a column.

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