JP2003138340A - Ultrahigh strength steel pipe with excellent toughness of weld zone, and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel pipe having excellent toughness at low temperature and >=900 MPa (exceeding API Standard X100) tensile strength and also to provide its manufacturing method. SOLUTION: In the high strength welded steel structure with excellent toughness of weld zone, integranular bainite is formed using compound inclusions as nuclei in a weld metal zone or in a weld heat-affected zone, and the nuclei are: compound sulfides which consist of two phases of oxides and sulfides composed essentially of Mn sulfide and sulfides composed essentially of Cu sulfide both precipitated in the periphery of the oxides; or compound inclusions in which nitrides are further precipitated in the compound sulfides.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-high strength steel sheet used for a high-strength welded steel structure excellent in low-temperature toughness of a welded portion (welded structure such as construction, bridge, trench, line pipe), and a method for producing the same. It is about.

[0002]

2. Description of the Related Art In recent years, HT80 and HT100 class high-strength steels have been manufactured with a low carbon equivalent due to the progress of the manufacturing technology, and the weldability, especially the cold cracking sensitivity, has been improved.

On the other hand, the welding of high-strength steel is also aimed at increasing the heat input of the welded portion in order to improve the efficiency. until now,
Multi-layer welding with small heat input was common HT80, HT10
0 double-sided one-pass welding is being considered. The same applies to seam welding of natural gas transportation line pipes. The American Petroleum Institute (AP) is the main line for long-distance transportation.
I) Standard X65 is the basis of design, and the actual amount used is overwhelmingly large.

However, there is a demand for a higher strength line pipe in order to 1) improve the transportation efficiency by increasing the pressure and 2) improve the on-site construction efficiency by reducing the outer diameter and weight of the line pipe. Up to now, line pipes up to X80 (tensile strength of 620 MPa or more) have been put into practical use, but there is a growing need for line pipes with even higher strength.

At present, research on a method for manufacturing an ultra-high strength line pipe is conducted by using a conventional X80 line pipe manufacturing technology (for example, N
KK Technical Report No. 138 (1992), pp24-31 and The 7Th Offshore Mechan.
ics and Arctic Engineerin
g (1988), Volume V, pp179-18.
5) is being considered as the basis, but this is at most X
The production of 100 (tensile strength of 760 MPa or more) line pipe is considered to be the limit. Regarding ultra-high strength line pipes exceeding X100, research on steel plate production has already been conducted (PCT / JP96 / 00155, 00157).

[0006] However, in such an ultra-high strength line pipe, it is difficult to secure the toughness of the welded portion in particular, and if the problems relating to this cannot be solved, the steel pipe can be manufactured even if the steel plate can be manufactured. The ultra-high strength of pipelines has many problems such as the balance of strength and low temperature toughness of the base metal, weld metal and weld heat affected zone (HAZ) toughness, field weldability, joint softening, and tube breakage due to burst test. Therefore, there is a demand for early development of an epoch-making ultra-high-strength line pipe (X100 or more) that overcomes these problems.

In a thick plate or the like, in order to improve the low temperature toughness of an ultra-high strength welded portion, particularly a weld heat affected zone and a weld metal, is it necessary to add a large amount of Ni (for example, a manual for welding and joining pp).
888), or multi-layer heaping with low heat input (for example, Nippon Steel CAT No. EXE332 (1973) pp
1 to 69) is preferable.

However, there remains a problem that it costs a great deal to apply these to an actual line pipe.

[0009]

DISCLOSURE OF THE INVENTION The present invention has a tensile strength of 900 MPa or more excellent in low temperature toughness (API standard X100.
And a method for manufacturing the same.

[0010]

[Means for Solving the Problems] The inventors of the present invention have conducted earnest researches on conditions to be satisfied by an ultra-high-strength seam weld having a weld metal having a tensile strength of 900 MPa or more and excellent low temperature toughness. The inventors have invented an ultra-high-strength weld and a method for manufacturing the same. The gist of the present invention is as follows. (1) In the weld metal part or in the heat affected zone of the weld, the intergranular bainite fraction of 50% or more is generated with the complex inclusion as a nucleus, and the complex inclusion is mainly composed of an oxide and Mn mainly precipitated around the oxide. An ultra-high strength steel pipe having excellent weld toughness, which is a composite sulfide consisting of two phases of sulfide and sulfide mainly composed of Cu. (2) In the weld metal part or in the heat affected zone of the weld, an intergranular bainite fraction of 50% or more is formed with the composite inclusion as a nucleus, and the composite inclusion is mainly composed of oxide and Mn mainly precipitated around the oxide. An ultrahigh-strength steel pipe having excellent weld toughness, which is a composite sulfide composed of two phases of sulfide and a sulfide mainly containing Cu and a composite inclusion in which nitride is precipitated on the composite sulfide. (3) The ultra-high-strength steel pipe excellent in weld toughness according to (1) or (2), characterized in that the oxide is a Ti-containing oxide. (4) The size of the composite sulfide composed of the oxide or sulfide or the composite inclusion composed of the oxide, sulfide, or nitride is 0.01 to 5 μm in terms of average circle equivalent diameter, and the average density. Is present in the weld metal in an amount of 1 × 10 ³ pieces / mm ² or more, the high-strength welded steel structure having excellent weld toughness according to any one of (1) to (3). (5) The ultrahigh-strength steel pipe having excellent weld toughness according to any one of (1) to (4), wherein the base material has a tensile strength of 900 MPa or more. (6) The ultra-high-strength steel pipe with excellent weld toughness according to any one of (1) to (5), wherein the weld metal has a bainite / martensite fraction of 50% or more. (7) The chemical composition of the weld metal is C: 0.04 in mass%.
~ 0.14%, Si: 0.05-0.40%, Mn:
1.2-2.2%, P: ≦ 0.01%, S: ≦ 0.01
0%, Ni: 1.3 to 3.2%, Cu: 0.1 to 1.0
%, Cr + Mo + V: 1.0 to 2.5%, Ti: 0.
003 to 0.050%, Al: ≤ 0.02%, B: ≤
An ultrahigh-strength steel pipe excellent in weld toughness according to any one of (1) to (6), characterized in that it contains 0.005% and the balance is Fe and unavoidable impurities. (8) The chemical composition of the base material is C: 0.03% by mass%.
0.10%, Si: ≤ 0.6%, Mn: 1.7 to 2.5
%, P: ≤ 0.015%, S: ≤ 0.003%, Ni:
0.1-2.0%, Cu: 0.1-1.0%, Mo:
0.15 to 2.5%, Nb: 0.01 to 0.10%, T
i: 0.005 to 0.030%, Al: ≤ 0.06%, and optionally, B: ≤ 0.005%, N: 0.
001 to 0.006%, V: ≦ 0.10%, Cr: ≦
0.8%, Ca: ≤0.01%, REM: ≤0.02
%, Mg: ≦ 0.006%, one or two or more, and the balance Fe and inevitable impurities are contained, and the welded part according to any one of (1) to (7). Ultra high strength steel pipe with excellent toughness. (9) C: 0.03 to 0.10% by mass%, Si: ≤
0.6%, Mn: 1.7 to 2.5%, P: ≤ 0.015
%, S: 0.003%, Ni: 0.1 to 2.0%, C
u: 0.1 to 1.0%, Mo: 0.15 to 2.5%, N
b: 0.01 to 0.10%, Ti: 0.005 to 0.0
30%, Al: ≤ 0.06%, and optionally,
B: ≤ 0.005%, N: 0.001 to 0.006%,
V: ≤0.10%, Cr: ≤0.8%, Ca: ≤0.0
1%, REM: ≤ 0.02%, Mg: ≤ 0.006%, one or more of them, and a balance of Fe and unavoidable impurities is used to form a steel plate into a tubular shape in the UO process, From the inner and outer surfaces of the butted portion, in mass%, C: 0.01 to
0.12%, Si: ≤ 0.3%, Mn: 1.2 to 2.4
%, Ni: 4.0 to 8.5%, Cu: 0.1 to 2.0
%, Cr + Mo + V: 3.0 to 5.0%, Ti: 0.0
05-0.150%, Al: ≤ 0.02%, submerged arc welding is performed using a welding wire containing the balance Fe as the main component and a baking type or melting type flux, and then
A method for producing an ultra-high strength steel pipe having excellent weld toughness, which is characterized by expanding the pipe.

[0011]

DETAILED DESCRIPTION OF THE INVENTION The contents of the present invention will be described in detail below.

The present invention has a tensile strength (T
It is an invention relating to an ultra-high strength weld having S) and excellent low temperature toughness.

As described above, as an application of this high-strength welded portion, there is an ultra-high-strength line pipe having a tensile strength of 900 MPa or more. This line pipe withstands about twice as much pressure as the conventional mainstream X65, so it is possible to transport about twice as much gas with the same size. In the case of X65, in order to increase the pressure, it is necessary to increase the wall thickness, which increases material costs, transportation costs, and local welding construction costs, resulting in a significant increase in pipeline construction costs. This is the reason why an ultrahigh strength line pipe having a tensile strength (TS) of 900 MPa or more and excellent in low temperature toughness is required.

On the other hand, when the strength becomes high, it becomes difficult to manufacture the steel pipe rapidly. In this case, in order to obtain the target properties including the seam weld, the low temperature toughness of the weld (here, the weld metal and the weld heat affected zone) must be improved.

The low temperature toughness of the weld metal has a high strength dependency, and as shown in FIG. 1, for example, considering the worst case to satisfy the Charpy absorbed energy of 84 J at −20 ° C., the weld metal strength is considered. Is required to be less than 1025 MPa, but in some cases it may exceed 1025 MPa, and there is a demand for the development of a weld metal having higher strength and higher toughness.

Regarding the low-temperature toughness of the heat-affected zone of welding, the coarse-grained portion produced after the inner surface welding is affected by the heat in the outer surface welding to produce the coarse-grained reheated portion, which significantly deteriorates the toughness. It has been known. This cause is caused by a coarse bainite structure, but it is desired to subdivide the structure in order to overcome this.

As a result of intensive studies by the inventors for improving the low temperature toughness of the heat-affected zone of the weld metal and the weld metal, when bainite is generated with inclusions as nuclei, the internal structure of the grains is subdivided and the Charpy fracture surface We have found that the unit is extremely small.

FIG. 2 shows an intragranular bainite structure having the inclusions of FIG. 2 as a nucleus.

The inclusions, which are the core of the intragranular bainite, are oxides containing Ti as a main component (Al, Si, Mn, Cr, Mg,
When Ca is also contained) and a sulfide containing Cu and Mn as main components (Ca and Mg may be contained), bainite is remarkably formed with these inclusions as nuclei. I found it.

The authors consider the reason why bainite is formed from inclusions in which CuS and MnS are complexly precipitated in the oxide containing Ti as the main component, as follows. Since the oxide of Ti is a cation vacancy type oxide, it is possible to take in a large amount of Mn ions, for example, and a Mn-deficient layer exists around it. Also,
When MnS is deposited around the oxide, a Mn-depleted layer exists around it.

Further, when CuS is deposited around the oxide, a Cu-depleted layer exists around it. Therefore, in the case of ferrite transformation from the high temperature austenite phase, this M
The presence of the n-deficient layer and the Cu-deficient layer facilitates the formation of bainite transformation with inclusions as nuclei. It is considered that the existence of the Mn-depleted layer and the Cu-depleted layer is the mechanism for the formation of intragrain bainite. Furthermore, if the cooling rate is fast or the quenchability is high, bainite transformation from inside the grains is likely to occur.

Next, the size and number of inclusions will be described. The size of the Ti-containing oxide may be in the range of 0.01 to 5 μm. If the density is 1 × 10 ³ particles / mm ² or more, it is considered that intragrain bainite is generated. Also,
The size of MnS and CuS that are compositely precipitated in this oxide may be in the range of 0.01 to 5 μm as in the Ti-containing oxide.

Around the oxides such as the above Ti-containing oxides, sulfides mainly composed of Mn or Cu, or Ti
Nitride such as N and VN is deposited.

As described above, the oxide composition often contains elements such as Mn, Si, Al, Ca, Mg, and Cr in addition to Ti, and the sulfide contains C in addition to Mn and Cu.
In some cases, elements such as a and Mg are contained.

The reasons for limiting the components of the weld metal will be described below.

The C content is limited to 0.04 to 0.14%.
C is extremely effective in improving the strength of the steel, and in order to obtain the target strength in the martensitic structure, at least 0.
04% is required. However, if the amount of C is too large, welding cold cracking is likely to occur and the HAZ maximum hardness of the so-called T-cross portion where the on-site weld and the seam weld intersect is increased, so the upper limit was made 0.14%. Desirably, the upper limit is 0.10%.

Si is 0.05 to prevent blowholes.
% Or more is necessary, but if the content is large, the low temperature toughness is significantly deteriorated, so the upper limit was made 0.6%. In particular, when performing inner / outer surface welding or multi-layer welding, the low temperature toughness of the reheated portion is deteriorated.

Mn is an essential element for ensuring an excellent balance between strength and low temperature toughness, and is also an essential element as an inclusion for forming intragranular bainite. The lower limit is 1.2%. However, if the Mn content is too large, segregation is promoted, which not only deteriorates the low temperature toughness but also makes it difficult to manufacture a welding material, so the upper limit was made 2.2%.

The purpose of adding Ni is to improve the hardenability, to secure the strength, and to improve the low temperature toughness. If it is 1.3% or less, 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%.

Cu is an indispensable element also as an inclusion for generating intragrain bainite. The lower limit is 0.1%. However, too much Cu not only deteriorates the low temperature toughness but also makes it difficult to manufacture a welding material, so the upper limit was made 1.0%.

Although it is not possible to strictly distinguish between the effects of Cr, Mo, and V, it is difficult to distinguish between them.
Add to obtain high strength. If the total of Cr + Mo + V is 1.2% or less, the effect is not sufficient, and if added in a large amount, the risk of cold cracking increases, so the upper limit was made 2.5%.

B is an element effective for the low temperature toughness of the weld metal by increasing the hardenability in a small amount, but if the content is too large, the low temperature toughness is rather deteriorated, so the content range is 0.00.
It was set to 5% or less.

Ti is indispensable as a main component of inclusions that form intragranular bainite, and its lower limit is 0.003%.
Is. If the amount of Ti is too large, a large amount of Ti carbide is generated and the low temperature toughness is deteriorated, so the upper limit was made 0.05%.

Other than the above, A is added to the weld metal as needed in order to favorably perform refining and solidification during welding.
In some cases, elements such as 1, Zr, Nb, and Mg are contained.
It is necessary to generate an oxide of Ti in order to generate intragranular bainite, and it is desirable that Al is as low as possible.
Desirably, 0.001 to 0.015% or less is preferable. Furthermore, in order to deteriorate the low temperature toughness and reduce the low temperature cracking susceptibility, it is desirable that the amounts of P and S be low.

Next, the bainite-martensite fraction must be 80% or more in order to obtain a tensile strength of 900 MPa or more of the weld metal strength that defines the structure of the weld metal. Further, in order to improve the low temperature toughness of the weld metal, the larger the intragrain bainite fraction is, the more preferable it is, and the more preferable ratio is 80% or more.

The steel sheet aimed at by the present invention is formed into a pipe in the O-shape by the UO method, and the butt portion is arc-welded.
The manufacturing method is established.

Among the arc welding, welding methods such as submerged arc welding, MIG welding, TIG welding and MAG welding can be considered.

Next, the reasons for limiting the components of the steel sheet will be described.

The amount of C is limited to 0.03 to 0.10%.
C is extremely effective in improving the strength of the steel, and in order to obtain the target strength in the martensitic structure, at least 0.
03% is required. However, if the C content is too high, the base metal,
Since the low temperature toughness of HAZ and the field weldability are remarkably deteriorated, the upper limit was made 0.10%. More desirably, the upper limit is preferably 0.07%.

Si is an element added for deoxidation and strength improvement, but if added in a large amount, HAZ toughness and field weldability are significantly deteriorated, so the upper limit was made 0.6%. Deoxidation of steel is sufficiently possible with Al or Ti, and Si is not necessarily added.

Mn is an element indispensable for ensuring a good balance between strength and low temperature toughness by making the microstructure of the steel of the present invention a structure mainly composed of martensite, and its lower limit is 1.7.
%. In addition, it is a main element that produces intragrain bainite as sulfide in the weld heat affected zone. However, Mn
If too much is added, not only the hardenability of the steel increases and HAZ toughness and field weldability deteriorate, but also the center segregation of continuously cast steel slabs is promoted and the low temperature toughness of the base metal also deteriorates, so the upper limit is 2 It was set to 0.5%.

The purpose of adding Ni is to improve the low carbon steel of the present invention without deteriorating the low temperature toughness and field weldability. Compared to Mn, Cr, and Mo additions, addition of Ni is less likely to form a hardened structure detrimental to low-temperature toughness in the rolling structure (especially the central segregation zone of continuously cast steel pieces), and is not less than 0.1%. It was found that the addition of a trace amount of Ni is also effective in improving the HAZ toughness (the amount of Ni added which is particularly effective in terms of HAZ toughness is 0.3% or more). However, if the addition amount is too large, not only the economical efficiency but also the HAZ toughness and field weldability are deteriorated, so the upper limit was made 2.0%. Further, addition of Ni is also effective for preventing Cu cracking during continuous casting and hot rolling. In this case, Ni is 1 / Cu
It is necessary to add 3 or more.

Cu is a main element for increasing the strength of the base material and the welded portion and, like Mn, for forming intragranular bainite as a sulfide in the weld heat affected zone. H is too much
It significantly deteriorates AZ toughness and field weldability. Therefore C
The upper limit of the amount of u is 1.0%.

The reason for adding Mo is to improve the hardenability of steel and to obtain the target structure mainly composed of martensite. In the B-added steel, the hardenability effect of Mo is enhanced, and the co-presence of Mo and Nb suppresses recrystallization of austenite during controlled rolling, and is also effective in refining the structure of austenite.

B is a very effective element in order to dramatically improve the hardenability of steel with a very small amount and to obtain the target structure mainly composed of martensite. Further, B enhances the hardenability improving effect of Mo and, together with Nb, synergistically increases the hardenability. On the other hand, if added excessively, not only the low temperature toughness is deteriorated, but also the hardenability improving effect of B may be lost, so the upper limit is 0.003.
It was set to 0%.

In the steel of the present invention, N is an essential element.
b: 0.01 to 0.10%, Ti: 0.005 to 0.0
Contains 30%. Nb coexists with Mo and not only suppresses recrystallization of austenite during controlled rolling to refine the structure, but also contributes to precipitation hardening and increase in hardenability and strengthens steel. In particular, coexistence of Nb and B synergistically enhances the hardenability improving effect. However, if the amount of Nb added is too large,
The HAZ toughness and field weldability are adversely affected, so the upper limit was made 0.10%. On the other hand, Ti addition is fine Ti
N is formed to suppress the coarsening of austenite grains of the HAZ during reheating of the slab and refine the microstructure to improve the low temperature toughness of the base material and the HAZ. It also has a role of fixing solid solution N, which is harmful to the effect of improving the hardenability of B, as TiN. For this purpose, it is desirable to add Ti in an amount of 3.4 N (each weight%) or more. Further, when the amount of Al is small (for example, 0.005% or less), Ti forms an oxide and acts as an intragranular ferrite formation nucleus in the HAZ, which also has the effect of refining the HAZ structure. In order to exert such an effect of TiN, at least 0.00
5% Ti addition is required. However, if the amount of Ti is too large, coarsening of TiN and precipitation hardening due to TiC occur,
Since the low temperature toughness is deteriorated, the upper limit is limited to 0.030%.

Al is an element usually contained in steel as a deoxidizing material, and has an effect on the refinement of the structure. However, the amount of Al is preferably as low as possible in order to generate intragrain bainite, and is preferably 0.001% to 0.015%. Deoxidation is T
It is also possible to use i or Si, and it is not always necessary to add Al.

N forms TiN and suppresses the coarsening of austenite grains in the HAZ during reheating of the slab and the base metal, H
Improve the low temperature toughness of AZ. The minimum amount 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 defects and solid solution N, and deterioration of hardenability of B, so the upper limit must be suppressed to 0.006%.

Further, in the present invention, the amounts of P and S which are 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. Reduction of the amount of P reduces the center segregation of the continuously cast slab, prevents grain boundary fracture, and improves the low temperature toughness. Further, the reduction of the amount of S has the effect of reducing MnS that is drawn by hot rolling and improving the ductility and toughness.

Next, the purpose of adding V, Cr, Ca, REM and Mg will be described.

The main purpose of adding these elements to the basic composition is to further improve the strength and toughness and to expand the size of steel that can be manufactured without impairing the excellent characteristics of the steel of the present invention. Is. Therefore, the amount added is of a nature that should be limited by itself.

V has almost the same effect as Nb, but the effect is weaker than that of Nb. However, the effect of V addition in the ultra-high strength steel is great, and the combined addition of Nb and V makes the excellent characteristics 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 addition of 0.03 to 0.08% is a particularly desirable range.

Cr increases the strength of the base material and the welded portion, but if it is too much, it significantly deteriorates HAZ toughness and field weldability. Therefore, the upper limit of the amount of Cr is 0.6%.

Ca and REM control the morphology of sulfide (MnS) and improve low temperature toughness (such as increase in absorbed energy in Charpy test). The amount of Ca is 0.006%,
CaO-CaS when REM is added over 0.02%
Alternatively, a large amount of REM-CaS is produced to form large clusters and large inclusions, which not only impairs the cleanliness of steel but also adversely affects the on-site weldability. Therefore, the upper limit of the amount of Ca added is limited to 0.006% or the condition of the amount of REM added is limited to 0.02%. In the ultra high strength line pipe, the S and O contents were reduced to 0.001% and 0.002% or less, respectively, and ESSP = (Ca) [1-124
It is particularly effective to set (O)] / 1.25S to 0.5 ≦ ESSP ≦ 10.0.

Mg forms an oxide that is finely dispersed, suppresses grain coarsening in the heat-affected zone of welding, and improves low temperature toughness.
If it is 0.006% or more, coarse oxides are produced and conversely the toughness is deteriorated.

In addition to the above limitations on the individual additive elements, P = 2.7C + 0.4Si + Mn + 0.8Cr + 0.
45 (Ni + Cu) + (1 + β) Mo-1 + β = 1.9
It is desirable to limit to ≦ P ≦ 4.0. However, B ≧ 3
β = 1 at ppm, β = 0 at B <3 ppm. This 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 strength of 900 MPa or more and excellent low temperature toughness. The upper limit of the P value is set to 4.0 in order to maintain excellent HAZ toughness and field weldability.

When the above steel plates are butted to each other to form a high-strength steel pipe, the mass% is measured from the inner and outer surfaces of the butted part.
C: 0.01 to 0.12%, Si: 0.3% or less,
Mn: 1.2 to 2.4%, Ni: 4.0 to 8.5%, C
u: 0.1 to 2.0%, Cr + Mo + V: 3.0 to 5.
0%, Ti: 0.005 to 0.15%, Al: 0.02
% Or less, with the balance being Fe as the main component, and submerged arc welding is preferably performed using a baking type or melting type flux. The high-strength steel pipe welded in this way can then be expanded into an ultra-high-strength steel pipe.

[0058]

EXAMPLES Next, examples of the present invention will be described.

After smelting in a 300 ton converter, it is made into a continuously cast steel slab, and then it is heated at 1100 ° C. and then finish-rolled at a cumulative reduction of 80% at 800 to 900 ° C. to obtain a 16 mm tensile strength of 900 MPa or more. A steel plate was produced. Using this steel plate, it was formed into a tubular shape at the UO factory, after tack welding, using a welding wire and flux, 3 electrodes,
Submerged arc welding of each pass on the inner and outer surfaces was performed under welding conditions of 1.75 m / min and heat input of 2.2 kJ / mm. After that, the tube was expanded by 1%. Tables 1 and 2 (continued from Table 1) show the chemical composition of the base metal, the mechanical properties, and the toughness of the weld heat affected zone.

In the tensile test of the base material, test pieces were taken from the circumferential direction of the steel pipe. For the base metal toughness, a Charpy test piece was sampled from the circumferential direction of the steel pipe, and the notch was placed in the plate thickness direction. For the toughness of the heat-affected zone of the weld, a Charpy test piece was sampled from the outer surface of the weld, and a notch was made so that the weld metal of the heat-affected zone of the weld zone was 50%.

Embodiment No. which is an example of the invention. From 1 to 20, the strength / low temperature toughness balance of the base material and the toughness of the weld heat affected zone are good.

Comparative Example No. Nos. 21 to 29 have chemical components outside the scope of the present invention, and the target strength / low temperature toughness balance of the base metal and the toughness of the weld heat affected zone are not satisfied.

No. 21,23,25 are C, Mn, Cu
Since the amount of each is low, the strength is not satisfactory.
Since Nos. 22, 24 and 26 have a high content of C, Mn and Cu, the strength / low temperature toughness balance of the base metal and the toughness of the weld heat affected zone are not satisfied.

No. Regarding No. 26, cracking occurred after rolling. No. In No. 27, since the amount of Al is large, intragranular bainite is not formed, and the toughness of the weld heat affected zone is deteriorated. No. With respect to No. 28, the amount of Ti was small and intragrain bainite was not formed, and the toughness of the weld heat affected zone deteriorated. Two
Since No. 9 has a large amount of Ti, a large amount of Ti carbide is generated, and the toughness of the base material and the weld heat affected zone is deteriorated.

Next, the weld metal will be described.

Tables 3 and 4 (continued from Table 3), Table 5 (Table 3)
(Continued) shows the chemical composition of the wire, the weld metal composition, the weld metal structure, and the strength / toughness of the weld metal.

Implementation No. which is an example of the invention. With Nos. 1 to 14, a good bead was obtained, and the weld metal had a good balance of strength and low temperature toughness.

Comparative Example No. Chemical compositions of 15 to 29 are out of the range of the present invention, and do not satisfy the target strength and low temperature toughness of the weld metal.

No. Nos. 15 and 17 did not satisfy the strength because the amounts of C, Mn, and Cu were low, respectively. In No. 25, since the amount of Cu is low, intragranular bainite is hard to be generated. Nos. 16, 18, and 26 have high contents of C, Mn, and Cu, so that the strength is too high and intragrain bainite is not formed, and the low temperature toughness is deteriorated.

No. Regarding No. 26, Cu cracking is called. No. In Nos. 19 to 23, since the amount of All is large, intragranular bainite is difficult to be generated, and intragranular bainite is not generated,
No. For No. 24, hot cracking occurred after welding,
No. No. 27 has a small amount of Ti and does not generate intragranular bainite, which deteriorates the low temperature toughness. Since No. 28 has a large amount of Ti, a large amount of Ti carbide is generated, which deteriorates the low temperature toughness. No. In No. 29, since the amount of Al was excessively large and the size of the oxide was large, intragranular bainite was not formed and the low temperature toughness was deteriorated.

[0071]

[Table 1]

[0072]

[Table 2]

[0073]

[Table 3]

[0074]

[Table 4]

[0075]

[Table 5]

[0076]

INDUSTRIAL APPLICABILITY According to the present invention, the chemical composition is limited to a specific range, and the oxide containing Ti as the main component and the sulfide containing Mn and Cu as the main components are finely dispersed. To generate the intragranular bainite. Due to this effect, it is possible to improve the low temperature toughness of the weld heat affected zone and the weld metal. As a result, the cost and workability for high strength welded steel structures are significantly improved.

[Brief description of drawings]

FIG. 1 Strength and toughness (charpy absorbed energy at −20 ° C.) of weld metal of conventional high-strength steel pipe of the present invention
It is a figure which shows the relationship with.

FIG. 2 is a diagram showing a weld metal structure according to the present invention having a structure in which intragranular bainite is formed.

FIG. 3 is a diagram showing a conventional weld metal structure having a structure in which intragranular bainite is not formed.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/58 C22C 38/58 // B23K 101: 06 B23K 101: 06 F term (reference) 4E001 AA03 BB05 CC03 EA05 4E081 BA05 BA19 BB01 BB13 CA05 FA03

Claims (9)

[Claims] 1. An intragranular bainite fraction of 50% or more is formed in the weld metal part or the heat-affected zone of a weld with a core of the composite inclusions, and the composite inclusions are oxides and Mn precipitated around the oxides. Main sulfide and Cu-based sulfide 2
Ultra high strength steel pipe with excellent weld toughness, which is a composite sulfide consisting of phases. 2. An intragranular bainite fraction of 50% or more is formed in the weld metal part or the heat-affected zone by the inclusion of nuclei, and the compound inclusions precipitate in the oxide and Mn precipitated around the oxide. Main sulfide and Cu-based sulfide 2
An ultrahigh-strength steel pipe having excellent weld toughness, which is a composite sulfide consisting of phases and a composite inclusion in which nitride is precipitated on the composite sulfide. 3. The ultra-high-strength steel pipe excellent in weld zone toughness according to claim 1, wherein the oxide is a Ti-containing oxide. 4. The average sphere-equivalent diameter of the composite sulfide consisting of the oxide or sulfide or the composite inclusion consisting of the oxide, sulfide and nitride is 0.01 to 5 μm.
And has an average density of 1 × 10 ³ pieces / mm ² or more and is present in the weld metal. Ultra-high strength excellent in weld zone toughness according to any one of claims 1 to 3. Steel pipe. 5. The ultrahigh strength steel pipe excellent in weld toughness according to any one of claims 1 to 4, wherein the base material has a tensile strength of 900 MPa or more. 6. The bainite / martensite fraction of the weld metal is 50% or more.
An ultrahigh-strength steel pipe having excellent weld zone toughness according to any one of items 1 to 5. 7. The chemical composition of the weld metal, in mass%, is C:
0.04 to 0.14%, Si: 0.05 to 0.40%,
Mn: 1.2 to 2.2%, P: ≤ 0.01%, S: ≤
0.010%, Ni: 1.3 to 3.2%, Cu: 0.1
~ 1.0%, Cr + Mo + V: 1.0-2.5%, T
i: 0.003 to 0.050%, Al: ≤ 0.02
%, B: ≤ 0.005%, and the balance Fe and unavoidable impurities.
An ultrahigh-strength steel pipe having excellent weld toughness according to any one of 6 above. 8. The chemical composition of the base material is C: 0.
03-0.10%, Si: ≤ 0.6%, Mn: 1.7-
2.5%, P: ≤ 0.015%, S: ≤ 0.003%,
Ni: 0.1-2.0%, Cu: 0.1-1.0%, M
o: 0.15 to 2.5%, Nb: 0.01 to 0.10.
%, Ti: 0.005 to 0.030%, Al: ≤0.0
6%, and optionally, B: ≤ 0.005%,
N: 0.001 to 0.006%, V: ≤ 0.10%, C
r: ≦ 0.8%, Ca: ≦ 0.01%, REM: ≦ 0.
The weld toughness according to any one of claims 1 to 7, which contains one or two or more of 02% and Mg: 0.006%, and the balance Fe and unavoidable impurities. Excellent super high strength steel pipe. 9. C: 0.03 to 0.10% by mass%,
Si: ≦ 0.6%, Mn: 1.7 to 2.5%, P: ≦
0.015%, S: ≤ 0.003%, Ni: 0.1
2.0%, Cu: 0.1-1.0%, Mo: 0.15-
2.5%, Nb: 0.01 to 0.10%, Ti: 0.0
05: 0.030%, Al: ≤ 0.06%, and optionally, B: ≤ 0.005%, N: 0.001%
0.006%, V: ≤0.10%, Cr: ≤0.8%,
Ca: ≤ 0.01%, REM: ≤ 0.02%, Mg: ≤
0.006% of 1 type or 2 types or more, and the balance Fe
And a steel sheet consisting of unavoidable impurities into a tubular shape in the UO process, and from the inner and outer surfaces of the butt portion of the steel sheet, in mass%,
C: 0.01 to 0.12%, Si: ≤ 0.3%, Mn:
1.2-2.4%, Ni: 4.0-8.5%, Cu:
0.1-2.0%, Cr + Mo + V: 3.0-5.0
%, Ti: 0.005 to 0.150%, Al: ≤0.0
Manufacture of ultra-high strength steel pipe with excellent weld toughness, characterized by performing submerged arc welding using a welding wire containing 2% of balance Fe as the main component and a firing type or melting type flux, and then expanding the pipe. Method.