JP2001355039A - Ultrahigh strength steel tube excellent in low temperature toughness of weld zone and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ultrahigh strength steel tube excellent in the low temperature toughness of the weld metal in the weld zone and having tensile strength of >=900 MPa (>100 X in the API standard) and to provide its economical production method. SOLUTION: In this ultrahigh strength steel tube excellent in the low temperature toughness of the weld zone, in the weld metal of the weld zone Ti- containing oxides at least containing Ti and blended particles in which at least Mn sulfides are precipitated around the Ti-containing oxides as the neclei are contained, and intragranular bainite formed with those as the heuclei are present in >=50% by fractional ratio, and production method.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention provides excellent low-temperature toughness for use in line pipes for transporting natural gas and crude oil.
The present invention relates to an ultra-high-strength steel pipe having a tensile strength (TS) of 00 MPa or more.

[0002]

2. Description of the Related Art In recent years, line pipes have become increasingly important as a long-distance transportation method of 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, development of higher-strength line pipes has been eagerly desired in order to improve transportation efficiency by increasing the pressure and to improve on-site construction efficiency by reducing the outer diameter and weight of the line pipes. X80 so far
(A tensile strength of at least 620 MPa) has been put to practical use.
There is a growing need for high-strength line pipes exceeding 0 MPa). Currently, research on ultra-high-strength linepipe manufacturing methods is based on conventional X80 linepipe manufacturing techniques (eg, NKK Technical Report No. 138 (1992), pp24-31, and The 7th Offshore Mechanics and Arctic Engineeri).
ng (1988), Volume V, pp. 179-185), but at most X100 (tensile strength of 760)
The production of line pipes is considered to be the limit. At present, methods for producing ultra-high-strength steel sheets having a strength level exceeding X100 have already been studied and reported, and patent applications have been filed (for example, PCT / JP96 / 00155, 0000).
157). However, when manufacturing an ultra-high-strength line pipe using an ultra-high-strength steel plate having a strength level exceeding X100, the strength of the base metal and the balance between the low-temperature toughness and the weld metal and the weld heat-affected zone (HAZ) are reduced. It has many problems such as toughness, on-site weldability, and softening of joints, and it is necessary to overcome these problems for practical use.

In particular, when manufacturing an ultra-high-strength (X100 or more) line pipe, the weld metal at the welded portion is
Since it is used as solidified and the toughness of the weld metal generally decreases with an increase in strength, it has been a major issue to secure the required low-temperature toughness of the weld metal as a line pipe. Conventionally, as a method of improving the low-temperature toughness of a weld metal when welding an ultrahigh-strength thick plate, a method of adding a large amount of Ni into a weld metal by using a welding wire or a flux or the like (welding joint handbook) (1990.9.3
Maruzen) p888) is known. A method of performing low-heat-input multi-layer welding by restricting the heat input during welding to a low level (Nippon Steel Cat. No. EXE332 (1973) p1 to 69) is also conventionally known. However, these methods have problems in that they use expensive alloy components and decrease welding efficiency, so that all methods increase the production cost of line pipes.

[0004]

SUMMARY OF THE INVENTION In view of the above-mentioned problems in the prior art, the present invention has a tensile strength excellent in low-temperature toughness of a weld metal particularly at a welded portion of 900 MPa or more (API standard X10).
0) and a method of manufacturing the same in an economical manner.

[0005]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems, and its gist is as follows. (1) An ultra-high-strength steel pipe excellent in low-temperature toughness of a weld, wherein intragranular bainite is present in the weld metal of the weld. (2) The ultra-high-strength steel pipe excellent in low-temperature toughness of a weld according to (1), wherein the fraction of bainite in the grains in the weld metal in the weld is 50% or more. (3) Ti containing at least Ti in the weld metal
The low temperature of the weld according to any one of the above (1) or (2), characterized in that the low temperature of the weld portion according to any one of the above (1) and (2), further comprising a composite particle in which at least Mn sulfide is precipitated around the titanium-containing oxide and the Ti-containing oxide. Ultra high strength steel pipe with excellent toughness. (4) The Ti-containing oxide has a mean circle equivalent diameter of 0.01 to 5 μm and an average density of 1 × 10 ³ / mm ² or more in the weld metal. The ultrahigh-strength steel pipe excellent in low-temperature toughness of a weld according to any one of the assistants (1) to (3). (5) As a component of the weld metal, C: 0.
04-0.14%, Si: 0.05-0.4%, Mn:
1.2 to 2.2%, P: 0.01% or less, S: 0.01
% Or less, Ti: 0.003 to 0.05%, Al: 0.0
2% or less, Ni: 1.3 to 3.2%, Cr, Mo and V
One or more of them: 1 to 2.5%, B: 0.0
The ultra-high-strength steel pipe according to any one of the above (1) to (4), wherein the ultra-high-strength steel pipe contains 0.05% or less and the balance contains Fe as a main component. (6) The weld metal according to any one of the above (1) to (5), wherein the weld metal has a bainite-martensite fraction of 50% or more. Strength steel pipe. (7) The ultra-high strength steel pipe according to any one of (1) to (6) above, wherein the weld metal has a tensile strength of 900 MPa or more. (8) In mass%, C: 0.03 to 0.1%, Si: 0.
6% or less, Mn: 1.7 to 2.5%, P: 0.015%
Hereinafter, S: 0.003% or less, Ti: 0.005 to 0.
03%, Ni: 0.1-1%, Mo: 0.15-0.6
%, Nb: 0.01 to 0.1%, and more selectively, Al: 0.06% or less, B: 0.005% or less,
N: 0.001 to 0.006% or less; V: 0.1% or less; Cu: 1% or less; Cr: 0.8% or less;
01% or less, REM: 0.02% or less, Mg: 0.006
% Or less, and a steel sheet comprising the balance of iron and unavoidable impurities is formed into a tube by the UO process, and C: 0.01 from the inner and outer surfaces of the butted portion of the steel sheet.
~ 0.12%, Si: 0.3% or less, Mn: 1.2 ~
2.4%, Ti: 0.005 to 0.15%, Ni: 4 to
8.5%, one or more of Cr, Mo and V: 3 to 5%, with the balance being the use of a welding wire mainly composed of Fe and a sintering or melting flux. A method for producing an ultra-high-strength steel pipe excellent in low-temperature toughness of a welded part, characterized by performing submerged arc welding and then expanding the pipe.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the contents of the present invention will be described in detail. In the present invention, the base material and the welded portion are 900
The present invention relates to an ultra-high-strength steel pipe having a tensile strength (TS) of not less than MPa and excellent in low-temperature toughness, and a method for producing the same. Generally, the tensile strength (TS) is 900 MPa or more (A
Ultra-high strength steel pipes for line pipes (PI standard X100 or more) can withstand about twice the pressure compared to the conventional mainstream X65, so it is possible to transport about twice the gas with the same size. become. Also, since it is not necessary to increase the wall thickness in order to withstand the required pressure increase as in the case of X65, the laying of material costs, transportation costs, on-site welding construction costs, etc., when laying pipelines is compared to X65. Costs can be reduced. In recent years, this has been used for long-distance transportation line pipes for crude oil and natural gas, and has a tensile strength (T
This is the reason why ultra-high-strength steel pipes having S) are increasingly important.

On the other hand, the higher the strength, the more rapidly it becomes difficult to manufacture a steel pipe. In particular, a tensile strength of 900 MPa or more (T
When manufacturing a steel pipe for an ultra-high-strength line pipe having S), it is difficult to ensure low-temperature toughness of a weld metal portion of a welded portion. As one evaluation criterion to secure target properties such as strength and toughness including seam welds of steel pipes,
It is considered essential to be able to break from the pipe body without breaking from the weld metal in a circumferential tensile test (burst test) including a seam weld portion.

In general, the low-temperature toughness of a weld metal in a steel pipe weld is used as it is after being welded, and the low-temperature toughness of the weld metal decreases with an increase in strength. Therefore, the strength of the weld metal is regulated in order to ensure necessary low-temperature toughness. . According to the experimental results of the present inventors,
The relationship between the strength of the weld metal and the low-temperature toughness is as shown in FIG. 1. In the conventional weld metal (の) of a high-strength steel pipe,
For example, the Charpy absorbed energy of 84 J at -20 ° C.
In order to obtain low-temperature toughness satisfying the following conditions, it was necessary to regulate the strength of the weld metal to at least less than 1025 MPa.

Therefore, the present inventors suppressed the decrease in the toughness of the weld metal due to the increase in the strength of the weld metal as described above, and obtained a 900MPa high strength and high toughness weld metal.
The inventors conducted intensive studies on a method for producing a steel pipe for an ultra-high-strength line pipe of a or more (API standard X100 or more). As a result, if specific inclusions are present in the crystal grains of the weld metal in the welded portion, the grains will be used as nuclei to generate a later-described intragranular bainite structure (hereinafter referred to as intragranular bainite), and the intragranular structure will be subdivided. As a result, it has been found that as a result, the Charpy fracture surface unit becomes extremely small, and the toughness of the weld metal is improved. In addition, as an inclusion that can be a nucleus for generating intragranular bainite, a Ti-containing oxide containing at least Ti (Al, Si, Mn, Cr, Ti, in addition to Ti)
Mg or Ca, etc.) and an Mn-containing sulfide containing at least Mn around the Ti-containing oxide as a nucleus (Ca, Cu, Mg, etc. may be contained in addition to Mn) It has been found that the composite particles in which are precipitated are effective, and when these inclusions are present in the crystal grains in the component-type weld metal having high hardenability, intragranular bainite is formed using these as nuclei.

[0010] The intragranular bainite mentioned here is a low Al-high Ti high quenching component system shown in Fig. 2 and is formed radially with a Ti-containing oxide or composite particles of this Ti-containing oxide and Mn-containing sulfide as nuclei. The bainite is formed, and is distinguished from a normal bainite structure formed by growing a bainite lath from a grain boundary in a conventional relatively high Al-low Ti high quenching component system shown in FIG.

According to the experimental results of the present inventors, as shown in FIG. 1, the weld metal (●) in which intragranular bainite is remarkably present is different from the weld metal (の) of the conventional high-strength steel pipe. In comparison, a decrease in the toughness of the weld metal with an increase in the strength of the intragranular weld metal is suppressed, and the Charpy absorbed energy at −20 ° C. at -20 ° C. is excellent in low-temperature toughness of 150 J or more even at a tensile strength exceeding 900 MPa. It has been found that a weld metal can be obtained.

The reason why intragranular bainite is formed from Ti-containing oxide or composite particles of this oxide and Mn-containing sulfide in the crystal grains of the weld metal is considered as follows. Since Ti oxide is a cation vacancy type oxide, it has a property of taking in a lot of Mn ions.
When the Ti-containing oxide is present in the grains, a Mn-deficient layer is formed around the Ti-containing oxide, or a Mn-containing sulfide is precipitated around the layer to form composite particles of the Ti-containing oxide and the Mn-containing sulfide. . In this case, the presence of the Ti-containing oxide and the Mn-containing sulfide as composite particles as compared with the case where the Ti-containing oxide is present alone in the grains has a smaller Mn around it.
The formation of a depletion layer is further promoted. This Mn deficient layer present in the grains of the weld metal promotes intragranular ferrite transformation when transforming from a high-temperature austenite phase to ferrite, and a component having high hardenability, such as the ultra-high strength steel pipe of the present invention. Under conditions where the cooling rate is relatively high in the system, bainite transformation occurs immediately after ferrite transformation, and intragranular bainite is formed. The formation of intragranular bainite by such a mechanism results in the fragmentation of the intragranular structure,
It is considered that the toughness of the weld metal is improved because the Charpy fracture surface unit is extremely small.

In the present invention, it is necessary to form intragranular bainite in the weld metal in order to improve the low-temperature toughness of the weld metal in the weld. In order to improve the low-temperature toughness of the weld metal, it is preferable that the intragranular bainite fraction (the area ratio (%) of the intragranular bainite in the weld metal structure) is large, and in the present invention, the effect is reduced. In order to obtain a sufficient amount, the content is set to 50% or more. In order to set the intragranular bainite fraction to 50% or more, when the cooling rate is relatively high under the condition where the Ti-containing oxide is present, for example, 30 ° C./s to 5 ° C.
C./s can be achieved.

Further, in the present invention, in order to generate intragranular bainite in the weld metal of the welded portion, a Ti-containing oxide containing at least Ti in the molten weld metal and a peripheral portion of the Ti-containing oxide using the Ti-containing oxide as a nucleus. Must contain at least composite particles in which Mn sulfide is precipitated. More preferably, the size of the Ti-containing oxide present in the weld metal of the weld is in the range of 0.01 to 5 μm in terms of an average circle equivalent diameter, and the average density is defined as 1 × 10 ³ pieces / mm ² or more. This is preferable from the viewpoint of promoting formation of bainite in the grains and improving the low-temperature toughness of the weld metal. In order to set the Ti-containing oxide in the above-described range, the heat input in the submerged arc welding can be obtained under the condition of 2.0 to 5.0 KJ / mm.

Further, the tensile strength of the weld metal is 900MP.
In order to achieve a value of at least a, the bainite-martensite fraction of the weld metal is set to 5%.
It is preferred to be 0% or more. Next, the reasons for limiting the components of the weld metal in the welded portion of the steel pipe of the present invention will be described. In addition,% shown below shall show a mass% unless there is particular description.

The amount of C is limited to 0.04 to 0.14%.
C is extremely effective in improving the strength of steel. To obtain a target strength in a martensitic structure, at least 0.1C is required.
04% is needed. However, when the C content is too large, low-temperature cracking of the weld is apt 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. Therefore, the upper limit is set to 0.14%. More preferably, the upper limit is set to 0.1%.

Si is 0.05 to prevent blowholes.
% Or more is necessary, but if the content is large, the low-temperature toughness is remarkably deteriorated, so the upper limit was made 0.4%. 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 an excellent balance between strength and low-temperature toughness, and also forms inclusions of Mn-containing sulfides and forms intragranular bainite. In particular,
When the cation vacancy type Ti-containing oxide is present in the grains,
Mn-containing sulfide precipitates around the Ti-containing oxide,
It promotes the generation of Mn-containing sulfide in the grains and promotes the formation of intragranular bainite. In order to obtain these effects, the lower limit of the added amount is set to 1.2%. However, if the Mn content is too large, segregation is promoted and not only deteriorates low-temperature toughness but also makes it difficult to produce a welding material, so the upper limit is 2.2%.
And

Ni is necessary for enhancing the hardenability and securing the strength, and for improving the low-temperature toughness. If it is less than 1.3%, it is difficult to obtain the target strength and low-temperature toughness, so the lower limit is made 1.3%. On the other hand, if the content is too large, there is a risk of hot cracking, so the upper limit is 3.2.
%. Cr, Mo, and V are elements necessary for enhancing hardenability and obtaining high strength, and one or more of these elements are added in the range of 1 to 2.5%. I do. If the content is less than 1%, the effect is not sufficient, so the lower limit is set to 1%. If the content is excessively large, the risk of low-temperature cracking increases, so the upper limit is set to 2.5%.

B is a very small element that enhances the hardenability and is effective for the low-temperature toughness of the weld metal. However, if the content is too large, the low-temperature toughness is rather deteriorated.
5% or less. Ti is T which forms intragranular bainite
It is an essential component for generating inclusions of i-containing oxides and composite particles of this oxide and Mn-containing sulfide, and forms intragranular bainite with these inclusions as nuclei in the grains to form a weld metal. Improves low temperature toughness. In order to sufficiently obtain these effects, the lower limit of the content is set to 0.003%.
On the other hand, if the Ti content is too large, a large amount of Ti carbide is generated and the low-temperature toughness is degraded. Therefore, the upper limit was made 0.05%.

Al is known as a deoxidizing component.
_Although an oxide such as ₂ O ₃ is generated, the oxide is an anionic vacancy type oxide and has poor bonding with Mn-containing sulfides such as MnS. It is desirable to make the temperature as low as possible so as not to inhibit the production of Mn-containing sulfide. Therefore, in the present invention, the upper limit of the content is defined as 0.02%.

In the present invention, the contents of P and S are desirably low in order to lower the low-temperature toughness of the weld metal and reduce the susceptibility to low-temperature cracking. The amount of oxygen contained in the weld metal is preferably at least 20 ppm. Further, in the present invention, in addition to the above components, Zr, Nb, Mg, etc. may be added to the weld metal as necessary in order to perform refining and solidification during welding well.

Next, a method for manufacturing a steel pipe according to the present invention will be described.
This will be described below. In the steel pipe of the present invention, the 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, and the butted portions are tack-welded by arc welding, then submerged arc welding is performed from the inner and outer surfaces, and then the pipe is expanded to increase the roundness, thereby efficiently manufacturing the pipe. be able to.

Submerged arc welding at the time of forming a steel sheet is a welding in which the base metal has a high dilution ratio. 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. is necessary. Hereinafter, the reasons for limiting the chemical composition of the welding wire used in the production of the steel pipe of the present invention will be described. In addition,% shown below shall show a mass% unless there is particular description.

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

In order to obtain a range of the amount of Ni required for the weld metal, Ni is set to 4 to 4 in consideration of dilution with a base metal component.
It was set to 8.5%. Cr, Mo, and V are contained in an amount of 3 to 5% in consideration of dilution with a base metal component in order to obtain a range of one or more of these contents required for the weld metal. And

Ti is set to 0.005 to 0.15% in consideration of dilution with a base metal component in order to obtain a range of Ti content required for the weld metal. Also, P, S, Al
Is an unavoidable component, and in the present invention, it is desirable to minimize the low-temperature toughness of the weld metal,
P and S are each 0.01% or less, and Al is 0.0% or less.
Restrict to 2% or less.

In the present invention, B in the welding wire
Although not particularly specified, a small amount may be added as a hardenable component as required for strength adjustment. Further, Zr, Nb, Mg, or the like may be added to the welding wire as a deoxidizing material. In the present invention, welding can be performed not only by using a single electrode but also by using a plurality of electrodes. In the case of welding with a plurality of electrodes, combinations of various wires are possible, and it is not necessary for each wire to be within the above-described component range.

The flux used in the submerged arc welding at the time of manufacturing the steel pipe according to the present invention is roughly classified into a sintering flux and a molten flux. The calcined flux has an advantage that the alloy can be added and the amount of diffusible hydrogen is low, but there is a disadvantage that it is easily powdered and it is difficult to use repeatedly. On the other hand, the molten flux has the advantages of being glassy, having a high particle strength, being less likely to absorb moisture, and having a high diffusible hydrogen content. In the present invention, both can be used essentially in industry.

Next, the reasons for limiting the components of the steel sheet used for producing the steel pipe of the present invention will be described. In addition,% shown below shall show a mass% unless there is particular description. The amount of C is limited to 0.03 to 0.1%. C 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.
%. More preferably, the upper limit is 0.07%.

Si is an element to be added for deoxidation and to improve the strength. However, if added in a large amount, the HAZ toughness and on-site weldability are remarkably deteriorated, so the upper limit was made 0.6%. Deoxidation of steel can be sufficiently performed with Al and Ti described later, and it is preferable to add the steel in accordance with the addition amount thereof for strength adjustment.
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, adding too much Mn increases the hardenability of steel.
In addition to deteriorating HAZ toughness and on-site weldability, it promotes center segregation of continuously cast steel slabs and also degrades the low-temperature toughness of the base metal.

Ni is added to improve the strength of the low carbon steel sheet without deteriorating the low temperature toughness. Ni is
Compared to quenched components such as Mn, Cr, and Mo, not only is it hard to form a hardened structure harmful to low-temperature toughness in the rolled structure (especially the central segregation zone of continuous cast steel slabs),
It has been found that the HAZ toughness can be improved with a very small addition amount of 0.1% or more (the Ni addition amount that is particularly effective in terms of 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 deteriorated, so the upper limit is set to 1%. Further, the addition of Ni is also effective in preventing Cu cracks generated during continuous casting and hot rolling of Cu-containing steel. In this case, Ni needs to be added at least 1/3 of the Cu content.

Mo is used in an amount of 0.1 to improve the hardenability of the steel sheet and obtain the desired structure mainly composed of martensite.
Add 15% or more. Particularly in the case of B-added steel, the quenching effect of Mo is enhanced, and coexistence of Mo and Nb suppresses austenite recrystallization during controlled rolling, and is also effective in refining the structure of austenite. However, excessive addition deteriorates HAZ toughness and on-site weldability, and further reduces the effect of improving the hardenability of B.
The upper limit is set to 0.6%.

By coexisting with Mo as described above, Nb not only suppresses austenite recrystallization during controlled rolling to refine the structure, but also contributes to precipitation hardening and increase in hardenability, and makes the steel sheet tough. . In particular, when Nb and B coexist, the effect of improving hardenability increases synergistically. In the present invention, 0.01% or more is added to obtain these effects. However, if the added amount of Nb is too large, it adversely affects the HAZ toughness and the on-site weldability, so the upper limit was made 0.1%.

Ti forms fine TiN in the steel, 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. Also, it has a role of fixing solid solution N 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 3.4N (each wt%) or more. When the amount of Al is small (for example, 0.005% or less), Ti forms an oxide and HA
H acts as an intragranular ferrite nucleus in Z
It also has the effect of making the structure finer. In order to exert such an effect of TiN, 0.005% or more of Ti must be added. However, if the Ti content is too large,
Since the coarsening of N and the precipitation hardening due to TiC occur to deteriorate the low-temperature toughness, the upper limit thereof is limited to 0.03%.

P and S are unavoidable impurity elements. In the present invention, in order to further improve the low-temperature toughness of the base material and the HAZ, the contents of P and S are each 0.015.
%, 0.003% or less. 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 above are the basic components of the steel sheet used in the present invention, and it is preferable that the following components be selectively added in the following ranges. 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, since the above-mentioned deoxidation is possible with Ti or Si, the content is adjusted by using these without necessarily being used.

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 added in excess, not only deteriorates the low-temperature toughness, but may also lose the effect of improving the hardenability of B, so the upper limit is 0.005.
%.

N forms TiN and suppresses the coarsening of austenite grains of the HAZ during reheating of the slab and in the HAZ.
Improves the low temperature toughness of HAZ. The minimum required for this is 0.001%. However, if the amount of N is too large, it causes deterioration of the HAZ toughness due to slab surface flaws or solid solution N, and a decrease in the effect of improving the hardenability of B.
006%.

V, Cu, Cr, Ca, REM and Mg are:
In order to further improve the strength and toughness and increase the size of a steel material that can be manufactured without impairing the excellent characteristics of the steel of the present invention, an appropriate amount can be added as follows. V has almost the same function and effect as Nb, but the degree of the effect is Nb.
Weaker than. 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. In the present invention, the upper limit of the addition amount is set at 0. 0 in view of the HAZ toughness of the steel sheet and the on-site weldability.
Specify 1%. Further, from the viewpoint of the above effects, 0.03-
A 0.08% addition is a more desirable range.

[0040] Cu increases the strength of the base material and the welded portion, but if it is too large, the HAZ toughness and on-site weldability are remarkably deteriorated. Therefore, Cu is added with the upper limit of the content being 1%. Cr increases the strength of the base material and weld,
If the amount is too large, the HAZ toughness and on-site weldability are significantly deteriorated. Therefore, the amount of Cr is added with the upper limit of the content being 0.8%.

Ca and REM control the morphology of sulfide (MnS) and improve low-temperature toughness (increase in energy absorbed in Charpy test, etc.). When the amount of Ca added is 0.
If the content exceeds 006% or the amount of REM exceeds 0.02%, CaO-CaS or REM-CaS is generated in a large amount in the steel, resulting in large clusters and large inclusions, which only impair the cleanliness of the steel. In addition, it has an adverse effect on the on-site weldability. Therefore, the upper limit of the amount of Ca added is 0.006.
%, And the upper limit of the amount of REM is set to 0.02%.
In the ultra-high-strength line pipe, the amounts of S and O are reduced to 0.001% and 0.002%, respectively, and the ESS is reduced.
P = (Ca) [1-124 (O)] / 1.25S to 0.
More preferably, 5 ≦ ESSP ≦ 10.0.

Mg forms a finely dispersed oxide, suppresses the coarsening of the weld heat affected zone, and improves the low temperature toughness. If it is added in excess of 0.006%, a coarse oxide is formed and, conversely, the toughness is deteriorated. In addition to the limitation of the individual additive elements described above, in order to achieve a balance between strength and low-temperature toughness, P = 2.7C + 0.4Si + Mn + 0.8Cr +
It is desirable to limit the P value of the hardenability index determined by 0.45 (Ni + Cu) + (1 + β) Mo−1 + β to a range of 1.9 ≦ P ≦ 4.0. However, when B ≧ 3 ppm, β
= 1, β = 0 for B <3 ppm. 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-mentioned P value represents an index of hardenability,
The higher the value of P, the greater the strength, which means that the structure tends to be a bainite-martensite structure.

[0043]

Next, an embodiment of the present invention will be described. 30
In a 0 ton converter, steel for ultra high strength steel pipe having the chemical composition shown in Table 1 was melted and made into a continuously cast steel slab. Then, after reheating to 1100 ° C, the accumulated rolling reduction at a temperature of 800 to 900 ° C was 80%. %, And then water-cooled from 800 ° C. to 200 ° C., and having a tensile strength of 900 MPa or more.
m was prepared. This steel sheet was formed into a tube in the UO process, and after tack welding, three electrodes, 1.7 using welding wires and fluxes of various components shown in Table 2.
Under the welding conditions of 5 m / min and heat input of 2.2 kJ / mm, submerged arc welding was performed on each of the inner and outer surfaces for one pass, and then 1% pipe expansion was performed. Table 3 shows the chemical composition, structure and properties of the weld metal in the welded portion of the obtained steel pipe.

In Table 3, Comparative Example No. 15-29
The chemical composition and structure of the weld metal were out of the range of the present invention, and the target strength or low-temperature toughness of the weld metal of the present invention could not be satisfied as described below. Execution No. 15, 17, 25 are C, Mn, Cr +
Since the total amount of Mn + V was low, the strength of the weld metal was low.

Implementation No. 16, 18, and 26 are C, M
Since the total amount of n, Cr + Mn + V was high, the strength was extremely high, but the low-temperature toughness of the weld metal was deteriorated without intragranular bainite. Execution No. 19-23
However, because of the high Al content, the generation of composite particles with the Ti-containing oxide and the Mn-containing oxide precipitated using the same as the nucleus was small, sufficient intragranular bainite was not generated, and the low-temperature toughness was deteriorated.

Implementation No. In No. 24, although the Ni content was too high, although the strength was high, hot cracking occurred after welding.
Execution No. In No. 27, since the Ti content was small, Ti-containing oxides and composite particles of this oxide and Mn-containing oxides were not sufficiently generated, and intragranular bainite was not sufficiently generated to deteriorate the low-temperature toughness.

Implementation No. In No. 28, since the Ti content was too large, a large amount of Ti carbide was generated, and the low-temperature toughness was deteriorated. Execution No. No. 29 was A due to too much Al content.
The amount of 1-oxide increased, and the size of the Ti-containing oxide was increased due to the compounding with the Ti-containing oxide. As a result, a sufficient number of generated Ti-containing oxides could not be obtained, and the generation of intragranular bainite was reduced, and the low-temperature toughness was deteriorated.

On the other hand, in the embodiment No. 1-14
As a result, the weld metal of an ultra-high-strength steel pipe having excellent strength and low-temperature toughness characteristics was obtained because both the composition and the structure of the weld metal satisfied the range of the present invention. From the above, according to the present invention, it is possible to obtain a weld metal of an ultra-high-strength steel pipe having a Charpy absorption energy at least at −20 ° C. of 150 J or more, a tensile strength of 900 MPa or more, and excellent low-temperature toughness characteristics. I understand.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

[Table 3]

[0052]

[Table 4]

[0053]

[Table 5]

[0054]

According to the present invention, it is possible to use expensive alloy components such as Ni in the weld metal as in the prior art, or to use a low-efficiency welding method such as low heat input multi-pass welding. By producing intragranular bainite using Ti-containing oxides or composite particles of Ti-containing oxides and Mn-containing sulfides as nuclei in a weld metal, excellent low-temperature toughness (-) can be obtained even in ultra-high-strength steel pipes exceeding 900 MPa. It is possible to manufacture a steel pipe for an ultra-high-strength line pipe excellent in low-temperature toughness of a welded portion capable of obtaining a Charpy absorbed energy at 20 ° C. of 150 J or more.

By applying the present invention to a line pipe, the laying cost of a long-distance pipeline is greatly reduced,
It can contribute to solving global energy problems.

[Brief description of the drawings]

FIG. 1 Strength and toughness of weld metal of ultra-high strength steel pipe of the prior art and the present invention (Charpy absorbed energy at −20 ° C.)
FIG.

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

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

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) B23K 9/23 B23K 9/23 C 35/30 320 35/30 320C 320F C22C 38/58 C22C 38/58 / / B23K 101: 10 B23K 101: 10 103: 04 103: 04 F term (reference) 4E001 AA03 BB05 CA05 CC03 EA05 EA07 4E028 CB04 CB06 4E081 YX15 YX16 YX20

Claims (8)

[Claims] 1. An ultra-high-strength steel pipe excellent in low-temperature toughness of a weld portion, wherein intragranular bainite is present in the weld metal of the weld portion. 2. The ultra-high-strength steel pipe according to claim 1, wherein the intragranular fraction of bainite in the weld metal of the weld is 50% or more. 3. The weld metal contains a Ti-containing oxide containing at least Ti and composite particles having at least Mn sulfide precipitated around the Ti-containing oxide as a nucleus. 3. An ultra-high-strength steel pipe excellent in low-temperature toughness of a weld according to any one of 1 and 2. 4. The Ti-containing oxide has an average equivalent circle diameter of 0.01 to 5 μm and an average density of 1 ×.
The ultrahigh-strength steel pipe excellent in low-temperature toughness of a welded part according to any one of claims 1 to 3, wherein the pipe is present in the weld metal at 10 ³ / mm ² or more. 5. As a component of the weld metal, in mass%,
C: 0.04 to 0.14%, Si: 0.05 to 0.4
%, Mn: 1.2 to 2.2%, P: 0.01% or less,
S: 0.01% or less, Ti: 0.003 to 0.05%,
Al: 0.02% or less, Ni: 1.3 to 3.2%, C
One or more of r, Mo and V: 1 to 2.5
%, B: 0.005% or less, the balance being Fe as a main component, the ultra-high strength excellent in low-temperature toughness of the welded part according to any one of claims 1 to 4, Steel pipe. 6. The weld metal according to claim 1, wherein a bainite-martensite fraction is 50% or more.
6. An ultra-high-strength steel pipe excellent in low-temperature toughness of a welded portion according to any one of items 1 to 5. 7. The tensile strength of the weld metal is 900MP.
The ultrahigh-strength steel pipe excellent in low-temperature toughness of a welded part according to any one of claims 1 to 6, wherein the steel pipe has at least a. 8. In mass%, C: 0.03-0.1%, S
i: 0.6% or less, Mn: 1.7 to 2.5%, P: 0.
015% or less, S: 0.003% or less, Ti: 0.00
5 to 0.03%, Ni: 0.1 to 1%, Mo: 0.15
-0.6%, Nb: 0.01-0.1%, and more selectively, Al: 0.06% or less, B: 0.005%
Below, N: 0.001 to 0.006% or less, V: 0.1
%, Cu: 1% or less, Cr: 0.8% or less, Ca:
0.01% or less, REM: 0.02% or less, Mg: 0.0
A steel sheet containing one or two or more of the following components of 0.6% or less, and the balance consisting of iron and unavoidable impurities is formed into a tube by the UO process, and C: 0.
01-0.12%, Si: 0.3% or less, Mn: 1.2
To 2.4%, Ti: 0.005 to 0.15%, Ni: 4
About 8.5%, one or more of Cr, Mo and V: 3 to 5%, with the balance being the use of a welding wire mainly composed of Fe and a sintering or melting flux. A method for producing an ultra-high strength steel pipe having excellent low-temperature toughness at a welded portion, wherein submerged arc welding is performed and then pipe expansion is performed.