JP2001113374A - Ultra-high-strength steel pipe with excellent low-temperature toughness at seam welds and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a line pipe having an excellent tenacity at low temperature and a hyper strength whose tensile strength is 900 MPa or higher (larger than X100 at API Standard), and which is easily welded at a job site, and a method for manufacturing therefore. SOLUTION: In the hyper strength seam welded steel pipe having an excellent low temperature tenacity, the a tensile strength of the base material is 900 MPa or larger, and the difference between the tensile strength of a welded metal portion and that of a base metal is -100 MPa or larger, the cap between an inner face welded metal portion formed by a regular welding executed after a tack welding of a butted portion of a steel plate in a pipe manufacturing process and an outer face welded metal portion is 0 mm or greater at the said welded metal zone of the seam welded steel pipe. The steel pipe is characterized by the overlapping of each of the inner face welded metal zone and the outer welded metal zone with a tack welded metal zone formed by the said tack welding.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention can be widely used as a line pipe for transporting natural gas and crude oil, and can improve the efficiency of on-site construction by improving the transport efficiency by increasing the pressure and reducing the outer diameter and weight to 900 MPa or more. TECHNICAL FIELD The present invention relates to an ultra-high-strength line pipe excellent in low-temperature toughness of a seam weld having a tensile strength of 10 mm and a method of manufacturing the same.

[0002]

2. Description of the Related Art In recent years, pipelines have become increasingly important as a long-distance transportation method for crude oil and natural gas.
At present, American Petroleum Institute (API) standard X65 is the basis of design for trunk line pipes for long-distance transportation, and actual usage is overwhelmingly large. However, higher-strength line pipes have been demanded for (1) improvement of transportation efficiency by increasing pressure and (2) improvement of on-site construction efficiency by reducing outer diameter and weight of line pipes. Until now, line pipes up to X80 (tensile strength of 620 MPa or more) have been put to practical use, but the need for higher-strength line pipes has increased. At present, research on ultrahigh-strength linepipe manufacturing methods is based on conventional X80 linepipe manufacturing techniques (eg, NKK Technical Report No. 138 (1992), pp24-31 and Th).
e 7th OffshoreMechanics and Arctic Engineering (19
88), Volume V, pp. 179-185), but at most X100 (tensile strength 760MP)
a) The production of line pipes is considered to be the limit. X1
For ultrahigh-strength line pipes exceeding 00, research on steel sheet production has already been conducted (PCT / JP96 / 00).
155, 00157). However, the conventional seam welding technology cannot be applied to such an ultra-high-strength line pipe, and if the problem relating to the seam welding technology cannot be solved, a steel plate can be manufactured but a steel pipe cannot be manufactured. Over-strengthening of the pipeline has many problems such as balance between strength and low temperature toughness, heat affected zone (HAZ) toughness, on-site weldability, softening of joints, and tube breakage due to burst test. There is a demand for early development of a revolutionary ultra-high-strength line pipe (X100 or more).

[0003]

DISCLOSURE OF THE INVENTION The present invention has an excellent balance of low-temperature toughness and a tensile strength (TS) that allows easy on-site welding.
An object of the present invention is to provide an ultra-high-strength line pipe of 900 MPa or more (exceeding API standard X100), in particular, an ultra-high-strength line pipe excellent in low-temperature toughness of a seam welded portion which does not have a welded portion fracture in a burst test and has a pipe body fracture, and a method of manufacturing the same It is.

[0004]

The gist of the present invention is as follows. (1) the base material has a tensile strength of 900 MPa or more;
And a seam welded steel pipe in which the difference between the tensile strength of the weld metal part and the tensile strength of the base metal part is -100 MPa or more, and in the weld metal part of the seam welded steel pipe, The distance between the inner weld metal portion and the outer weld metal portion formed by the main welding performed after the tack welding is greater than 0 mm, and the inner weld metal portion and the outer weld metal portion are formed by the tack welding. An ultra-high-strength welded steel pipe excellent in low-temperature toughness of a seam weld, characterized by overlapping with a weld metal part. (2) In terms of% by weight, C: 0.03 to 0.10%, Si: 0.6% or less, Mn: 1.7 to 2.5%, P: 0.015% or less, S: 0.003 %: Ni: 0.1 to 1.0%, Mo: 0.15 to 0.60%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.030%, Al: 0 B: 0.0030% or less;
N: 0.001 to 0.006%, V: 0.10% or less,
Cu: 1.0% or less, Cr: 1.0% or less, Ca: 0.
01% or less, REM: 0.02% or less, Mg: 0.00
A base material portion containing one or more kinds of 6% or less, and the balance being iron and unavoidable impurities; and C: 0.03 to 0.14%, Si: 0.05 to 040 by weight%. %, Mn: 1.2 to 2.2%, P: 0.010% or less, S: 0.010% or less, Ni: 1.3 to 3.2%, one of Cr, Mo, and V Or a total amount of two or more of 1.0 to 2.5%, B: 0.005% or less, the balance being a weld metal portion composed of iron and unavoidable impurities, and The Ni content is 1% or more higher than the Ni content of the base material, and the structure of the seam weld including the weld metal portion and the weld heat affected zone of the base metal is bainite.
The ultra-high strength welded steel pipe according to the above (1), which is made of martensite and has excellent low-temperature toughness at a seam weld. (3) In weight%, C: 0.03 to 0.10%, Si: 0.6% or less, Mn: 1.7 to 2.5%, P: 0.015% or less, S: 0.003 %: Ni: 0.1 to 1.0%, Mo: 0.15 to 0.60%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.030%, Al: 0 B: 0.0030% or less;
N: 0.001 to 0.006%, V: 0.10% or less,
Cu: 1.0% or less, Cr: 1.0% or less, Ca: 0.
01% or less, REM: 0.02% or less, Mg: 0.00
After joining both ends of a steel sheet containing 6% or less of one or more kinds and the balance consisting of iron and unavoidable impurities, the joined parts are expressed by weight% as C: 0.01-0. .
12%, Si: 0.3% or less, Mn: 1.2 to 2.4%
After performing tack welding from the outer surface using a welding wire containing Fe as a main component, the tack welded portion is expressed by weight: C: 0.01 to 0.12%, Si: 0 0.3% or less, Mn: 1.2 to 2.4%, Ni: 4.0 to 8.5
%, Total amount of one or more of Cr, Mo, and V 3.0
An inner weld metal portion and an outer surface formed by welding using a welding wire and a flux containing Fe as a main component and containing at least 5.0% and having a Ni content of at least 1% higher than the Ni content of the steel sheet. The distance between the weld metal parts is more than 0 mm,
And the main welding is performed from the inner surface and the outer surface of the tack welding portion so that the inner welding metal portion and the outer welding metal portion overlap with the tack welding metal portion formed by the tack welding, respectively. For producing ultra-high strength welded steel pipes with excellent low-temperature toughness at seam welds. (4) In the main welding, ultra-high strength welding with excellent low-temperature toughness of the seam weld according to (3), wherein the tack weld is welded in two passes or more from the inner surface and the outer surface, respectively. Manufacturing method of steel pipe. (5) MAG arc welding, MI
Using one of G arc welding and TIG arc welding, submerged arc welding, MA
(3) characterized in that any one of G arc welding, MIG arc welding and TIG arc welding is used.
Or the manufacturing method of the ultra high strength welded steel pipe excellent in low temperature toughness of the seam weld according to any one of (4).

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the contents of the present invention will be described in detail. The present invention has a tensile strength (T
The present invention relates to an ultra-high-strength line pipe having excellent low-temperature toughness of a seam weld having S). The ultra-high-strength line pipe of this strength level can withstand about twice the pressure as compared with the conventional mainstream X65, so that about twice the gas of the same size can be transported. On the other hand, when using X65 to achieve gas transport efficiency equivalent to that of the above ultra-high-strength line pipe, it is necessary to increase the wall thickness in order to increase the pressure, and material costs, transport costs, and on-site welding construction costs increase. This significantly increases the cost of laying pipelines. This is 900MPa
This is why an ultra-high-strength line pipe having the above-mentioned tensile strength (TS) and excellent in low-temperature toughness is required. Conventionally, in such an ultra-high-strength line pipe having a tensile strength of 900 MPa or more, it becomes extremely difficult to manufacture a steel pipe, and it is also difficult to secure particularly low-temperature toughness characteristics of the steel pipe.
As a guide to assure the target characteristics including the seam welded portion of the steel pipe, in the burst test, fracture in the pipe body is achieved without breaking in the weld heat affected zone and weld metal, etc. Improving toughness is an important technical issue. With conventional ultra-high strength line pipes,
During welding, coarse MA (Martensi) along the former austenite grain boundary from the junction of the heat affected zone of the seam weld to 1 mm
te-Austenite Constituent: a composite of martensite and austenite) was easily formed, which became the starting point of fracture and was a factor that significantly reduced the absorbed energy value. Therefore, the V-notch Charpy absorbed energy at the junction of the 1 / 2t part of the conventional welding heat affected zone or at the junction + 1 mm is as low as less than 50 J at -30 ° C, for example, satisfying the target of 64 J or more at -30 ° C. Was quite difficult.

The present inventors have conducted intensive studies through experiments and the like in order to improve the low-temperature toughness of a seam welded part in an ultrahigh-strength line pipe having a tensile strength of 900 MPa or more. FIG.
FIG. 2 shows a conventional welded portion and a welded portion according to the present invention in an ultrahigh-strength line pipe. Normally, in seam welding at the time of forming a steel pipe, after joining both ends of a steel plate, the joined portion is firstly tacked from the outer surface by MAG arc welding or the like (hereinafter referred to as tack welding), and thereafter, the tack is welded. The welded portion is further welded from the inner surface and the outer surface by submerged arc welding or the like (hereinafter referred to as main welding). At this time, in the conventional main welding, as shown in FIG. 1, a welding metal portion formed by welding from the outer surface in the main welding (hereinafter referred to as an outer surface welding metal portion).
And the weld metal part formed by welding from the inner surface (hereinafter referred to as the inner weld metal part) overlaps with each other. Higher austenite grains in the heat affected zone become coarser and form along the former austenite grain boundaries
MA is also coarsened, which lowers the Charpy absorbed energy of the heat affected zone and softens the heat affected zone, causing a break from the weld (not a tube break) in a burst test. I understood.

[0007] The present inventors have found that the low-temperature toughness of a weld portion caused by an excessive rise in welding heat input in the main welding from the inner and outer surfaces where the outer weld metal portion and the inner weld metal portion overlap each other as in the prior art. In order to solve such a problem, the optimal conditions of the main welding were examined in detail. as a result,
As shown in FIG. 2, in the final welding after the tack welding, the outer welding metal portion and the inner welding metal portion are not overlapped and the tack welding metal portion is left without being melted, and excessive welding input during the final welding is performed. By avoiding an increase in heat, coarsening of the prior-austenite grain size and coarsening of the MA in the coarse-grained portion in the heat-affected zone of the welding is suppressed, and the 1 / 2t portion of the welded heat-affected zone is associated with or
It was found that by improving the V-notch Charpy absorbed energy at 1 mm and suppressing the softened portion of the weld heat affected zone, it was possible to break the pipe (no break from the welded portion) in a burst test. In addition, when performing such main welding, in order to prevent the occurrence of welding defects in the welded portion, each of the inner surface welding metal portion and the outer surface welding metal portion formed by the main welding and the temporary welding before the same are performed. It was found that it was necessary to overlap the formed temporary weld metal part.

From the above findings, in the present invention, in the seam weld portion of an ultra-high strength line pipe having a tensile strength of 900 MPa or more, an inner weld metal portion and an outer weld metal formed by main welding after tack welding are provided. The interval between parts (Δd) is 0
mm (whereas the conventional Δd is less than 0 mm), and the inner weld metal portion and the outer weld metal portion respectively overlap with the tack weld metal portion formed by the tack welding. Is required. The above Δd is 0 mm
In the following, that is, when the inner weld metal portion and the outer weld metal portion formed by the main welding overlap, the tack weld metal portion performed before the main welding melts and disappears as described above, and the welding heat input becomes excessive. As a result, the former austenite grains in the weld heat affected zone become coarser and the MA generated along the former austenite grain boundaries also becomes coarser, lowering the Charpy absorbed energy of the weld heat affected zone and softening the weld heat affected zone. Happens.
In addition, if each of the inner surface weld metal portion and the outer surface weld metal portion formed by the main weld portion does not overlap with the tack weld metal portion formed by tack weld, a weld defect of the weld portion occurs. . Here, the method of tack welding performed first in the present invention may be MAG arc welding, MIG arc welding, or TIG welding, which is generally known. Also,
Inner / outer surface welding performed after tack welding may be submerged arc welding, MIG arc welding, or TIG welding. In addition, the welding from the inner surface and the outer surface of the tack welding portion in the final welding after the tack welding may be one-pass welding or two or more-pass welding, respectively. Is more preferable in terms of suppressing the softening of the heat affected zone by lowering the welding temperature as much as possible.

Further, from the results of a number of burst tests performed by the present inventors using steel pipes manufactured by the above-described welding method, the tensile strength of the weld metal was found to be [base material strength] −100 (MP).
a) It is known that if it is above, it does not break from the welded portion but breaks from the tube. Therefore, in the present invention, the average tensile strength of the welded portion is set to [the circumferential tensile strength of the base metal portion] −100.
(MPa) or more.

Next, the components and structures of the base metal and the weld metal constituting the steel pipe of the present invention will be described. First, the reasons for limiting the base material components of the present invention are as follows. C content is limited to 0.03 to 0.10%. Carbon is extremely effective in improving the strength of steel, and at least 0.03% is required to obtain the target strength in the martensite structure. However, if the C content is too large, the low-temperature toughness and the on-site weldability of the base material and HAZ are remarkably deteriorated, so the upper limit is set to 0.10%. Further, preferably, the upper limit value is 0.1.
08% is preferred.

[0011] Si is an element added for deoxidation and improvement of strength, but if added in a large amount, HAZ toughness and on-site weldability are significantly deteriorated, so the upper limit was made 0.6%. Deoxidation of steel is sufficiently possible with Al or Ti, and Si need not always be added. Mn is an element indispensable for making the microstructure of the steel of the present invention a martensite-based structure and ensuring excellent balance between strength and low-temperature toughness, and the lower limit thereof is 1.7%. However, if the Mn content is too large, the hardenability of the steel is increased to deteriorate not only the HAZ toughness and the on-site weldability, but also the central segregation of the continuously cast steel slab and the low-temperature toughness of the base material. 2.5%.

In the present invention, the amounts of P and S as impurity elements are set to 0.015% and 0.003% or less, respectively. The main reason for this is to further improve the low-temperature toughness of the base material and HAZ. The reduction of the P content reduces the segregation of the center of the continuously cast slab, prevents the grain boundary fracture, and improves the low-temperature toughness. Further, the reduction of the amount of S has the effect of reducing MnS to be elongated by hot rolling and improving ductility.

The purpose of adding Ni is to improve the low carbon steel of the present invention without deteriorating the low-temperature toughness and the on-site weldability. Compared with the addition of Mn, Cr, and Mo, Ni addition not only causes less formation of a hardened structure that is harmful to low-temperature toughness in the rolled structure (particularly, the central segregation zone of a continuously cast steel slab), but also 0.1% or more. It has been found that the addition of a small amount of Ni is also effective in improving the HAZ toughness (the amount of Ni particularly effective in terms of the HAZ toughness is 0.3% or more). However, if the addition amount is too large, not only economic efficiency but also HAZ toughness and on-site weldability are degraded, so the upper limit was made 1.0%.
Ni addition is performed during continuous casting and hot rolling.
It is also effective in preventing cracks. In this case, Ni needs to be added at least 1/3 of the Cu amount.

The reason for adding Mo is to improve the hardenability of steel and to obtain the desired structure mainly composed of martensite. In the B-added steel, the effect of improving the hardenability of Mo is enhanced, and Mo coexists with Nb to suppress recrystallization of austenite during controlled rolling, and is also effective in refining the austenite structure. To obtain such an effect, M
o must be at least 0.15%. However, excess M
o addition deteriorates HAZ toughness and on-site weldability,
In some cases, the effect of improving the hardenability of the steel may be lost, so the upper limit was set to 0.6%.

Nb contains 0.01 to 0.10%. Nb coexists with Mo to suppress the recrystallization of austenite during controlled rolling, not only to refine the structure, but also to contribute to precipitation hardening and hardenability, and to strengthen the steel,
It contains 0.01% or more. In particular, when Nb and B coexist, the effect of improving hardenability increases synergistically. However, if the added amount of Nb is too large, it adversely affects HAZ toughness and on-site weldability, so the upper limit was set to 0.10%.

[0016] Ti contains 0.005 to 0.030%. Addition of Ti forms fine TiN, suppresses coarsening of austenite grains in the HAZ during reheating of the slab and refines the microstructure, and improves the low-temperature toughness of the base material and the HAZ. In addition, solid solution N which is harmful to the effect of improving the hardenability of B
Also has the role of fixing as TiN. For this purpose, it is desirable to add Ti in an amount of 3.4N (each wt%) or more. When the amount of Al is small (for example, 0.0
0.05% or less), Ti forms an oxide, acts as an intragranular ferrite generation nucleus in the HAZ, and also has the effect of making the HAZ structure finer. In order to exert such an effect of TiN, at least 0.005% of Ti must be added. However, if the amount of Ti is too large, coarsening of TiN and T
Since precipitation hardening due to iC occurs and deteriorates low-temperature toughness, the upper limit thereof is limited to 0.030%.

Al is an element usually contained in steel as a deoxidizing material, and also has an effect on refining the structure. However, if the amount of Al exceeds 0.06%, Al-based nonmetallic inclusions increase and impair the cleanliness of the steel, so the upper limit was made 0.06%. However, deoxidation is possible with Ti or Si, and Al need not always be added. The above are the main components of the steel pipe base material of the present invention, and the following components are selectively contained as necessary.

B is a very effective element for dramatically improving the hardenability of steel in a very small amount and for obtaining a desired structure mainly composed of martensite. Further, B enhances the effect of improving the hardenability of Mo, and synergistically increases the hardenability together with Nb. On the other hand, if it is added excessively, it not only deteriorates the low-temperature toughness, but also sometimes loses the effect of improving the hardenability of B, so the upper limit is 0.003.
0%.

N forms TiN and suppresses coarsening of austenite grains of the HAZ during reheating of the slab and HAZ.
Improves the low temperature toughness of AZ. The minimum required for this is 0.001%. However, if the amount of N is too large, it causes deterioration of HAZ toughness due to slab surface flaws and solid solution N, and lowers the effect of improving the hardenability of B, so the upper limit is 0.0.
It is necessary to suppress it to 06%.

Next, V, Cu, Cr, Ca, REM,
The purpose of adding Mg will be described. The main purpose of further adding these elements to the basic components of the steel pipe base material of the present invention is to improve the strength and strength without impairing the excellent characteristics of the steel of the present invention.
This is because the toughness is further improved and the size of the steel material that can be manufactured is increased. Therefore, the amount of addition is of a nature that should be naturally restricted.

V has almost the same effect as Nb, but the effect is weaker than Nb. However, the effect of V addition on ultra-high strength steel is great, and the combined addition of Nb and V makes the excellent features of the steel of the present invention more remarkable. The upper limit is H
From the viewpoint of AZ toughness and on-site weldability, 0.10% is acceptable, but 0.03 to 0.08% is particularly preferable.

Although Cu increases the strength of the base material and the welded portion, if it is too large, the HAZ toughness and the on-site weldability are remarkably deteriorated. Therefore, the upper limit of the amount of Cu is 1.0%. Cr increases the strength of the base material and the weld, but if too much, HAZ
It significantly deteriorates toughness and on-site weldability. Therefore, the upper limit of the Cr content is 0.6%. Ca and REM control the sulfide (MnS) morphology and improve low temperature toughness (such as increasing the energy absorbed in the Charpy test). If the Ca content exceeds 0.006% and the REM exceeds 0.02%, CaO-CaS or REM-CaS is generated in large quantities to form large clusters and large inclusions, which not only impairs the cleanliness of the steel, It also has an adverse effect on local weldability. Therefore, the upper limit of the amount of Ca added was limited to 0.006%, and the condition of the amount of REM added was limited to 0.02%. In the case of the ultra-high strength line pipe, the S and O amounts are 0.001% and 0.0%, respectively.
ESP (EffestiveS), which is an index for controlling the shape of MnS clusters as shown below.
ulphide Shape Controlling Parameter) 0.5 ≦ E
It is particularly effective to adjust Ca, S, and O so as to satisfy SSP ≦ 10.0.

ESSP = (Ca) [1-124 (O)] / 1.25S (1) When the above-mentioned ESSP is less than 0.5, a large amount of CaO—CaS is generated to form coarse clusters and coarse inclusions. And the weldability such as weld cracking deteriorates, and the above ESSP becomes 10.0
Is exceeded, the effect of controlling the shape of MnS is lost.
The ESSP is defined as 0.5 to 10.0.

Mg forms a finely dispersed oxide, suppresses grain coarsening of the weld heat affected zone, and improves low temperature toughness.
When the content is 0.006% or more, a coarse oxide is generated, and on the contrary, the toughness is deteriorated. In addition to the limitation of the individual additional elements described above, P, which is an index indicating hardenability shown below, is set to 1.9 ≦
It is desirable to limit to P ≦ 4.0.

P = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + (1 + β) Mo-1 + β (2) where B = 1 when B ≧ 3 ppm and β = when B <3 ppm
Set to 0. The reason for controlling P as described above is to achieve the desired strength-low temperature toughness balance. The lower limit of the P value is set to 1.9 in order to obtain a strength of 900 MPa or more and excellent low-temperature toughness. Further, the upper limit of the P value is set to 4.0 in order to maintain excellent HAZ toughness and on-site weldability.

The above is the basis for limiting the components contained in the steel pipe base material of the present invention. However, even if it has the above-mentioned chemical components, the fine martensite-bainite-based structure of the present invention can be obtained. The desired characteristics cannot be obtained unless the manufacturing conditions are appropriate for obtaining the above. The principle method of obtaining a fine martensite-based structure is to process the recrystallized grains in the non-recrystallized temperature range to form austenite grains flattened in the sheet thickness direction, and convert them to a critical cooling rate that suppresses ferrite formation. Cooling at a cooling rate of

A preferred manufacturing method is to reheat a billet having the chemical composition of the present invention to 950 to 1250 ° C.
70 so that the cumulative reduction at 950 ° C. becomes 50% or more.
After rolling at a steel material temperature of 0 ° C. or more, it is cooled to 550 ° C. or less at a cooling rate of 10 ° C. or more. A _C1 if necessary
Tempering is performed at a temperature below the transformation point. The steel pipe of the present invention is obtained by forming the steel plate manufactured as described above into a tubular shape, arc welding the butted portions at both ends of the steel plate, and further expanding the tube to form a steel pipe.

Next, the reasons for limiting the components of the weld metal portion of the steel pipe of the present invention will be described. C content is 0.03-0.1
Limited to 4%. Carbon is extremely effective in improving the strength of steel, and at least 0.03% is necessary to obtain the target strength in the martensite structure. However, if the C content is too large, low-temperature cracking is liable to occur, and the so-called H-shape of the T-cross section where the on-site weld and seam weld intersect.
, The upper limit is set to 0.14%. Further, the upper limit is desirably 0.10%.

Si is 0.05 to prevent blow holes.
% Or more is necessary, but if the content is large, the low-temperature toughness is remarkably deteriorated, so the upper limit was made 0.40%. In particular, when performing inner / outer surface welding or multilayer welding, the low-temperature toughness of the reheated portion is deteriorated. Mn is an indispensable element for ensuring excellent balance between strength and low-temperature toughness, and its lower limit is 1.2%. However, if the Mn content is too large, not only segregation is promoted and the low-temperature toughness is deteriorated, but also the production of the welding material becomes difficult, so the upper limit was made 2.2%.

P and S are desirably low in the amount of P and S in order to lower the low-temperature toughness and reduce the susceptibility to low-temperature cracking. The purpose of adding Ni is to increase the hardenability, secure the strength, and further improve the low-temperature toughness. If it is less than 1.3%, it is difficult to obtain the target strength and low-temperature toughness. On the other hand, if the content is too large, there is a risk of hot cracking, so the upper limit was made 3.2%.

Although the differences in the effects of Cr, Mo, and V cannot be strictly distinguished, all of them are added to obtain high strength by improving hardenability. If the total amount of one or more of Cr, Mo, and V is 1.0% or less, the effect is not sufficient. On the other hand, if a large amount is added, the risk of low-temperature cracking increases, so the upper limit is made 2.5%. B is a very small element that enhances the hardenability and is effective in improving the low-temperature toughness of the weld metal. However, if the content is too large, the low-temperature toughness is rather reduced.
005% or less.

In some cases, the weld metal contains, as other components, elements such as Ti, Al, Zr, Nb, Mg, etc., which are added as necessary to improve the refining and solidification during welding. But the balance is iron and unavoidable impurities. The ultrahigh-strength steel sheet of the present invention is manufactured by casting a steel having the above-described components, hot-working, and then quenching or tempering in some cases. 900M tensile strength
In order to achieve an ultra-high strength of Pa or more, it is necessary to convert the steel into a microstructure mainly composed of a low-temperature transformation structure such as martensite / bainite to suppress the formation of ferrite.

The weld metal has an as-solidified structure after welding, and in the case of a weld metal having a low cooling rate, the present invention obtains the above-mentioned target strength and further obtains excellent low-temperature toughness similar to the steel sheet of the present invention. It is necessary to adjust the chemical composition and structure of the weld metal. Ni enhances hardenability so that high strength can be obtained even at a low cooling rate, and promotes the formation of retained austenite between martensite laths and improves low-temperature toughness.

In the present invention, the desired strength and low-temperature toughness can be attained by increasing the Ni content of the weld metal by 1% or more from the steel sheet component and forming the weld metal portion and the weld heat affected zone into a bainite-martensite structure. can get. N of weld metal
If the i content is 1% lower than the steel sheet component, the above effect cannot be obtained. Therefore, in the present invention, the lower limit is set to 1%. next,
The method for producing the steel pipe of the present invention will be described.

The line pipe aimed at by the present invention usually has a diameter of 450 mm to 1500 mm and a wall thickness of about 10 mm to 40 mm. As a method of producing a steel pipe of such a size at a high efficiency, a steel plate is formed into a U-shape and then an O-shape.
Pipes are formed in the UO process of forming into a shape, the ends of the steel plate are butted together, and the butted portion is tack-welded from the outer surface by MAG arc welding or the like. A method of manufacturing a steel pipe that is welded and then expanded to increase roundness has been established.

The arc welding method such as submerged arc welding is a welding in which the base metal is greatly diluted, and in order to obtain desired characteristics, that is, a weld metal composition, it is necessary to select a welding material in consideration of the dilution of the base metal. . Hereinafter, the reasons for limiting the chemical composition of the welding wire used for welding when manufacturing the ultra-high-strength line pipe of the present invention will be described. In addition, the tack welding of the jointed portion with the steel plate performed before the main welding according to the present invention has a small welding area and the influence of the quality of the weld metal part is smaller than that of the main welding. , All are defined as follows, but the components of the welding wire used for tack welding other than C, Si, and Mn need not be particularly defined.

In order to obtain the range of the amount of C required for the weld metal, C is obtained from the dilution with the base metal component and the atmosphere.
Was set to 0.01 to 0.12% in consideration of the mixing of Si
In order to obtain a range of the amount of Si required in the weld metal, the content was set to 0.3% or less in consideration of dilution with a base metal component. In order to obtain the range of the amount of Mn required for the weld metal, Mn is set to 1.2% or more in consideration of dilution with the base metal component.
2.4%.

Ni is used in consideration of dilution with a base metal component in order to obtain a range of the amount of Ni required for the weld metal.
0% to 8.5%. Cr, Mo, and V are 3.0 in consideration of dilution with a base metal component in order to obtain a range of the total amount of one or more of Cr, Mo, and V required in the weld metal. % To 5.0%.

In addition, it is desirable that the impurities of P and S are as small as possible, and B can be added for ensuring the strength. Further, Ti, Al, Zr, Nb, Mg and the like are used for the purpose of deoxidation. The tack welding and the main welding according to the present invention can be performed not only with a single pole but also with a plurality of electrodes.
In the case of welding with a plurality of electrodes, various kinds of wires can be combined, and the individual wires need not be in the above-described component range, and it is sufficient that the average composition from the respective wire components and the consumption is within the above-described component range.

The flux used in the main welding such as the submerged arc welding is roughly classified into a firing flux and a fusion flux. The calcined flux has an advantage that an alloy material can be added and the amount of diffusible hydrogen is low, but it has a disadvantage that it is easily powdered and is difficult to use repeatedly. On the other hand, the molten flux is in the form of glass powder, has the advantage of high grain strength, is unlikely to absorb moisture, and has the drawback that diffusible hydrogen is rather high. In the case of ultra-high strength as in the present invention, low-temperature cracking easily occurs,
From this point, a sintering mold is desirable. On the other hand, a molten mold that can be collected and used repeatedly has an advantage of low cost for mass production. Although the cost is high in the firing type, and the necessity of strict quality control is a problem in the melting type, it is within a range that can be industrially dealt with, and both types are essentially usable.

Desirable ranges for the welding conditions are as follows. The initial tack welding may be MAG arc welding, MIG arc welding, or TIG arc welding. Usually, it is MAG arc welding. Next, the main welding performed after the tack welding is usually a submerged arc welding.
G arc welding may be used. The appropriate welding speed is about 1-3 m / min. Welding at less than 1 m / min is inefficient for seam welding of line pipes, and bead shape is not stable at high speed welding at more than 3 m / min. If tack welding and subsequent main welding overlap, the welding heat input is preferably as low as possible. The arc welding of the main welding may be performed in any number of passes. Although the welding heat input varies depending on the plate thickness, for example, when the plate thickness is 16 mm, the welding heat input is 1.0 to 2.7.
A desirable range is kJ / mm. If the heat input is too small, the penetration will be insufficient, the number of weldings will be increased, and the working efficiency will be deteriorated. If the heat input is too large, the heat-affected zone will be softened greatly and the toughness of the welded portion will be reduced.

After these seam weldings, the roundness is improved by pipe expansion. In order to form a perfect circle, it is necessary to deform to the plastic region. However, in the case of the high-strength steel as in the present invention, the expansion ratio is about 0.7% or more (= (circumference after expansion−circle before expansion) / (Circumference before expansion) is necessary, but if a large expansion exceeding 2% is performed, the toughness deterioration due to plastic deformation in both the base metal and the welded portion will increase, so the expansion ratio should be 0.7 to 2% or less. Is desirable.

[0043]

The embodiments of the present invention and the effects thereof will be specifically described below. Inventive steel (Steel A to D steel) of the components satisfying the range of the present invention shown in Table 1 and comparative steels (E steel, F steel) of components out of the range of the present invention are melted in a 300-ton converter and then continuously cast steel. After being re-heated to 1100 ° C, rolled in a recrystallization region, and then subjected to controlled rolling to a cumulative reduction of 75% at 900 to 750 ° C to 16 mm, and then the water cooling stop temperature becomes 200 to 450 ° C. The steel plate was manufactured by water cooling as described above. As a result, as shown in Table 1, invention steels (A steel to D
Steel), the strength of the steel sheet is within the target range of the present invention (900 MPa).
), And the low-temperature toughness (absorbed energy at −30 ° C. in the Charpy test: 230 J or more) was also high. on the other hand,
The strength of the steel sheet of E steel having a high C content and not adding Ni is within the target range of the present invention, but the low-temperature toughness is low,
The low-temperature toughness of F steel sheet with low C content is within the target range,
Low strength.

[0044]

[Table 1]

[0045]

[Table 2]

These steel sheets are further formed into a tube shape at a UO factory, and the joined parts of the steel sheets are formed of 80% Ar + 20% CO _2.
After performing tack welding using MAG arc welding with the shielding gas of Table 1, temporary welding was performed using the welding wires and fluxes shown in Table 2 under the welding conditions of three electrodes, 2.0 m / min, and heat input of 1.5 KJ / mm. The inner and outer surfaces of the portion were subjected to main welding by one-pass submerged arc welding, and then expanded at a pipe expansion ratio of 1%. Table 3 shows the results of evaluating the properties of the obtained steel pipe.

An example of the invention (Example No. 1) in which welding was performed using steel and a welding wire having components satisfying the range of the present invention shown in Table 2.
In (6), a good weld bead is obtained at the steel pipe seam weld, and the chemical composition of the weld metal part satisfies the range of the present invention,
The weld metal strength (900MPa or more), the difference between the tensile strength of the weld metal and the tensile strength of the steel sheet and the appropriate range (-100MPa or more) are appropriate, and the distance between the inner weld metal part and the outer weld metal part formed by the main welding. (Δd) (Δd: 0 mm
Super)) was also suitable. In addition, the steel pipe of the invention example satisfying the scope of the present invention has mechanical strength such as target strength and low-temperature toughness in both the base material and the welded part, and the pipe body fracture can be achieved even in the burst test. .

On the other hand, in the comparative example, No. In Nos. 7 to 9, the base metal component is within the range of the present invention, but the wire component is out of the range of the present invention (No. 7: Ni is low, No. 8: C is high,
No. 9: Ni is higher), the components of the weld metal portion were outside the range of the present invention. As a result, no. In No. 7, the strength of the weld metal was low, and the difference between the tensile strength of the weld metal and the tensile strength of the steel sheet was out of an appropriate range (-100 MPa or more). Also, N
o. In No. 8, low-temperature cracking of the welded portion occurred. In No. 9, a tensile test and a burst test could not be performed because hot cracking occurred.

Example No. of Comparative Example In No. 10, the composition of the welding wire falls within the range of the present invention, but the composition of the steel sheet is out of the range of the present invention.
Did not reach. However, in Comparative Example No. 11 and No. In No. 12, the components of the base metal and the welding wire are within the range of the present invention, but the distance (Δd) between the inner surface weld metal portion and the outer surface weld metal portion in the weld formed by the main welding is within the range of the present invention. (Δd> 0), so that a burst in the weld heat affected zone occurred in the burst test. In addition, No. of the comparative example. 13
Does not satisfy the required characteristics of the line pipe because the toughness of the weld metal is low because the C of the weld material and the weld metal is outside the range of the present invention (high). No. 1 of the comparative example
No. 4 shows that the components of the base metal and the welding wire are within the range of the present invention, but the difference between the tensile strength of the weld metal and the tensile strength of the steel sheet was out of the proper range (−100 MPa or more), so that the burst test showed that the weld heat affected zone fractured. Occurred.

[0050]

[Table 3]

[0051]

[Table 4]

[0052]

[Table 5]

[0053]

According to the present invention, it is possible to realize an ultra-high-strength line pipe having a tensile strength of 900 MPa or more (API standard X100 or more) which is excellent in low-temperature toughness balance for both the base material and the welded portion, and is easily welded on site. Therefore, the cost of laying long-distance pipelines is reduced, which can contribute to solving the world's energy problems.

[Brief description of the drawings]

FIG. 1 is a sectional view of a steel pipe seam weld of the present invention.

FIG. 2 is a cross-sectional view of a conventional steel pipe seam weld.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Outer surface weld metal part of this weld metal part 2 ... Inner surface weld metal part of this weld metal part 3 ... Base material part of steel pipe 4 ... Temporary weld metal part

[Procedure amendment]

[Submission date] April 25, 2000 (200.4.2
5)

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] Fig. 1

[Correction method] Change

[Correction contents]

FIG.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shigeru Ohita 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Yoshio Terada 1 Kimitsu, Kimitsu-shi, Chiba New Japan F-term in Kimitsu Works (reference) 4E001 AA03 BB05 BB07 BB08 CA02 CC03 DC01 DC05 DG02

Claims (5)

[Claims] 1. A seam welded steel pipe having a base material having a tensile strength of 900 MPa or more and a difference between a tensile strength of a weld metal and a tensile strength of a base material of -100 MPa or more, wherein the seam welding is performed. In the weld metal part of the steel pipe, the interval between the inner weld metal part and the outer weld metal part formed by the main welding performed after the tack welding of the steel sheet-attached part in the pipe making process is more than 0 mm, and the inner weld An ultra-high-strength welded steel pipe excellent in low-temperature toughness of a seam weld portion, wherein a metal portion and an outer surface weld metal portion overlap with a tack weld metal portion formed by the tack weld, respectively. 2. In% by weight, C: 0.03 to 0.10%, Si: 0.6% or less, Mn: 1.7 to 2.5%, P: 0.015% or less, S: 0 0.003% or less, Ni: 0.1 to 1.0%, Mo: 0.15 to 0.60%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.030%, Al : 0.06% or less, and further, by weight%, B: 0.0030% or less,
N: 0.001 to 0.006%, V: 0.10% or less,
Cu: 1.0% or less, Cr: 1.0% or less, Ca: 0.
01% or less, REM: 0.02% or less, Mg: 0.00
A base material portion containing one or more of 6% or less, and the balance being iron and unavoidable impurities; and C: 0.03 to 0.14% by weight, Si: 0.05 to 0% by weight. 0.40%, Mn: 1.2 to 2.2%, P: 0.010% or less, S: 0.010% or less, Ni: 1.3 to 3.2%, Cr, Mo, and V A total of one or more of 1.0 to 2.5%, B: 0.005% or less, the balance being a weld metal portion composed of iron and unavoidable impurities, and a weld metal The Ni content of the portion is 1% or more higher than the Ni content of the base material portion, and the structure of the seam weld including the weld metal portion and the weld heat affected zone of the base material is made of bainite martensite. Item 1. An ultra-high strength welded steel pipe having excellent low-temperature toughness in a seam weld according to item 1. 3. In% by weight, C: 0.03 to 0.10%, Si: 0.6% or less, Mn: 1.7 to 2.5%, P: 0.015% or less, S: 0 0.003% or less, Ni: 0.1 to 1.0%, Mo: 0.15 to 0.60%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.030%, Al : 0.06% or less, and further, by weight%, B: 0.0030% or less,
N: 0.001 to 0.006%, V: 0.10% or less,
Cu: 1.0% or less, Cr: 1.0% or less, Ca: 0.
01% or less, REM: 0.02% or less, Mg: 0.00
After joining both ends of a steel sheet containing 6% or less of one or more kinds and the balance consisting of iron and unavoidable impurities, the joined parts are expressed by weight% as C: 0.01-0. .
12%, Si: 0.3% or less, Mn: 1.2 to 2.4%
After performing tack welding from the outer surface using a welding wire containing Fe as a main component, the tack welded portion is expressed by weight: C: 0.01 to 0.12%, Si: 0 0.3% or less, Mn: 1.2 to 2.4%, Ni: 4.0 to 8.5
%, Total amount of one or more of Cr, Mo, and V 3.0
An inner weld metal portion and an outer surface formed by welding using a welding wire and a flux containing Fe as a main component, the Fe content of which is greater than or equal to 1% as compared with the Ni content of the steel sheet. The distance between the weld metal parts is more than 0 mm,
And, the inner weld metal portion and the outer surface weld metal portion overlap with the tack weld metal portion formed by the tack weld, respectively, so that the tack weld portion is subjected to main welding from the inner surface and the outer surface. For producing ultra-high-strength welded steel pipes with excellent low-temperature toughness at seam welds. 4. The ultra-high strength with excellent low-temperature toughness of a seam weld according to claim 3, wherein in the main welding, the tack weld is welded in two or more passes from the inner surface and the outer surface, respectively. Manufacturing method of welded steel pipe. 5. The method according to claim 5, wherein the tack welding uses any one of MAG arc welding, MIG arc welding, and TIG arc welding, and the main welding includes submerged arc welding, MAG arc welding, MIG arc welding, and TIG arc. The method for producing an ultra-high-strength welded steel pipe excellent in low-temperature toughness of a seam weld according to any one of claims 3 and 4, wherein any one method of welding is used.