WO2024100939A1

WO2024100939A1 - Hot-rolled steel sheet, electric resistance welded steel pipe, rectangular steel pipe, line pipe, and building structure

Info

Publication number: WO2024100939A1
Application number: PCT/JP2023/027298
Authority: WO
Inventors: 晃英松本; 直道岩田; 信介井手
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2022-11-08
Filing date: 2023-07-26
Publication date: 2024-05-16
Also published as: JPWO2024100939A1; TW202419639A

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

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

The present invention relates to electric resistance welded steel pipes and square steel pipes, as well as hot-rolled steel sheets used as the raw materials for these pipes, and line pipes and building structures that use these pipes.

Electric resistance welded steel pipes and roll-formed square steel pipes used in line pipes and building structures are required to have high strength to withstand the internal pressure of the fluid flowing inside and external loads. At the same time, they are also required to have high buckling resistance from the perspective of earthquake resistance.

Electrical welded steel pipes and roll-formed rectangular steel pipes (hereinafter sometimes referred to as "rectangular steel pipes") are made from hot-rolled steel plate (hot-rolled steel strip). This is cold rolled to form a cylindrical open pipe, and the butt joint is electric resistance welded (sometimes referred to as electric resistance welding) to form a round steel pipe. Electric welded steel pipes are manufactured by adjusting the outer diameter and roundness using a forming roll placed on the outside of this round steel pipe. Square steel pipes are manufactured by further roll-forming this round steel pipe into a square shape using a roll with a desired polygonal hole shape. This method of manufacturing rectangular steel pipes by roll forming has the advantage of being more productive than the method of manufacturing steel pipes by press bending. However, since a large tensile strain is applied in the axial direction of the pipe during roll forming, electric welded steel pipes and roll-formed rectangular steel pipes have a problem of low ductility in the axial direction and low buckling resistance. In addition, it is required to select an appropriate hot-rolled steel plate (hot-rolled steel strip) for the material to be rolled, taking into account the decrease in ductility caused by roll forming.

Furthermore, the thicker the wall of electric resistance welded steel pipes and roll-formed square steel pipes, the greater the processing strain during roll forming, resulting in a further decrease in ductility and a further decrease in buckling resistance.

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

Patent Document 2 discloses a thick, hot-rolled steel sheet for rectangular steel pipes for architectural structural members with a low yield ratio, which has a composition, by mass%, of C: 0.07-0.18%, Mn: 0.3-1.5%, P: 0.03% or less, S: 0.015% or less, Al: 0.01-0.06%, N: 0.006% or less, with the balance being Fe and unavoidable impurities, and has a structure in which ferrite is the main phase and pearlite or pearlite and bainite are present as the second phase, the second phase frequency defined by a specific formula is 0.20-0.42, and the average crystal grain size including the main phase and the second phase is 7-15 μm.

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

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

Patent Document 5 discloses a square steel pipe manufactured by hot forming, characterized by high deformability and toughness.

Japanese Patent Application Laid-Open No. 7-224351 Patent No. 5589885 Japanese Patent No. 6052374 Patent No. 7031477 JP 2004-330222 A

However, these technologies were developed with a view to preventing necking and breakage in tensile sections, and no sufficient research was done into local buckling during bending or compressive deformation of electric resistance welded steel pipes and square steel pipes.

Furthermore, as in Patent Documents 3 and 5, steel pipes that have been heat-treated after pipe-making or steel pipes manufactured by hot forming have a large yield elongation, so they are prone to non-uniform deformation and are unable to fully demonstrate their buckling resistance.

The present invention has been made in consideration of the above circumstances, and has an object to provide electric resistance welded steel pipes and square steel pipes having excellent buckling resistance, as well as hot-rolled steel sheets used as raw materials for these pipes.
Another object of the present invention is to provide a line pipe and a building structure using the above-mentioned electric resistance welded steel pipe and square steel pipe.

Here, "excellent buckling resistance" in the present invention means that the yield strength 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. Here, t represents the wall thickness (mm) of the electric resistance welded steel pipe or square steel pipe, D represents the outer diameter (mm) of the electric resistance welded steel pipe, B represents the side length (mm) of the square steel pipe, σy represents the yield stress (N/mm ² (=MPa)) of the base material part of the electric resistance welded steel pipe or the flat part of the square steel pipe, and σmax represents the maximum stress intensity (N/mm ² ) in the axial compression test. 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 plate of the above-mentioned material includes a hot-rolled steel strip.

As a result of intensive research to solve the above problems, it was found that the buckling resistance of cold-formed electric resistance welded steel pipes and cold-formed rectangular steel pipes can be improved by giving them a low yield ratio and reducing the logarithmic standard deviation of the equivalent plastic strain distribution during deformation. In other words, it was found that the smaller the logarithmic standard deviation, the smaller the variation in plastic strain during deformation, the more uniform the distribution of plastic strain, and the less likely strain will concentrate in a specific part, making it less likely that local buckling will occur. It was also found that the electric resistance welded steel pipes and rectangular steel pipes can be obtained by using hot-rolled steel sheets with a small logarithmic standard deviation of the equivalent plastic strain distribution during deformation as their raw material.
The present invention has been completed based on these findings and comprises the following:
[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, with the balance being Fe and unavoidable impurities;
The steel structure at the center of the plate thickness is
The total volume fraction of ferrite and bainite is 70% or more and 98% or less,
The balance is one or more selected from pearlite, martensite, and austenite,
The average crystal grain size is 15.0 μm or less,
The CP value calculated by the following formula (1) is 0.090 or less,
The tensile strength is 400 MPa or more,
A hot-rolled steel sheet having a yield ratio of 90% or less.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1)
[2] In addition to the above-mentioned 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 hot-rolled steel sheet according to [1], comprising one or more selected from the following:
[3] The hot-rolled steel sheet according to [1] or [2], wherein the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 8.0% is 0.70 or less.
[4] An electric resistance welded steel pipe having a base metal portion and an electric resistance welded portion,
The composition is in mass percent:
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, the balance being Fe and unavoidable impurities;
The steel structure in the center of the wall thickness is
The total volume fraction of ferrite and bainite is 70% or more and 98% or less,
The balance is one or more selected from pearlite, martensite, and austenite,
The average crystal grain size is 15.0 μm or less,
The CP value calculated by the following formula (1) is 0.090 or less,
The tensile strength of the base material is 400 MPa or more,
An electric welded steel pipe having a base metal portion with a yield ratio of 97% or less.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1)
[5] In addition to the above-mentioned 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 welded steel pipe according to [4], comprising one or more selected from the following:
[6] An electric resistance welded steel pipe according to [4] or [5], in which the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 4.0% to the base metal portion is 0.60 or less.
[7] A line pipe, in which the electric resistance welded steel pipe according to [4] or [5] is used.
[8] A line pipe, in which the electric resistance welded steel pipe according to [6] is used.
[9] A square steel pipe having a flat portion and a corner portion,
The composition is in mass percent:
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, with the balance being Fe and unavoidable impurities;
The steel structure in the center of the wall thickness is
The total volume fraction of ferrite and bainite is 70% or more and 98% or less,
The balance is one or more selected from pearlite, martensite, and austenite,
The average crystal grain size is 15.0 μm or less,
The CP value calculated by the following formula (1) is 0.090 or less,
The tensile strength of the flat portion is 400 MPa or more,
A square steel pipe having a yield ratio of 97% or less in the flat portion.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1)
[10] In addition to the above-mentioned 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 the following:
[11] A square steel pipe according to [9] or [10], in which the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 4.0% to the flat portion is 0.60 or less.
[12] An architectural structure in which the square steel pipe according to [9] or [10] is used as a pillar material.
[13] An architectural structure in which the square steel pipe according to [11] is used as a pillar material.

According to the present invention, it is possible to provide electric resistance welded steel pipes and square steel pipes having excellent buckling resistance, as well as hot-rolled steel sheets used as raw materials for these pipes.
Furthermore, according to the present invention, it is possible to provide a line pipe and a building structure using the electric resistance welded steel pipe and the square steel pipe.

FIG. 1 is a schematic diagram of a tensile test piece used for measuring the distribution of equivalent plastic strain.

The present invention is described in detail below.

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

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

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

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

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, improves hardenability, contributes to the formation of martensite, and contributes to the stabilization of austenite, and therefore also contributes to the formation of a hard phase. In order to ensure the strength targeted in the present invention, C needs to be contained at 0.030% or more. However, if the C content exceeds 0.300%, the ratio 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 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. The C content is preferably 0.250% or less, more preferably 0.200% or less.

Si: 0.010% or more and 0.500% or less Si is an element that increases the strength of steel by solid solution strengthening. In order to obtain such an effect, it is desirable to contain Si at 0.010% or more. However, if the Si content exceeds 0.500%, the ratio of 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 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, more preferably 0.030% or more. The Si content is preferably 0.400% or less, more preferably 0.300% or less.

Mn: 0.30% or more and 2.50% or less Mn is an element that increases the strength of steel by solid solution strengthening. In addition, Mn is an element that contributes to fine structure by lowering the transformation start temperature. In order to ensure the strength and structure targeted in the present invention, Mn needs to be contained at 0.30% or more. However, if 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 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, more preferably 0.50% or more. The Mn content is preferably 2.30% or less, more preferably 2.10% or less.

P: 0.050% or less Since P segregates at grain boundaries and causes inhomogeneity in the material, it is preferable to reduce it as much as possible as an inevitable impurity, but a content of 0.050% or less is acceptable. Therefore, the P content is 0.050% or less. The P content is preferably 0.040% or less, and more preferably 0.030% or less. There is no particular lower limit for the P content, but since excessive reduction leads to high smelting costs, the P content is preferably 0.002% or more.

S: 0.0200% or less S is usually present in steel as MnS, but MnS is thinly drawn in the hot rolling process and has a negative effect on ductility and toughness. For this reason, in the present invention, it is preferable to reduce S as much as possible, but a content of 0.0200% or less is acceptable. For this reason, the S content is 0.0200% or less. The S content is preferably 0.0150% or less, and more preferably 0.0100% or less. Although there is no particular lower limit for the S content, since excessive reduction leads to an increase in smelting costs, the S content is preferably 0.0002% or more.

Al: 0.005% or more and 0.100% or less Al is an element that acts as a strong deoxidizer when added to molten steel. In order to obtain such an effect, it is necessary to contain 0.005% or more of Al. However, if the Al content exceeds 0.100%, the weldability deteriorates, and the amount of alumina-based inclusions increases, resulting in poor surface properties. For this reason, the Al content is set to 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. Moreover, the Al content is preferably 0.080% or less, and more preferably 0.060% or less.

N: 0.0100% or less N is an inevitable impurity and 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 an impurity as much as possible, but the N content 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.

The balance can be Fe and unavoidable impurities. Examples of unavoidable impurities in the balance include Sn, As, Sb, Bi, Co, Pb, Zn, and O. However, within the range that does not impair the effects of the present invention, this does not exclude the inclusion of 0.1% or less Sn, 0.05% or less As, Sb, and Co, and 0.005% or less Bi, Pb, Zn, and O.

The above components are the basic composition of the hot-rolled steel sheet, electric resistance welded steel pipe, and square steel pipe in this invention. The properties desired in this invention can be obtained with the essential elements listed above, but the following elements can be included as necessary within the content ranges listed below.

One or more selected from the following: 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

Nb: 0.100% or less, V: 0.100% or less, Ti: 0.150% or less Nb, Ti, and V are elements that form fine carbides and nitrides in steel and contribute to improving the strength of steel through strengthening by precipitation, and can be contained as necessary. The contents of Nb, Ti, and V may be 0%, but when Nb, Ti, and V are contained, the preferred contents are 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 Nb, Ti, and V are contained, it is preferable to set the Nb content to 0.100% or less, the V content to 0.100% or less, and the Ti content to 0.150% or less, respectively. More preferable contents are Nb: 0.070% or less, V: 0.070% or less, and Ti: 0.110% or less, respectively. Note that when two or more selected from Nb, Ti, and V are contained, there is a risk of increasing the yield ratio and increasing the logarithmic standard deviation of the equivalent plastic strain distribution, so it is preferable to set the total content (the total content of Nb + Ti + V) to 0.150% or less.

Cr: 0.50% or less, Mo: 0.50% or less Cr and Mo are elements that increase the hardenability of steel and increase the strength of steel, and can be contained as necessary. The contents of Cr and Mo may be 0%, but when Cr and Mo are contained, the preferred contents are 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 Cr and Mo are contained, it is preferable to set 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.

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 necessary. The contents of Cu and Ni may be 0%, but when Cu and Ni are contained, the preferred contents are Cu: 0.01% or more and Ni: 0.01% or more, respectively. More preferred contents are Cu: 0.10% or more and Ni: 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 Cu and Ni are contained, it is preferable to set Cu: 0.50% or less and Ni: 0.50% or less, respectively. More preferred contents are Cu: 0.35% or less and Ni: 0.35% or less, respectively.

Ca: 0.0050% or less Ca is an element that contributes to improving the ductility and toughness of steel by spheroidizing sulfides such as MnS that are thinly drawn in the hot rolling process, and can be contained as necessary. The Ca content may be 0%, but when Ca is contained, the preferred content is 0.0002% or more. A more preferred content is Ca: 0.0010% or more. However, when the Ca content exceeds 0.0050%, Ca oxide clusters are formed in the steel, and ductility and toughness may deteriorate. Therefore, when Ca is contained, the Ca content is preferably 0.0050% or less. A more preferred content is Ca: 0.0040% or less.

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 B is contained, the preferred content is 0.0001% or more. A more preferred content is B: 0.0005% or more. However, if 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. Therefore, when B is contained, the B content is preferably 0.0050% or less. A more preferred content is B: 0.0040% or less.

Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less Mg, Zr, and REM are elements that increase the strength of steel through grain refinement, and can be contained as necessary. The contents of Mg, Zr, and REM may each be 0%, but when Mg, Zr, and REM are contained, the preferred contents are Mg: 0.0005% or more, Zr: 0.0005% or more, and REM: 0.0005% or more, respectively. On the other hand, excessive content may cause an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when Mg, Zr, and REM are contained, it is preferable to set the contents to Mg: 0.020% or less, Zr: 0.020% or less, and REM: 0.020% or less, respectively. More preferable contents are Mg: 0.010% or less, Zr: 0.010% or less, and REM: 0.010% or less. Here, REM is a general term for 17 elements in total, 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.

Next, we will explain the reasons for limiting the steel structure of the hot-rolled steel sheet, electric resistance welded steel pipe, and square steel pipe in this invention. The limited steel structure described below refers to the steel structure at the center of the plate thickness or the center of the wall thickness, and exists at the 1/2t position of the plate thickness. In this invention, the 1/2t position of the plate thickness means the position 1/2 (middle) of the plate thickness t in the plate thickness direction.

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

Remainder: one or more selected from pearlite, martensite, and austenite Pearlite, martensite, and austenite are hard structures, and in particular they increase the strength of steel, and when mixed with soft ferrite, they can achieve a low yield ratio. In order to obtain such an effect, the remainder other than ferrite and bainite is one or more selected from pearlite, martensite, and austenite. The total volume fraction of pearlite, martensite, and austenite is 2% or more and 30% or less. The total of the volume fractions is preferably 3% or more, and more preferably 5% or more. The total of the volume fractions is preferably 25% or less, and more preferably 20% or less.

The volume fractions of ferrite, bainite, pearlite, martensite, and austenite can be measured by the method described in the examples below.

Average grain size: 15.0 μm or less If the average grain size of the grains exceeds 15.0 μm, the tensile strength targeted in the present invention cannot be obtained. In addition, the logarithmic standard deviation of the equivalent plastic strain distribution targeted in the present invention cannot be obtained. This is because, when the average grain size is large, the degree of connectivity between the coarse grains increases, so that the strains generated in the coarse grains during deformation are connected to each other, and the distribution of the strain becomes more non-uniform as the deformation progresses. For this reason, the average grain size of the grains is set to 15.0 μm or less. The average grain size of the grains is preferably set to 13.0 μm or less, more preferably set to 10.0 μm or less. Since the yield ratio increases when the average grain size is small, the average grain size is preferably 2.0 μm or more. The average grain size is more preferably 3.0 μm or more.

CP value: 0.090 or less The CP value is a value representing the degree of connectivity between coarse grains having a grain size of 20 μm or more, and is calculated by the following formula (1). The larger the CP value, the higher the proportion of grain boundaries between coarse crystal grains, so that the coarse grains are more connected to each other. If the CP value exceeds 0.090, the strain generated in the coarse grains during deformation will be connected to each other, and the distribution of strain will become more non-uniform as the deformation progresses, so that the logarithmic standard deviation of the equivalent plastic strain distribution, which is the object of the present invention, cannot be obtained. For this reason, the CP value is set to 0.090 or less. The CP value is preferably 0.080 or less, and more preferably 0.070 or less. The smaller the CP value, the more preferable it is, and there is no particular lower limit specified, but since excessive reduction leads to an increase in manufacturing costs and manufacturing load, the CP value is preferably set to 0.001 or more.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1)
In addition, in formula (1), "the total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm" refers to the total length of high-angle grain boundaries in the portion where crystal grains with a grain size of 20 μm or more are adjacent to each other.

The average crystal grain size and CP value can be measured by the SEM/EBSD method, and can be measured by the method described in the examples below.

Next, we will explain the reasons for limiting the properties in the tensile tests of the hot-rolled steel plate, electric resistance welded steel pipe, and square steel pipe in this invention.

Tensile strength of 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 targeted 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, more preferably 450 MPa or more. The upper limit of the tensile strength of the hot-rolled steel sheet is not particularly limited, but as an example, the tensile strength of the hot-rolled steel sheet is 700 MPa or less.

Yield ratio of 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 targeted 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, more preferably 85% or less. The lower limit of the yield ratio of the hot-rolled steel sheet is not particularly limited, but as an example, the yield ratio of the hot-rolled steel sheet is 60% or more.

Logarithmic standard deviation of equivalent plastic strain distribution after applying 8.0% tensile strain to hot-rolled steel sheet: 0.70 or less. The equivalent plastic strain distribution can be approximated by a logarithmic normal distribution with the horizontal axis representing equivalent plastic strain (unit: none) and the vertical axis representing the ratio (area ratio) (unit: %). In the logarithmic normal distribution, the logarithm of the variable (horizontal axis) follows a normal distribution. Therefore, it can be approximated by a normal distribution with the horizontal axis representing the natural logarithm of equivalent plastic strain (unit: none) and the vertical axis representing the ratio (area ratio) (unit: %). 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 becomes.

If 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 suitable logarithmic standard deviation of the equivalent plastic strain distribution of the electric resistance welded steel pipe and the logarithmic standard deviation of the equivalent plastic strain distribution of the square steel pipe that are the objective of the present invention. Therefore, it is preferable that the logarithmic standard deviation after applying 8.0% tensile strain to the hot-rolled steel sheet is 0.70 or less. The logarithmic standard deviation is more preferably 0.68 or less, and even more preferably 0.65 or less. The smaller the logarithmic standard deviation, the better, and no lower limit is specified, but since excessive reduction leads to increased manufacturing costs and manufacturing load, it is preferable that the logarithmic standard deviation is 0.050 or more.

Tensile strength of the base material of the electric resistance welded steel pipe and the flat plate of the square steel pipe: 400 MPa or more If the tensile strength of the base material of the electric resistance welded steel pipe and the tensile strength of the flat plate of the square steel pipe are less than 400 MPa, the buckling resistance performance decreases. Therefore, the tensile strength is set to 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, but as an example, the tensile strength is 700 MPa or less.

Yield ratio of the base material of the electric resistance welded steel pipe and the yield ratio of the flat plate of the square steel pipe: 97% or less If the yield ratio of the base material of the electric resistance welded steel pipe and the yield ratio of the flat plate of the square steel pipe exceed 97%, the buckling resistance performance decreases. Therefore, the yield ratio is set to 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, but as an example, the yield ratio is 75% or more.

Logarithmic standard deviation of equivalent plastic strain distribution after applying 4.0% tensile strain to the base material of the electric resistance welded steel pipe and the flat plate 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 to the base material of the electric resistance welded steel pipe and the flat plate of the square steel pipe is 0.60 or less, the buckling resistance performance is more easily 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. The smaller the logarithmic standard deviation, the more preferable, and there is no particular lower limit, but since excessive reduction leads to increased manufacturing costs and manufacturing loads, it is preferable that the logarithmic standard deviation is 0.050 or more.

The tensile strength and yield ratio can be measured by the tensile test described in the Examples below. The logarithmic standard deviation of the equivalent plastic strain distribution can be measured by combining the tensile test described in the Examples below and the SEM-DIC method. More specifically, the logarithmic standard deviation of the equivalent plastic strain distribution can be determined by the method described in the Examples below.

Next, we will explain the manufacturing method of hot-rolled steel sheet, electric resistance welded steel pipe, and square steel pipe in one embodiment of the present invention.

The hot-rolled steel sheet of the present invention can be obtained, for example, by subjecting a steel material having the above-mentioned composition to a heating process in which the material is heated to a temperature of 1100°C or higher and 1300°C or lower, followed by a hot rolling process in which the material is rolled to a finish rolling end temperature of 750°C or higher and 850°C or lower, and at an average cooling rate of 1.0°C/s or higher in the temperature range of 900°C or higher at the center of the plate thickness, to obtain a hot-rolled sheet, and after the hot rolling process, a cooling process is performed in which the material is cooled at an average cooling rate of 5°C/s or higher and 50°C/s or lower from the start of cooling to the end of cooling at the center of the plate thickness, and at a cooling end temperature of 400°C or higher and 650°C or lower, and after the cooling process, a winding process is performed in which the hot-rolled sheet is wound into a coil.

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 rolling, butting both circumferential ends of the cylindrical shape together and electric resistance welding, and then adjusting the outer diameter and roundness by cold forming using a roll with a circular hole shape.

The square steel pipe of the present invention is manufactured by forming the hot-rolled steel sheet into a cylindrical shape by cold rolling, butting both circumferential ends of the cylinder and electric resistance welding them, and then forming the flat plate portion and the corner portion by cold forming using a roll having a hole shape of the desired polygonal shape. The square steel pipe of the present invention includes regular polygons (equilateral triangle, square, regular pentagon, etc.), equilateral polygons with different combinations of interior angles (rhombus, star, etc.), and polygons with different combinations of side lengths (isosceles triangle, rectangle, parallelogram, trapezoid, etc.). When the square steel pipe of the present invention is used as a pillar material for a building, beams are usually joined on all four sides at 90 degree intervals, so it is preferable that the cross section is square or rectangular.

The cylindrical shape mentioned above refers to a circumferential cross section of the pipe that is "C" shaped. In addition, in the following explanation of the manufacturing method, the temperature indicated in "°C" refers to the surface temperature of the steel material or steel plate (hot-rolled plate) unless otherwise specified. These surface temperatures can be measured with a radiation thermometer or the like. The temperature at the center of the steel plate thickness can be found by calculating the temperature distribution in the cross section of the steel plate using heat transfer analysis, and correcting the result by the surface temperature of the steel plate.

In the present invention, the method of melting the steel material (steel slab) is not particularly limited, and any of the known melting methods such as converter, electric furnace, and vacuum melting furnace are suitable. The casting method is also not particularly limited, and the desired dimensions are produced by known casting methods such as continuous casting. Note that there is no problem in applying the ingot casting-blooming rolling method instead of the continuous casting method. The molten steel may further be subjected to secondary refining such as ladle refining.

Then, 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 in which the finish rolling end temperature is 750°C or higher and 850°C or lower, and the average cooling rate in the temperature range of 900°C or higher at the center of the plate thickness is 1.0°C/s or higher, to produce a hot-rolled plate.

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

In addition to the conventional method of producing a steel slab, first cooling it to room temperature and then reheating it, the present invention can also be used to easily apply energy-saving direct rolling processes in which the hot slab is loaded into the heating furnace without being cooled to room temperature, or is rolled immediately after a short period of heat retention.

Finish rolling end temperature: 750°C or more and 850°C or less When the finish rolling end temperature is less than 750°C, the steel sheet surface temperature becomes equal to or lower than the ferrite transformation start temperature during finish rolling, ferrite is generated, and the subsequent rolling results in processed ferrite grains elongated in the rolling direction, 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 grain size aimed at in the present invention. In addition, it becomes difficult to suppress the generation of coarse grains, and it becomes difficult to control the CP value to the range aimed at in the present invention. For this reason, the finish rolling end temperature is set to 750°C or more and 850°C or less. The finish rolling end temperature is more preferably 760°C or more. In addition, the finish rolling end temperature is more preferably 840°C or less.

Average cooling rate in the temperature range of 900°C or more at the plate thickness center temperature: 1.0°C/s or more In the present invention, by increasing the average cooling rate in the temperature range of 900°C or more at the plate thickness center temperature (hereinafter, sometimes referred to as the average cooling rate in hot rolling), it is possible to suppress the coarsening of austenite in the austenite recrystallization temperature range and obtain the average grain size and CP value targeted in the present invention. In order to achieve the average cooling rate, for example, the rolled material may be cooled using water cooling equipment during rolling. If the average cooling rate is less than 1.0°C/s, austenite will coarsen in the austenite recrystallization temperature range, making it difficult to ensure the average grain size targeted in the present invention. In addition, it becomes difficult to suppress the generation of coarse grains, making it difficult to control the CP value to 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. If 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.

The average cooling rate in the temperature range where the temperature at the center of the plate thickness is 900°C or higher is calculated 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 temperature at the center of the plate thickness reaches 900°C. In other words, the average cooling rate is calculated as [(Temperature at the center of the plate thickness when the steel material is extracted from the heating furnace (°C) - 900 (°C)) / Time (s) from when the steel material is extracted from the heating furnace until the temperature at the center of the plate thickness of the steel material reaches 900°C].

In the present invention, the upper limit of the finished plate thickness is not specified, but from the viewpoint of ensuring the necessary cooling rate and controlling the steel plate temperature, it is preferable that the thickness is 32 mm or less. In addition, the lower limit of the finished plate thickness is not particularly limited, but as an example, the plate thickness is 5 mm or more.

After the hot rolling process, the hot-rolled sheet is subjected to a cooling process. In the cooling process, the average cooling rate from the start of cooling to the end of cooling is 5°C/s or more and 50°C/s or less, and the cooling end temperature is 400°C or more and 650°C or less.

Average cooling rate from start of cooling to end of cooling (end of cooling): 5°C/s or more and 50°C/s or less When the average cooling rate in the temperature range from start of cooling to end of cooling (hereinafter, sometimes referred to as the average cooling rate in the cooling process) at the center temperature of the thickness of the hot-rolled sheet is less than 5°C/s, the frequency of ferrite nucleation decreases and the ferrite grains become coarse, making it difficult to ensure the average grain size targeted in the present invention. In addition, 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 ratio 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. In addition, 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 regarded as the start of cooling, and air cooling before that is not included in the cooling.

In the present invention, it is preferable to start cooling immediately after the end of finish rolling in order to suppress the formation of ferrite on the steel sheet surface before cooling.

Cooling stop temperature: 400°C or more and 650°C or less At the thickness center temperature of the hot-rolled sheet, if the cooling stop temperature is less than 400°C, a large amount of martensite is generated, and the total volume ratio of ferrite and bainite targeted in the present invention cannot be obtained. On the other hand, if the cooling stop temperature exceeds 650°C, the frequency of ferrite nucleation decreases and the ferrite grains become coarse, making it difficult to ensure the average grain size targeted in the present invention. In addition, 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. In addition, the cooling stop temperature is preferably 620°C or less, more preferably 600°C or less.

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

After the cooling process, the hot-rolled sheet is coiled and then cooled in the coiling process.

The hot-rolled steel sheet of the present invention is manufactured in this manner. The hot-rolled steel sheet of the present invention has the characteristics of a tensile strength of 400 MPa or more and a yield ratio of 90% or less. Furthermore, it can have the characteristics of a logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 8.0% of 0.70 or less.

In addition, electric resistance welded steel pipes and square steel pipes manufactured using the above-mentioned hot-rolled steel plate as a material have the characteristics of a tensile strength of 400 MPa or more and a yield ratio of 97% or less. Furthermore, they can have the characteristic of a logarithmic standard deviation of the equivalent plastic strain distribution after applying a 4.0% tensile strain of 0.60 or less. The electric resistance welded steel pipes and square steel pipes of the present invention have excellent buckling resistance.

Furthermore, line pipes and architectural structures using the electric resistance welded steel pipes and square steel pipes can have high buckling resistance. As a result, the architectural structures have high buckling resistance and can withstand external loads, making them suitable for use as pillar materials for architectural structures.

The present invention will be described in more detail below based on examples. Note that the present invention is not limited to the following examples.

Steel material (steel slab) with the composition shown in Table 1 was melted and subjected to the heating process, hot rolling process, and cooling process under the conditions shown in Table 2 to produce hot-rolled steel sheet with the finished thickness (mm) shown in Table 2.

The hot-rolled steel sheet thus obtained was formed into a cylindrical open pipe (round steel pipe) by cold rolling, and the butt joints of the open pipe were electric resistance welded to produce steel pipe material. The steel pipe material was then formed using rolls arranged above, below, left and right, to obtain electric resistance welded steel pipes with the outer diameter D (mm) and wall thickness t (mm) shown in Table 3, or square steel pipes with the side length B (mm) and wall thickness t (mm) shown in Table 3. The cross-sectional shape of the square steel pipes is square.

　Test pieces were taken from the obtained hot-rolled steel sheets, electric resistance welded steel pipes, and square steel pipes, and the following structural observations, tensile tests, and equivalent plastic strain distribution measurements were carried out.

[Structural Observation]
The test pieces for microstructure observation were prepared by taking the specimens so that the observation surface was a cross section in the rolling direction during hot rolling and at the 1/2t position of the plate thickness, polishing them, and then etching them with nital. For the microstructure observation, an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) was used to observe and image the structure at the 1/2t position of the plate thickness of the steel plate. From the obtained optical microscope images and SEM images, the area ratios of ferrite, bainite, and the remaining structure (pearlite, martensite, austenite) were obtained. The area ratios of each structure were calculated as the average value of the values obtained in five visual fields by observing the structure. Here, the area ratios obtained by the microstructure observation were taken as the volume ratios of each structure.

Here, ferrite is a product of diffusion transformation, and has a low dislocation density and a nearly restored structure. This includes polygonal ferrite and pseudo-polygonal ferrite. Bainite is a complex phase structure of lath-shaped ferrite and cementite with a high dislocation density. Pearlite is a structure in which cementite and ferrite are arranged in layers. Austenite does not contain cement, unlike bainite. Martensite and austenite were distinguished from each other by the brighter contrast of the SEM image compared to bainite.

Because it is difficult to distinguish between martensite and austenite in optical microscope images and SEM images, the area fraction of the structures observed as martensite or austenite in the obtained SEM images was measured, and the volume fraction of austenite, measured using the method described below, was subtracted from this to determine the volume fraction of martensite.

The austenite volume fraction was measured by X-ray diffraction. Test pieces for microstructural observation were prepared by grinding so that the diffraction surface was at 1/2t of the plate thickness, then chemically polishing to remove the surface treatment layer. Mo Kα radiation was used for the measurement, and the austenite volume fraction was calculated from the integrated intensity of the (200), (220), and (311) planes of fcc iron and the (200) and (211) planes of bcc iron.

The average grain size and CP value were measured using the SEM/EBSD method. The measurement area was 500 μm x 500 μm, and the measurement step size was 0.5 μm. Based on the obtained EBSD data, the crystal orientation analysis software OIM Analysis (trademark) was used to obtain the grain boundary distribution, with boundaries with an orientation difference of 15° or more considered as grain boundaries (high-angle grain boundaries). The average grain size was calculated as the arithmetic mean of the circle equivalent diameter (grain size) of each grain. The CP value was calculated by calculating the total length of the high-angle grain boundaries in the area excluding grains with a grain size of less than 20 μm, and the total length of the high-angle grain boundaries, and then calculating the ratio of these. Here, the total length of high-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 high-angle grain boundaries measured in the region excluding crystal grains with a grain size of less than 20 μm from the distribution of the grain boundaries in the measurement region, and the total length of high-angle grain boundaries is the sum of the lengths of high-angle grain boundaries measured from the distribution of the grain boundaries in the measurement region. Note that, in calculating the average grain size and CP value, grains with a grain size of 2.0 μm or less were excluded as measurement noise.

[Tensile test]
Tensile test pieces according to JIS No. 5 were taken so that the tensile direction was parallel to the rolling direction. The tensile test pieces were taken from a position 1/4W (W: plate width) in the width direction from the end in the width direction for the hot-rolled steel plate, from a position 90° away in the circumferential direction from the electric resistance welded part for the electric resistance welded steel pipe, and from a flat part adjacent to the flat part including the electric resistance welded part for the square steel pipe. 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, and the yield ratio defined as (yield stress σy)/(tensile strength) was calculated.

[Equivalent plastic strain distribution]
The equivalent plastic strain distribution was measured by the SEM-DIC method. Tensile test pieces shown in FIG. 1 were taken from the center of the plate thickness of the hot-rolled steel plate, the center of the plate thickness of the electric resistance welded steel pipe, and the center of the plate thickness of the square steel pipe so that the tensile direction was parallel to the rolling direction. For the plate width direction of the hot-rolled steel plate, 1/4W (W: plate width) in the width direction from the end in the width direction, for the circumferential direction of the electric resistance welded steel pipe, the sample was taken at a position 90° away from the electric resistance weld in the circumferential direction, and for the square steel pipe, the sample was taken at a flat plate part adjacent to the flat plate part including the electric resistance weld. One side of the obtained tensile test piece was polished and etched with nital, and the parallel part (tensile deformation part) was imaged in five fields of view using an SEM (magnification: 1000 times). Then, at a tensile speed of 5 mm/min, a tensile strain of 8.0% was applied to the test pieces taken from the hot-rolled steel plate, and a tensile strain of 4.0% was applied to the test pieces taken from the electric resistance welded steel pipe and the square steel pipe, respectively, and the load was removed. Then, the same field of view as before tension (before tensile strain was applied) was imaged using an SEM (magnification: 1000 times). Based on the obtained SEM images before and after tension, the equivalent plastic strain distribution of the imaged surface was calculated by the DIC method using image analysis software GOM Correlate (GOM Co., Ltd.). The DIC method is a method of measuring the displacement and strain at various points on the observation surface by comparing the random patterns on the surface of an object before and after deformation. Specifically, a square area called a subset is defined in the image before deformation, 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 throughout the entire image to obtain the displacement distribution and strain distribution. In the present invention, the nital corrosion marks of the metal structure are used as a random pattern, and the subset size is 80 pixels x 80 pixels (3.6 μm x 3.6 μm) and the measurement interval is 10 pixels (0.45 μm) for an image of 1910 pixels x 2560 pixels. The horizontal axis represents the natural logarithm of the obtained equivalent plastic strain (unit: none), and the vertical axis represents the ratio (area ratio) (unit: %). The standard deviation at this time was approximated by a normal distribution, and the logarithmic standard deviation (logarithmic standard deviation of the equivalent plastic strain distribution) was used. Specifically, the logarithmic standard deviation was calculated by the following method. First, the ratio (area ratio) (unit: %) of each class was calculated with a class width of 0.02 within the range of equivalent plastic strain from 0 to 0.20. At this time, the class with equivalent plastic strain of 0 to less than 0.02 was set as the first class, the class with equivalent plastic strain of 0 to less than 0.02 was set as the second class, ..., the class with equivalent plastic strain of 0 to less than 0.20 was set as the tenth class. The logarithmic standard deviation was calculated by the following formulas (4) and (5), with x _i as the natural logarithm of the class value of the i-th class and x ₀ as the average value of the natural logarithm of the equivalent plastic strain.

[Axial compression test]
Pressure 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 compression test device. The stress at which the compressive load was maximum was taken as the maximum stress intensity σmax (N/mm ² ). The yield stress σy obtained from the tensile test was used to calculate the yield strength increase rate τ (=σmax/σy).

The results obtained for hot-rolled steel sheets are shown in Table 4.

The results obtained for electric resistance welded steel pipes and square steel pipes are shown in Table 5.

In Tables 4 and 5, Nos. 1 to 6 are examples of the present invention, and Nos. 7 to 12 are comparative examples.

The hot-rolled steel plate of the present invention had a steel structure at the center of the plate thickness with a total volume fraction of ferrite and bainite of 70% to 98%, with the remainder consisting of one or more types selected from pearlite, martensite and austenite, an average crystal grain size of 15.0 μm or less, and a CP value calculated by the specified formula (1) of 0.090 or less. In addition, the tensile strength was 400 MPa or more, and the yield ratio was 90% or less. Furthermore, the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 8.0% was 0.70 or less.

Furthermore, the electric resistance welded steel pipe and square steel pipe of the present invention were manufactured from the hot rolled steel plate of the present invention, and the steel structure at the center of the wall thickness had a total volume fraction of ferrite and bainite of 70% to 98%, with the remainder consisting of one or more types selected from pearlite, martensite and austenite, an average crystal grain size of 15.0 μm or less, and a CP value calculated by the specified formula (1) of 0.090 or less. Furthermore, the tensile strength of the base material or flat plate was 400 MPa or more, and the yield ratio of the base material or flat plate was 97% or less. Furthermore, the logarithmic standard deviation of the equivalent plastic strain distribution after applying a 4.0% tensile strain to the base material or flat plate was 0.60 or less. In addition, the yield strength increase rate τ (=σmax/σy) in the axial compression test satisfied τ≧4.0×(t/D)+0.85...(2) for electric resistance welded steel pipes, and τ≧3.0×(t/B)+0.85...(3) for square steel pipes. In Table 5, the values obtained by substituting t and D for electric resistance welded steel pipes and t and B for square steel pipes into the right-hand sides of the above equations (2) and (3) are listed as the required lower limit value of τ.

On the other hand, in the comparative example, No. 7, the C content was below the range of the present invention, so the tensile strength was outside the range of the present invention.

Comparative example No. 8 had a C content exceeding the range of the present invention, so the total volume ratio of ferrite and bainite was below the range of the present invention. As a result, the yield ratio was outside the range of the present invention, and the logarithmic standard deviation was outside the preferred range, so the yield strength increase rate did not reach the desired value.

Comparative example No. 9 had a Si and Mn content below the range of the present invention, so the total volume fraction of ferrite and bainite exceeded the range of the present invention, and the average crystal grain size exceeded the range of the present invention. As a result, the tensile strength was outside the range of the present invention.

Comparative example No. 10 had a Si and Mn content exceeding the range of the present invention, so the total volume fraction of ferrite and bainite was below the range of the present invention. As a result, the yield ratio was outside the range of the present invention, and the logarithmic standard deviation was outside the preferred range, so the yield strength increase rate did not reach the desired value.

In comparative example No. 11, the average cooling rate in the temperature range of 900°C or higher in the hot rolling process was below the range of the preferred manufacturing method, so the average crystal grain size exceeded the range of the present invention, and the CP value exceeded the range of the present invention. As a result, the logarithmic standard deviation exceeded the preferred range of the present invention, and the yield strength increase rate did not reach the desired value. In addition, the tensile strength was below the range of the present invention.

Comparative example No. 12 had a cooling stop temperature in the hot rolling process that exceeded the range of the preferred manufacturing method, so the CP value exceeded the range of the present invention. As a result, the logarithmic standard deviation exceeded the preferred range of the present invention, and the yield strength increase rate did not reach the desired value.

From the above, by setting the steel composition and structure within the range of the present invention, it is possible to provide electric resistance welded steel pipes and square steel pipes with excellent buckling resistance, as well as hot rolled steel sheets used as materials for these pipes. It is also possible to provide line pipes and building structures with high buckling resistance using the electric resistance welded steel pipes and square steel pipes.

Claims

The composition is in mass percent:
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, with the balance being Fe and unavoidable impurities;
The steel structure at the center of the plate thickness is
The total volume fraction of ferrite and bainite is 70% or more and 98% or less,
The balance is one or more selected from pearlite, martensite, and austenite,
The average crystal grain size is 15.0 μm or less,
The CP value calculated by the following formula (1) is 0.090 or less,
The tensile strength is 400 MPa or more,
A hot-rolled steel sheet having a yield ratio of 90% or less.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1)
In addition to the above-mentioned component composition, the following is further added 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 hot-rolled steel sheet according to claim 1 , comprising one or more selected from the following:
The hot-rolled steel sheet according to claim 1 or 2, in which the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 8.0% is 0.70 or less.
An electric resistance welded steel pipe having a base metal portion and an electric resistance welded portion,
The composition is in mass percent:
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, the balance being Fe and unavoidable impurities;
The steel structure in the center of the wall thickness is
The total volume fraction of ferrite and bainite is 70% or more and 98% or less,
The balance is one or more selected from pearlite, martensite, and austenite,
The average crystal grain size is 15.0 μm or less,
The CP value calculated by the following formula (1) is 0.090 or less,
The tensile strength of the base material is 400 MPa or more,
An electric welded steel pipe having a base metal portion with a yield ratio of 97% or less.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1)
In addition to the above-mentioned component composition, the following is further added 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, comprising one or more selected from the following:
An electric resistance welded steel pipe as described in claim 4 or 5, in which the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 4.0% to the base material is 0.60 or less.
A line pipe in which the electric resistance welded steel pipe according to claim 4 or 5 is used.
A line pipe in which the electric resistance welded steel pipe according to claim 6 is used.
A square steel pipe having a flat portion and a corner portion,
The composition is in mass percent:
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, the balance being Fe and unavoidable impurities;
The steel structure in the center of the wall thickness is
The total volume fraction of ferrite and bainite is 70% or more and 98% or less,
The balance is one or more selected from pearlite, martensite, and austenite,
The average crystal grain size is 15.0 μm or less,
The CP value calculated by the following formula (1) is 0.090 or less,
The tensile strength of the flat portion is 400 MPa or more,
A square steel pipe having a yield ratio of 97% or less in the flat portion.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1)
In addition to the above-mentioned component composition, the following is further added 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 claim 9, comprising one or more selected from the following:
The square steel pipe according to claim 9 or 10, in which the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 4.0% to the flat portion is 0.60 or less.
An architectural structure in which the square steel pipe according to claim 9 or 10 is used as a pillar material.
An architectural structure, in which the square steel pipe according to claim 11 is used as a pillar material.