EP4675003A1

EP4675003A1 - Steel pipe and method for manufacturing same

Info

Publication number: EP4675003A1
Application number: EP24807243.1A
Authority: EP
Inventors: Kohei Ikeda; Kazuhiko Baba; Takashi Kawano; Daichi IZUMI; Itaru Samusawa
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2023-05-17
Filing date: 2024-05-15
Publication date: 2026-01-07
Also published as: JP7722599B2; WO2024237292A1; JPWO2024237292A1; KR20250141756A

Abstract

Provided are a steel pipe that has excellent SSCC resistance stably and a method for producing the same. The steel pipe has a chemical composition containing, in mass%, C: 0.020 to 0.080 %, Mn: 0.50 to 1.80 %, Mo: 0.01 to 0.50 %, N: 0.0010 to 0.0080 %, Si: 0.01 to 0.50 %, P: 0.015 % or less, S: 0.0015 % or less, Al: 0.010 to 0.080 %, and Ca: 0.0005 % to 0.0050 %, with the balance being Fe and inevitable impurities, and a bainitic microstructure at a position 0.25 mm outward in a radial direction from an inner circumferential surface of the steel pipe, in which an area ratio of bainite with a crystal plane orientation of {110} aligned within 15° is 30.0 % or less in the bainitic microstructure.

Description

TECHNICAL FIELD

This disclosure relates to a steel pipe suitable for line pipes used for transporting crude oil and natural gas, particularly a steel pipe that is suitably used in sour environments containing hydrogen sulfide, and has excellent sulfide stress corrosion cracking resistance (SSCC resistance). This disclosure also relates to a method for producing the aforementioned steel pipe.

BACKGROUND

Generally, a steel pipe is produced by forming a steel plate produced by a plate mill or hot rolling mill into a steel pipe by, for example, UOE forming, press bend forming, and roll forming.
Steel pipes (line pipes) used for transporting oil and natural gas require various strengths, toughness, and weldability depending on the operating environment. Furthermore, line pipes used in sour environments of petroleum and natural gas containing hydrogen sulfide (H₂S) are required to have properties that can suppress hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSCC), commonly referred to as sour resistance.
HIC is a phenomenon in which hydrogen ions generated by corrosion reactions adsorb onto the surface of steel, enter the steel as atomic hydrogen, diffuse and accumulate around non-metallic inclusions such as MnS and hard secondary phase within the steel, recombine into molecular hydrogen, and cause cracking due to the internal pressure generated thereby. This HIC is considered as a problem in line pipes with relatively low strength levels relative to oil well pipes or tubes, and many countermeasure techniques have been disclosed.
On the other hand, SSCC is known to occur in regions having high hardness in the welded portions of oil well pipes or tubes and line pipes, and has generally not been regarded as a significant issue in line pipes having relatively low hardness. However, it has been found that in line pipes with relatively high hardness, SSCC originating from local corrosion in the base metal can occur. Therefore, it is considered important to suppress local corrosion in the base metal and improve SSCC resistance in line pipes used in sour environments.
Typically, the production of high strength steel plates for line pipes employs a so-called Thermo-Mechanical Control Process (TMCP) technology that combines controlled rolling and controlled cooling. In order to enhance the strength of a steel plate by employing TMCP, it is effective to increase the cooling rate during controlled cooling. However, when controlled cooling is performed at a high cooling rate, the surface layer of the steel plate is rapidly cooled, resulting in an increase in the hardness of the surface layer compared to the inside of the steel plate. Furthermore, in the case of steel pipes, work hardening occurs when a steel plate is formed into a tubular shape to produce the steel pipe, resulting in an increase in the hardness of the surface layer of the steel pipe, which in turn leads to a reduction in SSCC resistance.
In order to solve the above-described problem, for example, WO2020/067209A1 (PTL 1) discloses a high strength steel plate for a sour-resistant line pipe having a tensile strength of 520 MPa or more and improved SSCC resistance, in which the steel microstructure at a depth of 0.25 mm below the surface of the steel plate is a bainitic microstructure having a dislocation density of 1.0 × 10¹⁴ (m^-2) to 7.0 × 10¹⁴ (m^-2), and variations in Vickers hardness is controlled, thereby improving SSCC resistance.
Additionally, WO2021/020220A1 (PTL 2) discloses a high strength steel plate for a sour-resistant line pipe having a tensile strength of 520 MPa or more and improved SSCC resistance, in which the steel microstructure at a depth of 0.25 mm below the surface of the steel plate is a bainitic microstructure, and the area ratio of crystal grains having a Kernel Average Misorientation (KAM) value of 0.4 or more in the bainite is set to 50 % or less, thereby improving SSCC resistance.

CITATION LIST

Patent Literature

PTL 1: WO2020/067209A1
PTL 2: WO2021/020220A1

SUMMARY

(Technical Problem)

Steel pipes produced using the steel plate described in PTL 1, in which dislocation density and hardness variation are controlled, exhibit excellent SSCC resistance. Although the steel pipe thus obtained possesses sufficient SSCC resistance, there remains room for improvement in terms of achieving this property consistently.
In addition, steel pipes produced using the steel plate described in PTL 2, in which the KAM value is controlled in the bainitic microstructure at a depth of 0.25 mm below the steel plate surface, exhibit excellent SSCC resistance. Although the steel pipe thus obtained possesses sufficient SSCC resistance, there remains room for improvement in terms of achieving this property consistently.
It could thus be helpful to provide a high strength steel pipe that stably has excellent SSCC resistance and a method for producing the same. Here, "high strength" means a tensile strength of 520 MPa or more.

(Solution to Problem)

To achieve the above objectives, we have conducted extensive study on the microstructure of the inner surface of the steel pipe before and after evaluating SSCC resistance, as well as the method for producing the steel pipe, and have obtained the following findings.

[1] SSCC originates from localized corrosion. The occurrence of such localized corrosion is significantly influenced by the crystal plane orientation. Therefore, it is important to control the crystal plane orientation of the microstructure at the outermost surface layer on the inner side of the steel pipe, which is exposed to sour environments, thereby suppressing localized corrosion that serves as the initiation point of SSCC.
[2] To consistently achieve excellent SSCC resistance, it is important to control the microstructure of the outermost surface layer on the inner side of the steel pipe, specifically the area ratio of bainite with a crystal plane orientation of {110} aligned within 15° in the bainitic microstructure at a depth of 0.25 mm below the steel plate surface to 30 % or less.
[3] To realize the aforementioned microstructure, it is important to appropriately control the total rolling reduction during rolling and the cooling rate after rolling when producing steel plates for steel pipes.

This disclosure is based on the aforementioned discoveries and further studies. We thus provide the following.

1. A steel pipe comprising a chemical composition containing (consisting of), in mass%,
- C: 0.020 % or more and 0.080 % or less,
- Mn: 0.50 % or more and 1.80 % or less,
- Mo: 0.01 % or more and 0.50 % or less,
- N: 0.0010 % or more and 0.0080 % or less,
- Si: 0.01 % or more and 0.50 % or less,
- P: 0.015 % or less,
- S: 0.0015 % or less,
- Al: 0.010 % or more and 0.080 % or less, and
- Ca: 0.0005 % or more and 0.0050 % or less, with the balance being Fe and inevitable impurities, and
- a bainitic microstructure at a position 0.25 mm outward in a radial direction from an inner circumferential surface of the steel pipe, wherein an area ratio of bainite with a crystal plane orientation of {110} aligned within 15° is 30.0 % or less in the bainitic microstructure.
2. The steel pipe according to 1., in which the chemical composition further contains, in mass%, at least one selected from the group consisting of:
- Cu: 0.30 % or less,
- Ni: 0.10 % or less,
- Cr: 0.50 % or less,
- Nb: 0.1 % or less,
- V: 0.1 % or less,
- Ti: 0.1 % or less,
- Zr: 0.02 % or less,
- Mg: 0.02 % or less, and
- REM: 0.02 % or less.
3. A method for producing a steel pipe, comprising:
- heating a steel material having the chemical composition according to 1. or 2. to a temperature of 1000 °C or higher and 1300 °C or lower;
- then hot rolling the steel material under the condition that total rolling reduction A in an austenite region is 95 % or less to obtain a hot-rolled steel plate;
- subjecting the hot-rolled steel plate to cooling, wherein the cooling is performed within a temperature range starting from a cooling start temperature of at least (Ar₃ - 10 °C) for a steel plate surface temperature, to a cooling stop temperature of 550 °C or lower for a steel plate temperature at a depth of 0.25 mm below a steel plate surface, average cooling rate B within a temperature range of 750 °C to 550 °C for a steel plate temperature at the depth of 0.25 mm below the steel plate surface being from 20 °C/s to 50 °C/s, and parameter F, defined by Equation (1) below in relation to the total rolling reduction A and the average cooling rate B, satisfying 0 or more and 30.00 or less, thereby obtaining a cooled steel plate, and
- forming the cooled steel plate into a steel pipe $F = 3.35 A + 0.03 B - 286.27$

(Advantageous Effect)

According to this disclosure, it is possible to stably achieve excellent SSCC resistance in the steel pipe. Moreover, according to the method for producing the steel pipe of this disclosure, it is possible to stably produce a steel pipe with excellent SSCC resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram illustrating the crystal plane orientation in the bainitic microstructure; and
FIG. 2 illustrates the sampling method of SSCC test pieces in the steel pipe.

DETAILED DESCRIPTION

The following specifically describes embodiments of this disclosure. The following description merely presents examples of preferred embodiments of this disclosure, and this disclosure is not limited to these embodiments.

[Steel Pipe]

The steel pipe in one of the disclosed embodiments is produced using a steel plate having a specific chemical composition and mechanical properties. Furthermore, the steel pipe obtained by forming the steel plate is characterized by having a specific microstructure in the outermost surface layer on the inside thereof. Below, the reasons for the limitations of the chemical composition, mechanical properties, and microstructure will be explained. In terms of chemical composition, since the steel pipe is produced using a steel plate, the chemical composition of the steel pipe is the same as that of the steel plate used.

[Chemical composition]

First, the appropriate range of the chemical composition of the steel pipe, that is, the steel plate, and the reasons for its limitation will be explained. In the following description, when the unit "%" relating to the content refers to "mass%" unless otherwise stated.
In the chemical composition of the steel pipe according to this disclosure, it is important to define the C content, Mn content, Mo content, and N content to obtain strength and SSCC resistance as a steel pipe.

C: 0.020 % or more and 0.080 % or less

C is an element that contributes to improving the strength of the steel plate. If the C content is less than 0.020 %, sufficient strength cannot be ensured; therefore, the C content should be 0.020 % or more. The C content is preferably 0.025 % or more. On the other hand, if the C content exceeds 0.080 %, the SSCC resistance and HIC resistance deteriorate due to increased hardness in the surface layer and central segregation area during accelerated cooling. In addition, the toughness of the steel plate also deteriorates. Therefore, the C content is set to 0.080 % or less. The C content is preferably 0.070 % or less.

Mn: 0.50 % or more and 1.80 % or less

Mn is an element that contributes to improving the strength and toughness of the steel plate. If the Mn content is less than 0.50 %, these effects are not sufficiently exhibited. Therefore, the Mn content is set to 0.50 % or more. The Mn content is preferably 0.80 % or more. On the other hand, if the Mn content exceeds 1.80 %, the SSCC resistance and HIC resistance deteriorate due to an increase in the hardness of the surface layer and the central segregation area during accelerated cooling. Weldability is also degraded. Therefore, the Mn content is set to 1.80 % or less. The Mn content is preferably 1.70 % or less.

Mo: 0.01 % or more and 0.50 % or less

Mo is an element that contributes to improving the strength and toughness of the steel plate, as well as enhancing SSCC resistance. If the Mo content is less than 0.01 %, these effects are not sufficiently exhibited. Therefore, the Mo content is set to 0.01 % or more. The Mo content is preferably 0.10 % or more. On the other hand, if the Mo content is too high, the hardenability becomes excessive, leading to an increase in hardness and a deterioration in SSCC resistance. Weldability is also degraded. Therefore, the Mo content is set to 0.50 % or less. The Mo content is preferably 0.40 % or less.

N: 0.0010 % or more and 0.0080 % or less

N is an element that contributes to improving the strength of the steel plate, and for this purpose, it is contained in an amount of 0.0010 % or more. The N content is preferably 0.0015 % or more. On the other hand, if the N content exceeds 0.0080 %, the SSCC resistance and HIC resistance deteriorate due to an increase in the hardness of the surface layer and the central segregation area during accelerated cooling. In addition, the toughness of the steel plate also deteriorates. Therefore, the N content is set to 0.0080 % or less. The N content is preferably 0.0070 % or less.
In the steel pipe according to this disclosure, it is important to control the C content, Mn content, Mo content, and N content. Furthermore, for steel pipes used in sour environments, it is desirable to ensure HIC resistance and toughness, so the chemical composition should include the above components along with the following components.

Si: 0.01 % or more and 0.50 % or less

Si is added for deoxidation, but when the Si content is less than 0.01 %, the deoxidizing effect is insufficient. Therefore, the Si content is set to 0.01 % or more. The Si content is preferably 0.05 % or more. On the other hand, if the Si content exceeds 0.50 %, the toughness and weldability of the steel plate deteriorate. Therefore, the Si content is set to 0.50 % or less. The Si content is preferably 0.45 % or less.

P: 0.015 % or less

P is an inevitable impurity element, which degrades weldability and increases the hardness of the central segregation area, thereby degrading HIC resistance. This tendency becomes more pronounced if the P content exceeds 0.015 %. Therefore, the P content is set to be 0.015 % or less. The P content is preferably 0.008 % or less. On the other hand, in order to obtain the aforementioned effects, it is desirable to reduce the P content as much as possible. Therefore, the lower limit of the P content is not particularly restricted and may be 0 %. However, excessive reduction leads to higher refining costs, so from the standpoint of industrial production, it is preferable to set the P content to 0.001 % or more.

S: 0.0015 % or less

S is an inevitable impurity element, and in the steel plate, it forms MnS inclusions that deteriorate HIC resistance. Therefore, it is preferable that the S content be as low as possible; however, an S content of up to 0.0015 % is allowable. The S content is preferably 0.0010 % or less. On the other hand, in order to obtain the aforementioned effects, it is desirable to reduce the S content as much as possible. Therefore, the lower limit of the S content is not particularly restricted and may be 0 %. However, excessive reduction leads to higher refining costs, so from the standpoint of industrial production, it is preferable to set the S content to 0.0002 % or more.

Al: 0.010 % or more and 0.080 % or less

Al is added as a deoxidizer, but its addition effect is not realized when the Al content is less than 0.010 %. Therefore, the Al content is set to 0.010 % or more, and is preferably 0.015 % or more. On the other hand, if the Al content exceeds 0.080 %, the cleanliness of the steel decreases, and the toughness of the steel plate deteriorates, so the Al content is set to 0.080 % or less. The Al content is preferably 0.070 % or less.

Ca: 0.0005 % or more and 0.0050 % or less

Ca is an effective element for improving HIC resistance through morphological control of sulfide inclusions, but its addition effect is not sufficient when the Ca content is less than 0.0005 %. Therefore, the Ca content is set to 0.0005 % or more. The Ca content is preferably 0.0008 % or more. On the other hand, if the Ca content exceeds 0.0050 %, not only does the aforementioned effect become saturated, but the HIC resistance deteriorates due to a decrease in the cleanliness of the steel plate; therefore, the Ca content is set to 0.0050 % or less. The Ca content is preferably 0.0045 % or less.
The steel plate in one of the disclosed embodiments has a chemical composition containing the above elements with the balance being Fe and inevitable impurities.
In addition, the chemical composition of the steel pipe in another embodiment of this disclosure may further contain at least one selected from the group consisting of Cu, Ni, Cr, Nb, V, Ti, Zr, Mg, and REM to further improve the properties as a steel pipe.

Cu: 0.30 % or less

Cu is an effective element for improving the toughness and increasing the strength of the steel plate. When Cu is added, it is preferable that the Cu content be 0.05 % or more in order to obtain the aforementioned effect. However, if the Cu content exceeds 0.30 %, fine cracks known as "fissures" are more likely to form in environments with a hydrogen sulfide partial pressure of less than 1 bar. Therefore, when Cu is added, the upper limit is set to 0.30 %. The Cu content is preferably 0.20 % or less.

Ni: 0.10 % or less

Ni is an effective element for improving the toughness and increasing the strength of the steel plate. When Ni is added, it is preferable that the Ni content be 0.01 % or more in order to obtain the aforementioned effect. However, if the Ni content exceeds 0.10 %, fine cracks known as "fissures" are more likely to form in environments with a hydrogen sulfide partial pressure of less than 1 bar. Therefore, when Ni is added, the upper limit is set to 0.10 %. The Ni content is preferably 0.02 % or less.

Cr: 0.50 % or less

Cr, like Mn, is an effective element for obtaining sufficient strength in the steel plate even with a low C content. When Cr is added, it is preferable that the Cr content be 0.05 % or more in order to obtain the aforementioned effect. However, if the Cr content exceeds 0.50 %, the hardenability becomes excessive, resulting in increased hardness and a deterioration in SSCC resistance. Weldability is also degraded. Therefore, when Cr is added, the upper limit is set to 0.50 %.

Nb: 0.1 % or less

Nb is an element that can be optionally added to increase the strength and toughness of the steel plate. When Nb is added, it is preferable that the Nb content be 0.005 % or more in order to obtain the aforementioned effect. However, if the Nb content exceeds 0.1 %, the toughness of the welded portion deteriorates. Therefore, when Nb is added, the upper limit is set to 0.1 %.

V: 0.1 % or less

V, like Nb, is an element that may be optionally added to improve the strength and toughness of the steel plate. When V is added, it is preferable that the V content be 0.005 % or more in order to obtain the aforementioned effect. However, if the V content exceeds 0.1 %, the toughness of the welded portion deteriorates. Therefore, when V is added, the upper limit is set to 0.1 %.

Ti: 0.1 % or less

Ti, like Nb and V, is an element that may be optionally added to increase the strength and toughness of the steel plate. When Ti is added, it is preferable that the Ti content be 0.005 % or more in order to obtain the aforementioned effect. However, if the Ti content exceeds 0.1 %, the toughness of the welded portion deteriorates. Therefore, when Ti is added, the upper limit is set to 0.1 %.

Zr: 0.02 % or less

Zr is an element that can be optionally added to improve the toughness of the steel plate through crystal grain refinement and to enhance cracking resistance by controlling the characteristics of inclusions. When Zr is added, it is preferable that the Zr content be 0.0005 % or more in order to obtain the aforementioned effects. However, if the Zr content exceeds 0.02 %, the effect becomes saturated. Therefore, when Zr is added, the upper limit is set to 0.02 %.

Mg: 0.02 % or less

Mg, like Zr, is an element that can be optionally added to improve the toughness of the steel plate through crystal grain refinement and to enhance cracking resistance by controlling the characteristics of inclusions. When Mg is added, it is preferable that the Mg content be 0.0005 % or more in order to obtain the aforementioned effects. However, if the Mg content exceeds 0.02 %, the effect becomes saturated. Therefore, when Mg is added, the upper limit is set to 0.02 %.

REM: 0.02 % or less

REM, like Zr and Mg, is an element that can be optionally added to improve the toughness of the steel plate through crystal grain refinement and to enhance cracking resistance by controlling the characteristics of inclusions. When REM is added, the REM content is set to 0.0005 % or more in order to obtain the aforementioned effects. However, if the REM content exceeds 0.02 %, the effect becomes saturated. Therefore, when REM is added, the upper limit is set to 0.02 %.
This disclosure discloses a technology for improving the SSCC resistance of steel pipes. From the viewpoint of sour resistance, it is preferable that HIC resistance is also achieved in addition to SSCC resistance. To improve the HIC resistance, it is preferable that the CP value, calculated according to Equation (2) below, is kept at or below 1.00 in the chemical composition of the steel pipe: CP = 4.46[%C] + 2.37[%Mn] / 6 + (1.74[%Cu] + 1.7[%Ni]) / 15 + (1.18[%Cr] + 1.95[%Mo] + 1.74[%V]) / 5 + 22.36[%P] (2).
However, in Equation (2) above, [%X] represents the content (mass%) of element X, and is taken to be 0 when the corresponding element is not contained.
The above-mentioned CP value is an equation devised to estimate the material property of the central segregation area based on the content of each alloying element. As the CP value defined by Equation (2) above increases, the concentration of components in the central segregation zone increases, leading to an increase in the hardness of the central segregation zone. Therefore, by keeping the CP value determined by Equation (2) above at or below 1.00, it becomes possible to suppress crack formation in the HIC test. Since the hardness of the central segregation area decreases with a lower CP value, the upper limit for the CP value can be set to 0.95 when higher HIC resistance is required.
The balance, other than the above elements, is Fe and inevitable impurities. However, there is no intention in this expression of precluding the inclusion of other trace elements, without impairing the action or effect of the present disclosure. For example, O is an inevitable element in the steel plate, but is acceptable in this disclosure when its content is 0.0050 % or less, preferably 0.0040 % or less.

[Microstructure]

Next, the microstructure of the steel pipe of this disclosure will be described. In one of the disclosed embodiments, the microstructure at a position 0.25 mm outward in a radial direction from an inner circumferential surface of the steel pipe (hereinafter referred to as at a depth of 0.25 mm below the inner surface of the steel pipe) is a bainitic microstructure mainly composed of bainite. Here, "mainly composed of bainite" means that the area ratio of bainite is 95 % or more.

• Bainitic microstructure

In order to improve the SSCC resistance by keeping the maximum hardness at 0.25 mm below the inner surface of the steel pipe constant, it is necessary for the microstructure at 0.25 mm below the inner surface of the steel pipe to be a bainitic microstructure. In particular, if hard phases such as martensite or martensite austenite constituent (MA) are formed in the surface layer up to a depth of 0.25 mm from the inner surface of the steel pipe, the hardness of the inner surface layer of the steel pipe increases, and the variation in the hardness of the inner surface layer increases, thereby hindering the material homogeneity. Therefore, the microstructure of the inner surface layer of the steel pipe should be a bainitic microstructure.
Here, the bainitic microstructure includes a microstructure referred to as lath-like bainite or granular bainite, which transforms during or after accelerated cooling and contributes to transformation strengthening. When different microstructures such as ferrite, martensite, pearlite, martensite austenite constituent, and retained austenite are mixed within the bainitic microstructure, it can lead to a decrease in the strength, deterioration of toughness, and an increase in surface layer hardness of the steel pipe. Therefore, a smaller area ratio of microstructures other than bainite is preferable. However, if the area fraction of microstructures other than bainite is sufficiently low, their effects can be ignored, so a certain amount is acceptable. Specifically, it is preferable that the total area ratio of microstructures other than bainite (ferrite, martensite, pearlite, martensite austenite constituent, retained austenite, etc.) is less than 5 %.
Furthermore, in the aforementioned bainitic microstructure, it is important that the area ratio of bainite with a crystal plane orientation of {110} aligned within 15° is 30.0 % or less. The reason for controlling the microstructure of the surface layer on the inner side of the steel pipe is that the inner surface is exposed to a sour environment and comes into contact with hydrogen sulfide. In addition, the reason for controlling the microstructure at a depth of 0.25 mm below the inner surface of the steel pipe is that the crack depth used to determine the presence or absence of SSCC occurs at this 0.25 mm position.

• Area ratio of bainite with crystal plane orientation of {110} aligned within 15°: 30.0 % or less

We have found that, in a sour environment, among the crystal plane orientations of the bainitic microstructure, the crystal plane orientation {110} undergoes corrosion (local corrosion) as the primary dissolution surface. Furthermore, detailed investigations were conducted on the crystal plane orientation of the bainitic microstructure at a depth of 0.25 mm below the inner surface of the steel pipe. That is, when the bainitic microstructure is considered as a collection of bainite with different crystal plane orientations across large-angle grain boundaries, where the grain boundaries are defined as those with an orientation difference of 15° or more, we have found that by ensuring the area ratio of bainite with a crystal plane orientation of {110} aligned within 15° is 30.0 % or less, excellent and stable SSCC resistance can be achieved.
Here, to facilitate understanding, the bainite with a crystal plane orientation of {110} aligned within 15° is explained by assuming the inner circumferential surface (curved surface) of the steel pipe as a flat plate, with reference to Fig. 1. That is, the aforementioned bainite refers to bainite that has a crystal plane orientation in which the axis 1, which is perpendicular to the {110} plane, is aligned 15° or less with respect to the axis L perpendicular to the plate surface, as shown in Fig. 1. Since the inner circumferential surface of the steel pipe is actually a curved surface, the axis L above can be considered as the normal to the inner peripheral surface.
It is presumed that limiting the area ratio of bainite with the specified crystal plane orientation to 30 % or less contributes to suppressing the initiation of local corrosion, which is considered the initiation point for SSCC occurrence. In other words, bainite with a crystal plane orientation of {110} aligned within 15° has an impact on SSCC resistance, and by controlling the area ratio of such bainite to 30 % or less, as demonstrated in the examples described later, excellent and stable SSCC resistance can be consistently achieved. On the other hand, in methods of the prior art that control the area ratio of bainite using the proportion of crystal grains with a KAM value of 0.4 or higher, or in prior art methods that control the area ratio of bainite using dislocation density and hardness, it has been found that the area ratio of bainite cannot be controlled to 30 % or less, and in such cases, the desired SSCC resistance is not achieved.

[Tensile strength of steel pipe]

The tensile strength (TS) of the steel pipe according to this disclosure is not particularly limited as long as the steel pipe can be produced. However, in steel pipes with low tensile strength, SSCC resistance is generally not a concern. Therefore, it is preferable that the steel pipe of this disclosure has a tensile strength of 520 MPa or higher. In particular, steel pipes with a high tensile strength of 520 MPa or higher are particularly suitable for applications such as line pipes.

[Method for producing steel pipe]

The following describes a method for producing a steel plate according to one of the disclosed embodiments.
The steel pipe of this disclosure can be produced by heating a steel material having the aforementioned chemical composition, followed by hot rolling to form a hot-rolled steel plate, then applying controlled cooling under predetermined conditions to the hot-rolled steel plate to form a steel plate, and finally forming the steel plate into a steel pipe.

• Steel material

Any form of material can be used as the steel material. The steel material may be, for example, a steel slab. The steel material may be produced by any method, but for example, can be produced by smelting steel having the aforementioned chemical composition by a conventional method and subjecting the smelted steel to casting. The smelting can be performed by any method using a converter, an electric furnace, an induction furnace, and the like. The casting is preferably performed by continuous casting in terms of productivity, but may be performed by ingot casting.

• Heating temperature: 1000 °C or higher and 1300 °C or lower

The steel material is heated prior to hot rolling. The heating may be carried out after the steel material obtained by casting or other methods has been once cooled, or the steel material obtained may be directly subjected to the heating without cooling.
If the heating temperature of the steel material is lower than 1000 °C, carbides are not sufficiently dissolved, and the necessary strength as a steel plate cannot be obtained. Therefore, the heating temperature is set to 1000 °C or higher. On the other hand, if the heating temperature exceeds 1300 °C, excessive energy will be required, resulting in reduced productivity. In addition, the toughness of the steel plate also deteriorates. Therefore, the heating temperature is set to 1300 °C or lower. Note that the heating temperature refers to the temperature inside the heating furnace, and the steel material is heated to the aforementioned temperature throughout, including the center.

• Total rolling reduction in austenite region: 95 % or less

Next, the heated steel material is hot rolled to obtain a hot-rolled steel plate. In this case, in order to produce a steel pipe that meets the conditions of this disclosure, the total rolling reduction during hot rolling in the austenite region (at or above the Ar₃ transformation point) is set to 95 % or less. If the total rolling reduction exceeds 95 %, shear deformation promotes the development of austenite with {111} crystal planes oriented along the plate surface. After hot rolling and subsequent cooling, this develops the formation of a bainitic texture with {110} crystal plane orientation. As a result, it becomes impossible to keep the area ratio of bainite with a crystal plane orientation of {110} aligned within 15° at or below 30 %, leading to a deterioration in the SSCC resistance of the steel pipe. For this reason, the total rolling reduction is set to 95 % or lower. On the other hand, to ensure toughness, it is desirable to set a total rolling reduction to 60 % or more in the austenite region. In addition, from the perspective of improving strength and HIC resistance, it is preferable to perform hot rolling in the austenite region.

• Rolling finish temperature

To achieve high toughness as the steel pipe, the hot-rolling finish temperature should be as low as possible. However, on the other hand, this results in a decrease in rolling efficiency. Therefore, the rolling finish temperature must be set considering both the required toughness of the steel pipe and the rolling efficiency. From the perspective of improving the strength and HIC resistance of the steel pipe, it is preferable to set the rolling finish temperature to be at or above the Ar₃ transformation point. The rolling finish temperature is measured as the surface temperature of the steel material, which can be determined using a radiation thermometer or the like.
Here, the Ar₃ transformation point refers to the start temperature of the ferrite transformation during cooling, calculated according to Equation (3) below: Ar₃(°C)=910 - 273[%C] - 74[%Mn] - 56[%Ni] - 16[%Cr] - 9[%Mo] - 5[%Cu] (3).
However, in Equation (3) above, [%X] represents the content (mass%) of element X and is taken to be 0 when the corresponding element is not contained.

• Cooling

Next, cooling is applied to the hot-rolled steel plate after hot rolling, within the temperature range from the cooling start temperature to the cooling stop temperature, as described later.

•• Cooling start temperature: at or above (Ar₃ - 10 °C) in terms of steel plate surface temperature

If the cooling start temperature is low, the amount of ferrite formed before cooling increases. In particular, if the temperature drop from the Ar₃ transformation point exceeds 10 °C, ferrite with a volume fraction exceeding 5 % is formed, resulting in a significant decrease in the strength of the steel pipe and a deterioration in its HIC resistance. Therefore, the cooling start temperature should be set to at least (Ar₃ - 10 °C). The cooling start temperature is also equal to or lower than the rolling finish temperature. The cooling start temperature is measured as the surface temperature of the hot-rolled steel plate, which can be determined using a radiation thermometer or the like.

•• Cooling rate: average cooling rate of 50 °C/s or less within a temperature range of 750 °C to 550 °C for a steel plate temperature at a depth of 0.25 mm below the steel plate surface

To enhance the strength of the steel plate, i.e., the steel pipe, while reducing the hardness variation within the steel plate and improving the material homogeneity, it is important to control the cooling rate of the steel plate surface layer. In particular, to obtain the desired microstructure 0.25 mm below the steel plate surface, it is necessary to control the average cooling rate to 50 °C/s or less within the temperature range of 750 °C to 550 °C for the steel plate temperature at a depth of 0.25 mm below the steel plate surface.
By minimizing the average cooling rate, it is possible to produce bainite whose {110} crystal planes are not oriented within 15° Furthermore, by reducing the average cooling rate, it is possible to lower the maximum hardness, thereby improving the SSCC resistance. If the average cooling rate exceeds 50 °C/s, the area ratio of bainite with a crystal plane orientation of {110} aligned within 15° increases, and the maximum hardness HV 0.5 at 0.25 mm below the steel plate surface exceeds 230. Furthermore, a transformation texture develops, and among the bainitic microstructure, the area ratio of bainite with a crystal plane orientation of {110} aligned within 15° increases, leading to a deterioration in the SSCC resistance after the steel plate is made into a pipe. Therefore, the average cooling rate should be set to 50 °C/s or less. Preferably, it is 30 °C/s or less. The lower limit of the average cooling rate is not particularly limited, but if the average cooling rate becomes excessively small, ferrite and pearlite will form, resulting in insufficient strength for the steel plate, and thus the steel pipe. From the perspective of preventing insufficient strength, it is preferable that the average cooling rate be 20 °C/s or higher. $F (= 3.35 A + 0.03 B - 286.27) : 30.00$ or less
In order to achieve an area ratio of bainite with a crystal plane orientation of {110} aligned within 15° at a depth of 0.25 mm below the steel plate surface of, with the area ratio being ≤ 30.0 %, the total rolling reduction in the austenite region and the average cooling rate within a temperature range of 750 °C to 550 °C for the steel plate temperature at a depth of 0.25 mm below the steel plate surface are critical. We further conducted investigations and found that it is insufficient to control the total rolling reduction and the average cooling rate within predetermined ranges., and when the total rolling reduction is denoted by A (%), the average cooling rate by B (°C/s), and the area ratio of bainite with a crystal plane orientation of {110} aligned within 15° is denoted by F (%), F is expressed by Equation (1) below. If F exceeds 30.00, the area ratio of bainite with a crystal plane orientation of {110} aligned within 15° exceeds 30.0 %, leading to a deterioration in SSCC resistance. On the other hand, the lower limit of F can be 0 or more. $F = 3.35 A + 0.03 B - 286.27$

•• Cooling stop temperature: 550 °C or lower for steel plate temperature at depth of 0.25 mm below steel plate surface

The hot-rolled steel plate after hot rolling is cooled from the cooling start temperature to the cooling stop temperature by controlled cooling. Here, by setting the cooling stop temperature to 550 °C or lower, which is the temperature range of the bainite transformation, it is possible to form the bainite phase. When the cooling stop temperature exceeds 550 °C, bainite transformation is incomplete and sufficient strength cannot be obtained. Therefore, the cooling stop temperature is set to 550 °C or lower. Furthermore, if the cooling stop temperature is below 250 °C, the dislocation density increases, which may lead to a significant rise in surface layer hardness when the pipe is formed, potentially degrading the SSCC resistance. Therefore, it is preferable that the cooling stop temperature is 250 °C or higher.
Furthermore, for cooling at 550 °C or lower for the steel plate temperature at a depth of 0.25 mm below the steel plate surface, if the cooling rate is too slow, cooling may not occur in a stable nucleate boiling state, leading to hardness variations in the outermost surface layer of the steel plate. This can cause an increase in the maximum Vickers hardness and potentially degrade the SSCC resistance. Therefore, the average cooling rate within a temperature range of 550 °C to the cooling stop temperature for the steel plate temperature at a depth of 0.25 mm below the steel plate surface is preferably 150 °C/s or more. A more preferable average cooling rate is 170 °C/s or higher. The upper limit of the average cooling rate is not particularly limited, but due to equipment constraints, it is preferable to be 250 °C/s or lower.
The steel plate temperature at a depth of 0.25 mm below the steel plate surface cannot be measured directly due to physical limitations. However, based on the surface temperature of the hot-rolled steel plate measured by a radiation thermometer at the start of cooling and the surface temperature of the hot-rolled steel plate at the target cooling stop, the temperature distribution within the cross section along the thickness direction can be calculated in real-time using, for example, a process computer and differential calculation, and the temperature at the specified depth can be derived from this result. The temperature at a depth of 0.25 mm below the steel plate surface in the temperature distribution is referred to as "the steel plate temperature at a depth of 0.25 mm below the steel plate surface" in this specification.

• Steel Pipe

Next, the cooled steel plate is made into a steel pipe. Methods for making the steel plate into a steel pipe include, for example, producing methods that form the steel plate into a tubular shape by press bend forming, roll forming, UOE forming, etc., and then welding the butt joint. By the aforementioned manufacturing method, the steel pipes, such as UOE steel pipes, electric resistance welded steel pipes, and spiral steel pipes, with excellent material homogeneity within the steel pipe, suitable for the transportation of crude oil and natural gas, can be produced.
For example, UOE steel pipes are produced by performing groove machining on the edges of the steel plate, forming the steel pipe shape using C press, U-ing press, and O-ing press, then seam welding the butt joint by internal and outer surface welding, and, if necessary, undergoing an expanding process. Any welding method that provides sufficient joint strength and joint toughness is acceptable, but submerged arc welding is preferred in terms of excellent weld quality and producing efficiency.

EXAMPLES

The following examples will describe the actions and effects of this disclosure. However, this disclosure is not limited to the following examples.
Steels with the chemical compositions listed in Table 1 were made into steel slabs as steel materials by continuous casting. The obtained steel slabs were heated to the heating temperatures listed in Table 2, and then hot rolled with a total rolling reduction in the austenite region listed in Table 2 to obtain hot-rolled steel plates with the thicknesses listed in Table 2. Subsequently, controlled cooling was performed on the obtained hot-rolled steel plates using a water-cooled controlled cooling system under the conditions listed in Table 2, resulting in steel plates. Subsequently, the edges of the obtained steel plates were subjected to groove machining, and the plates were formed into a pipe shape using C press, U-ing press, and O-ing press. The butt joints on both the inner and outer surfaces were seam welded by submerged arc welding, and then a pipe expanding process was performed to obtain steel pipes. For the actual pipe products, an outer surface coating process is applied. To simulate the outer surface coating process, aging heat treatment was conducted at 250 °C for 1 hour.

[Table 1]

Steel sample ID	Chemical Composition (mass%)																		CP value
Steel sample ID	C	Si	Mn	P	S	Al	N	Mo	Ca	Cu	Ni	Cr	Nb	v	Ti	Zr	Mg	REM	CP value
1	0.031	0.31	1.58	0.004	0.0010	0.032	0.0037	0.09	0.0032	-	-	-	-	-	-	-	-	-	0.89
2	0.070	0.33	1.32	0.004	0.0013	0.025	0.0041	0.16	0.0033	-	-	-	-	-	-	-	-	-	0.99
3	0091	0.42	1.34	0.004	0.0012	0.043	0.0042	0.09	0.0025	-	-	-	-	-	-	-	-	-	1.06
4	0.014	0.42	1.35	0.004	0.0011	0.043	0.0042	0.08	0.0032	-	-	-	-	-	-	-	-	-	0.72
5	0.080	0.42	1.19	0.004	0.0011	0.043	0.0042	0.08	0.0033	-	-	-	-	-	-	-	-	-	0.95
6	0.020	0.35	1.56	0.004	0.0013	0.040	0.0041	0.07	0.0027	-	-	-	-	-	-	-	-	-	0.82
7	0.043	0.05	1.28	0.004	0.0012	0.033	0.0042	0.09	0.0021	-	-	-	-	-	-	-	-	-	0.82
8	0.054	0.40	1.37	0.004	0.0011	0.034	0.0039	010	0.0031	-	-	-	-	-	-	-	-	-	0.91
9	0.054	0.36	0.80	0.004	0.0012	0.051	0.0041	0.09	0.0025	-	-	-	-	-	-	-	-	-	0.68
10	0.052	0.32	1.70	0.004	0.0010	0.034	0.0036	0.12	0.0027	-	-	-	-	-	-	-	-	-	1.04
11	0.056	0.27	1.90	0.004	0.0010	0.027	0.0042	0.09	0.0025	-	-	-	-	-	-	-	-	-	1.12
12	0.071	0.31	0.41	0.004	0.0012	0.042	0.0042	0.08	0.0021	-	-	-	-	-	-	-	-	-	0.60
13	0.033	0.34	1.80	0.004	0.0012	0.045	0.0049	0.08	0.0032	-	-	-	-	-	-	-	-	-	0.98
14	0.073	0.26	0.50	0.004	0.0013	0.041	0.0052	0.15	0.0036	-	-	-	-	-	-	-	-	-	0.67
15	0.035	0.28	1.50	0.014	0.0012	0.044	0.0035	0.09	0.0034	-	-	-	-	-	-	-	-	-	1.10
16	0.042	0.35	1.42	0.004	0.0013	0.035	0.0042	0.04	0.0032	-	-	-	-	-	-	-	-	-	0.85
17	0.053	0.31	1.36	0.004	0.0011	0.024	0.0036	0.05	0.0030	-	-	-	-	-	-	-	-	-	0.88

C	Si	Mn	P	s	Al	N	Mo	Ca	Cu	Ni	Cr	Nb	v	Ti	Zr	Mg	REM
18	0.057	0.29	1.36	0.004	0.0010	0.063	0.0038	0.09	0.0015	-	-	-	-	-	-	-	-	-	0.92
19	0.055	0.30	1.45	0.004	0.0012	0.027	0.0021	0.22	0.0031	-	-	-	-	-	-	-	-	-	0.99
20	0.035	0.34	1.39	0.004	0.0012	0.029	0.0067	0.06	0.0031	-	-	-	-	-	-	-	-	-	0.82
21	0.056	0.26	1.38	0.003	0.0013	0.037	0.0091	0.09	0.0028	-	-	-	-	-	-	-	-	-	0.90
22	0.071	0.29	1.29	0.004	0.0015	0.036	0.0004	0.08	0.0027	-	-	-	-	-	-	-	-	-	0.95
23	0.044	0.34	1.39	0.004	0.0015	0.031	0.0080	0.06	0.0031	-	-	-	-	-	-	-	-	-	0.86
24	0.052	0.30	1.45	0.004	0.0014	0.032	0.0010	0.22	0.0032	-	-	-	-	-	-	-	-	-	0.98
25	0.063	0.25	1.31	0.004	0.0010	0.025	0.0042	000	0.0034	-	-	-	-	-	-	-	-	-	0.89
26	0.044	0.25	1.37	0.004	0.0010	0.033	0.0037	0.03	0.0017	-	-	-	-	-	-	-	-	-	0.84
27	0.041	0.32	1.48	0.004	0.0011	0.027	0.0043	0.30	0.0032	-	-	-	-	-	-	-	-	-	0.97
28	0.045	0.31	1.41	0.004	0.0011	0.042	0.0044	0.60	0.0031	-	-	-	-	-	-	-	-	-	1.08
29	0.037	0.28	1.35	0.004	0.0013	0.036	0.0042	0.09	0.0005	-	-	-	-	-	-	-	-	-	0.82
30	0.042	0.32	1.35	0.004	0.0012	0.034	0.0039	0.15	0.0050	-	-	-	-	-	-	-	-	-	0.87
31	0.044	0.29	1.34	0.004	0.0010	0.015	0.0044	0.30	0.0023	-	-	-	-	-	-	-	-	-	0.93
32	0.043	0.31	1.37	0.004	0.0010	0.033	0.0043	0.09	0.0031	0.10	0.02	0.28	0.03	-	0.011	-	-	-	0.94
33	0.043	0.31	1.37	0.004	0.0011	0.030	0.0041	0.09	0.0033	0.10	0.02	0.28	0.03	0.008	0.011	0.013	0.009	0.008	0.94

(Measurement of crystal plane orientation)

To evaluate the microstructure of the steel pipes produced by the above producing method, measurements were performed using EBSD (Electron Backscatter Diffraction). That is, a test piece was taken from the inner side of each steel pipe, and after removing the scale from the inner surface, mirror polishing and electropolishing were carried out so that the measurement surface would be located at a depth of 0.25 mm from the surface after the removal of the scale. Subsequently, EBSD was performed on the measurement surface using a scanning electron microscope under the conditions of an accelerating voltage of 20 kV, a measurement area of 300 × 200 µm, and a step size of 0.2 µm. From the obtained measurement results, using EBSD analysis software, the area ratio of the low-index plane {100} oriented within 15°, the area ratio of the low-index plane {110} oriented within 15°, the area ratio of the low-index plane {111} oriented within 15°, and the area ratio of high-index planes other than the above were calculated. From these calculated results, the area ratio of crystal plane orientation {110} aligned within 15° was calculated. The results are listed in Table 2.
As listed in Table 2, the steel pipes in our examples and comparative examples, excluding No. 41 and 42, had a microstructure mainly composed of bainite. Here, "mainly composed of bainite" means that bainite accounts for 95 % or more. On the other hand, bainite content in Nos. 41 and 42 was less than 95 %.

(Evaluation of SSCC resistance)

The SSCC resistance of the steel pipes produced by the above producing method was evaluated. As illustrated in Fig. 2, a SSCC test piece of 5 × 25 × 125 mm was taken from the inner surface of each steel pipe. At this time, the inner surface of the steel pipe, which served as the test surface, was left non-pickled (with mill scale) in order to retain the state of the outermost surface layer. A four-point bending SSCC test was conducted on the sampled SSCC test piece by applying a stress equivalent to 90 % of the actual yield stress of each steel pipe, using NACE standard TM0177 Solution B under a hydrogen sulfide partial pressure of 0.15 bar and a carbon dioxide partial pressure of 0.75 bar. If no crack is observed after 720 hours of immersion, the SSCC resistance is determined to be "good," whereas if a crack occurs, the SSCC resistance is determined to be "poor." Here, the term "crack" refers to a crack of 0.25 mm or greater observed at the center in the longitudinal direction of the test piece after the four-point bending test. The evaluation results of the obtained SSCC resistance are also listed in Table 2.

(Measurement of tensile strength)

Tensile tests were conducted in accordance with ASTM A370 using full-thickness test pieces sampled in the circumferential direction of the above-mentioned steel pipes as the tensile test pieces, and the tensile strength was measured. The tensile strengths obtained for the steel pipes are also listed in Table 2.

[Table 2]

No.	Steel sample ID	Plate thickness (mm)	Heating temperature (°C)	Total rolling reduction (%)	Cooling start temperature (°C)	Cooling stop temperature (°C)	Average cooling rate within temperature range of 750°C to 550°C (°C/s)	F	Microstructure	{110}±15° fraction (%)	Tensile strength (MPa)	SSCC resistance of steel pipe	Remarks
1	1	30	1060	88	800	450	30	18.63	bainite	18.2	586	good	Ex.
2	2	30	1060	88	800	450	28	18.57	bainite	17.6	625	good	Ex.
3	3	30	1060	88	800	450	30	18.63	bainite	20.1	642	poor	Comp. Ex.
4	4	30	1060	88	800	450	30	18.63	bainite	22.3	515	good	Comp. Ex.
5	5	30	1060	88	800	450	33	18.72	bainite	13.1	630	good	Ex.
6	6	30	1060	88	800	450	30	18.63	bainite	18.9	589	good	Ex.
7	7	30	1060	88	800	450	30	18.63	bainite	18.3	615	good	Ex.
8	8	30	1060	88	800	450	35	18.78	bainite	18.5	615	good	Ex.
9	9	30	1060	88	850	450	29	18. 60	bainite	25.3	560	good	Ex.
10	10	30	1060	88	800	450	30	18.63	bainite	18.2	637	good	Ex.
11	11	30	1060	88	800	450	35	18.78	bainite	19.4	644	poor	Comp. Ex.
12	12	30	1060	88	850	450	30	18.63	bainite	19.2	510	good	Comp. Ex.
13	13	30	1060	88	800	450	30	18.63	bainite	18.3	621	good	Ex.
14	14	30	1060	88	850	450	29	18.60	bainite	18.3	574	good	Ex.
15	15	30	1060	88	800	450	30	18.63	bainite	18.5	615	good	Ex.
16	16	30	1060	88	800	450	30	18.63	bainite	18.4	614	good	Ex.
17	17	30	1060	88	800	450	33	18.72	bainite	19.2	622	good	Ex.
18	18	30	1060	88	800	450	30	18.63	bainite	18.3	620	good	Ex.
19	19	30	1060	88	800	450	30	18.63	bainite	18.3	615	good	Ex.
20	20	30	1060	88	800	450	29	18.60	bainite	17.8	612	good	Ex.
21	21	30	1060	88	800	450	30	18.63	bainite	18.3	638	poor	Comp. Ex.
22	22	30	1060	88	800	450	30	18.63	bainite	18.6	512	good	Comp. Ex.
23	23	30	1060	88	800	450	30	18.63	bainite	17.9	634	good	Ex.
24	24	30	1060	88	800	450	29	18.60	bainite	17.3	534	good	Ex.
25	25	30	1060	88	800	450	32	18.69	bainite	18.9	578	poor	Comp. Ex.
26	26	30	1060	88	800	450	30	18.63	bainite	18.2	615	good	Ex.
27	27	30	1060	88	800	450	31	18.66	bainite	18.6	655	good	Ex.
28	28	30	1060	88	800	450	30	18.63	bainite	17.9	664	poor	Comp. Ex.
29	29	30	1060	88	800	450	28	18.57	bainite	17.3	615	good	Ex.
30	30	30	1060	88	800	450	32	18.69	bainite	18.5	625	good	Ex.
31	31	30	1060	88	800	450	28	18.57	bainite	17.2	618	good	Ex.
32	32	30	1060	88	800	450	30	18.63	bainite	18.2	622	good	Ex.
33	33	30	1060	88	800	450	30	18.63	bainite	18.2	615	good	Ex.
34	33	30	1060	84	800	450	45	6.08	bainite	4.2	610	good	Ex.
35	33	30	1060	91	800	450	30	28.38	bainite	27.5	624	good	Ex.
36	33	30	1060	97	800	450	30	47.88	bainite	40.2	646	poor	Comp. Ex.
37	33	30	1060	88	800	300	30	18.63	bainite	18.3	632	good	Ex.
38	33	30	1060	88	800	500	30	18.63	bainite	18.2	603	good	Ex.
39	33	30	1060	88	800	450	25	18.48	bainite	17.5	606	good	Ex.
40	33	30	1060	91	800	450	45	28.83	bainite	26.8	615	good	Ex.
41	33	30	1060	88	770	450	30	18.63	bainite90%+ferrite10%	23.5	511	good	Comp. Ex.
42	33	30	1060	88	800	560	35	18.78	bainite91%+ferrite9%	24.5	515	good	Comp. Ex.
43	33	30	1060	92	800	450	58	32.47	bainite	33.1	648	poor	Comp. Ex.

As listed in Table 2, all of our examples in which the chemical compositions and producing conditions fall within the appropriate ranges defined by this disclosure exhibited high strength and good SSCC resistance.
In contrast, all of the comparative examples in which the chemical composition and/or producing conditions fall outside the appropriate ranges defined by this disclosure exhibited inferior tensile strength and/or SSCC resistance. Specifically, Nos. 3, 4, 11, 12, 21, 22, 25, and 28 had chemical compositions outside the ranges defined by this disclosure, and exhibited inferior tensile strength and/or SSCC resistance. In addition, although No. 36 had a chemical composition within the range of this disclosure, the total rolling reduction in the austenite region exceeded 95 %, and the F value exceeded 30.00. As a result, the area ratio of bainite with a crystal plane orientation of {110} aligned within 15° exceeded 30.0 %, leading to inferior SSCC resistance. Although No. 43 had a chemical composition within the range of this disclosure, the average cooling rate was 50 °C/s or higher, and the F value exceeded 30.00. As a result, the area ratio of bainite with a crystal plane orientation of {110} aligned within 15° exceeded 30.0 %, leading to inferior SSCC resistance. Furthermore, No. 41 exhibited a lower tensile strength because the cooling start temperature was below (Ar₃ - 10 °C) based on the surface temperature of the steel plate, resulting in a bainite content of less than 95 %, and therefore, the microstructure was not mainly composed of bainite. No. 42 exhibited a lower tensile strength because the cooling stop temperature exceeded 550 °C at a depth of 0.25 mm below the steel plate surface, resulting in a bainite content of less than 95 %, and therefore, the microstructure was not mainly composed of bainite.
As can be seen from the results listed in Table 2, the steel pipes of our examples had excellent tensile strength in the steel plates and SSCC resistance in the steel pipes. On the other hand, the steel pipes of the comparative example outside the scope of this disclosure had low tensile strength or inferior SSCC resistance.

Claims

A steel pipe comprising a chemical composition containing, in mass%,
C: 0.020 % or more and 0.080 % or less,

Mn: 0.50 % or more and 1.80 % or less,

Mo: 0.01 % or more and 0.50 % or less,

N: 0.0010 % or more and 0.0080 % or less,

Si: 0.01 % or more and 0.50 % or less,

P: 0.015 % or less,

S: 0.0015 % or less,

Al: 0.010 % or more and 0.080 % or less, and

Ca: 0.0005 % or more and 0.0050 % or less, with the balance being Fe and inevitable impurities, and

a bainitic microstructure at a position 0.25 mm outward in a radial direction from an inner circumferential surface of the steel pipe, wherein an area ratio of bainite with a crystal plane orientation of {110} aligned within 15° is 30.0 % or less in the bainitic microstructure.
The steel pipe according to claim 1, in which the chemical composition further contains, in mass%, at least one selected from the group consisting of:
Cu: 0.30 % or less,

Ni: 0.10 % or less,

Cr: 0.50 % or less,

Nb: 0.1 % or less,

V: 0.1 % or less,

Ti: 0.1 % or less,

Zr: 0.02 % or less,

Mg: 0.02 % or less, and

REM: 0.02 % or less.
A method for producing a steel pipe, comprising:
heating a steel material having the chemical composition according to claim 1 or 2 to a temperature of 1000 °C or higher and 1300 °C or lower;

then hot rolling the steel material under the condition that total rolling reduction A in an austenite region is 95 % or less to obtain a hot-rolled steel plate;

subjecting the hot-rolled steel plate to cooling, wherein the cooling is performed within a temperature range starting from a cooling start temperature of at least (Ar₃ - 10 °C) for a steel plate surface temperature, to a cooling stop temperature of 550 °C or lower for a steel plate temperature at a depth of 0.25 mm below a steel plate surface, average cooling rate B within a temperature range of 750 °C to 550 °C for a steel plate temperature at the depth of 0.25 mm below the steel plate surface being 50 °C/s or less, and parameter F, defined by Equation (1) below in relation to the total rolling reduction A and the average cooling rate B, satisfying 0 or more and 30.00 or less, thereby obtaining a cooled steel plate, and

forming the cooled steel plate into a steel pipe $F = 3.35 A + 0.03 B - 286.27$