JP7722599B2 - Steel pipe and its manufacturing method - Google Patents

Description

The present invention relates to a steel pipe suitable for use as a line pipe for transporting crude oil or natural gas, particularly a steel pipe with excellent resistance to sulfide stress corrosion cracking (SSCC resistance) that is suitable for use in sour environments containing hydrogen sulfide. The present invention also relates to a method for manufacturing such a steel pipe.

Generally, steel pipes are manufactured by forming steel plates produced by plate mills or hot rolling mills into steel pipes using UOE forming, press bending, roll forming, etc.

Steel pipes for pipelines (line pipes) used to transport oil and natural gas are required to have various strengths, toughnesses, and weldability depending on the environment they are used in. Furthermore, line pipes used in sour environments of oil and natural gas containing hydrogen sulfide (H ₂ S) are required to have properties capable of suppressing hydrogen induced cracking (HIC) and sulfide stress corrosion cracking (SSCC), i.e., sour resistance.

HIC occurs when hydrogen ions produced by corrosion reactions are adsorbed onto the steel surface, penetrate the steel as atomic hydrogen, and then diffuse and accumulate around non-metallic inclusions such as MnS and hard second-phase structures in the steel, turning into molecular hydrogen, which then generates internal pressure and causes cracks. HIC is a problem in linepipes, which have a relatively low level of strength compared to oil country tubular goods, and many countermeasures have been disclosed.

On the other hand, SSCC is known to occur in the high hardness range of welds in oil country tubular goods and line pipe, and has generally not been considered much of a problem in line pipe, which has a relatively low hardness. However, it has been found that in line pipe, which has a relatively high hardness, SSCC may occur due to localized corrosion in the base material. Therefore, for line pipe used in sour environments, it is considered important to suppress localized corrosion in the base material and improve SSCC resistance.

Typically, the so-called TMCP (Thermo-Mechanical Control Process) technology, which combines controlled rolling and controlled cooling, is applied to the production of high-strength steel plate for line pipe. Increasing the cooling rate during controlled cooling is an effective way to increase the strength of steel plate using this TMCP technology. However, when controlled cooling is performed at a high cooling rate, the surface layer of the steel plate is rapidly cooled, making the surface layer harder than the interior of the steel plate. Furthermore, in the case of steel pipes, work hardening occurs when the steel plate is formed into a tubular shape, increasing the hardness of the surface layer of the steel pipe and resulting in reduced SSCC resistance.

In order to solve the above problems, for example, Patent Document 1 discloses a high-strength steel plate for sour-resistant line pipes having a tensile strength of 520 MPa or more and improved ^SSCC resistance, in which the steel structure 0.25 mm below the surface of the steel plate is a bainite structure with a dislocation density of 1.0 × 10 ¹⁴ to 7.0 × 10 ¹⁴ (m -2 ) and the variation in Vickers hardness is controlled.

Patent Document 2 also discloses a high-strength steel plate for sour-resistant line pipes with a tensile strength of 520 MPa or more, which has improved SSCC resistance by making the steel structure 0.25 mm below the surface of the steel plate a bainite structure and reducing the area ratio of crystal grains in the bainite with a KAM (Kernel Average Misorientation) value of 0.4 or more to 50% or less.

International Publication No. 2020/067209 International Publication No. 2021/020220

Steel pipes manufactured using steel plates with controlled dislocation density and hardness variations, as described in Patent Document 1, have excellent SSCC resistance. While the steel pipes thus obtained have sufficient SSCC resistance, there is room for improvement in that these properties cannot be consistently achieved.

Furthermore, as described in Patent Document 2, steel pipes manufactured using steel plates in which the KAM value is controlled in the bainite structure 0.25 mm below the steel plate surface have excellent SSCC resistance. While the steel pipes thus obtained have sufficient SSCC resistance, there is room for improvement in that these properties cannot be consistently obtained.

The present invention was made in consideration of the above-mentioned circumstances, and aims to provide a high-strength steel pipe that stably exhibits excellent SSCC resistance and a method for manufacturing the same. Here, "high strength" means a tensile strength of 520 MPa or more.

In order to solve the above problems, the present inventors have conducted extensive research into the microstructure of the inner surface of a steel pipe before and after evaluation of SSCC resistance, and into the manufacturing method of the steel pipe, and have obtained the following findings.
[1] SSCC begins with localized corrosion. Furthermore, localized corrosion is significantly affected by crystal plane orientation. Therefore, it is important to control the crystal plane orientation of the structure of the extreme surface layer inside the steel pipe, which is exposed to a sour environment, and to suppress localized corrosion, which is the initiation point of SSCC.
[2] In order to stably obtain excellent SSCC resistance, it is important to control the area ratio of bainite having a crystal plane orientation of {110} within 15° in the structure of the extreme surface layer on the inside of the steel pipe, specifically, in the bainite structure 0.25 mm below the steel plate surface, to 30% or less.
[3] In order to realize the above-mentioned microstructure, it is important to appropriately control the total reduction rate during rolling when producing a steel plate for steel pipes and the cooling rate after rolling.

The present invention was made based on the above findings and further investigations, and the gist of the present invention is as follows.
1. In mass %,
C: 0.020-0.080%,
Mn: 0.50 to 1.80%,
Mo: 0.01-0.50%,
N: 0.0010-0.0080%,
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%
and the balance being Fe and inevitable impurities,
A steel pipe in which the structure at a position 0.25 mm radially outward from the inner peripheral surface of the steel pipe is a bainite structure, and in the bainite structure, the area ratio of bainite having a crystal plane orientation in which the {110} crystal plane orientation is oriented at an angle of 15° or less is 30.0% or less.

2. The composition further contains, in mass%, 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,
2. The steel pipe according to 1 above, containing one or more selected from the group consisting of 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 described in 1 or 2 above to a temperature of 1000°C or higher and 1300°C or lower, and then hot rolling the steel material under conditions where the total reduction A in the austenite region is ₉₅ % or lower to obtain a hot-rolled steel plate, and cooling the hot-rolled steel plate to obtain a steel plate, wherein the average cooling rate B in the temperature range of 750°C to 550°C at the steel plate temperature 0.25 mm below the steel plate surface is 20 to 50°C/s within a temperature range where the cooling start temperature is (Ar 3 -10°C) or higher at the steel plate surface and the cooling stop temperature is 550°C or lower at the steel plate temperature 0.25 mm below the steel plate surface, and F, as defined by the following formula (1) for the total reduction A and the average cooling rate B, satisfies 0 or higher and 30.00 or lower, and the cooled steel plate is used as a steel pipe.
F=3.35A+0.03B-286.27...(1)

According to the present invention, excellent SSCC resistance can be stably achieved in steel pipes. Furthermore, according to the steel pipe manufacturing method of the present invention, steel pipes with excellent SSCC resistance can be stably manufactured.

FIG. 2 is a diagram illustrating the crystal plane orientation in a bainite structure. FIG. 1 is a diagram showing a method for collecting SSCC test specimens from steel pipes.

The following describes in detail an embodiment of the present invention. Note that the following description shows an example of a preferred embodiment of the present invention, and the present invention is not limited to this.

[Steel pipe]
A steel pipe according to one embodiment of the present invention is manufactured using a steel plate having a specific chemical composition and mechanical properties. The steel pipe is characterized in that, after the steel plate is formed into a steel pipe, the innermost surface layer of the steel pipe has a specific microstructure. The reasons for limiting the chemical composition, mechanical properties, and microstructure are explained below. Regarding the chemical composition, since the steel pipe is manufactured using a steel plate, the chemical composition of the steel pipe is the same as that of the steel plate used.

[Component composition]
First, the appropriate range of the chemical composition of the steel pipe, i.e., the steel plate, and the reasons for limiting it will be explained. In the following explanation, "%" as a unit of content represents "% by mass" unless otherwise specified.

It is important that the chemical composition of the steel pipe of the present invention specifies the C content, Mn content, Mo content and N content in order to obtain the strength and SSCC resistance required for the steel pipe.

C: 0.020-0.080%
C is an element that contributes to improving the strength of steel plate. If the C content is less than 0.020%, sufficient strength cannot be ensured, so the C content is set to 0.020% or more. Preferably, the C content is set to 0.025% or more. On the other hand, if the C content exceeds 0.080%, the hardness of the surface layer and the central segregation portion increases during accelerated cooling, resulting in deterioration of SSCC resistance and HIC resistance. In addition, the toughness of the steel plate also deteriorates. For this reason, the C content is set to 0.080% or less. Preferably, the C content is set to 0.070% or less.

Mn: 0.50-1.80%
Mn is an element that contributes to improving the strength and toughness of steel plates, and if the Mn content is less than 0.50%, the above-mentioned effects are not fully exhibited. Therefore, the Mn content is set to 0.50% or more. Preferably, the Mn content is set to 0.80% or more. On the other hand, if the Mn content exceeds 1.80%, the hardness of the surface layer and central segregation increases during accelerated cooling, resulting in deterioration of SSCC resistance and HIC resistance. Weldability also deteriorates. Therefore, the Mn content is set to 1.80% or less. Preferably, the Mn content is set to 1.70% or less.

Mo: 0.01~0.50%
Mo is an element that contributes to improving the strength and toughness of steel sheets and improving SSCC resistance. If the Mo content is less than 0.01%, the above-mentioned effects are not fully exhibited. Therefore, the Mo content is set to 0.01% or more. Preferably, the Mo content is set to 0.10% or more. On the other hand, if the Mo content is too high, the hardenability becomes excessive, which increases hardness and deteriorates SSCC resistance. In addition, weldability also deteriorates. Therefore, the Mo content is set to 0.50% or less. Preferably, the Mo content is set to 0.40% or less.

N: 0.0010-0.0080%
N is an element that contributes to improving the strength of steel plate, and for this purpose, N is contained in an amount of 0.0010% or more. Preferably, the N content is 0.0015% or more. On the other hand, if the N content exceeds 0.0080%, the hardness of the surface layer and the central segregation portion increases during accelerated cooling, resulting in deterioration of SSCC resistance and HIC resistance. In addition, the toughness of the steel plate also deteriorates. For this reason, the N content is set to 0.0080% or less. Preferably, the N content is set to 0.0070% or less.

In the steel pipe of the present invention, it is important to control the C content, Mn content, Mo content, and N content. Furthermore, since it is desirable for steel pipes used in sour environments to ensure HIC resistance and toughness, the chemical composition is such that the following components are added to the above components.

Si: 0.01~0.50%
Si is added for deoxidation, but if the Si content is less than 0.01%, the deoxidation effect is insufficient. Therefore, the Si content is set to 0.01% or more. Preferably, the Si content is set to 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. Preferably, the Si content is set to 0.45% or less.

P: 0.015% or less P is an inevitable impurity element that deteriorates weldability and increases the hardness of the center segregation, thereby deteriorating HIC resistance. This tendency becomes more pronounced when the P content exceeds 0.015%, so the P content is set to 0.015% or less. Preferably, the P content is set to 0.008% or less. Meanwhile, since it is desirable to reduce P as much as possible to obtain the above-mentioned effects, the lower limit of the P content is not particularly limited and may be 0%. However, excessive reduction leads to increased refining costs, so from the viewpoint of industrial production, the P content is preferably set to 0.001% or more.

S: 0.0015% or less S is an inevitable impurity element, and since it forms MnS inclusions in the steel sheet and deteriorates HIC resistance, a small amount is preferable, but an S content of up to 0.0015% is acceptable. Preferably, the S content is 0.0010% or less. On the other hand, since it is desirable to reduce S as much as possible to obtain the above-mentioned effects, the lower limit of the S content is not particularly limited and may be 0%. However, excessive reduction leads to an increase in refining costs, so from the viewpoint of industrial production, the S content is preferably 0.0002% or more.

Al: 0.010-0.080%
Al is added as a deoxidizer, but if the Al content is less than 0.010%, the addition effect is ineffective. Therefore, the Al content is set to 0.010% or more, 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. Preferably, the Al content is set to 0.070% or less.

Ca: 0.0005-0.0050%
Ca is an element effective in improving HIC resistance by controlling the morphology of sulfide-based inclusions, but if the Ca content is less than 0.0005%, the effect of adding it is insufficient. Therefore, the Ca content is set to 0.0005% or more, preferably 0.0008% or more. On the other hand, if the Ca content exceeds 0.0050%, not only does the above-mentioned effect saturate, but the cleanliness of the steel sheet also decreases, resulting in a deterioration in HIC resistance. Therefore, the Ca content is set to 0.0050% or less, preferably 0.0045% or less.

In one embodiment of the present invention, the steel pipe has a chemical composition containing the above elements, with the remainder being Fe and unavoidable impurities.

In addition, in another embodiment of the present invention, the chemical composition of the steel pipe may optionally further contain one or more elements selected from the group consisting of Cu, Ni, Cr, Nb, V, Ti, Zr, Mg and REM, in order to further improve the properties of the steel pipe.

Cu: 0.30% or less Cu is an element effective in improving the toughness and increasing the strength of steel plate. When Cu is added, the Cu content is preferably 0.05% or more to obtain the above effects. However, if the Cu content exceeds 0.30%, microcracks called Fischer cracks are likely to occur in an environment with a hydrogen sulfide partial pressure of less than 1 bar. Therefore, when Cu is added, the upper limit is set to 0.30%. Preferably, it is set to 0.20% or less.

Ni: 0.10% or less Ni is an element effective in improving the toughness and increasing the strength of steel plate. When Ni is added, the Ni content is preferably 0.01% or more to obtain the above effects. However, if the Ni content exceeds 0.10%, microcracks called Fischer cracks are likely to occur in an environment with a hydrogen sulfide partial pressure of less than 1 bar. Therefore, when Ni is added, the upper limit is set to 0.10%. Preferably, it is set to 0.02% or less.

Cr: 0.50% or less Like Mn, Cr is an effective element for obtaining sufficient strength in steel sheets even with low C content. When Cr is added, the Cr content is preferably 0.05% or more to obtain the above effect. However, if the Cr content exceeds 0.50%, the hardenability becomes excessive, which increases hardness and deteriorates SSCC resistance. Weldability also deteriorates. 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 added optionally to increase the strength and toughness of steel sheets. When Nb is added, the Nb content is preferably 0.005% or more to obtain the above-mentioned 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 Like Nb, V is an element that can be added optionally to increase the strength and toughness of steel sheets. When V is added, the V content is preferably 0.005% or more to obtain the above-mentioned 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 Like Nb and V, Ti is an element that can be added optionally to increase the strength and toughness of steel sheets. When Ti is added, the Ti content is preferably 0.005% or more to obtain the above-mentioned 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 added as needed to improve the toughness of steel sheets by refining crystal grains and to improve crack resistance by controlling the properties of inclusions. When Zr is added, the Zr content is preferably 0.0005% or more to obtain the above effects. However, when the Zr content exceeds 0.02%, the above effects saturate. Therefore, when Zr is added, the upper limit is set to 0.02%.

Mg: 0.02% or less Like Zr, Mg is an element that can be added as needed to improve the toughness of steel sheets by refining crystal grains and to improve crack resistance by controlling the properties of inclusions. When Mg is added, the Mg content is preferably 0.0005% or more to obtain the above effects. However, when the Mg content exceeds 0.02%, the above effects become saturated. Therefore, when Mg is added, the upper limit is set to 0.02%.

REM: 0.02% or less Like Zr and Mg, REM is an element that can be added optionally to improve the toughness of steel plate by refining crystal grains and to improve crack resistance by controlling the properties of inclusions. When REM is added, the REM content is preferably 0.0005% or more to obtain the above effects. However, when the REM content exceeds 0.02%, the above effects saturate. Therefore, when REM is added, the upper limit is set to 0.02%.

The present disclosure discloses a technique for improving the SSCC resistance of steel pipes, but it is preferable that sour gas resistance performance also satisfies HIC resistance along with SSCC resistance. To improve HIC resistance, it is preferable that the CP value calculated by the following formula (2) in the chemical composition of the steel pipe be 1.00 or less.
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, [% X] in the above formula (2) represents the content (mass %) of the X element, and is set to 0 when the element is not contained.

The CP value is a formula devised to estimate the material quality of the center segregation region from the content of each alloying element. The higher the CP value defined by formula (2) above, the higher the element concentration in the center segregation region, and the harder the center segregation region becomes. Therefore, by setting the CP value calculated by formula (2) above to 1.00 or less, it is possible to suppress cracking during HIC testing. Furthermore, since the lower the CP value, the lower the hardness of the center segregation region, so if even higher HIC resistance is required, the upper limit can be set to 0.95.

The remainder of the steel sheet consists of iron and unavoidable impurities. However, other trace elements may be included as long as they do not impair the effects of the present invention. For example, oxygen is an element that is inevitably included in steel sheet, but its content is acceptable in the present invention as long as it is 0.0050% or less, preferably 0.0040% or less.

[Microstructure]
Next, the microstructure of the steel pipe of the present invention will be described. In one embodiment of the present invention, the microstructure at a position 0.25 mm radially outward from the inner peripheral surface of the steel pipe (hereinafter also referred to as 0.25 mm below the inner surface of the steel pipe) is a bainite structure mainly composed of bainite. Here, "mainly composed of bainite" means that the area ratio of bainite is 95% or more.

Bainite structure: In order to maintain a constant maximum hardness at 0.25 mm below the surface of the inner surface of the steel pipe and improve SSCC resistance, the microstructure at 0.25 mm below the surface of the inner surface of the steel pipe needs to be a bainite structure. In particular, if hard phases such as martensite or island martensite (MA) are formed in the surface layer from the surface of the inner surface of the steel pipe to a depth of 0.25 mm, the hardness of the surface layer on the inner surface of the steel pipe increases, increasing the hardness variation in the surface layer on the inner surface of the steel pipe and hindering material uniformity. Therefore, the microstructure of the surface layer on the inner surface of the steel pipe needs to be a bainite structure.

Here, the term "bainite structure" refers to structures known as lath bainite or granular bainite, which contribute to transformation strengthening and transform during or after accelerated cooling. The presence of heterogeneous structures such as ferrite, martensite, pearlite, island martensite, and retained austenite in the bainite structure can result in a decrease in the strength and toughness of the steel pipe, as well as an increase in surface hardness. Therefore, the smaller the area fraction of structures other than bainite, the better. However, if the area fraction of structures other than bainite is sufficiently low, their effects can be ignored, and a certain amount is acceptable. Specifically, it is preferable that the total area fraction of structures other than bainite (ferrite, martensite, pearlite, island martensite, retained austenite, etc.) be less than 5%.

Furthermore, it is essential that the area ratio of bainite in the above-mentioned bainite structure, with the crystal plane orientation {110} oriented within 15°, is 30% or less. The reason for controlling the microstructure in the surface layer region of the inner steel pipe is that the inside of the steel pipe is in a sour environment and comes into contact with hydrogen sulfide. The reason for controlling the microstructure 0.25 mm below the inner steel pipe surface is that the crack depth at which SSCC occurrence is determined is at this 0.25 mm position.

Area fraction of bainite having a crystal plane orientation in which the {110} crystal plane orientation is within 15°: 30.0% or less The present inventors discovered that in a sour environment, the {110} crystal plane orientation of the bainite structure serves as the preferential dissolution plane and corrodes (localized corrosion). Furthermore, detailed studies were conducted, focusing on the crystal plane orientation of the bainite structure 0.25 mm below the surface of the inner surface of a steel pipe. Specifically, the inventors discovered that when the bainite structure is an aggregate of bainite with different crystal plane orientations on either side of a high-angle grain boundary with a misorientation of 15° or more, excellent SSCC resistance can be consistently achieved by setting the area fraction of bainite having a crystal plane orientation in which the {110} crystal plane orientation is within 15° of the {110} crystal plane orientation.

Here, for ease of understanding, "bainite having a plane orientation in which the {110} crystal plane orientation is oriented within 15°" will be explained with reference to Figure 1, assuming that the inner surface (curved surface) of a steel pipe is flat. In other words, as shown in Figure 1, the above-mentioned bainite refers to bainite having a plane orientation in which the axis l perpendicular to the {110} plane is oriented within 15° with respect to the axis L perpendicular to the plate surface. Note that, since the inner surface of a steel pipe is actually a curved surface, the above-mentioned axis L can be considered to be the normal to the above-mentioned inner surface.

It is believed that keeping the area fraction of bainite with the above-mentioned specific plane orientation to 30% or less will suppress the occurrence of the embryonic stage of localized corrosion, which is believed to be the starting point of SSCC. In other words, "bainite with a plane orientation in which the {110} crystal plane orientation is oriented within 15°" affects SSCC resistance, and by controlling the area fraction of this bainite to 30% or less, excellent SSCC resistance can be consistently achieved, as shown in the examples described below. On the other hand, it has been found that conventional techniques that control the bainite area fraction by the proportion of crystal grains with a KAM value of 0.4 or more, or by controlling the dislocation density and hardness, sometimes fail to control the bainite area fraction 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 the present invention is not particularly limited as long as the steel pipe can be manufactured. However, since SSCC resistance is less likely to be a problem in steel pipes with low tensile strength, the steel pipe according to the present invention preferably has a tensile strength of 520 MPa or more. In particular, steel pipes having a high tensile strength of 520 MPa or more can be suitably used for applications such as line pipes.

[Method of manufacturing steel pipes]
Next, a method for manufacturing a steel pipe according to one embodiment of the present invention will be described.
The steel pipe of the present invention can be produced by heating a steel material having the above-mentioned chemical composition, hot-rolling the material to form a hot-rolled steel sheet, subjecting the hot-rolled steel sheet to controlled cooling under predetermined conditions to form a steel sheet, and forming the steel sheet 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 method for producing the steel material is not particularly limited, but the steel material can be produced, for example, by melting molten steel having the above-mentioned composition by a conventional method and casting it. The melting can be carried out by any method, such as a converter, an electric furnace, or an induction furnace. Furthermore, from the viewpoint of productivity, the casting is preferably carried out by a continuous casting method, but may also be carried out by an ingot casting method.

Heating temperature: 1000° C. or higher and 1300° C. or lower The steel material is heated prior to hot rolling. The heating may be performed after the steel material obtained by a method such as casting has been cooled once, or the obtained steel material may be directly subjected to the heating without being cooled.

If the heating temperature of the steel material is below 1000°C, the carbides will not dissolve sufficiently and the necessary strength for the steel plate will not be obtained. Therefore, the heating temperature must be 1000°C or higher. On the other hand, if the heating temperature exceeds 1300°C, excessive energy will be required, reducing productivity. The toughness of the steel plate will also deteriorate. Therefore, the heating temperature must be 1300°C or lower. Note that the heating temperature is the temperature inside the heating furnace, and the steel material is heated to the heating temperature all the way to its center.

Total reduction in the austenite region: 95% or less Next, the heated steel material is hot-rolled to obtain a hot-rolled steel sheet. In order to produce a steel pipe satisfying the conditions of the present invention, the total reduction in the austenite region (above the _Ar3 transformation point) during the hot rolling is set to 95% or less. If the total reduction exceeds 95%, austenite with a {111} crystal plane orientation develops on the sheet surface due to shear deformation, and a bainite texture with a {110} crystal plane orientation develops after hot rolling and cooling. As a result, it becomes impossible to suppress the area fraction of bainite with a {110} crystal plane orientation within 15° in the bainite structure to 30% or less, which deteriorates the SSCC resistance of the steel pipe. For this reason, the total reduction is set to 95% or less. On the other hand, in order to ensure toughness, it is desirable to set the total reduction in the austenite region to 60% or more. In addition, from the viewpoint of improving strength and HIC resistance, it is preferable to carry out hot rolling in the austenite region.

- Rolling end temperature To obtain high toughness as a steel pipe, the lower the hot rolling end temperature, the better, but on the other hand, the rolling efficiency decreases. Therefore, the rolling end temperature needs to be set taking into consideration the toughness and rolling efficiency required for the steel pipe. From the viewpoint of improving the strength and HIC resistance of the steel pipe, it is preferable that the rolling end temperature be set to the _Ar3 transformation point or higher. The rolling end temperature is the surface temperature of the steel material, and the surface temperature can be measured using a radiation thermometer or the like.

Here, the _Ar3 transformation point means the ferrite transformation start temperature during cooling, and is calculated according to the following formula (3).
Ar ₃ (°C) = 910-273[%C]-74[%Mn]-56[%Ni]-16[%Cr]-9[%Mo]-5[%Cu]...(3)
However, [% X] in the above formula (3) represents the content (mass %) of the X element, and is set to 0 when the element is not contained.

Cooling Next, the hot-rolled steel sheet after hot rolling is cooled in a temperature range from a cooling start temperature to a cooling stop temperature, which will be described later.
...Cooling start temperature: steel plate surface temperature of (Ar ₃ - 10°C) or higher If the cooling start temperature is low, the amount of ferrite generated before cooling will be large. In particular, if the temperature drop from the Ar ₃ transformation point exceeds 10°C, ferrite will be generated in a volume fraction of more than 5%, resulting in a significant decrease in strength as a steel pipe and deterioration of HIC resistance. Therefore, the cooling start temperature is set to (Ar ₃ - 10°C) or higher. The cooling start temperature is set to be the rolling end temperature or lower. The cooling start temperature is the surface temperature of the hot-rolled steel plate and can be measured using a radiation thermometer, etc.

Cooling rate: average cooling rate of 50°C/s or less from 750°C to 550°C at a steel plate temperature 0.25 mm below the surface. In order to increase the strength of the steel plate, i.e., the steel pipe, while reducing hardness variation within the steel plate and improving material uniformity, it is important to control the cooling rate of the surface layer of the steel plate. In particular, to obtain the desired microstructure 0.25 mm below the surface of the steel plate, it is necessary to control the average cooling rate of 50°C/s or less from 750°C to 550°C at a steel plate temperature 0.25 mm below the surface of the steel plate.

By minimizing the average cooling rate, it is possible to create bainite with a crystal plane orientation where the {110} crystal plane orientation is not within 15°. Furthermore, as the average cooling rate is slowed, the maximum hardness can be reduced, thereby improving SSCC resistance. If the average cooling rate exceeds 50°C/s, the area fraction of bainite with a crystal plane orientation where the {110} crystal plane orientation is within 15° increases, causing the maximum HV0.5 hardness 0.25 mm below the steel plate surface to exceed 230. Furthermore, the transformation texture develops, increasing the area fraction of bainite with a crystal plane orientation where the {110} crystal plane orientation is within 15°, resulting in a deterioration of SSCC resistance after the steel plate is formed into a pipe. Therefore, the average cooling rate is set to 50°C/s or less, preferably 30°C/s or less. Although there is no particular limitation on the lower limit of the average cooling rate, if the average cooling rate is too low, ferrite and pearlite are generated, resulting in a steel sheet, i.e., a steel pipe, having insufficient strength. From the viewpoint of preventing the insufficiency of strength, the average cooling rate is preferably 20°C/s or more.

F (= 3.35A + 0.03B - 286.27): 30.00 or less As described above, in order to achieve an area fraction of 30.0% or less for bainite having a crystal plane orientation in which the {110} crystal plane orientation is within 15° at 0.25 mm below the surface of the steel sheet, the total rolling reduction in the austenite region and the average cooling rate from 750°C to 550°C at the steel sheet temperature 0.25 mm below the surface of the steel sheet are important. The inventors conducted further studies and found that it is not sufficient to simply control the total rolling reduction and the average cooling rate within predetermined ranges, and that when the total rolling reduction is A (%), the average cooling rate is B (°C/s), and the area fraction of bainite having a crystal plane orientation in which the {110} crystal plane orientation is within 15° F (%), F can be expressed by the following formula (1): If F exceeds 30.00, the area ratio of bainite having a plane orientation in which the sheet surface {110} is oriented at an angle of 15° or less exceeds 30.0%, resulting in a deterioration in SSCC resistance. On the other hand, the lower limit of F may be 0 or more.
F=3.35A+0.03B-286.27...(1)

Cooling stop temperature: 550°C or less at the steel sheet temperature 0.25 mm below the steel sheet surface. The hot-rolled steel sheet 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 less, which is the temperature range of bainite transformation, it is possible to generate a bainite phase. If the cooling stop temperature exceeds 550°C, the bainite transformation becomes incomplete, and sufficient strength cannot be obtained. Therefore, the cooling stop temperature is set to 550°C or less. Furthermore, if the cooling stop temperature is less than 250°C, the dislocation density increases, which significantly increases the hardness of the surface layer when the steel sheet is made into a pipe, and there is a risk of deterioration in SSCC resistance. Therefore, it is preferable that the cooling stop temperature be 250°C or more.

Furthermore, when cooling to a steel plate temperature of 550°C or less at a depth of 0.25 mm below the steel plate surface, if the cooling rate is slow, the cooling will not occur in a stable nucleate boiling state, which may result in variations in hardness at the very surface layer of the steel plate, a higher maximum Vickers hardness value, and a deterioration in SSCC resistance. Therefore, the average cooling rate from a steel plate temperature of 550°C at a depth of 0.25 mm below the steel plate surface to the cooling stop temperature is preferably 150°C/s or more. A more preferred average cooling rate is 170°C/s or more. There is no particular upper limit to the average cooling rate, but due to equipment constraints, it is preferable to set it to 250°C/s or less.

The steel plate temperature 0.25 mm below the surface cannot be physically measured directly, but it can be calculated in real time using, for example, a process computer to calculate the temperature distribution within the plate thickness cross section by differential calculation based on the surface temperature of the hot-rolled steel plate at the start of cooling measured with a radiation thermometer and the surface temperature of the hot-rolled steel plate at the target time of cooling stop. The temperature 0.25 mm below the surface of the steel plate in the above temperature distribution is referred to as the "steel plate temperature 0.25 mm below the surface of the steel plate" in this specification.

Steel Pipe Next, the cooled steel plate is made into a steel pipe. Methods for making steel pipe from steel plate include, for example, forming the steel plate into a tubular shape by press bending, roll forming, UOE forming, etc., and then welding the butt joints. By using the above-mentioned manufacturing method, it is possible to manufacture steel pipes with excellent material uniformity within the steel plate that are suitable for transporting crude oil and natural gas, such as UOE steel pipes, electric resistance welded steel pipes, and spiral steel pipes.

For example, UOE steel pipes are manufactured by first beveling the ends of steel plates and then forming them into a steel pipe shape using a C press, U press, or O press. The butt joints are then seam welded using internal and external welding, and a pipe expansion process is performed as needed. Any welding method can be used as long as it provides sufficient joint strength and toughness, but submerged arc welding is preferred from the perspective of superior weld quality and manufacturing efficiency.

The functions and effects of the present invention will be described below using examples, but the present invention is not limited to the following examples.
A steel slab having the chemical composition shown in Table 1 was produced as a steel material by continuous casting. The obtained steel slab was heated to the heating temperature shown in Table 2 and then hot-rolled at the total reduction in the austenite region shown in Table 2 to produce a hot-rolled steel sheet having the thickness shown in Table 2. The obtained hot-rolled steel sheet was then subjected to controlled cooling using a water-cooled controlled cooling device under the conditions shown in Table 2 to produce a steel sheet. The end of the obtained steel sheet was then beveled and formed into a steel pipe shape using a C press, a U press, and an O press. The inner and outer butt joints were then seam-welded by submerged arc welding, and the steel pipe was produced through a pipe expansion process. Since actual pipe products are to be coated on the outside, an aging heat treatment was performed at 250°C for 1 hour to simulate the external coating process.

(Measurement of crystal plane orientation)
In order to evaluate the microstructure of the steel pipe manufactured by the above-mentioned manufacturing method, measurements were performed by EBSD (Electron Backscatter Diffraction). That is, a test piece was taken from the inside of the steel pipe, and after removing the scale from the inside surface, mirror polishing and electrolytic polishing were performed so that the measurement surface was located at a depth of 0.25 mm from the surface after descaling. Next, using a scanning electron microscope, EBSD measurements were performed on the measurement surface at an acceleration 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 fraction of low-index planes {100} oriented at an angle of 15° or less, the area fraction of low-index planes {110} oriented at an angle of 15° or less, the area fraction of low-index planes {111} oriented at an angle of 15° or less, and the area fraction of high-index planes other than the above were calculated. From these calculation results, the area ratio where the crystal plane orientation {110} is oriented within 15° was calculated. The results are shown in Table 2.

As shown in Table 2, the steel pipes in the invention examples and comparative examples, except for Nos. 41 and 42, had a microstructure primarily composed of bainite. Here, "primarily composed of bainite" means that bainite accounted for 95% or more. On the other hand, Nos. 41 and 42 had a microstructure primarily composed of bainite.

(Evaluation of SSCC resistance)
The SSCC resistance of steel pipes manufactured by the above-described manufacturing method was evaluated. As shown in Figure 2, 5 x 25 x 125 mm SSCC test specimens were taken from the inner surface of the steel pipe. The inner surface of the steel pipe, the test surface, was left with black scale to preserve the state of the outermost layer. A four-point bending SSCC test was performed on the taken SSCC test specimens, applying a stress of 90% of the actual yield strength of each steel pipe, using NACE Standard TM0177 Solution B at a hydrogen sulfide partial pressure of 0.15 bar and a carbon dioxide partial pressure of 0.75 bar. If no cracks were observed after 720 hours of immersion, the SSCC resistance was judged to be "good," and if cracks were observed, the SSCC resistance was judged to be "poor." Here, "cracks" refers to cracks of 0.25 mm or more at the center of the test specimen in the longitudinal direction after the four-point bending test. The obtained SSCC resistance evaluation results are also shown in Table 2.

(Measurement of tensile strength)
The full thickness test pieces in the circumferential direction of the steel pipes were used as tensile test pieces, and a tensile test was carried out in accordance with ASTM A370 to measure the tensile strength. The tensile strengths of the obtained steel pipes are also shown in Table 2.

As shown in Table 2, all of the invention examples whose component composition and manufacturing conditions satisfied the appropriate ranges of the present invention had high strength and good SSCC resistance.

In contrast, all of the comparative examples, whose composition and/or manufacturing conditions were outside the range of the present invention, exhibited poor tensile strength or SSCC resistance. Specifically, Nos. 3, 4, 11, 12, 21, 22, 25, and 28 had composition outside the range of the present invention, and thus exhibited poor tensile strength or SSCC resistance. Furthermore, No. 36, although its composition was within the range, had a total reduction in the austenite region exceeding 95% and an F value exceeding 30.00, resulting in an area fraction of bainite with a crystal plane orientation of {110} within 15° exceeding 30.0%, and thus exhibited poor SSCC resistance. No. 43, although its composition was within the range, had an average cooling rate of 50°C/s or higher and an F value exceeding 30.00, resulting in an area fraction of bainite with a crystal plane orientation of {110} within 15° exceeding 30.0%, and thus exhibited poor SSCC resistance. In addition, in No. 41, the cooling start temperature was lower than (Ar ₃ -10°C) at the steel plate surface temperature, and the structure was less than 95% bainite, which was not a structure mainly composed of bainite, and therefore the tensile strength was low. In No. 42, the cooling stop temperature was higher than 550°C at the steel plate temperature 0.25 mm below the steel plate surface, and the structure was less than 95% bainite, which was not a structure mainly composed of bainite, and therefore the tensile strength was low.

As can be seen from the results shown in Table 2, the steel pipes of the present invention had excellent tensile strength in the steel plate and excellent SSCC resistance in the steel pipes. On the other hand, the steel pipes of the comparative examples, which were outside the scope of the present invention, had low tensile strength or poor SSCC resistance.

Claims (3)

In mass%,
C: 0.020-0.080%,
Mn: 0.50 to 1.80%,
Mo: 0.03-0.30%,
N: 0.0010-0.0080%,
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%
and the balance being Fe and inevitable impurities,
A steel pipe having excellent resistance to sulfide stress corrosion cracking, wherein the structure at a position 0.25 mm radially outward from the inner peripheral surface of the steel pipe is a bainite structure, and the area ratio of bainite in the bainite structure having a crystal plane orientation in which the {110} crystal plane orientation is within 15° is 30.0% or less. The composition further includes, in mass%, 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,
2. The steel pipe having excellent resistance to sulfide stress corrosion cracking according to claim 1, which contains one or more selected from the group consisting of Mg: 0.02% or less and REM: 0.02% or less. A steel material having the component composition according to claim 1 or 2 is heated to a temperature of 1000°C or higher and 1300°C or lower, and then hot-rolled under the condition that the total reduction ratio A in the austenite region is 95% or lower to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is subjected to a cooling start temperature: a steel sheet surface temperature (Ar ₃ a cooling stop temperature: within a temperature range of 550°C or less, the steel plate temperature at 0.25 mm below the surface of the steel plate is 750°C to 550°C, the average cooling rate B is 50°C/s or less, and F, defined by the following formula (1) regarding the total rolling reduction A and the average cooling rate B, satisfies 0 or more and 30.00 or less, and a steel plate is cooled to obtain a steel plate, and the cooled steel plate is used as a steel pipe, wherein a structure at a position 0.25 mm outward in the pipe radial direction from the inner peripheral surface is a bainite structure, and in the bainite structure, an area ratio of bainite having a crystal plane orientation in which a crystal plane orientation of {110} is within 15° is 30.0% or less .
F=3.35A+0.03B-286.27...(1)