Hot-rolled steel sheets, electric welded steel pipes, square steel pipes, line pipes, and building structures

Abstract

The present invention provides an electric resistance welded steel pipe and a rectangular steel pipe which have excellent anti-buckling performance, a hot-rolled steel sheet which is used as a material for the same, and a line pipe and a building structure which use the same.　Provided is a hot-rolled steel sheet having a specific component composition, wherein: in the steel structure in the plate thickness center, the total of ferrite and bainite by volume ratio is 70-98%, with the remainder being one or more structures selected from pearlite, martensite, and austenite; the average crystal grain size is not more than 15.0 μm; the value of CP as determined by a specific formula is not more than 0.090; tensile strength is not less than 400 MPa; and yield ratio is not more than 90%. Also provided are an electric resistance welded steel pipe and a rectangular steel pipe which use the hot-rolled steel sheet, and a line pipe and a building structure which use these.

Description

Technical Field

[0001] The present invention relates to electric welded steel pipes and square steel pipes, hot-rolled steel sheets used as materials therefor, line pipes and building structures using them.

Background Art

[0002] Electric welded steel pipes and roll-formed square steel pipes used for line pipes and building structures are required to have high strength in order to withstand the internal pressure of the fluid flowing inside and the load from the outside. At the same time, it is also required to have high buckling resistance from the viewpoint of seismic resistance.

[0003] Electric welded steel pipes and roll-formed square steel pipes (hereinafter sometimes referred to as "square steel pipes") are made of hot-rolled steel sheets (hot-rolled steel strips). By cold-rolling this into a cylindrical open pipe and electric resistance welding (sometimes referred to as electric resistance welding) the butt joint, a round steel pipe is obtained. The electric welded steel pipe is manufactured by adjusting the outer diameter and roundness with forming rolls arranged outside this round steel pipe. The square steel pipe is manufactured by further roll-forming this round steel pipe into a square shape with rolls having a hole shape of the desired polygonal shape. This method of manufacturing square steel pipes by roll forming has the advantage of high productivity compared to the method of manufacturing steel pipes by press bending. However, since a large tensile strain is applied in the pipe axis direction during roll forming, electric welded steel pipes and roll-formed square steel pipes have problems of low ductility in the pipe axis direction and low buckling resistance. In addition, it is required to select an appropriate hot-rolled steel sheet (hot-rolled steel strip) for the material to be roll-formed in consideration of the decrease in ductility due to roll forming.

[0004] Furthermore, the greater the wall thickness of electric welded steel pipes and roll-formed square steel pipes, the greater the processing strain during roll forming, resulting in further reduced ductility and further reduced buckling resistance.

[0005] In response to such requirements, for example, Patent Document 1 discloses a high-strength hot-rolled steel sheet excellent in uniform elongation after cold working, which contains C: 0.04 to 0.25%, N: 0.0050 to 0.0150%, and Ti: 0.003 to 0.050% by weight, and has a carbon equivalent (Ceq.) of 0.10 to 0.45% determined by a predetermined formula, and the pearlite phase is in the range of 5 to 20% in area fraction, and further TiN with an average particle size of 1 to 30 μm is dispersed in the steel at a ratio of 0.0008 to 0.015% by weight.

[0006] Patent Document 2 discloses a thick-walled hot-rolled square steel pipe for building structural members with a low yield ratio, which contains C: 0.07 to 0.18%, Mn: 0.3 to 1.5%, P: 0.03% or less, S: 0.015% or less, Al: 0.01 to 0.06%, N: 0.006% or less by mass%, and the balance is Fe and inevitable impurities, and has a structure with ferrite as the main phase and, as the second phase, pearlite, or pearlite and bainite, and the second-phase frequency defined by a predetermined formula is 0.20 to 0.42, and the average crystal grain size including the main phase and the second phase is 7 to 15 μm.

[0007] Patent Document 3 discloses an electric resistance welded steel pipe for a line pipe with a low yield ratio, characterized in that dislocations introduced in the forming process are pinned by carbon atom clusters, fine carbides, and Nb carbides by tempering after pipe making.

[0008] Patent Document 4 discloses a square steel pipe with a low yield ratio made of a hot-rolled steel sheet characterized in that ferrite is the main phase, the second-phase frequency is 0.05 to 0.15, and the second-phase area ratio is 3 to 15%, and the average crystal grain size of the main phase and the second phase at 1 / 4 thickness of the steel sheet is 10 to 25 μm.

[0009] Patent Document 5 discloses a square steel pipe manufactured by hot forming and characterized by having high deformation performance and toughness.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0011] However, these technologies have been studied regarding the characteristics during tensile deformation, that is, suppression of necking and fracture in the tensile part, and the study on local buckling in bending deformation and compression deformation of electric welded steel pipes and square steel pipes has not been sufficiently carried out.

[0012] In addition, as in Patent Documents 3 and 5, steel pipes subjected to heat treatment after pipe manufacturing or steel pipes manufactured by hot forming have a large yield elongation, so non-uniform deformation is likely to occur and the buckling resistance performance cannot be fully exhibited.

[0013] The present invention has been made in view of the above circumstances, and an object thereof is to provide an electric welded steel pipe and a square steel pipe having excellent buckling resistance performance, and a hot-rolled steel sheet used as a material thereof. Another object of the present invention is to provide a line pipe and a building structure using the above electric welded steel pipe and square steel pipe.

[0014] Here, "excellent in buckling resistance" as referred to in the present invention means that the stress increase rate τ (= σmax / σy) in the axial compression test satisfies τ ≧ 4.0×(t / D) + 0.85 for electric resistance welded steel pipes and τ ≧ 3.0×(t / B) + 0.85 for square steel pipes, respectively. However, t is the wall thickness (mm) of the electric resistance welded steel pipe or square steel pipe, D is the outer diameter (mm) of the electric resistance welded steel pipe, B is the side length (mm) of the square steel pipe, σy is the yield stress (N / mm 2 (= MPa)) of the base metal part of the electric resistance welded steel pipe or the flat part of the square steel pipe, and σmax is the maximum stress (N / mm 2 ) in the axial compression test, respectively. However, when the cross-sectional shape of the square steel pipe is a polygon with different side lengths, the average value of each side length is taken as the side length B of the square steel pipe. In the present invention, the hot-rolled steel sheet of the above material includes a hot-rolled steel strip.

Means for Solving the Problems

[0015] As a result of intensive studies to solve the above problems, it has been found that by making the cold-formed electric resistance welded steel pipe and the cold-formed square steel pipe have a low yield ratio and reducing the logarithmic standard deviation of the equivalent plastic strain distribution during deformation, their buckling resistance can be improved. That is, it has been found that the smaller the logarithmic standard deviation, the smaller the variation in plastic strain during deformation, the plastic strain is uniformly distributed, and it is difficult for strain to concentrate in a specific part, so local buckling is less likely to occur. It has also been found that the electric resistance welded steel pipe and the square steel pipe can be obtained by using a hot-rolled steel sheet with a small logarithmic standard deviation of the equivalent plastic strain distribution during deformation as the material. The present invention has been completed based on these findings and consists of the following gist. [1] The component composition is in mass%, C: 0.030% or more and 0.300% or less, Si: 0.010% or more and 0.500% or less, Mn: 0.30% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less containing, with the balance consisting of Fe and inevitable impurities, The steel structure at the center of the plate thickness is In terms of volume ratio, the total of ferrite and bainite is 70% or more and 98% or less, and the balance consists of one or more selected from pearlite, martensite, and austenite, the average crystal grain size is 15.0 μm or less, the value of CP obtained by the following formula (1) is 0.090 or less, the tensile strength is 400 MPa or more, and the yield ratio is 90% or less. A hot-rolled steel sheet. CP = (total length of large-angle grain boundaries in the region excluding crystal grains with a grain size less than 20 μm) / (total length of large-angle grain boundaries) ··· (1) [2] In addition to the above component composition, in terms of mass%, Nb: 0.100% or less, V: 0.100% or less, Ti: 0.150% or less, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ca: 0.0050% or less, B: 0.0050% or less, Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less, The hot-rolled steel sheet according to [1], containing one or more selected from the above. [3] The hot-rolled steel sheet according to [1] or [2], wherein the logarithmic standard deviation of the equivalent plastic strain distribution after applying 8.0% tensile strain is 0.70 or less. [4] An electric resistance welded steel pipe having a base metal part and an electric resistance welded part, and the component composition is, in terms of mass%, C: 0.030% or more and 0.300% or less, Si: 0.010% or more and 0.500% or less, Mn: 0.30% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less containing the balance being Fe and inevitable impurities, the steel structure at the center of the wall thickness is in terms of volume ratio, the total of ferrite and bainite is 70% or more and 98% or less, the balance being composed of one or more selected from pearlite, martensite, and austenite, the average crystal grain size is 15.0 μm or less, the value of CP obtained by the following formula (1) is 0.090 or less, the tensile strength of the base material part is 400 MPa or more, the yield ratio of the base material part is 97% or less, an electric welded steel pipe. CP = (total length of large-angle grain boundaries in the region excluding crystal grains with a grain size less than 20 μm) / (total length of large-angle grain boundaries) ··· (1) [5] In addition to the above component composition, in terms of mass%, Nb: 0.100% or less, V: 0.100% or less, Ti: 0.150% or less, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ca: 0.0050% or less, B: 0.0050% or less, Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less, including one or more selected from the above, the electric welded steel pipe according to [4]. [6] The electric welded steel pipe according to [4] or [5], wherein the logarithmic standard deviation of the equivalent plastic strain distribution after applying 4.0% tensile strain to the base material part is 0.60 or less. [7] A line pipe in which the electric welded steel pipe according to [4] or [5] is used. A line pipe using the electric resistance welded steel pipe described in [6]. [9] A square steel pipe having a flat part and a corner part, The component composition is in mass%, C: 0.030% or more and 0.300% or less, Si: 0.010% or more and 0.500% or less, Mn: 0.30% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less containing, with the balance being Fe and inevitable impurities, The steel structure at the center of the wall thickness is, in terms of volume ratio, the total of ferrite and bainite is 70% or more and 98% or less, the balance being composed of one or more selected from pearlite, martensite, and austenite, the average crystal grain size is 15.0 μm or less, the value of CP obtained by the following formula (1) is 0.090 or less, the tensile strength of the flat part is 400 MPa or more, A square steel pipe in which the yield ratio of the flat part is 97% or less. CP = (total length of large-angle grain boundaries in the region excluding crystal grains with a grain size less than 20 μm) / (total length of large-angle grain boundaries) ··· (1)

[10] In addition to the above component composition, further in mass%, Nb: 0.100% or less, V: 0.100% or less, Ti: 0.150% or less, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ca: 0.0050% or less, B: 0.0050% or less, Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less, The square steel pipe according to [9], comprising one or more selected from among them.

[11] The square steel pipe according to [9] or

[10] , wherein the logarithmic standard deviation of the equivalent plastic strain distribution after applying a 4.0% tensile strain in the flat plate portion is 0.60 or less.

[12] A building structure in which the square steel pipe according to [9] or

[10] is used as a column member.

[13] A building structure in which the square steel pipe according to

[11] is used as a column member. [Effect of the Invention]

[0016] According to the present invention, it is possible to provide an electric welded steel pipe and a square steel pipe having excellent buckling resistance, and a hot-rolled steel sheet used as a material therefor. Further, according to the present invention, it is possible to provide a line pipe and a building structure using the electric welded steel pipe and the square steel pipe. [Brief Description of the Drawings]

[0017]

Figure 1

[0018] Hereinafter, the present invention will be described in detail.

[0019] The hot-rolled steel sheet of the present invention contains, by mass%, C: 0.030% or more and 0.300% or less, Si: 0.010% or more and 0.500% or less, Mn: 0.30% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less, and the balance consists of Fe and unavoidable impurities, having a component composition. The steel structure at the center of the sheet thickness has a total volume ratio of ferrite and bainite of 70% or more and 98% or less, and the balance consists of one or more selected from pearlite, martensite, and austenite, with an average crystal grain size of 15.0 μm or less, and the value of CP obtained by the following formula (1) is 0.090 or less. Further, the tensile strength is 400 MPa or more, and the yield ratio is 90% or less. Further, the logarithmic standard deviation of the equivalent plastic strain distribution after applying 8.0% tensile strain is preferably 0.70 or less. CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size less than 20 μm) / (total length of high-angle grain boundaries) ··· (1)

[0020] The electric resistance welded steel pipe of the present invention has a base metal part and an electric resistance welded part, and contains, by mass%, C: 0.030% or more and 0.300% or less, Si: 0.010% or more and 0.500% or less, Mn: 0.30% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less, and the balance consists of Fe and unavoidable impurities, having a component composition. The steel structure at the center of the wall thickness has a total volume ratio of ferrite and bainite of 70% or more and 98% or less, and the balance consists of one or more selected from pearlite, martensite, and austenite, with an average crystal grain size of 15.0 μm or less, and the value of CP obtained by the above formula (1) is 0.090 or less. Further, the tensile strength of the base metal part is 400 MPa or more, and the yield ratio of the base metal part is 97% or less. Further, the logarithmic standard deviation of the equivalent plastic strain distribution after applying 4.0% tensile strain to the base metal part is preferably 0.60 or less.

[0021] The square steel pipe of the present invention has a flat part and a corner part, and in terms of mass%, C: 0.030% or more and 0.300% or less, Si: 0.010% or more and 0.500% or less, Mn: 0.30% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less, and the balance consists of Fe and inevitable impurities. The steel structure at the center of the wall thickness has a total of ferrite and bainite of 70% or more and 98% or less in terms of volume ratio, and the balance consists of one or more selected from pearlite, martensite, and austenite, the average crystal grain size is 15.0 μm or less, and the value of CP obtained by the above formula (1) is 0.090 or less. In addition, the tensile strength of the flat part is 400 MPa or more, and the yield ratio of the flat part is 97% or less. Further, the logarithmic standard deviation of the equivalent plastic strain distribution after applying a 4.0% tensile strain in the flat part is preferably 0.60 or less.

[0022] First, in the present invention, the reasons for limiting the component compositions of the hot-rolled steel sheet, the electric resistance welded steel pipe, and the square steel pipe will be explained below. In this specification, unless otherwise specified, "%" indicating the content of each component means "mass%".

[0023] C: 0.030% or more and 0.300% or less C is an element that increases the strength of steel by solid solution strengthening. In addition, C promotes the formation of pearlite, increases hardenability and contributes to the formation of martensite, and contributes to the stabilization of austenite, so it is also an element that contributes to the formation of hard phases. In order to ensure the strength targeted in the present invention, it is necessary to contain C at 0.030% or more. However, when the C content exceeds 0.300%, the proportion of the hard phase increases and the yield ratio targeted in the present invention cannot be obtained. Furthermore, the strain distribution during deformation when applying tensile strain becomes non-uniform, and the logarithmic standard deviation of the suitable equivalent plastic strain distribution targeted in the present invention cannot be obtained. For this reason, the C content is set to 0.030% or more and 0.300% or less. The C content is preferably 0.035% or more, more preferably 0.040% or more. Also, the C content is preferably 0.250% or less, more preferably 0.200% or less.

[0024] Si: 0.010% or more and 0.500% or less Si is an element that increases the strength of steel by solid solution strengthening. To obtain such an effect, it is desirable that Si be contained in an amount of 0.010% or more. However, when the Si content exceeds 0.500%, the proportion of the hard phase increases and the yield ratio targeted in the present invention cannot be obtained. Furthermore, the strain distribution during deformation when tensile strain is applied becomes non-uniform, and the logarithmic standard deviation of the suitable equivalent plastic strain distribution targeted in the present invention cannot be obtained. For this reason, the Si content is set to 0.500% or less. The Si content is preferably 0.020% or more, and more preferably 0.030% or more. Also, the Si content is preferably 0.400% or less, and more preferably 0.300% or less.

[0025] Mn: 0.30% or more and 2.50% or less Mn is an element that increases the strength of steel by solid solution strengthening. Also, Mn is an element that contributes to the refinement of the structure by lowering the transformation start temperature. To ensure the strength and structure targeted in the present invention, it is necessary that Mn be contained in an amount of 0.30% or more. However, when the Mn content exceeds 2.50%, the yield ratio targeted in the present invention cannot be obtained. Furthermore, the strain distribution during deformation when tensile strain is applied becomes non-uniform, and the logarithmic standard deviation of the suitable equivalent plastic strain distribution targeted in the present invention cannot be obtained. For this reason, the Mn content is set to 0.30% or more and 2.50% or less. The Mn content is preferably 0.40% or more, and more preferably 0.50% or more. Also, the Mn content is preferably 2.30% or less, and more preferably 2.10% or less.

[0026] P: 0.050% or less Since P segregates at grain boundaries and causes inhomogeneity in the material, it is preferably reduced as much as possible as an inevitable impurity, but a content of 0.050% or less is acceptable. Therefore, the P content is set to 0.050% or less. The P content is preferably 0.040% or less, and more preferably 0.030% or less. Although the lower limit of the P content is not particularly defined, excessive reduction leads to an increase in smelting cost, so the P content is preferably 0.002% or more.

[0027] S: 0.0200% or less In steel, S usually exists as MnS. However, MnS is thinly stretched in the hot rolling process and has an adverse effect on ductility and toughness. Therefore, in the present invention, it is preferable to reduce S as much as possible, but a content of 0.0200% or less is acceptable. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0150% or less, and more preferably 0.0100% or less. Although the lower limit of the S content is not particularly defined, excessive reduction leads to an increase in smelting cost, so the S content is preferably 0.0002% or more.

[0028] Al: 0.005% or more and 0.100% or less Al is an element that acts as a powerful deoxidizer when added to molten steel. To obtain such an effect, it is necessary for Al to contain 0.005% or more. However, when the Al content exceeds 0.100%, the weldability deteriorates, the amount of alumina-based inclusions increases, and the surface properties deteriorate. Therefore, the Al content is 0.005% or more and 0.100% or less. The Al content is preferably 0.010% or more, and more preferably 0.020% or more. Also, the Al content is preferably 0.080% or less, and more preferably 0.060% or less.

[0029] N: 0.0100% or less N is an inevitable impurity and is an element that has the effect of increasing the yield ratio by firmly fixing the movement of dislocations. In the present invention, it is desirable to reduce N as much as possible as an impurity, but the content of N can be tolerated up to 0.0100%. For this reason, the N content is set to 0.0100% or less. The N content is preferably 0.0090% or less, and more preferably 0.0080% or less. Note that excessive reduction leads to an increase in smelting costs, so the N content is preferably 0.0010% or more, and more preferably 0.0015% or more.

[0030] The balance can be Fe and inevitable impurities. Examples of the inevitable impurities in the balance include Sn, As, Sb, Bi, Co, Pb, Zn, and O. However, as long as the effects of the present invention are not impaired, it does not exclude containing Sn at 0.1% or less, As, Sb, and Co at 0.05% or less each, and Bi, Pb, Zn, and O at 0.005% or less each.

[0031] The above components are the basic component compositions of the hot-rolled steel sheet, electric resistance welded steel pipe, and square steel pipe in the present invention. Although the characteristics targeted in the present invention can be obtained with the above essential elements, the following elements can be contained within the following content ranges as required.

[0032] One or more selected from Nb: 0.100% or less, V: 0.100% or less, Ti: 0.150% or less, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ca: 0.0050% or less, B: 0.0050% or less, Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less

[0033] Nb: 0.100% or less, V: 0.100% or less, Ti: 0.150% or less Nb, Ti, and V are all elements that can form fine carbides and nitrides in steel and contribute to the improvement of the strength of steel through precipitation strengthening, and can be contained as needed. The content of Nb, Ti, and V may each be 0%, but when containing Nb, Ti, and V, the preferred content is Nb: 0.001% or more, Ti: 0.001% or more, and V: 0.001% or more, respectively. More preferred contents are Nb: 0.008% or more, V: 0.008% or more, and Ti: 0.008% or more, respectively. On the other hand, excessive content may lead to an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when containing Nb, Ti, and V, it is preferably Nb: 0.100% or less, V: 0.100% or less, and Ti: 0.150% or less, respectively. More preferred contents are Nb: 0.070% or less, V: 0.070% or less, and Ti: 0.110% or less, respectively. In addition, when containing two or more selected from Nb, Ti, and V, there is a risk of increasing the yield ratio and the logarithmic standard deviation of the equivalent plastic strain distribution, so the total amount (total content of Nb + Ti + V) is preferably 0.150% or less.

[0034] Cr: 0.50% or less, Mo: 0.50% or less Cr and Mo are each elements that can increase the hardenability of steel and increase the strength of steel, and can be contained as needed. The content of Cr and Mo may each be 0%, but when containing Cr and Mo, the preferred content is Cr: 0.01% or more and Mo: 0.01% or more, respectively. More preferred contents are Cr: 0.10% or more and Mo: 0.10% or more, respectively. On the other hand, excessive content may lead to an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when containing Cr and Mo, it is preferably Cr: 0.50% or less and Mo: 0.50% or less, respectively. More preferred contents are Cr: 0.30% or less and Mo: 0.30% or less, respectively.

[0035] Cu: 0.50% or less, Ni: 0.50% or less Cu and Ni are elements that increase the strength of steel by solid solution strengthening and can be contained as needed. The content of Cu and Ni may each be 0%, but when containing Cu and Ni, the preferred content is each Cu: 0.01% or more and Ni: 0.01% or more. The more preferred content is each Cu: 0.10% or more and Ni: 0.10% or more. On the other hand, excessive content may lead to an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when containing Cu and Ni, it is preferable that each is Cu: 0.50% or less and Ni: 0.50% or less. The more preferred content is each Cu: 0.35% or less and Ni: 0.35% or less.

[0036] Ca: 0.0050% or less Ca is an element that contributes to the improvement of the ductility and toughness of steel by spheroidizing sulfides such as MnS that are thinly stretched in the hot rolling process and can be contained as needed. The content of Ca may be 0%, but when containing Ca, the preferred content is 0.0002% or more. The more preferred content is Ca: 0.0010% or more. However, when the Ca content exceeds 0.0050%, Ca oxide clusters may be formed in the steel, and the ductility and toughness may deteriorate. For this reason, when containing Ca, the Ca content is preferably 0.0050% or less. The more preferred content is Ca: 0.0040% or less.

[0037] B: 0.0050% or less B is an element that contributes to the refinement of the structure by lowering the ferrite transformation start temperature. The content of B may be 0%, but when containing B, the preferred content is 0.0001% or more. The more preferred content is B: 0.0005% or more. However, when the B content exceeds 0.0050%, there is a risk of an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. For this reason, when containing B, the B content is preferably 0.0050% or less. The more preferred content is B: 0.0040% or less.

[0038] Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less Mg, Zr, and REM are each elements that increase the strength of steel through grain refinement and can be contained as needed. The content of Mg, Zr, and REM may each be 0%, but when containing Mg, Zr, and REM, the preferred content is Mg: 0.0005% or more, Zr: 0.0005% or more, and REM: 0.0005% or more, respectively. On the other hand, excessive content may lead to an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when containing Mg, Zr, and REM, it is preferably Mg: 0.020% or less, Zr: 0.020% or less, and REM: 0.020% or less, respectively. More preferred contents are Mg: 0.010% or less, Zr: 0.010% or less, and REM: 0.010% or less, respectively. Here, REM is a general term for a total of 17 elements including Sc, Y, and lanthanoid elements. One or more of these 17 elements can be contained in the steel, and the REM content means the total content of these elements.

[0039] Next, the reasons for limiting the steel structure of the hot-rolled steel sheet, electric resistance welded steel pipe, and square steel pipe in the present invention will be explained. Also, the limited steel structure described below is the steel structure at the center of the plate thickness or the center of the wall thickness and refers to the structure existing at the 1 / 2t position of the plate thickness. In the present invention, the 1 / 2t position of the plate thickness means the position at 1 / 2 (middle) of the plate thickness t in the plate thickness direction.

[0040] Total volume ratio of ferrite and bainite: 70% or more and 98% or less Ferrite and bainite are soft tissues, and by mixing them with other hard tissues, the yield ratio can be lowered. In order to obtain the low yield ratio targeted in the present invention due to such an effect, the total volume ratio of ferrite and bainite needs to be 70% or more. The total volume ratio of ferrite and bainite is preferably 75% or more, and more preferably 80% or more. However, when the total volume ratio of ferrite and bainite exceeds 98%, the tensile strength targeted in the present invention cannot be obtained, so the total volume ratio of ferrite and bainite needs to be 98% or less. The total volume ratio of ferrite and bainite is preferably 97% or less, and more preferably 95% or less.

[0041] The balance: one or more selected from pearlite, martensite, and austenite Pearlite, martensite, and austenite are hard tissues, and in particular, they can increase the strength of steel and achieve a low yield ratio by mixing with soft ferrite. In order to obtain such an effect, the balance other than ferrite and bainite is one or more selected from pearlite, martensite, and austenite. Pearlite, martensite, and austenite are 2% or more and 30% or less in total volume ratio. The total volume ratio is preferably 3% or more, and more preferably 5% or more. Also, the total volume ratio is preferably 25% or less, and more preferably 20% or less.

[0042] Note that the volume ratios of ferrite, bainite, pearlite, martensite, and austenite can be measured by the method described in the examples below.

[0043] Average crystal grain size: 15.0 μm or less When the average crystal grain size of the crystal grains exceeds 15.0 μm, the tensile strength targeted in the present invention cannot be obtained. Also, the logarithmic standard deviation of the suitable equivalent plastic strain distribution targeted in the present invention cannot be obtained. This is because when the average crystal grain size is large, the connectivity between coarse grains increases, so the strains generated in the coarse grains during deformation are connected to each other, and the strain distribution becomes more non-uniform as the deformation progresses. Therefore, the average crystal grain size of the crystal grains should be 15.0 μm or less. The average crystal grain size of the crystal grains is preferably 13.0 μm or less, and more preferably 10.0 μm or less. Note that when the average crystal grain size is small, the yield ratio increases, so the average crystal grain size is preferably 2.0 μm or more. The average crystal grain size is more preferably 3.0 μm or more.

[0044] CP value: 0.090 or less The CP value is a numerical value representing the connectivity between coarse grains with a particle size of 20 μm or more, and is obtained by the following formula (1). The larger the CP value, the higher the proportion of grain boundaries between coarse crystal grains, so the state where the coarse grains are more connected. When the CP value exceeds 0.090, the strains generated in the coarse grains during deformation are connected to each other, and the strain distribution becomes more non-uniform as the deformation progresses. Therefore, the logarithmic standard deviation of the suitable equivalent plastic strain distribution targeted in the present invention cannot be obtained. Therefore, the CP value should be 0.090 or less. The CP value is preferably 0.080 or less, and more preferably 0.070 or less. Note that the smaller the CP value, the more preferable it is. Although the lower limit is not particularly defined, excessive reduction will lead to an increase in manufacturing cost and manufacturing load. Therefore, the CP value is preferably 0.001 or more. CP = (total length of large-angle grain boundaries in the region excluding crystal grains with a particle size less than 20 μm) / (total length of large-angle grain boundaries) ···(1) Note that the "total length of large-angle grain boundaries in the region excluding crystal grains with a particle size less than 20 μm" in formula (1) refers to the total length of large-angle grain boundaries where crystal grains with a particle size of 20 μm or more are adjacent to each other.

[0045] Note that the average crystal grain size and the CP value can be measured by the SEM / EBSD method, and here they can be measured by the method described in the examples below.

[0046] Next, the reasons for limiting the properties in the tensile tests of the hot-rolled steel sheet, electric resistance welded steel pipe, and square steel pipe in the present invention will be explained.

[0047] Tensile strength of the hot-rolled steel sheet: 400 MPa or more If the tensile strength of the hot-rolled steel sheet is less than 400 MPa, the tensile strength of the electric resistance welded steel pipe and the tensile strength of the square steel pipe aimed at in the present invention cannot be obtained. Therefore, the tensile strength of the hot-rolled steel sheet is set to 400 MPa or more. The tensile strength of the hot-rolled steel sheet is preferably 420 MPa or more, and more preferably 450 MPa or more. The upper limit of the tensile strength of the hot-rolled steel sheet is not particularly limited. As an example, the tensile strength of the hot-rolled steel sheet is 700 MPa or less.

[0048] Yield ratio of the hot-rolled steel sheet: 90% or less If the yield ratio of the hot-rolled steel sheet exceeds 90%, the yield ratio of the electric resistance welded steel pipe and the yield ratio of the square steel pipe aimed at in the present invention cannot be obtained. Therefore, the yield ratio of the hot-rolled steel sheet is set to 90% or less. The yield ratio of the hot-rolled steel sheet is preferably 88% or less, and more preferably 85% or less. The lower limit of the yield ratio of the hot-rolled steel sheet is not particularly limited. As an example, the yield ratio of the hot-rolled steel sheet is 60% or more.

[0049] Logarithmic standard deviation of the equivalent plastic strain distribution after applying 8.0% tensile strain to the hot-rolled steel sheet: 0.70 or less The equivalent plastic strain distribution can be approximated by a lognormal distribution with the horizontal axis being the equivalent plastic strain (unit: none) and the vertical axis being the ratio (area ratio) (unit: %). In a lognormal distribution, the logarithm of the variable (horizontal axis) follows a normal distribution. Therefore, when the horizontal axis is the natural logarithm of the equivalent plastic strain (unit: none) and the vertical axis is the ratio (area ratio) (unit: %), it can be approximated by a normal distribution. In the present invention, the standard deviation at this time is defined as the "logarithmic standard deviation". The smaller the logarithmic standard deviation, the smaller the spread of the peak of the equivalent plastic strain distribution and the more uniform the distribution of plastic strain.

[0050] When the logarithmic standard deviation of the equivalent plastic strain distribution after applying 8.0% tensile strain to the hot-rolled steel sheet is 0.70 or less, it becomes easier to obtain the logarithmic standard deviation of the equivalent plastic strain distribution of the suitable electric resistance welded steel pipe and the logarithmic standard deviation of the equivalent plastic strain distribution of the square steel pipe, which are the objects of the present invention. Therefore, the logarithmic standard deviation after applying 8.0% tensile strain to the hot-rolled steel sheet is preferably 0.70 or less. The logarithmic standard deviation is more preferably 0.68 or less, and even more preferably 0.65 or less. Note that the smaller the logarithmic standard deviation, the more preferable it is. Although the lower limit is not particularly defined, excessive reduction will lead to an increase in manufacturing cost and manufacturing load. Therefore, the logarithmic standard deviation is preferably 0.050 or more.

[0051] Tensile strength of the base material part of the electric resistance welded steel pipe and tensile strength of the flat part of the square steel pipe: 400 MPa or more When the tensile strength of the base material part of the electric resistance welded steel pipe and the tensile strength of the flat part of the square steel pipe are less than 400 MPa, the buckling resistance performance deteriorates. Therefore, the tensile strength is 400 MPa or more. The tensile strength is preferably 420 MPa or more, and more preferably 450 MPa or more. The upper limit of the tensile strength is not particularly limited. As an example, the tensile strength is 700 MPa or less.

[0052] Yield ratio of the base material part of the electric resistance welded steel pipe and yield ratio of the flat part of the square steel pipe: 97% or less When the yield ratio of the base material part of the electric resistance welded steel pipe and the yield ratio of the flat part of the square steel pipe exceed 97%, the buckling resistance performance deteriorates. Therefore, the yield ratio is 97% or less. The yield ratio is preferably 96% or less, and more preferably 95% or less. The lower limit of the yield ratio is not particularly limited. As an example, the yield ratio is 75% or more.

[0053] Logarithmic standard deviation of the equivalent plastic strain distribution after applying 4.0% tensile strain to the base material part of the electric resistance welded steel pipe and the flat part of the square steel pipe: 0.60 or less When the logarithmic standard deviation of the equivalent plastic strain distribution after applying 4.0% tensile strain in the base material part of the electric resistance welded steel pipe and the flat plate part of the square steel pipe is 0.60 or less, the buckling resistance performance is more likely to be improved. Therefore, it is preferable that the logarithmic standard deviation is 0.60 or less. The logarithmic standard deviation is more preferably 0.58 or less, and even more preferably 0.55 or less. Note that the smaller the logarithmic standard deviation, the more preferable it is. Although the lower limit is not particularly defined, excessive reduction will lead to an increase in manufacturing cost and manufacturing load. Therefore, it is preferable that the logarithmic standard deviation is 0.050 or more.

[0054] Note that the tensile strength and yield ratio can be measured by the tensile test described in the examples below. Also, the logarithmic standard deviation of the equivalent plastic strain distribution can be measured by combining the tensile test and the SEM-DIC method described in the examples below. More specifically, the logarithmic standard deviation of the equivalent plastic strain distribution can be obtained by the method described in the examples below.

[0055] Next, a method for manufacturing a hot-rolled steel sheet, an electric resistance welded steel pipe, and a square steel pipe according to an embodiment of the present invention will be described.

[0056] The hot-rolled steel sheet of the present invention is obtained, for example, by subjecting a steel material having the above-described component composition to a heating step of heating to a heating temperature of 1100°C or higher and 1300°C or lower, followed by a hot rolling step of rolling at a finish rolling end temperature of 750°C or higher and 850°C or lower and an average cooling rate in the temperature range of 900°C or higher at the center temperature of the plate thickness of 1.0°C / s or higher to obtain a hot-rolled sheet. After the hot rolling step, a cooling step of cooling at an average cooling rate of 5°C / s or higher and 50°C / s or lower from the start of cooling to the stop of cooling at the center temperature of the plate thickness and a cooling stop temperature of 400°C or higher and 650°C or lower is performed. After the cooling step, a winding step of winding the hot-rolled sheet into a coil shape is performed.

[0057] Furthermore, the electric resistance welded steel pipe of the present invention is manufactured by forming the hot-rolled steel sheet into a cylindrical shape by cold roll forming, butting both circumferential ends of the cylindrical shape and performing electric resistance welding, and then adjusting the outer diameter and roundness by cold forming using a roll having a true circular hole shape.

[0058] In addition, the square steel pipe of the present invention is manufactured by forming the hot-rolled steel sheet into a cylindrical shape by cold roll forming, butting both circumferential ends of the cylindrical shape and performing electric seam welding, and then forming a flat portion and a corner portion by cold forming using a roll having a hole shape of a target polygonal shape. The square steel pipe of the present invention includes regular polygons (equilateral triangle, square, regular pentagon, etc.), equilateral polygons having combinations of different interior angles (rhombus, star shape, etc.), and polygons having combinations of different side lengths (isosceles triangle, rectangle, parallelogram, trapezoid, etc.). When the square steel pipe of the present invention is used as a column material for a building, since beams are usually joined in four directions at 90-degree intervals, it is preferable that the cross section is square or rectangular.

[0059] Note that the cylindrical shape refers to a shape in which the cross section of the pipe circumference is a "C" shape. In addition, in the description of the following manufacturing method, the "°C" display regarding temperature refers to the surface temperature of the steel material and the steel sheet (hot-rolled sheet) unless otherwise specified. These surface temperatures can be measured with a radiation thermometer or the like. In addition, the temperature at the center of the steel sheet thickness can be obtained by calculating the temperature distribution within the steel sheet cross section by heat transfer analysis and correcting the result by the surface temperature of the steel sheet.

[0060] In the present invention, the melting method of the steel material (steel slab) is not particularly limited, and any known melting method such as a converter, an electric furnace, or a vacuum melting furnace is applicable. The casting method is also not particularly limited, but it is manufactured to a desired size by a known casting method such as a continuous casting method. Note that there is no problem even if an ingot-block rolling method is applied instead of the continuous casting method. The molten steel may be further subjected to secondary refining such as ladle refining.

[0061] Next, the obtained steel material (steel slab) is heated to a heating temperature of 1100°C or higher and 1300°C or lower, and then subjected to a hot rolling process with a finish rolling end temperature of 750°C or higher and 850°C or lower, and an average cooling rate in a temperature range of 900°C or higher at the center temperature of the sheet thickness of 1.0°C / s or higher to obtain a hot-rolled sheet.

[0062] Heating temperature: 1100°C or higher and 1300°C or lower When the heating temperature is less than 1100°C, the deformation resistance of the material to be rolled (steel slab) increases, making rolling difficult. On the other hand, when the heating temperature exceeds 1300°C, the austenite grains coarsen, and fine austenite grains cannot be obtained in subsequent rolling (rough rolling and finish rolling), making it difficult to ensure the average crystal grain size targeted in the present invention. Also, it becomes difficult to suppress the generation of coarse grains, and it is difficult to control the CP value within the range targeted in the present invention. For this reason, the heating temperature in the heating furnace before hot rolling is set to be 1100°C or higher and 1300°C or lower. The heating temperature is more preferably 1120°C or higher. Also, the heating temperature is more preferably 1280°C or lower.

[0063] In addition, in the present invention, in addition to the conventional method of manufacturing a steel slab, cooling it to room temperature once and then reheating it, it can be charged into the heating furnace as a warm slab without cooling it to room temperature, or rolling can be performed immediately after slight heat retention. These energy-saving processes for direct rolling can also be applied without problems.

[0064] Finish rolling end temperature: 750°C or higher and 850°C or lower When the finish rolling end temperature is less than 750°C, the surface temperature of the steel plate during finish rolling becomes lower than the ferrite transformation start temperature, ferrite is generated, and it becomes processed ferrite grains elongated in the rolling direction by subsequent rolling, which causes an increase in the yield ratio. On the other hand, when the finish rolling end temperature exceeds 850°C, the reduction amount in the austenite non-recrystallization temperature range is insufficient, fine austenite grains cannot be obtained, and it becomes difficult to ensure the average crystal grain size targeted in the present invention. Also, it becomes difficult to suppress the generation of coarse grains, and it becomes difficult to control the CP value within the range targeted in the present invention. For this reason, the finish rolling end temperature is set to be 750°C or higher and 850°C or lower. The finish rolling end temperature is more preferably 760°C or higher. Also, the finish rolling end temperature is more preferably 840°C or lower.

[0065] Average cooling rate in the temperature range of 900°C or higher at the center temperature of the plate thickness: 1.0°C / s or higher In the present invention, by increasing the average cooling rate (hereinafter, may also be referred to as the average cooling rate in hot rolling) in the temperature range of 900 °C or higher at the center temperature of the plate thickness, the coarsening of austenite in the austenite recrystallization temperature range can be suppressed, and the average crystal grain size and CP value targeted in the present invention can be obtained. In order to achieve the average cooling rate, for example, the material to be rolled may be cooled using a water cooling facility during rolling. When the average cooling rate is less than 1.0 °C / s, austenite coarsens in the austenite recrystallization temperature range, making it difficult to secure the average crystal grain size targeted in the present invention. Further, it becomes difficult to suppress the generation of coarse grains, and it becomes difficult to control the CP value within the range targeted in the present invention. The average cooling rate is preferably 1.2 °C / s or more, more preferably 1.5 °C / s or more. When the average cooling rate exceeds 5.0 °C / s, the equipment load increases, so the average cooling rate is preferably 5.0 °C / s or less.

[0066] Note that the average cooling rate in the temperature range of 900 °C or higher at the center temperature of the plate thickness is determined as the average cooling rate at the center of the plate thickness from when the steel material (steel slab) is extracted from the heating furnace until the center temperature of the plate thickness reaches 900 °C. That is, the average cooling rate is obtained by [(the center temperature of the plate thickness (°C) when the steel material is extracted from the heating furnace - 900 (°C)) / the time (s) from when the steel material is extracted from the heating furnace until the center temperature of the steel material reaches 900 °C].

[0067] In the present invention, the upper limit of the finished plate thickness is not particularly defined, but from the viewpoint of ensuring the required cooling rate and steel plate temperature management, it is preferably 32 mm or less. Also, the lower limit of the finished plate thickness is not particularly limited, but as an example, the plate thickness is 5 mm or more.

[0068] After the hot rolling process, a cooling process is performed on the hot rolled sheet. In the cooling process, the average cooling rate from the start of cooling to the stop of cooling: 5 °C / s or more and 50 °C / s or less, and the cooling stop temperature: cooling is performed at 400 °C or more and 650 °C or less.

[0069] Average cooling rate from the start of cooling to the stop of cooling (end of cooling): 5 °C / s or more and 50 °C / s or less At the center temperature of the hot-rolled sheet thickness, if the average cooling rate (hereinafter, may also be referred to as the average cooling rate in the cooling process) in the temperature range from the start of cooling to the stop of cooling described below is less than 5 °C / s, the ferrite nucleation frequency decreases and the ferrite grains coarsen, making it difficult to ensure the average crystal grain size targeted in the present invention. Also, it becomes difficult to suppress the generation of coarse grains, and it is difficult to control the CP value within the range targeted in the present invention. On the other hand, when the average cooling rate exceeds 50 °C / s, a large amount of martensite is generated, and the total volume fraction of ferrite and bainite targeted in the present invention cannot be obtained. The average cooling rate is preferably 7 °C / s or more, more preferably 10 °C / s or more. Also, the average cooling rate is preferably 45 °C / s or less, more preferably 40 °C / s or less. In the cooling process, the start of intentional cooling such as water cooling is defined as the start of cooling, and the air cooling before that is not included in the cooling.

[0070] In the present invention, from the viewpoint of suppressing ferrite formation on the surface of the steel sheet before cooling, it is preferable to start cooling immediately after the finish of finish rolling.

[0071] Cooling stop temperature: 400 °C or more and 650 °C or less At the center temperature of the hot-rolled sheet thickness, if the cooling stop temperature is less than 400 °C, a large amount of martensite is generated, and the total volume fraction of ferrite and bainite targeted in the present invention cannot be obtained. On the other hand, when the cooling stop temperature exceeds 650 °C, the ferrite nucleation frequency decreases and the ferrite grains coarsen, making it difficult to ensure the average crystal grain size targeted in the present invention. Also, it becomes difficult to suppress the generation of coarse grains, and it is difficult to control the CP value within the range targeted in the present invention. The cooling stop temperature is preferably 420 °C or more, more preferably 450 °C or more. Also, the cooling stop temperature is preferably 620 °C or less, more preferably 600 °C or less.

[0072] In the present invention, unless otherwise specified, the average cooling rate in the cooling process is defined as the value obtained by ((the center temperature of the plate thickness of the hot-rolled plate before cooling - the center temperature of the plate thickness of the hot-rolled plate after cooling) / cooling time). Examples of the cooling method include water cooling such as water injection from nozzles and cooling by injection of cooling gas. In the present invention, it is preferable to perform a cooling operation (treatment) on both surfaces of the hot-rolled plate so that both surfaces of the hot-rolled plate are cooled under the same conditions.

[0073] After the cooling process, a coiling process is performed in which the hot-rolled plate is coiled and then air-cooled.

[0074] As described above, the hot-rolled steel sheet of the present invention is manufactured. The hot-rolled steel sheet of the present invention has a tensile strength of 400 MPa or more and a yield ratio of 90% or less. Furthermore, it can have a characteristic that the logarithmic standard deviation of the equivalent plastic strain distribution after applying 8.0% tensile strain is 0.70 or less.

[0075] In addition, the electric resistance welded steel pipe and the square steel pipe manufactured using the hot-rolled steel sheet as a raw material have a tensile strength of 400 MPa or more and a yield ratio of 97% or less. Furthermore, it can have a characteristic that the logarithmic standard deviation of the equivalent plastic strain distribution after applying 4.0% tensile strain is 0.60 or less. The electric resistance welded steel pipe and the square steel pipe of the present invention have excellent buckling resistance performance.

[0076] In addition, the line pipe and the building structure using the electric resistance welded steel pipe and the square steel pipe can have high buckling resistance performance. As a result, the building structure has high buckling resistance performance and can withstand external loads, so it is suitable for use as a column material for building structures.

Examples

[0077] Hereinafter, the present invention will be described in more detail based on examples. Note that the present invention is not limited to the following examples.

[0078] A steel material (steel slab) having the component composition shown in Table 1 was melted, and subjected to a heating process, a hot rolling process, and a cooling process under the conditions shown in Table 2 to obtain a hot-rolled steel sheet with the finish plate thickness (mm) shown in Table 2.

[0079]

Table 1

[0080]

Table 2

[0081] The hot-rolled steel sheet thus obtained was formed into a cylindrical open tube (round steel pipe) by cold roll forming, and the butt joint portion of the open tube was electric resistance welded to obtain a steel pipe material. Thereafter, the steel pipe material was formed by rolls arranged above, below, left, and right thereof to obtain an electric resistance welded steel pipe with the outer diameter D (mm) and wall thickness t (mm) shown in Table 3, or a square steel pipe with the side length B (mm) and wall thickness t (mm). The cross-sectional shape of the square steel pipe is square.

[0082]

Table 3

[0083] Test pieces were taken from the obtained hot-rolled steel sheet, electric resistance welded steel pipe, and square steel pipe, and the following microstructure observation, tensile test, and measurement of equivalent plastic strain distribution were carried out.

[0084] 〔Microstructure Observation〕 The test pieces for microstructure observation were sampled so that the observation surface was the cross-section in the rolling direction during hot rolling and at the 1 / 2t position of the plate thickness. After polishing, they were nitrided and prepared. Microstructure observation was carried out using an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) to observe and image the microstructure at the 1 / 2t position of the steel plate thickness. From the obtained optical microscope images and SEM images, the area ratios of ferrite, bainite, and the remaining microstructure (pearlite, martensite, austenite) were determined. The area ratio of each microstructure was observed in 5 fields of view and calculated as the average value of the values obtained in each field of view. Here, the area ratio obtained by microstructure observation was taken as the volume ratio of each microstructure.

[0085] Here, ferrite is a product by diffusion transformation and exhibits a microstructure with a low dislocation density and almost recovered state. Polygonal ferrite and pseudo-polygonal ferrite are included in this. Also, bainite is a duplex structure of lath-like ferrite with a high dislocation density and cementite. Also, pearlite is a microstructure in which cementite and ferrite are arranged in layers. Also, austenite does not have cement compared with bainite. Also, martensite and austenite were discriminated because the contrast of the SEM image was brighter compared with bainite.

[0086] Note that since it is difficult to distinguish martensite and austenite in the optical microscope image and SEM image, the area ratio of the microstructure 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 taken as the volume ratio of martensite.

[0087] The measurement of the volume ratio of austenite was carried out by X-ray diffraction. The test pieces for microstructure observation were ground so that the diffraction plane was at the 1 / 2t position of the steel plate thickness, and then chemically polished to remove the surface processing layer and prepared. For the measurement, Mo Kα rays were used, and the volume ratio of austenite was determined from the integrated intensities of the (200), (220), (311) planes of fcc iron and the (200), (211) planes of bcc iron.

[0088] The average crystal grain size and CP value were measured using the SEM / EBSD method. The measurement area was 500 μm × 500 μm, and the measurement step size was 0.5 μm. Based on the obtained EBSD data, using the crystal orientation analysis software OIM Analysis (trademark), boundaries with an orientation difference of 15° or more were defined as crystal grain boundaries (large-angle grain boundaries), and the distribution of grain boundaries was obtained. The average crystal grain size was determined as the arithmetic mean of the equivalent circle diameters (grain sizes) of each crystal grain. Also, the CP value was calculated as the ratio of the total length of large-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm and the total length of large-angle grain boundaries. Here, the total length of large-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm is the sum of the lengths of large-angle grain boundaries measured in the region excluding crystal grains with a grain size of less than 20 μm from the grain boundary distribution in the measurement area, and the total length of large-angle grain boundaries is the sum of the lengths of large-angle grain boundaries measured from the grain boundary distribution in the measurement area. In addition, in the calculation of the average crystal grain size and CP value, crystal grains with a grain size of 2.0 μm or less were excluded as measurement noise.

[0089] 〔Tensile Test〕 JIS No. 5 tensile test specimens were taken so that the tensile direction was parallel to the rolling direction. For hot-rolled steel sheets, the specimens were taken from a position 1 / 4W (W: sheet width) in the width direction from the end in the width direction, for electric resistance welded steel pipes, from a position 90° circumferentially away from the electric resistance welding part, and for square steel pipes, from the flat part adjacent to the flat part including the electric resistance welding part. The tensile test was carried out in accordance with the provisions of JIS Z 2241 (2011), and the yield stress σy and tensile strength were measured respectively, and the yield ratio defined by (yield stress σy) / (tensile strength) was calculated.

[0090] 〔Equivalent Plastic Strain Distribution〕 The equivalent plastic strain distribution was measured by the SEM-DIC method. Tensile test specimens shown in Fig. 1 were taken from the center of the thickness of the hot-rolled steel sheet, the center of the wall thickness of the electric resistance welded steel pipe, and the center of the wall thickness of the square steel pipe so that the tensile direction was parallel to the rolling direction. For the width direction of the hot-rolled steel sheet, it was taken at a position 1 / 4W (W: sheet width) in the width direction from the end of the width direction. For the circumferential direction of the electric resistance welded steel pipe, it was taken at a position 90° away from the electric resistance welding part in the circumferential direction. For the square steel pipe, it was taken at a flat part adjacent to the flat part including the electric resistance welding part. One surface of the obtained tensile test specimen was polished, nital-etched, and the parallel part (tensile deformation part) was imaged in 5 fields of view using SEM (magnification: 1000 times). Then, a tensile strain of 8.0% was applied to the test specimen taken from the hot-rolled steel sheet and a tensile strain of 4.0% was applied to the test specimens taken from the electric resistance welded steel pipe and the square steel pipe at a tensile speed of 5 mm / min, and then unloaded. Then, the same field of view as before the tension (before applying the tensile strain) was imaged using SEM (magnification: 1000 times). Based on the obtained SEM images before and after the tension, the equivalent plastic strain distribution of the imaged surface was calculated by the DIC method using the image analysis software GOM Correlate (GOM). The DIC method is a technique for measuring displacements and strains at various locations on the observation surface by comparing the random patterns on the object surface before and after deformation. Specifically, a square region called a subset is defined in the pre-deformation image, and the subset is tracked before and after deformation based on the random pattern inside the subset, and the displacement of the center point of the subset is calculated. This operation is performed comprehensively over the entire image to obtain the displacement distribution and strain distribution. In the present invention, the nital etching marks of the metal structure are used as the random pattern, and for an image of 1910 pixels × 2560 pixels, the subset size is 80 pixels × 80 pixels (3.6 μm × 3.6 μm) and the measurement interval is 10 pixels (0.45 μm). The one with the natural logarithm of the obtained equivalent plastic strain (unit: none) on the horizontal axis and the ratio (area ratio) (unit: %) on the vertical axis was approximated by a normal distribution, and the standard deviation at this time was defined as the logarithmic standard deviation (logarithmic standard deviation of the equivalent plastic strain distribution). Specifically, the logarithmic standard deviation was obtained by the following method. First, within the range where the equivalent plastic strain is 0 to 0.20, the ratio (area ratio) (unit: %) of each class was obtained with a class width of 0.02.At this time, the class where the equivalent plastic strain is 0 or more and less than 0.02 is the first class, the class where it is 0.02 or more and less than 0.04 is the second class, ···, and the class where it is 0.18 or more and less than 0.20 is the tenth class. x. i Taking the natural logarithm of the class value of the i-th class as i and the average value of the natural logarithm of the equivalent plastic strain as x0, the logarithmic standard deviation was obtained by the following formulas (4) and (5).

[0091]

Equation

[0092]

Equation

[0093] 〔Axial Compression Test〕 Pressure-resistant plates were attached to both ends of the electric resistance welded steel pipe and the square steel pipe, and an axial compression test was carried out using a large-scale compression test device. The stress when the compression load reached the maximum was defined as the maximum stress degree σmax (N / mm 2 ). Also, using the yield stress σy obtained by the above tensile test, the yield strength increase rate τ (= σmax / σy) was calculated.

[0094] The results obtained for the hot-rolled steel sheet are shown in Table 4.

[0095]

Table 4

[0096] The results obtained for the electric resistance welded steel pipe and the square steel pipe are shown in Table 5.

[0097]

Table 5

[0098] In Tables 4 and 5, No. 1 to 6 are the examples of the present invention, and No. 7 to 12 are the comparative examples.

[0099] In the hot-rolled steel sheet of the present invention example, the steel structure at the center of the sheet thickness has a total volume ratio of ferrite and bainite of 70% or more and 98% or less, and the balance is composed of one or more selected from pearlite, martensite, and austenite, the average crystal grain size is 15.0 μm or less, and the value of CP obtained by a predetermined formula (1) is 0.090 or less. Also, the tensile strength is 400 MPa or more, and the yield ratio is 90% or less. Further, the logarithmic standard deviation of the equivalent plastic strain distribution after applying 8.0% tensile strain was 0.70 or less.

[0100] Also, the electric resistance welded steel pipe and the square steel pipe of the present invention example are manufactured from the hot-rolled steel sheet of the present invention example. The steel structure at the center of the wall thickness has a total volume ratio of ferrite and bainite of 70% or more and 98% or less, and the balance is composed of one or more selected from pearlite, martensite, and austenite, the average crystal grain size is 15.0 μm or less, and the value of CP obtained by a predetermined formula (1) is 0.090 or less. Also, the tensile strength of the base material part or the flat plate part is 400 MPa or more, and the yield ratio of the base material part or the flat plate part is 97% or less. Further, the logarithmic standard deviation of the equivalent plastic strain distribution after applying 4.0% tensile strain to the base material part or the flat plate part was 0.60 or less. Also, the rate of increase in yield strength τ (= σmax / σy) in the axial compression test satisfied τ ≧ 4.0×(t / D) + 0.85 ··· (2) for the electric resistance welded steel pipe and τ ≧ 3.0×(t / B) + 0.85 ··· (3) for the square steel pipe. In Table 5, the values obtained by substituting the t and D of the electric resistance welded steel pipe and the t and B of the square steel pipe into the right sides of the above formulas (2) and (3) respectively are described as the required lower limit values of τ.

[0101] On the other hand, in Comparative Example No. 7, since the C content was below the range of the present invention, the tensile strength was outside the range of the present invention.

[0102] In Comparative Example No. 8, since the content of C exceeded the scope of the present invention, the total volume ratio of ferrite and bainite was lower than the scope of the present invention. As a result, the yield ratio was outside the scope of the present invention, and the logarithmic standard deviation was outside the preferable range, so the yield strength increase rate did not reach the desired value.

[0103] In Comparative Example No. 9, since the contents of Si and Mn were lower than the scope of the present invention, the total volume ratio of ferrite and bainite exceeded the scope of the present invention, and the average crystal grain size exceeded the scope of the present invention. As a result, the tensile strength was outside the scope of the present invention.

[0104] In Comparative Example No. 10, since the contents of Si and Mn exceeded the scope of the present invention, the total volume ratio of ferrite and bainite was lower than the scope of the present invention. As a result, the yield ratio was outside the scope of the present invention, and the logarithmic standard deviation was outside the preferable range, so the yield strength increase rate did not reach the desired value.

[0105] In Comparative Example No. 11, since the average cooling rate in the temperature range of 900 °C or higher in the hot rolling process was lower than the scope of the suitable manufacturing method, the average crystal grain size exceeded the scope of the present invention, and the CP value exceeded the scope of the present invention. As a result, the logarithmic standard deviation exceeded the preferable range of the present invention, and the yield strength increase rate did not reach the desired value. Also, the tensile strength was lower than the scope of the present invention.

[0106] In Comparative Example No. 12, since the cooling stop temperature in the hot rolling process exceeded the scope of the suitable manufacturing method, the CP value exceeded the scope of the present invention. As a result, the logarithmic standard deviation exceeded the preferable range of the present invention, and the yield strength increase rate did not reach the desired value.

[0107] From the above, by making the steel composition and structure within the scope of the present invention, it is possible to provide an electric resistance welded steel pipe and a square steel pipe excellent in buckling resistance performance, and a hot-rolled steel sheet used as a material thereof. Further, it is possible to provide a line pipe and a building structure having high buckling resistance performance using the above electric resistance welded steel pipe and square steel pipe.

Claims

1. The component composition is by mass percentage: C: 0.030% or more and 0.300% or less, Si: 0.010% or more and 0.500% or less, Mn: 0.30% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less is contained, and the balance consists of Fe and inevitable impurities, The steel structure at the center of the plate thickness is such that the total of ferrite and bainite is 70% or more and 98% or less by volume ratio, and the balance consists of one or more selected from pearlite, martensite, and austenite, the average crystal grain size is 15.0 μm or less, the value of CP obtained by the following formula (1) is 0.090 or less, the tensile strength is 400 MPa or more, and the yield ratio is 90% or less. A hot-rolled steel sheet. CP = (total length of large-angle grain boundaries in the region excluding crystal grains with a grain size less than 20 μm) / (total length of large-angle grain boundaries) ··· (1)

2. In addition to the above component composition, further by mass percentage, Nb: 0.100% or less, V: 0.100% or less, Ti: 0.150% or less, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ca: 0.0050% or less, B: 0.0050% or less, Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less, The hot-rolled steel sheet according to claim 1, containing one or more selected from the above.

3. The hot-rolled steel sheet according to claim 1 or 2, wherein the logarithmic standard deviation of the equivalent plastic strain distribution after applying 8.0% tensile strain is 0.70 or less.

4. An electric resistance welded steel pipe having a base metal part and an electric resistance welded part, The component composition is by mass percentage, C: 0.030% or more and 0.300% or less, Si: 0.010% or more and 0.500% or less, Mn: 0.30% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less is contained, and the balance consists of Fe and inevitable impurities, The steel structure at the center of the wall thickness is such that the total of ferrite and bainite is 70% or more and 98% or less by volume ratio, and the balance consists of one or more selected from pearlite, martensite, and austenite, the average crystal grain size is 15.0 μm or less, the value of CP obtained by the following formula (1) is 0.090 or less, the tensile strength of the base metal part is 400 MPa or more, Electric resistance welded steel pipe with a yield ratio of the base metal part of 97% or less. CP = (total length of large-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of large-angle grain boundaries) ··· (1)

5. In addition to the above component composition, further, in mass%, Nb: 0.100% or less, V: 0.100% or less, Ti: 0.150% or less, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ca: 0.0050% or less, B: 0.0050% or less, Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less, The electric resistance welded steel pipe according to claim 4, containing one or more selected from the above.

6. The electric resistance welded steel pipe according to claim 4 or 5, wherein the logarithmic standard deviation of the equivalent plastic strain distribution after applying a 4.0% tensile strain in the base metal part is 0.60 or less.

7. Line pipe in which the electric resistance welded steel pipe according to claim 4 or 5 is used.

8. Line pipe in which the electric resistance welded steel pipe according to claim 6 is used.

9. Square steel pipe having a flat part and a corner part, The component composition is, in mass%, C: 0.030% or more and 0.300% or less, Si: 0.010% or more and 0.500% or less, Mn: 0.30% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less is contained, and the balance consists of Fe and inevitable impurities, The steel structure at the center of the wall thickness is, in volume ratio, the total of ferrite and bainite is 70% or more and 98% or less, the balance consists of one or more selected from pearlite, martensite, and austenite, the average crystal grain size is 15.0 μm or less, the value of CP obtained by the following formula (1) is 0.090 or less, the tensile strength of the flat part is 400 MPa or more, the yield ratio of the flat part is 97% or less. Square steel pipe. CP = (total length of large-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of large-angle grain boundaries) ··· (1)

10. In addition to the above component composition, further, in mass%, Nb: 0.100% or less, V: 0.100% or less, Ti: 0.150% or less, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ca: 0.0050% or less, B: 0.0050% or less, Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less, The rectangular steel pipe according to claim 9, comprising one or more selected from among them.

11. The rectangular steel pipe according to claim 9 or 10, wherein the logarithmic standard deviation of the equivalent plastic strain distribution after applying a 4.0% tensile strain in the flat plate portion is 0.60 or less.

12. A building structure in which the rectangular steel pipe according to claim 9 or 10 is used as a column member.

13. A building structure in which the rectangular steel pipe according to claim 11 is used as a column member.

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