EP4632097A1

EP4632097A1 - Hot-rolled steel sheet and electric resistance welded steel pipe

Info

Publication number: EP4632097A1
Application number: EP23921306.9A
Authority: EP
Inventors: Akihide MATSUMOTO; Naomichi IWATA; Shinsuke Ide
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2023-02-09
Filing date: 2023-11-17
Publication date: 2025-10-15
Also published as: CN120603972A; JPWO2024166485A1; EP4632097A4; JP7635881B2; TW202432269A; KR20250132542A; TWI887866B; WO2024166485A1

Abstract

An object of the present invention is to provide an electric resistance welded steel pipe with excellent anti-flattening performance, and a hot-rolled steel sheet used as the material for such pipes.

A hot-rolled steel sheet, in a specific C-shaped flattening test where a test specimen formed by bending the steel sheet is flattened, is free from a crack of 0.50 mm or longer until a direct contact is reached where the inner surfaces of the bent test specimen are in contact with each other, and has a ratio of a normalized load calculated from expression (1) below to a normalized displacement calculated from expression (2) below of 100 MPa or more when the normalized displacement is in the range of 0.20 to 0.30,

(Normalized load (MPa)) = (P/L) × (r/t²) × (1 - ((x₀ - x)/2r)²)^1/2

(Normalized displacement) = (x_{0} - x) / 2 r

where P: load (N), L: initial length (mm) of the flattening test specimen, r: initial radius of curvature (mm) of the outer surface of the bent portion of the flattening test specimen, t: initial sheet thickness (mm) of the flattening test specimen, x₀: initial distance (mm) between two flat plates, and x: distance (mm) between the two flat plates.

Description

Technical Field

The present invention relates to electric resistance welded steel pipes suitably used in, for example, building structures and line pipes, and to hot-rolled steel sheets for producing the electric resistance welded steel pipes.

Background Art

Electric resistance welded steel pipes used in building structures and line pipes suffer a sudden drop in proof stress and are broken when buckling occurs in response to external forces, such as earthquake forces. Thus, these electric resistance welded steel pipes are desirably resistant to buckling.
In order to make electric resistance welded steel pipes resistant to buckling, it is effective to suppress cross-sectional flattening of the steel pipes that is a pre-buckling phenomenon.
While there have been few studies that address this requirement, for example, Patent Literature 1 discloses a steel pipe resistant to buckling during bending that is obtained by heating a parent electric resistance welded steel pipe and subjecting it to diameter reduction hot rolling so as to control the r value in the longitudinal direction of the pipe to 1.0 or more.

Citation List

Patent Literature

PTL 1: Japanese Patent No. 6954504

Non Patent Literature

NPL 1: Hidetoshi Kobayashi, Hiroshi Okubo, Masashi Daimaruya, KEIKINZOKU (Journal of the Japan Institute of Light Metals), 39 (1989), p. 8

Summary of Invention

Technical Problem

As described in Patent Literature 1, increasing the r value in the steel pipe axis direction promotes deformation in the circumferential direction of the pipe and reduces the change in wall thickness, thereby making it possible to suppress the cross-sectional flattening of the steel pipe. However, this approach has a problem in which the steel pipe shrinks in cross-sectional diameter and undergoes necking, which reduces the sectional area and lowers the proof stress of structures.
The present invention has been made in consideration of the circumstances discussed above. It is therefore an object of the present invention to provide an electric resistance welded steel pipe with excellent anti-flattening performance, and a hot-rolled steel sheet used as the material for such pipes.
In the present invention, "excellent anti-flattening performance" means that in a flattening test, a flattening test specimen sampled from an electric resistance welded steel pipe is flattened without a crack of 0.50 mm or longer until a direct contact is reached where the inner surfaces of the pipe are in contact with each other and that (normalized load calculated from expression (1) below/normalized displacement calculated from expression (2)) is 100 MPa or more when the normalized displacement is in the range of 0.20 to 0.30. The phrase that the inner surfaces of a flattening test specimen sampled from an electric resistance welded steel pipe are in contact with each other means, for example, that the upper inner surface and the lower inner surface of a flattening test specimen sampled from an electric resistance welded steel pipe come into contact with each other when the flattening test specimen sampled from an electric resistance welded steel pipe is flattened by the application of a load from above and below. (Normalized load (MPa)) = (P/L) × (r/t²) × (1 - ((x₀ - x)/2r)²)^1/2 $(Normalized displacement) = (x_{0} - x) / 2 r$ where:

P: load (N),
L: initial length (mm) of the flattening test specimen in the pipe axis direction,
r: initial radius of curvature (mm) of the outer surface of the bent portion of the flattening test specimen,
t: initial sheet thickness (mm) of the flattening test specimen,
x₀: initial distance (mm) between the two flat plates,
x: distance (mm) between the two flat plates.

Solution to Problem

As a result of extensive studies, the present inventors have found that increasing the proportion of fine grains in the microstructures of an electric resistance welded steel pipe enhances toughness and can reduce the occurrence of cracking in a flattening test. On the other hand, the present inventors have also found that an excessively high proportion of fine grains leads to a decrease in ductility and facilitates the occurrence of cracking before a direct contact is reached in a flattening test.
Furthermore, the present inventors have found that increasing the connectivity of fine grains allows the steel pipe to withstand an increased load for a certain displacement (a certain amount of compression) in a flattening test, that is, to attain an increased flattening resistance. On the other hand, the present inventors have also found that an excessively high connectivity of fine grains leads to a decrease in ductility and gives rise to the occurrence of cracking before a direct contact is reached in a flattening test.
Furthermore, the present inventors have found that a hot-rolled steel sheet exhibiting high flattening resistance in a C-shaped flattening test gives an electric resistance welded steel pipe with excellent anti-flattening performance.
The present invention has been completed based on the above findings and provides the following [1] to [6]:

[1] A hot-rolled steel sheet, wherein a C-shaped flattening test where a test specimen formed by bending the steel sheet into a U-shape is pressed between two flat plates shows that:
- the test specimen is free from a crack of 0.50 mm or longer until a direct contact is reached where the inner surfaces of the bent test specimen are in contact with each other,
  and
- the ratio of a normalized load calculated from expression (1) below to a normalized displacement calculated from expression (2) below is 100 MPa or more when the normalized displacement is in the range of 0.20 to 0.30, (Normalized load (MPa)) = (P/L) × (r/t²) × (1 - ((x₀ - x)/2r)²)^1/2 $(Normalized displacement) = (x_{0} - x) / 2 r$ where:
  - P: load (N),
  - L: initial length (mm) of the flattening test specimen,
  - r: initial radius of curvature (mm) of the outer surface of the bent portion of the flattening test specimen,
  - t: initial sheet thickness (mm) of the flattening test specimen,
  - x₀: initial distance (mm) between the two flat plates,
  - x: distance (mm) between the two flat plates.
[2] The hot-rolled steel sheet according to [1], wherein the hot-rolled steel sheet has a chemical composition including, 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, and
- N: 0.0100% or less,
or further including one, or two or more selected from:
- 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, and
- Sn: 0.100% or less,
- the balance being Fe and incidental impurities.
[3] The hot-rolled steel sheet according to [1] or [2], wherein a steel microstructure at half the sheet thickness is such that:
- the average grain size of grains observed as regions surrounded by a high-angle grain boundary is 15.0 µm or less,
- the area fraction of 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 the grains,
- the connectivity of fine grains calculated from expression (3) below is 0.05 or more and 0.50 or less,
- the volume fraction of bainite is 10% or more,
- the total volume fraction of ferrite and bainite is 80% or more, and
- the balance is one, or two or more selected from pearlite, martensite, and austenite representing a volume fraction of 20% or less in total, (Connectivity of fine grains) = (total length of high-angle grain boundaries of grains having a grain size smaller than the average grain size)/(total length of high-angle grain boundaries) wherein the numerator on the right side of expression (3) does not include the length of high-angle grain boundaries between a grain smaller than the average grain size and a grain equal to or larger than the average grain size.
[4] An electric resistance welded steel pipe including a base metal material and an electric resistance weld, wherein a flattening test where a flattening test specimen sampled from the electric resistance welded steel pipe is pressed between two flat plates shows that:
- the test specimen is free from a crack of 0.50 mm or longer until a direct contact is reached where the inner surfaces of the flattening test specimen are in contact with each other, and
- the ratio of a normalized load calculated from expression (1) below to a normalized displacement calculated from expression (2) below is 100 MPa or more when the normalized displacement is in the range of 0.20 to 0.30, (Normalized load (MPa)) = (P/L) × (r/t²) × (1 - ((x₀ - x)/2r)²)^1/2 $(Normalized displacement) = (x_{0} - x) / 2 r$ where:
  - P: load (N),
  - L: initial length (mm) of the flattening test specimen in the pipe axis direction,
  - r: initial radius of curvature (mm) of the outer surface of the bent portion of the flattening test specimen,
  - t: initial sheet thickness (mm) of the flattening test specimen,
  - x₀: initial distance (mm) between the two flat plates,
  - x: distance (mm) between the two flat plates.
[5] The electric resistance welded steel pipe according to [4], wherein the base metal material has a chemical composition including, 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, and
- N: 0.0100% or less,
or further including one, or two or more selected from:
- 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, and
- Sn: 0.100% or less,
- the balance being Fe and incidental impurities.
[6] The electric resistance welded steel pipe according to [4] or [5], wherein a steel microstructure of the base metal material at half the wall thickness is such that:
- the average grain size of grains observed as regions surrounded by a high-angle grain boundary is 15.0 µm or less,
- the area fraction of 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 the grains,
- the connectivity of fine grains calculated from expression (3) below is 0.05 or more and 0.50 or less,
- the volume fraction of bainite is 10% or more,
- the total volume fraction of ferrite and bainite is 80% or more, and
- the balance is one, or two or more selected from pearlite, martensite, and austenite representing a volume fraction of 20% or less in total, (Connectivity of fine grains) = (total length of high-angle grain boundaries of grains having a grain size smaller than the average grain size)/(total length of high-angle grain boundaries) wherein the numerator on the right side of expression (3) does not include the length of high-angle grain boundaries between a grain smaller than the average grain size and a grain equal to or larger than the average grain size. Advantageous Effects of Invention

The present invention can provide an electric resistance welded steel pipe with excellent anti-flattening performance, and a hot-rolled steel sheet used as the material for such pipes.

Brief Description of Drawings

[Fig. 1] Fig. 1 is a schematic diagram illustrating a relationship between the normalized load and the normalized displacement in a flattening test.
[Fig. 2(a)] Fig. 2(a) is a view illustrating a sampling direction in which a C-shaped flattening test specimen, viewed from the side, is sampled from a hot-rolled steel sheet.
[Fig. 2(b)] Fig. 2(b) is a side view of a flattening test of a C-shaped flattening test specimen of a hot-rolled steel sheet.
[Fig. 3] Fig. 3 is a side view of a flattening test of an electric resistance welded steel pipe.
[Fig. 4] Fig. 4 is a schematic view of a circumferential cross section of an electric resistance weld of an electric resistance welded steel pipe.

Description of Embodiments

A hot-rolled steel sheet and an electric resistance welded steel pipe of the present invention, and methods for manufacturing them will be described below.
First, there will be described the reasons as to why the mechanical properties of the hot-rolled steel sheet and of the electric resistance welded steel pipe of the present invention are limited.
The hot-rolled steel sheet of the present invention is characterized in that a C-shaped flattening test shows that the steel sheet that has been bent is free from a crack of 0.50 mm or longer until a direct contact is reached where the inner surfaces of the bent steel sheet are in contact with each other, and the value of normalized load/normalized displacement is 100 MPa or more when the normalized displacement is in the range of 0.20 to 0.30.
The electric resistance welded steel pipe of the present invention is characterized in that a flattening test shows that a flattening test specimen sampled from the electric resistance welded steel pipe is free from cracks until a direct contact is reached where the inner surfaces of the test specimen are in contact with each other, and the value of normalized load/normalized displacement is 100 MPa or more when the normalized displacement is in the range of 0.20 to 0.30.
The normalized load and the normalized displacement are calculated from expression (1) and expression (2), respectively: (Normalized load (MPa)) = (P/L) × (r/t²) × (1 - ((x₀ - x)/2r)²)^1/2 $(Normalized displacement) = (x_{0} - x) / 2 r$ where:

P: load (N),
L: initial length (mm) of the flattening test specimen,
r: initial radius of curvature (mm) of the outer surface of the bent portion of the flattening test specimen,
t: initial sheet thickness (mm) of the flattening test specimen,
x₀: initial distance (mm) between the two flat plates,
x: distance (mm) between the two flat plates.

The phrase "free from cracks until a direct contact is reached" in the flattening test means that cracks do not occur until the inner surfaces of the flattening test specimen sampled from the electric resistance welded steel pipe come into contact with each other, and the phrase that the inner surfaces of the flattening test specimen sampled from the electric resistance welded steel pipe come into contact with each other means, for example, that the upper inner surface and the lower inner surface of the flattening test specimen sampled from the electric resistance welded steel pipe come into contact with each other when the flattening test specimen sampled from the electric resistance welded steel pipe is flattened by the application of a load from above and below. Incidentally, the above-mentioned cracks are of a size that can be confirmed visually. A size that can be confirmed visually is 0.50 mm or more in length. While there is no particular upper limit to the length of cracks, the length of interest is preferably equal to or less than the length of the test specimen in the pipe axis direction in view of the fact that cracks are linear. The length means the longitudinal size of a crack.
In the C-shaped flattening test and the flattening test, the flattening resistance can be considered greater with increasing load for a certain distance (a certain amount of compression) between the two flat plates. Because the load and the displacement for a material are variable depending on the dimensions or the shape of the test specimen, the load and the displacement are normalized by expression (1) and expression (2), respectively, as described in Non Patent Literature 1.
In the C-shaped flattening test and the flattening test, the test specimen is deformed elastically until the normalized displacement reaches a certain value, after which the test specimen yields and undergoes plastic deformation. Fig. 1 is a schematic diagram of a relationship between the normalized load and the normalized displacement in a flattening test. In Fig. 1, the curve 1 illustrates changes in normalized load with the normalized displacement. As illustrated in Fig. 1, the normalized load and the normalized displacement are in a proportional relationship in the elastic zone 2 before the test specimen yields. After that, the test specimen yields and the deformation enters the plastic zone 3, in which the slope (normalized load/normalized displacement) of the curve becomes smaller, and the testing proceeds at a constant slope. The testing further proceeds while the slope (normalized load/normalized displacement) of the curve is increased compared to the above slope (the slope after the entry into the plastic zone 3), and the testing ends when cracks occur on the outer surface of the bent portion or when a direct contact is reached where the inner surfaces of the test specimen are in contact with each other (the upper inner surface and the lower inner surface are in contact with each other). Hereafter, the state of the test specimen at the end of the testing will be described as cracked or direct contact 4.
Cracking that occurs before a direct contact is reached gives rise to a sudden drop in load. In order to ensure the safety of structures, it is important in the present invention to prevent cracks from occurring until a direct contact is reached.
As an index of anti-flattening performance, the present invention employs (normalized load/normalized displacement) in the plastic zone 3. Specifically, the larger the (normalized load/normalized displacement), the greater the flattening resistance and the higher the anti-flattening performance. In the present invention, in particular, the (normalized load/normalized displacement) is limited to 100 MPa or more in the first half of the plastic zone 3 where the normalized displacement is in the range of 0.20 to 0.30. The (normalized load/normalized displacement) is preferably 120 MPa or more. The (normalized load/normalized displacement) is more preferably 140 MPa or more. The (normalized load/normalized displacement) is even more preferably 150 MPa or more. When the (normalized load/normalized displacement) is more than 600 MPa, ductility is lowered and cracks tend to occur before a direct contact is reached. Thus, it is preferable that the value of (normalized load/normalized displacement) be 600 MPa or less. The (normalized load/normalized displacement) is more preferably 550 MPa or less. The value of (normalized load/normalized displacement) is even more preferably 500 MPa or less. The (normalized load/normalized displacement) is most preferably 450 MPa or less.
The range of the normalized displacement is limited to 0.20 to 0.30 because the value of (normalized load/normalized displacement) in that plastic zone is almost constant and stable.
The C-shaped flattening test is a test in which a test specimen that is a U-shape bent steel sheet is pressed between two flat plates.
In the C-shaped flattening test, a sheet material having a width of 50 mm × t (t: sheet thickness) and a length of 100 mm is sampled from a hot-rolled steel sheet in such a manner that the long side direction of the test specimen will be the width direction of the hot-rolled steel sheet. Next, the sheet material is bent into a C-shaped test specimen by the press bending method described in JIS Z 2248 (2006), and the test specimen is tested by the method described in JIS G 3441 (2021). Fig. 2(a) is a view illustrating a sampling direction in which a C-shaped flattening test specimen is sampled from a hot-rolled steel sheet, and also illustrating the C-shaped flattening test specimen viewed from the side. Numeral 5A indicates a test specimen before being bent into a C-shaped flattening test specimen. The test specimen is bent into a C-shaped flattening test specimen. Numeral 5 indicates the C-shaped flattening test specimen thus formed. The initial length and the initial thickness do not change before and after the test specimen is formed into a C-shaped flattening test specimen. The initial length 100 of the C-shaped flattening test specimen (the flattening test specimen) 5 is the length described above, that is, 50 mm, and the longitudinal direction of the C-shaped flattening test specimen 5 is the same as the rolling direction 102 of the hot-rolled steel sheet. Numeral 101 indicates the initial thickness of the C-shaped flattening test specimen (the flattening test specimen) 5. Fig. 2(b) is a side view of the flattening test of the C-shaped flattening test specimen of the hot-rolled steel sheet. The C-shaped flattening test is a test method in which, as illustrated in Fig. 2(a) and Fig. 2(b), a test specimen 5A that has been cut out is bent into a U-shape to form a C-shaped flattening test specimen 5, the C-shaped flattening test specimen 5 is sandwiched between two flat plates 6, and a load is applied in a compressing direction 7 perpendicular to the flat plates 6 until the test specimen reaches the above-mentioned state. In the press bending into a C-shaped flattening test specimen 5, the inside radius of the tip of the pressing element is 9 × t (mm). The initial radius of curvature r (mm) of the outer surface of the bent portion of the C-shaped flattening test specimen is determined by adding the sheet thickness t (mm) to the inside radius of the tip of the pressing element in the above press bending, as shown in expression (4): $r = 10 \times t$
In the flattening test, an electric resistance welded steel pipe including an electric resistance weld is cut to give a round test specimen that extends along the pipe axis direction and has a length in the pipe axis direction of 100 mm, and the test specimen is tested by the method described in JIS G 3441 (2021). As illustrated in Fig. 3, a flattening test specimen 8 is placed in such a manner that a line connecting an electric resistance weld 9 and the center 10 of the electric resistance welded steel pipe will be directed parallel to a compressing direction 12. The initial radius of curvature r of the outer surface of the bent portion of the flattening test specimen 8 is 1/2 of the outer diameter of the electric resistance welded steel pipe.
The hot-rolled steel sheet and the base metal material of the electric resistance welded steel pipe of the present invention preferably have a chemical composition including, 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, and
N: 0.0100% or less,

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, and
Sn: 0.100% or less,
the balance being Fe and incidental impurities.

In the present specification, "%" in the steel composition is "mass%" unless otherwise specified.

C: 0.020% or more and 0.200% or less

Carbon is an element that increases the strength of steel by solid solution strengthening. Furthermore, carbon enhances the hardenability of steel to increase the proportion of fine grains and to enhance toughness, thus contributing to suppressing the occurrence of cracks in a flattening test, and also increases the connectivity of fine grains to contribute to increasing the flattening resistance. In order to obtain these effects, it is preferable to add 0.020% or more carbon. The C content is more preferably 0.025% or more, and even more preferably 0.030% or more. The C content is most preferably 0.035% or more. When, however, the C content is more than 0.200%, hard pearlite, martensite, and austenite are excessively formed to cause a decrease in ductility, which leads to easy occurrence of cracks before a direct contact is reached in a C-shaped flattening test or a flattening test. Thus, the C content is preferably 0.200% or less. The C content is more preferably 0.180% or less, and even more preferably 0.170% or less. The C content is most preferably 0.165% or less.

Si: 0.50% or less

Silicon is an element that increases the strength of steel by solid solution strengthening. In order to obtain this effect, it is preferable to add 0.02% or more silicon. 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. When, however, the Si content is more than 0.50%, ductility is lowered and cracks tend to occur before a direct contact is reached in a flattening test. Thus, the Si content is preferably 0.50% or less. The Si content is more preferably 0.40% or less, and 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

Manganese is an element that increases the strength of steel by solid solution strengthening. Furthermore, manganese enhances the hardenability of steel to increase the proportion of fine grains and to enhance toughness, thus contributing to suppressing the occurrence of cracks in a flattening test, and also increases the connectivity of fine grains to contribute to increasing the flattening resistance. In order to obtain these effects, it is preferable to add 0.30% or more manganese. 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. When, however, the Mn content is more than 2.00%, hard pearlite, martensite, and austenite are excessively formed to cause a decrease in ductility, which leads to easy occurrence of cracks before a direct contact is reached in a flattening test. Thus, the Mn content is preferably 2.00% or less. The Mn content is more preferably 1.90% or less, and even more preferably 1.80% or less. The Mn content is most preferably 1.75% or less.

P: 0.050% or less

Phosphorus is segregated at grain boundaries and lowers toughness. Thus, phosphorus is regarded as an incidental impurity and is preferably removed as much as possible. The P content is preferably in the range of 0.050% and less. The P content is more preferably 0.040% or less, and even more preferably 0.030% or less. The P content is most preferably 0.020% or less. While the lower limit of the P content is not particularly specified, excessive dephosphorization increases the smelting costs and thus the P content is preferably 0.001% or more.

S: 0.0200% or less

In steel, sulfur is usually present as MnS. MnS is thinly elongated in the hot rolling process to adversely affect ductility and toughness. Thus, in the present invention, it is preferable to remove sulfur 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 even more preferably 0.0050% or less. The S content is most preferably 0.0030% or less. While the lower limit of the S content is not particularly specified, excessive desulfurization increases the smelting costs and thus the S content is preferably 0.0001% or more.

Al: 0.005% or more and 0.100% or less

Aluminum is an element that acts as a powerful deoxidizing agent. In order to obtain such an effect, it is preferable to add 0.005% or more aluminum. 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, more than 0.100% aluminum deteriorates weldability and forms an increased amount of alumina inclusions, thereby deteriorating surface properties. Thus, the Al content is preferably 0.100% or less. The Al content is more preferably 0.080% or less, and even more preferably 0.070% or less. The Al content is most preferably 0.065% or less.

N: 0.0100% or less

Nitrogen is an incidental impurity and is an element that acts to reduce ductility and toughness by strongly pinning the movement of dislocations. In the present invention, nitrogen is regarded as an impurity and is desirably removed as much as possible. However, up to 0.0100% nitrogen is acceptable. Thus, the N content is controlled 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 even more preferably 0.0065% or less. The N content is most preferably 0.0060% or less. While the lower limit is not particularly limited, excessive denitrification increases the refining costs and thus the N content is preferably 0.0010% or more.
The chemical composition of the electric resistance welded steel pipe and that of the hot-rolled steel sheet of the present invention may further include one, or two or more selected from Nb, V, Ti, Cu, Ni, Cr, Mo, Ca, B, Mg, Zr, REM, and Sn.

Nb: 0.080% or less

Niobium may be added as required. Niobium is an element that contributes to enhancing the strength of steel by forming fine carbides and nitrides in the steel, and further suppresses the coarsening of austenite during hot rolling, thereby contributing to reducing the size of microstructures. When niobium is added in order to obtain the above effects, it is preferable that the Nb content be 0.002% or more. The Nb content is more preferably 0.005% or more, and even more preferably 0.010% or more. The Nb content is most preferably 0.012% or more. However, more than 0.080% niobium deteriorates ductility and toughness. Thus, when niobium is added, the Nb content is controlled to 0.080% or less. The Nb content is more preferably 0.070% or less, and even more preferably 0.065% or less. The Nb content is most preferably 0.060% or less.

V: 0.080% or less

Vanadium may be added as required. Vanadium is an element that contributes to enhancing the strength of steel by forming fine carbides and nitrides in the steel. When vanadium is added in order to obtain the above effect, it is preferable that the V content be 0.002% or more. The V content is more preferably 0.005% or more, and even more preferably 0.010% or more. The V content is most preferably 0.015% or more. However, more than 0.080% vanadium deteriorates ductility and toughness. Thus, when vanadium is added, the V content is controlled to 0.080% or less. The V content is more preferably 0.070% or less, and even more preferably 0.065% or less. The V content is most preferably 0.060% or less.

Ti: 0.080% or less

Titanium may be added as required. Titanium is an element that contributes to enhancing the strength of steel by forming fine carbides and nitrides in the steel, and, due to its high affinity with nitrogen, also contributes to reducing the amount of solute nitrogen in steel. When titanium is added in order to obtain the above effects, it is preferable that the Ti content be 0.002% or more. The Ti content is more preferably 0.005% or more, and even more preferably 0.010% or more. The Ti content is most preferably 0.012% or more. However, more than 0.080% titanium deteriorates ductility and toughness. Thus, when titanium is added, the Ti content is controlled to 0.080% or less. The Ti content is more preferably 0.070% or less, and even more preferably 0.065% or less. The Ti content is most preferably 0.060% or less.

Cu: 0.50% or less, Ni: 0.50% or less

Copper and nickel may be added as required. Copper and nickel are elements that increase the strength of steel by solid solution strengthening. Furthermore, these elements increase the hardenability of steel and contribute to reducing the size of microstructures. When copper is added in order to obtain the above effects, it is preferable that the Cu content be 0.01% or more. The Cu content is more preferably 0.05% or more, and even more preferably 0.10% or more. When nickel is added in order to obtain the above effects, it is preferable that the Ni content be 0.01% or more. The Ni content is more preferably 0.05% or more, and even more preferably 0.10% or more. On the other hand, excessive addition may deteriorate ductility and toughness, and may lead to excessive formation of hard pearlite, martensite, and austenite. Thus, when copper is added, the Cu content is controlled to 0.50% or less. The Cu content is preferably 0.40% or less, and more preferably 0.30% or less. When nickel is added, the Ni content is controlled to 0.50% or less. The Ni content is preferably 0.40% or less, and more preferably 0.30% or less.

Cr: 0.50% or less, Mo: 0.50% or less

Chromium and molybdenum may be added as required. Chromium and molybdenum are elements that increase the hardenability of steel and contribute to reducing the size of microstructures. When chromium is added in order to obtain the above effects, it is preferable that the Cr content be 0.01% or more. The Cr content is more preferably 0.05% or more, and even more preferably 0.10% or more. When molybdenum is added in order to obtain the above effects, it is preferable that the Mo content be 0.01% or more. The Mo content is more preferably 0.05% or more, and even more preferably 0.10% or more.
On the other hand, excessive addition may lead to excessive formation of hard pearlite, martensite, and austenite.
Thus, when chromium is added, the Cr content is controlled to 0.50% or less. The Cr content is preferably 0.40% or less, and more preferably 0.30% or less. When molybdenum is added, the Mo content is controlled to 0.50% or less. The Mo content is preferably 0.40% or less, and more preferably 0.30% or less.

Ca: 0.0050% or less

Calcium may be added as required. Calcium is an element that contributes to enhancing the toughness of steel by spheroidizing sulfides, such as MnS, that are thinly elongated in the hot rolling process. When calcium is added in order to obtain the above effect, it is preferable that the Ca content be 0.0005% or more. The Ca content is more preferably 0.0008% or more, and even more preferably 0.0010% or more. The Ca content is most preferably 0.0015% or more. When, however, the Ca content is more than 0.0050%, calcium oxide clusters are formed in the steel to deteriorate toughness. Thus, when calcium is added, the Ca content is controlled to 0.0050% or less. The Ca content is preferably 0.0040% or less, and more preferably 0.0035% or less. The Ca content is even more preferably 0.0030% or less.

B: 0.0050% or less

Boron may be added as required. Boron is an element that lowers transformation start temperature and thereby contributes to reducing the size of microstructures. When boron is added in order to obtain the above effects, it is preferable that the B content be 0.0002% or more. The B content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The B content is most preferably 0.0010% or more. However, more than 0.0050% boron deteriorates ductility and toughness. Thus, when boron is added, the B content is controlled to 0.0050% or less. The B content is more preferably 0.0040% or less, and even more preferably 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

Magnesium, zirconium, and REM (rare earth metal(s)) may be added as required. These elements increase the strength of steel by reducing the size of grains. The Mg content may be 0%. When magnesium is added, the lower limit of the Mg content is preferably 0.0005% or more. The Mg content is more preferably 0.0008% or more. The Zr content may be 0%. When zirconium is added, the lower limit of the Zr content is preferably 0.0005% or more. The Zr content is more preferably 0.0008% or more. The REM content may be 0%. When REM is added, the lower limit of the REM content is preferably 0.0005% or more. The REM content is more preferably 0.0008% or more. On the other hand, excessive addition may cause an increase in yield ratio and an increase in logarithmic standard deviation of equivalent plastic strain distribution. Thus, when magnesium is added, the Mg content is controlled to 0.020% or less. The Mg content is preferably 0.010% or less. When zirconium is added, the Zr content is controlled to 0.020% or less. The Zr content is preferably 0.010% or less. When REM is added, the REM content is controlled to 0.020% or less. The REM content is preferably 0.010% or less. Here, REM is a general term for a total of 17 elements including scandium, yttrium, and the lanthanoid elements. One or more of these 17 elements may be added to the steel, and the REM content means the total content of these elements.

Sn: 0.100% or less

Tin is an element that suppresses decarburization resulting from nitridation or oxidation of the steel sheet surface and thereby prevents a decrease in strength. In order to obtain the above effects, it is preferable that the Sn content be 0.001% or more. The Sn content is more preferably 0.002% or more, and even more preferably 0.005% or more. However, excessive addition deteriorates the ductility and the toughness of steel. Thus, the Sn content is preferably controlled 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.
The balance is Fe and incidental impurities. Incidental impurities are impurities that are incidentally mixed in from raw materials, manufacturing processes, production facilities, and the like, and may be contained as long as they do not impair the object of the present invention. Examples of the incidental impurities in the balance include As, Sb, Bi, Co, Pb, Zn, O, Ta, W, Te, Hf, Ge, Sr, and Cs. Examples of the raw materials for steel sheets include iron ore, reduced iron, and scrap.
Preferably, the steel microstructure at half the sheet thickness of the hot-rolled steel sheet, and the steel microstructure at half the wall thickness of the base metal material of the electric resistance welded steel pipe of the present invention are such that the average grain size of grains observed as regions surrounded by a high-angle grain boundary is 15.0 µm or less, the area fraction of 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 the grains, the connectivity of fine grains calculated from expression (3) below is 0.05 or more and 0.50 or less, the volume fraction of bainite is 10% or more, the total volume fraction of ferrite and bainite is 80% or more, and the balance is one, or two or more selected from pearlite, martensite, and austenite representing a volume fraction of 20% or less in total. (Connectivity of fine grains) = (total length of high- angle grain boundaries of grains having a grain size smaller than the average grain size)/(total length of high-angle grain boundaries) wherein the numerator on the right side of expression (3) does not include the length of high-angle grain boundaries between a grain smaller than the average grain size and a grain equal to or larger than the average grain size.
The average grain size, the area fraction of grains having a grain size equal to or smaller than the average grain size, and the connectivity of fine grains are measured by the SEM/EBSD method. The measurement region is 500 µm × 500 µm, and the measurement step size is 0.5 µm. Values measured from at least five fields of view are averaged. The EBSD data obtained are analyzed with crystal orientation analysis software OIM Analysis (trademark) to determine the distribution of grain boundaries and the distribution of grain sizes assuming that boundaries with a misorientation of 15° or more are grain boundaries (high-angle grain boundaries). The average grain size is calculated as the diameter of a circle that has an area equal to the total of the measured areas divided by the number of grains (the equivalent circular diameter). To determine the connectivity of fine grains, calculations are made to obtain the total length of high-angle grain boundaries in regions excluding grains having a grain size equal to or larger than the average grain size (that is, high-angle grain boundaries of only grains having a grain size smaller than the average grain size), and the total length of all the high-angle grain boundaries, and the ratio of the former to the latter is calculated. Incidentally, the high-angle grain boundaries in regions excluding grains having a grain size equal to or larger than the average grain size do not include high-angle grain boundaries between a grain smaller than the average grain size and a grain equal to or larger than the average grain size. In the calculation of the average grain size, the area fraction of grains having a grain size equal to or smaller than the average grain size, and the connectivity of fine grains, grains with a grain size of 1.0 µm or less are excluded as a measurement noise.
A large average grain size comes with low toughness and facilitates the occurrence of cracks in a flattening test. In addition, the value of (normalized load/normalized displacement) may be lowered. Thus, it is preferable that the average grain size be 15.0 µm or less. The average grain size is more preferably 12.0 µm or less, and even more preferably 10.0 µm or less. The average grain size is most preferably 9.0 µm or less. A small average grain size lowers ductility and facilitates the occurrence of cracks in a flattening test. Thus, it is preferable that the average grain size be 2.0 µm or more. The average grain size is more preferably 3.0 µm or more, and even more preferably 3.5 µm or more.
When the area fraction of grains having a grain size equal to or smaller than the average grain size is low, the connectivity of fine grains equal to or smaller than the average grain size is lowered and the flattening resistance may become low. Furthermore, toughness is lowered and cracks occur easily in a flattening test. Thus, the above area fraction is preferably 10% or more. The area fraction is more preferably 12% or more, and even more preferably 15% or more. When the area fraction is more than 50%, the connectivity of fine grains is so increased that ductility is lowered and cracks occur easily in a flattening test. Thus, the area fraction is preferably 50% or less. The area fraction is more preferably 45% or less, and even more preferably 40% or less. The area fraction is most preferably 35% or less.
When the connectivity of fine grains is low, coarse grains are so highly connected to one another that strains in the coarse grains tend to be connected to one another. Because coarse grains are soft and relatively large strains occur within them, connection of such coarse grains produces stress concentration and may result in low flattening resistance. Thus, the connectivity of fine grains is preferably 0.05 or more, and more preferably 0.10 or more. The connectivity of fine grains is even more preferably 0.11 or more. The connectivity of fine grains is most preferably 0.12 or more. When the connectivity of fine grains is high, hard fine grains act as parent phases to cause a decrease in ductility and facilitate the occurrence of cracks in a flattening test. Thus, the connectivity of fine grains is preferably 0.50 or less. The connectivity of fine grains is more preferably 0.47 or less, and even more preferably 0.40 or less. The connectivity of fine grains is most preferably 0.35 or less.
Ferrite is a soft microstructure. Bainite is a microstructure that is harder than ferrite and softer than pearlite, martensite, and austenite. From the points of view of strength, ductility, and toughness, microstructure control of bainite is important.
When the volume fraction of bainite is low, soft ferrite represents a high proportion and strength is lowered. In addition, the value of (normalized load/normalized displacement) may be lowered. Thus, it is preferable that the volume fraction of bainite be 10% or more. The volume fraction of bainite is more preferably 12% or more, and even more preferably 20% or more. The volume fraction of bainite is most preferably 25% or more. The upper limit is not particularly limited but is preferably 75% or less to avoid a decrease in ductility.
When the total volume fraction of ferrite and bainite is low, hard pearlite, martensite, and austenite represent a high proportion to cause a decrease in ductility and a decrease in toughness. Thus, it is preferable that the total volume fraction of ferrite and bainite be 80% or more. The total volume fraction of ferrite and bainite is more preferably 85% or more, and even more preferably 88% or more.
For the reason described above, the total volume fraction of hard pearlite, martensite, and austenite is preferably 20% or less.
On the other hand, ductility is lowered when the total of hard pearlite, martensite, and austenite represents less than 1%. Thus, it is preferable that the total volume fraction of ferrite and bainite be 99% or less. The total volume fraction is more preferably 98% or less, and even more preferably 97% or less.
The above-described microstructures except austenite nucleate from austenite grain boundaries or deformation bands within austenite grains. Hot rolling that applies a large amount of rolling reduction at low temperatures where austenite recrystallization is difficult to occur can introduce a large number of dislocations into austenite to reduce the size of austenite grains and can introduce a large amount of deformation bands within the grains. Consequently, the treatment increases the area of nucleation sites and thus increases the nucleation frequency, making it possible to reduce the size of steel microstructures.
Here, the steel microstructures can be observed as follows. First, a test specimen for microstructure observation is sampled in such a manner that the observation surface will be a cross section that is parallel to both the rolling direction and the sheet thickness direction of the hot-rolled steel sheet and is at half the sheet thickness or will be a cross section that is parallel to both the axis direction and the wall thickness direction of the electric resistance welded steel pipe and is at half the wall thickness. The test specimen is then polished and is etched with Nital. The microstructures at half the sheet thickness (or wall thickness) are observed and photographed with an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times). Next, the optical microscopic images and the SEM images obtained are analyzed to determine the area fractions of bainite and the remaining microstructures (ferrite, pearlite, martensite, austenite). The area fraction of each microstructure is calculated as the average of values obtained by observation of at least five fields of view. In the present invention, the area fractions obtained by microstructure observation are taken as the same as the volume fractions of the respective microstructures. The microstructures are identified according to the following facts.
Ferrite is a product of diffusion transformation and appears as a microstructure in which dislocations are substantially recovered and have a low density. Ferrite includes polygonal ferrite and quasi-polygonal ferrite. Regions that do not show cementite and are free from lath substructures when observed with an optical microscope or SEM are identified as ferrite.
Bainite is a multiphase microstructure containing lath ferrite with a high dislocation density, and cementite. Regions that contain dispersed cementite or show lath substructures when observed with an optical microscope or SEM are identified as bainite.
Pearlite is a eutectoid microstructure of iron and iron carbide (ferrite + cementite), and appears as a lamellar microstructure composed of alternating layers of ferrite and cementite. Regions that show the above characteristics on SEM are identified as pearlite.
Martensite is a lath low-temperature transformation microstructure having a very high dislocation density. In SEM images, martensite appears bright compared to ferrite or bainite.
In optical microscopic images and SEM images, martensite and austenite are difficult to distinguish from each other. Thus, the area fractions of microstructures identified as martensite or austenite in the obtained SEM images are combined, and the volume fraction of austenite measured by the method described below is subtracted from the combined value to give the volume fraction of martensite.
Austenite is fcc phase. The volume fraction of austenite is measured by X-ray diffractometry of a test specimen prepared in the same manner as the test specimen for the measurement of dislocation density. The volume fraction of austenite is determined from the integrated intensities of (200), (220), and (311) planes of fcc iron, and (200) and (211) planes of bcc iron.
Next, a method for manufacturing the hot-rolled steel sheet and a method for manufacturing the electric resistance welded steel pipe according to an embodiment of the present invention will be described.
For example, the hot-rolled steel sheet of the present invention is manufactured as follows, but the manufacturing conditions are not particularly limited thereto. A steel material having the chemical composition described hereinabove is heated to a heating temperature of 1100°C or above and 1300°C or below and is subsequently subjected to hot rolling in which the average cooling rate at temperatures from 900°C to 1100°C is 0.5°C/s or more and 3.0°C/s or less, the finish rolling delivery temperature is 750°C or above and 850°C or below, and the total rolling reduction in the finish rolling is 45% or more and 75% or less. Next, the steel sheet is cooled in such a manner that the average cooling rate at half the sheet thickness from the finish rolling delivery temperature to a cooling stop temperature is 5°C/s or more and 40°C/s or less, the minimum cooling rate from the completion of the finish rolling to the end of cooling is 2°C/s or more, the cooling stop temperature is 400°C or above and 650°C or below, the duration of air-cooling during the process from the completion of the finish rolling to the end of cooling is 15 seconds or less, and the total air-cooling time is 50 seconds or less. The steel sheet is then coiled into a coil.
The electric resistance welded steel pipe of the present invention is manufactured by cold roll-forming the hot-rolled steel sheet into a cylindrical shape and welding the ends by electric resistance welding.
In the following description of the manufacturing method, the temperature "°C" indicates the surface temperature of a steel material or a steel sheet (a hot-rolled sheet) unless otherwise specified. These surface temperatures may be measured with, for example, a radiation thermometer. The temperature at half the thickness of a steel sheet may be determined by calculating the temperature distribution in a cross section of the steel sheet by heat transfer analysis and correcting the results with the surface temperature of the steel sheet. The term "hot-rolled steel sheet" includes hot-rolled sheet and hot-rolled steel strip.
In the present invention, the steel material (a steel slab) may be melted by any method without limitation. Any known melting methods, such as a converter, an electric arc furnace, or a vacuum melting furnace, may be adopted. The casting method is not particularly limited, and the steel slab may be produced with desired dimensions by a known casting method, such as a continuous casting method. In place of the continuous casting method, an ingot making-blooming method may be applied without any problems. The molten steel may be further subjected to secondary refining, such as ladle refining.
Next, the steel material (the steel slab) obtained is heated, hot-rolled, cooled, and coiled into a coil of the hot-rolled steel sheet.
When the heating temperature is low, the rolling workpiece exhibits high deformation resistance and is difficult to roll. Thus, the heating temperature is preferably 1100°C or above, more preferably 1120°C or above, even more preferably 1130°C or above, and most preferably 1150°C or above. When, on the other hand, the heating temperature is high, austenite grains are coarsened and fine austenite grains cannot be obtained in the subsequent rolling (rough rolling, finish rolling), resulting in a final product having a large average grain size. Thus, the heating temperature in the hot rolling step is preferably 1300°C or below, more preferably 1280°C or below, even more preferably 1270°C or below, and most preferably 1250°C or below.
In the present invention, the steel slab (the slab) that has been manufactured may be treated in the conventional manner, that is, may be cooled to room temperature and subsequently reheated. Alternatively, an energy-saving hot direct rolling process may be applied without problems in which the hot slab is introduced into the heating furnace without being cooled to room temperature or is rolled immediately after short hot idling.
When the average cooling rate at temperatures from 900°C to 1100°C is low, austenite is coarsened and the final product has an increased average grain size. Thus, the average cooling rate at temperatures from 900°C to 1100°C is preferably 0.5°C/s or more, more preferably 0.8°C/s or more, even more preferably 0.9°C/s or more, and most preferably 1.0°C/s or more. When, on the other hand, the average cooling rate is high, the recrystallization of austenite is insufficient and leaves coarse austenite; the steel microstructures in the final product will contain coarse grains and consequently the area fraction of grains having a grain size equal to or smaller than the average grain size is lowered. Thus, the average cooling rate at temperatures from 900°C to 1100°C is preferably 3.0°C/s or less, more preferably 2.5°C/s or less, even more preferably 2.4°C/s or less, and most preferably 2.2°C/s or less.
When the finish rolling delivery temperature is low, the surface temperature of the steel sheet falls to or below the ferrite start temperature during the finish rolling and a large amount of deformed ferrite is formed to cause a decrease in ductility. Thus, the finish rolling delivery temperature is preferably 750°C or above, more preferably 770°C or above, and even more preferably 780°C or above. When, on the other hand, the finish rolling delivery temperature is high, fine austenite grains cannot be obtained and the average grain size is increased. Thus, the finish rolling delivery temperature is preferably 850°C or below, more preferably 830°C or below, and even more preferably 820°C or below.
When the total rolling reduction in the finish rolling is low, sufficient working strains cannot be introduced at the hot rolling step and the final product has an increased average grain size. Thus, the total rolling reduction in the finish rolling is preferably 45% or more. The total rolling reduction in the finish rolling is more preferably 50% or more. The total rolling reduction in the finish rolling is even more preferably 52% or more. The total rolling reduction in the finish rolling is most preferably 54% or more. When, on the other hand, the total rolling reduction in the finish rolling is high, the average grain size is reduced and the area fraction of grains having a grain size equal to or smaller than the average grain size is increased. Thus, the total rolling reduction in the finish rolling is preferably 75% or less, and more preferably 70% or less. The total rolling reduction in the finish rolling is even more preferably 68% or less. The total rolling reduction in the finish rolling is most preferably 66% or less.
The total rolling reduction in the finish rolling is the total of the rolling reductions of the rolling passes in the finish rolling.
From the points of view of ensuring the required rolling reduction and controlling the steel sheet temperature, the finish sheet thickness is preferably 5 mm or more. The finish sheet thickness is more preferably 6 mm or more, and even more preferably 7 mm or more. The finish sheet thickness is preferably 40 mm or less. The finish sheet thickness is more preferably 35 mm or less, and even more preferably 30 mm or less.
After the hot rolling, the hot-rolled sheet is cooled.
When the average cooling rate at half the sheet thickness from the finish rolling delivery temperature to the cooling stop temperature is low, the microstructures are coarsened and the final product has an increased average grain size. Furthermore, the bainite fraction is lowered. Thus, the average cooling rate is preferably 5°C/s or more, more preferably 10°C/s or more, even more preferably 12°C/s or more, and most preferably 15°C/s or more. When, on the other hand, the average cooling rate at half the sheet thickness is high, the martensite fraction is increased to cause a decrease in ductility. Thus, the average cooling rate is preferably 40°C/s or less, more preferably 35°C/s or less, even more preferably 33°C/s or less, and most preferably 30°C/s or less.
When the minimum cooling rate at half the sheet thickness from the completion of the finish rolling to the end of cooling is low, the microstructures are coarsened and the average grain size is increased. Furthermore, the area fraction of grains having a grain size equal to or smaller than the average grain size is lowered. Furthermore, the bainite fraction is lowered. Thus, the minimum cooling rate is preferably 2°C/s or more, more preferably 3°C/s or more, even more preferably 4°C/s or more, and most preferably 5°C/s or more. While the upper limit is not particularly limited, the minimum cooling rate is preferably 15°C/s or less, more preferably 12°C/s or less, even more preferably 10°C/s or less, and most preferably 8°C/s or less. The time from the completion of the finish rolling to the end of cooling is divided into three seconds long sections, the average cooling rate in each section is calculated, and the minimum value among the average cooling rates is obtained as the minimum cooling rate.
When the duration of air-cooling during the process from the completion of the finish rolling to the end of cooling is long, ferrite and bainite grains are excessively grown and the microstructures are coarsened to increase the average grain size of the final product. Furthermore, the area fraction of grains having a grain size equal to or smaller than the average grain size is lowered. Furthermore, the bainite fraction may be lowered. Thus, the duration of air-cooling is preferably 15 seconds or less, more preferably 14 seconds or less, even more preferably 12 seconds or less, and most preferably 11 seconds or less. A shorter duration of air-cooling is more preferable. However, shortening the duration of air-cooling to less than 6 seconds produces less effects on the size reduction and only increases the facility load. Thus, the duration of air-cooling is preferably 6 seconds or more, more preferably 7 seconds or more, and even more preferably 8 seconds or more. Between the completion of the finish rolling and the end of cooling, the steel sheet is cooled by air cooling and water cooling.
When the total air-cooling time from the completion of the finish rolling to the end of cooling is long, ferrite and bainite grains are excessively grown and the microstructures are coarsened to increase the average grain size of the final product. Furthermore, the area fraction of grains having a grain size equal to or smaller than the average grain size is lowered. Furthermore, the bainite fraction may be lowered. Thus, the total air-cooling time is preferably 50 seconds or less, more preferably 45 seconds or less, even more preferably 40 seconds or less, and most preferably 38 seconds or less. A shorter duration of air-cooling is more preferable. However, shortening the total air-cooling time to less than 10 seconds produces less effects on the size reduction and only increases the facility load. Thus, the total air-cooling time is preferably 10 seconds or more, more preferably 12 seconds or more, and even more preferably 15 seconds or more.
When the cooling stop temperature is low, the martensite fraction is increased to cause a decrease in ductility. Thus, the cooling stop temperature is preferably 400°C or above, more preferably 420°C or above, even more preferably 450°C or above, and most preferably 470°C or above. When, on the other hand, the cooling stop temperature is high, the bainite fraction is lowered. Thus, the cooling stop temperature is preferably 650°C or below, more preferably 620°C or below, even more preferably 600°C or below, and most preferably 580°C or below.
In the electric resistance welded steel pipe of the present invention, it is preferable that the solidified melt zone of the weld (the electric resistance weld) have a width in the circumferential direction of 1 µm or more throughout the entire wall thickness of the pipe. This advantageously reduces the density of inclusions in the weld. It is also preferable that the width of the solidified melt zone of the weld (the electric resistance weld) in the circumferential direction be 1000 µm or less throughout the entire wall thickness of the pipe.
The outer diameter of the electric resistance welded steel pipe is preferably 80 mm or more. The outer diameter is preferably 800 mm or less. The wall thickness of the electric resistance welded steel pipe is preferably 3 mm or more. The wall thickness is preferably 40 mm or less.
Here, an etching solution may be selected appropriately in accordance with the steel components and the type of steel pipe. Fig. 4 is a schematic view of a circumferential cross section of the electric resistance weld of the electric resistance welded steel pipe. In the cross section after etching, as schematically illustrated in Fig. 4, a solidified melt zone 15 can be recognized as a region differing in microstructures and contrast from a base metal material 13 and a heat-affected zone 14 in Fig. 4. For example, a solidified melt zone 15 of an electric resistance welded steel pipe made of carbon steel or low-alloy steel can be identified as a region that appears white in a Nital-etched cross section under an optical microscope. A solidified melt zone 15 of a UOE steel pipe made of carbon steel or low-alloy steel can be identified as a region containing cellular or dendritic solidified microstructures in a Nital-etched cross section under an optical microscope.

EXAMPLES

Hereinbelow, the present invention will be described in greater detail based on Examples. The present invention is not limited to the following Examples.
Molten steels having a chemical composition described in Table 1 were melted and cast into slabs (steel materials). The slabs obtained were subjected to a hot rolling step and a cooling step under conditions described in Table 2, and further to a coiling step. Hot-rolled steel sheets with a finish sheet thickness (mm) described in Table 2 were thus obtained.
After the coiling step, the hot-rolled steel sheets were roll-formed into cylindrical round steel pipes. The butt joints were electric resistance welded. Subsequently, the pipes were reduced in diameter using rolls arranged above, below, left, and right of the round steel pipe. Thus, electric resistance welded steel pipes having an outer diameter (mm) and a wall thickness (mm) described in Table 4 were obtained.
Test specimens were sampled from the hot-rolled steel sheets and the electric resistance welded steel pipes described in Tables 3 and 4, respectively, and were subjected to the following C-shaped flattening test, flattening test, measurement of the average grain size, measurement of the area fraction of grains having a grain size equal to or smaller than the average grain size, measurement of the connectivity of fine grains, and observation of microstructures. The test specimens of the hot-rolled steel sheets were sampled from half the sheet thickness at the center in the width direction, and those of the electric resistance welded steel pipes were sampled from half the sheet thickness of the base metal material 90° away from the electric resistance weld in the circumferential direction.

[C-shaped flattening test]

The hot-rolled steel sheet was cut to give a sheet material (a test specimen) having the whole thickness, a width of 50 × t (t: sheet thickness), and a length of 100 mm. This sampling was performed in such a manner that the long side direction of the test specimen would be the width direction of the hot-rolled steel sheet. Next, the sheet material was bent into a C-shaped test specimen by the press bending method described in JIS Z 2248 (2006), and a C-shaped flattening test was performed by the method described in JIS G 3441 (2021). In the press bending, the inside radius of the tip of the pressing element was 9 × t. The initial radius of curvature r of the outer surface of the bent portion of the flattening test specimen was determined by adding the sheet thickness to the inside radius of the tip of the pressing element in the above press bending, as shown in expression (4): $r = 10 \times t$
The test specimen was determined as cracked when the test specimen in a direct contact had a crack of 0.50 mm or longer on the outer surface of the bent portion, and was determined as free from cracks when there was no such cracking on the outer surface of the bent portion.

[Flattening test]

The electric resistance welded steel pipe including an electric resistance weld was cut to give a round test specimen that extended along the pipe axis direction and had a length of 100 mm, and a flattening test was performed by the method described in JIS G 3441 (2021).
As illustrated in Fig. 3 , the test specimen was placed in such a manner that the weld would be directed in the compressing direction. The initial radius of curvature r of the outer surface of the bent portion of the flattening test specimen was 1/2 of the outer diameter of the electric resistance welded steel pipe. The test specimen was determined as cracked when the pipe in a direct contact had a crack of 0.50 mm or longer on the outer surface, and was determined as free from cracks when there was no such cracking on the outer surface of the pipe.

[Measurement of the average grain size]

A test specimen for measurement was sampled in such a manner that the measurement surface would be a cross section parallel to both the rolling direction and the sheet thickness direction of the hot-rolled steel sheet or would be a cross section parallel to both the axis direction and the wall thickness direction of the electric resistance welded steel pipe. The cross section was mirror-polished and was analyzed by the SEM/EBSD method. The misorientation between adjacent grains was determined, and boundaries with a misorientation of 15° or more were taken as grain boundaries. The grain size was measured assuming that a region surrounded by a boundary with 15° or more misorientation was a single grain. The grain sizes determined based on the grain boundaries were arithmetically averaged to give the average grain size. The acceleration voltage was 15 kV, the measurement region was 500 µm × 500 µm, and the measurement step size was 0.5 µm. Values measured from at least five fields of view were averaged. The EBSD data obtained were analyzed with crystal orientation analysis software OIM Analysis (trademark) to determine the distribution of grain boundaries and the distribution of grain sizes assuming that boundaries with a misorientation of 15° or more were grain boundaries (high-angle grain boundaries). The grain size and the average grain size were calculated as the diameter of a circle that had an area equal to the total of the measured areas divided by the number of grains (the equivalent circular diameter). In the calculation of the average grain size, grains with a grain size of 1.0 µm or less were excluded as a measurement noise.

[Measurement of the area fraction of grains having a grain size equal to or smaller than the average grain size]

The area fraction was determined from the average grain size and the grain size distribution obtained above. The area of each grain having a grain size equal to or smaller than the average grain size was calculated from its equivalent circular diameter. The total area of grains having a grain size equal to or smaller than the average grain size was calculated and was divided by the area of the measurement regions. In the calculation of the area fraction of grains having a grain size equal to or smaller than the average grain size, grains with a grain size of 1.0 µm or less were excluded as a measurement noise.

[Connectivity of fine grains]

To determine the connectivity of fine grains, calculations were made to obtain the total length of high-angle grain boundaries in regions excluding grains having a grain size equal to or larger than the average grain size, and the total length of high-angle grain boundaries of all the grains, and the ratio of the former to the latter was calculated. In the numerator on the right side of expression (3) described hereinabove, the total length of high-angle grain boundaries of grains having a grain size smaller than the average grain size does not include the length of high-angle grain boundaries between a grain smaller than the average grain size and a grain equal to or larger than the average grain size.
In the calculation of the connectivity of fine grains, grains with a grain size of 1.0 µm or less were excluded as a measurement noise.

[Observation of microstructures]

A test specimen for microstructure observation was sampled in such a manner that the observation surface would be a cross section parallel to both the rolling direction and the sheet thickness direction of the hot-rolled steel sheet or would be a cross section parallel to both the axis direction and the wall thickness direction of the electric resistance welded steel pipe. The cross section was then mirror-polished and was etched with Nital. The microstructures at half the sheet thickness of the hot-rolled steel sheet or at half the wall thickness of the electric resistance welded steel pipe were observed and photographed with an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times). In the SEM observation, the acceleration voltage was 15 kV. The optical microscopic images and the SEM images obtained were analyzed to determine the area fractions of bainite and the remaining microstructures (ferrite, pearlite, martensite, austenite). The area fraction of each microstructure was calculated as the average of values obtained by observation of at least five fields of view. Here, the area fractions obtained by the microstructure observation were taken as the same as the volume fractions of the respective microstructures.
Ferrite, pearlite, and martensite were identified from the microstructures as described in the embodiments.
The volume fraction of austenite was measured by X-ray diffractometry. The hot-rolled steel sheet and the electric resistance welded steel pipe were ground so that the center of the sheet thickness of the hot-rolled steel sheet and the center of the wall thickness of the electric resistance welded steel pipe would be diffraction surfaces. These surfaces were chemically polished to remove the worked surface layer. Test specimens for measurement at the center of the sheet thickness of the hot-rolled steel sheet and the center of the wall thickness of the electric resistance welded steel pipe were thus prepared. Mo-Kα radiation was used for the measurement. The volume fraction of austenite was determined from the integrated intensities of (200), (220), and (311) planes of fcc iron, and (200) and (211) planes of bcc iron.

The results obtained are described in Tables 3 and 4. The balance in the chemical composition is Fe and incidental impurities.

[Table 2]

Nos.	Heating step	Hot rolling step			Cooling step					Finish sheet thickness (mm)
Nos.	Heating temperature (°C)	Average cooling rate at temperatures from 900°C to 1100°C (°C/s)	Finish rolling delivery temperature (°C)	Total rolling reduction in finish rolling (%)	Average cooling rate (°C/s)	Minimum cooling rate (°C/s)	Duration of air-cooling (s)	Total air-cooling time (s)	Cooling stop temperature (°C)	Finish sheet thickness (mm)
1	1180	1.8	760	61	16	4	6	19	540	20
2	1180	2.7	830	57	22	3	16	43	620	16
3	1180	0.7	790	63	8	1	11	32	530	9
4	1200	0.9	810	56	9	6	8	21	510	28
5	1200	1.3	770	68	14	4	13	53	640	17
6	1230	1.0	770	49	23	4	7	16	470	25
7	1150	1.5	830	60	38	3	9	38	380	12
8	1180	1.1	820	73	37	5	6	25	600	6
9	1200	2.4	800	58	18	4	7	30	570	22
10	1150	1.2	840	77	16	2	12	15	450	16
11	1200	1.9	790	55	16	4	11	33	550	16
12	1200	2.2	780	50	24	3	8	22	560	16
13	1320	2.9	820	62	22	5	13	15	630	25
14	1200	1.5	780	66	44	5	5	24	380	12
15	1160	1.1	840	48	30	1	19	37	560	16
16	1250	2.9	800	79	35	5	14	45	410	12
17	1200	1.5	810	49	24	6	7	18	530	16
18	1180	2.9	820	73	9	5	11	45	480	22
19	1280	0.6	780	55	13	9	9	32	550	25
20	1200	1.7	770	59	37	7	13	40	500	16
21	1190	2.3	820	72	32	7	7	21	550	19
22	1210	2.0	810	50	27	5	5	20	490	25
23	1220	0.9	810	58	11	9	14	18	620	28
24	1170	2.0	820	58	19	5	7	39	610	22

[Table 3]

Steel sheets Nos.	Steel microstructure in hot-rolled steel sheet							Results of C-shaped flattening test of hot-rolled steel sheet		Remarks
Steel sheets Nos.	Average grain size (µm)	Area fraction of grains having a grain size equal to or smaller than the average grain size (%)	Connectivity of fine grains	F fraction (%)	B fraction (%)	F+B fraction (%)	Remaining microstructures	Occurrence of cracking until a direct contact is reached	Normalized load/normalized displacement (MPa)	Remarks
1	6.0	17	0.28	23	74	97	M	Absent	243	Inv. Ex.
2	8.9	22	0.15	83	8	91	P	Absent	81	Comp. Ex.
3	15.2	18	0.29	65	23	88	P	Present	96	Comp. Ex.
4	11.9	46	0.08	78	12	90	P,M	Absent	244	Inv. Ex.
5	8.8	19	0.35	61	23	84	P	Absent	77	Comp. Ex.
6	5.4	12	0.47	17	64	81	P,M	Absent	346	Inv. Ex.
7	4.9	33	0.30	15	63	78	M,A	Present	291	Comp. Ex.
8	8.1	35	0.11	82	3	85	P	Absent	118	Inv. Ex.
9	5.8	23	0.45	48	44	92	M,A	Absent	169	Inv. Ex.
10	6.3	53	0.53	45	50	95	M	Present	225	Comp. Ex.
11	5.8	26	0.16	43	48	91	P	Absent	188	Inv. Ex.
12	5.5	18	0.11	34	61	95	P,M	Absent	194	Inv. Ex.
13	15.4	32	0.23	85	13	98	M	Absent	112	Inv. Ex.
14	4.2	43	0.30	6	69	75	M	Absent	105	Inv. Ex.
15	6.1	7	0.24	48	36	84	M	Absent	154	Inv. Ex.
16	5.1	44	0.52	46	41	87	M	Absent	129	Inv. Ex.
17	5.3	17	0.18	36	54	90	M	Absent	286	Inv. Ex.
18	4.8	30	0.16	82	15	97	M	Absent	172	Inv. Ex.
19	5.4	18	0.37	70	19	89	P,M	Absent	181	Inv. Ex.
20	5.7	25	0.40	35	58	93	P,M	Absent	128	Inv. Ex.
21	6.9	41	0.22	41	49	90	M	Absent	201	Inv. Ex.
22	5.7	32	0.16	44	39	83	M	Absent	182	Inv. Ex.
23	7.8	40	0.31	44	51	95	P,M	Absent	281	Inv. Ex.
24	4.9	32	0.27	70	25	95	P,M	Absent	271	Inv. Ex.

· F fraction: volume fraction of ferrite, B fraction: volume fraction of bainite, F+B fraction: total volume fraction of ferrite and bainite.
. In the remaining microstructures, P: pearlite, M: martensite, A: austenite.
· The underlines indicate being outside the range of the present invention.

[Table 4]

Steel pipes Nos.	Dimensions of electric resistance welded steel pipe		Steel microstructure of base metal material of electric resistance welded steel pipe							Results of flattening test of electric resistance welded steel pipe		Remarks
Steel pipes Nos.	Outer diameter (mm)	Wall thickness (mm)	Average grain size (µm)	Area fraction of grains having a grain size equal to or smaller than the average grain size (%)	Connectivity of fine grains	F fraction (%)	B fraction (%)	F+B fraction (%)	Remaining microstructures	Occurrence of cracking until a direct contact is reached	Normalized load/normalized displacement (MPa)	Remarks
1	610	20	5.9	20	0.29	21	73	94	M	Absent	166	Inv. Ex.
2	500	16	8.7	16	0.11	81	9	90	P	Absent	94	Comp. Ex.
3	250	9	15.4	25	0.36	68	20	88	P	Present	91	Comp. Ex.
4	600	28	12.0	44	0.07	79	14	93	P,M	Absent	335	Inv. Ex.
5	500	17	8.7	8	0.33	60	25	85	P	Absent	80	Comp. Ex.
6	550	25	5.5	13	0.48	19	64	83	P,M	Absent	269	Inv. Ex.
7	500	12	5.0	37	0.31	16	61	77	M,A	Present	297	Comp. Ex.
8	150	6	8.2	38	0.15	77	4	81	P	Absent	241	Inv. Ex.
9	400	22	6.0	22	0.42	41	49	90	M,A	Absent	158	Inv. Ex.
10	500	16	6.0	51	0.54	52	45	97	M	Present	202	Comp. Ex.
11	500	16	5.7	27	0.22	44	48	92	P	Absent	303	Inv. Ex.
12	550	16	5.4	13	0.15	32	64	96	P,M	Absent	208	Inv. Ex.
13	600	25	15.3	34	0.21	86	12	98	M	Absent	120	Inv. Ex.
14	400	12	4.2	40	0.30	9	67	76	M	Absent	123	Inv. Ex.
15	500	16	6.0	8	0.25	48	40	88	M	Absent	124	Inv. Ex.
16	500	12	5.2	44	0.51	44	44	88	M	Absent	142	Inv. Ex.
17	500	16	5.3	20	0.16	42	50	92	M	Absent	292	Inv. Ex.
18	610	22	5.0	31	0.16	86	13	99	M	Absent	249	Inv. Ex.
19	610	25	5.4	20	0.39	76	15	91	P,M	Absent	291	Inv. Ex.
20	550	16	5.6	20	0.38	41	55	96	P,M	Absent	214	Inv. Ex.
21	600	19	7.0	44	0.24	42	46	88	M	Absent	207	Inv. Ex.
22	600	25	5.8	35	0.14	42	39	81	M	Absent	203	Inv. Ex.
23	610	28	7.4	37	0.33	44	52	96	P,M	Absent	241	Inv. Ex.
24	550	22	5.2	32	0.25	72	27	99	P,M	Absent	255	Inv. Ex.

In Tables 3 and 4, the hot-rolled steel sheets and the electric resistance welded steel pipes Nos. 1, 4, 6, 8, 9, and 11 to 24 represent Inventive Examples, and the hot-rolled steel sheets and the electric resistance welded steel pipes Nos. 2, 3, 5, 7, and 10 represent Comparative Examples.
In the C-shaped flattening test, all the hot-rolled steel sheets of Inventive Examples were flattened into a direct contact without suffering a crack of 0.50 mm or longer, and achieved a value of (normalized load/normalized displacement) of 100 MPa or more when the normalized displacement was in the range of 0.20 to 0.30.
In the flattening test, all the electric resistance welded steel pipes of Inventive Examples were flattened into a direct contact without suffering a crack of 0.50 mm or longer, and achieved a value of (normalized load/normalized displacement) of 100 MPa or more when the normalized displacement was in the range of 0.20 to 0.30.

Reference Signs List

1 CURVE SHOWING CHANGES IN NORMALIZED LOAD WITH NORMALIZED DISPLACEMENT
2 ELASTIC ZONE
3 PLASTIC ZONE
4 CRACKED OR DIRECT CONTACT
5A TEST SPECIMEN
5 C-SHAPED FLATTENING TEST SPECIMEN (FLATTENING TEST SPECIMEN)
6 FLAT PLATE
7 COMPRESSING DIRECTION
8 FLATTENING TEST SPECIMEN
9 ELECTRIC RESISTANCE WELD
10 CENTER OF ELECTRIC RESISTANCE WELDED STEEL PIPE
11 FLAT PLATE
12 COMPRESSING DIRECTION
13 BASE METAL MATERIAL
14 HEAT-AFFECTED ZONE
15 SOLIDIFIED MELT ZONE
100 INITIAL LENGTH L OF TEST SPECIMEN
101 INITIAL SHEET THICKNESS t OF TEST SPECIMEN
102 ROLLING DIRECTION OF HOT-ROLLED STEEL SHEET

Claims

A hot-rolled steel sheet, wherein a C-shaped flattening test where a test specimen formed by bending the steel sheet into a U-shape is pressed between two flat plates shows that:
the test specimen is free from a crack of 0.50 mm or longer until a direct contact is reached where the inner surfaces of the bent test specimen are in contact with each other,
and

the ratio of a normalized load calculated from expression (1) below to a normalized displacement calculated from expression (2) below is 100 MPa or more when the normalized displacement is in the range of 0.20 to 0.30, (Normalized load (MPa)) = (P/L) × (r/t²) × (1 - ((x₀ - x)/2r)²)^1/2 $(Normalized displacement) = (x_{0} - x) / 2 r$ where:
P: load (N),

L: initial length (mm) of the flattening test specimen,

r: initial radius of curvature (mm) of the outer surface of the bent portion of the flattening test specimen,

t: initial sheet thickness (mm) of the flattening test specimen,

x₀: initial distance (mm) between the two flat plates,

x: distance (mm) between the two flat plates.
The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet has a chemical composition comprising, 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, and

N: 0.0100% or less,
or further comprising one, or two or more selected from:
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, and

Sn: 0.100% or less,

the balance being Fe and incidental impurities.
The hot-rolled steel sheet according to claim 1 or 2, wherein a steel microstructure at half the sheet thickness is such that:
the average grain size of grains observed as regions surrounded by a high-angle grain boundary is 15.0 µm or less,

the area fraction of 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 the grains,

the connectivity of fine grains calculated from expression (3) below is 0.05 or more and 0.50 or less,

the volume fraction of bainite is 10% or more,

the total volume fraction of ferrite and bainite is 80% or more, and

the balance is one, or two or more selected from pearlite, martensite, and austenite representing a volume fraction of 20% or less in total, (Connectivity of fine grains) = (total length of high-angle grain boundaries of grains having a grain size smaller than the average grain size)/(total length of high-angle grain boundaries) wherein the numerator on the right side of expression (3) does not include the length of high-angle grain boundaries between a grain smaller than the average grain size and a grain equal to or larger than the average grain size.
An electric resistance welded steel pipe comprising a base metal material and an electric resistance weld, wherein a flattening test where a flattening test specimen sampled from the electric resistance welded steel pipe is pressed between two flat plates shows that:
the test specimen is free from a crack of 0.50 mm or longer until a direct contact is reached where the inner surfaces of the flattening test specimen are in contact with each other, and

the ratio of a normalized load calculated from expression (1) below to a normalized displacement calculated from expression (2) below is 100 MPa or more when the normalized displacement is in the range of 0.20 to 0.30, (Normalized load (MPa)) = (P/L) × (r/t²) × (1 - ((x₀ - x)/2r)²)^1/2 $(Normalized displacement) = (x_{0} - x) / 2 r$ where:
P: load (N),

L: initial length (mm) of the flattening test specimen in the pipe axis direction,

r: initial radius of curvature (mm) of the outer surface of the bent portion of the flattening test specimen,

t: initial sheet thickness (mm) of the flattening test specimen,

x₀: initial distance (mm) between the two flat plates,

x: distance (mm) between the two flat plates.
The electric resistance welded steel pipe according to claim 4, wherein the base metal material has a chemical composition comprising, 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, and

N: 0.0100% or less,
or further comprising one, or two or more selected from:
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, and

Sn: 0.100% or less,

the balance being Fe and incidental impurities.
The electric resistance welded steel pipe according to claim 4 or 5, wherein a steel microstructure of the base metal material at half the wall thickness is such that:
the average grain size of grains observed as regions surrounded by a high-angle grain boundary is 15.0 µm or less,

the area fraction of 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 the grains,

the connectivity of fine grains calculated from expression (3) below is 0.05 or more and 0.50 or less,

the volume fraction of bainite is 10% or more,

the total volume fraction of ferrite and bainite is 80% or more, and

the balance is one, or two or more selected from pearlite, martensite, and austenite representing a volume fraction of 20% or less in total, (Connectivity of fine grains) = (total length of high-angle grain boundaries of grains having a grain size smaller than the average grain size)/(total length of high-angle grain boundaries) wherein the numerator on the right side of expression (3) does not include the length of high-angle grain boundaries between a grain smaller than the average grain size and a grain equal to or larger than the average grain size.