JP2001207242A - Thick-walled sour-resisting steel tube excellent in toughness at low temperature in circumferential weld zone, and pipeline - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel tube having sour resistance and excellent electron beam weldability. SOLUTION: A composite of MnS and oxides composed essentially of Ti, Al, and Ca is incorporated into the circumferential weld zone of a low C-low Mn-extra low S-Nb-Ti-low Al-Ca steel tube. The resultant high strength steel tube excellent in HIC resistance as well as in toughness in circumferential weld zone can greatly improve the safety of pipeline.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a low-temperature toughness of a circumferentially welded portion welded by an electron beam or a laser, a resistance to hydrogen induced cracking (HIC) and a resistance to sulfide stress corrosion cracking (SSC). The present invention relates to a thick-walled sour-resistant steel pipe and pipeline excellent in quality.

[0002]

2. Description of the Related Art Large diameter line pipes for transporting crude oil and natural gas in cold regions and offshore are required to have high strength, excellent low-temperature toughness and on-site weldability. further,
With sourcing of crude oil and gas wells by seawater injection and development of inferior resources, sourcing of pipelines is progressing, and excellent resistance to HIC and SSC (sour resistance)
Is required.

Conventionally, line pipes having excellent sour resistance have been produced by purifying steel, reducing inclusions, controlling morphology by adding Ca to sulfide-based inclusions, and central segregation due to light pressure during continuous casting (CC). It has been manufactured by making full use of techniques such as microstructure control by reduction and accelerated cooling after rolling (for example, Japanese Patent Publication No. 63-001369,
2-112722, JP-A-61-124555, JP-A-3-236420). However, due to an increase in welding heat input due to an increase in thickness and an increase in toughness required from the viewpoint of safety, the low temperature toughness of HAZ of these steels has not always been sufficient. JP-A-3-
JP-A-236420 discloses a method for producing a steel sheet for the purpose of improving sour resistance and HAZ toughness.

On the other hand, when a thick wall sour resistant line pipe is welded on site, application studies of electron beam welding and laser welding are being promoted for the purpose of improving welding efficiency. In electron beam welding or laser welding, the base metal component has a large effect on toughness because the steel pipe base material is melted and joined. As a method for improving the toughness of an electron beam or a laser weld, Japanese Patent Laid-Open No. 22418/1990 is disclosed. However, it is not possible to obtain a steel pipe having sour resistance and excellent toughness of an electron beam or a laser weld. Can not. For this reason, there has been a strong demand for the development of a steel pipe having good sour resistance and excellent toughness in an electron beam or laser weld.

[0005]

SUMMARY OF THE INVENTION The present invention provides a thick-walled sour-resistant steel pipe having excellent low-temperature toughness and excellent sour resistance in a circumferentially welded portion welded by an electron beam or a laser. is there.

[0006]

The gist of the present invention is that the mass%
And C: 0.02 to 0.08%, Si: 0.5% or less,
Mn: 0.8-1.5%, P: 0.010% or less, S:
0.001% or less, Nb: 0.01 to 0.05%, T
i: 0.005 to 0.03%, Al: 0.005% or less, Ca: 0.001 to 0.004%, N: 0.001
0.005%, O: 0.003% or less, and 2.0 ≦ [Ca] (1-124 [O]) / 1.25
In the circumferentially welded metal portion of a steel pipe obtained by welding a steel pipe having a base material made of steel consisting of iron and unavoidable impurities in the circumferential direction by electron beam welding or laser welding, the balance satisfying [S] ≦ 7.0, S: 0.002-0.01%,
And 0.1~5μm of Ti, Al, oxides composed mainly of Ca and the complex of the MnS 5 to 1000 pieces / mm ² containing to be.

[0007] In mass%, C: 0.02-0.08%, S
i: 0.5% or less, Mn: 0.8 to 1.5%, P: 0.
010% or less, S: 0.001% or less, Nb: 0.01
-0.05%, Ti: 0.005-0.03%, Al:
0.005% or less, Ca: 0.001 to 0.004%,
N: 0.001 to 0.005%, O: 0.003% or less Ni: 0.1 to 1.0%, Cu: 0.
1 to 1.0%, Cr: 0.1 to 1.0%, Mo: 0.1
~ 1.0%, V: 0.01 ~ 0.10%, REM: 0.
0005-0.005% of one or more kinds, and 2.0 ≦ [Ca] (1-124 [O]) /
Circumferential weld metal of a steel pipe obtained by welding a steel pipe whose base material, which satisfies 1.25 [S] ≤ 7.0, is iron and unavoidable impurities as a base material by electron beam welding or laser welding in the circumferential direction. In the part, S: 0.002-0.
0.1% to 5 μm of a complex of an oxide mainly composed of Ti, Al, and Ca and MnS is 5 to 1000 pieces / m 2.
m ² .

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. The feature of the present invention is that low C-
Based on a low Mn-ultra low S-Nb-Ti-low Al-Ca system, and furthermore, 0.002 to 0.01
%, And a composite of a fine oxide mainly composed of Ti, Al, and Ca and MnS is dispersed in the welded portion, whereby a sour resistance and a markedly superior circumferential welded portion are obtained. The point is that low-temperature toughness can be achieved at the same time.

By reducing the amounts of C, Mn, and P, C
The center segregation of the C slab can be improved, and the generation and propagation of HIC can be prevented. In addition, by strictly limiting the amounts of S, Ca and O, it is possible to prevent the generation of MnS that can be a starting point of HIC, and obtain good sour resistance.
However, when electron beam welding or laser welding is applied in the circumferential direction welding of a steel pipe to which Ca is added at an extremely low S, an excellent toughness cannot be obtained due to a coarse upper bainite (Bu) structure.

Therefore, in order to refine the microstructure of the circumferentially welded portion, the welded portion contains 0.002 to 0.01% of S and 0.1 to 0.1% mainly containing Ti, Al and Ca. The composite of 5 μm oxide and MnS is 5-10
By adding it to the 00 / mm ² weld, an intragranular transformed ferrite having a core of a complex of an oxide mainly composed of Ti, Al, and Ca and MnS is generated in an electron beam or a laser weld, and The inventors have found that the structure has been refined and good toughness has been obtained, which has led to the present invention.

First, the toughness of an electron beam or a laser weld will be described. The low-temperature toughness of an electron beam or the like depends on the size of crystal grains, high-carbon island-like martensite (M *), and Bu.
And other metallurgical factors such as the dispersed state of the hardened phase, the presence or absence of grain boundary embrittlement, and the microsegregation of elements. In particular, it is known that the size of the crystal grains at the welded portion has a significant effect on the low-temperature toughness.

In the present invention, the crystal grain size of the circumferentially welded portion is refined, thereby significantly improving low-temperature toughness. That is, the use of a composite of an oxide and MnS as an intragranular transformation nucleus was studied to refine the structure of the weld. As a result, the weld contains 0.002 to 0.01% of S, and 0.1 to 5 mainly containing Ti, Al, and Ca.
5 to 1000 composites of a μm oxide and MnS / m
By containing the m ² weld microstructure is fine, it was found that the low temperature toughness is remarkably improved. At this time, when the S content contained in the weld is less than 0.002%, the amount of MnS effective for generating intragranular transformed ferrite is small, and the structure of the weld is not refined. If the S content exceeds 0.01%, a large amount of MnS is generated, and the toughness is deteriorated.

In addition, 0.1 is mainly composed of Ti, Al and Ca.
A composite of 1-5 μm oxide and MnS is 5-1000
Pieces / mm ² must be contained in the weld. 0.1-5
A composite of an oxide having a size of μm and MnS effectively acts as a nucleus for forming intragranular transformed ferrite. If the composite of these oxides and MnS is less than 5 / mm ² , the effect of refining the structure of the weld is small, and if it is 1000 / mm ² or more, the cleanliness is deteriorated and the low-temperature toughness is deteriorated. Also,
The smaller the amount of Al in the circumferential weld, the more the formation of intragranular transformed ferrite is promoted. In this case, it is necessary to reduce the Al content of the steel pipe base material in advance. Base material Al
If the amount is 0.005% or less, generation of intragranular transformed ferrite in the circumferentially welded portion is not suppressed.

0.002 to 0.01 for the electron beam weld
% Of S, and a composite of MnS and 0.1 to 5 μm oxide mainly composed of Ti, Al, Ca
In order to make the content contained in the 1000 welds / mm ² welded portion, for example, there is a method of adding a foil containing Ti, Al, Ca-based oxide or S at the time of electron beam welding, but the addition method is not limited.

Next, the sour resistance of the steel pipe will be described.
In the present invention, the amount of S, which is an impurity element, is set to 0.001% or less, and Ca is added to make 2.0 ≦ [Ca] (1-1
24 [O]) / 1.25 [S] ≦ 7.0. MnS
The system inclusion expands by rolling and becomes a starting point of HIC generation. In order to prevent this, it is necessary to reduce the absolute amount of inclusions, control the form of sulfides, and use CaS or CaO, which is difficult to elongate by rolling. Therefore, the S content is set to 0.001% or less, and Ca is set to 0.001 to 0.001%.
[Ca] (1-124 [O]) / 1.2 in order to sufficiently control the sulfide morphology by Ca.
The ESSP value represented by 5 [S] was set to 2.0 or more. However, if the ESSP value is too large, Ca-based inclusions increase and become the starting point of HIC. Therefore, the upper limit was set to 7.0. In relation to the above, the amount of O was limited to 0.003% or less. This is because the sulfide morphology is controlled with Ca by reducing oxide-based inclusions that are the starting points of HIC.

Incidentally, S is set to 0.00 in the circumferentially welded portion.
MnS is generated by adding 2 to 0.01%, but since the MnS of the weld is not elongated, there is no problem in the sour resistance of the weld. In order to obtain excellent sour resistance, it is necessary to further limit the amounts of C, Mn, and P. The reason is CC
This is to improve center segregation of the slab and prevent generation and propagation of HIC. For high-strength steel of X65 or more, C
Although the amount increases, the increase in the amount of C enhances the segregation of Mn and P in the central segregation zone at the time of solidification of the CC slab, promotes the formation of a hardened structure, and significantly reduces the sour resistance.

In order to prevent this, the upper limit of the C content is 0.08
%. The lower limit of 0.02% of the C content is the minimum amount for securing strength and low-temperature toughness. In addition to reducing the C content, further reducing the Mn and P contents is effective in reducing center segregation, that is, in suppressing the formation of a hardened structure. Therefore, the upper limits of Mn and P amounts are 1.5% and 0.01%, respectively.
Limited to 0%. The lower limit of 0.8% of the Mn content is a minimum value for securing the strength of the base material and the welded portion. On the other hand, the lower the P content, the better the sour resistance.

In the steel of the present invention, Nb: 0.
01-0.05%, Ti: 0.005-0.03%. Nb contributes to refinement of crystal grains and precipitation hardening in controlled rolling, and has an effect of toughening steel. But N
If b is added in an amount of 0.05% or more, on-site weldability and HAZ toughness are adversely affected, so the upper limit was made 0.05%. Further, the addition of Ti forms fine TiN, suppresses the coarsening of γ grains in the slab during reheating and in the welded HAZ, refines the microstructure, and improves the low-temperature toughness of the base material and the HAZ. In order to exhibit such an effect of TiN,
A minimum of 0.005% Ti addition is required. But Ti
If the amount is too large, coarsening of TiN and precipitation hardening due to TiC occur, and the low-temperature toughness deteriorates.
Must be limited to 3%.

Next, the reasons for limiting other elements will be described. Si is an element added for deoxidation and improvement of strength, but if added in a large amount, the on-site weldability and HAZ toughness are deteriorated, so the upper limit was made 0.5%. Al alone is sufficient for deoxidizing steel, and Si need not always be added. N
Forms TiN to suppress coarsening of γ grains during slab reheating and welding, thereby improving the low-temperature toughness of the base material and HAZ. The minimum required for this is 0.001%. However, if it is too large, it causes HAZ toughness deterioration due to slab surface flaws and solid solution N, so its upper limit must be suppressed to 0.005%. Next, Ni, Cu, C
The reason for adding r, Mo, V, and REM will be described. The main purpose of adding these elements to the basic components is to improve properties such as strength and low-temperature toughness without impairing the excellent characteristics of the steel sheet obtained by the present invention. Therefore, the amount of addition is of a nature restricted by itself.

The purpose of adding Ni is to improve the strength of the low carbon steel of the present invention without deteriorating the low-temperature toughness and the on-site weldability. Addition of Ni is less likely to form a hardened structure that is detrimental to low-temperature toughness and sour resistance in a rolled structure (especially the center segregation zone of the slab) as compared with Mn, and increases the strength. To achieve this effect, 0.1%
The above addition is necessary. However, if the addition amount is too large, not only the economic efficiency but also the on-site weldability and the HAZ toughness are deteriorated, so the upper limit is set to 1.0%. Ni is also effective in preventing Cu cracks during continuous casting and hot rolling.

Cu is 0.1% or more, and H is almost the same as Ni.
It has the effect of improving strength and low-temperature toughness without significantly affecting AZ toughness, and also has an effect on improving corrosion resistance and hydrogen-induced cracking resistance. Also, the strength is greatly increased by Cu precipitation hardening. However, if added in excess, precipitation hardening causes a decrease in the toughness of the base material and HAZ and Cu cracks during hot rolling, so the upper limit was made 1.0%. Cr is M
The center segregation is less likely to occur in the CC slab than in the case of n, which is effective for increasing the strength without impairing the low-temperature toughness and the sour resistance. To achieve this effect, 0.1
% Or more is required. However, if it is too large, the on-site weldability and the HAZ toughness are remarkably deteriorated. Therefore, the upper limit of the Cr content is set to 1.0%.

Mo, like Cr, is less likely to segregate in the center of the CC slab than Mn, and is an effective element for increasing the strength without impairing the low-temperature toughness and sour resistance. In order to obtain such an effect, Mo should be at least 0.1.
%is necessary. However, excessive Mo addition degrades HAZ toughness and on-site weldability, so the upper limit was made 1.0%.

V has almost the same effect as Nb, and has an improved low-temperature toughness and hardenability due to the refinement of the microstructure.
It is effective in increasing the strength by precipitation hardening. In order to exhibit this effect, 0.01% or more must be added. However, if the addition amount is too large, on-site weldability and HAZ toughness deteriorate, so the upper limit was made 0.10%. V
The lower limit of the addition amount is the minimum amount for exhibiting the above-mentioned effects. REM, like Ca, is effective in controlling the morphology of MnS. In order to exhibit this effect, 0.0005% or more must be added. If too much is added, REM
In order to increase the amount of system oxides and degrade the HIC resistance, the upper limit value was set to 0.005%.

[0024]

Next, an embodiment of the present invention will be described. Table 1
Were welded in the circumferential direction of steel pipes (thickness: 25 to 38 mm) of various steel components shown in FIG. At the time of electron beam welding, a Ti, Al, Ca-based oxide or iron foil containing S was added to change the amount of S in the weld and the size and number of the composite of oxide and MnS. The strength, low temperature toughness and HIC resistance of the steel pipe were investigated. Further, a Charpy test piece was sampled from the center of the thickness of the electron beam weld. Notches were made at the center of the electron beam weld and at the fusion line to evaluate the low temperature toughness of the weld. Table 2 shows the results.

[0025]

[Table 1]

[0026]

[Table 2]

The steel pipes produced according to the invention (steel according to the invention) all have good properties. On the other hand, the comparative steel pipe not according to the present invention is not suitable for the chemical composition of the base metal, the S content of the circumferentially welded portion, or the existence state of the composite of oxide and MnS, and the strength, low temperature toughness, and HIC resistance Either of the properties or the toughness in the circumferential direction of the weld is inferior. No. 13 is inferior in base metal toughness and HIC resistance because the C content is too high. No1
No. 4 has inferior HIC resistance because the Mn content is too high. No1
In No. 5, since the S content is too high, the ESSP value is low and the HIC resistance is low.
Poor nature. In No. 16, since the Al content is too high, the weld toughness in the circumferential direction is inferior. No. 17 is S of the electron beam weld.
Due to the small amount, the toughness of the weld is degraded. In No. 18, since the S content of the electron beam welded portion is too large, the toughness of the welded portion is deteriorated. No. 19 is Ti, Al, Ca based oxide and M
Since the number of composites with nS is small, the toughness of the weld is deteriorated. No. 20 is Ti, Al, Ca based oxide and MnS
The toughness of the welded portion is deteriorated because the number of composites with the above is too large.

[0028]

According to the present invention, there is provided a thick-walled sour-resistant steel pipe having excellent low-temperature toughness and excellent sour resistance in a circumferentially welded portion welded by an electron beam or a laser. The safety of the pipeline has been significantly improved.

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Claims (2)

[Claims] 1. Mass%, C: 0.02 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 1.5%, P: 0.010% or less, S: 0 0.001% or less, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.03%, Al: 0.005% or less, Ca: 0.001 to 0.004%, N: 0. 001-0.005%, O: 0.003% or less, and 2.0 ≦ [Ca] (1-124
[O]) / 1.25 [S] ≦ A steel pipe whose base material is steel consisting of iron and unavoidable impurities and whose circumference satisfies 7.0 is welded in the circumferential direction by electron beam welding or laser welding. In the circumferential weld metal, S: 0.0
02-0.01% and 0.1-5 μm of Ti, Al,
Complex of oxide mainly composed of Ca and MnS is 5 to 10
A thick-walled sour-resistant steel pipe and pipeline excellent in low-temperature toughness of a circumferentially welded portion characterized by containing 00 pieces / mm ² . 2. In mass%, C: 0.02 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 1.5%, P: 0.010% or less, S: 0 0.001% or less, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.03%, Al: 0.005% or less, Ca: 0.001 to 0.004%, N: 0. 001-0.005%, O: 0.003% or less Ni: 0.1-1.0%, Cu: 0.1-1.0%, Cr: 0.1-1. 0%, Mo: 0.1 to 1.0%, V: 0.01 to 0.10%, REM: 0.0005 to 0.005%, and 2.0% or more. ≤ [C
a] (1-124 [O]) / 1.25 [S] ≦ 7.0 The circumference of a steel pipe whose base material is steel consisting of iron and unavoidable impurities is made by electron beam welding or laser welding. S: 0.002 to 0.01% and 0.1 to 5μ in the circumferentially welded metal portion of the steel pipe welded in the direction
A thick-walled sour-resistant steel pipe excellent in low-temperature toughness of a circumferentially welded part, comprising a composite of MnS and an oxide mainly composed of m, Ti, Al and Ca in an amount of 5 to 1000 pieces / mm ^2. And pipeline.