JP5028761B2 - Manufacturing method of high strength welded steel pipe - Google Patents

Description

The present invention relates to a method of producing a high strength welded steel pipe, particularly beyond the tensile strength of the material is 900 MPa, and the joint tensile strength of the longitudinal seam weld is suitable as the method of producing a high strength welded steel pipe that satisfactory over 950MPa About.

In recent years, line pipes used for transportation of natural gas and crude oil have been strengthened year by year in order to improve transport efficiency by increasing pressure and to improve local welding efficiency by reducing wall thickness.
So far, X100 grade line pipes have been put into practical use in accordance with API standards, but further demands for X120 grades with a tensile strength exceeding 900 MPa are being realized.

  For example, in Patent Literature 1, two-stage cooling is performed after hot rolling, and the second-stage cooling stop temperature is set to 300. A technique for achieving high strength by setting the temperature to be equal to or lower than ° C. is disclosed.

  Patent Document 2 discloses a technique relating to accelerated cooling and tempering conditions for obtaining high strength and high toughness while reducing the amount of expensive alloy element addition.

Patent Document 3 discloses a component design technique that reduces the amount of alloying elements added to the base material in the same manner as Patent Document 1 and that provides high strength and high toughness in the weld metal of the longitudinal seam weld. .
Japanese Patent Laid-Open No. 2003-293089 JP 2002-173710 A JP 2000-355729 A

  However, when increasing the strength by means such as accelerated cooling while keeping the amount of alloying elements in the base metal low, depending on the welding conditions, the strength of the heat affected zone (Heat Affected Zone, hereinafter HAZ) of vertical seam welding Due to the divergence, there is concern about the safety of the welded part, such as fracture in the HAZ part with low strength in the actual pipe test for the water pressure test, and it is necessary to newly select appropriate welding conditions according to the high-strength base metal Is done.

  Also, when the accelerated cooling stop temperature is lowered to increase the strength, the generated low-temperature transformation structure may be cold-formed during pipe forming, and there is concern that the mechanical properties may change over time due to strain aging behavior. The

  The present invention achieves a joint strength higher than that of the base metal without greatly changing the vertical seam welding method used in the production of low-grade line pipes, and can suppress fluctuations in mechanical properties due to aging. It aims at providing the manufacturing method of high strength welded steel pipe.

The present inventors have intensively studied to solve the above problems, and found that the following basic technique is effective.
1) Ensuring the base material Pcm value necessary for the HAZ strength to achieve joint strength ≧ 950 MPa, and regulating the amount of individual alloy elements added to eliminate adverse effects on weldability and toughness.

  Fig. 1 shows the outer surface side of a welded steel pipe in which the base material Pcm = 0.22 and the tube thickness of 14 mm is subjected to inner / outer surface one layer submerged arc welding (after inner surface side one layer welding, outer surface side one layer welding). The distribution of Vickers hardness at a position of about 1 mm below the surface layer is shown.

  In HAZ, a decrease in hardness is recognized, and the position where the softening occurs is a region close to the boundary with the base material portion. As a result of observation of the microstructure of the softest part, the region heated immediately above the austenitizing temperature (Ac3 point) by outer surface welding is most softened.

  By designing the component to increase the hardness of the softest part, the HAZ hardness in the region where the heating temperature during welding is higher also increases, and joint softening is greatly reduced.

  It has also been found that when the hardness of the HAZ softened part in the welded steel pipe is about 90% or more of the base metal part, the joint strength does not decrease due to the HAZ softening.

  2) Selection of controlled rolling and accelerated cooling conditions to obtain high strength and high toughness without causing strain aging due to the above-mentioned base material component restrictions.

3) High Mn-Cu-Ni-Cr-M for proper strength matching between base metal and weld
o Selection of additive weld metal.
The present invention has been made by further study based on the above knowledge, that is, the present invention,
1. % By mass
C: 0.03-0.12%
Si: ≦ 0.5%
Mn: 1.7-3.0%
P ≦ 0.010%, S ≦ 0.003%
Al: 0.01 to 0.08%
Cu: ≦ 0.8%
Ni: 0.1 to 1.0%
Cr: ≦ 0.8%
Mo: ≦ 0.8%
Nb: 0.01 to 0.08%
V: ≦ 0.10%
Ti: 0.005-0.025%
B: ≦ 0.003%
Ca: ≦ 0.01%
REM: ≦ 0.02%
N: 0.001 to 0.006%
Containing
Pcm value calculated by the following formula (1) satisfies 0.21 ≦ Pcm ≦ 0.30, and the steel composed of the balance Fe and inevitable impurities,
Reheat to 1000-1200 ° C, perform hot rolling with a cumulative reduction amount ≧ 67% in a temperature range of 950 ° C or lower, and after completion of rolling, perform accelerated cooling at a cooling rate of 20-80 ° C / s from 700 ° C or higher. Start, stop cooling at 250 ° C or lower, air cool, and reheat to 250-400 ° C. Produced by applying 2% tensile pre-strain in the direction perpendicular to the rolling direction of steel plate , then heated at 250 ° C for 30 minutes The difference between the yield strength and the yield strength before strain aging in the full-thickness tensile specimen in the direction perpendicular to the steel sheet rolling direction is 20 MPa or less, and a tensile prestrain of 2% in the direction perpendicular to the steel sheet rolling direction. A steel plate having a Charpy absorbed energy at -30 ° C. at −30 ° C. in a Charpy impact test perpendicular to the direction of rolling of the steel plate when heated at 250 ° C. for 30 minutes after being formed into a tubular shape, and its butt portion The submerged After the steel pipe and click welded, further characterized by performing the tube expansion, the method of producing a high strength welded steel pipe.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 * B (1)
However, the content of each component (%).

2. The chemical composition of the weld metal obtained by submerged arc welding of the butt portion is
C: 0.05-0.09%
Si: 0.1 to 0.4%
Mn: 2.0 to 3.0%
P: ≦ 0.020%
S: ≦ 0.010%
Al: ≦ 0.015%
Cu: ≦ 0.5%
Ni: ≦ 2.0%
Cr: ≦ 1.0%
Mo: ≦ 1.0%
V: ≦ 0.1%
Ti: 0.003 to 0.03%
B: ≦ 0.0010%
O: ≦ 0.03%
N: ≦ 0.008%
2. The method for producing a high- strength welded steel pipe according to 1, which is the remaining Fe and inevitable impurities.

3. The method for producing a high- strength welded steel pipe according to 1 or 2, wherein the butt portion is subjected to submerged arc welding with one inner and outer surface after temporary welding.

  According to the present invention, the X120 grade (tensile strength of 900 MPa or more) excellent in joint tensile strength of the welded portion, excellent in strain aging characteristics, by the welding method that has been used in the manufacture of low strength grade welded steel pipes. Welded steel pipes can be manufactured and are extremely useful in industry.

Hereinafter, the present invention will be described in detail.
1 Material steel plate [component composition]
C: 0.03-0.12%
C contributes to an increase in strength by being supersaturated in a low temperature transformation structure. In order to obtain the effect, addition of 0.03% or more is necessary, but if added over 0.12%, the hardness increase of the circumferential welded portion of the pipe is remarkably increased, and the cold cracking resistance is reduced. The upper limit is 0.12%.

Si: ≦ 0.5%
Since Si is strengthened by solid solution regardless of the transformation structure, it is effective in increasing the strength of the base material and HAZ. However, if added over 0.5%, the toughness is significantly reduced, so the upper limit is made 0.5%.

Mn: 1.7-3.0%
Mn acts as a hardenability improving element. In particular, in order to obtain a low temperature transformation structure for achieving high strength in HAZ, addition of 1.7% or more is necessary. However, in the continuous casting process, the concentration of the central segregation part is remarkably increased, and the addition exceeds 3.0%. If this is performed, the toughness of the base metal and the HAZ is deteriorated, and delayed fracture at the segregation part is caused, so the upper limit is made 3.0%.

Al: 0.01 to 0.08%
Al acts as a deoxidizing element. A sufficient deoxidation effect can be obtained with addition of 0.01% or more, but if added over 0.08%, the cleanliness in the steel decreases and the toughness deteriorates, so the upper limit is made 0.08% .

Cu: ≦ 0.8%, Cr: ≦ 0.8%, Mo: ≦ 0.8%
Cu, Cr, and Mo all act as hardenability improving elements. These are used as an alternative to adding a large amount of Mn by obtaining a low temperature transformation structure like Mn and contributing to the strengthening of the base metal / HAZ. Since it is an expensive element and the effect of increasing the strength is saturated even when 0.8% or more is added, the upper limit is set to 0.8%.

Ni: 0.1 to 1.0%
Ni is a useful element because it acts as a hardenability improving element and does not cause toughness deterioration even when added. To obtain this effect, 0.1% or more is added, but it is an expensive element, and the upper limit is made 1.0%.

Nb: 0.01-0.08%, V: ≦ 0.10%
Nb and V are effective for preventing temper softening of HAZ that forms carbides and undergoes two or more welding heat cycles, and is added to obtain necessary HAZ strength.

  Nb also has an effect of expanding the austenite non-recrystallized region at the time of hot rolling. In particular, Nb needs to be added in an amount of 0.01% or more in order to make the non-recrystallized region up to 950 ° C. On the other hand, if added over 0.08%, the toughness of HAZ is significantly impaired, so the upper limit is made 0.08%.

  V forms carbides and is added because it prevents temper softening particularly in HAZ that is subjected to two or more thermal cycles. If added over 0.10%, the toughness of the HAZ is significantly impaired, so the upper limit is made 0.10%.

Ti: 0.005-0.025%
Ti forms a nitride, reduces the amount of solute N in the steel, and the precipitated TiN suppresses the coarsening of austenite grains due to the pinning effect, thereby contributing to the improvement of the toughness of the base material and HAZ. In order to obtain the necessary pinning effect, 0.005% or more is added, but if added over 0.025%, carbides are formed, and the toughness is significantly deteriorated by precipitation hardening, so the upper limit is 0.025%. And

B: ≦ 0.003%
B segregates at the austenite grain boundaries and suppresses ferrite transformation, thereby contributing particularly to the prevention of HAZ strength reduction. Even if added over 0.003%, the effect is saturated, so the upper limit is made 0.003%.

Ca: ≦ 0.01%
Ca is an element effective in controlling the form of sulfide in steel, and suppresses the generation of MnS harmful to toughness. However, if added over 0.01%, a CaO-CaS cluster is formed and the toughness is deteriorated, so the upper limit is made 0.01%.

REM: ≦ 0.02%
REM is an effective element for controlling the morphology of sulfides in steel, and suppresses the generation of MnS harmful to toughness. However, even if it is an expensive element and added over 0.02%, the effect is saturated, so the upper limit is made 0.02%.

N: 0.001 to 0.006%
N is usually present as an inevitable impurity in steel, and Ti addition forms TiN that suppresses austenite coarsening. In order to obtain the required pinning effect, it is necessary to be present in the steel in an amount of 0.001% or more. However, if it exceeds 0.006%, it is heated to 1450 ° C. or more near the weld, particularly in the vicinity of the melting line. Since the TiN decomposes in the HAZ and the solute N lowers the toughness, the upper limit is made 0.006%.

0.21 ≦ Pcm ≦ 0.30
Pcm (= C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 * B, where each component is content%) is a weld cracking sensitive composition, but in the present invention, the joint strength is ≧ 950 MPa. In order to achieve this, the lower limit is set to 0.21.

Fig. 2 shows the results obtained by assigning a reproducible heat cycle with the maximum heating temperature just above the Ac3 point to obtain the softest HAZ hardness in various experimental steel ingots, obtaining the Vickers hardness, and arranging them by the Pcm value. .
The softest HAZ hardness correlates with the Pcm value of the steel, and is set to a Vickers hardness of 270 or more (90% of the Vickers hardness conversion value of 950 MPa 300) to ensure a joint tensile strength of 950 MPa. 21.

  On the other hand, an increase in the Pcm value of steel promotes cold cracking, which is a problem during circumferential welding of steel pipes. From the y-type weld crack test results simulating circumferential welding, the upper limit of Pcm necessary for preventing low temperature cracking at 100 ° C preheating is 0.30. Was 0.30.

P: ≦ 0.010%, S: ≦ 0.003%
Both P and S are present as inevitable impurities in the steel. In particular, the segregation at the center segregation portion is an element, and the upper limit is set to 0.010% and 0.003%, respectively, in order to suppress a decrease in toughness due to the segregation portion of the base material.
[Production method]
Heating temperature: 1000-1200 ° C
In order to completely austenite the slab at the start of hot rolling, the lower limit temperature is set to 1000 ° C. On the other hand, when the steel slab is heated to a temperature exceeding 1200 ° C., the austenite grain growth is remarkable even by TiN pinning, and the base material toughness deteriorates, so the upper limit temperature is set to 1200 ° C.

Cumulative reduction at 950 ° C or lower ≧ 67%
In hot rolling, a large reduction with a cumulative reduction of 67% or more is performed at 950 ° C. or less, which is an austenite non-recrystallized region.

  The austenite grains are extended, and the parent phase of bainite that is transformed by the subsequent accelerated cooling is refined. The toughness of the bainite steel obtained by accelerated cooling is good. In particular, when the reduction amount is 67% or more, the fineness of the bainite is remarkably improved and the toughness is remarkably improved.

Cooling start temperature of accelerated cooling ≧ 700 ° C
If the temperature at which accelerated cooling is started after hot rolling is low, pro-eutectoid ferrite is generated from the austenite grain boundaries in the air cooling process, and the base metal strength is reduced. Therefore, the lower limit temperature for suppressing pro-eutectoid ferrite formation is 700. ℃.

Accelerated cooling rate: 20-80 ° C / s
The steel sheet strength tends to increase as the cooling rate of accelerated cooling increases. When the cooling rate during accelerated cooling is less than 20 ° C./s, the transformed structure is transformed at a relatively high temperature, so that sufficient strength cannot be obtained. On the other hand, when the cooling rate exceeds 80 ° C./s, martensitic transformation occurs in the vicinity of the surface, and tempering in a relatively low temperature region does not sufficiently recover the base material toughness and the base material toughness is reduced. The cooling rate during accelerated cooling is 20 to 80 ° C./s.

Cooling stop temperature for accelerated cooling: ≤250 ° C
The steel sheet strength tends to increase as the cooling stop temperature for accelerated cooling decreases. When the cooling stop temperature for accelerated cooling exceeds 250 ° C., sufficient strength cannot be obtained after tempering, which will be described later, so the cooling stop temperature for accelerated cooling is set to 250 ° C. or lower.

Tempering temperature: 250-400 ° C
This process is an important component in the present invention. In the production of high strength steel, when the strength is increased by lowering the stop temperature, the generated low temperature transformation structure may be subjected to plastic deformation by cold working during pipe forming. For this reason, we are anxious about the mechanical property of steel materials changing over time by strain aging.

  In order to reduce strain aging, it is effective to fix solute C and N by tempering before cold forming, but when the tempering temperature is less than 250 ° C., a sufficient strain aging inhibitory effect cannot be obtained, On the other hand, in the case of tempering exceeding 400 ° C., the strength decreases and the DWTT characteristics deteriorate due to the growth of carbides, so the tempering temperature is set to 250 to 400 ° C.

In addition, although it does not specifically limit about the steel making method of steel, From a viewpoint of economical efficiency, it is desirable to perform the steel making process by a converter method, and casting of the steel piece by a continuous casting process.
There is no particular limitation on the method of forming the steel plate produced by the above method into a steel pipe, and any of conventionally used UOE forming, press bend forming, and roll forming can be used.

2 Weld metal C: 0.05 to 0.09%
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 needs to have a tensile strength ≧ 950 MPa, and is set to 0.05% or more.

  On the other hand, if it exceeds 0.09%, hot cracking of the weld metal tends to occur, so the upper limit is made 0.09%.

Si: 0.1 to 0.4%
Si is necessary to ensure deoxidation of weld metal and good workability. If it is less than 0.1%, sufficient deoxidation effect cannot be obtained. On the other hand, if it exceeds 0.4%, welding workability deteriorates. Therefore, the upper limit is made 0.4%.

Mn: 2.0 to 3.0%
Mn is an important element for increasing the strength of the weld metal. In particular, an ultrahigh strength such as a tensile strength ≧ 950 MPa cannot be achieved with a conventional acicular ferrite structure, and can be achieved by containing a large amount of Mn to form a bainite structure.

  In order to acquire this effect, it is necessary to make it contain 2.0% or more, but if it exceeds 3.0%, weldability will deteriorate, so the upper limit is made 3.0%.

P: ≦ 0.020%, S: ≦ 0.010%
P and S are segregated at the grain boundaries in the weld metal and deteriorate their toughness, so the upper limits are made 0.020% and 0.010%, respectively.

Al: ≦ 0.015%
Although Al acts as a deoxidizing element, deoxidation with Ti has a great effect of improving toughness in weld metals. Further, when the Al oxide inclusions increase, the Charpy absorbed energy of the weld metal decreases, so it is not actively added and the upper limit is made 0.015%.

Cu: ≦ 0.5%, Ni: ≦ 2.0%, Cr: ≦ 1.0%, Mo: ≦ 1.0%
Similarly to the base material, Cu, Ni, Cr, and Mo improve the hardenability even in the weld metal, and thus are contained in order to obtain a bainite structure.

  However, if the amount added to the welding wire is increased, the wire strength is remarkably increased and the wire feedability during submerged arc welding is impaired, so the upper limit of the content is 0.5% for Cu and 2.0% for Ni. %, Cr is 1.0%, and Mo is 1.0%.

V: ≦ 0.1%
An appropriate amount of V addition is an effective element because it increases strength without deteriorating toughness and weldability. If it exceeds 0.1%, the toughness of the reheated portion of the weld metal is remarkably deteriorated, so the upper limit is made 0.1%.

Ti: 0.003 to 0.03%
Ti acts as a deoxidizing element in the weld metal and is effective in reducing oxygen in the weld metal. In order to obtain this effect, the content of 0.003% or more is necessary. However, if it exceeds 0.03%, excess Ti forms carbides and deteriorates the toughness of the weld metal, so the upper limit is set. 0.03%.

B: ≦ 0.0010%
B is contained because the weld metal has a bainite structure. However, if the amount of B in the weld metal exceeds 0.0010%, a martensitic structure with low toughness is generated, so the upper limit is made 0.0010%.

O: ≦ 0.03%
Reducing the amount of oxygen in the weld metal improves toughness. In particular, when the content is 0.03% or less, the upper limit is set to 0.03%.

N: ≦ 0.008%
When the amount of solute N in the weld metal is reduced, the toughness is improved. In particular, since it is remarkably improved by setting it to 0.008% or less, the upper limit is made 0.008%.

  The joint strength improving technique according to the present invention is particularly effective in a welding method with relatively high heat input in which submerged arc welding is performed on the inner surface and the outer surface layer by layer after tack welding.

  In the present invention, the flux used for submerged arc welding is not particularly limited, and may be a molten type or a fired type. Moreover, even if preheating or post-welding heat treatment is performed before welding for the purpose of preventing cold cracking of the welded portion, the effect of the present application is not impaired.

  Using the steels A to J having the chemical compositions shown in Table 1, the steel plate No. 1 was subjected to the hot rolling / accelerated cooling / tempering conditions shown in Table 2. 1-16 were produced. In Table 1, steel types A to F are component compositions within the scope of the present invention, and steel types G to J are component compositions outside the scope of the present invention.

  From the obtained steel plate, a full thickness tensile test piece according to API-5L, a DWTT test piece, and a V-notch Charpy impact test piece of JIS Z2202 from the central position of the plate thickness were sampled in a direction perpendicular to the steel plate rolling direction. Tensile test, DWTT test and Charpy impact test were conducted to evaluate strength and toughness.

  The base material has a tensile strength of 900 MPa or more, the base material has a toughness of 150 J or more in Charpy absorbed energy (vE-30) at −30 ° C., and a ductile fracture surface ratio at −20 ° C. in the DWTT test ( SA-20) made 85% or more good.

  Next, after applying a tensile pre-strain of 2% in the direction perpendicular to the steel plate rolling direction, heating was performed at 250 ° C. for 30 minutes, and a full thickness tensile test piece and a Charpy impact test piece were taken in a direction perpendicular to the steel plate rolling direction. A tensile test and a Charpy impact test were performed to evaluate strain aging characteristics.

  The strain aging characteristics were such that the difference in yield strength before and after strain aging was 20 MPa or less and the Charpy absorbed energy at −30 ° C. after strain aging was 150 J or more.

  Steel plate No. 1 to 16, the welding wire and the welding flux were variously changed, the steel plates were butt welded by submerged arc welding, and the welded joint Nos. Shown in Table 3 were used. 1-16 were produced. From the weld metal part of the obtained joint, an analysis sample was collected and subjected to chemical analysis. The analysis results are shown in Table 3.

  In addition, a joint tensile test piece (with surplus) conforming to API-5L and a JIS Z2202 V-notch Charpy impact test specimen were collected, and a welded joint tensile test and Charpy impact test (notch position: weld metal, HAZ) was carried out to evaluate the strength and toughness of the weld.

  The strength of the welded portion was 950 MPa or higher in terms of tensile strength and the fracture position was the base metal, and the toughness of the welded portion was 100 J or higher in terms of absorbed energy at −30 ° C. of the weld metal and HAZ.

  Further, a y-type weld cracking test was performed in accordance with JIS Z 3158. The test environment was controlled at a temperature of 30 ° C. and a humidity of 80%.

  A test bead was welded to a test body with a preheating temperature of 100 ° C. using a hand-welding rod for 100 kgf class high-strength steel left in this environment for 1 hour. Weld cracking susceptibility was evaluated by the cross-sectional crack rate obtained from the observation result of the cross-section orthogonal to the test bead.

  Table 4 summarizes the results of the base metal strength and toughness investigation, the weld joint strength and toughness investigation results, and the weld crack susceptibility evaluation results.

  The steel pipe No. of the present invention in which the composition of the base metal, the steel plate production method and the composition of the weld metal are within the scope of the present invention. Nos. 1 to 8 have good base material characteristics, strain aging characteristics, weld zone characteristics, and weldability.

  On the other hand, comparative steel pipe No. in which the base metal composition is within the scope of the present invention but the steel plate production conditions or the weld metal composition is outside the scope of the present invention. 9-12 are inferior compared with the example of this invention in any of a base material characteristic, a strain aging characteristic, a weld part characteristic, and weldability.

  Although the steel plate production conditions are within the scope of the present invention, the comparative steel pipe No. 1 in which the base metal composition and the weld metal composition are outside the scope of the present invention. Nos. 13 to 16 are inferior to the examples of the present invention in any of the base material characteristics, strain aging characteristics, welded part characteristics, and weldability.

The figure which shows the outer surface side hardness distribution of the welded steel pipe which performed inner and outer surface 1 layer submerged arc welding. The correlation diagram of the softening HAZ hardness and Pcm value which were obtained by the reproduction | regeneration thermal cycle test.

Claims (3)

% By mass
C: 0.03-0.12%
Si: ≦ 0.5%
Mn: 1.7-3.0%
P ≦ 0.010%, S ≦ 0.003%
Al: 0.01 to 0.08%
Cu: ≦ 0.8%
Ni: 0.1 to 1.0%
Cr: ≦ 0.8%
Mo: ≦ 0.8%
Nb: 0.01 to 0.08%
V: ≦ 0.10%
Ti: 0.005-0.025%
B: ≦ 0.003%
Ca: ≦ 0.01%
REM: ≦ 0.02%
N: 0.001 to 0.006%
Containing
Pcm value calculated by the following formula (1) satisfies 0.21 ≦ Pcm ≦ 0.30, and the steel composed of the balance Fe and inevitable impurities,
Reheat to 1000-1200 ° C, perform hot rolling with a cumulative reduction amount ≧ 67% in a temperature range of 950 ° C or lower, and after completion of rolling, perform accelerated cooling at a cooling rate of 20-80 ° C / s from 700 ° C or higher. Start, stop cooling at 250 ° C or lower, air cool, and reheat to 250-400 ° C. Produced by applying 2% tensile pre-strain in the direction perpendicular to the rolling direction of steel plate , then heated at 250 ° C for 30 minutes The difference between the yield strength and the yield strength before strain aging in the full-thickness tensile specimen in the direction perpendicular to the steel sheet rolling direction is 20 MPa or less, and a tensile prestrain of 2% in the direction perpendicular to the steel sheet rolling direction. A steel plate having a Charpy absorbed energy at -30 ° C. at −30 ° C. in a Charpy impact test perpendicular to the direction of rolling of the steel plate when heated at 250 ° C. for 30 minutes after being formed into a tubular shape, and its butt portion The submerged After the steel pipe and click welded, further characterized by performing the tube expansion, the method of producing a high strength welded steel pipe.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 * B (1)
However, the content of each component (%). The chemical composition of the weld metal obtained by submerged arc welding of the butt portion is
C: 0.05-0.09%
Si: 0.1 to 0.4%
Mn: 2.0 to 3.0%
P: ≦ 0.020%
S: ≦ 0.010%
Al: ≦ 0.015%
Cu: ≦ 0.5%
Ni: ≦ 2.0%
Cr: ≦ 1.0%
Mo: ≦ 1.0%
V: ≦ 0.1%
Ti: 0.003 to 0.03%
B: ≦ 0.0010%
O: ≦ 0.03%
N: ≦ 0.008%
The method for producing a high- strength welded steel pipe according to claim 1, wherein the balance is Fe and inevitable impurities. The method for producing a high- strength welded steel pipe according to claim 1 or 2, wherein the butt portion is subjected to submerged arc welding with one layer on the inner and outer surfaces after tack welding.

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