JP7635881B2 - Hot-rolled steel sheets and electric resistance welded steel pipes - Google Patents

Description

The present invention relates to electric resistance welded steel pipes and hot-rolled steel sheets that are the raw materials for such pipes, which are suitable for building structures, line pipes, etc.

Electric resistance welded steel pipes used in building structures and line pipes suffer a sudden drop in strength and break when buckling occurs when subjected to external forces such as earthquake forces. Therefore, it is desirable to suppress the occurrence of buckling in these electric resistance welded steel pipes.
In order to suppress buckling of electric resistance welded steel pipes, it is effective to suppress flattening of the steel pipe cross section, which is a precursor phenomenon to buckling.

There have been few studies conducted to address such requirements, but for example, Patent Document 1 discloses a steel pipe in which the parent electric-welded steel pipe is heated and subjected to hot diameter reduction rolling to set the r-value in the longitudinal direction of the pipe to 1.0 or more, thereby suppressing buckling during bending.

Patent No. 6954504

Hidetoshi Kobayashi, Hiroshi Okubo, Masashi Taimaruya: Light Metals, 39, (1989), p.8

As in Patent Document 1, by increasing the r-value in the axial direction of the steel pipe, it is possible to promote the circumferential deformation of the pipe and reduce the change in wall thickness, thereby suppressing the flattening of the steel pipe cross section. However, this causes the diameter of the steel pipe cross section to shrink, resulting in necking and a reduced cross-sectional area, which reduces the strength of the structure.

The present invention has been made in consideration of the above circumstances, and aims to provide an electric welded steel pipe with excellent flattening resistance, and a hot-rolled steel sheet used as the material for the electric welded steel pipe.

In the present invention, "excellent flattening resistance" means that no cracks of 0.50 mm or more in length occur until the flattened test specimens taken from the electric resistance welded steel pipe come into contact with each other in a flattening test, and the normalized load/normalized displacement calculated by the formula (1) below is 100 MPa or more when the normalized displacement calculated by the formula (2) is in the range of 0.20 to 0.30. The fact that the inner surfaces of the flattened test specimens taken from the electric resistance welded steel pipe come into contact with each other means, for example, that when a flattening test is performed by applying a load from above and below the flattened test specimen taken from the electric resistance welded steel pipe, the inner surfaces of the upper and lower parts of the flattened test specimen taken from the electric resistance welded steel pipe come into contact with each other.
(Normalized load (MPa)) = (P/L) x (r/t ² ) x (1-((x ₀ -x)/2r) ² ) ^1/2 ... (1)
(Normalized displacement)=(x ₀ −x)/2r (2)
Where:
P: Load (N)
L: Initial length of the flattened test piece in the tube axial direction (mm)
r: initial radius of curvature of the outer surface of the bent flattened test piece (mm)
t: initial thickness of flattened test piece (mm)
x ₀ : Initial distance between two plates (mm)
x: Distance between two plates (mm)

As a result of intensive research, the inventors have found that by increasing the proportion of fine grains in the microstructure of an electric resistance welded steel pipe, toughness can be improved and the occurrence of cracks in a flattening test can be suppressed. On the other hand, they have also found that if the proportion of fine grains becomes too high, ductility decreases, making it easier for cracks to occur before adhesion in a flattening test.

They also found that by increasing the connectivity of fine grains, it is possible to increase the increase in load relative to displacement (indentation amount) in flattening tests, and to increase resistance to flattening. On the other hand, they also found that if the connectivity of fine grains becomes too high, ductility decreases, and cracks occur before adhesion in flattening tests.

Furthermore, it was discovered that electric welded steel pipes manufactured using hot-rolled steel sheets, which have high resistance to flattening in C-type flattening tests, have excellent flattening resistance.

The present invention has been completed based on the above findings, and provides the following [1] to [6].
[1] In a C-type flattening test, a test piece made by bending a steel plate into a U-shape is clamped between two flat plates,
A hot-rolled steel sheet in which no cracks having a length of 0.50 mm or more occur until the inner surfaces of the bent test pieces come into contact with each other in a tight contact state, and in which the normalized load/normalized displacement calculated by the following formula (1) is 100 MPa or more when the normalized displacement calculated by the following formula (2) is in the range of 0.20 to 0.30.
(Normalized load (MPa)) = (P/L) x (r/t ² ) x (1 - ((x ₀ - x) / 2r) ² ) ^1/2 ... (1)
(Normalized displacement)=(x ₀ −x)/2r (2)
Where:
P: Load (N)
L: Initial length of the flattened test piece (mm)
r: initial radius of curvature of the outer surface of the bent flattened test piece (mm)
t: initial thickness of flattened test piece (mm)
x ₀ : Initial distance between two plates (mm)
x: Distance between two plates (mm)
[2] The component composition is, in 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 even more so:
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: 0.020% or less,
Zr: 0.020% or less,
REM: 0.020% or less,
Sn: 0.100% or less, one or more of the following are included;
The hot-rolled steel sheet according to [1], wherein the balance is Fe and unavoidable impurities.
[3] The steel structure at the center of the plate thickness is
The average grain size of the grains in the region surrounded by the high-angle grain boundaries is 15.0 μm or less,
The crystal grains having a grain size equal to or smaller than the average crystal grain size account for 10% or more and 50% or less of the total crystal grains,
The fine grain connectivity calculated by the following formula (3) is 0.05 or more and 0.50 or less, and bainite is 10% or more in volume fraction,
The total volume fraction of ferrite and bainite is 80% or more,
The balance is one or more selected from pearlite, martensite and austenite, the total of which is 20% or less by volume.
The hot-rolled steel sheet according to [1] or [2].
(Fine grain connectivity)=(total length of high angle grain boundaries in grains having 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 equation (3) does not include the length of the high-angle grain boundary between crystal grains smaller than the average crystal grain size and crystal grains larger than or equal to the average crystal grain size.
[4] An electric resistance welded steel pipe having a base metal portion and an electric resistance welded portion,
In a flattening test, a flattened test piece taken from an electric resistance welded steel pipe is clamped between two flat plates.
The electric resistance welded steel pipe is one in which no cracks having a length of 0.50 mm or more occur until the flattened test specimens reach a tight contact state where their inner surfaces come into contact with each other, and in which the normalized load/normalized displacement calculated by the following formula (1) is 100 MPa or more when the normalized displacement calculated by the following formula (2) is in the range of 0.20 to 0.30.
(Normalized load (MPa)) = (P/L) x (r/t ² ) x (1 - ((x ₀ - x) / 2r) ² ) ^1/2 ... (1)
(Normalized displacement)=(x ₀ −x)/2r (2)
Where:
P: Load (N)
L: Initial length of the flattened test piece in the tube axial direction (mm)
r: initial radius of curvature of the outer surface of the bent flattened test piece (mm)
t: initial thickness of flattened test piece (mm)
x ₀ : Initial distance between two plates (mm)
x: Distance between two plates (mm)
[5] The composition of the base material is, in 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 even more so:
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: 0.020% or less,
Zr: 0.020% or less,
REM: 0.020% or less,
Sn: 0.100% or less, one or more of the following are included;
The electric resistance welded steel pipe according to [4], the balance being Fe and unavoidable impurities.
[6] The steel structure at the center of the thickness of the base material portion is
The average grain size of the grains in the region surrounded by the high-angle grain boundaries is 15.0 μm or less,
The crystal grains having a grain size equal to or smaller than the average crystal grain size account for 10% or more and 50% or less of the total crystal grains,
The fine grain connectivity calculated by the following formula (3) is 0.05 or more and 0.50 or less, and bainite is 10% or more in volume fraction,
The total volume fraction of ferrite and bainite is 80% or more,
The balance is one or more selected from pearlite, martensite and austenite, the total of which is 20% or less by volume.
The electric resistance welded steel pipe according to [4] or [5].
(Fine grain connectivity)=(total length of high angle grain boundaries in grains having 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 equation (3) does not include the length of the high-angle grain boundary between crystal grains smaller than the average crystal grain size and crystal grains larger than or equal to the average crystal grain size.

According to the present invention, it is possible to provide electric welded steel pipes with excellent flattening resistance, and hot-rolled steel sheets used as the raw materials for such pipes.

FIG. 1 is a schematic diagram showing the relationship between standardized load and standardized displacement in a flattening test. FIG. 1 is a diagram showing the sampling direction of a C-shaped flattened test piece of a hot-rolled steel sheet, observed from the side. FIG. 1 is a side view of a flattening test of a C-shaped flattened test piece of a hot-rolled steel sheet. This is a side view of the flattening test of an electric resistance welded steel pipe. 1 is a schematic diagram of a circumferential cross section of an electric resistance welded portion of an electric resistance welded steel pipe.

The following describes the hot-rolled steel sheet and electric-resistance welded steel pipe of the present invention and their manufacturing methods.

First, we will explain the reasons for limiting the mechanical properties of the hot-rolled steel sheet and electric-resistance welded steel pipe of the present invention.

The hot-rolled steel sheet of the present invention is characterized in that, in a C-type flattening test, no cracks having a length of 0.50 mm or more occur until the inner surfaces of the bent steel sheets come into contact with each other in a tight contact state, and the value of normalized load/normalized displacement in the normalized displacement range of 0.20 to 0.30 is 100 MPa or more.
The electric-resistance welded steel pipe of the present invention is also characterized in that, in a flattening test, no cracks occur until the inner surfaces of flattened test pieces taken from the electric-resistance welded steel pipe come into contact with each other in a tight adhesion state, and the value of normalized load/normalized displacement in the normalized displacement range of 0.20 to 0.30 is 100 MPa or more.
The normalized load and normalized displacement are calculated by the following equations (1) and (2), respectively.
(Normalized load (MPa)) = (P/L) x (r/t ² ) x (1 - ((x ₀ - x) / 2r) ² ) ^1/2 ... (1)
(Normalized displacement)=(x ₀ −x)/2r (2)
Where:
P: Load (N)
L: Initial length of the flattened test piece (mm)
r: initial radius of curvature of the outer surface of the bent flattened test piece (mm)
t: initial thickness of flattened test piece (mm)
x ₀ : Initial distance between two plates (mm)
x: Distance between two plates (mm)
Furthermore, the value of normalized load/normalized displacement when the normalized displacement is in the range of 0.20 to 0.30 is calculated by ((normalized load when the normalized displacement is 0.30)-(normalized load when the normalized displacement is 0.20))/(0.30-0.20).

In addition, "no cracks occur until the flattening test reaches a tight contact state" means that no cracks occur until the inner surfaces of the flattened test pieces taken from the electric resistance welded steel pipe come into contact with each other, and "the inner surfaces of the flattened test pieces taken from the electric resistance welded steel pipe come into contact with each other" means that, for example, when a flattening test is performed by applying a load from above and below the flattened test piece taken from the electric resistance welded steel pipe, the inner surface of the upper part of the flattened test piece taken from the electric resistance welded steel pipe comes into contact with the inner surface of the lower part of the flattened test piece taken from the electric resistance welded steel pipe. In addition, the above-mentioned crack refers to a crack of a size that can be visually confirmed. A size that can be visually confirmed is a length of 0.50 mm or more. There is no particular limit to the upper limit of the length of the crack, but since the crack is linear, it is preferable to target a length equal to or less than the length of the test piece in the tube axis direction. In addition, the length refers to the longitudinal size of the crack.

In the C-type flattening test and flattening test, it can be considered that the greater the increase in load relative to the distance (push-in amount) between the two flat plates, the greater the resistance to flattening.
However, even if the material is the same, the load and displacement change depending on the dimensions and shape of the test piece. Therefore, as described in Non-Patent Document 1, the load and displacement are standardized by equations (1) and (2), respectively.

In the C-shaped flattening test and the flattening test, the test piece undergoes elastic deformation until the standardized displacement reaches a certain value, after which the test piece yields and undergoes plastic deformation. Figure 1 shows a schematic diagram of the relationship between the standardized load and the standardized displacement in the flattening test. In the curve 1 showing the change in the standardized load with the standardized displacement shown in Figure 1, in the elastic region 2 until the test piece yields, as shown in Figure 1, the standardized load and the standardized displacement are in a proportional relationship. After that, when the test piece yields and enters the plastic region 3, the slope of the curve (standardized load / standardized displacement) becomes smaller, and the test proceeds while maintaining a constant slope. After that, the test proceeds while the slope of the curve (standardized load / standardized displacement) shows a higher value than the slope (slope after entering the plastic region 3), and the test ends when cracks occur on the outer bent surface or when the inner surfaces of the test pieces come into contact with each other (the inner surfaces of the upper and lower parts come into contact) and come into close contact. Hereinafter, the state of the test piece at the end of the test will be referred to as a cracked or adhered state 4.

If cracks occur before the bond is tightly sealed, a sudden drop in load will occur. Therefore, in the present invention, it is important to prevent cracks from occurring until the bond is tightly sealed, from the viewpoint of ensuring the safety of the structure.

In the present invention, the (normalized load/normalized displacement) in the plastic region 3 is used as an index of 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, the (normalized load/normalized displacement) is set to 100 MPa or more, especially in the range of 0.20 to 0.30, which corresponds to the first half of the plastic region 3. Preferably, the (normalized load/normalized displacement) is 120 MPa or more. More preferably, the (normalized load/normalized displacement) is 140 MPa or more. Even more preferably, the (normalized load/normalized displacement) is 150 MPa or more. However, if the (normalized load/normalized displacement) exceeds 600 MPa, the ductility decreases, and cracks are more likely to occur before the adhesion state is reached. Therefore, it is preferable that the (normalized load/normalized displacement) value is 600 MPa or less. More preferably, the (normalized load/normalized displacement) is 550 MPa or less. Even more preferably, the (normalized load/normalized displacement) is 500 MPa or less. Most preferably, the (normalized load/normalized displacement) is 450 MPa or less.

The reason why the range of normalized displacement is set to 0.20 to 0.30 is because the value of (normalized load/normalized displacement) becomes almost constant and stable in the plastic region.

The C-type flattening test refers to a test in which a test piece made by bending a single steel plate into a U-shape is clamped between two flat plates.

In the C-type flattening test, a plate material having a width of 50 mm x t (t: plate thickness) and a length of 100 mm is taken from a hot-rolled steel plate so that the longitudinal direction of the test piece is the plate width direction of the hot-rolled steel plate, and then this is made into a C-type test piece by the press-bending method described in JIS Z 2248 (2006). Then, the test piece is subjected to the method described in JIS G 3441 (2021). FIG. 2(a) shows a diagram of the state of the taking direction of the C-type flattened test piece of the hot-rolled steel plate and the state after being made into a C-type flattened test piece observed from the side. The test piece before being made into a C-type flattened test piece is designated by the symbol 5A, and the C-type flattened test piece after being made into a C-type flattened test piece is designated by the symbol 5. The initial length and initial thickness do not change before and after being made into a C-type flattened test piece. The initial length 100 of the C-shaped flattened test piece (flattened test piece) 5 refers to the above-mentioned length of 50 mm, and the length direction of the C-shaped flattened test piece 5 is the rolling direction 102 of the hot-rolled steel sheet. The initial thickness of the C-shaped flattened test piece (flattened test piece) 5 is indicated by the symbol 101. FIG. 2(b) shows a diagram of a flattened test of a C-shaped flattened test piece of a hot-rolled steel sheet observed from the side. The C-shaped flattened test is a test method in which, as shown in FIG. 2(a) and FIG. 2(b), a cut-out test piece 5A is bent into a U-shape to form a C-shaped flattened test piece 5, and the C-shaped flattened test piece 5 is sandwiched between two flat plates 6, and a load is applied in the compression direction 7 perpendicular to the flat plates 6, while testing the test piece until it reaches the above-mentioned state. In the press bending of the C-shaped flattened test piece 5, the inner radius of the tip of the press fitting is 9×t (mm). The initial radius of curvature r (mm) of the outer bent surface of the C-shaped flattened test piece is calculated by adding the plate thickness t (mm) to the inner radius of the tip of the pressing metal fitting in the above-mentioned pressing and bending, as shown in formula (4).
r=10×t...(4)
The flattening test is carried out by taking a test piece cut into a ring having a length of 100 mm in the axial direction from an electric resistance welded steel pipe including an electric resistance welded portion, from the axial direction of the pipe, according to the method described in JIS G 3441 (2021). However, the flattening test piece 8 is placed so that the line connecting the electric resistance welded portion 9 and the center 10 of the electric resistance welded steel pipe faces in a direction parallel to the compression direction 12, as shown in FIG. 3. The initial radius of curvature r of the bent outer surface of the flattening test piece 8 is 1/2 the outer diameter of the electric resistance welded steel pipe.

The composition of the base material of the hot-rolled steel sheet and electric resistance welded steel pipe of the present invention is, in 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 even more so:
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: 0.020% or less,
Zr: 0.020% or less,
REM: 0.020% or less,
Sn: 0.100% or less, one or more of the following are included;
The balance preferably consists of Fe and unavoidable impurities.

In this specification, unless otherwise specified, "%" indicating steel composition means "mass %".

C: 0.020% or more and 0.200% or less C is an element that increases the strength of steel by solid solution strengthening. In addition, C improves the hardenability of steel, thereby increasing the proportion of fine grains, improving toughness, and contributing to suppressing the occurrence of cracks in a flattening test, and also contributes to increasing the resistance to flattening by increasing the connectivity of fine grains. In order to obtain such an effect, it is preferable to contain 0.020% or more of C. More preferably, the C content is 0.025% or more, and even more preferably, 0.030% or more. Most preferably, the C content is 0.035% or more. However, if the C content exceeds 0.200%, hard pearlite, martensite, and austenite are excessively generated, ductility is reduced, and cracks are likely to occur before the steel is in a tight contact state in a C-type flattening test or a flattening test. Therefore, the C content is preferably 0.200% or less. The C content is more preferably 0.180% or less. The C content is more preferably 0.170% or less, and most preferably 0.165% or less.

Si: 0.50% or less Si is an element that increases the strength of steel by solid solution strengthening. In order to obtain such an effect, it is preferable to contain 0.02% or more of Si. The Si content is more preferably 0.05% or more, and even more preferably 0.08% or more. The Si content is most preferably 0.10% or more. However, if the Si content exceeds 0.50%, the ductility decreases, and cracks tend to occur before the steel is in a tight contact state in the flattening test. Therefore, the Si content is preferably 0.50% or less. The Si content is more preferably 0.40% or less. Even more preferably 0.30% or less. The Si content is most preferably 0.28% or less.

Mn: 0.30% or more and 2.00% or less Mn is an element that increases the strength of steel by solid solution strengthening. In addition, Mn improves the hardenability of steel, thereby increasing the proportion of fine grains, improving toughness, and contributing to suppressing the occurrence of cracks in a flattening test, and also contributes to increasing the resistance to flattening by increasing the connectivity of fine grains. In order to obtain such effects, it is preferable 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 Mn content is most preferably 0.60% or more. However, if the Mn content exceeds 2.00%, hard pearlite, martensite, and austenite are excessively generated, ductility is reduced, and cracks are likely to occur before the flattening test is in a tight contact state. Therefore, the Mn content is preferably 2.00% or less. The Mn content is more preferably 1.90% or less. Even more preferably 1.80% or less. The Mn content is most preferably not more than 1.75%.

P: 0.050% or less Since P segregates at grain boundaries and reduces toughness, it is preferable to reduce P as an inevitable impurity as much as possible, and the P content is preferably in the range of 0.050% or less. The P content is more preferably 0.040% or less, and further preferably 0.030% or less. The P content is most preferably 0.020% or less. Although there is no particular lower limit for P, since excessive reduction leads to an increase in smelting costs, it is preferable that P is 0.001% or more.

S: 0.0200% or less S is usually present in steel as MnS, but MnS is thinly drawn in the hot rolling process and has a negative effect on ductility and toughness. For this reason, in the present invention, it is preferable to reduce S as much as possible, and the S content is preferably 0.0200% or less. The S content is more preferably 0.0100% or less, and further preferably 0.0050% or less. The S content is most preferably 0.0030% or less. Although there is no particular lower limit for S, since excessive reduction leads to an increase in smelting costs, it is preferable that S is 0.0001% or more.

Al: 0.005% or more and 0.100% or less Al is an element that acts as a strong deoxidizer. In order to obtain such an effect, it is preferable to contain 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 Al content is most preferably 0.020% or more. However, if the Al content exceeds 0.100%, the weldability deteriorates, and the amount of alumina-based inclusions increases, resulting in deterioration of the surface properties. For this reason, the Al content is preferably 0.100% or less. The Al content is more preferably 0.080% or less. Even more preferably 0.070% or less. The Al content is most preferably 0.065% or less.

N: 0.0100% or less N is an inevitable impurity and an element that has the effect of reducing ductility and toughness by firmly fixing the movement of dislocations. In the present invention, it is desirable to reduce N as an impurity as much as possible, but the N content can be tolerated up to 0.0100%. For this reason, the N content is 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. The lower limit is not particularly limited, but since excessive reduction leads to an increase in refining costs, it is preferable to set it to 0.0010% or more.

In addition to the above-mentioned chemical composition, the electric welded steel pipe and hot-rolled steel sheet 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.

Nb: 0.080% or less Nb is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel, and also contributes to refining the structure by suppressing the coarsening of austenite during hot rolling, and can be contained as necessary. In order to obtain the above-mentioned effects, when Nb is contained, it is preferable to contain 0.002% or more of Nb. More preferably, the Nb content is 0.005% or more, and even more preferably, 0.010% or more. The Nb content is most preferably 0.012% or more. However, when the Nb content exceeds 0.080%, the ductility and toughness decrease. Therefore, when Nb is contained, the Nb content is 0.080% or less. More preferably, the Nb content is 0.070% or less. Even more preferably, the Nb content is 0.065% or less. The Nb content is most preferably 0.060% or less.

V: 0.080% or less 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 necessary. In order to obtain the above-mentioned effects, when V is contained, it is preferable to contain 0.002% or more of V. More preferably, the V content is 0.005% or more, and even more preferably, 0.010% or more. The V content is most preferably 0.015% or more. However, when the V content exceeds 0.080%, the ductility and toughness decrease. Therefore, when V is contained, the V content is 0.080% or less. More preferably, it is 0.070% or less. Even more preferably, it is 0.065% or less. The V content is most preferably 0.060% or less.

Ti: 0.080% or less Ti is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel, and also contributes to reducing the amount of solute N in the steel because of its high affinity with N, and can be contained as necessary. In order to obtain the above-mentioned effects, when Ti is contained, it is preferable to contain 0.002% or more of Ti. More preferably, the Ti content is 0.005% or more, and even more preferably, 0.010% or more. The Ti content is most preferably 0.012% or more. However, if the Ti content exceeds 0.080%, the ductility and toughness decrease. Therefore, when Ti is contained, the Ti content is 0.080% or less. More preferably, the Ti content is 0.070% or less. Even more preferably, the Ti content is 0.065% or less. The Ti content is most preferably 0.060% or less.

Cu: 0.50% or less, Ni: 0.50% or less Cu and Ni are elements that increase the strength of steel by solid solution strengthening, and also increase the hardenability of steel and contribute to fine structure, and can be contained as necessary. In order to obtain the above-mentioned effects, when Cu is contained, the Cu content is preferably 0.01% or more. More preferably, it is 0.05% or more. Even more preferably, it is 0.10% or more. In order to obtain the above-mentioned effects, when Ni is contained, the Ni content is preferably 0.01% or more. More preferably, it is 0.05% or more. Even more preferably, it is 0.10% or more. On the other hand, excessive inclusion may cause a decrease in ductility and toughness. In addition, it may cause excessive generation of hard pearlite, martensite, and austenite. Therefore, when Cu is contained, the Cu content is 0.50% or less. Preferably, it is 0.40% or less. More preferably, it is 0.30% or less. When Ni is contained, the Ni content is set to 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

Cr: 0.50% or less, Mo: 0.50% or less Cr and Mo are elements that improve the hardenability of steel and contribute to refining the structure, and can be contained as necessary. In order to obtain the above-mentioned effects, when Cr is contained, the Cr content is preferably 0.01% or more. More preferably, it is 0.05% or more. Even more preferably, it is 0.10% or more. In addition, in order to obtain the above-mentioned effects, when Mo is contained, the Mo content is preferably 0.01% or more. More preferably, it is 0.05% or more. Even more preferably, it is 0.10% or more.
On the other hand, excessive inclusion of Cr may lead to excessive formation of hard pearlite, martensite, and austenite. Therefore, when Cr is contained, the Cr content is set to 0.50% or less. Preferably, it is set to 0.40% or less. More preferably, it is set to 0.30% or less. When Mo is contained, the Mo content is set to 0.50% or less. Preferably, it is set to 0.40% or less. More preferably, it is set to 0.30% or less.

Ca: 0.0050% or less Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly drawn in the hot rolling process, and can be contained as necessary. In order to obtain the above-mentioned effects, when Ca is contained, it is preferable to contain 0.0005% or more of Ca. More preferably, the Ca content is 0.0008% or more, and even more preferably, 0.0010% or more. The Ca content is most preferably 0.0015% or more. However, when the Ca content exceeds 0.0050%, Ca oxide clusters are formed in the steel, and the toughness deteriorates. Therefore, when Ca is contained, the Ca content is 0.0050% or less. Preferably, the Ca content is 0.0040% or less. More preferably, it is 0.0035% or less. The Ca content is even more preferably 0.0030% or less.

B: 0.0050% or less B is an element that contributes to the refinement of the structure by lowering the transformation start temperature, and can be contained as necessary. In order to obtain the above-mentioned effects, when B is contained, it is preferable to contain 0.0002% or more of B. More preferably, the B content is 0.0005% or more, and even more preferably, 0.0008% or more. The B content is most preferably 0.0010% or more. However, if the B content exceeds 0.0050%, the ductility and toughness deteriorate. Therefore, when B is contained, the B content is 0.0050% or less. More preferably, it is 0.0040% or less. Even more preferably, it is 0.0030% or less. The B content is most preferably 0.0025% or less.

Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less Mg, Zr, and REM are elements that increase the strength of steel through grain refinement, and may be contained as necessary. The Mg content may be 0%, but when Mg is contained, the preferred lower limit is 0.0005% or more. The Mg content is more preferably 0.0008% or more. The Zr content may be 0%, but when Zr is contained, the preferred lower limit is 0.0005% or more. The Zr content is more preferably 0.0008% or more. The REM content may be 0%, but when REM is contained, the preferred lower limit is 0.0005% or more. The REM content is more preferably 0.0008% or more. On the other hand, excessive content may lead to an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when Mg is contained, the Mg content is set to 0.020% or less. The Mg content is preferably 0.010% or less. When Zr is contained, the Zr content is set to 0.020% or less. The Zr content is preferably 0.010% or less. When REM is contained, the REM content is 0.020% or less. The REM content is preferably 0.010% or less. Here, REM is a general term for 17 elements in total, including Sc, Y, and lanthanoid elements. One or more of these 17 elements can be contained in the steel, and the REM content means the total content of these elements.

Sn: 0.100% or less Sn is an element that suppresses decarburization caused by nitriding or oxidation of the steel sheet surface and suppresses the decrease in strength. In order to obtain the above-mentioned effects, it is preferable to contain 0.001% or more of Sn. The Sn content is more preferably 0.002% or more, and even more preferably 0.005% or more. However, if it is contained excessively, the ductility and toughness of the steel will decrease. Therefore, the Sn content is preferably 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.

The balance is Fe and unavoidable impurities. Inevitable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, manufacturing equipment, etc., and are permitted to be included to the extent that they do not impair the purpose of the present invention. Examples of the unavoidable impurities in the balance include As, Sb, Bi, Co, Pb, Zn, O, Ta, W, Te, Hf, Ge, Sr, and Cs. Examples of raw materials for steel plate include iron ore, reduced iron, and scrap.

It is preferable that the steel structure at the center of thickness of the hot-rolled steel plate of the present invention and the base material part of the electric resistance welded steel pipe has an average grain size of 15.0 μm or less, which is an area surrounded by high-angle grain boundaries, and grains having a grain size equal to or less than the average grain size account for 10% to 50% of the total grains in terms of area ratio, the degree of fine grain connectivity calculated by the following formula (3) is 0.05 to 0.50, and bainite accounts for 10% or more by volume, the total of ferrite and bainite accounts for 80% or more by volume, and the remainder is one or more types selected from pearlite, martensite and austenite in total account for 20% or less by volume.

(Fine grain connectivity)=(total length of high angle grain boundaries in grains having 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 equation (3) does not include the length of the high-angle grain boundary between crystal grains smaller than the average crystal grain size and crystal grains larger than or equal to the average crystal grain size.

The average grain size, the area ratio of grains with a grain size equal to or smaller than the average grain size, and the degree of fine grain connectivity are measured using the SEM/EBSD method. The measurement area is 500 μm × 500 μm, the measurement step size is 0.5 μm, and the measured values of 5 or more fields are averaged. Based on the obtained EBSD data, the grain boundary and grain size distribution are obtained using the crystal orientation analysis software OIM Analysis (trademark), with boundaries with an orientation difference of 15° or more as grain boundaries (high-angle grain boundaries). The average grain size is calculated as the diameter (equivalent circle diameter) of a circle with an area equal to the value obtained by dividing the total measured area by the number of grains. The degree of fine grain connectivity is calculated by calculating the total length of the high-angle grain boundaries in the area excluding grains with a grain size equal to or larger than the average grain size (i.e., only grains with a grain size smaller than the average grain size), and the total length of all high-angle grain boundaries, and then calculating the ratio of these. However, the high-angle boundaries in the region excluding the grains having a grain size equal to or larger than the average grain size do not include the high-angle boundaries between grains smaller than the average grain size and grains having a grain size equal to or larger than the average grain size. In calculating the average grain size, the area ratio of grains having a grain size equal to or smaller than the average grain size, and the fine grain connectivity, grains having a grain size of 1.0 μm or less are excluded as measurement noise.

When the average grain size is large, the toughness decreases and cracks are more likely to occur in the flattening test. In addition, the value of (standardized load/standardized displacement) may also decrease. Therefore, the average grain size is preferably 15.0 μm or less. The average grain size is more preferably 12.0 μm or less. Even more preferably, it is 10.0 μm or less. The average grain size is most preferably 9.0 μm or less. In addition, when the average grain size is small, the ductility decreases and cracks are more likely to occur in the flattening test, so the average grain size is preferably 2.0 μm or more. More preferably, it is 3.0 μm or more. Even more preferably, the average grain size is 3.5 μm or more.

When the area ratio of crystal grains having a grain size equal to or smaller than the average grain size is low, the degree of connectivity of fine grains having a grain size equal to or smaller than the average grain size is low, and the resistance to flattening may be reduced. In addition, the toughness is reduced, and cracks are more likely to occur in the flattening test. Therefore, the above area ratio is preferably 10% or more. The above area ratio is more preferably 12% or more. Even more preferably, it is 15% or more. When the above area ratio exceeds 50%, the degree of connectivity of fine grains is high, ductility is reduced, and cracks are more likely to occur in the flattening test. Therefore, the above area ratio is preferably 50% or less. The above area ratio is more preferably 45% or less. Even more preferably, it is 40% or less. The above area ratio is most preferably 35% or less.

When the degree of fine grain connectivity is low, the degree of connectivity between coarse grains is high, and the strains in the coarse grains are more likely to connect to each other. Since the coarse grains are soft and relatively large strains occur within the grains, when they connect, stress concentration occurs and resistance to flattening may be reduced. Therefore, the degree of fine grain connectivity is preferably 0.05 or more. More preferably, it is 0.10 or more. The degree of fine grain connectivity is even more preferably 0.11 or more. The degree of fine grain connectivity is most preferably 0.12 or more. When the degree of fine grain connectivity is high, the hard fine grains act as the parent phase, so the ductility decreases and cracks are more likely to occur in the flattening test. Therefore, the degree of fine grain connectivity is preferably 0.50 or less. The degree of fine grain connectivity is more preferably 0.47 or less. Even more preferably, it is 0.40 or less. The degree of fine grain connectivity is most preferably 0.35 or less.

Ferrite is a soft structure. Bainite is harder than ferrite and softer than pearlite, martensite, and austenite, so controlling the structure of bainite is important from the standpoint of strength, ductility, and toughness.

As the volume fraction of bainite decreases, the proportion of soft ferrite increases, resulting in a decrease in strength. Also, the value of (standardized load/standardized displacement) may decrease. For this reason, it is preferable that the volume fraction of bainite be 10% or more. It is more preferable that the volume fraction of bainite be 12% or more. Even more preferable is 20% or more. It is most preferable that the volume fraction of bainite be 25% or more. There is no particular upper limit, but it is preferable that it be 75% or less because of the decrease in ductility.

When the total volume fraction of ferrite and bainite is small, the ratio of hard pearlite, martensite and austenite is high, and ductility and toughness are reduced. Therefore, the total volume fraction of ferrite and bainite is preferably 80% or more, more preferably 85% or more, and further preferably 88% or more.
For the above reasons, the total volume percentage of hard pearlite, martensite and austenite is preferably 20% or less.
On the other hand, if the total of hard pearlite, martensite and austenite is less than 1%, ductility decreases, so the total of ferrite and bainite is preferably 99% or less in volume fraction, more preferably 98% or less, and even more preferably 97% or less.

The above-mentioned various structures, except for austenite, have austenite grain boundaries or deformation bands within austenite grains as nucleation sites. In hot rolling, by increasing the reduction at low temperatures where austenite recrystallization is difficult to occur, a large number of dislocations can be introduced into the austenite, refining the austenite and introducing a large number of deformation bands within the grains. This increases the area of the nucleation sites, increasing the frequency of nucleation and making it possible to refine the steel structure.

Here, the observation of the steel structure can be performed by the method described below. First, test pieces for structure observation are taken so that the observation surface is a cross section parallel to both the rolling direction and the thickness direction and the center of the plate thickness of a hot-rolled steel sheet, and a cross section parallel to both the axial direction and the thickness direction and the center of the plate thickness of an electric resistance welded steel pipe, and then polished and etched with nital. For structure observation, an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) is used to observe and photograph the structure in the center of the plate thickness (or wall thickness). Next, the area ratio of bainite and the remainder (ferrite, pearlite, martensite, austenite) is obtained from the obtained optical microscope image and SEM image. The area ratio of each structure is calculated as the average value of the values obtained in five or more visual fields by observing each visual field. In the present invention, the area ratio obtained by structure observation is the volume ratio of each structure. The judgment of whether or not it is each structure is carried out according to the following contents.

Ferrite is a product of diffusion transformation and has a low dislocation density and a nearly restored structure. This includes polygonal ferrite and pseudo-polygonal ferrite. Areas where no cementite is observed with an optical microscope or SEM and where the lath structure, which is the substructure, is not observed, are judged to be ferrite.

Bainite is a complex phase structure of lath-shaped ferrite and cementite with a high dislocation density. Areas where cementite is dispersed or where the substructure, a lath structure, is observed using an optical microscope or SEM are judged to be bainite.

Pearlite is a eutectoid structure of iron and iron carbide (ferrite + cementite), and has a lamellar structure in which linear ferrite and cementite are arranged alternately. The areas observed as above with an SEM are determined to be pearlite.

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

Since it is difficult to distinguish between martensite and austenite in optical microscope images and SEM images, the area fraction of the structures observed as martensite or austenite in the obtained SEM images is measured, and the volume fraction of martensite is determined by subtracting from this measurement the volume fraction of austenite, which is measured using the method described below.

Austenite is an fcc phase, and the volume fraction of austenite is measured by X-ray diffraction using a test piece prepared in the same manner as the test piece used to measure dislocation density. The volume fraction of austenite is calculated from the integrated intensities of the (200), (220), and (311) planes of the obtained fcc iron and the (200) and (211) planes of the bcc iron.

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

The hot-rolled steel sheet of the present invention is manufactured, for example, by heating a steel material having the above-mentioned composition to a heating temperature of 1100°C to 1300°C, followed by hot rolling at an average cooling rate of 900°C to 1100°C, an average cooling rate of 0.5°C/s to 3.0°C/s, a finish rolling end temperature of 750°C to 850°C, and a total reduction in finish rolling of 45% to 75%, followed by cooling at an average cooling rate at the center of the plate thickness from the finish rolling end temperature to the cooling stop temperature of 5°C/s to 40°C/s, a minimum cooling rate from the end of finish rolling to the stop of cooling of 2°C/s or more, a cooling end temperature of 400°C to 650°C, a continuous air-cooling time from the end of finish rolling to the stop of cooling of 15 s or less, and a total air-cooling time of 50 s or less, and then winding the steel into a coil.

The electric-welded steel pipe of the present invention is manufactured by forming the hot-rolled steel sheet into a cylindrical shape by cold roll forming and then electric-welding the same.

In the following explanation of the manufacturing method, the temperature indicated in "℃" refers to the surface temperature of the steel material or steel plate (hot-rolled plate) unless otherwise specified. These surface temperatures can be measured with a radiation thermometer or the like. The temperature at the center of the steel plate thickness can be determined by calculating the temperature distribution in the cross section of the steel plate using heat transfer analysis and correcting the result by the surface temperature of the steel plate. Additionally, "hot-rolled steel plate" includes hot-rolled plate and hot-rolled steel strip.

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

The resulting steel material (steel slab) is then heated, hot rolled, cooled, and then wound into a coil to produce hot-rolled steel sheet.

If the heating temperature is low, the deformation resistance of the rolled material increases, making rolling difficult. For this reason, the heating temperature is preferably 1100°C or higher. More preferably, it is 1120°C or higher. Even more preferably, it is 1130°C or higher. Most preferably, it is 1150°C or higher. On the other hand, if the heating temperature is high, the austenite grains become coarse, and fine austenite grains cannot be obtained in the subsequent rolling (rough rolling, finish rolling), and the average crystal grain size of the final product becomes large. For this reason, the heating temperature in the hot rolling process is preferably 1300°C or lower. More preferably, it is 1280°C or lower. Even more preferably, it is 1270°C or lower. Most preferably, it is 1250°C or lower.

In addition to the conventional method of producing a steel slab, first cooling it to room temperature and then reheating it, the present invention can also easily apply energy-saving direct rolling processes in which the slab is not cooled to room temperature, but is instead loaded into a heating furnace while still hot, or is rolled immediately after a short period of heat retention.

If the average cooling rate from 900°C to 1100°C is small, the austenite will coarsen and the average grain size of the final product will become large. For this reason, it is preferable that the average cooling rate from 900°C to 1100°C be 0.5°C/s or more. More preferably, it is 0.8°C/s or more. Even 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 average cooling rate is large, the recrystallization of austenite will be insufficient, coarse austenite will remain, and the final product will have a steel structure in which coarse grains are mixed. As a result, the area ratio of grains having a grain size smaller than the average grain size will be low. For this reason, it is preferable that the average cooling rate from 900°C to 1100°C be 3.0°C/s or less. More preferably, it is 2.5°C/s or less. Even more preferably, it is 2.4°C/s or less. Most preferably, it is 2.2°C/s or less.

If the finish rolling end temperature is low, the surface temperature of the steel sheet during finish rolling will fall below the ferrite transformation start temperature, a large amount of processed ferrite will be generated, and ductility will decrease. Therefore, it is preferable that the finish rolling end temperature be 750°C or higher. More preferably, it is 770°C or higher. Even more preferably, it is 780°C or higher. On the other hand, if the finish rolling end temperature is high, fine austenite grains will not be obtained and the average crystal grain size will become large. Therefore, it is preferable that the finish rolling end temperature be 850°C or lower. More preferably, it is 830°C or lower. Even more preferably, it is 820°C or lower.

If the total reduction in the finish rolling is low, sufficient processing strain cannot be introduced in the hot rolling process, and the average grain size of the final product becomes large. Therefore, it is preferable that the total reduction in the finish rolling is 45% or more. The total reduction in the finish rolling is more preferably 50% or more. The total reduction in the finish rolling is even more preferably 52% or more. The total reduction in the finish rolling is most preferably 54% or more. On the other hand, if the total reduction in the finish rolling is high, the average grain size becomes small. In addition, the area ratio of grains having a grain size equal to or smaller than the average grain size becomes high. For this reason, the total reduction in the finish rolling is preferably 75% or less. More preferably, it is 70% or less. The total reduction in the finish rolling is even more preferably 68% or less. The total reduction in the finish rolling is most preferably 66% or less.

The total reduction in the above-mentioned finish rolling refers to the sum of the reduction in each rolling pass in the finish rolling.

From the viewpoint of ensuring the necessary rolling reduction rate and managing the steel plate temperature, it is preferable that the finished plate thickness be 5 mm or more. The finished plate thickness is more preferably 6 mm or more. Even more preferably 7 mm or more. In addition, it is preferable that the finished plate thickness be 40 mm or less. The finished plate thickness is more preferably 35 mm or less. Even more preferably 30 mm or less.

After hot rolling, the hot-rolled sheet is cooled.

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 will become coarse and the average crystal grain size of the final product will become large. The bainite fraction will also be low. Therefore, it is preferable that the average cooling rate be 5°C/s or more. More preferably, it is 10°C/s or more. Even 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 will increase and ductility will decrease. Therefore, it is preferable that the average cooling rate be 40°C/s or less. More preferably, it is 35°C/s or less. Even more preferably, it is 33°C/s or less. Most preferably, it is 30°C/s or less.

If the minimum cooling rate at the center of the plate thickness from the end of finish rolling to the end of cooling is low, the structure will become coarse and the average grain size will become large. In addition, the area ratio of grains having a grain size smaller than the average grain size will be low. In addition, the bainite fraction will be low. Therefore, it is preferable that the minimum cooling rate is 2°C/s or more. More preferably, it is 3°C/s or more. Even more preferably, it is 4°C/s or more. Most preferably, it is 5°C/s or more. There is no particular upper limit, but it is preferable that the minimum cooling rate is 15°C/s or less. More preferably, it is 12°C/s or less. Even more preferably, it is 10°C/s or less. Most preferably, it is 8°C/s or less. The minimum cooling rate is calculated by dividing the time from the end of finish rolling to the end of cooling into sections of 3 seconds each, calculating the average cooling rate in each section, and taking the minimum value of these.

If the continuous air-cooling time from the end of finish rolling to the end of cooling is long, ferrite and bainite will grow excessively, the structure will become coarse, and the average crystal grain size of the final product will become large. In addition, the area ratio of crystal grains having a grain size smaller than the average crystal grain size will decrease. In addition, the bainite fraction may decrease. Therefore, the continuous air-cooling time is preferably 15 seconds or less. More preferably, it is 14 seconds or less. Even more preferably, it is 12 seconds or less. Most preferably, it is 11 seconds or less. The shorter the continuous air-cooling time, the better, but if it is less than 6 seconds, the effect of reducing the continuous air-cooling time will be small, and the equipment load will only increase, so the continuous air-cooling time is preferably 6 seconds or more. More preferably, it is 7 seconds or more. Even more preferably, it is 8 seconds or more. Between the end of finish rolling and the end of cooling, cooling is performed using air cooling and water cooling.

If the total air-cooling time from the end of finish rolling to the end of cooling is long, ferrite and bainite will grow excessively, the structure will become coarse, and the average crystal grain size of the final product will become large. In addition, the area ratio of crystal grains having a grain size equal to or smaller than the average crystal grain size will decrease. In addition, the bainite fraction may decrease. Therefore, the total air-cooling time is preferably 50 seconds or less. More preferably, it is 45 seconds or less. Even more preferably, it is 40 seconds or less. Most preferably, it is 38 seconds or less. The shorter the continuous air-cooling time, the better, but if it is less than 10 seconds, the effect of refining the grains in relation to the reduction in the total air-cooling time will be small, and the equipment load will only increase, so the total air-cooling time is preferably 10 seconds or more. More preferably, it is 12 seconds or more. Even more preferably, it is 15 seconds or more.

If the cooling stop temperature is low, the proportion of martensite increases and ductility decreases. Therefore, it is preferable that the cooling stop temperature be 400°C or higher. More preferably, it is 420°C or higher. Even more preferably, it is 450°C or higher. Most preferably, it is 470°C or higher. On the other hand, if the cooling stop temperature is high, the proportion of bainite decreases. Therefore, it is preferable that the cooling stop temperature be 650°C or lower. More preferably, it is 620°C or lower. Even more preferably, it is 600°C or lower. Most preferably, it is 580°C or lower.

In the electric resistance welded steel pipe of the present invention, in order to reduce the inclusion density of the welded portion, it is preferable that the width of the molten solidified portion of the welded portion (electric resistance welded portion) in the circumferential direction of the pipe is 1 μm or more throughout the entire thickness of the pipe. It is also preferable that the width of the molten solidified portion of the welded portion (electric resistance welded portion) in the circumferential direction of the pipe is 1000 μm or less throughout the entire thickness of the pipe.

The outer diameter of the electric welded steel pipe is preferably 80 mm or more. The outer diameter is preferably 800 mm or less. The wall thickness of the electric welded steel pipe is preferably 3 mm or more. The wall thickness is preferably 40 mm or less.

Here, the etching solution may be selected appropriately depending on the steel components and the type of steel pipe. Figure 4 shows a schematic diagram of the circumferential cross section of an electric resistance welded part of an electric resistance welded steel pipe. The molten solidified part 15 can be visually recognized as a region having a different structure form and contrast from the base material part 13 and the heat-affected part 14 in Figure 4, as shown in the schematic diagram of the cross section after corrosion in Figure 4. For example, the molten solidified part 15 of an electric resistance welded steel pipe of carbon steel and low alloy steel can be identified as a region observed as white under an optical microscope in the cross section corroded with nital. Also, the molten solidified part 15 of a UOE steel pipe of carbon steel and low alloy steel can be identified as a region containing a cellular or dendritic solidified structure under an optical microscope in the cross section corroded with nital.

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

Molten steel having the chemical composition shown in Table 1 was produced as a slab (steel material). The obtained slab was subjected to a hot rolling process and a cooling process under the conditions shown in Table 2, and then a coiling process to produce a hot-rolled steel sheet having the finished thickness (mm) shown in Table 2.

After the coiling process, the hot-rolled steel sheet was rolled into a cylindrical round steel pipe, and the butt joint was electric resistance welded. The round steel pipe was then reduced in diameter by rolls placed above, below, left and right of the round steel pipe, to obtain electric resistance welded steel pipes with the outer diameter (mm) and wall thickness (mm) shown in Table 4.

Test pieces were taken from the obtained hot-rolled steel sheets and electric resistance welded steel pipes shown in Tables 3 and 4, respectively, and the following C-type flattening tests, flattening tests, average grain size measurements, area ratio measurements of grains with grain sizes smaller than the average grain size, fine grain connectivity measurements, and structural observations were performed. The various test pieces were taken from the center of the plate thickness in the width direction for the hot-rolled steel sheets, and from the center of the plate thickness of the base material part 90° away from the electric resistance weld in the circumferential direction of the pipe for the electric resistance welded steel pipes.

[C-type flattening test]
In the C-type flattening test, a plate material (test piece) having a total thickness of 50×t (t: plate thickness) and a length of 100 mm was taken from the hot-rolled steel plate so that the longitudinal direction of the test piece was the plate width direction of the hot-rolled steel plate, and then this was made into a C-type test piece by the press-bending method described in JIS Z 2248 (2006), and then it was carried out by the method described in JIS G 3441 (2021). In the press-bending, the inner radius of the tip of the press fitting was 9×t. The initial curvature radius r of the bent outer surface of the flattened test piece was obtained by adding the plate thickness to the inner radius of the tip of the press fitting in the press-bending as shown in formula (4).
r=10×t...(4)
When a crack having a length of 0.50 mm or more was generated on the outer bent surface in the tightly attached state, it was judged that there was a crack, and when no crack was generated on the outer bent surface, it was judged that there was no crack.

[Flattening test]
The flattening test was performed by taking a test piece cut into a ring having a length of 100 mm from an electric resistance welded steel pipe including an electric resistance welded portion in the axial direction of the pipe, and performing the flattening test according to the method described in JIS G 3441 (2021).
However, the test specimen was placed so that the welded portion faced the compression direction, as shown in Figure 3. The initial radius of curvature r of the bent outer surface of the flattened test specimen was set to 1/2 the outer diameter of the electric resistance welded steel pipe. If a crack of 0.50 mm or more in length was generated on the outer surface of the pipe in a tightly sealed state, it was judged that there was a crack, and if no crack was generated on the outer surface of the pipe, it was judged that there was no crack.

[Average grain size measurement]
The average grain size was measured by taking a test piece for measurement so that the measurement surface was a cross section parallel to both the rolling direction and the plate thickness direction of the hot-rolled steel sheet, and a cross section parallel to both the pipe axis direction and the wall thickness direction of the electric resistance welded steel pipe, mirror polishing, and then using the SEM/EBSD method. The grain size was measured by determining the orientation difference between adjacent grains, and the boundary with an orientation difference of 15° or more was considered as a grain boundary, and the area surrounded by an orientation difference of 15° or more was considered as one grain. The arithmetic average of the grain size was calculated from the obtained grain boundaries, and the average grain size was obtained. The acceleration voltage was 15 kV, the measurement area was 500 μm × 500 μm, the measurement step size was 0.5 μm, and the measured values of 5 fields or more were averaged. Based on the obtained EBSD data, the grain boundary and grain size distribution were obtained by using the crystal orientation analysis software OIM Analysis (trademark), with the boundary with an orientation difference of 15° or more being considered as a grain boundary (high angle grain boundary). The grain size and the average grain size were calculated as the diameter of a circle having an area equal to the total area measured divided by the number of grains (equivalent circle diameter). In calculating the average grain size, grains having a grain size of 1.0 μm or less were excluded as measurement noise.

[Measurement of area ratio of crystal grains having a grain size equal to or smaller than the average crystal grain size]
The area ratio was calculated from the average crystal grain size and the grain size distribution obtained above. For each crystal grain with a grain size equal to or smaller than the average crystal grain size, the area was calculated from its circle-equivalent diameter, the total area of the crystal grains with a grain size equal to or smaller than the average crystal grain size was calculated, and the total area was divided by the area of the measurement region. In calculating the area ratio of the crystal grains with a grain size equal to or smaller than the average crystal grain size, crystal grains with a grain size of 1.0 μm or less were excluded as measurement noise.

[Fine Grain Connectivity]
The fine grain connectivity was calculated by calculating the total length of high-angle boundaries in the region excluding grains having a grain size equal to or larger than the average grain size, and the total length of high-angle boundaries in all grains, and then calculating the ratio between them. However, the total length of high-angle boundaries in grains having a grain size smaller than the average grain size, which is written in the numerator on the right side of the above formula (3), does not include the length of high-angle boundaries between grains having a grain size smaller than the average grain size and grains having a grain size equal to or larger than the average grain size.
In addition, in calculating the fine grain connectivity, crystal grains having a crystal grain size of 1.0 μm or less were excluded as measurement noise.

[Structural Observation]
The test pieces for microstructural observation were taken so that the observation surface was a cross section parallel to both the rolling direction and the plate thickness direction of the hot-rolled steel sheet, and a cross section parallel to both the pipe axis direction and the wall thickness direction of the electric resistance welded steel pipe, respectively, and were prepared by mirror polishing and then etching with nital. The microstructural observation was performed using an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) to observe and image the structure at the plate thickness center position of the hot-rolled steel sheet and the wall thickness center position of the electric resistance welded steel pipe. The SEM observation was performed at an acceleration voltage of 15 kV. From the obtained optical microscope image and SEM image, the area ratio of bainite and the remainder (ferrite, pearlite, martensite, austenite) was obtained. The area ratio of each structure was calculated as the average value of the values obtained in each field of view by observing five or more fields of view. Here, the area ratio obtained by the microstructural observation was taken as the volume ratio of each structure.

From the above, the determination of ferrite, pearlite, and martensite is carried out using the method described in the embodiment.

The austenite volume fraction was measured by X-ray diffraction. The test pieces for measuring the center of thickness of the hot-rolled steel plate and the center of thickness of the electric resistance welded steel pipe were prepared by grinding so that the diffraction surface was at the center of thickness of the hot-rolled steel plate and the center of thickness of the electric resistance welded steel pipe, respectively, and then chemically polishing to remove the surface treatment layer. The measurement used Mo Kα radiation, and the austenite volume fraction was calculated from the integrated intensity of the (200), (220), and (311) planes of fcc iron and the (200) and (211) planes of bcc iron.

The results obtained are shown in Tables 3 and 4.

In Tables 3 and 4, hot-rolled steel plates and electric-resistance welded steel pipes No. 1, 4, 6, 8, 9, 11 to 24 are examples of the present invention, and hot-rolled steel plates and electric-resistance welded steel pipes No. 2, 3, 5, 7, and 10 are comparative examples.

In the C-type flattening test, none of the hot-rolled steel sheets of the present invention showed any cracks measuring 0.50 mm or more in length until they reached a tightly adhered state, and the (standardized load/standardized displacement) value was 100 MPa or more when the standardized displacement was in the range of 0.20 to 0.30.

In the flattening test, none of the electric welded steel pipes of the present invention showed any cracks greater than 0.50 mm in length until they reached a tightly sealed state, and the (standardized load/standardized displacement) value was 100 MPa or greater when the standardized displacement was in the range of 0.20 to 0.30.

1 Curve showing change in normalized load with normalized displacement 2 Elastic region 3 Plastic region 4 Crack or adhesion state 5A Test piece 5 C-shaped flattened test piece (flattened test piece)
6 Flat plate 7 Compression direction 8 Flattened test piece 9 Electric resistance welded part 10 Center of electric resistance welded steel pipe 11 Flat plate 12 Compression direction 13 Base material part 14 Heat-affected part 15 Melted and solidified part 100 Initial length L of test piece
101 Initial thickness of test piece t
102 Rolling direction of hot-rolled steel sheet

Claims (2)

The composition is in mass percent:
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 even more so:
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: 0.020% or less,
Zr: 0.020% or less,
REM: 0.020% or less,
Sn: 0.100% or less, one or more of the following are included;
The balance is Fe and unavoidable impurities,
In the C-type flattening test, a steel plate is bent into a U-shape and a test piece is clamped between two flat plates.
No cracks having a length of 0.50 mm or more are generated until the inner surfaces of the bent test pieces come into contact with each other, and
When the normalized displacement calculated by the following formula (2) is in the range of 0.20 to 0.30, the normalized load/normalized displacement calculated by the following formula (1) is 100 MPa or more,
The steel structure at the center of the plate thickness is
The average grain size of the grains in the region surrounded by the high-angle grain boundaries is 15.0 μm or less,
The crystal grains having a grain size equal to or smaller than the average crystal grain size account for 10% or more and 50% or less of the total crystal grains,
The fine grain connectivity calculated by the following formula (3) is 0.05 or more and 0.50 or less, and bainite is 10% or more in volume fraction,
The total volume fraction of ferrite and bainite is 80% or more,
The balance is one or more selected from pearlite, martensite and austenite, the total of which is 20% or less by volume.
Hot-rolled steel sheet.
(Normalized load (MPa)) = (P/L) x (r/t ² ) x (1 - ((x ₀ - x) / 2r) ² ) ^1/2 ... (1)
(Normalized displacement)=(x ₀ −x)/2r (2)
Where:
P: Load (N)
L: Initial length of the flattened test piece (mm)
r: initial radius of curvature of the outer surface of the bent flattened test piece (mm)
t: initial thickness of flattened test piece (mm)
x ₀ : Initial distance between two plates (mm)
x: Distance between two plates (mm)
(Fine grain connectivity)=(total length of high angle grain boundaries in grains having 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 equation (3) does not include the length of the high-angle grain boundary between crystal grains smaller than the average crystal grain size and crystal grains larger than or equal to the average crystal grain size. An electric resistance welded steel pipe having a base metal portion and an electric resistance welded portion,
The composition of the base material is, in 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 even more so:
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: 0.020% or less,
Zr: 0.020% or less,
REM: 0.020% or less,
Sn: 0.100% or less, one or more of the following are included;
The balance is Fe and unavoidable impurities,
In a flattening test, a flattened test piece taken from an electric resistance welded steel pipe is clamped between two flat plates.
No cracks having a length of 0.50 mm or more are generated until the inner surfaces of the flattened test pieces come into contact with each other, and
When the normalized displacement calculated by the following formula (2) is in the range of 0.20 to 0.30, the normalized load/normalized displacement calculated by the following formula (1) is 100 MPa or more,
The steel structure at the center of the thickness of the base material portion is
The average grain size of the grains in the region surrounded by the high-angle grain boundaries is 15.0 μm or less,
The crystal grains having a grain size equal to or smaller than the average crystal grain size account for 10% or more and 50% or less of the total crystal grains,
The fine grain connectivity calculated by the following formula (3) is 0.05 or more and 0.50 or less, and bainite is 10% or more in volume fraction,
The total volume fraction of ferrite and bainite is 80% or more,
The balance is one or more selected from pearlite, martensite and austenite, the total of which is 20% or less by volume.
Electric resistance welded steel pipe.
(Normalized load (MPa)) = (P/L) x (r/t ² ) x (1 - ((x ₀ - x) / 2r) ² ) ^1/2 ... (1)
(Normalized displacement)=(x ₀ −x)/2r (2)
Where:
P: Load (N)
L: Initial length of the flattened test piece in the tube axial direction (mm)
r: initial radius of curvature of the outer surface of the bent flattened test piece (mm)
t: initial thickness of flattened test piece (mm)
x ₀ : Initial distance between two plates (mm)
x: Distance between two plates (mm)
(Fine grain connectivity)=(total length of high angle grain boundaries in grains having 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 equation (3) does not include the length of the high-angle grain boundary between crystal grains smaller than the average crystal grain size and crystal grains larger than or equal to the average crystal grain size.

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