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

Abstract

Provided are: an electric-resistance-welded steel pipe and an angular steel pipe having excellent buckling resistance; a hot-rolled steel sheet used as a raw material for the electric-resistance-welded steel pipe and the angular steel pipe; and a line pipe and a building structure using the electric-resistance-welded steel pipe and the angular steel pipe. A hot-rolled steel sheet having a specific component composition and having a steel structure in the center of the sheet thickness in which the total of ferrite and bainite is 70-98% by volume fraction, and the steel structure in the center of the sheet thickness is 70-98% by volume fraction, and the steel structure in the center of the sheet thickness in the steel structure in the center of the sheet thickness in the steel structure in the center of the sheet thickness in the steel structure in the center of the sheet thickness in the steel structure in the center of the sheet thickness in the steel structure in the center of the sheet thickness in the steel structure. The hot-rolled steel sheet has an average crystal grain size of 15.0 [mu] m or less and a CP value of 0.090 or less as determined by a specific formula, and has a tensile strength of 400 MPa or more and a yield ratio of 90% or less, with the remainder comprising one or more elements selected from pearlite, martensite, and austenite, and the hot-rolled steel sheet having an average crystal grain size of 15.0 [mu] m or less and a CP value of 0.090 or less as determined by a specific formula.

Description

Technical Field

[0001] The present invention relates to electric resistance welded steel pipes and angle steel pipes, hot rolled steel plates used as raw materials thereof, and line pipes and building structures using the same. Background Art

[0002] Electric resistance welded steel pipes and roll-formed angle steel pipes used for pipelines and building structures are required to have high strength to withstand the internal pressure of the fluid flowing through them and the load from the outside. At the same time, they are also required to have high buckling resistance from the perspective of earthquake resistance.

[0003] Electric resistance welded steel pipes and roll-formed angled steel pipes (hereinafter sometimes referred to as "angled steel pipes") use hot-rolled steel plates (hot-rolled steel strips) as raw materials. They are formed into cylindrical open pipes by cold roll forming, and the butt joints are resistance welded (sometimes referred to as resistance welding) to form round steel pipes. Electric resistance welded steel pipes are manufactured by adjusting the outer diameter and roundness by forming rollers arranged on the outside of the round steel pipes. Angle steel pipes are manufactured by further roll forming the round steel pipe into an angle using a roller having a hole shape of a target polygonal shape. Compared with the manufacturing method of steel pipes based on press bending, the manufacturing method of angled steel pipes based on roll forming has the advantage of high productivity. However, since a large tensile strain is applied in the tube axis direction during roll forming, electric resistance welded steel pipes and roll-formed angled steel pipes have the problem of low ductility in the tube axis direction and low buckling resistance. In addition, for a material to be roll-formed, it is required to select an appropriate hot-rolled steel sheet (hot-rolled steel strip) in consideration of a decrease in ductility due to roll-forming.

[0004] Furthermore, in the case of electric resistance welded steel pipes and roll-formed angle steel pipes, as the wall thickness increases, the processing strain during roll forming increases, and thus the ductility further decreases, and the buckling resistance further decreases.

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

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

[0007] Patent Document 3 discloses an electric resistance welded steel pipe for line pipe having a low yield ratio, wherein dislocations introduced during 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 low yield ratio angular steel pipe using hot-rolled steel plate as raw material, 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 grain size of the main phase and the second phase at 1 / 4 thickness of the steel plate is 10 to 25 μm.

[0009] Patent Document 5 discloses an angled steel pipe characterized by being manufactured by hot forming and having high deformability and toughness.

[0010] Prior art literature

[0011] Patent Literature

[0012] Patent Document 1: Japanese Patent Application Laid-Open No. 7-224351

[0013] Patent Document 2: Japanese Patent No. 5589885

[0014] Patent Document 3: Japanese Patent No. 6052374

[0015] Patent Document 4: Japanese Patent No. 7031477

[0016] Patent Document 5: Japanese Patent Application Publication No. 2004-330222 Summary of the invention

[0017] Problems to be solved by the invention

[0018] However, these technologies focus on the characteristics during tensile deformation, that is, the suppression of necking and fracture of the tensile portion, but the research on the bending deformation and local buckling during compression deformation of electric resistance welded steel pipes and angled steel pipes is still insufficient.

[0019] In addition, as in Patent Documents 3 and 5, steel pipes subjected to heat treatment after pipe making and steel pipes manufactured by hot forming have large yield elongation and are prone to uneven deformation, and cannot fully demonstrate buckling resistance.

[0020] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an electric resistance welded steel pipe and a square steel pipe having excellent buckling resistance, and a hot-rolled steel plate used as a material thereof.

[0021] Another object of the present invention is to provide a line pipe and a building structure using the electric resistance welded steel pipe and the angle steel pipe.

[0022] Here, the term "excellent buckling resistance" as used in the present invention means that the resistance increase rate τ (=σmax / σy) in the axial compression test satisfies τ≥4.0×(t / D)+0.85 for the electric resistance welded steel pipe and τ≥3.0×(t / B)+0.85 for the angle steel pipe. Here, t represents the wall thickness (mm) of the electric resistance welded steel pipe or the angle steel pipe, D represents the outer diameter (mm) of the electric resistance welded steel pipe, B represents the side length (mm) of the angle steel pipe, and σy represents the yield stress (N / mm) of the base material of the electric resistance welded steel pipe or the flat plate of the angle steel pipe. 2 (=MPa)), σmax represents the maximum stress in the axial compression test (N / mm 2 ). Wherein, when the cross-sectional shape of the angled 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 angled steel pipe. It should be noted that in the present invention, the hot-rolled steel sheet of the above-mentioned raw material includes a hot-rolled steel strip.

[0023] Methods used to solve problems

[0024] In order to solve the above problems, we conducted in-depth research and found that by making the cold-formed electric resistance welded steel pipe and cold-formed angle 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. In other words, it was found that the smaller the logarithmic standard deviation, the smaller the fluctuation of the plastic strain during deformation, the plastic strain is evenly distributed, and the strain is not easy to concentrate on a specific part, so it is not easy to cause local buckling. In addition, it was also found that the above-mentioned electric resistance welded steel pipe and angle steel pipe can be obtained by using hot-rolled steel plates with a small logarithmic standard deviation of the equivalent plastic strain distribution during deformation as raw materials.

[0025] The present invention has been accomplished based on these findings, and includes the following gist.

[0026] [1] A hot-rolled steel sheet,

[0027] The composition is as follows: containing, 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% to 0.005% to 0.100%, N: 0.0100% to 0.0100%, the balance being Fe and unavoidable impurities.

[0028] The steel structure at the center of the plate thickness is as follows: the total volume percentage of ferrite and bainite is 70% or more and 98% or less, the balance is composed of one or more selected from pearlite, martensite, and austenite, the average grain size is 15.0 μm or less, and the CP value obtained by the following formula (1) is 0.090 or less,

[0029] The hot-rolled steel sheet has a tensile strength of 400 MPa or more and a yield ratio of 90% or less.

[0030] CP = (Total length of high-angle grain boundaries in the region excluding grains with a grain size of less than 20 μm) / (Total length of high-angle grain boundaries) …(1)

[0031] [2] The hot-rolled steel sheet according to [1], further comprising, in addition to the above-mentioned component composition, one or more selected from the group consisting of, in mass %, Nb: less than 0.100%, V: less than 0.100%, Ti: less than 0.150%, Cr: less than 0.50%, Mo: less than 0.50%, Cu: less than 0.50%, Ni: less than 0.50%, Ca: less than 0.0050%, B: less than 0.0050%, Mg: less than 0.020%, Zr: less than 0.020%, and REM: less than 0.020%.

[0032] [3] The hot-rolled steel sheet according to [1] or [2], wherein the logarithmic standard deviation of the equivalent plastic strain distribution after imparting 8.0% tensile strain is 0.70 or less.

[0033] [4] An electric resistance welded steel pipe comprising a base material portion and an electric resistance welded portion, wherein:

[0034] The electric resistance welded steel pipe has a composition as follows: containing, 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% to 0.005% to 0.100%, N: 0.0100% to 0.0100%, and the balance is Fe and inevitable impurities.

[0035] The steel structure at the center of the wall thickness is as follows: the total volume percentage of ferrite and bainite is 70% or more and 98% or less, the balance is composed of one or more selected from pearlite, martensite, and austenite, the average grain size is 15.0 μm or less, and the CP value obtained by the following formula (1) is 0.090 or less,

[0036] The tensile strength of the base material portion is 400 MPa or more, and the yield ratio of the base material portion is 97% or less.

[0037] CP = (Total length of high-angle grain boundaries in the region excluding grains with a grain size of less than 20 μm) / (Total length of high-angle grain boundaries) …(1)

[0038] [5] The electric resistance welded steel pipe according to [4], further comprising, in addition to the above component composition, one or more selected from the group consisting of, 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, and REM: 0.020% or less.

[0039] [6] The electric-resistance welded steel pipe according to [4] or [5], wherein the logarithmic standard deviation of the equivalent plastic strain distribution after imparting a 4.0% tensile strain to the base material portion is 0.60 or less.

[0040] [7] A line pipe using the electric resistance welded steel pipe described in [4] or [5].

[0041] [8] A line pipe using the electric resistance welded steel pipe described in [6].

[0042] [9] An angled steel pipe having a flat portion and an angled portion, wherein:

[0043] The angled steel pipe has a composition as follows: containing, 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% to 0.005% to 0.100%, N: 0.0100% to 0.0100%, and the balance is Fe and inevitable impurities.

[0044] The steel structure at the center of the wall thickness is as follows: the total volume percentage of ferrite and bainite is 70% or more and 98% or less, the balance is composed of one or more selected from pearlite, martensite, and austenite, the average grain size is 15.0 μm or less, and the CP value obtained by the following formula (1) is 0.090 or less,

[0045] 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.

[0046] CP = (Total length of high-angle grain boundaries in the region excluding grains with a grain size of less than 20 μm) / (Total length of high-angle grain boundaries) …(1)

[0047]

[10] The angled steel pipe according to [9], which, in addition to the above-mentioned component composition, further contains one or more selected from the group consisting of 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, and REM: 0.020% or less, in terms of mass %.

[0048]

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

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

[0049]

[12] A building structure, wherein the angled steel pipe described in [9] or

[10] is used as a column material.

[0050]

[13] A building structure, wherein the angled steel pipe described in

[11] is used as a column material.

[0051] Effects of the Invention

[0052] According to the present invention, it is possible to provide an electric resistance welded steel pipe and a square steel pipe having excellent buckling resistance, and a hot-rolled steel plate used as a material thereof.

[0053] 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 angle steel pipe. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 Schematic diagram of a tensile test piece used to determine the equivalent plastic strain distribution. DETAILED DESCRIPTION

[0055] Hereinafter, the present invention will be described in detail.

[0056] The hot rolled steel sheet of the present invention has the following composition: containing, 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 is Fe and inevitable impurities. The steel structure at the center of the plate thickness is as follows: the total volume fraction of ferrite and bainite is 70% or more and 98% or less, the balance is composed of one or more selected from pearlite, martensite, and austenite, the average grain size is 15.0 μm or less, and the value of CP obtained 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 imparting 8.0% tensile strain is preferably 0.70 or less.

[0057] CP = (Total length of high-angle grain boundaries in the region excluding grains with a grain size of less than 20 μm) / (Total length of high-angle grain boundaries) …(1)

[0058] The electric resistance welded steel pipe of the present invention has a base material portion and an electric resistance welded portion, and has a composition containing, 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 is Fe and inevitable impurities. The steel structure at the center of the wall thickness is as follows: the total volume fraction of ferrite and bainite is 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 the above formula (1) is 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, in the base material, the logarithmic standard deviation of the equivalent plastic strain distribution after applying 4.0% tensile strain is preferably 0.60 or less.

[0059] The angled steel pipe of the present invention has a flat plate portion and an angle portion, and has a composition containing, 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 is Fe and inevitable impurities. The steel structure at the center of the wall thickness is as follows: the total volume percentage of ferrite and bainite is 70% or more and 98% or less, and the balance is composed of one or more selected from pearlite, martensite, and austenite, the average 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 plate portion is 400 MPa or more, and the yield ratio of the flat plate portion is 97% or less. In addition, in the flat plate portion, the logarithmic standard deviation of the equivalent plastic strain distribution after imparting a 4.0% tensile strain is preferably 0.60 or less.

[0060] First, the reasons for limiting the composition of the hot-rolled steel sheet, electric resistance welded steel pipe and angled steel pipe in the present invention are described below. In this specification, "%" indicating the content of each component means "mass %" unless otherwise specified.

[0061] C: 0.030% or more and 0.300% or less

[0062] C is an element that increases the strength of steel by solid solution strengthening. In addition, C promotes the formation of pearlite, and improves hardenability and contributes to the formation of martensite, and contributes to the stabilization of austenite, and is therefore also an element that contributes to the formation of hard phase. In order to ensure the intensity of the purpose in the present invention, C needs to contain more than 0.030%. However, when C content exceeds 0.300%, the ratio of hard phase becomes high, and the yield ratio as the purpose in the present invention cannot be obtained. And then, the strain distribution in the deformation when tensile strain is given becomes uneven, and the logarithmic standard deviation of the appropriate equivalent plastic strain distribution as the purpose in the present invention cannot be obtained. Therefore, C content is set to more than 0.030% and less than 0.300%. C content is preferably more than 0.035%, more preferably more than 0.040%. In addition, C content is preferably less than 0.250%, more preferably less than 0.200%.

[0063] Si: 0.010% or more and 0.500% or less

[0064] Si is an element that increases the strength of steel by solid solution strengthening. In order to obtain such an effect, Si preferably contains more than 0.010%. However, when the Si content exceeds 0.500%, the proportion of hard phase becomes high, and the yield ratio as the purpose in the present invention cannot be obtained. Furthermore, the strain distribution in the deformation when the tensile strain is given becomes uneven, and the logarithmic standard deviation of the appropriate equivalent plastic strain distribution as the purpose in the present invention cannot be obtained. Therefore, the Si content is set to less than 0.500%. The Si content is preferably more than 0.020%, more preferably more than 0.030%. In addition, the Si content is preferably less than 0.400%, more preferably less than 0.300%.

[0065] Mn: 0.30% or more and 2.50% or less

[0066] Mn is an element that increases the strength of steel by solid solution strengthening. In addition, Mn is an element that contributes to the refinement of the organization by lowering the phase transformation starting temperature. In order to ensure the strength and organization as the purpose in the present invention, Mn needs to contain more than 0.30%. However, when the Mn content exceeds 2.50%, the yield ratio as the purpose in the present invention cannot be obtained. Furthermore, the strain distribution in the deformation when the tensile strain is given becomes uneven, and the logarithmic standard deviation of the appropriate equivalent plastic strain distribution as the purpose in the present invention cannot be obtained. Therefore, the Mn content is set to more than 0.30% and less than 2.50%. The Mn content is preferably more than 0.40%, more preferably more than 0.50%. In addition, the Mn content is preferably less than 2.30%, more preferably less than 2.10%.

[0067] P: 0.050% or less

[0068] P segregates at the grain boundaries and causes inhomogeneity of the material. Therefore, as an inevitable impurity, it is preferably reduced as much as possible, but a content of 0.050% or less is allowed. 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. It should be noted that there is no particular lower limit for the P content, but excessive reduction will lead to a rise in smelting costs. Therefore, the P content is preferably set to 0.002% or more.

[0069] S: 0.0200% or less

[0070] S usually exists in the form of MnS in steel, but MnS is stretched thinner in the hot rolling process, which has an adverse effect on ductility and toughness. Therefore, in the present invention, it is preferred to reduce S as much as possible, but a content of less than 0.0200% can be allowed. Therefore, the S content is set to less than 0.0200%. The S content is preferably less than 0.0150%, more preferably less than 0.0100%. It should be noted that the lower limit of the S content is not particularly specified, but excessive reduction will lead to a surge in smelting costs, so the S content is preferably set to more than 0.0002%.

[0071] Al: 0.005% or more and 0.100% or less

[0072] Al is an element that acts as a strong deoxidizer when added to molten steel. In order to obtain such an effect, Al needs to contain 0.005% or more. However, when the Al content exceeds 0.100%, weldability deteriorates, and alumina inclusions increase, and surface properties deteriorate. Therefore, the Al content is set to be greater than 0.005% and less than 0.100%. The Al content is preferably greater than 0.010%, and more preferably greater than 0.020%. In addition, the Al content is preferably less than 0.080%, and more preferably less than 0.060%.

[0073] N: 0.0100% or less

[0074] 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, N is preferably reduced as much as possible as an impurity, but the N content can be allowed to be up to 0.0100%. Therefore, the N content is set to less than 0.0100%. The N content is preferably less than 0.0090%, and more preferably less than 0.0080%. It should be noted that excessive reduction will lead to a surge in smelting costs, and therefore, the N content is preferably set to more than 0.0010%, and more preferably set to more than 0.0015%.

[0075] The balance may be Fe and inevitable impurities. Examples of the inevitable impurities in the balance include Sn, As, Sb, Bi, Co, Pb, Zn, and O. However, within the range that does not impair the effect of the present invention, it is not rejected to contain less than 0.1% of Sn, less than 0.05% of As, Sb, and Co, and less than 0.005% of Bi, Pb, Zn, and O.

[0076] The above components are the basic components of the hot-rolled steel sheet, electric resistance welded steel pipe and angled steel pipe of the present invention. The above essential elements can achieve the desired properties of the present invention, but the following elements may be contained within the following content ranges as required.

[0077] One or more selected from the group consisting of 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, and REM: 0.020% or less

[0078] Nb: 0.100% or less, V: 0.100% or less, Ti: 0.150% or less

[0079] Nb, Ti, and V are all elements that form fine carbides and nitrides in steel and contribute to the improvement of the strength of steel by strengthening based on precipitates, and can be contained as needed. The contents of Nb, Ti, and V can be 0%, respectively, but in the case of containing Nb, Ti, and V, the preferred contents are Nb: more than 0.001%, Ti: more than 0.001%, and V: more than 0.001%. The more preferred contents are Nb: more than 0.008%, V: more than 0.008%, and Ti: more than 0.008%. On the other hand, excessive inclusion may lead to an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, in the case of containing Nb, Ti, and V, it is preferably set to Nb: less than 0.100%, V: less than 0.100%, and Ti: less than 0.150%, respectively. The more preferred contents are Nb: less than 0.070%, V: less than 0.070%, and Ti: less than 0.110%. It should be noted that when two or more selected from Nb, Ti, and V are contained, there is a possibility of increasing the yield ratio and the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, it is preferred that the total amount (total content of Nb+Ti+V) is set to 0.150% or less.

[0080] Cr: 0.50% or less, Mo: 0.50% or less

[0081] Cr and Mo are elements that improve the hardenability of steel and increase the strength of steel, and can be contained as needed. The contents of Cr and Mo can be 0% respectively, but in the case of containing Cr and Mo, the preferred contents are Cr: 0.01% or more and Mo: 0.01% or more respectively. The more preferred contents are Cr: 0.10% or more and Mo: 0.10% or more respectively. On the other hand, excessive inclusion may lead to an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, in the case of containing Cr and Mo, it is preferably set to Cr: 0.50% or less and Mo: 0.50% or less respectively. The more preferred contents are Cr: 0.30% or less and Mo: 0.30% or less respectively.

[0082] Cu: 0.50% or less, Ni: 0.50% or less

[0083] 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 can be 0%, respectively, but in the case of containing Cu and Ni, the preferred content is Cu: more than 0.01% and Ni: more than 0.01%. The more preferred content is Cu: more than 0.10% and Ni: more than 0.10%. On the other hand, excessively containing may lead to an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, in the case of containing Cu and Ni, it is preferably set to Cu: less than 0.50% and Ni: less than 0.50%, respectively. The more preferred content is Cu: less than 0.35% and Ni: less than 0.35%.

[0084] Ca: 0.0050% or less

[0085] Ca is an element that helps improve the ductility and toughness of steel by spheroidizing sulfides such as MnS that are stretched thinner in the hot rolling process, and can be contained as needed. The Ca content can 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 sometimes formed in the steel, and ductility and toughness deteriorate. Therefore, when Ca is contained, the Ca content is preferably set to 0.0050% or less. A more preferred content is Ca: 0.0040% or less.

[0086] B: 0.0050% or less

[0087] B is an element that contributes to the refinement of the structure by lowering the starting temperature of the ferrite transformation. The content of B can be 0%, but in the case of containing B, the preferred content is 0.0001% or more. A more preferred content is B: 0.0005% or more. However, when the B content exceeds 0.0050%, it is possible to cause an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, in the case of containing B, the B content is preferably set to 0.0050% or less. A more preferred content is B: 0.0040% or less.

[0088] Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less

[0089] Mg, Zr and REM are elements that increase the strength of steel by grain refinement, and can be contained as needed. The contents of Mg, Zr and REM can be 0%, respectively, but in the case of containing Mg, Zr, and REM, the preferred contents are Mg: 0.0005% or more, Zr: 0.0005% or more, 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, in the case of containing Mg, Zr and REM, it is preferably set to Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less, respectively. The more preferred contents are Mg: 0.010% or less, Zr: 0.010% or less, REM: 0.010% or less, respectively. It should be noted that, here, REM is a general term for 17 elements including Sc, Y and lanthanide elements. One or more of these 17 elements may be contained in the steel, and the REM content refers to the total content of these elements.

[0090] Next, the reasons for limiting the steel structure of the hot-rolled steel plate, electric resistance welded steel pipe and angled steel pipe in the present invention are explained. In addition, the steel structure limited below is the steel structure in the center of the plate thickness or the center of the wall thickness, which means that it exists at the position of 1 / 2t of the plate thickness. It should be noted that in the present invention, the position of 1 / 2t of the plate thickness refers to the position of 1 / 2 (middle) of the plate thickness t in the plate thickness direction.

[0091] Total volume ratio of ferrite and bainite: 70% or more and 98% or less

[0092] Ferrite and bainite are soft structures, and the yield ratio can be reduced by mixing with other hard structures. In order to obtain the low yield ratio as the purpose of the present invention through such an effect, the total volume ratio of ferrite and bainite needs to be set to 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 as the purpose of the present invention cannot be obtained, and therefore, the total volume ratio of ferrite and bainite needs to be set to 98% or less. The total volume ratio of ferrite and bainite is preferably 97% or less, and more preferably 95% or less.

[0093] Balance: One or more of pearlite, martensite and austenite

[0094] Pearlite, martensite and austenite are hard structures, and in particular, low yield ratio can be achieved by mixing with soft ferrite while increasing the strength of steel. In order to obtain such an effect, the remainder other than ferrite and bainite is set to be selected from one or more of pearlite, martensite and austenite. Pearlite, martensite and austenite are calculated as a total of 2% or more and 30% or less in terms of each volume rate. The total of the above volume rates is preferably 3% or more, more preferably 5% or more. In addition, the total of the above volume rates is preferably 25% or less, more preferably 20% or less.

[0095] In addition, the volume ratios of ferrite, bainite, pearlite, martensite, and austenite can be measured by the method described in the examples described later.

[0096] Average crystal grain size: 15.0 μm or less

[0097] When the average crystal grain size of the grains is greater than 15.0 μm, the tensile strength as the purpose of the present invention cannot be obtained. In addition, the logarithmic standard deviation of the appropriate equivalent plastic strain distribution as the purpose of the present invention cannot be obtained. This is because, when the average crystal grain size is large, the degree of connectivity between coarse grains becomes higher, so the strains generated in the coarse grains during deformation are connected to each other, and as the deformation proceeds, the distribution of strain becomes more uneven. Therefore, the average crystal grain size of the grains is set to less than 15.0 μm. The average crystal grain size of the grains is preferably set to less than 13.0 μm, and more preferably set to less than 10.0 μm. It should be noted that when the average crystal grain size is small, the yield ratio becomes higher, and therefore, the average crystal grain size is preferably more than 2.0 μm. The average crystal grain size is more preferably more than 3.0 μm.

[0098] CP value: below 0.090

[0099] The CP value is a numerical value indicating the degree of connectivity between coarse grains with a particle 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 grains, so that the coarse grains become more connected to each other. When the CP value exceeds 0.090, the strains generated in the coarse grains during deformation are connected to each other, and as the deformation proceeds, the distribution of the strain becomes more uneven, so the logarithmic standard deviation of the appropriate equivalent plastic strain distribution for the purpose of the present invention cannot be obtained. Therefore, the CP value is set to less than 0.090. The CP value is preferably less than 0.080, and more preferably less than 0.070. It should be noted that the smaller the CP value, the better, and there is no special lower limit, but excessive reduction leads to an increase in manufacturing cost and manufacturing load, so the CP value is preferably set to more than 0.001.

[0100] CP = (Total length of high-angle grain boundaries in the region excluding grains with a grain size of less than 20 μm) / (Total length of high-angle grain boundaries) …(1)

[0101] It should be noted that the “total length of high-angle grain boundaries in the region excluding grains having a grain size of less than 20 μm” in formula (1) refers to the total length of high-angle grain boundaries in the portion where grains having a grain size of 20 μm or more are adjacent to each other.

[0102] 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 described later.

[0103] Next, the reasons for limiting the properties of the hot-rolled steel sheet, the electric-resistance-welded steel pipe, and the angled steel pipe in the present invention in the tensile test will be described.

[0104] Tensile strength of hot-rolled steel plate: 400MPa or more

[0105] When the tensile strength of the hot-rolled steel plate is less than 400 MPa, the tensile strength of the electric resistance welded steel pipe and the tensile strength of the angled steel pipe, which are the objectives of the present invention, cannot be obtained. Therefore, the tensile strength of the hot-rolled steel plate is set to 400 MPa or more. The tensile strength of the hot-rolled steel plate 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 plate is not particularly limited, and as an example, the tensile strength of the hot-rolled steel plate is 700 MPa or less.

[0106] Yield ratio of hot-rolled steel plate: less than 90%

[0107] When 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 angled steel pipe, which are the objectives of 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, and as an example, the yield ratio of the hot-rolled steel sheet is 60% or more.

[0108] Logarithmic standard deviation of equivalent plastic strain distribution after applying 8.0% tensile strain to hot-rolled steel sheet: 0.70 or less

[0109] The equivalent plastic strain distribution can be approximated by lognormal distribution with the horizontal axis set to equivalent plastic strain (unit: none) and the vertical axis set to ratio (area ratio) (unit: %). In the lognormal distribution, the logarithm of the variable (horizontal axis) follows the normal distribution. Therefore, if the horizontal axis is set to the natural logarithm of the equivalent plastic strain (unit: none) and the vertical axis is set to ratio (area ratio) (unit: %), it is possible to approximate with normal distribution. In the present invention, the standard deviation at this time is defined as "logarithmic standard deviation". The smaller the logarithmic standard deviation, the smaller the expansion of the peak of the equivalent plastic strain distribution, and the more uniform the distribution of plastic strain.

[0110] When the logarithmic standard deviation of the equivalent plastic strain distribution after the hot-rolled steel plate is given an 8.0% tensile strain is less than 0.70, it is easy to obtain the 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 angled steel pipe as the purpose of the present invention. Therefore, the logarithmic standard deviation after the hot-rolled steel plate is given an 8.0% tensile strain is preferably set to less than 0.70. The above logarithmic standard deviation is more preferably less than 0.68, and further more preferably less than 0.65. It should be noted that the smaller the above logarithmic standard deviation, the better, and the lower limit is not particularly specified, but excessive reduction leads to an increase in manufacturing cost and manufacturing load, so the above logarithmic standard deviation is preferably set to more than 0.050.

[0111] Tensile strength of the base material of the electric resistance welded steel pipe and the flat plate of the angled steel pipe: 400MPa or more

[0112] When the tensile strength of the base material of the electric resistance welded steel pipe and the tensile strength of the flat plate of the angled steel pipe are less than 400 MPa, the buckling resistance is reduced. 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, and as an example, the tensile strength is 700 MPa or less.

[0113] Yield ratio of the parent material of the electric resistance welded steel pipe and the flat plate of the angled steel pipe: 97% or less

[0114] When the yield ratio of the base material of the electric resistance welded steel pipe and the yield ratio of the flat plate of the angled steel pipe exceeds 97%, the buckling resistance performance is reduced. 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, and as an example, the yield ratio is 75% or more.

[0115] 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 angled steel pipe: 0.60 or less

[0116] When the logarithmic standard deviation of the equivalent plastic strain distribution after imparting 4.0% tensile strain to the base material portion of the electric resistance welded steel pipe and the flat plate portion of the angled steel pipe is less than 0.60, it is easy to further improve the buckling resistance. Therefore, the logarithmic standard deviation is preferably set to less than 0.60. The logarithmic standard deviation is more preferably less than 0.58, and further more preferably less than 0.55. It should be noted that the smaller the logarithmic standard deviation, the better, and there is no special lower limit, but excessive reduction leads to an increase in manufacturing cost and manufacturing load, so the logarithmic standard deviation is preferably set to more than 0.050.

[0117] It should be noted that tensile strength and yield ratio can be measured by the tensile test described in the embodiment described later. In addition, the logarithmic standard deviation of the equivalent plastic strain distribution can be measured by the tensile test and SEM-DIC method described in the embodiment described later by combination. More specifically, the logarithmic standard deviation of the equivalent plastic strain distribution can be obtained by the method described in the embodiment described later.

[0118] Next, a method for producing a hot-rolled steel plate, an electric-resistance-welded steel pipe, and a square steel pipe according to an embodiment of the present invention will be described.

[0119] The hot-rolled steel sheet of the present invention is obtained, for example, by the following steps: a steel raw material having the above-mentioned component composition is subjected to a heating step of heating to a heating temperature of 1100°C to 1300°C, and then subjected to a hot rolling step of rolling under conditions where the finish rolling end temperature is 750°C to 850°C and the average cooling rate in the temperature range of 900°C to the center temperature of the plate thickness is 1.0°C / s or more to produce a hot-rolled sheet; after the hot rolling step, a cooling step of cooling under conditions where the average cooling rate from the start of cooling to the stop of cooling is 5°C / s to 50°C / s and the cooling stop temperature is 400°C to 650°C; after the cooling step, a coiling step of coiling the hot-rolled sheet into a coil is performed, thereby obtaining the hot-rolled steel sheet of the present invention.

[0120] Furthermore, the electric resistance welded steel pipe of the present invention is produced by forming the above-mentioned hot-rolled steel plate into a cylindrical shape by cold rolling, 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 hole shape of a perfect circle, thereby producing the electric resistance welded steel pipe of the present invention.

[0121] In addition, the present invention is manufactured as follows: the above-mentioned hot-rolled steel plate is formed into a cylindrical shape by cold rolling, the two circumferential ends of the cylinder are butted and resistance welded, and then the flat plate portion and the corner portion are formed by cold forming using a roller with a target polygonal hole shape, thereby manufacturing the angled steel pipe of the present invention. It should be noted that the angled steel pipe of the present invention includes regular polygons (regular triangles, squares, regular pentagons, etc.), equilateral polygons with different combinations of internal angles (rhombuses, stars, etc.), and polygons with different combinations of side lengths (isosceles triangles, rectangles, parallelograms, trapezoids, etc.). When the angled steel pipe of the present invention is used as a column material for a building, beams are usually joined on four sides at 90-degree intervals, so the cross-section is preferably square or rectangular.

[0122] It should be noted that the cylindrical shape means that the cross section of the tube is in a "C" shape. In addition, in the following description of the manufacturing method, unless otherwise specified, "°C" related to temperature refers to the surface temperature of the steel raw material or steel plate (hot-rolled plate). These surface temperatures can be measured using a radiation thermometer or the like. In addition, the temperature at the center of the steel plate thickness can be obtained by calculating the temperature distribution in the cross section of the steel plate using heat transfer analysis and correcting the result using the surface temperature of the steel plate.

[0123] In the present invention, the smelting method of the steel raw material (steel billet) is not particularly limited, and any of the known smelting methods such as converter, electric furnace, vacuum melting furnace, etc. is suitable. The casting method is also not particularly limited, and the desired size can be manufactured by a known casting method such as continuous casting. It should be noted that there is no problem in applying the ingot casting-blowing rolling method instead of the continuous casting method. The molten steel can be further subjected to secondary refining such as ladle refining.

[0124] Next, the obtained steel raw material (steel billet) is heated to a heating temperature of 1100°C to 1300°C, and then subjected to a hot rolling process in which the finishing temperature of the finish rolling is 750°C to 850°C and the average cooling rate in the temperature range of 900°C or more in terms of the plate thickness center temperature is 1.0°C / s or more to produce a hot-rolled plate.

[0125] Heating temperature: 1100°C or higher and 1300°C or lower

[0126] When the heating temperature is lower than 1100°C, the deformation resistance of the rolled material (steel billet) becomes larger and rolling becomes difficult. On the other hand, when the heating temperature exceeds 1300°C, the austenite grains coarsen, and fine austenite grains cannot be obtained in the subsequent rolling (rough rolling, finish rolling), and it is difficult to ensure the average crystal grain size as the purpose of the present invention. In addition, it is difficult to suppress the formation of coarse grains, and it is difficult to control the CP value within the range of the purpose of the present invention. Therefore, the heating temperature based on the heating furnace before hot rolling is set to be above 1100°C and below 1300°C. The above-mentioned heating temperature is more preferably above 1120°C. In addition, the above-mentioned heating temperature is more preferably below 1280°C.

[0127] It should be noted that in the present invention, after manufacturing the steel billet, in addition to the existing method of temporarily cooling it to room temperature and then heating it again, it is also possible to apply energy-saving direct rolling processes such as loading it into a heating furnace in a warm sheet state without cooling it to room temperature, or rolling it immediately after slightly keeping it warm.

[0128] Finish rolling end temperature: 750℃ or higher and 850℃ or lower

[0129] When the finishing temperature of the finishing rolling is lower than 750°C, the surface temperature of the steel plate during the finishing rolling is lower than the ferrite transformation start temperature, ferrite is generated, and the subsequent rolling becomes processed ferrite grains elongated in the rolling direction, which becomes the reason for the increase in yield ratio. On the other hand, when the finishing temperature of the finishing rolling exceeds 850°C, the reduction amount within the austenite non-recrystallization temperature range is insufficient, and fine austenite grains cannot be obtained, making it difficult to ensure the average crystal grain size as the purpose of the present invention. In addition, it is difficult to suppress the formation of coarse grains, and it is difficult to control the CP value within the range as the purpose of the present invention. Therefore, the finishing temperature of the finishing rolling is set to be above 750°C and below 850°C. The finishing temperature of the finishing rolling is more preferably above 760°C. In addition, the finishing temperature of the finishing rolling is more preferably below 840°C.

[0130] Average cooling rate in the temperature range of 900°C or above at the center of the plate thickness: 1.0°C / s or more

[0131] In the present invention, by increasing the average cooling rate in the temperature range of 900°C or more measured by the center temperature of the plate thickness (hereinafter sometimes referred to as the average cooling rate in hot rolling.), the coarsening of austenite in the austenite recrystallization temperature range can be suppressed, and the average grain size and CP value used as the purpose of the present invention can be obtained. In order to achieve the above-mentioned average cooling rate, for example, a water cooling device can be used to cool the rolled material during rolling. When the above-mentioned average cooling rate is less than 1.0°C / s, austenite coarsens in the austenite recrystallization temperature range, and it is difficult to ensure the average grain size used as the purpose of the present invention. In addition, it is difficult to suppress the formation of coarse grains, and it is difficult to control the CP value within the range used as the purpose of the present invention. The above-mentioned average cooling rate is preferably greater than 1.2°C / s, and more preferably greater than 1.5°C / s. When the above-mentioned average cooling rate exceeds 5.0°C / s, the equipment load increases, so the above-mentioned average cooling rate is preferably less than 5.0°C / s.

[0132] It should be noted that the average cooling rate in the temperature range of 900°C or more in terms of the plate thickness center temperature is obtained as the average cooling rate of the plate thickness center from the time when the steel material (billet) is taken out of the heating furnace until the plate thickness center temperature reaches 900°C. That is, the above average cooling rate is obtained by [(plate thickness center temperature (°C) when the steel material is taken out of the heating furnace - 900 (°C)) / time (s) from the time when the steel material is taken out of the heating furnace until the plate thickness center temperature of the steel material reaches 900°C].

[0133] In the present invention, the upper limit of the finished plate thickness is not particularly specified, but is preferably 32 mm or less from the viewpoint of ensuring the necessary cooling rate and steel plate temperature management. In addition, the lower limit of the finished plate thickness is also not particularly limited, and as an example, the plate thickness is 5 mm or more.

[0134] After the hot rolling step, the hot rolled sheet is subjected to a cooling step in which the average cooling rate from the start of cooling to the stop of cooling is 5°C / s to 50°C / s and the cooling stop temperature is 400°C to 650°C.

[0135] Average cooling rate from the start of cooling to the end of cooling (cooling end): 5°C / s or more and 50°C / s or less

[0136] When the average cooling rate in the temperature range from the start of cooling to the stop of cooling described later (hereinafter, sometimes referred to as the average cooling rate in the cooling process) is less than 5°C / s, the nucleation frequency of ferrite is reduced, and the ferrite grains are coarsened, so it is difficult to ensure the average crystal grain size as the purpose of the present invention. In addition, it is difficult to suppress the formation of coarse grains and it is difficult to control the CP value within the range as the purpose of the present invention. On the other hand, when the above-mentioned average cooling rate exceeds 50°C / s, a large amount of martensite is generated, and the total volume ratio of ferrite and bainite as the purpose of the present invention cannot be obtained. The above-mentioned average cooling rate is preferably 7°C / s or more, and more preferably 10°C / s or more. In addition, the above-mentioned average cooling rate is preferably 45°C / s or less, and more preferably 40°C / s or less. It should be noted that in the cooling process, the intentional cooling start time such as water cooling is used as the start of cooling, and the air cooling before it is not included in the cooling.

[0137] In addition, in the present invention, from the viewpoint of suppressing the formation of ferrite on the surface of the steel sheet before cooling, it is preferred to start cooling immediately after the finish rolling is completed.

[0138] Cooling stop temperature: 400°C or higher and 650°C or lower

[0139] When the cooling stop temperature is lower than 400°C, a large amount of martensite is generated, and the total volume ratio of ferrite and bainite as the purpose of the present invention cannot be obtained. On the other hand, when the cooling stop temperature exceeds 650°C, the nucleation frequency of ferrite is reduced, and the ferrite grains are coarsened, so it is difficult to ensure the average crystal grain size as the purpose of the present invention. In addition, it is difficult to suppress the formation of coarse grains, and it is difficult to control the CP value within the range of the purpose of the present invention. The cooling stop temperature is preferably above 420°C, and more preferably above 450°C. In addition, the cooling stop temperature is preferably below 620°C, and more preferably below 600°C.

[0140] It should be noted that, in the present invention, unless otherwise specified, the average cooling rate in the cooling process is set to a value obtained by ((the plate thickness center temperature of the hot-rolled plate before cooling - the plate thickness center temperature of the hot-rolled plate after cooling) / cooling time). The cooling method may include water cooling such as spraying water from a nozzle, cooling based on the spraying of a cooling gas, and the like. In the present invention, in order to cool both sides of the hot-rolled plate under the same conditions, it is preferred to perform a cooling operation (treatment) on both sides of the hot-rolled plate.

[0141] After the cooling step, a coiling step is performed in which the hot-rolled sheet is coiled and then allowed to cool.

[0142] As described above, the hot-rolled steel sheet of the present invention is manufactured. 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. In addition, the hot-rolled steel sheet can have the characteristics of a logarithmic standard deviation of an equivalent plastic strain distribution of 0.70 or less after imparting 8.0% tensile strain.

[0143] In addition, the electric resistance welded steel pipe and the angle steel pipe manufactured using the hot rolled steel plate as a raw material have the characteristics of a tensile strength of 400 MPa or more and a yield ratio of 97% or less. Furthermore, the logarithmic standard deviation of the equivalent plastic strain distribution after imparting a 4.0% tensile strain is 0.60 or less. The electric resistance welded steel pipe and the angle steel pipe of the present invention have excellent buckling resistance.

[0144] In addition, the pipeline pipe and building structure using the electric resistance welded steel pipe and the angle steel pipe can have high buckling resistance. Therefore, the building structure has high buckling resistance and can withstand external loads, so it is suitable for use as a column material of the building structure.

[0145] Example

[0146] Hereinafter, the present invention will be described in further detail based on examples. It should be noted that the present invention is not limited to the following examples.

[0147] Steel raw materials (steel billets) having the chemical compositions shown in Table 1 were melted, and subjected to a heating step, a hot rolling step, and a cooling step under the conditions shown in Table 2 to produce hot rolled steel sheets having the finished sheet thickness (mm) shown in Table 2.

[0148]

[0149]

[0150] The hot rolled steel sheet thus obtained is formed into a cylindrical open tube (round steel tube) by cold rolling, and the butt joint of the open tube is subjected to resistance welding to obtain a steel tube raw material. Then, the steel tube raw material is formed by rollers arranged above, below, left and right thereof to obtain a resistance welded steel tube with an outer diameter D (mm) and a wall thickness t (mm) as shown in Table 3, or a square steel tube with a side length B (mm) and a wall thickness t (mm). It should be noted that the cross-sectional shape of the above-mentioned square steel tube is a square.

[0151]

[0152] Test pieces were cut out from the obtained hot-rolled steel sheets, electric-resistance-welded steel pipes, and angled steel pipes, and the following microstructure observations, tensile tests, and measurements of equivalent plastic strain distribution were performed.

[0153] [Organization Observation]

[0154] Regarding the test piece for organization observation, it is cut in such a way that the observation surface is the cross section in the rolling direction during hot rolling and the position of the plate thickness 1 / 2t, and after grinding, it is corroded with nitric acid ethanol solution to make it. Regarding organization observation, an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) is used to observe the structure of the steel plate at the position of the plate thickness 1 / 2t and take pictures. According to the obtained optical microscope image and SEM image, the area ratio of ferrite, bainite and residual structure (pearlite, martensite, austenite) is calculated. The area ratio of each organization is calculated as the average value of the values ​​obtained in each field of view by observing in 5 fields of view. Here, the area ratio obtained by organization observation is used as the volume ratio of each organization.

[0155] Here, ferrite is a product caused by diffusion phase transformation, and presents a structure with low dislocation density and basically recovered. Polygonal ferrite and quasi-polygonal ferrite are also included. In addition, bainite is a multiphase structure of lath-shaped ferrite and cementite with high dislocation density. In addition, pearlite is a structure in which cementite and ferrite are arranged in layers. In addition, compared with bainite, austenite does not have carbides. In addition, martensite and austenite are distinguished by brighter contrast in SEM images compared with bainite.

[0156] It should be noted that it is difficult to distinguish martensite and austenite using optical microscope images and SEM images. Therefore, the area ratio of the structure observed as martensite or austenite is measured based on the obtained SEM image, and then the volume ratio of austenite measured by the method described later is subtracted, and the value thus obtained is used as the volume ratio of martensite.

[0157] The volume fraction of austenite is determined by X-ray diffraction. The test piece for structural observation is ground so that the diffraction surface is at the position of 1 / 2t of the plate thickness of the steel plate, and then chemically polished to remove the surface processing layer. The Kα ray of Mo is used in the measurement, and the volume fraction of austenite is calculated based on the integrated intensity of the (200), (220), and (311) planes of fcc iron and the (200) and (211) planes of bcc iron.

[0158] The average grain size and CP value are measured using the SEM / EBSD method. The measurement area is set to 500μm×500μm, and the measurement step is set to 0.5μm. Based on the obtained EBSD data, the crystal orientation analysis software OIM Analysis (trademark) is used to take the boundary with an orientation difference of 15° or more as the grain boundary (large-angle grain boundary) to obtain the distribution of the grain boundary. The average grain size is obtained in the form of the arithmetic mean of the equivalent circular diameter (grain size) of each grain. In addition, with respect to the CP value, the total length of the large-angle grain boundaries in the region other than the grains with a grain size of less than 20μm and the total length of the large-angle grain boundaries are calculated respectively, and the total length of the large-angle grain boundaries is obtained in the form of their ratio. Here, the total length of the large-angle grain boundaries in the region other than the grains with a grain size of less than 20μm is the sum of the lengths of the large-angle grain boundaries measured in the region after removing the grains with a grain size of less than 20μm from the distribution of the above-mentioned grain boundaries in the measurement area, and the total length of the large-angle grain boundaries is the sum of the lengths of the large-angle grain boundaries measured from the distribution of the above-mentioned grain boundaries in the measurement area. In addition, in the calculation of the average crystal grain size and the CP value, crystal grains having a grain size of 2.0 μm or less were excluded as measurement noise.

[0159] [Tensile test]

[0160] The tensile test piece of JIS No. 5 was cut in such a way that the tensile direction was parallel to the rolling direction. For the hot-rolled steel plate, the tensile test piece was cut from a position 1 / 4W (W: plate width) from the end in the width direction; for the electric resistance welded steel pipe, the tensile test piece was cut from a position 90° away from the resistance welded part in the circumferential direction; for the angle steel pipe, the tensile test piece was cut from the flat plate part adjacent to the flat plate part containing the resistance welded 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.

[0161] [Equivalent plastic strain distribution]

[0162] The equivalent plastic strain distribution was measured by SEM-DIC method. The samples were cut from the center of the plate thickness of hot-rolled steel plate, the center of the wall thickness of electric resistance welded steel pipe and the center of the wall thickness of angled steel pipe in such a way that the tensile direction was parallel to the rolling direction. Figure 1The tensile test piece shown. For the hot-rolled steel plate, the plate width direction is cut at a position 1 / 4W (W: plate width) away from the end in the width direction; for the circumferential direction of the electric resistance welded steel pipe, the plate is cut at a position 90° away from the resistance welded part in the circumferential direction; for the angle steel pipe, the plate is cut from the flat plate portion adjacent to the flat plate portion containing the resistance welded part. One surface of the obtained tensile test piece is grinded, corroded with nitric acid ethanol solution, and the parallel portions (tensile deformation portions) of 5 fields of view are photographed using SEM (magnification: 1000 times). Then, at a tensile speed of 5 mm / min, a tensile strain of 8.0% is applied to the test piece cut from the hot-rolled steel plate, and a tensile strain of 4.0% is applied to the test pieces cut from the electric resistance welded steel pipe and the angle steel pipe, and then unloaded. Then, a SEM (magnification: 1000 times) is used to photograph the same field of view as before stretching (before the tensile strain is applied). Based on the obtained SEM images before and after stretching, the image analysis software GOM Correlate (GOM) is used to calculate the equivalent plastic strain distribution of the photographed surface by the DIC method. The DIC method is a method of comparing the random patterns on the surface of an object before and after deformation and measuring the displacement and strain of each part of the observation surface. 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 central point of the subset is calculated. This operation is performed on the entire image to obtain the displacement distribution and strain distribution. In the present invention, the nitric acid ethanol solution corrosion marks of the metal structure are used as random patterns. For an image of 1910 pixels × 2560 pixels, the subset size is set to 80 pixels × 80 pixels (3.6μm × 3.6μm), and the measurement interval is set to 10 pixels (0.45μm). The graph with the horizontal axis as the natural logarithm of the equivalent plastic strain obtained (unit: none) and the vertical axis as the proportion (area ratio) (unit: %) is approximated with a normal distribution, and the standard deviation at this time is taken as the logarithmic standard deviation (logarithmic standard deviation of the equivalent plastic strain distribution). Specifically, the logarithmic standard deviation is calculated by the following method. First, in the range of equivalent plastic strain of 0 to 0.20, the grade width is set to 0.02 and the proportion (area ratio) of each grade is calculated (unit: %). At this time, the grade with an equivalent plastic strain of greater than 0 and less than 0.02 is set as the first grade, the grade with an equivalent plastic strain of greater than 0.02 and less than 0.04 is set as the second grade, ..., and the grade with a value of greater than 0.18 and less than 0.20 is set as the tenth grade. x i Let x0 be the natural logarithm of the grade value of the i-th grade, let x0 be the average value of the natural logarithm of the equivalent plastic strain, and the logarithmic standard deviation be calculated by the following equations (4) and (5).

[0163]

[0164]

[0165] [Axial compression test]

[0166] Pressure plates were installed at both ends of the electric resistance welded steel pipe and the angle steel pipe, and an axial compression test was performed using a large compression test device. The stress at the maximum compression load was taken as the maximum stress σmax (N / mm 2 ). In addition, the yield stress σy obtained by the above-mentioned tensile test is used to calculate the yield strength increase rate τ (=σmax / σy).

[0167] Table 4 shows the results obtained for the hot-rolled steel sheets.

[0168]

[0169] Table 5 shows the results obtained for the electric resistance welded steel pipes and the angle steel pipes.

[0170]

[0171] In Table 4 and Table 5, No. 1 to 6 are examples of the present invention, and No. 7 to 12 are comparative examples.

[0172] In the hot-rolled steel sheet of the present invention, the steel structure in the center of the plate thickness is 70% or more and 98% or less in volume ratio of ferrite and bainite, the remainder is composed of one or more selected from pearlite, martensite, and austenite, the average grain size is 15.0 μm or less, and the value of CP obtained by the prescribed 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 imparting 8.0% tensile strain is 0.70 or less.

[0173] In addition, the electric resistance welded steel pipe and the angle steel pipe of the present invention are manufactured from the hot-rolled steel plate of the present invention, and the steel structure in the center of the wall thickness is 70% or more and 98% or less in total of ferrite and bainite by volume, and the remainder is composed of one or more selected from pearlite, martensite, and austenite, and the average grain size is 15.0 μm or less, and the CP value obtained by the prescribed (1) formula is 0.090 or less. In addition, the tensile strength of the parent material part or the flat plate part is 400 MPa or more, and the yield ratio of the parent material part or the flat plate part is 97% or less. Furthermore, in the parent material part or the flat plate part, the logarithmic standard deviation of the equivalent plastic strain distribution after imparting a 4.0% tensile strain is 0.60 or less. In addition, the yield strength increase rate τ (=σmax / σy) in the axial compression test satisfies 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 angled steel pipe. It should be noted that in Table 5, the values ​​​​of t and D substituted for the electric resistance welded steel pipe and t and B substituted for the angled steel pipe are recorded on the right side of the above formula (2) and (3) as the necessary lower limit values ​​of τ.

[0174] On the other hand, in Comparative Example No. 7, the C content was lower than the range of the present invention, and thus the tensile strength was out of the range of the present invention.

[0175] In Comparative Example No. 8, the C content exceeds the range of the present invention, so the total volume fraction of ferrite and bainite is lower than the range of the present invention. As a result, the yield ratio is outside the range of the present invention, and the logarithmic standard deviation is also outside the preferred range, so the yield strength improvement rate does not reach the desired value.

[0176] In Comparative Example No. 9, the contents of Si and Mn are lower than the range of the present invention, so the total volume ratio of ferrite and bainite exceeds the range of the present invention, and the average grain size exceeds the range of the present invention. As a result, the tensile strength is outside the range of the present invention.

[0177] In Comparative Example No. 10, the contents of Si and Mn exceeded the range of the present invention, so the total volume fraction of ferrite and bainite was lower than 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 improvement rate did not reach the desired value.

[0178] In Comparative Example No. 11, the average cooling rate in the temperature range of 900°C or more in the hot rolling process is lower than the range of the appropriate manufacturing method, so the average grain size exceeds the range of the present invention, and the CP value exceeds the range of the present invention. As a result, the logarithmic standard deviation exceeds the preferred range of the present invention, and the yield strength increase rate does not reach the desired value. In addition, the tensile strength is lower than the range of the present invention.

[0179] In Comparative Example No. 12, the cooling stop temperature in the hot rolling process exceeded the range of the appropriate 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.

[0180] As can be seen from the above, by making the steel composition and structure within the scope of the present invention, it is possible to provide electric resistance welded steel pipes and angle steel pipes with excellent buckling resistance, and hot rolled steel plates used as raw materials thereof. In addition, it is possible to provide pipelines and building structures with high buckling resistance using the electric resistance welded steel pipes and angle steel pipes.

Claims

1. A hot rolled steel plate, The composition is as follows: containing, 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% to 0.005% to 0.100%, N: 0.0100% to 0.0100%, the balance being Fe and unavoidable impurities. The steel structure at the center of the plate thickness is as follows: the total volume percentage of ferrite and bainite is 70% or more and 98% or less, the balance is composed of one or more selected from pearlite, martensite, and austenite, the average grain size is 15.0 μm or less, and the CP value obtained by the following formula (1) is 0.090 or less, The hot-rolled steel sheet has a tensile strength of 400 MPa or more and a yield ratio of 90% or less. CP = (total length of high-angle grain boundaries in regions excluding grains having a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1).

2. The hot rolled steel sheet according to claim 1, wherein: In addition to the above composition, the present invention further contains, 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: One or more of 0.020% or less, Zr: 0.020% or less, and REM: 0.020% or less.

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 was 0.70 or less.

4. An electric resistance welded steel pipe, comprising a base material portion and an electric resistance welded portion, wherein: The electric resistance welded steel pipe has a composition as follows: containing, 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% to 0.005% to 0.100%, N: 0.0100% to 0.0100%, and the balance is Fe and inevitable impurities. The steel structure at the center of the wall thickness is as follows: the total volume percentage of ferrite and bainite is 70% or more and 98% or less, the balance is composed of one or more selected from pearlite, martensite, and austenite, the average grain size is 15.0 μm or less, and the CP value obtained by the following formula (1) is 0.090 or less, The tensile strength of the base material is 400 MPa or more, and the yield ratio of the base material is 97% or less. CP = (total length of high-angle grain boundaries in regions excluding grains having a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1).

5. The electric resistance welded steel pipe according to claim 4, wherein: In addition to the above composition, the present invention further contains, 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: One or more of 0.020% or less, Zr: 0.020% or less, and REM: 0.020% or less.

6. The electric resistance welded steel pipe according to claim 4 or 5, wherein: In the base material portion, the logarithmic standard deviation of the equivalent plastic strain distribution after applying a 4.0% tensile strain was less than 0.

60.

7. A line pipe using the electric resistance welded steel pipe according to claim 4 or 5.

8. A line pipe using the electric resistance welded steel pipe according to claim 6.

9. An angular steel pipe having a flat portion and an angle portion, wherein: The angled steel pipe has a composition as follows: containing, 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% to 0.005% to 0.100%, N: 0.0100% to 0.0100%, and the balance is Fe and inevitable impurities. The steel structure at the center of the wall thickness is as follows: the total volume percentage of ferrite and bainite is 70% or more and 98% or less, the balance is composed of one or more selected from pearlite, martensite, and austenite, the average grain size is 15.0 μm or less, and the CP value obtained by the following formula (1) is 0.090 or less, The tensile strength of the flat plate is 400 MPa or more, and the yield ratio of the flat plate is 97% or less. CP = (total length of high-angle grain boundaries in regions excluding grains having a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1).

10. The angled steel tube according to claim 9, wherein: In addition to the above composition, the present invention further contains, 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: One or more of 0.020% or less, Zr: 0.020% or less, and REM: 0.020% or less.

11. The angled steel pipe according to claim 9 or 10, wherein: In the flat plate portion, the logarithmic standard deviation of the equivalent plastic strain distribution after applying a 4.0% tensile strain was less than 0.

60.

12. A building structure, wherein: The angled steel pipe according to claim 9 or 10 is used as a column material.

13. A building structure, wherein: The angled steel pipe according to claim 11 is used as a column material.

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