JPS5919179B2 - Steel for line pipes with excellent wet carbon dioxide corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a line pipe steel that has excellent mechanical properties and weldability as well as corrosion resistance, particularly corrosion resistance against wet carbon dioxide gas.

Steel for line pipes that transport oil or natural gas include:
In addition to the specified mechanical properties, it must have excellent weldability.

Furthermore, as oil and gas sources containing corrosive substances such as hydrogen sulfide and carbon dioxide gas have been increasingly extracted from oil and gas sources in recent years, strict requirements have been placed on line pipes in terms of corrosion resistance.

An object of the present invention is to provide a steel having excellent corrosion resistance as a line pipe material for oil and gas, particularly containing carbon dioxide gas (hereinafter referred to as Co2) or hydrogen sulfide (hereinafter referred to as H2S).

Corrosion caused by CO2 is corrosion in a normal neutral humid environment,
It is characterized by a much faster corrosion rate than atmospheric corrosion.

Corrosion caused by CO2 is, in principle, as shown below, in that CO2 dissolved in aqueous solution participates in the corrosion reaction, increases the concentration of hydrogen ions involved in the reaction, or facilitates the replenishment of hydrogen ions, promoting corrosion. It is considered to be a corrosion phenomenon different from normal corrosion in a neutral humid environment that depends on oxygen and atmospheric corrosion. For example, American Petroli
Trainirg published by um In5titude
It is also mentioned in Book etc. that it is distinguished from corrosion caused by dissolved oxygen.

H2O-1-CO22H2C03=cod-)1:O,,:
2Mori O: ``In this way, CO2 corrosion often becomes a problem not only as general corrosion, but also as localized corrosion in areas with different thermal histories, such as welds.

CO2 corrosion in line pipes is expected to become even more serious as oil and gas resources diversify in the future, but currently the only countermeasures against this problem are injection of inhibitors, an incomplete and high running cost method. No fundamental preventive measures have been taken to improve the product itself.

Even when improving the material itself, as mentioned above, there is no point in impairing basic properties such as weldability, strength, and toughness, and for line pipe steel that is produced and used in large quantities, manufacturing costs are low. Being low is also an indispensable condition.

Based on many years of research and development of steel for line pipes and extensive production results, the present inventor has found that it fully satisfies the above conditions, is particularly resistant to CO2 corrosion, and has the ability to prevent cracking caused by H2S (hydrogen-induced steel). We have obtained a steel that exhibits high resistance to cracking.

Its composition is as follows.

C0.06% or less, S i O,03-0.50%, M
n0.10-1.20%, Cr 1.0-3.0%, N
b001~0.08%, AtO,005~0.10%
, the balance is Fe and impurities, the weld cracking susceptibility index POM expressed by the following formula is 0.23% or less, S in the impurities is 0.009% or less, and Cu6Z is 0.04% or less.

In addition to the above ingredients, one or two elements from the following three groups
It is possible to select and contain more than one species.

1st group Mo 0.02-0.20%, VO, 01-0.10
% 2nd group Zr O,005~0.10%, Ca O,0005
~0.05 Rare earth elements 0.0005~0.05% 3rd group Co O, 5~2.0%, Ti O, 005~0.1
0% This invention steel has a C
Although it exhibits comprehensively excellent properties such as corrosion resistance against O2 corrosion, mechanical properties, and weldability, the reason for limiting the content of each component is as follows.

It is preferable that C be as low as possible from the viewpoint of carbon dioxide corrosion resistance and weldability.

0.06% is the allowable upper limit.

It is preferable to suppress the content to 0.04% or less in order to prevent the formation of pearlite during rolling or normalizing and improve corrosion resistance.

In addition, with recent steel manufacturing technology, it is possible to manufacture ultra-low carbon steel with a carbon content of 0.01% or less, and it is not unlikely that such ultra-low carbon steel will also be included in the steel of this invention.

0.03% or more of Si is required as a deoxidizing agent and as a component to compensate for the decrease in strength due to low carbonization.

However, to ensure stable toughness, the content should be 0.5% or less, preferably 0.3% or less.

Mn is effective in improving strength and toughness, and must be contained in an amount of 0.1% or more.

However, if it exceeds 1.2%, the corrosion rate due to 002 increases, so the upper limit must be kept at 1.2%.

Cr significantly reduces corrosion due to CO2 at a content of 1.0%.

It is particularly effective in reducing local corrosion at welded parts.

However, if it exceeds 3.0%, weldability will be impaired and it will be difficult to design an alloy without the risk of welding cold cracking.

Addition of a small amount of Nb has the effect of improving the strength and toughness of as-rolled steel in particular.

It also contributes to increased resistance to CO2 corrosion.

These effects become noticeable when the content is 0.01% or more, but when the content exceeds 0.08%, undesirable effects such as deterioration of the toughness of the weld zone occur.

At is added to stabilize the deoxidation of steel, and contributes to improving toughness.

0.005% is the minimum amount that ensures this effect.

On the other hand, if it exceeds 0.10%, it will cause scratches and deterioration of toughness.

In addition to the above components, the basic steel of the present invention consists of the balance Fe and impurities, and the POM value is adjusted to 0.23% or less.

It is important to strictly control S and Cu among impurities.

Since S has a negative effect on toughness and corrosion resistance,
It is better to have as few as possible.

In particular, in order to prevent local corrosion of welded parts and hydrogen-induced cracking due to H2S, it is necessary to suppress the content to 0.009% or less.

In order to further stabilize corrosion resistance, it is desirable that the content be 0.006% or less.

It is also possible to manufacture ultra-low S steel of 0.001% or less.

Cu is known as a component that improves corrosion resistance against atmospheric corrosion.

However, according to the inventor's test results (see Examples),
It has a rather large negative effect on C02 corrosion.

The figure shows the corrosion rate index C of comparative steels AI to A4 in the example.
This is a plot of u content.

It is clear that corrosion decreases as the Cu content decreases, and the corrosion rate index decreases to 10 at CuO, 0.4% or less.
It becomes 0 or less.

Based on these results, in this invention, the upper limit permissible value of Cu as an impurity was set to 0.04%.

Note that it is more preferable to suppress it to 0.02% or less.

The above-mentioned Group 1 elements are added to improve the strength and toughness of the steel without impairing its corrosion resistance.

If Mo and V are less than 0.02% and less than 0.01%, respectively, the effect cannot be obtained.

However, if Mo exceeds 0.2% and 0.10%, respectively, it will adversely affect toughness, weldability, and workability.
02 to 0.2%, ■ 0.01 to 0.10%.

Group 2 elements are added to make inclusions mainly composed of MnS spheroidal, eliminate anisotropy of the steel material, and improve toughness and elongation.

Ca and rare earth elements improve the shape of nonmetallic inclusions mainly composed of MnS and reduce anisotropy in mechanical properties.

In addition, hydrogen-induced cracking caused by trace amounts of H2S contained in oil and gas is prevented.

However, if any of them is less than 0.0005%, no effect will be obtained, and if Ca exceeds 0.05% and rare earth elements exceeds 0.05%, weldability and toughness will be impaired, so CaO00005~0.0
5%, and rare earth elements 0.0005 to 0.05%.

Although La and C8 are typical rare earth elements, they may actually be used as Mitsushi metal.

When Zr is 0.005% or more, it is effective in controlling the shape of inclusions, improves the anisotropy of mechanical properties, and improves the toughness of the weld bond.

However, if it exceeds 0.10%, the toughness will deteriorate, so the component range is set to 0.005 to 0.10%.

Group 3 elements are added mainly to improve corrosion resistance, especially corrosion resistance against wet carbon dioxide gas.

Co has a particularly great effect in reducing the rate of carbon dioxide corrosion, but if it is less than 0.5%, this effect cannot be obtained, but even if it increases beyond 2%, the effect is almost saturated, and the price of steel is not wasted. It increases to 0.5 to 2.0%.

Addition of Ti improves carbon dioxide corrosion resistance, but if it is less than 0.005% it has no effect, and if it is less than 0.10%
If it exceeds 0.005 to 0.10%, the toughness will deteriorate.
And so.

By controlling POM to a low level, weld cracking caused by hydrogen in the welded area can be suppressed.

In addition, POM exhibits good hardenability of welds, and a trace amount of H2
POM≦0.23% to prevent sulfide cracking due to S
And so.

The steel of this invention can be used as hot-worked, or after being subjected to heat treatment such as normalizing, quenching, and further resetting.

Whether or not to apply heat treatment and its conditions are determined depending on the performance required of the line pipe to be manufactured.

The line pipe may be formed by forming and welding a hot-rolled steel plate made of this invention steel, or a seamless steel pipe made by punching and elongating a billet of this invention steel.

Next, embodiments of the invention will be described.

Steel having the composition shown in Table 1 was melted in an electric furnace, formed into an ingot, and then bloomed to obtain a seamless steel pipe (4i x 8mmt) and a hot rolled steel plate (900m cell lx20mmt).

. The manufacturing methods for both seamless steel pipes and hot rolled steel sheets are as-rolled materials (indicated by the symbol A in Table 2) and quenched and tempered materials (indicated by the symbol Q in Table 2).

The heat treatment conditions were as follows: heating at 920°C for 1 hour, cooling with water, and then tempering at 600°C for 1 hour.

The steel plate was manufactured as a material for high-toughness welded large-diameter steel pipes.

The steel pipes thus produced were welded using a cellulose acetate welding rod under the conditions of circumferential welding normally used for steel pipes for transportation.

Next, let's talk about performance.

(1) Carbon dioxide corrosion performance of base material A test piece of 4 d' x 2 mm thick was cut from the base material, and the surface was polished with 320 mesh emery paper.
A circulating fluid immersion test for 500 h was conducted in artificial seawater saturated with CO2 at 80°C with a flow rate of 500 h.

This assumes corrosion of a pipeline that transports high-speed fluid containing CO2.

Comparative steels A1 to A4 are APIX-6 with varying Cu content.
For class 5 line pipes, as the Cu content increases, the corrosion rate index [(corrosion rate of target steel/corrosion of AI steel) x 100] due to carbon dioxide corrosion increases as the Cu content increases.

That is, as the Cu content increases, the corrosion resistance deteriorates.

In this sense, limiting the amount of Cu in steel is effective in improving corrosion resistance.

It is clear from the comparison in Table 2 that the corrosion resistance of the steel of the present invention, which has undergone component control, is better than that of the comparative steel containing ordinary Cr-containing steel.

When the corrosion rate index is set to 100 based on steel A1, which is a normal X-65 class line pipe material, the corrosion resistance of the steel of the present invention is 25 or less, indicating that the corrosion resistance is improved by more than 4 times. .

(2) Local corrosion resistance of welded parts In order to examine the localized corrosion of the weld heat affected zone, a 70 mmW
After descaling the surface of a 4.5m71Lt arc-shaped specimen by pickling, a circulating fluid immersion test was conducted for 500h in CO2-saturated artificial seawater at a flow rate of 2.5m/sec and 80°C, and the welding heat-affected zone was local dust.

The presence or absence of phagocytosis was evaluated.

While local corrosion occurs in the comparative steel, there is no local corrosion in the steel of the present invention.

Such localized corrosion becomes a starting point for the generation of turbulence, especially in pipelines where high-speed fluid flows, and has the effect of extremely accelerating the corrosion rate, so it is important to stop its occurrence.

(3) Resistance to sulfide cracking in the weld zone The base material is 701 cg/mrtF or less, and sulfide cracking does not occur due to the presence of a small amount of H2S coexisting with CO2.

Welds are susceptible to sulfide cracking due to thermal effects and increased strength.

In order to investigate the sulfide cracking performance due to the presence of a trace amount of H2S in the coexistence of CO2, the flow rate was set at 2.5 m/sec.
e, H2S 50ppm in 5% NaCt below 40℃,
Stress corrosion cracking tests were carried out in a fluid containing 180 ppm of CO2.

The test piece is a four-point bending test piece with a notch in which the welded part is placed in the center of the test piece, and the applied stress is 1σy of the yield point of the base metal.

As shown in Table 2, the results are as follows: No cracking occurred in the inventive steel, while cracking occurred in the comparative steel. The excellent effects of

(4) Toughness One of the performance requirements for line pipes is toughness.

Table 2 shows that the steel according to the invention has excellent toughness.

This shows the effectiveness of component element adjustment such as low S-low C.

As described above, compared to conventional steels, the steel of this invention exhibits excellent properties against corrosion cracking in environments containing CO2 or CO2 and trace amounts of H2Sk, and has excellent mechanical properties and weldability. It is clear that it is also excellent.

[BRIEF DESCRIPTION OF THE DRAWINGS] The drawing is a chart showing the influence of Cu on wet carbon dioxide corrosion in line pipe steel.

Claims (1)

[Claims] I C 0.06% or less, Si 0.03 to 0.50%,
Mn0.10-1.20%, Cr1. O~3.0%, N
b0101~0.08%, Ato, 005~0.10%
, the balance is Fe and unavoidable impurities, and the following weld cracking susceptibility index PCM value is 0.23% or less, and the impurities include S-b''-0,OO 9% or less and Cu 0.04% or less. Steel for line pipes with excellent gas corrosion resistance. 2 C0.06% or less, Si 0.03-0.5
0%, Mn0, 10-1.20%, Cr 1.0-3.
0%, NbO, 01-0.08%, A40.005
~0.10%, further Moo, 02~0.20%, Vo,
01 to 0.10% of one kind or two kinds, the balance consists of Fe and unavoidable impurities, and has the following weld cracking susceptibility index P
CM value is 0.23% or less and S in impurities is 0.00
Steel for line pipes with excellent wet carbon dioxide corrosion resistance and Cu content of 9% or less and 0.04% or less. 3C0.06% or less, SiO,03~0.50%, Mn
0.10-1.2%, Cr1.0-3.0%, Nb0
101~0.08%, AtO, 005~0.10%, further CaO, 0005~0.05%, rare earth elements 0.0
005-0.05%, ZrO, 005-0.10%
Contains one or more of the following, the remainder consists of Fe and unavoidable impurities, the following weld cracking susceptibility index PCM value is 0.23% or less, S in the impurities is 0.009% or less, and Cu is 0. Steel for line pipes with excellent wet carbon dioxide corrosion resistance of .04% or less. 4 C0.06% or less, 3i0.03-0.50%,
Mn 0.10-1.20%, Cr 1.0-3.0%
, Nb0.01~0.08%, AtO,005~0.1
0%, further Co O, 5-2.0%, T i O, 00
Contains 5 to 0.10% of one or two types, with the balance consisting of Fe and unavoidable impurities, and has the following weld cracking susceptibility index PC
M value is 0.23% or less and S-b''-0 in impurities and substances
,009% or less, and a line pipe steel having excellent wet carbon dioxide corrosion resistance and containing Cu of 0.04% or less. 5C 0.06% or less, Si 0.03-0.50%, Mn
0.10-1.20%, Cr 1.0-3.0%, N
b0901~0.08%, AtO,005~0.10%
, further M o 0.02-0.20%, V O,01-
0.10'% of one or two types and CaO,0O05~0
．． 05%, rare earth elements 0.0005-0.05%, Zr
Containing 0.005 to 0.10% of one or more types,
The balance consists of Fe and unavoidable impurities, and the following weld cracking susceptibility index PCM value is 0.23% or less and S in the impurities
Steel for line pipes that has excellent wet carbon dioxide corrosion resistance and has Cu content of 0.009% or less and Cu content of 0.04% or less. 6 C0.06% or less, Si0.03-0.50%,
Mn 0.10-1.20%, Cr 1.0-3.0%,
Nb0001~0.08%, A40.005~0.10
%, further Mo 0.02-0.20%, Vo 01-
0.10% of one or two types and Co 0.5-2.0%
, 0.005 to 0.10% of Ti, and the remainder consists of Fe and unavoidable impurities, and the following weld cracking susceptibility index PcM value is 0.23% or less and S in the impurities is 0.005 to 0.10%. Steel for line pipes with excellent wet carbon dioxide corrosion resistance, containing 0.009% or less and Cu6'0.04% or less. 7C 0.06% or less, Si 0.03-0.50%, Mn
0.10-1.20%, Cr 1.0-3.0%, N
b0801~0.08%, A40.005~0.10%
, furthermore Ca0.0005-0.05%, rare earth element 0.
0005-0.05%, Z r O, 005-0.10
% and one or more of them and co 0.5-2.0%
, 0.005 to 0.10% of Ti, and the remainder consists of Fe and unavoidable impurities, and the following weld crack susceptibility index PCM value is 0.23% or less and S in the impurities is 0.005 to 0.10%. Steel for line pipes with excellent wet carbon dioxide corrosion resistance, with Cu#Z of 0.09% or less and Cu#Z of 0.04% or less. 8 C0.06% or less, SiO,03-0.50%,
Mn 0.10-1.20%, Cr1.0-3.0%,
Nb0.01~0.08%, AtO,005~0.10
%, further Mo 0.02-0.20%, Vo, 01-0
．． 10% of one or two kinds and Ca O,0005-0.
05%, rare earth elements 0.0005-0.05%, Zr0
．． 005 to 0.10% and one or more kinds of Co
0.5-2.0%, TiO, 0.005-0.10% or two, the balance is Fe and unavoidable impurities, and the following weld crack susceptibility index PCM value is 0.23% or less. And S in impurities is 0.009% or less, Cu is 0.0%
Steel for line pipes with excellent wet carbon dioxide corrosion resistance of 4% or less.