JP7211168B2 - ERW steel pipe - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electric resistance welded steel pipe.

BACKGROUND ART In recent years, pipelines are becoming more and more important as a long-distance transportation method for crude oil and natural gas. At present, trunk line pipes for long-distance transportation are designed according to the standards of the American Petroleum Institute (API).

In particular, line pipes used in cold regions require not only strength but also excellent low-temperature toughness. Conventionally, UOE steel pipes have generally been used for such applications, but application of less expensive electric resistance welded steel pipes is desired.

For example, Patent Literatures 1 and 2 disclose high-strength hot-rolled steel sheets having excellent base material and weld zone toughness, which are suitable for X56 class electric resistance welded steel pipes in thick materials having a thickness of 18 mm or more. .

JP 2007-138289 A Japanese Unexamined Patent Application Publication No. 2007-138290

However, Patent Literatures 1 and 2 do not consider the structure control of the weld zone, and the low temperature toughness is evaluated by the Charpy impact test. In order to ensure reliability when used in cold regions, it is essential to evaluate not by the Charpy impact test but by the CTOD (Crack Tip Opening Displacement) test at the lowest operating temperature.

An object of the present invention is to provide an electric resistance welded steel pipe having high strength and excellent low temperature toughness.

The present invention has been made to solve the above problems, and the gist thereof is the following electric resistance welded steel pipe.

(1) An electric resistance welded steel pipe having a base material and a heat-treated seam portion,
The chemical composition of the base material is, in mass%,
C: 0.03 to 0.09%,
Si: 0.01 to 0.50%,
Mn: 0.80-1.60%,
P: 0.020% or less,
S: 0.003% or less,
Al: 0.060% or less,
Ti: 0.001 to 0.030%,
Nb: 0.01 to 0.04%,
N: 0.001 to 0.008%,
O: 0.005% or less,
Cu: 0-0.80%,
Ni: 0 to 0.80%,
Cr: 0 to 0.80%,
Mo: 0-0.80%,
V: 0 to 0.10%,
B: 0 to 0.0020%,
Ca: 0 to 0.0050%,
REM: 0-0.010%,
Sb: 0 to 0.10%,
Sn: 0-0.10%,
Co: 0-0.10%,
As: 0 to 0.10%,
Pb: 0 to 0.005%,
Bi: 0 to 0.005%,
H: 0 to 0.0005%,
balance: Fe and impurities,
Ceq represented by the following formula (i) is 0.20 to 0.53,
Pcm represented by the following formula (ii) is 0.150 to 0.250,
When the thickness of the base material is t _B , the metal structure at a position 1 mm in the thickness direction from the outer surface of the base material and at a position 1/2 t _B in the thickness direction from the outer surface of the base material has an area % contains 0% or more and 50% or less of ferrite, the balance is bainite, and has an average grain size of 15 μm or less,
In the base material, the difference between the hardness at a position 1 mm in the thickness direction from the outer surface and the hardness at a position 1/2 t _B in the thickness direction from the outer surface is 30 HV10 or less,
The metal structure at a position 3 mm in the thickness direction from the outer surface of the heat-treated seam portion contains more than 20% ferrite in terms of area%, the balance being bainite, and having an average grain size of 20 μm or less,
Assuming that the thickness of the heat-treated seam portion is _tS , the metal structure at a position 1/ _2tS in the thickness direction from the outer surface of the heat-treated seam portion contains more than 50% ferrite in terms of area%, and the balance is bainite. and has an average crystal grain size of 15 μm or less,
The maximum hardness _Hvmax at a position 3 mm in the thickness direction from the outer surface of the heat-treated seam portion is 220HV10 or less,
The CTOD value at −20° C. of the seam heat-treated portion is 0.40 mm or more,
ERW steel pipe.
Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 (i)
Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5×B (ii)
However, each element symbol in the formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained.

(2) the chemical composition of the base material, in mass %,
Cu: 0.01 to 0.80%,
Ni: 0.01 to 0.80%,
Cr: 0.01 to 0.80%,
Mo: 0.01 to 0.80%,
V: 0.001 to 0.10%,
B: 0.0001 to 0.0020%,
Ca: 0.0001 to 0.0050%, and
REM: 0.0001 to 0.010%,
The electric resistance welded steel pipe according to (1) above, containing one or more selected from.

(3) the thickness of the base material is 25.4 mm or less,
The electric resistance welded steel pipe according to (1) or (2) above.

According to the present invention, it is possible to obtain an electric resistance welded steel pipe having high strength and excellent low temperature toughness.

The present inventors have studied a method for obtaining an electric resistance welded steel pipe having high strength and excellent low-temperature toughness, and have obtained the following findings.

An electric resistance welded steel pipe is generally manufactured by roll-forming a steel plate, heating both ends of the steel plate by high-frequency induction heating, and performing butt welding. After that, heat treatment is applied under predetermined conditions to a predetermined range including the welded portion (matching surface). Therefore, a heat-treated region is formed in the welded portion and the base material on both sides of the welded portion. In the specification of the present application, the above treatment is referred to as seam heat treatment, and a region subjected to seam heat treatment and heated to a temperature range of Ac ₃ or higher is referred to as a seam heat treatment portion.

In order to achieve both high strength and excellent low-temperature toughness, it is important to control the metallographic structures of the base metal and the seam heat-treated portion, in addition to adjusting the chemical composition of the steel pipe.

In particular, the metallographic structure of the heat-treated seam portion is controlled by performing heat treatment for heating from the outer surface side and then cooling with water from the outer surface side. Therefore, a difference in cooling rate occurs between the outer surface side of the heat-treated seam portion and the center portion of the wall thickness, resulting in variations in structure.

If there is a large difference in hardness between the outer surface side of the heat-treated seam portion and the thickness central portion due to variations in the structure, low-temperature toughness cannot be ensured. Therefore, it is necessary to reduce variations in hardness over the entire heat-treated seam portion by strictly controlling the heat treatment and cooling conditions for the heat-treated seam portion.

The present invention has been made based on the above findings.

An electric resistance welded steel pipe according to the present invention has a base material, and a heat-treated seam portion including the heat-treated base material and a welded portion. The base material is cylindrical, and the weld extends parallel to the axial direction of the steel pipe. Further, as described above, the heat-treated seam portion includes the base material heat-treated on both sides of the welded portion in the circumferential direction of the electric resistance welded steel pipe. In other words, the heat-treated seam portion includes the base material that has been subjected to heat treatment, but in the following description, simply referring to the "base material" refers to the portion excluding the welded portion and the heat-treated seam portion. .

In the present invention, it is important to adjust the chemical composition of the base material and seam heat-treated portion and to control the metal structure. When steel plates are electric resistance welded to form an electric resistance welded steel pipe, no welding material or the like is used, so that the chemical composition of the base material and the weld zone are substantially the same. Each requirement of the present invention will be described in detail below.

1. Chemical Composition of Base Material The reasons for limiting each element are as follows. In addition, "%" about content in the following description means "mass %."

C: 0.03-0.09%
C is an element that improves the base material strength of steel. In order to obtain the above effect, it is necessary to contain 0.03% or more of C. On the other hand, when the C content exceeds 0.09%, the weldability and low-temperature toughness of the steel are deteriorated. Therefore, the C content should be 0.03 to 0.09%. The C content is preferably 0.035% or more, more preferably 0.04% or more. Also, the C content is preferably 0.08% or less, more preferably 0.075% or less.

Si: 0.01-0.50%
Si is an element necessary as a deoxidizing element in steelmaking. In order to obtain the above effects, it is necessary to contain 0.01% or more of Si. On the other hand, when the Si content exceeds 0.50%, the toughness of the heat-treated seam portion is lowered. Therefore, the Si content should be 0.01 to 0.50%. The Si content is preferably 0.015% or more. Also, the Si content is preferably 0.40% or less, more preferably 0.30% or less.

Mn: 0.80-1.60%
Mn is an element necessary for ensuring the strength and low-temperature toughness of the base material. In order to obtain the above effect, it is necessary to contain 0.80% or more of Mn. On the other hand, if the Mn content exceeds 1.60%, the toughness of the heat-treated seam portion is significantly impaired. Therefore, the Mn content should be 0.80 to 1.60%. The Mn content is preferably 1.00% or more and preferably 1.50% or less.

P: 0.020% or less P is an element that is contained as an impurity and affects the low-temperature toughness of steel. significantly impede. Therefore, the P content is set to 0.020% or less. The P content is preferably 0.018% or less. Although the P content is preferably as small as possible, it can be made 0.001% or more from the viewpoint of manufacturing cost.

S: 0.003% or less S is an element contained as an impurity, and if the content exceeds 0.003%, it causes the formation of coarse sulfides and impairs low-temperature toughness. Therefore, the S content is made 0.003% or less. The S content is preferably 0.002% or less. Although the S content is preferably as small as possible, it can be made 0.0001% or more from the viewpoint of manufacturing cost.

Al: 0.060% or less Al is an element usually added as a deoxidizer. However, when the Al content exceeds 0.060%, the toughness of the base material and the heat-treated seam portion deteriorates. Therefore, the Al content is set to 0.060% or less. The Al content is preferably 0.030% or less. There is no lower limit to the Al content, and it may be 0%. In order to enhance the deoxidizing effect, the Al content is preferably 0.001% or more.

Ti: 0.001-0.030%
Ti is an element that exerts an effect of refining crystal grains as a nitride-forming element. In order to obtain the above effect, it is necessary to contain 0.001% or more of Ti. On the other hand, when the Ti content exceeds 0.030%, the formation of carbides causes a significant decrease in low temperature toughness. Therefore, the Ti content should be 0.001 to 0.030%. The Ti content is preferably 0.003% or more, more preferably 0.005% or more, and preferably 0.025% or less.

Nb: 0.01-0.04%
Nb is an element that forms carbides and/or nitrides and contributes to strength improvement. In addition, by suppressing recrystallization in the austenite region, expanding the non-recrystallized rolling temperature region, and further improving the hardenability of austenite, ferrite generated during accelerated cooling after rolling and after seam heat treatment It has the effect of finely homogenizing the structure of bainite, etc., and improving the toughness of the steel pipe base material part and the seam heat treatment part. In order to obtain the above effect, it is necessary to contain 0.01% or more of Nb. On the other hand, when the Nb content exceeds 0.04%, the low temperature toughness is lowered. Therefore, the Nb content is set to 0.01 to 0.04%. The Nb content is preferably 0.015% or more and preferably 0.03% or less.

N: 0.001 to 0.008%
N is an element that affects the low temperature toughness of steel. The N content is set to 0.001% or more in order to form nitrides, refine crystal grains, and improve low-temperature toughness. On the other hand, if the N content exceeds 0.008%, the toughness of not only the base material but also the heat-treated seam portion is significantly impaired. Therefore, the N content is set to 0.001 to 0.008%. The N content is preferably 0.006% or less.

O: 0.005% or less O is an element that is contained as an impurity and affects the low-temperature toughness of steel. significantly impede. Therefore, the O content is set to 0.005% or less. The O content is preferably 0.003% or less. Although the O content is preferably as small as possible, it can be 0.001% or more from the viewpoint of production costs.

Cu: 0-0.80%
Cu is an element effective in increasing the strength without lowering the low-temperature toughness, so it may be contained as necessary. However, if the content exceeds 0.80%, cracks tend to occur during billet heating and welding. Therefore, the Cu content is set to 0.80% or less. The Cu content is preferably 0.50% or less. In order to obtain the above effects, the Cu content is preferably 0.01% or more, more preferably 0.10% or more.

Ni: 0-0.80%
Ni is an element effective in improving low-temperature toughness and strength, so it may be contained as necessary. However, when the content exceeds 0.80%, the weldability deteriorates. Therefore, the Ni content is set to 0.80% or less. The Ni content is preferably 0.50% or less. In order to obtain the above effects, the Ni content is preferably 0.01% or more, more preferably 0.10% or more.

Cr: 0-0.80%
Cr is an element that improves the strength of steel by precipitation strengthening, so it may be contained as necessary. However, if its content exceeds 0.80%, it increases the hardenability, produces a bainite structure, and lowers the low temperature toughness. Therefore, the Cr content is set to 0.80% or less. The Cr content is preferably 0.50% or less. To obtain the above effects, the Cr content is preferably 0.01% or more, more preferably 0.10% or more.

Mo: 0-0.80%
Mo is an element that improves hardenability and at the same time forms carbonitrides to improve strength, so it may be contained as necessary. In addition, by containing Nb in a composite manner, recrystallization in the austenite region is suppressed, the non-recrystallization rolling temperature range is expanded, and the hardenability of austenite is improved. It has the effect of finely homogenizing the structure of ferrite, bainite, etc., which are generated during subsequent accelerated cooling, and improving the toughness of the steel pipe base metal portion and seam heat treated portion. However, if the content exceeds 0.80%, excessive strengthening and a significant decrease in low-temperature toughness are caused. Therefore, Mo content shall be 0.80% or less. Mo content is preferably 0.50% or less. In order to obtain the above effects, the Mo content is preferably 0.01% or more, more preferably 0.05% or more.

V: 0-0.10%
V is an element that forms carbides and/or nitrides and contributes to the improvement of strength, so it may be contained as necessary. However, when the content exceeds 0.10%, the low temperature toughness is lowered. Therefore, the V content is set to 0.10% or less. The V content is preferably 0.060% or less. To obtain the above effect, the V content is preferably 0.001% or more, more preferably 0.010% or more.

B: 0 to 0.0020%
B is an element that improves hardenability, so it may be contained as necessary. However, when the content exceeds 0.0020%, precipitates of B are formed, deteriorating the low temperature toughness. Therefore, the B content is set to 0.0020% or less. The B content is preferably 0.0015% or less. In order to obtain the above effects, the B content is preferably 0.0001% or more, more preferably 0.0003% or more.

Ca: 0-0.0050%
Ca suppresses the formation of elongated MnS by forming sulfides and improves the properties of the steel material in the plate thickness direction, particularly lamellar tear resistance, so that it may be contained as necessary. However, when the content exceeds 0.0050%, the number of Ca oxides in the base material and seam heat-treated portion increases. As a result, the Ca oxide becomes the starting point of fracture, and the low temperature toughness is greatly reduced. Therefore, the Ca content is set to 0.0050% or less. The Ca content is preferably 0.0040% or less. In order to obtain the above effect, the Ca content is preferably 0.0001% or more, more preferably 0.0010% or more.

REM: 0-0.0100%
Like Ca, REM suppresses the formation of elongated MnS by forming sulfides and improves the properties of the steel material in the plate thickness direction, especially the lamellar tear resistance. good. However, when the content exceeds 0.0100%, the number of REM oxides increases and the low temperature toughness decreases. Therefore, the REM content is set to 0.0100% or less. The REM content is preferably 0.0050% or less, more preferably 0.0040% or less. To obtain the above effect, the REM content is preferably 0.0001% or more, more preferably 0.0010% or more.

Here, REM refers to a total of 17 elements of Sc, Y and lanthanoids, and the REM content means the total content of these elements.

Sb: 0-0.10%
Sb may be included as an impurity. Sb is an element that affects the strength and low-temperature toughness of steel, and if its content exceeds 0.10%, it significantly impairs not only the toughness of the base metal but also the toughness of the heat-treated seam portion. Therefore, the Sb content is set to 0 to 0.10%. The Sb content is preferably 0-0.01%.

Sn: 0-0.10%
Sn may be included as an impurity. Sn is an element that affects the strength and low-temperature toughness of steel, and if its content exceeds 0.10%, it significantly impairs not only the toughness of the base metal but also the toughness of the heat-treated seam portion. Therefore, the Sn content is set to 0 to 0.10%. The Sn content is preferably 0-0.01%.

Co: 0-0.10%
Co is an element that is contained as an impurity and affects the strength and low-temperature toughness of steel, and if its content exceeds 0.10%, weldability deteriorates. Therefore, the Co content is set to 0.10% or less. The Co content is preferably 0-0.07%.

As: 0-0.10%
As is contained as an impurity and is an element that affects the strength and low-temperature toughness of steel. When the content exceeds 0.10%, the toughness of not only the base material but also the heat-treated seam portion is significantly impaired. Therefore, the As content is set to 0 to 0.10%. The As content is preferably 0-0.01%.

Pb: 0-0.005%
Pb may be included as an impurity. Pb is an element that affects the low-temperature toughness of steel, and when its content exceeds 0.005%, it significantly impairs the toughness not only of the base metal but also of the heat-treated seam portion. Therefore, the Pb content should be 0 to 0.005%. The Pb content is preferably 0-0.001%.

Bi: 0-0.005%
Bi may be contained as an impurity. Bi is an element that affects the low-temperature toughness of steel, and when its content exceeds 0.005%, it significantly impairs the toughness of not only the base metal but also the heat-treated seam portion. Therefore, the Bi content should be 0 to 0.005%. The Bi content is preferably 0-0.001%.

H: 0-0.0005%
H may be included as an impurity. H is an element that affects the low-temperature toughness of steel, and if its content exceeds 0.0005%, the toughness of not only the base metal but also the heat-treated seam portion is significantly impaired. Therefore, the H content should be 0 to 0.0005%. The H content is preferably 0-0.0001%.

In the above chemical composition, the balance is Fe and impurities. Here, the term "impurities" refers to components mixed in by various factors in raw materials such as ores, scraps, etc., and in the manufacturing process when steel is manufactured industrially. means something As for P, S, O, Sb, Sn, Co, As, Pb, Bi, and H, it is necessary to limit the contents as described above.

Ceq: 0.20-0.53
Ceq is a value that serves as an index of hardenability and is represented by the following formula (i). If the Ceq is less than 0.20, the required strength cannot be obtained. On the other hand, when the Ceq exceeds 0.53, the low temperature toughness deteriorates. Therefore, Ceq is set to 0.20 to 0.53. Ceq is preferably 0.30 or more and preferably 0.50 or less.
Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 (i)
However, each element symbol in the formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained.

Pcm: 0.150-0.250
Pcm is a value that serves as an index of weldability and is represented by the following formula (ii). In addition, each element on the right side of Pcm has the effect of improving the strength of steel, so if Pcm is small, the required strength may not be obtained. In particular, when Pcm is less than 0.150, the required strength cannot be obtained. On the other hand, when Pcm exceeds 0.250, the low temperature toughness deteriorates. Therefore, Pcm is set to 0.150 to 0.250. Pcm is preferably 0.152 or more and preferably 0.245 or less.
Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5×B (ii)
However, each element symbol in the formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained.

2. Metallographic structure As described above, in order to improve the strength and low-temperature toughness of a steel pipe, it is important to control the metallographic structure in the base metal and the seam heat-treated portion. Each of the base material and the seam heat treated portion will be described in detail below.

In order to ensure the strength and low-temperature toughness of steel pipes, it is important to control the metallographic structure of the base material. Specifically, when the thickness of the base material is t _B , the metal structure of the outer surface layer part and 1/2t _B part of the base material contains 0 to 50% ferrite in terms of area%, and the balance is It must be bainite. If the area ratio of ferrite contained in the base material exceeds 50%, the strength may decrease.

Here, ferrite in the present invention includes not only polygonal ferrite but also acicular ferrite. The acicular ferrite refers to ferrite in which the prior austenite grain boundaries are unclear and the grain interiors are in the form of lumps or needles. In addition, bainite includes not only a mixed structure of lath-like ferrite and carbide, but also a mixed structure containing one or two selected from pearlite and MA (Martensite-Austenite Constituent) in the structure. Granular bainite in which the lath-like morphology is broken is also included.

In the present invention, the outer surface layer portion and the 1/2t _B portion of the base material mean a depth position of 1 mm and a depth position of 1/2t _B in the thickness direction from the outer surface of the base material, respectively. .

In order to ensure good low-temperature toughness, it is necessary to set the average crystal grain size in the outer surface layer portion and 1/2t _B portion of the base material to 15 μm or less. The average crystal grain size is preferably 13 μm or less. When simply referring to the metal structure of the base material, the area ratio of the ferrite of the base material, and the average crystal grain size of the base material, the metal structure of both the surface layer portion and the 1/2t _B portion of the base material, the area ratio of ferrite, the average It means grain size.

In addition, from the viewpoint of ensuring low-temperature toughness in the heat-treated seam portion, the metallographic structure of the heat-treated seam portion needs to be composed of a predetermined area ratio of ferrite and the balance of bainite. Here, ferrite in the present invention includes not only polygonal ferrite but also acicular ferrite. The acicular ferrite refers to ferrite in which the prior austenite grain boundaries are unclear and the grain interiors are in the form of lumps or needles. Also, bainite refers to a structure containing at least one selected from lath-like ferrite (including granular bainite), carbide, and pearlite.

As will be described later, the metallographic structure of the heat treated seam portion is controlled by water cooling after heat treatment from the outer surface side. Therefore, a difference occurs in the cooling rate between the heat-treated seam portion and the center portion of the thickness on the outer surface side, resulting in variation in the structure. Therefore, the ferrite area ratio differs depending on the depth.

Specifically, when the thickness of the heat-treated seam portion is _tS , the area ratio of ferrite is more than 20% at a position 3 mm in the thickness direction from the outer surface of the heat-treated seam portion. In addition, the area ratio of ferrite shall be more than 50% at the position of 1/ _2tS from the surface of the heat-treated seam portion. Moreover, the balance other than ferrite is bainite in any part. Although no particular upper limit is defined for the area ratio of ferrite, it is preferably less than 40% at a position 3 mm in the thickness direction from the outer surface of the heat-treated seam portion, and at a position 1/2 _tS from the surface of the heat-treated seam portion. is preferably less than 80%.

In the present invention, the depth position of 3 mm and the depth position of 1/2t _S from the outer surface of the heat treated seam portion may be referred to as the outer surface layer portion and the 1/2t _S portion of the heat treated seam portion, respectively.

In order to ensure good low-temperature toughness, it is important to _refine the crystal grains. It is necessary to control the diameter to 15 μm or less. The average grain size is preferably 13 μm or less at any position.

In the present invention, the metal structure is obtained as follows. First, two test pieces are cut out from each of the base material and the heat-treated seam section in the thickness direction, and used for structure observation and grain size measurement.

In the present invention, the thickness direction cross section of the base material is a cross section parallel to the length direction and thickness direction of the steel pipe at a position 180° away from the welded portion in the circumferential direction of the steel pipe. . Further, the thickness direction cross section of the heat-treated seam portion is a cross-section that passes through the heat-treated seam portion including the welded portion and is perpendicular to the length direction of the steel pipe. The welded portion is the contact surface of electric resistance welding, and the position in the circumferential direction of the steel pipe can be specified from the metal flow.

Moreover, in the present invention, the area ratio of ferrite is measured by the following method. The collected sample for tissue observation is polished with a colloidal silica abrasive for 30 to 60 minutes. The polished sample is analyzed using EBSP-OIM to determine the area ratio of ferrite. The visual field range is 200 μm (base material: length direction, seam heat-treated portion: circumferential direction)×500 μm (thickness direction). The observation magnification is 400 times and the measurement step is 0.3 μm.

Specifically, the area ratio of ferrite is obtained by the KAM (Kernel Average Misorientation) method provided in EBSP-OIM.

In the KAM method, any one regular hexagonal pixel in the measurement data is taken as the central pixel. First approximation using 6 pixels adjacent to this central pixel (7 pixels in total), or second approximation using 12 pixels outside of these 6 pixels (19 pixels in total) , or the third approximation (total 37 pixels) using 18 pixels further outside of these 12 pixels, obtain the orientation difference between each pixel. The obtained orientation differences are averaged, and the obtained average value is taken as the value of the central pixel. This operation is performed for all pixels.

In the present embodiment, pixels with an orientation difference of 5° or less between adjacent pixels by the third approximation are displayed. In this embodiment, the ferrite area ratio is defined as the area ratio of pixels calculated to have a misorientation of 1° or less in the third approximation with respect to the entire area of the viewing range. A structure other than ferrite such as bainite has a misorientation exceeding 1° in the third approximation.

The area ratio of ferrite in the base material portion is the average value of the area ratio of ferrite measured at 7 locations at intervals of 0.5 mm in the length direction at each plate thickness position of the outer surface layer portion and 1/2t _B portion. be. In addition, the ferrite area ratio of the heat-treated seam portion is the average value of the ferrite area ratios measured at a total of 7 points, including the center position of the welded part and three points on each side of the center position in the circumferential direction at a pitch of 0.5 mm. , outer surface layer and 1/2t _S section. Here, the center position of the welded part is the above-mentioned welded part (contact surface of electric resistance welding), and the position is specified by leaving an impression of a Vickers hardness tester, for example, before polishing with a colloidal silica abrasive. is preferred.

Also, the average crystal grain size is measured using EBSP-OIM. Specifically, a sample is taken and polished in the same manner as in the measurement of the area ratio of ferrite. Polished samples are analyzed using EBSP-OIM. More specifically, the grain boundary is defined as a position where the orientation difference between adjacent measurement points exceeds 15° in the orientation measurement at each fixed measurement step. 15° is the threshold value of the large tilt angle grain boundary, which is generally recognized as the grain boundary.

Without specifying whether the metal structure is ferrite or a structure other than ferrite, the grain size and the surface area of the crystal grain are obtained by regarding the region surrounded by the high-angle grain boundary as the crystal grain. The area-average particle size is determined from the obtained particle size and surface area. In this specification, the determined area average grain size is defined as the average crystal grain size. The visual field range is 200 μm (base material: longitudinal direction, heat-treated seam portion: circumferential direction)×500 μm (thickness direction). The observation magnification is 400 times and the measurement step is 0.3 μm.

The average crystal grain size of the base material is the average value of the average crystal grain sizes measured at 7 points in the rolling direction at a pitch of 0.5 mm at each plate thickness position of the outer surface layer portion and the 1/2t _B portion. The average crystal grain size of the seam heat-treated part is the average value of the average crystal grain sizes measured at a total of 7 points, ie, the central position of the welded part and three points on each side in the circumferential direction with a pitch of 0.5 mm from the central position. , outer surface layer and 1/2t _S section.

3. Mechanical Properties A large variation in hardness in the base metal degrades the low temperature toughness. Therefore, the difference in hardness between the outer surface layer portion and the 1/2t _B portion in the cross section in the thickness direction of the base material is set to 30HV10 or less.

In the heat-treated seam portion, the low-temperature toughness deteriorates when the hardness of the outer surface side, which has a high cooling rate after the heat treatment, increases. In the _CTOD test, which will be described later, the stress condition is the most severe at a position 3 mm from the surface of the test piece in the center direction. exceeds 220HV10, the low temperature toughness deteriorates. Therefore, the maximum hardness Hv _max at a position 3 mm in the thickness direction from the outer surface of the heat-treated seam portion is set to 220HV10 or less. "HV10" means a "hardness symbol" when a Vickers hardness test is performed with a test force of 98 N (10 kgf) (see JIS Z 2244:2009).

The hardness of the base material portion is the average value of the hardness measured at 7 locations in the rolling direction at a pitch of 0.5 mm at each plate thickness position. The hardness of the heat-treated seam portion is the average value of hardness measured at a total of 7 points, ie, the central position of the welded part and three points on both sides of the central position in the circumferential direction at a pitch of 0.5 mm.

In the present invention, the difference in hardness is determined as follows. First, a test piece is cut out from each cross section of the base material in the thickness direction, and used for hardness measurement. Then, in the cross section in the thickness direction of the base material, the hardness is measured at the outer surface layer portion and the 1/2t _B portion, and the difference is calculated.

In order to ensure reliability when used in cold regions, it is necessary to set the CTOD value of the heat-treated seam portion to 0.40 mm or more. The CTOD test is performed using a CTOD test piece that includes the heat treated seam portion and is cut to a length of 300 mm in the longitudinal direction and 300 mm in the circumferential direction including the heat treated seam portion. The CTOD value of the heat-treated seam portion may be measured by conducting a CTOD test at a test temperature of -20°C in accordance with the BS7448 standard.

In the electric resistance welded steel pipe of the present invention, other mechanical properties are not particularly limited. However, when used as a line pipe, it is preferable that the yield stress is 440 MPa or more and the tensile strength is 500-700 MPa.

4. Thickness There is no particular limitation on the thickness of the electric resistance welded steel pipe of the present invention. However, when used as a line pipe, the wall thickness is preferably 10.0 mm or more, more preferably 15.0 mm or more, from the viewpoint of improving the transportation efficiency of fluid passing through the pipe. On the other hand, the upper limit of the thickness of the electric resistance welded steel pipe is generally 25.4 mm.

5. Manufacturing Method The electric resistance welded steel pipe according to the present invention can be manufactured, for example, by the following method, but is not limited to this method.

After the steel having the chemical composition described above is melted in a furnace, a slab is produced by casting. After that, the slab is heated to a temperature range of 1000° C. or higher and hot rolled. If the heating temperature is less than 1000° C., the load on the rolling mill becomes high and the rolling efficiency is remarkably lowered. On the other hand, if the heating temperature exceeds 1200° C., the austenite grains become coarse and a fine structure may not be obtained. More preferably, the temperature is 1150° C. or lower.

In order to refine the metal structure of the base material, it is preferable to set the rolling reduction ratio in the recrystallized region to 2 or more and the rolling reduction ratio in the non-recrystallized region to 3 or more during hot rolling. In particular, by setting the reduction ratio in the non-recrystallized region to 3 or more, it is possible to make the average crystal grain size of the base metal 15 μm or less. The reduction ratio in the non-recrystallized region is preferably 4 or more. The boundary between the recrystallized region and the non-recrystallized region is approximately 900 to 950° C., although it depends on the composition of the steel.

Furthermore, in the finish rolling, it is preferable that the rolling in the austenite region is performed in one pass or more and the rolling reduction in one pass is 20% or less. Moreover, it is preferable that finish rolling finish temperature shall be 770 degreeC or more. After finishing rolling, it is water-cooled to a temperature range of 450 to 550° C. and wound up within the temperature range.

Then, the obtained hot-rolled steel sheet is roll-formed and high-frequency welding is performed to form an electric resistance welded steel pipe, thereby manufacturing an electric resistance welded steel pipe. Subsequently, seam heat treatment is applied to the welded portion and its surroundings.

In the seam heat treatment, heat treatment is performed from the outer surface side, heating to a temperature range of 900 to 1000° C., and then water cooling to 600° C. or less. The heat treatment can be performed by, for example, high-frequency induction heating, burner heating, or electric resistance heating. High-frequency induction heating, which is excellent in heating responsiveness and uniformity, is preferably employed.

At this time, the outer surface portion is generally heated in many cases. Hard martensite forms in the structure of the seam heat treated zone during subsequent cooling, which may reduce weld zone toughness.

In addition, in order to reduce the maximum hardness in the heat-treated seam portion and ensure good low-temperature toughness, it is necessary to limit the amount of bainite produced and ensure a predetermined amount or more of ferrite.

In order to suppress the formation of these hard martensite and promote the formation of ferrite, it is effective to perform multistage accelerated cooling in the cooling process after the heat treatment. Multi-stage accelerated cooling is a method of reducing the water density in accelerated cooling, shortening the cooling time, and dividing the accelerated cooling into two or three or more times.

In particular, in order to limit the amount of bainite produced, it is important that the first cooling stop temperature does not fall below the Bs point. After that, after the surface temperature rises to 750° C. or more by reheating, the second cooling is started. By setting the water cooling stop temperature after multistage accelerated cooling to 600°C or less, the formation of hard martensite is suppressed, the increase in hardness of the outer surface layer of the weld zone is suppressed, and low-temperature toughness can be secured. can. There are no particular restrictions on the cooling stop temperature and the temperature after reheating in the accelerated cooling after the second time.

Here, the Bs point (°C) is represented by the following formula (iii) and means the temperature at which bainite is formed.
Bs=830-270C-90Mn-37Ni-70Cr-83Mo (iii)
However, each element symbol in the formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained.

By performing the above processes, it is possible to control the metal structure and hardness of the heat-treated seam portion within the ranges described above.

EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to these examples.

A 240 mm thick steel ingot having the chemical composition shown in Table 1 was heated to the temperature shown in Table 2. Then, hot rolling was performed in the recrystallized zone to a thickness of 55 to 100 mm. After that, hot rolling was performed to the thickness shown in Table 2 in the non-recrystallized region from the starting temperature shown in Table 2 to 800°C. Subsequently, water cooling was performed under the conditions shown in Table 2.

After that, the hot-rolled steel sheet was cut into slits to form a coil, and electric resistance welding was performed. Then, under the conditions shown in Table 2, the welded portion and its periphery were heated by high-frequency induction heating, and then cooled with water.

Three test pieces were cut out from each of the thickness direction cross-sections of the base material and the heat-treated seam portion of each of the obtained electric resistance welded steel pipes, and used for structure observation, grain size measurement, and hardness measurement.

After polishing the collected sample for structural observation with a colloidal silica abrasive for 30 to 60 minutes, it was analyzed using EBSP-OIM to determine the area ratio of ferrite. The field of view was 200 μm (base material: rolling direction, seam heat-treated portion: circumferential direction)×500 μm (thickness direction), the observation magnification was 400, and the measurement step was 0.3 μm.

Specifically, the area ratio of ferrite was determined by the KAM (Kernel Average Misorientation) method provided in EBSP-OIM.

In the KAM method, any one regular hexagonal pixel in the measurement data is taken as the central pixel. First approximation using 6 pixels adjacent to this central pixel (7 pixels in total), or second approximation using 12 pixels outside of these 6 pixels (19 pixels in total) , or the third approximation (total 37 pixels) using 18 pixels further outside of these 12 pixels, obtain the orientation difference between each pixel. The obtained orientation differences are averaged, and the obtained average value is taken as the value of the central pixel. This operation is performed for all pixels.

In this embodiment, the third approximation is used to display the orientation difference of 5° or less between adjacent pixels. Then, the area ratio of pixels calculated to have an orientation difference of 1° or less in the third approximation with respect to the entire area of the viewing range was defined as the area ratio of ferrite. On the other hand, those with a misorientation exceeding 1° in the third approximation were treated as structures other than ferrite such as bainite.

Then, the area ratio of ferrite in the base material portion was the average value of the area ratios of ferrite measured at 7 locations at a pitch of 0.5 mm in the rolling direction at each plate thickness position of the outer surface layer portion and the 1/2t _B portion. . In addition, the area ratio of ferrite in the seam heat treatment part is, at each plate thickness position of the outer surface layer part and the 1/2t _S part, the center position of the welded part and three points each in the circumferential direction at a pitch of 0.5 mm on both sides from the center position. , and the average value of the area ratio of ferrite measured at a total of seven locations.

Also, the average grain size was measured using EBSP-OIM. Specifically, samples were taken, polished, and analyzed using EBSP-OIM in the same manner as in the measurement of the area ratio of ferrite. More specifically, the position where the orientation difference between adjacent measurement points exceeded 15° in the orientation measurement at each fixed measurement step was defined as the grain boundary.

Then, the grain size and the surface area of the crystal grains were obtained by using the region surrounded by the grain boundaries as the crystal grains. The area-average particle size was obtained from the obtained particle size and surface area. In this example, the obtained area average grain size was used as the average crystal grain size. The visual field range was 200 μm (base material: rolling direction, seam heat-treated portion: circumferential direction)×500 μm (thickness direction), the observation magnification was 400 times, and the measurement step was 0.3 μm.

The average crystal grain size of the base metal portion was the average value of the average crystal grain sizes measured at 7 locations at 0.5 mm intervals in the rolling direction at each plate thickness position of the outer surface layer portion and the 1/2t _B portion. In addition, the average crystal grain size of the seam heat treated part is three points each at the center position of the welded part and at a pitch of 0.5 mm on both sides in the circumferential direction from the center position at each plate thickness position of the outer surface layer part and 1/2t _S part. , and the average value of the average crystal grain size measured at a total of seven locations.

Furthermore, for the test piece for hardness measurement, in the thickness direction cross section of the base material and the seam heat treatment part, the thickness direction from the outer surface layer part of the base material and the 1/2t _B part, The hardness at a position of 3 mm was measured. Then, the difference in hardness between the outer surface layer portion of the base material and the 1/2t _B portion and the maximum hardness value Hv _max of the outer surface layer portion of the heat-treated seam portion were calculated. The Vickers hardness was measured according to JIS Z 2244:2009 with a test force of 98 N (10 kgf).

Table 3 shows the measurement results for the base material, and Table 4 shows the measurement results for the heat-treated seam portion.

Furthermore, two full-thickness test pieces conforming to the API5L standard were sampled in the circumferential direction at a position 180° away from the welded portion in the circumferential direction of the steel pipe. Then, a tensile test was performed at room temperature to measure yield stress (YS) and tensile strength (TS). Tensile tests were performed according to API standards.

Next, a DWTT test piece was taken at a position 90° away from the welded portion in the circumferential direction of the steel pipe so that the longitudinal direction of the test piece coincided with the circumferential direction of the steel pipe. Then, a DWTT test was performed at a test temperature of -20°C to measure the DWTT ductile fracture surface ratio. DWTT testing was performed according to ASTM E436.

Furthermore, a CTOD test piece including the heat-treated seam portion was obtained by cutting it into a length of 300 mm in the longitudinal direction and a length of 300 mm in the circumferential direction including the heat-treated seam portion. Then, a CTOD test was performed at a test temperature of −20° C., and the CTOD value of the heat-treated seam portion was measured. The CTOD test was performed according to the BS7448 standard.

These results are summarized in Table 5. In the present invention, when the yield stress is 440 MPa or more and the tensile strength is 500 MPa or more, it is judged to have high strength. Also, when the DWTT of the base material at -10°C is 85% or more and the CTOD value of the seam heat-treated portion at -20°C is 0.40 mm or more, the low temperature toughness is judged to be excellent.

As is clear from the results in Table 5, Test No. 1 satisfies the provisions of the present invention. 1-21 are found to have high strength and excellent low temperature toughness. On the other hand, Test No. In Nos. 22 to 34, the chemical composition of the steel did not satisfy the requirements of the present invention, resulting in poor strength or low temperature toughness. Also, test no. In Nos. 35 to 42, at least one of strength and low temperature toughness was inferior due to inappropriate manufacturing conditions.

According to the present invention, it is possible to obtain an electric resistance welded steel pipe having high strength and excellent low temperature toughness. Therefore, the electric resistance welded steel pipe according to the present invention can be suitably used as a line pipe laid in cold regions.

Claims (3)

An electric resistance welded steel pipe having a base material and a heat-treated seam portion,
The chemical composition of the base material is, in mass%,
C: 0.03 to 0.09%,
Si: 0.01 to 0.50%,
Mn: 0.80-1.60%,
P: 0.020% or less,
S: 0.003% or less,
Al: 0.060% or less,
Ti: 0.001 to 0.030%,
Nb: 0.01 to 0.04%,
N: 0.001 to 0.008%,
O: 0.005% or less,
Cu: 0-0.80%,
Ni: 0 to 0.80%,
Cr: 0 to 0.80%,
Mo: 0-0.80%,
V: 0 to 0.10%,
B: 0 to 0.0020%,
Ca: 0 to 0.0050%,
REM: 0-0.010%,
Sb: 0 to 0.10%,
Sn: 0-0.10%,
Co: 0-0.10%,
As: 0 to 0.10%,
Pb: 0 to 0.005%,
Bi: 0 to 0.005%,
H: 0 to 0.0005%,
balance: Fe and impurities,
Ceq represented by the following formula (i) is 0.20 to 0.53,
Pcm represented by the following formula (ii) is 0.150 to 0.250,
When the thickness of the base material is t _B , the metal structure at a position 1 mm in the thickness direction from the outer surface of the base material and at a position 1/2 t _B in the thickness direction from the outer surface of the base material has an area % contains 0% or more and 50% or less of ferrite, the balance is bainite, and has an average grain size of 15 μm or less,
In the base material, the difference between the hardness at a position 1 mm in the thickness direction from the outer surface and the hardness at a position 1/2 t _B in the thickness direction from the outer surface is 30 HV10 or less,
The metal structure at a position 3 mm in the thickness direction from the outer surface of the heat-treated seam portion contains more than 20% ferrite in terms of area%, the balance being bainite, and having an average grain size of 20 μm or less,
Assuming that the thickness of the heat-treated seam portion is _tS , the metal structure at a position 1/ _2tS in the thickness direction from the outer surface of the heat-treated seam portion contains more than 50% ferrite in terms of area%, and the balance is bainite. and has an average crystal grain size of 15 μm or less,
The maximum hardness _Hvmax at a position 3 mm in the thickness direction from the outer surface of the heat-treated seam portion is 220HV10 or less,
The CTOD value at −20° C. of the seam heat-treated portion is 0.40 mm or more,
ERW steel pipe.
Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 (i)
Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5×B (ii)
However, each element symbol in the formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained. The chemical composition of the base material is, in mass%,
Cu: 0.01 to 0.80%,
Ni: 0.01 to 0.80%,
Cr: 0.01 to 0.80%,
Mo: 0.01 to 0.80%,
V: 0.001 to 0.10%,
B: 0.0001 to 0.0020%,
Ca: 0.0001 to 0.0050%, and
REM: 0.0001 to 0.010%,
containing one or more selected from
The electric resistance welded steel pipe according to claim 1. The thickness of the base material is 25.4 mm or less,
The electric resistance welded steel pipe according to claim 1 or 2.