JP2013204103A - High strength welded steel pipe for low temperature use having superior buckling resistance, and method for producing the same, and method for producing steel sheet for high strength welded steel pipe for low temperature use having superior buckling resistance - Google Patents

Abstract

An API X100 grade high-strength steel pipe excellent in buckling resistance and low-temperature toughness after coating treatment, a manufacturing method thereof, and a manufacturing method of a steel plate as a base material are provided.
SOLUTION: The base material is mass%, C: more than 0.03%, 0.08% or less, Si: 0.01 to 0.5%, Mn: 1.5 to 3.0%, P, S , Al: 0.01 to 0.08%, Nb: 0.005 to 0.025%, Ti: 0.005 to 0.025%, N: 0.001 to 0.010%, O: 0.005 %, More than one or more of Cu, Ni, Cr, Mo, V, B, 0.19 ≦ P _CM ≦ 0.25, balance Fe and unavoidable impurities, TS760-930 MPa, 5% or more Elongation, YR 85% or less, microstructure is mainly bainite containing island martensite, lower bainite 1-10%, residual austenite 3% or less, seam welding by submerged arc welding is heat input Specific seam weld metal components at 80 kJ / cm or less Steel pipe with a formed.
[Selection figure] None

Description

  The present invention relates to a high-temperature welded steel pipe for low temperature with excellent buckling resistance having API class X100 strength suitable for natural gas and crude oil transport steel pipes used in seismic zones and frozen land zones with severe ground fluctuations, and its manufacture. The present invention relates to a method and a method for producing a steel sheet for high-strength welded steel pipe for low temperature, and particularly relates to a material having excellent properties after coating treatment.

  In recent years, welded steel pipes used for transporting natural gas and crude oil have been challenged to improve transport efficiency by increasing pressure and to improve the efficiency of on-site welding. The thickening is also progressing to cope with the high pressure in the component composition.

  In addition, since the environment in which steel pipes are used is expanding to cold and ground-fluctuating zones, it is also required that the base metal part has excellent low-temperature toughness and buckling resistance from the viewpoint of preventing high-speed ductile fracture of the base metal part. Yes.

  Patent Document 1 relates to a high-temperature steel pipe for low temperature having such characteristics and excellent in buckling performance and welding heat-affected zone toughness of API standard X100 class, and a method for producing the same, and has a tensile strength of 620 MPa to 930 MPa. In order to achieve excellent buckling resistance, the compression buckling limit strain on the bending compression side and the breaking limit strain on the bending tension side at the start of buckling have a uniform elongation of 5% or more and a yield ratio (0 .5% yield strength / tensile strength) is improved to 85% or less.

Further, in order to improve the toughness of the weld heat affected zone, B is utilized in the composition of the base material, the weld heat affected zone is made into a specific microstructure, and the Charpy absorbed energy of the weld bond at −30 ° C. is 100 J or more. It is disclosed that it is obtained.
As the low temperature toughness of the base metal part, a Charpy absorbed energy of −40 ° C. and an SA value at −20 ° C. by a DWTT test piece are disclosed.

  Patent Document 2 relates to a steel pipe having high strength of API standard X100 or higher and excellent weld heat-affected zone toughness and deformability. In order to improve the weld heat-affected zone toughness, alloy element content in an extremely low C-Nb-Ti system In order to improve the deformability, the uniform elongation in the tube axis direction tensile test is 3% or more, and the yield strength in the base material tube axis direction tensile test is the yield strength in the circumferential direction tensile test. It is described that the strength is 0.9 times or more.

JP 2009-57629 A JP 2003-293078 A

  By the way, welded steel pipes such as UOE steel pipes, ERW steel pipes, SPL steel pipes, and press bend steel pipes used for line pipes are formed on the outer surface of the steel pipe from the viewpoint of corrosion prevention, etc. A coating process is performed in which coating is performed by heating at 250 ° C. for about 30 minutes.

  0.5% proof stress is increased by strain aging due to processing distortion during coating treatment and pipe making, and the yield ratio in steel pipe after coating treatment is larger than the yield ratio in steel plate, and the plastic deformability and buckling resistance of steel pipe Although there is concern that the characteristics will be adversely affected, Patent Documents 1 and 2 are unclear because there is no description of this point.

  In addition, in the case of APIX 100 grade steel sheets, the influence of the microstructure on the deformation performance and toughness of the base metal part has not been sufficiently elucidated, such as the factors affecting the plastic deformability of steel pipes differ in Patent Documents 1 and 2.

  Therefore, the present invention provides a DWTT test at −15 ° C. at a tensile strength of the base material of 760 MPa to 930 MPa, uniform elongation of 5% or more, yield ratio of 90% or less before and after the coating treatment. Buckling resistance, with a ductile fracture surface ratio of 85% or more, a Charpy absorbed energy of the base material at −15 ° C. of 230 J or more, and a Charpy absorbed energy of 100 J or more at a test temperature of −15 ° C. in HAZ coarse particles (CGHAZ), It aims at providing the manufacturing method of the high strength welded steel pipe for low temperature which is excellent in low temperature toughness, its manufacturing method, and the high strength welded steel pipe for low temperature.

  The high-strength welded steel pipe according to the present invention satisfies all of the API X100 class standards, and has some API X100 class tensile strength, but some other characteristics are outside the API standard range. Also included are those adjusted to.

In order to solve the above-mentioned problems, the present inventors conducted intensive research and obtained the following knowledge.
1. By making the base material structure a bainite structure having island martensite, the strain aging resistance is improved, and excellent buckling resistance can be secured even after the coating treatment. For this purpose, it is important to control the area fraction of island martensite with high accuracy. Further, it has been newly found that the same effect can be obtained when the second phase structure includes residual austenite (γ) and lower bainite in addition to hard island martensite.
2. Coarse island martensite structure promotes the generation and propagation of fractures and tends to deteriorate low temperature toughness.In order to ensure the desired low temperature toughness, the structure size of island martensite and tempered martensite is increased. It is important to control the accuracy. It is also important to control the fraction of retained austenite and lower bainite in the second phase structure described in 1 with high accuracy.
3. In order to achieve an absorption energy of 230 J or more at −15 ° C. in a Charpy test in a high-strength steel sheet of X100 grade, it is necessary to refine the microstructure.

The present invention has been made by further study based on the above knowledge, that is, the present invention,
1. Ingredient composition is mass%,
C: more than 0.03%, 0.08% or less,
Si: 0.01 to 0.5%,
Mn: 1.5-3.0%
P: 0.015% or less,
S: 0.003% or less,
Al: 0.01 to 0.08%,
Nb: 0.005 to 0.025%,
Ti: 0.005 to 0.025%,
N: 0.001 to 0.010%,
O: 0.005% or less,
Further,
Cu: 0.01 to 1.0%,
Ni: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Mo: 0.01 to 1.0%,
V: 0.01 to 0.1%
B: 0.0003 to 0.0020%
Containing one or more of
_{P CM} value calculated by the following formula (1) (unit: mass%) satisfies the 0.19 ≦ _{P CM} ≦ 0.25, the balance of Fe and unavoidable impurities, the tensile strength is more than 760 MPa 930 MPa or less, A base material part having a uniform elongation of 5% or more, a yield ratio of 90% or less, and a Charpy absorbed energy at a test temperature of −15 ° C. of 230 J or more;
Ingredient composition is mass%,
C: 0.03-0.10%,
Si: 0.5% or less,
Mn: 1.5-3.0%
P: 0.015% or less,
S: 0.005% or less,
Al: 0.05% or less,
Nb: 0.005 to 0.05%,
Ti: 0.005 to 0.03%,
N: 0.010% or less,
O: 0.015-0.045%,
B: 0.0003 to 0.0050%
Further,
Cu: 0.01 to 1.0%,
Ni: 0.01 to 2.5%,
Cr: 0.01 to 1.0%,
Mo: 0.01 to 1.5%,
V: contains one or more of 0.1% or less,
Comprising the weld metal of the seam weld with the balance being Fe and inevitable impurities,
The microstructure of the base metal part is mainly a bainite structure including island martensite, the island martensite having a major axis diameter of 2 μm or less, and the bainite structure being surrounded by a boundary having a misorientation angle of 15 ° or more. The major axis diameter of the tick ferrite is 20 μm or less, the lower bainite has an area fraction of 1 to 10%, and the remaining austenite has an area fraction of 3% or less. The area ratio of the island martensite with respect to the entire microstructure Is a high-strength welded steel pipe for low temperature with excellent buckling resistance, characterized in that is from 3% to 12%.
P _CM (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (1)
However, each element shows content (mass%).
2. 2. The high-strength welded steel pipe for low temperature excellent in buckling resistance performance according to 1, wherein the joint strength of the seam weld is 760 MPa or more and 930 MPa or less.
3. The chemical composition of the base metal and / or the weld metal of the seam weld is further in% by mass,
Ca: 0.0005 to 0.01%,
REM: 0.0005 to 0.02%,
Zr: 0.0005 to 0.03%,
Mg: 0.0005 to 0.01%
The high-strength welded steel pipe for low temperature excellent in buckling resistance performance according to 1 or 2, containing at least one of the above.
4. After the strain aging treatment for 30 minutes or less at a temperature of 250 ° C. or less, the uniform elongation is 5% or more and the yield ratio is 90% or less, as described in any one of 1 to 3 High strength welded steel pipe for low temperature with excellent buckling resistance.
The steel having the component composition of the base material described in 5.1 or 3 is heated to a temperature of 1000 to 1300 ° C., and the cumulative reduction rate at over 950 ° C. is 10% or more and the cumulative reduction rate at 750 ° C. or less is After hot rolling at a rolling finish temperature of 650 ° C. or higher to 75% or higher, accelerated cooling to a temperature of 450 ° C. or higher and lower than 600 ° C. is performed at a cooling rate of 10 ° C./s or higher, and immediately after stopping the accelerated cooling, 0 High temperature for low temperature with excellent buckling resistance characterized by reheating so that the reheating temperature is 500 to 650 ° C. and the cooling stop temperature + 50 ° C. or more at a temperature rising rate of 5 ° C./s or more. A method of manufacturing a steel plate for strength welded steel pipes.
6). The steel sheet for high-strength welded steel pipes for low-temperature use having excellent buckling resistance according to 5, wherein the cumulative rolling reduction at 750 ° C to 950 ° C is 20% or more in the hot rolling. Manufacturing method.
When the steel plate according to the manufacturing method described in 7.5 or 6 is formed into a cylindrical shape and the butt portion is welded layer by layer from the inner and outer surfaces, the welding heat input of each of the inner and outer surfaces is 80 kJ / cm or less, and the following formula ( 2) The manufacturing method of the high strength welded steel pipe for low temperature excellent in the buckling-proof performance characterized by satisfy | filling.
Inner surface heat input ≦ Outer surface heat input (2)
8). The high-temperature welding for low temperature with excellent buckling resistance according to 7, wherein the butt portion is welded one layer at a time from the inner and outer surfaces and then expanded at a tube expansion ratio of 0.4% or more and 2.0% or less. Steel pipe manufacturing method.

  According to the present invention, an APIX100 grade high-strength welded steel pipe for low temperature with excellent buckling resistance and base metal toughness, a manufacturing method thereof, and a steel plate for high-strength welded steel pipe for low temperature are obtained, which is extremely useful industrially. .

It is a figure explaining the sampling position and notch position in a welded joint Charpy test, (a) is a Charpy test piece of outer surface FL notch, (b) is a Charpy test piece of Root-FL notch.

  In the present invention, the composition of the base metal constituting the steel pipe, the base metal microstructure and the tensile strength characteristics, the constituent composition of the weld metal in the seam weld of the steel pipe, and the manufacturing method are defined. Hereinafter, it demonstrates in order.

[Component composition of base material]
In the description,% is mass%.
C: Exceeding 0.03% and 0.08% or less C contributes to an increase in strength by dissolving in supersaturation in a low-temperature transformation structure such as a martensite structure or an island-like martensite structure in the second phase. In order to obtain this effect, it is necessary to contain more than 0.03%. However, if it exceeds 0.08%, the hardness of the circumferential welded portion of the steel pipe is significantly increased and cold cracking is likely to occur. Therefore, the upper limit is made 0.08%. In order to secure the amount of island-like martensite, which is a hard phase necessary for controlling the yield ratio to be low, it is preferable to contain 0.05% or more, and preferably 0.05% or more and 0 0.08% or less.

Si: 0.01 to 0.5%
Si acts as a deoxidizer and further increases the strength of the steel by solid solution strengthening. However, if it is less than 0.01%, there is no effect, and if it exceeds 0.5%, the toughness is significantly reduced. Therefore, the upper limit is made 0.5%. More preferably, it is 0.01 to 0.2%. By suppressing it to 0.2% or less, it is possible to suppress excessive generation of island-like martensite in the steel pipe base metal part microstructure, and to improve the base material toughness. Therefore, more preferably, the upper limit is 0.2%.

Mn: 1.5 to 3.0%
Mn acts as a hardenability improving element. The effect can be obtained by the content of 1.5% or more, but in the continuous casting process, the concentration rises at the center segregation part remarkably, and if the content exceeds 3.0%, it causes the delayed fracture at the center segregation part. Therefore, the upper limit is made 3.0%. More preferably, it is 1.6 to 2.5%.

Al: 0.01 to 0.08%
Al acts as a deoxidizing element. A sufficient deoxidation effect can be obtained with a content of 0.01% or more. However, if the content exceeds 0.08%, the cleanliness in the steel decreases and the toughness deteriorates, so the upper limit is 0.08%. And More preferably, it is 0.02 to 0.06%.

Nb: 0.005 to 0.025%
Nb has an effect of expanding the austenite non-recrystallized region at the time of hot rolling, and is contained at 0.005% or more in order to make 950 ° C. or less an unrecrystallized region. On the other hand, if the content exceeds 0.025%, the Charpy absorbed energy among the toughness of the HAZ and the toughness of the base material is significantly impaired, so the upper limit is made 0.025%. More preferably, it is 0.010 to 0.025%.

Ti: 0.005-0.025%
Ti forms nitrides and is effective in reducing the amount of solute N in the steel. The precipitated TiN suppresses the coarsening of austenite grains by the pinning effect, and contributes to the improvement of the toughness of the base material and HAZ. In order to obtain the pinning effect, it is necessary to contain 0.005% or more, but if it exceeds 0.025%, carbides are formed, and the toughness is significantly deteriorated by precipitation hardening. Is 0.025%. More preferably, it is 0.008 to 0.020%.

N: 0.001 to 0.010%
N usually exists as an inevitable impurity in steel, but TiN is formed by containing Ti. In order to suppress the coarsening of austenite grains due to the pinning effect by TiN, it is necessary to be present in the steel in an amount of 0.001% or more, but if it exceeds 0.010%, 1450 in the vicinity of the weld, particularly in the vicinity of the weld bond. Since TiN decomposes in a region heated to a temperature higher than or equal to 0 ° C., and the adverse effect of solute N is significant, the upper limit is made 0.010%. More preferably, it is 0.002 to 0.005%.

O: 0.005% or less, P: 0.015% or less, S: 0.003% or less In the present invention, O, P, and S are inevitable impurities and define the upper limit of the content. O is 0.005% or less in order to suppress the formation of inclusions that are coarse and adversely affect toughness. If the P content is large, the central segregation is remarkable and the base material toughness is deteriorated. If the content of S is large, the amount of MnS produced increases remarkably and the toughness of the base material deteriorates. More preferably, they are O: 0.003% or less, P: 0.01% or less, and S: 0.001% or less.

One or more of Cu, Ni, Cr, Mo, V, and B Cu, Ni, Cr, Mo, V, and B all act as a hardenability improving element. Therefore, for the purpose of increasing the strength, one of these elements Contains the above.

Cu: 0.01 to 1.0%
Cu contributes to the hardenability improvement of steel by containing 0.01% or more. However, if containing 1.0% or more, toughness deterioration occurs, the upper limit is made 1.0%, and when Cu is contained, the content is made 0.01 to 1.0%. More preferably, it is 0.1 to 0.5%.

Ni: 0.01 to 1.0%
Ni contributes to the improvement of hardenability of steel by containing 0.01% or more. In particular, it is effective for toughening because it does not cause toughness deterioration even if contained in a large amount, but it is an expensive element, so if it contains Ni, the upper limit is 1.0%, and if it contains Ni Is 0.01 to 1.0%. More preferably, it is 0.1 to 0.5%.

Cr: 0.01 to 1.0%
Containing 0.01% or more of Cr also contributes to improving the hardenability of steel. On the other hand, if the content exceeds 1.0%, the toughness deteriorates, so the upper limit is made 1.0%, and when Cr is contained, the content is made 0.01 to 1.0%. More preferably, it is 0.1 to 0.5%.

Mo: 0.01 to 1.0%
Mo also contributes to improving the hardenability of steel by containing 0.01% or more. On the other hand, if the content exceeds 1.0%, the toughness deteriorates, so the upper limit is made 1.0%, and when it contains Mo, the content is made 0.01 to 1.0%. More preferably, it is 0.1 to 0.5%.

V: 0.01 to 0.1%
V forms precipitation strengthening by forming carbonitride, and contributes especially to the softening prevention of a weld heat affected zone. This effect can be obtained when the content is 0.01% or more. However, if the content exceeds 0.1%, precipitation strengthening is remarkably reduced in toughness. Therefore, the upper limit is set to 0.1%. 0.01 to 0.1%. More preferably, it is 0.01 to 0.05%.

B: 0.0003 to 0.0020%
When B is contained in an amount of 0.0003% or more, the formation of polygonal ferrite is suppressed. For this reason, compared to steel not containing B, it becomes possible to carry out austenite zone rolling even at a lower temperature range, and as a result, the toughness evaluated in the DWTT test or the like is improved. In addition, B segregates at the austenite grain boundary in the heat affected zone, has the effect of improving hardenability, suppresses the formation of upper bainite containing MA harmful to toughness, and facilitates the formation of lower bainite or martensite. To do. If the B content exceeds 0.0020%, the base material toughness deteriorates, so the upper limit is made 0.0020%.

P _CM (%): 0.19 to 0.25
P _CM in weld crack susceptibility composition expressed by C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B, each element and content (mass%), the element-free is 0.

In the present invention, 760 MPa or more in tensile strength of the base material, and, in order to achieve the above 760 MPa in joint _strength, the _{P CM} was 0.19% or more, whereas, 0.25% in terms of the circumferential weld ensuring The following. More preferably, it is 0.23% or less.

The above is the basic component composition of the base metal part of the steel pipe according to the present invention, the remaining Fe and unavoidable impurities, but when further improving the toughness of the welded part, it contains one or more of Ca, REM, Zr, Mg Can do.
Ca, REM, Zr, Mg
Ca, REM, Zr, and Mg form oxysulfides or carbonitrides in steel, and are mainly contained for the purpose of suppressing austenite grain coarsening in the weld heat affected zone by the pinning effect and improving toughness. it can.

Ca: 0.0005 to 0.01%
In the steelmaking process, when the Ca content is less than 0.0005%, it is difficult to secure CaS due to the deoxidation reaction, and the effect of improving toughness cannot be obtained. When Ca is contained, the lower limit of Ca is 0.0005%. And

  On the other hand, when the Ca content exceeds 0.01%, coarse CaO is likely to be generated, and the toughness including the base material is lowered, and the nozzle of the ladle is blocked and the productivity is hindered. The upper limit is 0.01%, and if contained, 0.0005 to 0.01%. More preferably, it is 0.001 to 0.005%.

REM: 0.0005 to 0.02%
REM forms an oxysulfide in steel and contains 0.0005% or more, thereby providing a pinning effect that prevents the weld heat affected zone from becoming coarse. However, since it is an expensive element and the effect is saturated even if it contains more than 0.02%, the upper limit is made 0.02%, and if it is contained, it is made 0.0005 to 0.02%. More preferably, it is 0.001 to 0.005%.

Zr: 0.0005 to 0.03%
Zr forms carbonitrides in steel and brings about a pinning effect that suppresses the coarsening of austenite grains, particularly in the weld heat affected zone. In order to obtain a sufficient pinning effect, the content of 0.0005% or more is necessary. However, if the content exceeds 0.03%, the cleanliness in the steel is remarkably lowered and the toughness is lowered. Therefore, the upper limit is made 0.03%, and when it is contained, the content is made 0.0005 to 0.03%. More preferably, it is 0.001 to 0.01%.

Mg: 0.0005 to 0.01%
Mg is produced as fine oxides in the steel during the steelmaking process, and in particular, has a pinning effect that suppresses the coarsening of austenite grains in the weld heat affected zone. In order to obtain a sufficient pinning effect, the content of 0.0005% or more is necessary. However, if the content exceeds 0.01%, the cleanliness in the steel decreases and the toughness decreases. The upper limit is 0.01%, and when it is contained, the content is 0.0005 to 0.01%. More preferably, it is 0.001 to 0.005%.

[Composition composition of weld metal in seam welds]
In the description,% is mass%.
C: 0.03-0.10%
Also in the weld metal, C is an important element as a steel strengthening element. In particular, in order to achieve overmatching of the joint portion, the weld metal portion also needs to have a tensile strength of 760 MPa or more, and in order to obtain this strength, it is necessary to contain 0.03% or more. On the other hand, if it exceeds 0.10%, hot cracking of the weld metal tends to occur, so the upper limit was made 0.10%. More preferably, it is 0.05 to 0.08%.

Si: 0.5% or less Si is useful for deoxidizing the weld metal and ensuring good workability. However, if it exceeds 0.5%, the weld workability is deteriorated. 5%. More preferably, it is 0.3% or less.

Mn: 1.5 to 3.0%
Mn is an important element for increasing the strength of the weld metal. In particular, in order to make the tensile strength 760 MPa or more, it is necessary to contain 1.5% or more. However, if it exceeds 3.0%, the weldability deteriorates, so the upper limit was made 3.0%. More preferably, it is 1.6 to 2.5%.

P: 0.015% or less, S: 0.005% or less P and S are segregated at the grain boundaries in the weld metal and deteriorate their toughness, so the upper limits were made 0.015% and 0.005%, respectively. . More preferably, they are 0.01% or less and 0.003% or less, respectively.

Al: 0.05% or less Al acts as a deoxidizing element, but in the weld metal part, deoxidation with Ti has a greater effect of improving toughness, and when there are more inclusions of Al oxide-based weld metal Since the absorption energy of Charpy is lowered, it is not actively contained, and the upper limit is made 0.05%. More preferably, it is 0.03% or less.

Nb: 0.005 to 0.05%
Nb is an element effective for increasing the strength of the weld metal. In particular, in order to make the tensile strength 760 MPa or more, it is necessary to contain 0.005% or more, but if it exceeds 0.05%, the toughness deteriorates, so the upper limit was made 0.05%. More preferably, it is 0.005-0.04%, More preferably, it is 0.005-0.03%.

Ti: 0.005 to 0.03%
Ti acts as a deoxidizing element in the weld metal and is effective in reducing oxygen in the weld metal. To obtain this effect, 0.005% or more must be contained, but when it exceeds 0.03%, the excess Ti forms carbides and degrades the toughness of the weld metal. Was 0.03%. More preferably, it is 0.005 to 0.02%.

N: 0.010% or less Reduction of the solid solution N in the weld metal also has an effect of improving toughness, and the upper limit is made 0.010% because it is remarkably improved especially by setting it to 0.010% or less. More preferably, it is 0.008% or less.

O: 0.015-0.045%
Reduction of the amount of oxygen in the weld metal has an effect of improving toughness, and the upper limit is made 0.045% because it is remarkably improved especially by setting it to 0.045% or less. On the other hand, if the amount of oxygen in the weld metal is less than 0.015%, the amount of oxide effective for refining the structure of the weld metal decreases, and conversely the toughness of the weld metal deteriorates, so the lower limit was made 0.015%. . More preferably, it is 0.015 to 0.035%.

B: 0.0003 to 0.0050%
In a welded pipe for a line pipe with a strength grade of 760 MPa or more and 930 MPa or less, it is effective to contain B in order to make the microstructure of the weld metal a fine bainite main structure. Content of 0003% or more and 0.0050% or less is necessary. In addition, a more preferable range is 0.0005 to 0.0050%, and a more preferable range is 0.0005 to 0.0030% or less. More preferably, it is 0.0007 to 0.0020%.

When containing one or more of Cu, Ni, Cr, Mo and V, one or more of Cu, Ni, Cr, Mo and V, Cu: 0.01 to 1.0%, Ni: 0.01 to 2.5% Cr: 0.01-1.0%, Mo: 0.01-1.5%, V: 0.1% or less.

  Since Cu, Ni, Cr, and Mo improve the hardenability in the weld metal as well as the base material, one or more of them are contained in an amount of 0.01% or more for bainite organization. However, as the amount increases, the alloying element content in the welding wire increases, and the wire strength increases significantly. As a result, the wire feedability at the time of submerged arc welding is obstructed, so Cu, Ni, Cr, Mo are each The upper limits were 1.0%, 2.5%, 1.0%, and 1.5%. More preferably, Cu: 0.01 to 0.5%, Ni: 0.01 to 2.3%, Cr: 0.01% or more and less than 0.5%, Mo: 0.01 to 1.2% is there. More preferable ranges of Ni and Mo are Ni: 0.01 to 2.0% and Mo: 0.01 to 1.0%, respectively, and even more preferable ranges are Ni: 0.5 to 2 respectively. 0.0%, Mo: 0.1-1.0%.

V: 0.1% or less An appropriate amount of V is an effective element because it increases the strength without degrading toughness and weldability. In order to exert this effect, 0.01% or more must be contained. preferable. On the other hand, if it exceeds 0.1%, the toughness of the reheated portion of the weld metal deteriorates significantly, so the upper limit was made 0.1%. More preferably, it is 0.05% or less.

  The above is the basic component composition of the weld metal part of the seam welding of a steel pipe according to the present invention. When the toughness of the weld metal part is further improved, one or more of Ca, REM, Zr, and Mg can be contained.

Ca, REM, Zr, Mg
Ca, REM, Zr, and Mg can be contained for the purpose of forming an oxysulfide or carbonitride in the steel, suppressing austenite grain coarsening in the weld metal part by a pinning effect, and improving toughness.

Ca: 0.0005 to 0.01%
When the Ca content is less than 0.0005% in the weld metal melt solidification process, it is difficult to secure CaS due to the deoxidation reaction control, and the effect of improving toughness cannot be obtained. 0.0005%.

  On the other hand, when the Ca content exceeds 0.01%, coarse CaO is likely to be generated, and the toughness is reduced. Therefore, the upper limit is set to 0.01%, and if contained, 0.0005 to 0.01%. And More preferably, it is 0.001 to 0.005%.

REM: 0.0005 to 0.02%
REM forms an oxysulfide in steel and contains 0.0005% or more, thereby providing a pinning effect that prevents austenite grain coarsening in the weld metal part. However, since it is an expensive element and the effect is saturated even if it contains more than 0.02%, the upper limit is made 0.02%, and if it is contained, it is made 0.0005 to 0.02%. More preferably, it is 0.001 to 0.01%.

Zr: 0.0005 to 0.03%
Zr forms carbonitride in steel and brings about a pinning effect that suppresses coarsening of austenite grains in the weld metal part. In order to obtain a sufficient pinning effect, it is necessary to contain 0.0005% or more. However, if it exceeds 0.03%, the cleanliness of the weld metal part is remarkably lowered, and the toughness is lowered. Therefore, the upper limit is made 0.03%, and when it is contained, the content is made 0.0005 to 0.03%. More preferably, it is 0.001 to 0.01%.

Mg: 0.0005 to 0.01%
Mg is formed as a fine oxide and has a pinning effect that suppresses the coarsening of austenite grains in the weld metal part. In order to obtain a sufficient pinning effect, the content of 0.0005% or more is necessary. However, if the content exceeds 0.01%, the cleanliness in the weld metal is lowered and the toughness is lowered. Therefore, the upper limit is made 0.01%, and when it is contained, the content is made 0.0005 to 0.01%. More preferably, it is 0.001 to 0.005%.

[Microstructure of base material]
In order to obtain excellent buckling resistance as a steel pipe, absorbed energy of 230 J or more in the Charpy impact test of the base material at a test temperature of −15 ° C., and excellent strain aging characteristics of the base material, the microstructure of the base material In addition, it is defined as a structure mainly composed of a bainite structure including island martensite. The bainite structure including island martensite is a two-phase structure in which the bainite structure of the parent phase is a soft phase and the island martensite is a hard phase, and the yield ratio (0.5% yield strength / tensile strength) is reduced.

  In the present invention, in order to obtain a steel pipe having buckling resistance, the area ratio of island martensite which is a hard phase is set to 3 so that the yield ratio (0.5% yield strength / tensile strength) is 90% or less. % Or more.

  However, in the present invention, the area ratio of the island martensite is the area of the island martensite with respect to the entire microstructure including the bainite and the island martensite as well as the remaining structure within the allowable range as described later. It shall refer to the rate.

  The buckling resistance requires that the tensile strength characteristic of the base material be a round house type and an SS curve having a high work hardening index (n value). In the present invention, an effect equivalent to the n value is obtained. The microstructure is defined with the goal of reducing the yield ratio (0.5% yield strength / tensile strength) to 90% or less. In the microstructure of the base material, bainite refers to bainitic ferrite in a narrow sense.

  In order to set Charpy absorbed energy at a test temperature of −15 ° C. to 230 J or more, the major axis diameter of bainitic ferrite surrounded by a boundary of island-shaped martensite of 2 μm or less and an orientation difference angle of 15 ° or more The fine microstructure is 20 μm or less.

  If the area ratio of the island martensite exceeds 12%, it is difficult to achieve the above-mentioned base material toughness and a yield ratio of 90% or less even when the microstructure is refined. To do.

  The microstructure of the base steel sheet is mainly composed of a bainite structure including island martensite, and the island martensite has an area ratio of 3% to 12%, so that C becomes an untransformed austenite phase of island martensite. Concentration and the amount of solute C in the bainite phase are reduced, so that excellent strain aging resistance can be obtained.

  As a result, the general steel pipe coating process suppresses the increase in yield stress due to strain aging and the accompanying decrease in uniform elongation even after a heat history of 30 minutes at 250 ° C, which is equivalent to a high temperature for a long time. After coating, uniform elongation: 5% or more and yield ratio: 90% or less can be ensured.

  Furthermore, by defining the microstructure of the base material as described above, a ductile fracture surface ratio of 85% or more can be achieved in the DWTT test at a test temperature of −15 ° C.

  The phrase “mainly composed of a bainite structure including island-like martensite” means that 80% or more of the entire structure is the structure, and allows the remainder to include lower bainite and retained austenite. In the present invention, the Charpy absorbed energy of the base material can be improved by containing these structures in an area fraction of 1 to 10% and 1 to 3%, respectively. The tissue is identified by using a scanning electron microscope at the center of the plate thickness and observing at a magnification of 2000 times, and the area ratio is randomly observed for 10 or more fields of view, and the average value is obtained.

[Manufacturing conditions of base steel sheet]
In the present invention, the steel having the above-described component composition is hot-rolled, accelerated and cooled, and then immediately reheated to produce a base steel plate.

  In the following description, temperatures such as heating temperature, rolling end temperature, cooling end temperature, and reheating temperature are the average temperature of the steel sheet. The average temperature is obtained by calculation from the measured surface temperature of the slab or steel plate, taking into account parameters such as plate thickness and thermal conductivity. Moreover, a cooling rate is an average cooling rate which remove | divided the temperature difference required in order to cool to the cooling completion temperature (less than 450-600 degreeC) after the hot rolling completion by the time required to perform the cooling.

  Moreover, a heating rate is an average temperature increase rate which remove | divided by the time required to reheat the temperature difference required for reheating to reheating temperature (500-650 degreeC) after cooling. Hereinafter, each manufacturing condition will be described in detail.

Heating temperature: 1000-1300 ° C
In performing hot rolling, the lower limit is set to 1000 ° C. in order to form austenite. On the other hand, when the steel slab is heated to a temperature exceeding 1300 ° C., even if TiN pinning is performed, the austenite grain growth is remarkable and the base material toughness deteriorates, so the upper limit was set to 1300 ° C. More preferably, it is 1000-1150 degreeC.

Cumulative rolling reduction above 950 ° C .: Rolling in the austenite recrystallization region for 10% or more suppresses mixing of grains such as formation of coarse austenite grains. Since the effect cannot be expected when the cumulative rolling reduction is less than 10%, the cumulative rolling reduction over 950 ° C. is set to 10% or more.

  By performing rolling at a relatively high temperature side of the austenite non-recrystallized region, mixing of grains such as formation of coarse austenite grains is further suppressed. If the cumulative rolling reduction at 750 ° C. and 950 ° C. corresponding to this temperature range is less than 20%, the effect is small. Therefore, the cumulative rolling reduction at 750 ° C. and 950 ° C. or less is preferably 20% or more.

Cumulative rolling reduction at 750 ° C. or lower: 75% or more Baini that austenite grains are expanded by accumulating large pressure in the temperature range on the low temperature side of the austenite non-recrystallized region, and then transformed by accelerated cooling. Tick ferrite and island martensite are refined and toughness is greatly improved.

  In the present invention, in order to achieve a low yield ratio, island martensite is dispersed as a hard phase. On the other hand, in order to prevent a reduction in toughness due to this, the reduction rate is set to 75% or more to promote bainite refinement. More preferably, it is 80% or more.

  By accumulating large pressures on the low temperature side of such an austenite non-recrystallized region, the effect of improving toughness through refinement of the structure becomes remarkable.

Rolling end temperature: 650 ° C. or more If the hot rolling end temperature is less than 650 ° C., proeutectoid ferrite is generated from the austenite grain boundaries in the subsequent air cooling process, and the base metal strength is reduced. In order to suppress pro-eutectoid ferrite formation, the lower limit temperature was set to 650 ° C. More preferably, it is 650-700 degreeC.

  Accelerated cooling after hot rolling and subsequent reheating treatment are important processes in the present invention. Accelerated cooling is terminated during bainite transformation, that is, in a temperature range where untransformed austenite exists, and then immediately reheated to cause transformation from untransformed austenite to bainite.

  Bainitic ferrite in bainite produced at a relatively high temperature has a small amount of C solid solution that can be tolerated, so that C is discharged into the surrounding untransformed austenite, and with the progress of bainite transformation during reheating, untransformed The amount of C in austenite increases.

  At this time, if Mn, Si or the like, which is an austenite stabilizing element, is contained in a certain amount or more, untransformed austenite in which C is concentrated remains even at the end of reheating, and MA is cooled in the cooling process after reheating (air cooling). As a result of the transformation to the end, the base material structure finally becomes a bainite structure including island martensite.

Accelerated cooling rate: 10 ° C./s or higher To achieve a high strength of 760 MPa or higher, the microstructure needs to be a bainite-based structure. For this reason, accelerated cooling is performed after hot rolling. When the cooling rate is less than 10 ° C./s, the bainite transformation starts at a relatively high temperature, so that sufficient strength cannot be obtained. Therefore, the cooling rate of accelerated cooling is set to 10 ° C./s or more. More preferably, it is 12-50 degreeC / s.

Cooling stop temperature of accelerated cooling: 450 ° C. or higher and lower than 600 ° C. Accelerated cooling stops in a temperature range where untransformed austenite existing during bainite transformation exists. If the cooling stop temperature is less than 450 ° C, it is difficult to secure sufficient untransformed austenite, and sufficient island martensite cannot be obtained during air cooling after reheating, and a low yield ratio of 85% or less is achieved. Can not.

  On the other hand, when the cooling stop temperature is 600 ° C. or higher, C is consumed in the pearlite that precipitates during cooling and no island-like martensite is generated, so the upper limit was made lower than 600 ° C. More preferably, it is 500-550 degreeC from a viewpoint of intensity | strength and toughness balance.

Heating rate of reheating after cooling stop: Concentrate C in untransformed austenite by reheating immediately after accelerated cooling of 0.5 ° C / s or more, and form island martensite in the subsequent air cooling process Can do. In addition, reheating immediately after accelerated cooling means starting reheating at a temperature rising rate of 0.5 ° C./s or more within 3 minutes after stopping accelerated cooling.

  When the rate of temperature increase is less than 0.5 ° C./s, cementite in the bainite becomes coarse and the base material toughness decreases, so the rate of temperature increase is 0.5 ° C./s or more. More preferably, it is 1.0-10 degreeC / s.

Reheating temperature: 500 to 650 ° C. and cooling stop temperature +50
If the reheating temperature is less than 500 ° C. and the cooling stop temperature is less than + 50 ° C., C concentration to austenite does not occur sufficiently, and the required island-like martensite area ratio cannot be ensured. And the cooling stop temperature + 50 ° C. or higher.

  On the other hand, when the reheating temperature exceeds 650 ° C., the area fraction of lower bainite and retained austenite cannot be obtained sufficiently as the second phase other than island martensite, and the effect of improving the Charpy absorbed energy of the base material is reduced. Therefore, the temperature is regulated to 650 ° C. or lower, and the cooling stop temperature is set. From the viewpoint of the balance between strength and toughness, the temperature is preferably 580 ° C. and the cooling stop temperature + 50 ° C. or higher to 640 ° C.

There is no need to set the temperature holding time at the reheating temperature. In the cooling process after reheating, island-shaped martensite is generated regardless of the cooling rate. Therefore, it is preferable that the cooling after reheating is basically air cooling. If reheating after accelerated cooling is performed with a high-frequency heating device arranged on the same line as the accelerated cooling device, it is easy to heat immediately after accelerated cooling, which is preferable.
In addition, although it does not specifically limit about the steel making method of steel, From the viewpoint of quality and economical efficiency, it is desirable to cast the steel piece by the steel making process by a converter method, and the continuous casting process.

  With the above manufacturing process, it has a tensile strength of 760 MPa or more and 930 MPa or less, a uniform elongation of 5% or more, and a high deformation performance with a yield ratio (0.5% proof stress / tensile strength × 100) of 90% or less. However, it is possible to obtain a steel sheet having a low temperature toughness with a ductile fracture surface ratio of 85% or more and a Charpy absorbed energy at −15 ° C. of 230 J or more in the DWTT test at −15 ° C.

[Production conditions for steel pipes]
The high-strength steel pipe excellent in buckling resistance and low-temperature toughness of the base metal according to the present invention uses the base steel sheet having the above-described tensile strength characteristics as a steel pipe according to a conventional method. The manufacturing method of the steel pipe is cylindrical by U-pressing and O-pressing, then performing seam welding and then expanding the pipe. There are various methods such as a press manufacturing method for expanding the pipe after seam welding, or roll forming for forming a cylindrical shape by roll forming and welding the seam portion, but is not particularly limited. The welded portion may be either straight seam welding that is straight in the longitudinal direction of the steel pipe, or the spiral spiral welding of the welded portion.

  Seam welding is performed by submerged arc welding on the inner and outer surfaces one after another by tack welding, and the flux used for submerged arc welding is not particularly limited, and it may be a melt-type flux or a fired-type flux. Absent. Further, preheating before welding or heat treatment after welding is performed as necessary.

Submerged arc welding of the welding heat input (kJ / cm) is, _{P CM} is from 0.19 to 0.25% of the base material steel plate in the above-mentioned composition of ingredients, in the range of less heat input 80 kJ / cm, adjusted.

  In particular, if the heat input balance between the outer surface side welding and the inner surface side welding satisfies the following formula (2), the γ grain coarsening of the HAZ coarse grain (CGHAZ) portion on the inner surface side can be suppressed, and the outer surface As a joint HAZ toughness taken from the FL position on the side and the root side (sometimes referred to as a bond part), it is possible to stably secure a Charpy absorbed energy of 100 J or more at a test temperature of -15 ° C. in HAZ coarse particles (CGHAZ) It becomes.

“Securing stably” means that the cumulative failure probability when the joint HAZ Charpy test is performed 100 times or more at a test temperature of −15 ° C. or less is 1% or less.
Inner surface heat input ≦ Outer surface heat input (2)
After seam welding, pipe expansion is performed at a pipe expansion rate of 0.4% or more and 2.0% or less according to the required roundness. When the tube expansion ratio is less than 0.4%, it is difficult to achieve the usually required roundness particularly when the plate thickness is 20 mm or more. Moreover, when the pipe expansion rate is more than 2.0%, there is a concern that the strain concentration at the bond portion at the boundary between the weld metal and the weld heat affected zone is excessively increased and the pipe expansion cracks. Moreover, there is a concern that the joint characteristics may deteriorate due to excessive strain introduction. From the viewpoint of improving the roundness and securing of joint strength and toughness, it is preferably 0.5 to 1.5%.

  The microstructure of HAZ coarse grains (CGHAZ) with a prior austenite grain size of 50 μm or more near the bond part is randomly observed with a scanning electron microscope (magnification 5000 times) at a position of 6 mm from the outer surface. To identify.

  Steels having various chemical compositions shown in Table 1 were melted in a converter and made into slabs having a thickness of 170 to 250 mm by continuous casting, and then steel plates 1 to 1 were subjected to hot rolling, accelerated cooling, and reheating conditions shown in Table 2. 15 was produced. The reheating was performed using an induction heating type heating device installed on the same line as the accelerated cooling facility.

  Further, these steel sheets were formed by U press and O press, then subjected to inner seam welding by submerged arc welding and then outer seam welding. Thereafter, the pipe was expanded at a pipe expansion rate of 0.6 to 1.2% to obtain a steel pipe having an outer diameter of 400 to 1626 mm. Table 3 shows the chemical compositions of the weld metal parts of the inner surface seam welding and outer surface seam welding of the steel pipes 1 to 15 (two steel pipes made of the steel plates 3, 6, and 8).

  In order to evaluate the joint strength of the obtained steel pipe, a full thickness tensile test piece based on API-5L was taken from the pipe axis direction for the base metal part and from the pipe circumferential direction for the seam welded part, and subjected to a tensile test. Carried out.

  JIS Z2202 (1980) V-notch Charpy impact test specimens 1 and 3 were taken from two positions of outer surface FL and Root-FL shown in FIG. 1-A and FIG. A Charpy impact test was conducted at a test temperature of ° C. The notch position 2 was a position where HAZ and weld metal exist at a ratio of 1: 1. Reference numeral 1 denotes a Charpy test piece having an outer surface FL notch, reference numeral 2 denotes a notch position of the Charpy test piece, reference numeral 3 denotes a Charpy test piece having a root-FL notch, reference numeral 5 denotes an outer surface weld metal, reference numeral 6 denotes an inner surface weld metal, reference numeral Reference numeral 7 denotes a melting line (FL, bond portion).

  Further, a V-notch Charpy impact test piece of JIS Z2202 (1980) was collected from the center position of the thickness of the base material portion of the steel pipe, and a Charpy impact test was performed at a test temperature of −15 ° C. Further, a DWTT test piece conforming to API-5L was taken from the steel pipe and tested at a test temperature of −15 ° C. to determine the SA value (Shear Area: ductile fracture surface ratio).

  In the present invention, the base steel sheet has a tensile strength of 760 MPa or more and 930 MPa or less, a uniform elongation of 5% or more, a ratio of 0.5% proof stress to the tensile strength of 90% or less, and a test temperature in the base material − Charpy absorption at 15 ° C. is 230 J or more, DWTSA-15 ° C. is 85% or more, steel pipe seam weld joint strength is 760 MPa or more and 930 MPa or less, Charpy absorption at HAZ coarse grain (CGHAZ) at test temperature of −15 ° C. The target performance is an energy of 100 J or more.

  Table 4 shows the test results. Test No. 1 to 8 are examples of the base material and the welded portion satisfying the provisions of the present invention, showing the desired base material strength, yield ratio, uniform elongation and toughness, and the high HAZ toughness of the seam welded portion, In the microstructure of the base metal part, the main component is a bainite structure including island martensite having an area ratio of 3% or more and 12% or less, the major axis diameter of the contained island martensite is 2 μm or less, and the orientation difference angle The major axis diameter of bainitic ferrite surrounded by a boundary of 15 ° or more was 20 μm or less, the lower bainite area ratio was 1 to 10%, and the retained austenite area ratio was 3% or less.

  On the other hand, test no. In No. 9, the base material component is within the scope of the invention described in claim 1, but the base metal toughness decreased because the rolling reduction of 750 ° C. or less was less than 75% in rolling the steel sheet (see Table 2). . The microstructure of the welded portion satisfies the provisions of claim 1 and good toughness is obtained.

  Test No. No. 10 is within the scope of the invention as claimed in claim 1, but because the cooling stop temperature at the time of steel plate production was too low, the required MA area ratio could not be secured, and strain aging was maintained at 250 ° C. for 30 minutes. Prior to treatment, the evaluation criteria for the yield ratio of the steel pipe base material were not met.

  Test No. Nos. 11 and 12 have a base material component within the scope of the invention described in claim 1, but the base material toughness decreased because the reheating temperature was higher than 650 ° C. in rolling the steel sheet (see Table 2).

  Test No. 13, 14, and 15 have a base metal component within the scope of the invention described in claim 1, but because the welding heat input is high and the fraction of the upper bainite structure is high, both the outer surface side and the inner surface side root part have HAZ toughness. Declined.

Test No. _{16, P CM} is below the lower limit of the present invention, the tensile strength and the tensile strength of the joint of the base metal is less than 760 MPa, In addition, joint HAZ coarse (CGHAZ) tissue becomes upper bainite, outer side, the inner side The HAZ toughness was reduced in both the root parts.

Test No. _{17, P CM} value exceeds the upper limit of the present invention, HAZ coarse (CGHAZ) structure becomes martensite, the outer surface side, HAZ toughness on the inner surface side Root portion both decreased. Furthermore, the MA area ratio of the base material portion was too high, and the base material toughness was reduced.

  Test No. 18 is a welding heat input of 80 kJ / cm or less on the inner surface side and the outer surface side, but the welding heat input on the inner surface side is higher than the welding heat input on the outer surface side, and the austenite grain size is large in the microstructure of the Root part. Therefore, a coarse upper bainite structure was obtained, and the HAZ toughness on the root side was lowered.

1: Charpy test piece of outer surface FL notch 2: Notch position of Charpy test piece 3: Charpy test piece of Root-FL notch 5: Outer surface weld metal 6: Inner surface weld metal 7: Melting line

Claims (8)

Ingredient composition is mass%,
C: more than 0.03%, 0.08% or less,
Si: 0.01 to 0.5%,
Mn: 1.5-3.0%
P: 0.015% or less,
S: 0.003% or less,
Al: 0.01 to 0.08%,
Nb: 0.005 to 0.025%,
Ti: 0.005 to 0.025%,
N: 0.001 to 0.010%,
O: 0.005% or less,
Further,
Cu: 0.01 to 1.0%,
Ni: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Mo: 0.01 to 1.0%,
V: 0.01 to 0.1%
B: 0.0003 to 0.0020%
Containing one or more of
_{P CM} value calculated by the following formula (1) (unit: mass%) satisfies the 0.19 ≦ _{P CM} ≦ 0.25, the balance of Fe and unavoidable impurities, the tensile strength is more than 760 MPa 930 MPa or less, A base material part having a uniform elongation of 5% or more, a yield ratio of 90% or less, and a Charpy absorbed energy at a test temperature of −15 ° C. of 230 J or more;
Ingredient composition is mass%,
C: 0.03-0.10%,
Si: 0.5% or less,
Mn: 1.5-3.0%
P: 0.015% or less,
S: 0.005% or less,
Al: 0.05% or less,
Nb: 0.005 to 0.05%,
Ti: 0.005 to 0.03%,
N: 0.010% or less,
O: 0.015-0.045%,
B: 0.0003 to 0.0050%
Further,
Cu: 0.01 to 1.0%,
Ni: 0.01 to 2.5%,
Cr: 0.01 to 1.0%,
Mo: 0.01 to 1.5%,
V: contains one or more of 0.1% or less,
Comprising the weld metal of the seam weld with the balance being Fe and inevitable impurities,
The microstructure of the base metal part is mainly a bainite structure including island martensite, the island martensite having a major axis diameter of 2 μm or less, and the bainite structure being surrounded by a boundary having a misorientation angle of 15 ° or more. The major axis diameter of the tick ferrite is 20 μm or less, the lower bainite has an area fraction of 1 to 10%, and the remaining austenite has an area fraction of 3% or less. The area ratio of the island martensite with respect to the entire microstructure Is a high-strength welded steel pipe for low temperature with excellent buckling resistance, characterized in that is from 3% to 12%.
P _CM (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (1)
However, each element shows content (mass%).   The high-strength welded steel pipe for low temperature excellent in buckling resistance according to claim 1, wherein the joint strength of the seam weld zone is 760 MPa or more and 930 MPa or less. The chemical composition of the base metal and / or the weld metal of the seam weld is further in% by mass,
Ca: 0.0005 to 0.01%,
REM: 0.0005 to 0.02%,
Zr: 0.0005 to 0.03%,
Mg: 0.0005 to 0.01%
The high-strength welded steel pipe for low temperature excellent in buckling resistance according to claim 1 or 2, comprising at least one of the following.   4. The uniform elongation is 5% or more and the yield ratio is 90% or less after a strain aging treatment at a temperature of 250 ° C. or less for 30 minutes or less. 5. High strength welded steel pipe for low temperature with excellent buckling resistance.   The steel having the component composition of the base material according to claim 1 or 3 is heated to a temperature of 1000 to 1300 ° C, and the cumulative reduction rate at over 950 ° C is 10% or more and the cumulative reduction rate at 750 ° C or less. After hot rolling at a rolling finish temperature of 650 ° C. or higher to 75% or higher, accelerated cooling to a temperature of 450 ° C. or higher and lower than 600 ° C. is performed at a cooling rate of 10 ° C./s or higher, and immediately after stopping the accelerated cooling, 0 High temperature for low temperature with excellent buckling resistance characterized by reheating so that the reheating temperature is 500 to 650 ° C. and the cooling stop temperature + 50 ° C. or more at a temperature rising rate of 5 ° C./s or more. A method of manufacturing a steel plate for strength welded steel pipes.   6. The high-strength welded steel pipe for low temperature with excellent buckling resistance according to claim 5, wherein, in the hot rolling, the cumulative rolling reduction at more than 750 ° C. and not more than 950 ° C. is 20% or more. Steel plate manufacturing method. When the steel plate by the manufacturing method according to claim 5 or 6 is formed into a cylindrical shape and the butt portion is welded one layer at a time from the inner and outer surfaces, the welding heat input of each of the inner and outer surfaces is 80 kJ / cm or less, and the following formula ( 2) The manufacturing method of the high strength welded steel pipe for low temperature excellent in the buckling-proof performance characterized by satisfy | filling.
Inner surface heat input ≦ Outer surface heat input (2)   8. The high temperature for low temperature use with excellent buckling resistance according to claim 7, wherein the butt portion is welded one layer at a time from the inner and outer surfaces and then expanded at a tube expansion ratio of 0.4% or more and 2.0% or less. A manufacturing method of strength welded steel pipe.