JPH08269566A - Production of high strength and high toughness uoe steel pipe excellent in sr characteristic - Google Patents

Abstract

PURPOSE: To produce a high strength and high toughness UOE steel pipe excellent in SR characteristics by subjecting a base material having a specified compsn. to submerged arc welding to obtain a weld metal having a specified compsn. and thereafter executing specified stress relief annealing. CONSTITUTION: A base material contg., by weight, 0.02 to 0.10% C, <=0.50% Si, <=2.00% Mn, <=0.020% P, <=0.0020% S, <=0.045% Nb, <=0.08% V, <=0.05% Ti, <=0.0010% B, <=0.50% Mo, <=0.0050% N, <=0.0030% O and Fe is subjected to submerged arc welding of one pass on the inside and outside faces with an Mo-Ti-B welding wire. Thus, a weld metal having a compsn. constituted of 0.01 to 0.06% C, <=0.6% Si, <=1.60% Mn, <=0.02% P, S and Nb, <=0.1% V, <=0.02% Al, <=1.5% Ni, <=0.05% Ti, <=0.0030% B, <=0.5% Mo, <=0.10% N, 0.015 to 0.050% O and Fe and in which Pcm in formula I is regulated to 0.13 to 0.18 and α in formula II is regulated to <=1 is obtd. The UOE steel pipe obtd. by the same is subjected to stress relief annealing of heating the same at 580 to 750 deg.C for >=10min and thereafter executing cooling at <=1 deg.C/sec cooling rate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing large diameter welded steel pipes, large diameter welded bend steel pipes and the like, which are subjected to SR (stress relief annealing) after being welded by a submerged arc welding method, and more particularly to a welded portion. The present invention relates to a method for manufacturing a UOE steel pipe excellent in low temperature toughness and strength.

[0002]

2. Description of the Related Art A line pipe is used as a means for mass-transporting crude oil or natural gas mined as an industrial raw fuel, or a liquid or gas obtained by refining them, or other liquid, gas, slurry or the like. It is well known that this is done. This line pipe is an extremely effective means as a method for mass transportation of industrial raw fuel, and a line pipe that can withstand more severe environments is required. In particular, as a measure when deploying a line pipe in a cold region, steel materials and weld metals for line pipes are required to have high tensile strength and at the same time ensure excellent low temperature impact toughness.

In order to meet such demands, for example, Japanese Patent Laid-Open No. 54-161517 discloses that the embrittlement due to SR treatment is caused by the hardening of Nb or V carbonitrides in the weld metal due to precipitation. It has been proposed to use a base material with the content controlled to 40 ppm or less. Further, JP-A-62-151545 discloses a steel plate having a Pcm of 0.19% or less as a thick steel plate for a line pipe, which has good weldability and excellent toughness.

However, in a steel pipe used in a harsh environment, especially in a place where a load is applied to the entire line pipe, the residual stress in the weld metal may be a problem even if the steel sheet disclosed in the above publication is used. Many. Welding residual stress means that welding causes expansion in the vicinity of the weld due to temperature rise and subsequent contraction, but at this time, stress changes due to complicated restraint in the vicinity of the joint during the temperature change due to welding, and cooling occurs. The stress remains after that.

Residual welding stress is a factor that has an important influence on various characteristics including fracture, corrosion, cracking and fatigue characteristics. That is, the welding residual stress is not only one of the factors that cause welding defects such as welding cracks, but also often deteriorates the appearance thereof. Originally, it is said that the effect of this welding residual stress is small in a material having high ductility such as a low alloy steel used for a large diameter steel pipe.

However, even in such a material having high ductility, it is known that if there are small defects such as undercuts and poor penetration in the welded portion, various characteristics are significantly deteriorated by residual tensile stress. In order to remove this residual welding stress, it is effective to perform heat treatment after welding. That is, stress relief annealing (SR)
It is possible to eliminate the residual welding stress by carrying out, and in order to remove the stress in the weld metal, it is necessary to heat at about 600 ° C or higher and then cool it in the furnace.

However, in a commonly used large-diameter steel pipe, when subjected to SR treatment and heated at about 600 ° C. or higher, extremely fine precipitates are formed in the weld metal after SR treatment. As a result, an unfavorable phenomenon occurs in the toughness and the toughness is significantly deteriorated. As a weld metal capable of performing SR treatment on a large diameter steel pipe,
It is disclosed in JP-A-60-61194. This makes it possible to reduce the embrittlement of the weld metal at the welded portion even if SR treatment is performed by defining the test steel sheet, the welding wire and the welding flux.

[0008]

However, as described above, recently, in steel materials and weld metals for line pipes for cold regions, it is necessary to secure not only higher strength but also excellent low temperature impact toughness. These requirements have been met, and it is difficult for the conventionally considered weld metal to meet these requirements at the same time.

The present invention has been made in view of the above circumstances. In the production of a straight pipe, the inner and outer surfaces of the inner and outer surfaces are welded by submerged arc welding, and then the large diameter steel pipe is subjected to SR treatment to obtain a weld metal. It is a method for producing a bend pipe, which is capable of obtaining a high-strength steel pipe having excellent low temperature toughness in a welded portion after removing residual stress therein. The strength of the bend pipe targeted by the manufacturing method of the present invention is TS 600 MPa or more, and the toughness is not less than 200 J in notch toughness value (vE-30 ° C).

[0010]

The present invention reduces the welding residual stress of a steel pipe and secures strength and toughness by performing SR treatment by comprehensively defining not only the chemical composition of the weld metal but also the heat treatment conditions. This is what has been made possible, and the main points are as follows. According to the present invention, in the UOE steel pipe, the chemical composition of the base metal is weight (w
t)% (hereinafter simply referred to as%) C: 0.02 to 0.10%, Si
: 0.50% or less, Mn: 2.00% or less, P: 0.020% or less, S: 0.0020% or less, Nb: 0.045% or less, V:
0.080% or less, Ti: 0.05% or less, B: 0.0010% or less, Mo: 0.50% or less, N: 0.0050% or less, O: 0.
0030% or less, the balance consisting of unavoidable impurities and Fe, 1-pass latent arc welding is performed on the inner and outer surfaces of the above base metal using a Mo-Ti-B welding wire, and C: 0.01 to 0.06% by weight.
%, Si: 0.6% or less, Mn: 1.60% or less, P: 0.02
% Or less, S: 0.02% or less, Nb: 0.02% or less, V:
0.1% or less, Al: 0.02% or less, Ni: 1.5% or less, Ti
: 0.05% or less, B: 0.0030% or less, Mo: 0.5% or less, N: 0.010% or less, O: 0.015 to 0.050%, and the balance is unavoidable impurities and Fe, and Pcm of the following formula is 0.13. To 0.18 and α in the following formula is 1 or less, and a UOE steel pipe composed of the base metal and the weld metal is heated in the temperature range of 580 to 750 ° C. for 10 minutes or more and then 1
This is a method for producing a high-strength, high-toughness UOE steel pipe excellent in SR characteristics which has been subjected to stress relief annealing by cooling at a cooling rate of ° C / sec or less.

Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 +
Ni / 60 + Mo / 15 + V / 10 + 5 × B α = 38.2 × Nb + 7.5 × V

[0012]

The reason for limiting the chemical composition of the base material as described above is as follows. C: An element which has a great influence on the strength and toughness of the base metal, and if the content is less than 0.02%, the required strength cannot be obtained.
If it exceeds 0.10%, the toughness and ductility are adversely affected.
The range was 02 to 0.10%.

Si: Si is an element that is inevitably contained in the deoxidizing process of steel, but is 0.50 in order to improve the toughness of the HAZ part.
%. Mn: It is an extremely important element that improves the strength and toughness of the base material at the same time, but if it exceeds 2.00%, it will adversely affect the steel material due to segregation, etc., so the upper limit was made 2.00%. P, S: P and S are elements that promote center segregation and are preferably low, and the upper limits were 0.020% and 0.0020%, respectively.

Nb, V: Nb and V are added to secure the strength and toughness of the base metal and HAZ part. Nb and V in these base metals are the toughness of the weld metal after SR treatment. Remarkably deteriorate. As a result of various studies by the inventors, Nb was 0.045% in order to secure the toughness of the weld metal after SR treatment.
Hereinafter, it was found that V should be 0.080% or less.

Ti: Ti is an element necessary for ensuring the toughness of the base material, but if it is contained in excess of 0.05%, the toughness of the base material is deteriorated, so the upper limit is 0.05%. B: B segregates at the austenite grain boundaries during rolling to improve hardenability, but if it exceeds 0.0010%, HAZ
The upper limit was made 0.0010% in order to deteriorate the toughness of the part.

Mo: Mo is an element necessary to secure the strength of the base metal, but if it exceeds 0.50%, HAZ
The upper limit was 0.50% for the part to harden. N: If N exceeds 0.0050%, the toughness of the weld metal deteriorates due to N that dissolves in the weld metal due to dilution of the base metal,
Should be less than 0.0050%. O: O is preferably as small as possible in order to secure the toughness of the base material, and the upper limit was 0.0030%.

The reasons for limiting the chemical components in the above-described weld metal of the present invention will be described. C: It is an element having a great influence on the strength and toughness of the weld metal. If it is less than 0.01%, the required strength cannot be obtained, and if it exceeds 0.06%, high toughness is obtained even if the strength is satisfactory. In addition, since the solidification cracking susceptibility of the weld metal is increased, the range is set to 0.01 to 0.06%.

Si: Si enters the weld metal from the base metal, welding wire and flux, but if Si exceeds 0.6%, the toughness of the weld metal decreases, so 0.6% was made the upper limit. Mn: Mn is an essential element for deoxidizing the weld metal, and at the same time is an important element for strength and toughness.
If it exceeds 60%, the strength becomes high, but the hardenability becomes too large to form a lath-like structure, and the toughness of the SR-treated weld metal deteriorates, so 1.60% was made the upper limit.

P: P is an element that deteriorates the toughness of the weld metal, so it is preferable that its content be small, and in order to prevent the toughness of the weld metal from decreasing, it should be 0.02% or less. S: S, like P, is an element that deteriorates the toughness of the weld metal, so it is preferable that the content be small, and in order to prevent the toughness of the weld metal from decreasing, it should be 0.02% or less.

Nb, V: As a result of various studies by the inventors,
It has been found that the amounts of Nb and V in the weld metal are the most important elements for ensuring the toughness of the weld metal in the steel pipe subjected to SR treatment. Fig. 1 shows the relationship between the Nb and V contents in the weld metal and the fracture surface transition temperature of the SR-treated weld metal. When the Nb and V contents in the weld metal are increased, the SR treatment is performed. It is clear that the fracture surface transition temperature of the weld metal subjected to the heat treatment markedly increases and the toughness deteriorates. That is, in order to ensure the toughness of the SR-treated weld metal at -30 ° C, Nb should be 0.02% or less and V should be 0.1%.
It should be clear that:

As a result of further examination, Nb and V in the weld metal
It was found that the toughness of the SR portion cannot be ensured unless the α value shown in the following equation is 1 or less even if the amount is within the above range. α = 38.2 × Nb (%) + 7.5 × V (%) That is, Fig. 2 shows the relationship between the α value and the toughness of the weld metal subjected to SR treatment, but when the α value exceeds 1, SR The toughness of the treated weld metal deteriorates to 20 at -30 ℃.
The α value is set to 1 or less because 0 J cannot be secured.

Al: Al is contained for deoxidizing the weld metal, but if it exceeds 0.02%, the toughness of the weld metal deteriorates, so it should be 0.02% or less. Ni: Ni is a good element in terms of the toughness of the weld metal, but if it is contained in excess of 1.5%, hot cracking easily occurs during welding, so the upper limit was made 1.5%. Ti: 0.01% is required because Ti has a remarkable effect of suppressing the growth of austenite grains during high temperature heating and finely dividing ferrite grains generated during cooling. However, if it exceeds 0.05%, the structure changes and the toughness deteriorates significantly, so the content was made 0.05% or less.

B: B is an element effective for improving the toughness of the weld metal, but if it exceeds 0.0030%, the structure changes and the toughness deteriorates, so the content was made 0.0030% or less. Mo: Mo is an element effective in improving the toughness of the weld metal as it is welded, but if it is contained in a large amount, toughness will be reduced, so from the balance of these, it should be 0.5% or less.

N: N is preferably harmful because it is harmful for improving the toughness of the weld metal, and its upper limit should be 0.010%. O: O significantly affects the toughness of the weld metal, and when it exceeds 0.050%, the toughness of the weld metal is deteriorated, so 0.050% was made the upper limit. Further, in a weld metal of less than 0.015%, the structure is so hardened that it becomes a structure called upper bainite and the toughness is significantly deteriorated, so 0.015% was made the lower limit.

The reason for limiting Pcm is as follows. It is well known that Pcm has been conventionally used as an index showing the properties of steel pipes, but it has been found that Pcm greatly affects the toughness of the weld metal after SR treatment. That is,
In a weld metal having a Pcm of less than 0.13%, the hardenability of the weld metal is low, so the structure after SR treatment becomes a structure mainly composed of pro-eutectoid ferrite, and sufficient toughness cannot be obtained. It is also difficult to satisfy the strength.
If the weld metal has a Pcm of more than 0.18%,
Since extremely fine precipitates are formed in the weld metal after SR treatment and the toughness is unfavorably caused and the toughness is significantly deteriorated, 0.13% ≤ Pcm ≤ 0.18%.

The SR treatment method is a method of heating at 580 ° C. to 750 ° C. for 10 minutes or more and then cooling at a cooling rate of 1 ° C./sec or less. The reason for this limitation is as follows. Figure 3 shows the relationship between the SR treatment temperature and the residual stress of the weld metal. When the SR temperature is 580 ° C or higher, the residual stress is remarkably reduced, and should be 580 ° C or higher. Although the residual stress was removed at a too high temperature, the structure of the weld metal changed and the toughness deteriorated, so the temperature was set to 750 ° C or lower.

Further, FIG. 4 shows the relationship between the cooling rate and the residual welding stress after SR was performed at 600 ° C.
The cooling rate was set to 1 ° C / sec or less because the residual stress of the weld metal is not sufficiently reduced by cooling at 1 ° C / sec or more. Further, if the SR treatment time is too short, the effect of removing stress disappears, but if the treatment time is 10 minutes or more, it is considered that the effect appears. Therefore, the minimum SR time was set to 10 minutes.

[0028]

EXAMPLES Specific production examples according to the present invention will be described below. The test steel plate (base material) used by the inventors of the present invention had a plate thickness of 24 mm having the chemical components shown in Table 1.

[0029]

[Table 1]

This test steel sheet was welded to both sides by the 4-electrode submerged arc welding method under the welding conditions shown in Table 2 to obtain a test piece. Further, a groove having a groove angle of 45 ° and a groove angle of 40 ° was formed with 4.9 mm on the inner surface side and 7.8 mm on the outer surface side, respectively, and welded.

[0031]

[Table 2]

As the welding wire for submerged arc welding, a Mo-Ti-B type welding wire having the chemical composition shown in Table 3 is used, and as the flux, a highly basic flux having the composition shown in Table 4 is used. It was used.

[0033]

[Table 3]

[0034]

[Table 4]

Table 5 shows the chemical composition of the weld metal thus obtained. In Table 5, a to o are within the scope of the present invention, and kato are out of the range.

[0036]

[Table 5]

Each of these weld metals was subjected to SR treatment and its characteristics were examined. This is one in which the weld metal was heated at 600 ° C. for 1 hour and then cooled to room temperature at 0.1 ° C./sec to examine the mechanical properties of the weld metal. The results are shown in Table 6.
Is shown in.

[0038]

[Table 6]

In Table 6, Examples Nos. 1 to 5 are within the range specified in the present invention, and Comparative Examples Nos. 6 to 20 are those in which the heat treatment range is out of the range specified in the present invention. In each of Examples 1 to 5, the residual stress was reduced to 100 MPa or less by SR. Furthermore, in each of the examples, the Charpy absorbed energy at −30 ° C. is 200 J or more, an extremely high toughness value is obtained, and sufficient tensile strength is obtained. That is, it is clear that the weld metal after SR has high strength and high toughness within the range specified in the present invention.

On the other hand, in Comparative Example No. 6, C is a comparative example.
No. 7 is Si, Comparative Example No. 8 is Mn, Comparative Example No. 9 is Nb, Comparative Example No. 10 is V, Comparative Example No. 11 is Al, and Comparative Example No. 13 is Mo.
However, Comparative Example No. 14 has Ti, Comparative Example No. 15 has B, Comparative Example No. 16
Is higher than N in the present invention, and Comparative Example No. 17 is higher than O in the present invention. Therefore, the toughness of the weld metal after SR treatment is 100 J
The following values indicate that satisfactory toughness is not obtained. Further, in Comparative Example No. 12, although the toughness and strength of the weld metal after SR treatment were secured, the weld metal cracked because the Ni content exceeded the range of the present invention.

Further, in Comparative Example No. 18, although the chemical composition of the weld metal is within the range of the present invention, the toughness of the weld metal is remarkably deteriorated because the α value exceeds the range of the present invention. I understand. Comparative Example No. 19, although the chemical composition of the weld metal is within the scope of the present invention, the toughness is significantly deteriorated because the Pcm value exceeds the range of the present invention, and Comparative Example No. 20 has a Pcm value of It can be seen that the strength of the weld metal is insufficient because it is below the range of the invention of the present application, and that both are unsuitable.

[0042]

According to the present invention, when a large-diameter welded steel pipe, a large-diameter welded bend steel pipe, and the like which are subjected to SR treatment after being welded by the submerged arc welding method are manufactured,
The residual stress can be reduced by the treatment, and the weld metal after SR treatment can be made to have a high tensile strength and at the same time excellent cold-temperature impact toughness can be satisfied, and the effect is great.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between Nb and V amounts (%) and a fracture surface transition temperature.

FIG. 2 is a diagram showing a relationship between an α value (%) and a Charpy absorbed energy value (J / cm ² ) at −30 ° C. of a weld metal after SR treatment.

FIG. 3 is a diagram showing the relationship between the temper temperature (° C.) and the residual stress (MPa) in the welding line direction.

[Fig. 4] Cooling rate (℃ / sec) and residual stress of weld line (MP
It is a diagram which shows the relationship with a).

Claims (1)

[Claims] 1. In a UOE steel pipe, the chemical composition of the base material is C: 0.02 to 0.10%, Si: 0.50% or less, Mn: 2.00% or less, P: 0.020% or less, S: 0.0020% by weight (wt)%. Below, Nb: 0.045% or less, V: 0.080% or less, Ti: 0.05% or less, B: 0.0010% or less, Mo: 0.50% or less, N: 0.0050% or less, O: 0.0030% or less, and the balance is unavoidable. Containing impurities and Fe, 1-pass latent arc welding was performed on the inner and outer surfaces of the above base metal using a Mo-Ti-B welding wire, and C: 0.01-0.06%, Si: 0.6% or less in weight (wt)%. , Mn: 1.60% or less, P: 0.02% or less, S: 0.02% or less, Nb: 0.02% or less, V: 0.1% or less, Al: 0.02% or less, Ni: 1.5% or less, Ti: 0.05% or less, B: : 0.0030% or less, Mo: 0.5% or less, N: 0.010% or less, O: 0.015 to 0.050%, the balance being unavoidable impurities and Fe, and the formula below. After obtaining a weld metal having Pcm of 0.13 to 0.18 and α of 1 or less in the following formula, heating the UOE steel pipe composed of the base metal and the weld metal in the temperature range of 580 to 750 ° C. for 10 minutes or more, A method for producing a high-strength, high-toughness UOE steel pipe excellent in SR characteristics, which is subjected to stress relief annealing by cooling at a cooling rate of 1 ° C / sec or less. Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo /
15 + V / 10 + 5 × B α = 38.2 × Nb + 7.5 × V