JPS5817808B2 - Method for producing welded steel pipes with excellent stress corrosion cracking resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to welded steel pipes (U
The present invention relates to a manufacturing method for providing OE steel pipes, spiral steel pipes, electric resistance welded steel pipes, etc.).

Large diameter welded steel pipes with excellent stress corrosion cracking resistance are required as line pipes for transporting sour crude oil or natural gas.

In this case, stress corrosion cracking refers to a part of the steel material being corroded by H2S in a humid environment, and the H generated at that time penetrating into the steel and causing cracks under the action of operating stress or various residual stresses. This refers to a phenomenon that occurs, and it is widely known that problems actually occur in line pipes such as those described above.

Conventionally, the following methods have been used to improve the performance of the base material, and some results have been achieved.

(1) Addition of corrosion-resistant alloying elements such as Cu and Cr, (...)S
(g) Adjustment of the shape of inclusions (same as above); (5) Improvement of the structure through heat treatment; however, stress corrosion cracking occurs in actual line pipes. It was found that most of the locations were welded areas, and that it was less likely to occur in other locations.

This is based on the conventional knowledge that a mixed structure of bainite + pro-eutectoid ferrite and increased hardness tend to increase stress corrosion cracking susceptibility. + Since it is known that it becomes a pro-eutectoid ferrite mixed structure,
Similarly, in line pipe materials, the heat affected zone has almost the same mixed structure, and at the same time its fusion structure
It is thought that the hardness increases near the ne (fusion line), which increases the stress corrosion cracking susceptibility.

Therefore, in order to obtain a welded steel pipe with excellent stress corrosion cracking resistance, both the base metal and the welded part must have the same performance. Since no effective method has been found to date, corrosion cracking does not occur; 6' is a major problem when used in an environment.

This invention was devised to solve the above-mentioned problems, and the purpose of this invention is to improve the stress corrosion cracking resistance of welded parts, which is a problem with conventional manufacturing methods. In order to do this, after pipe making is completed, the welded part or the entire pipe including the welded part is quenched under predetermined temperature conditions.
By doing this, the increased hardness that occurs during welding is eliminated by heating the □-austenite region prior to quenching and then uniformly cooling it. The overall structure of the hem weld changes to a uniform tempered martensite or a fine-grained ferrite+pearlite structure, and both effects improve stress corrosion cracking resistance.

The welded steel pipe targeted by the present invention is used particularly as a line pipe for transporting sour crude oil or natural gas.

UOE steel pipes, spiral steel pipes, electric resistance welded steel pipes, etc. have common compositions to date, typically C: 0.05 to 0.2.
0%, Si: 0.01~0.8%, Mn: 0.5~
1.6%, P: 0.03% or less, S2 0.04% or less,
Cu: tr~0.8%, Ni: tr~1.0%
, AI: tr ~ 0.1%, Cr: tr ~ 1.0%, N
b: tr~0.1%, V: tr~0.1%, Mo
: tr ~ 1.0%, Ca: tr ~ 0.005%
, La+Ce: tr ~0.005%, Ti:
tr ~ 0.1%, B: tr ~ 0.001%, the balance covers all welded steel pipes made using steel with a normal composition as a base material consisting of Fe and unavoidable impurities. It is based on the fact that it is not intended to improve the performance of the weld itself, but to make the performance of the welded part equivalent to that of the base metal.

In the present invention, the welded portion or the entire steel pipe including the welded portion is
Quenched from a temperature of 0 to 1000℃, then 550℃
Tempering is performed at a temperature below °C'4Ac, but in this case, the quenching temperature has almost no effect on stress corrosion cracking resistance, so it can be considered to be the same as general quenching, and therefore there are no restrictions from this point of view. However, from the viewpoint of strength and low-temperature toughness, it is 850 to ioo.

It is desirable that the temperature is ℃.

Regarding the tempering temperature, if the tempering temperature is lower than 550°C, the stress corrosion cracking resistance will not improve because the tempering will not be performed sufficiently.
If it exceeds 1, it will become a mixed structure of ferrite + hardened structure and the stress corrosion cracking resistance will be greatly deteriorated.
c, with a particularly desirable range of 600°C to A
It is c1.

As for this tempering temperature, it is preferable to select a temperature as high as possible within Ac 1' in order to sufficiently soften the quenched structure and impart stress corrosion cracking resistance, but this also takes into consideration the required strength and toughness. Needless to say, this should be done in a consistent manner.

In order to confirm the effectiveness of the method of this invention against stress corrosion cracking, as an example, C: 0.06 to 0.1
5%, Si: 0.15-0.35%, Mn: 0.6
7-1.53%, P: 0.012-0.075%,
S: 0.004-0.026%, Cu: tr-0
．． 18%, N i : t r ~0.23% 1Mo:
tr~0.14%, Nb: tr~0.041%, C
r: tr~0.31%, V: tr~0.09
A UOE steel pipe is made of steel with a composition of 5% Fe, the balance being Fe and unavoidable impurities as a base material, and after pipe making, the entire pipe including the bath contact part is quenched and tempered according to the temperature conditions of the method of this invention. Regarding the base metal part (parts other than the welded part)
Table 1 shows the hardness of the welded part and the results of a small 5-hell three-point bending test.

The above-mentioned 5-hell method small three-point bending test is a 5-hour test.
This is a stress corrosion cracking test method developed by EL1 Oil Company, and is a widely used test method.

The test piece was L near 5 x W: 4.5 as shown in Figure 4.
mmXT: 1 from both widthwise ends of the 1.5 mm test piece
Drill holes with a diameter of 0.02 g inch at two points apart, and glass slots that will serve as fulcrums for the test piece during pushing up, which will be described later, at positions spaced apart by an inch from both ends in the length direction. Then, the test piece is pushed up with a push-up screw located at the lower surface in the center in the longitudinal direction to give a strain to the test piece.
The S value is defined as kilopound square inch) × -.

Test specimens with various water content and multiple loads applied at various load levels are □ while the loads are still being applied.
After immersing in the test liquid for 28 days, the test piece is pulled out and observed with the naked eye.

The test chamber uses a solution of 0.5% acetic acid saturated with H2S.

In this case, if stress corrosion cracking occurs, cracks are observed near the drill hole.

The Sc value indicates the upper limit of the S value at which cracks are no longer observed.

Comparing the test results of the steel pipe according to the method of the present invention shown in Table 1 above with the performance of the steel pipe according to the conventional method in Fig. 1, the stress corrosion cracking resistance performance of the welded part of the steel pipe according to the method of the present invention is It can be seen that the stress corrosion cracking resistance of the base material is almost the same as that of the base metal, and it is significantly improved compared to the welded parts of steel pipes made using conventional methods.

Further, according to the method of the present invention, even if only the welded portion is subjected to the quenching/tempering treatment, excellent performance can be obtained that is as good as when the entire steel pipe is subjected to the heat treatment.

FIGS. 2A and 2B are graphs showing examples of the quenching pattern and tempering pattern of the method of the present invention shown in Table 1.

As already mentioned, the strength and toughness of the steel pipe must also be taken into account when considering the heat treatment conditions.
3A and 3B are graphs showing the results of a tensile test and a Charpy test conducted in parallel with the Sc value measurement in the example of the method of the present invention shown in Table 1. As can be seen, in this example, the strength (YS, TS) increases as the quenching temperature rises during quenching, but the austenite grain size increases, so the toughness (vTs) according to the Charpy test increases at -1050°C or higher. It showed a decreasing trend.

Also, as seen in Figure B, the influence of the tempering temperature is particularly high at 60°C.
Good results are shown at 0°C-=Ac1.

Therefore, considering the balance of Sc value, strength, and toughness, the quenching temperature is 850 ~
It will be understood that a high temperature of 1000° C., a tempering temperature of 550° C.”A c1 or less, and a high temperature are desirable.

In addition, A shown in Table 2 below for the same machine as the above example,
Welded steel pipes made from four types of steel: B, , C, and D were subjected to a 5-hell small-scale 3-point bending test on the base metal and welded parts before and after quenching and tempering according to the present invention, and the results are shown in Table 3. The results were obtained.

Incidentally, compared to the steels of Examples shown in Table 1 above, steel type A contains slightly more Mo and also has Ti added thereto.

Steel type B contains slightly more Cu and has B added,
Steel type C contains slightly more Cu and T1 and Cu are added, and steel type corresponds to one with a slightly higher content of Cu and Cr, but Table 3 shows the effects of the present invention on welded steel pipes made of either steel type. This is obtained as follows.

It is known from experience that welded steel pipes with an Sc value of >10 can be safely used for transporting sour crude oil or natural gas, and from this point of view, the welded steel pipes produced by the method of the present invention are applicable to the above-mentioned applications. It has a remarkable effect.

[Brief explanation of the drawing]

Fig. 1 is a graph showing the relationship between Sc value and hardness for the conventional method and the present invention. Fig. 2 A and B are graphs showing the quenching pattern and tempering pattern of the present invention, respectively. FIG. 4 is a graph showing the relationship between strength and toughness and the quenching temperature and tempering temperature of the present invention. FIG. 4 is an explanatory diagram showing a small-sized 3-point bending test method using a 5-hell system.

Claims (1)

[Claims] I UOE steel pipes, spiral steel pipes, electric resistance welded steel pipes, etc.
:0.05~0.20%, Si:0.01~0.8%,
Mn: 0.5-1.6%, P: 0.03% or less, S
: 0.04% or less, Cu: tr~0.8%, Ni:
tr~1.0%, Al: tr~0.1%, Cr:t
r~1.0%, Nb: tr~0.1%, V: tr~0.
1%, Mo: tr ~ 1.0%, Ca: tr ~ 0
．． 005%, La+Ce: tr ~0.005
%, Ti: tr~0.1%, B: tr~0.00
A welded steel pipe made using a steel having a composition of 1% Fe, the balance Fe and unavoidable impurities as a base metal, or the entire steel pipe including the welded part, is quenched at a temperature of 850 to 1000°C, and then quenched at a temperature of 550°C to Ac1. A method for manufacturing a welded steel pipe with excellent stress corrosion cracking resistance, characterized by tempering at the following temperature.