Hot-rolled steel sheet and electric resistance welded steel pipe

Abstract

The purpose of the present invention is to provide: an electric-resistance-welded steel pipe having excellent flattening resistance; and a hot-rolled steel sheet used as a raw material for the electric-resistance-welded steel pipe. Provided is a hot-rolled steel sheet in which cracks having a length of 0.50 mm or more do not occur in a specific C-shape flattening test until the inside surfaces of a test piece after bending are in contact with each other, and in which the normalized load / normalized displacement determined by formula (1) is 100 MPa or more in a range in which the normalized displacement determined by formula (2) is 0.20-0.30, and the normalized load / normalized displacement determined by formula (1) is 0.20-0.30. (normalized load (MPa)) = (P / L) * (r / t2) * (1-((x0-x) / 2r) 2) 1 / 2 (normalized displacement) = (x0-x) / 2r... (2) (where P is load (N), L is the initial length (mm) of the flattened test piece, r is the initial radius of curvature (mm) of the curved outer surface of the flattened test piece, t is the initial sheet thickness (mm) of the flattened test piece, x0 is the initial distance (mm) between the two flat plates, and x is the distance (mm) between the two flat plates.

Description

Technical Field

[0001] The present invention relates to an electric resistance welded steel pipe suitable for building structures, pipelines, etc., and a hot-rolled steel plate as a raw material thereof. Background Art

[0002] If electric resistance welded steel pipes used for building structures and line pipes buckle when subjected to external forces such as earthquakes, their yield strength will drop sharply and they may break. Therefore, it is desirable to suppress the occurrence of buckling in these electric resistance welded steel pipes.

[0003] In order to suppress the buckling of the electric resistance welded steel pipe, it is effective to suppress the flattening of the cross section of the steel pipe, which is a precursor phenomenon to the buckling.

[0004] While there are relatively few examples of research conducted to address such requirements, Patent Document 1 discloses a steel pipe in which an electric resistance welded steel pipe serving as a mother pipe is heated and subjected to hot reduction rolling to reduce the r value in the longitudinal direction of the pipe to 1.0 or greater, thereby suppressing buckling during bending.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent No. 6954504

[0008] Non-patent literature

[0009] Non-patent document 1: Kobayashi Hidetoshi, Okubo Hiroshi, Taimaruya Masashi: Light Metal, 39, (1989), p.8 Summary of the Invention

[0010] Problems to be solved by the invention

[0011] As described in Patent Document 1, by increasing the r-value of the steel pipe in the axial direction, circumferential deformation of the pipe is promoted, and variations in wall thickness are reduced, thereby suppressing cross-sectional flattening of the steel pipe. However, this reduction in cross-sectional diameter results in necking, which reduces the cross-sectional area and, consequently, lowers the yield strength of the structure.

[0012] 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 having excellent crushing resistance, and a hot-rolled steel plate used as a raw material thereof.

[0013] It should be noted that the term "excellent crush resistance" as used in the present invention means that, in a crush test, no cracks of 0.50 mm or longer are generated until the inner surfaces of the crush test pieces cut from the electric resistance welded steel pipe are in close contact with each other, and the normalized displacement calculated by equation (2) is within the range of 0.20 to 0.30 (normalized load / normalized displacement calculated by equation (1) below) is 100 MPa or greater. The term "the inner surfaces of the crush test pieces cut from the electric resistance welded steel pipe are in contact with each other" means, for example, that, when a load is applied from the top and bottom of the crush test pieces cut from the electric resistance welded steel pipe to perform the crush test, the inner surfaces of the upper portion and the lower portion of the crush test pieces cut from the electric resistance welded steel pipe are in contact with each other.

[0014] (Normalized load (MPa)) = (P / L) × (r / t 2 )×(1-((x0-x) / 2r) 2 ) 1 / 2 …(1)

[0015] (Normalized displacement) = (x0-x) / 2r …(2)

[0016] in,

[0017] P: load (N)

[0018] L: Initial length of the flattened test piece in the tube axis direction (mm)

[0019] r: Initial curvature radius of the curved outer surface of the flattened test piece (mm)

[0020] t: Initial thickness of the flattened test piece (mm)

[0021] x0: initial distance between the two plates (mm)

[0022] x: distance between two plates (mm)

[0023] Methods used to solve problems

[0024] The present inventors conducted extensive research and discovered that increasing the proportion of fine grains in the microstructure of electric resistance welded steel pipes improves toughness and suppresses cracking during flattening tests. On the other hand, they also discovered that excessively high proportions of fine grains reduce ductility, making cracking more likely to occur before sealing during flattening tests.

[0025] Furthermore, it was found that increasing the degree of connectivity of fine grains can increase the load increase relative to displacement (indentation) in the flattening test, thereby increasing the resistance to flattening. On the other hand, it was also found that excessive connectivity of fine grains reduces ductility, leading to cracking before adhesion in the flattening test.

[0026] Furthermore, it was found that electric resistance welded steel pipes produced using hot-rolled steel sheets having high resistance to flattening in a C-shaped flattening test as raw materials have excellent flattening resistance.

[0027] The present invention has been completed based on the above findings and provides the following [1] to [6].

[0028] [1] A hot-rolled steel plate, wherein:

[0029] In the C-shaped crush test, a steel plate is bent into a U-shaped test piece and then clamped between two flat plates.

[0030] No cracks with a length of 0.50 mm or more are generated until the inner surfaces of the bent test pieces are in close contact with each other, and

[0031] When the normalized displacement obtained by the following formula (2) is within the range of 0.20 to 0.30, the normalized load obtained by the following formula (1) / the normalized displacement is 100 MPa or more.

[0032] (Normalized load (MPa)) = (P / L) × (r / t 2 )×(1-((x0-x) / 2r) 2 ) 1 / 2 …(1)

[0033] (Normalized displacement) = (x0-x) / 2r …(2)

[0034] in,

[0035] P: load (N)

[0036] L: Initial length of the flattened test piece (mm)

[0037] r: Initial curvature radius of the curved outer surface of the flattened test piece (mm)

[0038] t: Initial thickness of the flattened test piece (mm)

[0039] x0: initial distance between the two plates (mm)

[0040] x: distance between two plates (mm)

[0041] [2] The hot-rolled steel sheet according to [1], wherein the composition comprises, in mass%, C: 0.020% to 0.200%, Si: 0.50% to 0.50%, Mn: 0.30% to 2.00%, P: 0.050% to 0.0200%, S: 0.005% to 0.100%, N: 0.0100% to 0.050%, or further comprises Nb: 0.080% to 0.080%, V: 0.080% to 0.080%, or further comprises Nb: 0.080% to 0.080%, or further comprises Nb: 0.080% to 0.080%, or further comprises V: 0.080% to 0.050%, or further comprises Nb ... % or less, Ti: 0.080% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 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, Sn: 0.100% or less, and the balance being Fe and inevitable impurities.

[0042] [3] The hot-rolled steel sheet according to [1] or [2], wherein

[0043] In the steel structure at the center of the plate thickness,

[0044] The average grain size of the grains in the region surrounded by the high-angle grain boundaries is 15.0 μm or less.

[0045] The area ratio of crystal grains having a grain size equal to or smaller than the above average grain size is 10% or more and 50% or less relative to all crystal grains.

[0046] The fine grain connectivity obtained by the following formula (3) is 0.05 or more and 0.50 or less, and

[0047] Bainite is 10% or more by volume,

[0048] The total volume fraction of ferrite and bainite is 80% or more.

[0049] The remainder, which is one or more selected from pearlite, martensite, and austenite, is 20% or less in total by volume.

[0050] (Connectivity of fine grains) = (Total length of high-angle grain boundaries of grains with a grain size smaller than the average grain size) / (Total length of high-angle grain boundaries) …(3)

[0051] However, the numerator on the right side of the formula (3) does not include the length of the high-angle grain boundary between the grains smaller than the average grain size and the grains larger than the average grain size.

[0052] [4] An electric resistance welded steel pipe having a base material portion and an electric resistance welded portion, wherein:

[0053] In the flattening test, a flattening test piece cut from an electric resistance welded steel pipe is clamped between two flat plates.

[0054] No cracks with a length of 0.50 mm or more are generated until the inner surfaces of the flattened test pieces are in close contact with each other, and

[0055] When the normalized displacement obtained by the following formula (2) is within the range of 0.20 to 0.30, the normalized load obtained by the following formula (1) / the normalized displacement is 100 MPa or more.

[0056] (Normalized load (MPa)) = (P / L) × (r / t 2 )×(1-((x0-x) / 2r) 2 ) 1 / 2 …(1)

[0057] (Normalized displacement) = (x0-x) / 2r …(2)

[0058] in,

[0059] P: load (N)

[0060] L: Initial length of the flattened test piece in the tube axis direction (mm)

[0061] r: Initial curvature radius of the curved outer surface of the flattened test piece (mm)

[0062] t: Initial thickness of the flattened test piece (mm)

[0063] x0: initial distance between the two plates (mm)

[0064] x: distance between two plates (mm)

[0065] [5] The electric resistance welded steel pipe according to [4], wherein the base material portion comprises, in mass%, C: 0.020% to 0.200%, Si: 0.50% to 2.00%, Mn: 0.30% to 2.00%, P: 0.050% to 0.0200%, S: 0.005% to 0.100%, N: 0.0100% to 0.050%, or further comprises Nb: 0.080% to 0.080%, V: 0.0000% to 0.0000%. One or more of the following: 0.080% or less, Ti: 0.080% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 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, Sn: 0.100% or less, and the balance being Fe and inevitable impurities.

[0066] [6] The electric resistance welded steel pipe according to [4] or [5], wherein

[0067] In the steel structure at the center of the wall thickness of the above-mentioned base material,

[0068] The average grain size of the grains in the region surrounded by the high-angle grain boundaries is 15.0 μm or less.

[0069] The area ratio of crystal grains having a grain size equal to or smaller than the above average grain size is 10% or more and 50% or less relative to all crystal grains.

[0070] The fine grain connectivity obtained by the following formula (3) is 0.05 or more and 0.50 or less, and

[0071] Bainite is 10% or more by volume,

[0072] The total volume fraction of ferrite and bainite is 80% or more.

[0073] The remainder, which is one or more selected from pearlite, martensite, and austenite, is 20% or less in total by volume.

[0074] (Connectivity of fine grains) = (Total length of high-angle grain boundaries of grains with a grain size smaller than the average grain size) / (Total length of high-angle grain boundaries) …(3)

[0075] However, the numerator on the right side of the formula (3) does not include the length of the high-angle grain boundary between the grains smaller than the average grain size and the grains larger than the average grain size.

[0076] Effects of the Invention

[0077] According to the present invention, it is possible to provide an electric resistance welded steel pipe having excellent crushing resistance and a hot-rolled steel plate used as a raw material thereof. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 It is a schematic diagram of the relationship between normalized load and normalized displacement in the flattening test.

[0079] FIG. 2( a ) is a side view showing the direction in which a C-shaped flattening test piece of a hot-rolled steel sheet is cut.

[0080] FIG2( b ) is a side view of a flattening test of a C-shaped flattening test piece of a hot-rolled steel sheet.

[0081] Figure 3 This is a diagram showing the flattening test of an electric resistance welded steel pipe as viewed from the side.

[0082] Figure 4 This is a schematic diagram of a cross-section of the resistance welded portion of an electric resistance welded steel pipe. DETAILED DESCRIPTION

[0083] Hereinafter, the hot-rolled steel sheet and electric-resistance-welded steel pipe of the present invention and their production methods will be described.

[0084] First, the reasons for limiting the mechanical properties of the hot-rolled steel sheet and electric-resistance-welded steel pipe of the present invention will be described.

[0085] The hot-rolled steel sheet of the present invention is characterized in that, in a C-shaped flattening test, no cracks with a length of 0.50 mm or more are generated until the inner surfaces after bending are in close contact with each other, and the value of normalized load / normalized displacement within the range of normalized displacement of 0.20 to 0.30 is 100 MPa or more.

[0086] Furthermore, the electric resistance welded steel pipe of the present invention is characterized in that, in a flattening test, no cracks are generated until the inner surfaces of flattening test pieces cut from the electric resistance welded steel pipe are in close contact with each other, and the normalized load / normalized displacement value is 100 MPa or greater when the normalized displacement is within the range of 0.20 to 0.30.

[0087] The normalized load and normalized displacement are obtained by equations (1) and (2), respectively.

[0088] (Normalized load (MPa)) = (P / L) × (r / t 2 )×(1-((x0-x) / 2r) 2 ) 1 / 2 …(1)

[0089] (Normalized displacement) = (x0-x) / 2r …(2)

[0090] in,

[0091] P: load (N)

[0092] L: Initial length of the flattened test piece (mm)

[0093] r: Initial curvature radius of the curved outer surface of the flattened test piece (mm)

[0094] t: Initial thickness of the flattened test piece (mm)

[0095] x0: initial distance between the two plates (mm)

[0096] x: distance between two plates (mm)

[0097] The value of normalized load / normalized displacement in the range of normalized displacement of 0.20 to 0.30 is obtained by ((normalized load when normalized displacement is 0.30)-(normalized load when normalized displacement is 0.20)) / (0.30-0.20).

[0098] It should be noted that the phrase "no cracks are generated until a close fit is achieved" during the flattening test means that no cracks are generated until the inner surfaces of the flattening test pieces cut from the electric resistance welded steel pipe come into contact with each other. The aforementioned "inner surfaces of the flattening test pieces cut from the electric resistance welded steel pipe coming into contact with each other" means, for example, that when a flattening test is performed by applying loads from the upper and lower directions to the flattening test pieces cut from the electric resistance welded steel pipe, the inner surface of the upper portion of the flattening test piece cut from the electric resistance welded steel pipe comes into contact with the inner surface of the lower portion. It should be noted that the aforementioned cracks refer to cracks of a size that can be visually confirmed. The size that can be visually confirmed refers to a length of 0.50 mm or more. There is no particular upper limit on the length of the crack, but since the crack is linear, it is preferably less than the length of the test piece in the pipe axis direction. In addition, the length refers to the size of the long side of the crack.

[0099] In the C-shaped crush test and the crush test, the greater the increase in load relative to the distance between the two flat plates (the amount of indentation), the greater the resistance to crushing.

[0100] However, even for the same material, the load and displacement vary depending on the size and shape of the test piece. Therefore, as described in Non-Patent Document 1, the load and displacement are normalized using equations (1) and (2).

[0101] In the C-shaped flattening test and the flattening test, the test piece undergoes elastic deformation until the normalized displacement reaches a certain value, after which the test piece yields and undergoes plastic deformation. Figure 1 The diagram in Figure 2 shows the relationship between the normalized load and the normalized displacement in the flattening test. Figure 1 In the curve 1 showing the change of normalized load caused by normalized displacement, as shown in FIG. Figure 1 In that way, in the elastic region 2 until the test piece yields, the normalized load and the normalized displacement are proportional. Then, when the test piece yields and enters the plastic region 3, the slope of the curve (normalized load / normalized displacement) becomes smaller, and the test is carried out while maintaining a certain slope. Then, when the slope of the curve (normalized load / normalized displacement) shows a value higher than the above-mentioned slope (the slope after becoming the plastic region 3), the test is carried out, cracks are generated on the curved outer surface, or the inner surfaces of the test piece contact each other (the inner surface of the upper part contacts the inner surface of the lower part) and become a close-fitting state, thereby ending the test. Hereinafter, the state of the test piece at the end of the test will also be referred to as crack or close-fitting state 4.

[0102] If cracks occur before a close-fitting state is achieved, a sudden load drop occurs. Therefore, in the present invention, it is important to prevent cracks from occurring before a close-fitting state is achieved from the perspective of ensuring safety as a structure.

[0103] In the present invention, (normalized load / normalized displacement) in the plastic zone 3 is used as an indicator of the flattening resistance. That is, the larger the (normalized load / normalized displacement), the greater the resistance to flattening and the higher the flattening resistance. In the present invention, in particular, the (normalized load / normalized displacement) in the range of 0.20 to 0.30 corresponding to the normalized displacement in the first half of the plastic zone 3 is set to 100 MPa or more. Preferably, the above-mentioned (normalized load / normalized displacement) is 120 MPa or more. More preferably, the above-mentioned (normalized load / normalized displacement) is 140 MPa or more. Further preferably, the above-mentioned (normalized load / normalized displacement) is 150 MPa or more. However, when the above-mentioned (normalized load / normalized displacement) exceeds 600 MPa, the ductility decreases, and therefore cracks are likely to occur before reaching a tight state. Therefore, the value of the above-mentioned (normalized load / normalized displacement) is preferably set to 600 MPa or less. More preferably, the (normalized load / normalized displacement) is 550 MPa or less. Still more preferably, the (normalized load / normalized displacement) is 500 MPa or less. Most preferably, the (normalized load / normalized displacement) is 450 MPa or less.

[0104] Note that the range of the normalized displacement is set to 0.20 to 0.30 because the value of (normalized load / normalized displacement) is substantially constant and stable in the plastic region.

[0105] The C-shaped flattening test is a test in which a test piece of steel plate is bent into a U-shape and then clamped between two flat plates.

[0106] Regarding the C-shaped flattening test, a plate with a width of 50 mm × t (t: plate thickness) and a length of 100 mm is cut from the hot-rolled steel plate so that the length direction of the test piece is the plate width direction of the hot-rolled steel plate. Then, it is made into a C-shaped test piece by the bending method described in JIS Z 2248 (2006), and then implemented by the method described in JIS G 3441 (2021). Figure 2 (a) shows a side view of the cutting direction of the C-shaped flattening test piece of the hot-rolled steel plate and the situation after the C-shaped flattening test piece is made. The test piece before being made into the C-shaped flattening test piece is designated by symbol 5A, and the C-shaped flattening test piece after being made into the C-shaped flattening test piece is designated by symbol 5. The initial length and initial thickness do not change before and after the C-shaped flattening test piece is made. The initial length 100 of the C-shaped flattening test piece (flattening test piece) 5 refers to the length 50 mm described above, and the length direction of the C-shaped flattening test piece 5 is the rolling direction 102 of the hot-rolled steel plate. The initial thickness of the C-shaped flattening test piece (flattening test piece) 5 is denoted by symbol 101. FIG. 2(b) shows a side view of the flattening test of the C-shaped flattening test piece of the hot-rolled steel plate. The C-shaped flattening test refers to the following test method: as shown in FIG. 2(a) and FIG. 2(b), a cut test piece 5A is bent into a U-shape to form a C-shaped flattening test piece 5, and the C-shaped flattening test piece 5 is clamped between two flat plates 6. While applying a load in a compression direction 7 perpendicular to the flat plates 6, the test is performed until the state described above is reached. Regarding the C-shaped flattening test piece 5, during bending, the inner radius of the front end of the pressing piece is set to 9×t (mm). The initial curvature radius r (mm) of the bent outer surface of the C-shaped flattening test piece is obtained by adding the plate thickness t (mm) to the inner radius of the front end of the pressing piece during the bending as shown in formula (4).

[0107] r=10×t …(4)

[0108] Regarding the flattening test, a circular test piece with a length of 100 mm in the pipe axis direction was cut from the electric resistance welded steel pipe including the electric resistance welded portion, and the flattening test was carried out according to the method described in JIS G 3441 (2021). Figure 3 The test piece 8 was placed so that the line connecting the resistance welded portion 9 and the center 10 of the resistance welded steel pipe was parallel to the compression direction 12. The initial curvature radius r of the curved outer surface of the flattened test piece 8 was set to 1 / 2 the outer diameter of the resistance welded steel pipe.

[0109] In addition, the base material portion of the hot-rolled steel sheet and electric-resistance-welded steel pipe of the present invention preferably contains, in mass%, C: 0.020% to 0.200%, Si: 0.50% to 0.30% to 2.00%, P: 0.050% to 0.0200%, S: 0.005% to 0.100%, and N: 0.0100%, or further contains Nb: 0.080% to 0.100%, and V: 0.0000%. One or more of the following: 0.080% or less, Ti: 0.080% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 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, and Sn: 0.100% or less, and the balance being Fe and inevitable impurities.

[0110] In this specification, “%” indicating the steel composition means “mass %” unless otherwise specified.

[0111] C: 0.020% or more and 0.200% or less

[0112] C is an element that increases the strength of steel through solid solution strengthening. Furthermore, C is an element that, by improving the hardenability of steel and increasing the proportion of fine grains, improves toughness, helps suppress cracking during flattening tests, and by increasing the connectivity of fine grains, also helps increase resistance to flattening. To achieve these effects, a C content of 0.020% or more is preferred. A C content of 0.025% or more is more preferred, and 0.030% or more is even more preferred. A C content of 0.035% or more is most preferred. However, if the C content exceeds 0.200%, hard pearlite, martensite, and austenite are excessively formed, reducing ductility and making cracking more likely to occur before achieving a tight fit during C-shaped flattening tests and flattening tests. Therefore, the C content is preferably 0.200% or less. A C content of 0.180% or less is more preferred. A C content of 0.170% or less is even more preferred. A C content of 0.165% or less is most preferred.

[0113] Si: 0.50% or less

[0114] Si is an element that increases the strength of steel through solid solution strengthening. To achieve this effect, it is preferably contained at least 0.02% Si. The Si content is more preferably at least 0.05%, and even more preferably at least 0.08%. The most preferred Si content is at least 0.10%. However, when the Si content exceeds 0.50%, ductility decreases, and cracks are likely to occur before a tight fit is achieved in a flattening test. Therefore, the Si content is preferably set to 0.50% or less. The Si content is more preferably at most 0.40%. Even more preferably, it is at most 0.30%. The most preferred Si content is at most 0.28%.

[0115] Mn: 0.30% or more and 2.00% or less

[0116] Mn is an element that increases the strength of steel through solid solution strengthening. Furthermore, Mn is an element that helps suppress cracking during flattening tests by increasing the hardenability of steel, increasing the proportion of fine grains, and improving toughness. Furthermore, by increasing the connectivity of fine grains, it also helps increase resistance to flattening. To achieve this effect, it is preferred to contain 0.30% or more of Mn. The Mn content is more preferably 0.40% or more, and even more preferably 0.50% or more. The most preferred Mn content is 0.60% or more. However, when the Mn content exceeds 2.00%, hard pearlite, martensite, and austenite are excessively generated, reducing ductility and making it easy to crack before achieving a tight fit during a flattening test. Therefore, the Mn content is preferably set to 2.00% or less. The Mn content is more preferably 1.90% or less. It is even more preferably 1.80% or less. The most preferred Mn content is 1.75% or less.

[0117] P: 0.050% or less

[0118] P segregates at grain boundaries and reduces toughness, so it is preferable to reduce it as much as possible as an unavoidable impurity. The P content is preferably set within a range of 0.050% or less. The P content is more preferably 0.040% or less, and even more preferably 0.030% or less. The most preferred P content is 0.020% or less. It should be noted that there is no particular lower limit for P, but excessive reduction leads to increased smelting costs, so P is preferably set to 0.001% or more.

[0119] S: 0.0200% or less

[0120] S is usually present in the form of MnS in steel, but MnS is thinly extended during the hot rolling process, which has a negative impact on ductility and toughness. Therefore, in the present invention, it is preferred to reduce S as much as possible, and the S content is preferably set to 0.0200% or less. The S content is more preferably 0.0100% or less, and even more preferably 0.0050% or less. The S content is most preferably 0.0030% or less. It should be noted that there is no particular lower limit for S, but excessive reduction leads to a rise in smelting costs, so S is preferably set to 0.0001% or more.

[0121] Al: 0.005% or more and 0.100% or less

[0122] Al is an element that acts as a powerful deoxidizer. To achieve this effect, it is preferably present in an amount of 0.005% or more of Al. The Al content is more preferably 0.010% or more, and even more preferably 0.015% or more. The most preferred Al content is 0.020% or more. However, if the Al content exceeds 0.100%, weldability deteriorates, and the presence of alumina inclusions increases, degrading surface properties. Therefore, the Al content is preferably set to 0.100% or less. The Al content is more preferably 0.080% or less. Even more preferably, it is 0.070% or less. The most preferred Al content is 0.065% or less.

[0123] N: 0.0100% or less

[0124] N is an unavoidable impurity and an element that has the effect of reducing ductility and toughness 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 0.0100% or less. The N content is preferably 0.0080% or less. The N content is more preferably 0.0070% or less. The N content is further preferably 0.0065% or less. The N content is most preferably 0.0060% or less. There is no particular lower limit, but excessive reduction leads to an increase in refining costs, so it is preferably set to 0.0010% or more.

[0125] The electric resistance welded steel pipe and hot-rolled steel plate of the present invention may further contain one or more of Nb, V, Ti, Cu, Ni, Cr, Mo, Ca, B, Mg, Zr, REM, and Sn in addition to the above-mentioned composition.

[0126] Nb: 0.080% or less

[0127] Nb is an element that contributes to the improvement of the strength of steel by forming fine carbides and nitrides in steel, and also contributes to the refinement of the structure by suppressing the coarsening of austenite during hot rolling. It can be contained as needed. In order to obtain the above-mentioned effect, when Nb is contained, it is preferably contained at least 0.002% Nb. More preferably, the Nb content is at least 0.005%, and further preferably at least 0.010%. The most preferred Nb content is at least 0.012%. However, when the Nb content exceeds 0.080%, ductility and toughness decrease. Therefore, when Nb is contained, the Nb content is set to 0.080% or less. More preferably, the Nb content is 0.070% or less. More preferably, the Nb content is 0.065% or less. The most preferred Nb content is 0.060% or less.

[0128] V: 0.080% or less

[0129] V is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel, and can be contained as needed. To achieve the above-mentioned effects, when V is contained, it is preferably contained at least 0.002% V. More preferably, the V content is at least 0.005%, and even more preferably at least 0.010%. The most preferred V content is at least 0.015%. However, when the V content exceeds 0.080%, ductility and toughness decrease. Therefore, when V is contained, the V content is set to 0.080% or less. More preferably, it is 0.070% or less. Even more preferably, it is 0.065% or less. The most preferred V content is 0.060% or less.

[0130] Ti: 0.080% or less

[0131] Ti is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in steel. It is also an element that contributes to reducing the solid-solution N in steel due to its high affinity with N, and can be contained as needed. In order to obtain the above-mentioned effect, when containing Ti, it is preferred to contain 0.002% or more of Ti. More preferably, the Ti content is 0.005% or more, and further preferably 0.010% or more. The most preferred Ti content is 0.012% or more. However, when the Ti content exceeds 0.080%, ductility and toughness decrease. Therefore, when containing Ti, the Ti content is set to 0.080% or less. More preferably, the Ti content is 0.070% or less. More preferably, the Ti content is 0.065% or less. The most preferred Ti content is 0.060% or less.

[0132] Cu: 0.50% or less, Ni: 0.50% or less

[0133] Cu and Ni are elements that increase the strength of steel through solid solution strengthening, and are elements that improve the hardenability of steel and also contribute to the refinement of the structure. They can be contained as needed. To achieve the above-mentioned effects, when Cu is contained, the Cu content is preferably set to 0.01% or more. More preferably, it is 0.05% or more. More preferably, it is 0.10% or more. To achieve the above-mentioned effects, when Ni is contained, the Ni content is preferably set to 0.01% or more. More preferably, it is 0.05% or more. More preferably, it is 0.10% or more. On the other hand, excessive inclusion may lead to a decrease in ductility and toughness. In addition, it may lead to excessive formation of hard pearlite, martensite, and austenite. Therefore, when Cu is contained, the Cu content is set to 0.50% or less. Preferably, it is 0.40% or less. More preferably, it is 0.30% or less. In addition, when Ni is contained, the Ni content is set to 0.50% or less. Preferably, it is 0.40% or less. More preferably, it is 0.30% or less.

[0134] Cr: 0.50% or less, Mo: 0.50% or less

[0135] Cr and Mo are elements that improve the hardenability of steel and contribute to microstructure refinement, and may be included as needed. To achieve these effects, when Cr is included, the Cr content is preferably set to 0.01% or greater. It is more preferably set to 0.05% or greater. It is even more preferably set to 0.10% or greater. Furthermore, to achieve these effects, when Mo is included, the Mo content is preferably set to 0.01% or greater. It is more preferably set to 0.05% or greater. It is even more preferably set to 0.10% or greater.

[0136] On the other hand, excessive inclusion may lead to excessive formation of hard pearlite, martensite, and austenite. Therefore, when Cr is included, the Cr content is set to 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less. Furthermore, when Mo is included, the Mo content is set to 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

[0137] Ca: 0.0050% or less

[0138] Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly stretched during the hot rolling process, and can be contained as needed. To achieve the above-mentioned effects, when Ca is contained, it is preferably contained at least 0.0005% Ca. A more preferred Ca content is 0.0008% or more, and even more preferably 0.0010% or more. The most preferred Ca content is 0.0015% or more. However, when the Ca content exceeds 0.0050%, Ca oxide clusters form in the steel, degrading toughness. Therefore, when Ca is contained, the Ca content is set to 0.0050% or less. A preferred Ca content is 0.0040% or less. A more preferred Ca content is 0.0035% or less. A more preferred Ca content is 0.0030% or less.

[0139] B: 0.0050% or less

[0140] B is an element that contributes to the refinement of the structure by lowering the phase transformation start temperature and can be contained as needed. In order to achieve the above-mentioned effect, when B is contained, it is preferably contained at least 0.0002%. More preferably, the B content is at least 0.0005%, and even more preferably at least 0.0008%. The most preferably B content is at least 0.0010%. However, when the B content exceeds 0.0050%, ductility and toughness deteriorate. Therefore, when B is contained, the B content is set to 0.0050% or less. More preferably, it is 0.0040% or less. More preferably, it is 0.0030% or less. The most preferably B content is 0.0025% or less.

[0141] Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less

[0142] Mg, Zr, and REM are elements that increase the strength of steel by refining the grains and can be included as needed. The Mg content can be 0%, but when Mg is included, the preferred lower limit is 0.0005% or more. The Mg content is more preferably 0.0008% or more. The Zr content can be 0%, but when Zr is included, the preferred lower limit is 0.0005% or more. The Zr content is more preferably 0.0008% or more. The REM content can be 0%, but when REM is included, the preferred lower limit is 0.0005% or more. The REM content is more preferably 0.0008% or more. On the other hand, excessive inclusion may lead to an increase in the yield ratio and the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when Mg is included, the Mg content is set to 0.020% or less. The Mg content is preferably 0.010% or less. When Zr is included, the Zr content is set to 0.020% or less. The Zr content is preferably 0.010% or less. When REM is present, the REM content is 0.020% or less. The REM content is preferably 0.010% or less. It should be noted that REM is a general term for 17 elements, including Sc, Y, and lanthanides. Steel may contain one or more of these 17 elements, and the REM content refers to the total content of these elements.

[0143] Sn: 0.100% or less

[0144] Sn is an element that suppresses decarburization caused by nitridation or oxidation on the steel sheet surface and reduces strength. To achieve this effect, it is preferably present in an amount of 0.001% or more. The Sn content is more preferably 0.002% or more, and even more preferably 0.005% or more. However, excessive Sn content reduces the ductility and toughness of the steel. Therefore, the Sn content is preferably set to 0.100% or less. The Sn content is more preferably 0.070% or less. The Sn content is even more preferably 0.040% or less.

[0145] The balance is Fe and unavoidable impurities. Unavoidable impurities are impurities that inevitably enter from raw materials, manufacturing processes, or manufacturing equipment, and their presence is permitted as long as they do not hinder the objectives of the present invention. Examples of unavoidable impurities in the balance include As, Sb, Bi, Co, Pb, Zn, O, Ta, W, Te, Hf, Ge, Sr, and Cs. Raw materials for steel sheets include iron ore, reduced iron, and scrap.

[0146] The steel structure at the center of the plate thickness of the hot-rolled steel plate and the center of the wall thickness of the base material portion of the electric resistance welded steel pipe of the present invention is preferably such that the average grain size of the grains in the region surrounded by high-angle grain boundaries is 15.0 μm or less, the area ratio of grains having a grain size less than the average grain size is 10% or more and 50% or less relative to all grains, the fine grain connectivity determined by the following formula (3) is 0.05 or more and 0.50 or less, and the volume ratio of bainite is 10% or more, the volume ratio of ferrite and bainite is 80% or more, and the balance is 20% or less of one or more selected from pearlite, martensite, and austenite.

[0147] (Connectivity of fine grains) = (Total length of high-angle grain boundaries of grains with a grain size smaller than the average grain size) / (Total length of high-angle grain boundaries) …(3)

[0148] However, the numerator on the right side of the formula (3) does not include the length of the high-angle grain boundary between the grains smaller than the average grain size and the grains larger than the average grain size.

[0149] The average grain size, the area ratio of the grains with a grain size below the average grain size and the fine grain connectivity are measured using the SEM / EBSD method. The measurement area is set to 500 μm × 500 μm, the measurement step is set to 0.5 μm, and the measured values ​​of more than 5 fields of view are averaged. Based on the obtained EBSD data, the crystal orientation analysis software OIM Analysis (trademark) is used, and the boundary with an orientation difference of more than 15 ° is used as the grain boundary (large-angle grain boundary) to obtain the distribution of grain boundaries and grain size. The average grain size is obtained in the form of the diameter (equivalent circle diameter) of a circle with an area equal to the value obtained by dividing the total area measured by the number of grains. In addition, the fine grain connectivity is to calculate the total length of the large-angle grain boundaries in the region other than the grains with a grain size above the average grain size (that is, only the grains with a grain size smaller than the average grain size) and the total length of all large-angle grain boundaries and to obtain them in the form of their ratio. However, the high-angle grain boundaries in the region other than the grains having a grain size greater than the average grain size do not include the high-angle grain boundaries between grains smaller than the average grain size and grains greater than the average grain size. It should be noted that in the calculation of the average grain size, the area ratio of grains having a grain size less than the average grain size, and the fine grain connectivity, grains with a grain size of 1.0 μm or less are excluded as measurement noise.

[0150] When the average crystal grain size becomes larger, the toughness decreases and cracks are easily generated in the flattening test. In addition, the value of (normalized load / normalized displacement) is sometimes reduced. Therefore, the average crystal grain size is preferably less than 15.0 μm. The average crystal grain size is more preferably less than 12.0 μm. More preferably, it is less than 10.0 μm. The average crystal grain size is most preferably less than 9.0 μm. It should be noted that when the average crystal grain size becomes smaller, the ductility decreases and cracks are easily generated in the flattening test. Therefore, the average crystal grain size is preferably more than 2.0 μm. More preferably, it is more than 3.0 μm. The average crystal grain size is more preferably more than 3.5 μm.

[0151] When the area ratio of grains having a particle size below the average crystal grain size decreases, the connectivity of fine grains below the average crystal grain size sometimes decreases, and the resistance to flattening becomes smaller. In addition, the toughness decreases, and cracks are easily generated during the flattening test. Therefore, the above-mentioned area ratio is preferably 10% or more. The above-mentioned area ratio is more preferably 12% or more. Further preferably, it is 15% or more. When the above-mentioned area ratio exceeds 50%, the connectivity of fine grains increases, the ductility decreases, and cracks are easily generated during the flattening test. Therefore, the above-mentioned area ratio is preferably 50% or less. The above-mentioned area ratio is more preferably 45% or less. Further preferably, it is 40% or less. The above-mentioned area ratio is most preferably 35% or less.

[0152] When the degree of connectivity of fine grains decreases, the degree of connectivity between coarse grains increases, and the strains in the coarse grains are easily connected to each other. Coarse grains are soft and produce large strains within the grains. Therefore, if they are connected, stress concentration sometimes occurs, and the resistance to flattening becomes smaller. Therefore, the degree of connectivity of fine grains is preferably greater than 0.05. More preferably, it is greater than 0.10. The degree of connectivity of fine grains is further preferably greater than 0.11. The degree of connectivity of fine grains is most preferably greater than 0.12. When the degree of connectivity of fine grains increases, the hard fine grains act as the parent phase, so the ductility decreases and cracks are easily generated in the flattening test. Therefore, the degree of connectivity of fine grains is preferably less than 0.50. The degree of connectivity of fine grains is more preferably less than 0.47. More preferably, it is less than 0.40. The degree of connectivity of fine grains is most preferably less than 0.35.

[0153] Ferrite is a soft structure. Bainite is a structure that is harder than ferrite and softer than pearlite, martensite, and austenite. From the perspectives of strength, ductility, and toughness, it is important to control the structure of bainite.

[0154] When the volume fraction of bainite decreases, the proportion of soft ferrite increases, and the strength decreases. In addition, the value of (normalized load / normalized displacement) may also decrease. Therefore, the volume fraction of bainite is preferably set to 10% or more. The volume fraction of bainite is more preferably set to 12% or more. More preferably, it is 20% or more. The volume fraction of bainite is most preferably set to 25% or more. The upper limit is not particularly limited, but is preferably 75% or less for the reason of reduced ductility.

[0155] As the combined volume fraction of ferrite and bainite decreases, the proportions of hard pearlite, martensite, and austenite increase, reducing ductility and toughness. Therefore, the combined volume fraction of ferrite and bainite is preferably 80% or greater. More preferably, it is 85% or greater. Even more preferably, the combined volume fraction of ferrite and bainite is 88% or greater.

[0156] Furthermore, for the reasons described above, the total volume fraction of hard pearlite, martensite, and austenite is preferably 20% or less.

[0157] On the other hand, if the total volume fraction of hard pearlite, martensite, and austenite is less than 1%, ductility decreases. Therefore, the total volume fraction of ferrite and bainite is preferably 99% or less, more preferably 98% or less, and even more preferably 97% or less.

[0158] The various structures mentioned above, excluding austenite, use austenite grain boundaries or deformation bands within austenite grains as nucleation sites. During hot rolling, increasing the reduction at low temperatures, where austenite recrystallization is less likely to occur, introduces a large number of dislocations into the austenite, resulting in austenite refinement, and introduces a large number of deformation bands within the grains. This increases the area of ​​nucleation sites and the frequency of nucleation, making it possible to refine the steel structure.

[0159] Here, the observation of the steel structure can be carried out by the method described below. First, the test piece for structural observation is made as follows: the observation surface is cut in a manner that becomes a cross-section parallel to the rolling direction and the plate thickness direction of the hot-rolled steel plate and the center of the plate thickness, and a cross-section parallel to the tube axis direction and the wall thickness direction of the electric resistance welded steel pipe and the center of the wall thickness, and after grinding, it is corroded with nitric acid solution. For structural observation, an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) is used to observe the structure of the center of the plate thickness (or wall thickness) and take pictures. Then, based on the obtained optical microscope image and SEM image, the area ratio of bainite and the remainder (ferrite, pearlite, martensite, austenite) is calculated. The area ratio of each structure is observed in more than 5 fields of view and calculated as the average value of the values ​​obtained in each field of view. It should be noted that in the present invention, the area ratio obtained by structural observation is used as the volume ratio of each structure. The judgment of whether it is each structure is implemented according to the following content.

[0160] Ferrite is a product of diffusion transformation, exhibiting a low dislocation density and a largely restored structure. This includes polygonal ferrite and quasi-polygonal ferrite. Areas where no cementite is observed using an optical microscope or SEM, and where no lath structure, the underlying structure, is observed, are considered ferrite.

[0161] Bainite is a multiphase structure of lath-shaped ferrite and cementite with a high dislocation density. Regions where cementite is dispersed or a lath structure as the underlying structure is observed using an optical microscope or SEM are judged to be bainite.

[0162] Pearlite is a eutectoid structure of iron and iron carbide (ferrite + cementite), and exhibits a layered structure in which linear ferrite and cementite are alternately arranged. The region observed by SEM as described above was determined to be pearlite.

[0163] Martensite is a lath-shaped, low-temperature transformation structure with a very high dislocation density. In SEM images, it exhibits brighter contrast than ferrite and bainite.

[0164] It should be noted that it is difficult to distinguish martensite and austenite in 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 the value obtained by subtracting the volume ratio of austenite measured by the method described later from the measured value is used as the volume ratio of martensite.

[0165] Austenite is an fcc phase. The volume fraction of austenite was measured by X-ray diffraction using a test piece prepared in the same manner as the test piece used for the dislocation density measurement. The volume fraction of austenite was determined based on the integrated intensities of the (200), (220), and (311) planes of fcc iron and the (200) and (211) planes of bcc iron.

[0166] Next, a method for producing a hot-rolled steel sheet and an electric-resistance-welded steel pipe according to an embodiment of the present invention will be described.

[0167] The hot-rolled steel sheet of the present invention is not particularly limited to the following manufacturing conditions, but is manufactured, for example, by the following method: after heating a steel material having the above-mentioned chemical composition to a heating temperature of 1100° C. to 1300° C., hot rolling is performed at an average cooling rate of 0.5° C. / s to 3.0° C. / s from 900° C. to 1100° C., a finish rolling end temperature of 750° C. to 850° C., and a total reduction ratio during the finish rolling of 45% to 75%. Subsequently, cooling is performed at an average cooling rate of 5° C. / s to 40° C. / s at the center of the sheet thickness from the finish rolling end temperature to the cooling stop temperature, a minimum cooling rate of 2° C. / s from the finish rolling end to the cooling stop, a cooling stop temperature of 400° C. to 650° C., a continuous air cooling time of 15 s or less from the finish rolling end to the cooling stop, and a total air cooling time of 50 s or less, and the steel sheet is coiled into a coil.

[0168] The electric resistance welded steel pipe of the present invention is produced by forming the hot-rolled steel sheet into a cylindrical shape by cold rolling and then performing electric resistance welding.

[0169] It should be noted that, in the following description of the manufacturing method, unless otherwise specified, temperature references in "°C" refer to the surface temperature of the steel material or steel plate (hot-rolled plate). These surface temperatures can be measured using a radiation thermometer, etc. Furthermore, the temperature at the center of the steel plate thickness can be determined by calculating the temperature distribution within the cross-section of the steel plate using heat transfer analysis and correcting the result using the surface temperature of the steel plate. Furthermore, "hot-rolled steel plate" also includes hot-rolled plate and hot-rolled steel strip.

[0170] In the present invention, the method for melting the raw steel (steel billet) is not particularly limited; any known melting method, such as a converter, electric furnace, or vacuum melting furnace, is suitable. The casting method is also not particularly limited; the desired dimensions can be obtained using known casting methods, such as continuous casting. It should be noted that the ingot casting and bloom rolling method can be used instead of continuous casting without any problems. The molten steel may be further subjected to secondary refining, such as ladle refining.

[0171] Next, the obtained steel material (steel slab) is heated, hot-rolled, cooled, and then coiled into a coil to produce a hot-rolled steel sheet.

[0172] If the heating temperature is low, the deformation resistance of the rolled material becomes larger and rolling becomes difficult. Therefore, the heating temperature is preferably above 1100°C. More preferably, it is above 1120°C. More preferably, it is above 1130°C. Most preferably, it is above 1150°C. On the other hand, if the heating temperature is high, the austenite grains coarsen, and fine austenite grains cannot be obtained in the subsequent rolling (rough rolling, finish rolling), and the average grain size of the final product becomes larger. Therefore, the heating temperature in the hot rolling process is preferably set to below 1300°C. More preferably, it is below 1280°C. More preferably, it is below 1270°C. Most preferably, it is below 1250°C.

[0173] It should be noted that in the present invention, after the slab is manufactured, it is temporarily cooled to room temperature and then reheated. In addition to such existing methods, energy-saving direct rolling processes such as loading the slab into a heating furnace in a warm state without cooling to room temperature or rolling the slab immediately after slight insulation can also be applied without any problems.

[0174] If the average cooling rate of 900°C or more and 1100°C or less is small, the austenite coarsens and the average grain size of the final product becomes larger. Therefore, the average cooling rate of 900°C or more and 1100°C or less is preferably set to 0.5°C / s or more. More preferably, it is 0.8°C / s or more. More preferably, it is 0.9°C / s or more. Most preferably, it is 1.0°C / s or more. On the other hand, if the above-mentioned average cooling rate is large, the recrystallization of the austenite becomes insufficient, and coarse austenite remains, resulting in a steel structure in which coarse grains are mixed in the final product. As a result, the area ratio of grains having a grain size below the average grain size becomes low. Therefore, the average cooling rate of 900°C or more and 1100°C or less is preferably set to 3.0°C / s or less. More preferably, it is 2.5°C / s or less. More preferably, it is 2.4°C / s or less. Most preferably, it is 2.2°C / s or less.

[0175] If the finish rolling end temperature is too low, the steel sheet surface temperature will fall below the ferrite transformation start temperature during finish rolling, resulting in the formation of a large amount of deformed ferrite and reduced ductility. Therefore, the finish rolling end temperature is preferably set to 750°C or higher. More preferably, it is set to 770°C or higher. Even more preferably, it is set to 780°C or higher. On the other hand, if the finish rolling end temperature is too high, fine austenite grains cannot be obtained, and the average grain size increases. Therefore, the finish rolling end temperature is preferably set to 850°C or lower. More preferably, it is set to 830°C or lower. Even more preferably, it is set to 820°C or lower.

[0176] If the total reduction rate in the finishing rolling is low, sufficient processing strain cannot be introduced in the hot rolling process, so the average grain size of the final product becomes larger. Therefore, the total reduction rate in the finishing rolling is preferably set to more than 45%. The total reduction rate in the finishing rolling is more preferably more than 50%. The total reduction rate in the finishing rolling is further preferably more than 52%. The total reduction rate in the finishing rolling is most preferably more than 54%. On the other hand, if the total reduction rate in the finishing rolling is high, the average grain size becomes smaller. In addition, the area ratio of grains with a grain size below the average grain size increases. Therefore, the total reduction rate in the finishing rolling is preferably less than 75%. More preferably, it is less than 70%. The total reduction rate in the finishing rolling is further preferably less than 68%. The total reduction rate in the finishing rolling is most preferably less than 66%.

[0177] The total rolling reduction ratio in the above-mentioned finish rolling refers to the total rolling reduction ratios in the respective rolling passes in the finish rolling.

[0178] From the perspective of ensuring the necessary reduction ratio and controlling the steel plate temperature, the finished plate thickness is preferably set to 5 mm or greater. The finished plate thickness is more preferably set to 6 mm or greater. Even more preferably, it is set to 7 mm or greater. Furthermore, the finished plate thickness is preferably set to 40 mm or less. The finished plate thickness is more preferably set to 35 mm or less. Even more preferably, it is set to 30 mm or less.

[0179] After hot rolling, the hot rolled sheet is cooled.

[0180] If the average cooling rate at the center of the plate thickness from the finish rolling end temperature to the cooling stop temperature is low, the structure coarsens and the average grain size of the final product becomes larger. In addition, the bainite fraction becomes lower. Therefore, the average cooling rate is preferably set to 5°C / s or more. More preferably, it is 10°C / s or more. More preferably, it is 12°C / s or more. Most preferably, it is 15°C / s or more. On the other hand, if the average cooling rate at the center of the plate thickness is high, the martensite fraction becomes higher and the ductility decreases. Therefore, the average cooling rate is preferably set to 40°C / s or less. More preferably, it is 35°C / s or less. More preferably, it is 33°C / s or less. Most preferably, it is 30°C / s or less.

[0181] If the minimum cooling rate at the center of the plate thickness from the end of finishing rolling to the stop of cooling is low, the structure coarsens and the average grain size becomes larger. In addition, the area ratio of grains with a grain size below the average grain size becomes lower. In addition, the bainite fraction becomes lower. Therefore, the minimum cooling rate is preferably set to 2°C / s or more. More preferably, it is 3°C / s or more. Further preferably, it is 4°C / s or more. Most preferably, it is 5°C / s or more. There is no particular upper limit, and the minimum cooling rate is preferably 15°C / s or less. More preferably, it is 12°C / s or less. Further preferably, it is 10°C / s or less. Most preferably, it is 8°C / s or less. It should be noted that, with respect to the minimum cooling rate, the time from the end of finishing rolling to the stop of cooling is divided into intervals of 3s each, the average cooling rate is calculated in each interval, and the minimum cooling rate is obtained as the minimum value among them.

[0182] If the continuous air cooling time from the end of finish rolling to the stop of cooling is long, the ferrite and bainite grains grow excessively, the structure coarsens, and the average grain size of the final product becomes larger. In addition, the area ratio of grains with a grain size below the average grain size decreases. In addition, sometimes the bainite fraction decreases. Therefore, the above-mentioned continuous air cooling time is preferably set to 15s or less. More preferably, it is 14s or less. More preferably, it is 12s or less. Most preferably, it is 11s or less. The shorter the above-mentioned continuous air cooling time, the more preferred it is. However, when it is less than 6s, the effect of miniaturization becomes smaller relative to the shortening of the continuous air cooling time, and only the equipment load increases. Therefore, the above-mentioned continuous air cooling time is preferably set to more than 6s. More preferably, it is 7s or more. More preferably, it is 8s or more. During the period from the end of finish rolling to the stop of cooling, air cooling and water cooling are used for cooling.

[0183] If the total air cooling time from the end of finish rolling to the stop of cooling is long, ferrite and bainite will grow excessively, the structure will coarsen, and the average grain size of the final product will become larger. In addition, the area ratio of grains with a grain size below the average grain size will decrease. In addition, the bainite fraction may sometimes decrease. Therefore, the above-mentioned total air cooling time is preferably set to 50s or less. More preferably, it is 45s or less. More preferably, it is 40s or less. Most preferably, it is 38s or less. The shorter the above-mentioned continuous air cooling time, the more preferred it is. However, when it is less than 10s, the effect of miniaturization becomes smaller relative to the shortening of the total air cooling time, and only the equipment load increases. Therefore, the above-mentioned total air cooling time is preferably set to 10s or more. More preferably, it is 12s or more. More preferably, it is 15s or more.

[0184] If the cooling stop temperature is too low, the martensite fraction increases, reducing ductility. Therefore, the cooling stop temperature is preferably set to 400°C or higher. It is more preferably set to 420°C or higher. It is even more preferably set to 450°C or higher. It is most preferably set to 470°C or higher. On the other hand, if the cooling stop temperature is too high, the bainite fraction decreases. Therefore, the cooling stop temperature is preferably set to 650°C or lower. It is more preferably set to 620°C or lower. It is even more preferably set to 600°C or lower. It is most preferably set to 580°C or lower.

[0185] It should be noted that in the electric resistance welded steel pipe of the present invention, in order to reduce the inclusion density of the weld, the circumferential width of the melted and solidified portion of the weld (electric resistance weld) is preferably 1 μm or greater relative to the total pipe thickness. Furthermore, the circumferential width of the melted and solidified portion of the weld (electric resistance weld) is preferably 1000 μm or less relative to the total pipe thickness.

[0186] The outer diameter of the electric resistance welded steel pipe is preferably 80 mm or greater. Furthermore, the outer diameter is preferably 800 mm or less. The wall thickness of the electric resistance welded steel pipe is preferably 3 mm or greater. Furthermore, the wall thickness is preferably 40 mm or less.

[0187] Here, the etching liquid may be selected appropriately according to the steel composition and the type of steel pipe. Figure 4 Schematic diagram of the pipe circumference cross section of the resistance welded portion of the electric resistance welded steel pipe is shown in FIG. Figure 4 As shown in the schematic diagram above, the cross section after corrosion is Figure 4 The melted solidified portion 15 can be identified as a region having a different microstructure and contrast from the base material portion 13 and the heat-affected zone 14. For example, in the cross-section of a carbon steel or low-alloy steel electric resistance welded pipe after etching with Nital, the melted solidified portion 15 can be identified as a relatively white region observed using an optical microscope. Furthermore, in the cross-section of a carbon steel or low-alloy steel UOE pipe after etching with Nital, the melted solidified portion 15 can be identified as a region containing a cellular or dendritic solidified structure using an optical microscope.

[0188] Example

[0189] Hereinafter, the present invention will be described in further detail based on examples. However, it should be noted that the present invention is not limited to the following examples.

[0190] Molten steel having the chemical composition shown in Table 1 was melted to produce steel slabs (steel raw materials). The obtained steel slabs were subjected to hot rolling and cooling processes under the conditions shown in Table 2, and further subjected to coiling processes to produce hot-rolled steel sheets having the finished sheet thickness (mm) shown in Table 2.

[0191] After the coiling process, the hot-rolled steel sheet was formed into a cylindrical round steel pipe by roll forming, and the butted portions were resistance welded. Rollers positioned above, below, and to the left and right of the round steel pipe then applied diameter reduction to produce electric resistance welded steel pipes having the outer diameters (mm) and wall thicknesses (mm) shown in Table 4.

[0192] Test specimens were cut from the resulting hot-rolled steel sheets and electric-resistance-welded steel pipes shown in Tables 3 and 4, respectively, and subjected to the following C-shaped flattening test, flattening test, average grain size measurement, measurement of the area ratio of grains having a size less than the average grain size, measurement of fine grain connectivity, and microstructure observation. Each test specimen was cut from the center of the plate thickness in the middle of the width direction for the hot-rolled steel sheets and from the center of the plate thickness in the electric-resistance-welded steel pipe, 90° from the electric-resistance-welded portion in the pipe circumferential direction, for the base material.

[0193] [C-shaped flattening test]

[0194] For the C-shaped flattening test, a plate (test piece) with a total thickness of 50 × t (t: plate thickness) and a length of 100 mm was cut from the hot-rolled steel plate so that the length direction of the test piece was the plate width direction of the hot-rolled steel plate. Then, it was formed into a C-shaped test piece using the bending method described in JIS Z 2248 (2006), and then the test was carried out using the method described in JIS G 3441 (2021). During the bending, the inner radius of the front end of the pressing member was set to 9 × t. The initial curvature radius r of the bent outer surface of the flattened test piece was calculated by adding the plate thickness to the inner radius of the front end of the pressing member during the bending, as shown in formula (4).

[0195] r=10×t …(4)

[0196] When cracks with a length of 0.50 mm or more occurred on the curved outer surface in the closely adhered state, it was considered to be cracked, and when no cracks occurred on the curved outer surface, it was considered to be crack-free.

[0197] [Flattening test]

[0198] The flattening test was conducted by cutting a 100 mm long circular test piece from an electric resistance welded steel pipe including an electric resistance welded portion in the pipe axial direction and performing the test according to the method described in JIS G 3441 (2021).

[0199] However, the test piece Figure 3 Place the test piece so that the welded portion faces the compression direction. The initial radius of curvature r of the curved outer surface of the flattened test piece is set to 1 / 2 the outer diameter of the electric resistance welded steel pipe. If cracks of 0.50 mm or greater develop on the outer surface of the pipe in the tightly bonded state, the pipe is considered cracked; if no cracks develop on the outer surface of the pipe, the pipe is considered crack-free.

[0200] [Average crystal particle size measurement]

[0201] Regarding the average grain size, the test pieces for measurement were cut in such a way that the measurement surface was a cross section parallel to the rolling direction and the plate thickness direction of the hot-rolled steel plate, and a cross section parallel to the tube axis direction and the wall thickness direction of the electric resistance welded steel pipe. After mirror polishing, the SEM / EBSD method was used for measurement. Regarding the grain size, the orientation difference between adjacent grains was obtained, and the boundary with an orientation difference of 15° or more was used as a grain boundary, and the area surrounded by an orientation difference of 15° or more was measured as a grain. The arithmetic mean of the grain size was obtained according to the obtained grain boundaries as the average grain size. The acceleration voltage was set to 15 kV, the measurement area was set to 500 μm × 500 μm, the measurement step was set to 0.5 μm, and the measured values ​​of more than 5 fields of view were averaged. Based on the obtained EBSD data, the crystal orientation analysis software OIM Analysis (trademark) was used, and the boundary with an orientation difference of 15° or more was used as a grain boundary (high-angle grain boundary) to obtain the distribution of grain boundaries and grain size. The particle size and average crystal grain size are calculated as the diameter of a circle having an area equal to the value obtained by dividing the total area measured by the number of crystal grains (equivalent circle diameter). It should be noted that in the calculation of the average crystal grain size, crystal grains with a grain size of less than 1.0 μm are excluded as measurement noise.

[0202] [Measurement of the Area Ratio of Grains Having a Grain Size Less Than the Average Grain Size]

[0203] Obtained from the average crystal grain size and particle size distribution obtained above. For each crystal grain below the average crystal grain size, the area is calculated from its equivalent circle diameter, the total area of ​​the crystal grains below the average crystal grain size is calculated, and the total area is divided by the area of ​​the measurement region to obtain. It should be noted that in the calculation of the area ratio of the crystal grains having a particle size below the average crystal grain size, the crystal grains with a particle size of 1.0 μm or less are excluded as measurement noise.

[0204] [Fine grain connectivity]

[0205] Regarding the degree of fine grain connectivity, the total length of the high-angle grain boundaries in the region excluding the grains having a grain size greater than the average grain size and the total length of the high-angle grain boundaries in all the grains are calculated and obtained as a ratio thereof. The total length of the high-angle grain boundaries of the grains having a grain size smaller than the average grain size, as described in the numerator on the right side of the above formula (3), does not include the length of the high-angle grain boundaries between the grains having a grain size smaller than the average grain size and the grains having a grain size greater than the average grain size.

[0206] In addition, in the calculation of the fine grain connectivity, grains having a grain size of 1.0 μm or less were excluded as measurement noise.

[0207] [Organization Observation]

[0208] Test pieces for microstructure observation were prepared as follows: the observation surfaces were cut into sections parallel to both the rolling direction and the thickness direction of the hot-rolled steel sheet, and parallel to both the tube axis and the wall thickness direction of the electric resistance welded steel pipe. After mirror polishing, the specimens were etched with nital. For microstructure observation, an optical microscope (magnification: 1000x) or a scanning electron microscope (SEM, magnification: 1000x) was used to observe and photograph the microstructure at the center of the thickness of the hot-rolled steel sheet and the center of the wall thickness of the electric resistance welded steel pipe. SEM observations were performed at an accelerating voltage of 15 kV. Based on the obtained optical microscope and SEM images, the area ratios of bainite and the remainder (ferrite, pearlite, martensite, and austenite) were determined. The area ratio of each microstructure was observed in five or more fields of view and calculated as the average of the values ​​obtained in each field of view. Here, the area ratio obtained by microstructure observation was used as the volume ratio of each microstructure.

[0209] As described above, the determination of ferrite, pearlite, and martensite is performed by the method described in the embodiment.

[0210] The volume fraction of austenite was measured by X-ray diffraction. Test specimens for measurement, such as those for hot-rolled steel plates and electric-resistance-welded steel pipes, were ground so that the diffraction surface was aligned with the center of the plate thickness of the hot-rolled steel plates and the center of the wall thickness of the electric-resistance-welded steel pipes, respectively. These specimens were then chemically polished to remove the surface processing layer. Mo Kα radiation was used for the measurement, and the volume fraction of austenite was determined based on the integrated intensities of the (200), (220), and (311) planes of fcc iron and the (200) and (211) planes of bcc iron.

[0211] The obtained results are shown in Tables 3 and 4.

[0212]

[0213]

[0214]

[0215]

[0216] In Tables 3 and 4, the hot-rolled steel plates and electric-resistance-welded steel pipes of Nos. 1, 4, 6, 8, 9, 11 to 24 are examples of the present invention, and the hot-rolled steel plates and electric-resistance-welded steel pipes of Nos. 2, 3, 5, 7, and 10 are comparative examples.

[0217] The hot-rolled steel sheets of the examples of the present invention all have the following characteristics: in the C-shaped flattening test, no cracks with a length of 0.50 mm or more are generated until the close contact state is achieved, and the value of (normalized load / normalized displacement) within the range of 0.20 to 0.30 is 100 MPa or more.

[0218] All the electric resistance welded steel pipes of the examples of the present invention had no cracks of 0.50 mm or longer in length until the close contact state in the flattening test, and the value of (normalized load / normalized displacement) was 100 MPa or more when the normalized displacement was within the range of 0.20 to 0.30.

[0219] Explanation of symbols

[0220] 1 Curve showing the change in normalized load associated with normalized displacement

[0221] 2 Elastic Area

[0222] 3 Plastic zone

[0223] 4 Cracks or tightness

[0224] 5A test piece

[0225] 5 C-shaped flattening test piece (flattening test piece)

[0226] 6 tablets

[0227] 7 Compression direction

[0228] 8 Flattening test piece

[0229] 9 Resistance welding part

[0230] 10 Center of electric resistance welded steel pipe

[0231] 11 Tablet

[0232] 12 Compression direction

[0233] 13. Parent Material Department

[0234] 14 Heat-affected zone

[0235] 15 Melting and solidification section

[0236] 100 Initial length of the test piece L

[0237] 101 Initial thickness of the test piece t

[0238] 102 Rolling direction of hot rolled steel plate

Claims

1. A hot-rolled steel plate, wherein: In the C-shaped crush test, a steel plate is bent into a U-shaped test piece and then clamped between two flat plates. No cracks with a length of 0.50 mm or more are generated until the inner surfaces of the bent test pieces are in close contact with each other, and When the normalized displacement obtained by the following formula (2) is within the range of 0.20 to 0.30, the normalized load / normalized displacement obtained by the following formula (1) is 100 MPa or more. (Normalized load (MPa)) = (P / L) × (r / t 2 )×(1-((x0-x) / 2r) 2 ) 1 / 2 …(1) (Normalized displacement) = (x0-x) / 2r …(2) in, P: load (N); L: initial length of the flattened test piece (mm); r: initial radius of curvature of the curved outer surface of the flattened test piece (mm); t: initial thickness of the flattened test piece (mm); x0: initial distance between the two plates (mm); x: distance between two plates (mm).

2. The hot-rolled steel sheet according to claim 1, wherein the composition comprises, in mass %, C: 0.020% to 0.200%, Si: 0.50% to 0.50%, Mn: 0.30% to 2.00%, P: 0.050% to 0.0200%, S: 0.005% to 0.100%, N: 0.0100% to 0.050%, or further comprises Nb: 0.080% to 0.080%, V: 0.080% to 0.080%, and % or less, Ti: 0.080% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 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, Sn: 0.100% or less, and the balance being Fe and inevitable impurities.

3. The hot-rolled steel sheet according to claim 1 or 2, wherein: In the steel structure at the center of the plate thickness, The average grain size of the grains in the region surrounded by the high-angle grain boundaries is 15.0 μm or less. The area ratio of crystal grains having a grain size equal to or smaller than the average grain size is 10% or more and 50% or less relative to all crystal grains. The fine grain connectivity obtained by the following formula (3) is 0.05 or more and 0.50 or less, and Bainite is 10% or more by volume, The total volume fraction of ferrite and bainite is 80% or more. The balance is one or more selected from pearlite, martensite and austenite, and the total volume fraction is 20% or less. (Connectivity of fine grains) = (Total length of high-angle grain boundaries of grains with a grain size smaller than the average grain size) / (Total length of high-angle grain boundaries) …(3) However, the numerator on the right side of the formula (3) does not include the length of the high-angle grain boundary between the grains smaller than the average grain size and the grains larger than the average grain size.

4. An electric resistance welded steel pipe comprising a base material portion and an electric resistance welded portion, wherein: In the flattening test, a flattening test piece cut from an electric resistance welded steel pipe is clamped between two flat plates. No cracks with a length of 0.50 mm or more are generated until the inner surfaces of the flattened test pieces are in close contact with each other, and When the normalized displacement obtained by the following formula (2) is within the range of 0.20 to 0.30, the normalized load / normalized displacement obtained by the following formula (1) is 100 MPa or more. (Normalized load (MPa)) = (P / L) × (r / t 2 )×(1-((x0-x) / 2r) 2 ) 1 / 2 …(1) (Normalized displacement) = (x0-x) / 2r …(2) in, P: load (N); L: initial length of the flattened test piece in the tube axis direction (mm); r: initial radius of curvature of the curved outer surface of the flattened test piece (mm); t: initial thickness of the flattened test piece (mm); x0: initial distance between the two plates (mm); x: distance between two plates (mm).

5. The electric resistance welded steel pipe according to claim 4, wherein: The base material portion may include, by mass%, C: 0.020% or more and 0.200% or less, Si: 0.50% or less, Mn: 0.30% or more and 2.00% 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, or may further include Nb: 0.080% or less, V: 0.080% or less, Ti: 0.080% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, Ca: 0.0050% or less, B: 0.0050% or less, Mg: One or more of the following: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less, and Sn: 0.100% or less, with the balance being Fe and inevitable impurities.

6. The electric resistance welded steel pipe according to claim 4 or 5, wherein: In the steel structure at the center of the wall thickness of the base material part, The average grain size of the grains in the region surrounded by the high-angle grain boundaries is 15.0 μm or less. The area ratio of crystal grains having a grain size equal to or smaller than the average grain size is 10% or more and 50% or less relative to all crystal grains. The fine grain connectivity obtained by the following formula (3) is 0.05 or more and 0.50 or less, and Bainite is 10% or more by volume, The total volume fraction of ferrite and bainite is 80% or more. The balance is one or more selected from pearlite, martensite and austenite, and the total volume fraction is 20% or less. (Connectivity of fine grains) = (Total length of high-angle grain boundaries of grains with a grain size smaller than the average grain size) / (Total length of high-angle grain boundaries) …(3) However, the numerator on the right side of the formula (3) does not include the length of the high-angle grain boundary between the grains smaller than the average grain size and the grains larger than the average grain size.

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