JPS6035982B2 - Steel for line pipes with excellent hydrogen sulfide cracking resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a line pipe steel that has excellent cracking resistance in a humid environment containing hydrogen sulfide.

In recent years, line pipes have been increasingly used to transport crude oil and natural gas, but crude oil and natural gas often contain hydrogen sulfide, and if hydrogen sulfide coexists with moisture such as seawater or underground mine water, the steel surface will deteriorate. There is a high probability that it will be involved in corrosion, and the formation of arms due to corrosion has become a major problem.

Defects that occur in steel in a humid hydrogen sulfide environment include hydrogen-induced cracking and sulfide stress corrosion cracking, and both warpages are herein collectively referred to as hydrogen sulfide cracking. Hydrogen-induced cracking occurs even in the absence of external stress, and can be seen in both high-strength materials such as oil well pipes and low-strength materials such as line pipe steel.

These hydrogen sulfide cracks cause bulges on the steel surface, which are also called blisters, but even if there is no bulge on the surface, internal cracks can propagate in a step-like manner and develop into through-thickness cracks. Sulfide stress corrosion cracking is a crack that occurs when stress is applied to high-strength steel, such as steel pipe for oil wells, but even for low-strength copper, such as steel for line pipes, the heat-affected zone due to welding generally has a hardened structure. Easy to form, weld residual stress and weld defects become a stress concentration source under pipeline operating pressure, and sulfide corrosion cracking becomes a serious problem.

Line pipes are often manufactured by welding, such as ERW pipes and supmerged arc welded pipes.If you include circumferential welding for joining pipes during pipeline installation, all line pipes are formed by welding. It can be said that

Therefore, for line pipe steel used in a humid hydrogen sulfide environment, it is important not only to prevent hydrogen-induced cracking, but also to reduce hydrogen sulfide cracking, including sulfidation stress corrosion cracking in welds. By the way, when manufacturing line pipe materials that are resistant to hydrogen sulfide cracking, it is necessary to fully consider environmental conditions.

Conventionally, in order to evaluate the hydrogen sulfide cracking properties of materials, a bonding test using an artificial seawater solution saturated with hydrogen sulfide has been carried out, but under such conditions the pH value is usually between 4.5 and 5.5. in range. However, line pipes are actually used in a wide variety of environments, and are not limited to relatively mild environments as described above. Therefore, in developing line pipe materials with excellent hydrogen sulfide cracking resistance, it is necessary to consider even more severe conditions. According to conventional research, it is known that hydrogen-induced cracking is closely related to nonmetallic inclusions in steel and polarization of component elements.

In other words, hydrogen-induced cracking is particularly likely to occur starting from sulfide-based inclusions (such as manganese sulfide) elongated by rolling.
Furthermore, the presence of abnormal structures due to micro-polarization of manganese and phosphorus increases the susceptibility to hydrogen-induced cracking. For this reason, methods are being used to reduce the amount of sulfur in the steel in order to reduce the number of sulfide inclusions that can be the starting point for cracks, but this alone is not enough to prevent inclusions such as inverted V segregation in large steel ingots from being concentrated. It is not possible to completely prevent hydrogen-induced cracking in areas with .

Furthermore, recently, a method has been proposed that controls the shape of inclusions by adding rare elements such as cerium, and combines this shape control with reducing the amount of inclusions by reducing the amount of oxygen in the steel.

However, with this method, the specific gravity of rare elements is high and it is difficult to float and remove inclusions during solidification of molten steel, making it difficult to prevent cracks caused by the concentration of inclusions in the bottom precipitated crystal part of large steel ingots. It is. Furthermore, the addition of large amounts of rare earth elements causes many problems in industrial production, such as the tendency to cause nozzle chatter during casting and ingot formation. In addition, as countermeasures against abnormal structures, high-temperature long-time soaking treatment before hot rolling and cooling rate control after hot rolling are known, but these methods are complicated and do not provide sufficient effects. Furthermore, although limiting manganese and phosphorus is effective, this alone is not completely effective in preventing hydrogen-induced cracking, and adding copper is effective in suppressing hydrogen intrusion into steel, but it is not effective in harsh environments. Not enough. On the other hand, reducing the hardness of high-strength materials is effective against sulfide stress cracking, and Rockwell hardness
However, this reduction in hardness alone cannot completely prevent cracking in harsh environments.

In view of the above-mentioned current situation, the present inventors conducted various studies on the development of line pipe steel that does not cause hydrogen sulfide cracking even under severe environmental conditions, and as a result, discovered the following.

In other words, in order to prevent hydrogen-induced cracking, the content of manganese and phosphorus should be limited, calcium should be added within a specific range depending on the content, and non-metallic inclusions in steel should be prevented. By limiting the amount, hydrogen-induced cracking can be completely prevented even in harsh environments.

At this time, it is necessary to limit the contents of manganese and phosphorus in order to prevent the formation of abnormal tissues in the normal process. Although calcium is effective in controlling the shape of inclusions in steel,
There is an optimal content range in relation to the amount of sulfur, and in order to exhibit the effects of this invention, it is necessary to limit the amount of sulfur to a low level. In addition, calcium sulfide has a low specific gravity and is easily removed by flotation during solidification of molten steel, thereby preventing inclusions from forming in the precipitated crystal portion at the bottom of a steel ingot treated with rare earth elements. Furthermore, it is important to limit the cleanliness of nonmetallic inclusions to prevent hydrogen-induced cracking. Excellent effects can only be achieved when the above elements are combined. In addition, to prevent sulfide stress corrosion cracking in welds, it is important to take measures during welding, such as reducing welding residual stress and reducing weld defects. If it can be provided, the industrial value is extremely high.

Conventionally, measures to improve the sulfide stress corrosion cracking resistance of low-strength materials such as line pipe materials have not always been clear;
With conventional techniques, it has been difficult to completely prevent sulfide stress corrosion cracking in welds. Therefore, as a result of conducting a systematic study on this point, the present inventors found that, in addition to the measures against hydrogen-induced cracking described above, carbon, poppy, elemental, manganese, and chromium and molybdenum added as necessary were added to the steel. I learned that it is effective to limit nickel to a specific range. Furthermore, in recent years, in order to provide inexpensive line pipes, large steel ingots and continuous casting slabs with a thickness of 400 mm or more have been increasingly used, but such materials are prone to inclusions and microscopic fracture of component elements, In order to impart excellent hydrogen sulfide cracking resistance to the entire pipe, it is necessary to avoid these segregations.

Based on the above findings, the present invention provides a steel for line pipes that can completely prevent hydrogen sulfide cracking in a caustic environment. That is, this invention [1] CO. 02-0.15%, Sio. 01~0.
50%, Mno. 50-1.20%, PO. 012% or less, SO. 003% or less, Cuo. 20-0.50%,
AIO. 01 to 0.10%, VO. 01-0.10%
, Nbo. One or two of 01 to 0.05%, Ca
in the range of 2SCa%/S%SIO, and C%10Si%/3000Mn%/2000Cu%/20≦0.20
%, the balance substantially consists of Fe, and the cleanliness of non-metallic inclusions is 40×10-3% or less, and has excellent resistance to hydrogen flow cracking.

■ In addition to the steel described in item '11 above, Cro. 01
~0.50%, Moo. Steel for line pipes with excellent resistance to hydrogen sulfide cracking, containing one or more of 0.01 to 0.50%.

■ In addition to the copper described in item '11, Nio. 01
Steel for line pipes with excellent hydrogen sulfide cracking resistance containing ~0.10%.

■ In addition to the copper described in (2) above, Nio.01
Steel for line pipes with excellent hydrogen sulfide cracking resistance containing ~0.10%.

The gist is:

This invented steel is most effective for manufacturing line pipe materials from large steel ingots or continuous cast slabs with a thickness of 40 m or more that are prone to unevenness, and is a line pipe material with excellent hydrogen sulfide cracking resistance. can be provided at low cost.

Further, the present invention can be used as is after finish rolling, but if necessary, it may be used after being subjected to quenching and tempering treatment or normalizing treatment. The reason why the chemical components are limited in this invention is as follows.

If C is less than 0.02%, the necessary strength cannot be obtained;
If it exceeds 5%, weldability deteriorates, and the hardness of the weld increases and susceptibility to sulfide corrosion cracking increases, so 0.02
~0.15%.

Si is necessary as a deoxidizing agent during steel manufacturing, but 0.01%
If it is less than 0.01%, there is no effect, and 0.01% or more is required, but if it exceeds 0.50%, the toughness deteriorates, so 0.0.
1 to 0.50%.

However, it is desirable that the content be as low as possible within a range that provides a deoxidizing effect. Mn is necessary to impart strength, but if it is less than 0.50%, the required strength cannot be obtained;
If it exceeds 20%, it is easy to form abnormal structures in large steel ingots and continuous cast materials mainly due to microscopic dense polarization of manganese and phosphorous, making it difficult to prevent hydrogen-induced cracking especially in harsh environments, and the hardness of welded parts may deteriorate. Since the susceptibility to sulfide corrosion cracking increases, the content is set at 0.05 to 1.20%. Since P is easily polarized and promotes the formation of abnormal tissue, it is desirable to have a low content, and in order to exhibit the effects of the present invention, it is desirable to have a content of 0.012% or less. Since S forms sulfide-based inclusions and has a large effect on hydrogen-induced cracking susceptibility, it is desirable to reduce it as much as possible, and in this invention, it is preferably 0.003% or less. Mountain is necessary as a deoxidizing agent, but 0.01%
If it is less than 0.1%, the effect will not be enhanced, and if it exceeds 0.10%, the properties will deteriorate, so the content should be 0.01 to 0.10%.

Ca is an effective element for controlling the shape of inclusions, but when the value of Ca%/S% is less than 2, the effect is insufficient,
Hydrogen-induced cracking cannot be completely prevented due to residual manganese sulfide, and when the concentration exceeds 10, susceptibility to hydrogen-induced cracking increases due to the accumulation of calcium sulfides, oxides, and their composites (see Figure 1). Cu is effective in increasing strength and reducing hydrogen-induced cracking, but if it is less than 0.20%, the effect will not be sufficient, and if it exceeds 0.50%, weldability and hot workability will deteriorate. .20 to 0.50%.

Although V and Nb are effective in obtaining the required strength level,
If it is less than 0.01%, there is no effect, and V is 0.10%.
, Nb exceeding 0.05% is not desirable because it is economically disadvantageous. Cr is effective in improving strength and hydrogen-induced cracking resistance, but if it is less than 0.01%, it has no effect.
If it exceeds 0.50*%, it is undesirable because rutting performance deteriorates.

Mo increases strength and toughness, and is effective in improving corrosion resistance at the same time, but if it is less than 0.01%, it has no effect, and if it is less than 0.50%, it has no effect.
% is undesirable because toughness deteriorates. Since Ni is harmful to hydrogen sulfide cracking, it is desirable to have a small amount, but if it contains copper, it should be 0.01 to 0.0 to prevent copper embrittlement.
It is contained within a range of 10%.

Within this range, the effects of the present invention are not adversely affected.
Although N is not particularly limited, since it has an adverse effect on hydrogen sulfide cracking, it is preferably limited to 1 pm or less, preferably 4 pm or less. C%+Si%/300 Mn%/
The condition of satisfying 0.20% of 200 Cu% is necessary to prevent sulfide corrosion cracking in the weld, and as shown in Figure 3, it is necessary to satisfy 0.20% of Cu%. and sulfide corrosion cracking cannot be completely prevented.

This is because if the content exceeds 0.20%, martensitic and bentnite structures, which are highly susceptible to sulfide corrosion cracking, tend to appear in relation to the structure of the weld heat-affected zone. The reason why the cleanliness of non-metallic inclusions was set to 40x10-3% or less is because, as shown in Figure 2, if it exceeds 40x10-3%, hydrogen-induced cracking cannot be prevented in harsh environments. .

Next, embodiments of the invention will be described.

Pipes and plates were manufactured from large steel ingots or continuously cast slabs whose chemical composition is shown in Table 1. 1st Idada X is C Tsutomu Si Tano 300 Mn Tano 200 Cu
As shown in Fig. 4, test pieces were taken from each of these steels, which were approximately /200C /200Mo /150N /60, along the rolling direction of the plate and pipe, and hydrogen sulfide was removed. After being immersed in the following solution saturated with hydrogen for 300 hours, the presence or absence of cracks was examined by ultrasonic deep scratching, and the cut cross section was examined using a microscope to determine whether there were hydrogen-induced cracks.

A: A solution of artificial seawater or freshwater saturated with hydrogen sulfide (p
B: A solution of 0.5% acetic acid and 5% sodium chloride solution saturated with hydrogen sulfide (pH value 3.5);
0 to 3.5), and the test piece was placed at the top of the steel ingot in the steel ingot village.
From the position corresponding to the middle and bottom, collect 3 pieces each from a total of 9 locations at the edge, 1/4 position, and 1/2 (center) position of the board. Three samples were taken from each of three locations, 4 positions and 1/2 position, and tested.

The results are shown in Table 2. , Table 2 Test Results However, the crack evaluation is shown as 0: present, △: small crack, and ×: large crack.

From the above results, the ratio* is.

In the case of only one B sail, large cracks occurred in most cases, and A steel treated with rare earth elements, B steel
Although steel cannot prevent hydrogen-induced cracking in harsh environments, all of the steels of this invention have no cracking, indicating that they have excellent resistance to hydrogen sulfide cracking.

In addition, a test piece shown in Figure 5 was made from a sample taken from the welded part, and after applying a stress equivalent to the yield stress of the steel using the loading device shown in Figure 5, it was immersed in the solution B for 20 hours. After that, a test was conducted.

The welding conditions were the strictest, 9000 kcal/skin. The results are shown in Table 2, and it can be seen that this invention steel can completely prevent sulfide corrosion cracking. From this, as shown in Figure 3, C%+Si%/
30Mn%/20Cu%/20Cr%/20Mo
%/150Ni%/60≦0.20% and limiting the steel to a low hardness range is extremely effective in preventing sulfide corrosion cracking.

[Brief explanation of the drawing]

Figure 1 shows CO. 14%, Sio. 25%, Mno. 65
%, PO. 008%, Cuo. 30%, AIO. 03%
, Nbo. SO.025% basic ingredients. 001~0.0
Figure 2 is a chart showing the influence of the content ratio of Ca and S on hydrogen-induced cracking for steels in the range of 0.3% and 0.5SCa/S≦12. 03%, Sio. 25%, Mn
l. 15%. P.O. 008%, Cuo. 30%, AI
O. 03%, Nbo. 025%, VO. 035% basic ingredients, SO. 0001~0.003%3SCa/S≦
Figure 3 is a diagram showing the influence of non-metallic inclusions on hydrogen-induced cracking for steels in the range of Sio. 25%, P.O.
．． 008%, SO. 001%, Cuo. 30%, Cro
．． 30%, Moo. 10%, Ni0.08%, NO. 0
3%, Nb0.025%, Ca no S 3 basic components.
O. 06-0.14%, Mno. For copper in the range of 60-1.20%, C+Si%/300Nh%/2 in steel
00 Cu%/200 Cr%/200 Mo%/150 Ni
A chart showing the relationship between the value of %/60 and load stress/yield stress,
FIG. 4 is an explanatory diagram showing the state of specimen collection, and FIG. 5 is an explanatory diagram of the test specimen collected from the welded part and the load testing device. Figure 1 Figure 2 Figure 3 Figure 5 Figure 4

Claims (1)

[Claims] 1 C0.02-0.15%, Si0.01-0.50
%, Mn0.5-1.20%, P0.012% or less, S
0.003% or less, Cu0.20-0.50%, Al0
．． 01-0.10%, V0.01-0.10% and Nb
One or two of 0.01-0.05%, Ca 2≦
Contains in the range of Ca%/S%≦10, and C%+Si%
/30+Mn%/20+Cu%/20≦0.20%, the remainder substantially consists of Fe, and the cleanliness of non-metallic inclusions is 40×10^-^3% or less. Steel for line pipes with excellent hydrogen sulfide cracking resistance. 2 C0.02-0.15%, Si0.01-0.50
%, Mn0.5-1.20%, P0.012% or less, S
0.003% or less, Cu0.20-0.50%, Al0
．． 01-0.10%, V0.01-0.10% and Nb
One or two of 0.01-0.05%, Ca 2≦
In the range of Ca%/S%≦10, and further Cr0.01 to 0
．． 50%, one or two of Mo0.01-0.50%
Contains seeds and C%+Si%/30+Mn%/20+
Cu%/20+Cr%/20+Mo%/15≦0.20
%, the remainder substantially consists of Fe, and the cleanliness of non-metallic inclusions is 40 x 10^-^3% or less, and has excellent hydrogen flow cracking resistance. . 3 C0.02-0.15%, Si0.01-0.50
%, Mn0.5-1.20%, P0.012% or less, S
0.003% or less, Cu0.20-0.50%, Al0
．． 01-0.10%, V0.01-0.10% and Nb
One or two of 0.01-0.05%, Ca 2≦
In the range of Ca%/S%≦10, and further Ni0.01 to 0
．． 10% and C%+Si%/30+Mn%/
A flow-resistant material that satisfies 20+Cu%/20+Ni%/60≦0.20%, the remainder is substantially made of Fe, and the cleanliness of nonmetallic inclusions is 40×10^-^3% or less. Steel for line pipes with excellent hydrogen cracking resistance. 4 C0
．． 02-0.15%, Si0.01-0.50%, Mn
0.50-1.20%, P0.012% or less, S0.0
03% or less, Cu0.20-0.50%, Al0.01
~0.10%, V0.01~0.10%, Nb0.0
One or two of 1 to 0.05% and Cr0.01 to 0
．． 50%, one or two of Mo0.01-0.50%, Ca in the range of 2≦Ca%/S%≦10, further containing Ni0.01-0.10%, and C %+Si%
/30+Mn%/20+Cu%/30+Mn%/20+
Cu%/20+Cr%/20+Mo%/15+Ni%/
60≦0.20%, the remainder substantially consists of Fe, and the cleanliness of non-metallic inclusions is 40×10^-^3% or less, and has excellent resistance to hydrogen flow cracking. steel for line pipes.