JPS5945750B2 - Steel for line pipes with excellent wet carbon dioxide corrosion resistance and weldability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention has high corrosion resistance against wet carbon dioxide gas (
Concerning line pipe steel with excellent weldability.

It is well known that steel for line pipes for transporting oil, natural gas, etc. is required to have a certain level of strength and excellent weldability.

In addition to these properties, in recent years, the influence of corrosive substances contained in oil and natural gas has become more important.

Corrosion caused by carbon dioxide gas is one of these, and steel materials that come into contact with oil or natural gas containing carbon dioxide gas and water are subject to severe corrosion due to the action of H+ generated by the reaction.

Such corrosion is said to occur when the partial pressure of carbon dioxide is 30 Psi or more, and it has also been reported that it occurs even when the partial pressure is 7 Psi or less (L, 5peel: Material Per
1976. A8. p46) is also available.

Corrosion in line pipes is expected to become an increasingly serious problem with the diversification of oil and gas resources, but current countermeasures include injection of inhibitors and dew point control, which are cumbersome and costly.

This invention improves the steel material itself for line pipes,
This was done with the aim of completely eliminating corrosion caused by the carbon dioxide gas.

As mentioned above, there are strict requirements for line pipe steel not only in terms of corrosion resistance, but also in terms of mechanical properties and weldability.

Therefore, when determining its composition, sufficient consideration must be given not only to improving one property but also to creating a steel type that is comprehensively superior and inexpensive.

The present inventor has completed the invention based on a large number of experimental results and manufacturing results regarding steel for line pipes.

That is, this invention has C0.10% or less, Si0.1
0-1.00%, Mn 0.10-1.20%, Cr1
．． 0-3.0%, NbO, 01-0.08%, AIo,
01~0.10%, Bo, 0002~0.0050%,
A steel consisting of 10 to 0.015% NO, 0O, the balance Fe and unavoidable impurities, or M in addition to the above components.

0.02-0.20%, Vo, 0.01-0.10%, or Zr0.005-0.00%.
10%, CaO, 0005-0.01%, rare earth elements 0.0005-0.05%, or further contains Mo 0.02-0.20%, Vo, 01-0.10
% and VO, 01 to 0.10%, Z
rO, 005-0.10%, Ca O, 0005-0.
01%, rare earth elements 1 out of 0゜0005~0.05%
Steel containing seeds, B/N (ratio of B and N content) of 0.1 to 0.6, 80.010% or less of impurities, Cu 0.04% or less, and weldable. Cracking susceptibility index is 0
．． The key point is steel that satisfies 23% or less.

The steel of the present invention has comprehensively excellent properties due to the combination of the above components, and the effects of each component will be described below.

From the viewpoint of weldability and carbon dioxide corrosion resistance, it is desirable to have a small amount of C.

0.10% is the allowable upper limit. The decrease in strength due to the decrease in C can be compensated for by other components described below.

Therefore, C may be contained in a very small amount of 0.01% or less.

Si is required as a deoxidizing agent in an amount of 0.10% or more.

If it exceeds 1.00%, workability and toughness are adversely affected.

0.10% or more of Mn is required to improve strength and toughness.

However, when it exceeds 1.20%, the rate of corrosion due to carbon dioxide gas increases.

Cr is a component that improves resistance to carbon dioxide corrosion.

The effect becomes significant at 1.0% or more, and the higher the content, the greater the effect.

However, if it exceeds 3.0%, it will have a negative effect on toughness and weldability, so a range of 1.0 to 3.0% is preferable.

Addition of a small amount (0.01% or more) of aluminum significantly improves the strength and toughness of steel, and also contributes to increased resistance to carbon dioxide corrosion.

However, if it exceeds 0.08%, the toughness of the weld will deteriorate.

AI is added to stabilize the deoxidation of steel.

It should be added so that it remains at least 0.01%, and 0.1% or more remains.
If it exceeds 0%, it may cause flaws or a decrease in toughness (.

B (boron) is effective in improving weld bond toughness if it is contained in an appropriate ratio together with N (nitrogen), which will be described later.

If it is less than 0.0002%, the effect is small, and 0.005
If it exceeds 0%, it will lead to deterioration of the toughness of the base material.

N (nitrogen) combines with B during the cooling process after welding to generate BN.

This BN acts as a nucleus for ferrite transformation (thereby making ferrite grains finer and helping to improve the toughness of the weld bond).

This effect is small when N is less than 0.0010%, and 0.0
If it exceeds 15%, it will cause cracking of the steel ingot, so 0.001
A range of 0 to 0.015% is suitable.

Note that it is necessary to maintain the following B/N ratio appropriately within this range.

The reason why B/N is set in the range of 0.1 to 0.6 is to improve the weld bond toughness.

Since the bond part at the welding part is heated to over 1300℃,
BIN and AI are in solid solution.

During the cooling process after welding, B and N have a high diffusion rate, so they segregate at austenite grain boundaries or on the surface of nonmetallic inclusions, locally increasing their concentration, and as the temperature decreases, BN exceeds its solubility. Precipitate.

Other elements that tend to form nitrides, such as AI, have a lower diffusion rate than B and N, so they remain solidly dissolved in the matrix.

In the above mechanism, solid solution N, which is originally harmful to toughness, is not only dissolved as BN, but this BN acts as a ferrite generation nucleus and contributes to refinement of ferrite grains and improvement of toughness.

If B/N is less than 0.1, there is too much N, which increases the amount of solid solution N and deteriorates bond toughness, and if B/N exceeds 0.6, B
The effect of improving hardenability appears, and bainite, which is harmful to bond toughness, is generated.

The basic steel of the present invention consists of the above-mentioned components Q and the remainder Fe and unavoidable impurities.

Here, it is necessary to pay particular attention to P, S, and Cu among the impurities.

S generates nonmetallic inclusions mainly composed of MuS, leading to deterioration of toughness (as well as causing lamellar tear during welding, so it is better to have as little as possible).

0.010% is the allowable upper limit.

Cu is an undesirable component that reduces resistance to carbon dioxide corrosion.

Even if it is unavoidably mixed, it should be suppressed to 0.04% or less, preferably 0.02% or less.

Needless to say, the less the better.

Among the additional elements contained in addition to the above basic steel components, M
o, V are effective in improving strength and toughness without deteriorating corrosion resistance by solid solution in alloy components or precipitating carbides, but Mo is less than 0.02%, ■
If Mo is less than 0.01%, the effect cannot be obtained, and Mo
If MO exceeds 0.20% and 0.10%, it will have an unfavorable effect on toughness and weldability, so MO should be 0.02 to 0.
20%, and 0.01 to 0.10%.

Zr, Ca, and rare earth elements are effective in reducing sulfide inclusions and making sulfide inclusions spheroidal, thereby improving toughness, and contributing to improving corrosion resistance by reducing impurities.

However, Zr is 0. If the content is less than 0.5%, the effect cannot be obtained, and if it exceeds 0.10%, the toughness deteriorates.

Ca and rare earth elements are particularly effective in improving toughness and removing anisotropy of steel, but if both are less than 0.0005%, this effect cannot be obtained; If it exceeds 0.05%, weldability deteriorates, so Ca O, 0005 to 0.01%, and rare earth elements 0.005 to 0.01%. Q
OO was set at 5% to 0.05%.

Note that La and Ce are typical rare earth elements, but in practice they may be added as minsh metal.

In either case, care must be taken to keep the following PCM value below 0.23%.

This is P.

This is because when the M value exceeds 0.23%, the susceptibility to weld cracking due to hydrogen in the weld zone increases, and due to hardening, the susceptibility to sulfide cracking due to trace amounts of H2S also increases.

This invention steel can be used as it is after hot working or after being subjected to heat treatment such as normalizing, quenching and tempering.

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

Line pipes are made by forming and welding hot-rolled steel sheets made from ToΩ invention steel, or by punching and welding billets of this invention steel.
Any elongated seamless steel pipe may be used.

Example 1 Steel plate produced by melting the steel shown in Table 1 and rolling it (as rolled)
The mechanical properties and weld bond toughness were investigated.

For the weld bond toughness test, a test piece 2 was used, which was cut from a plate obtained by submerged arc welding with a plate thickness t of 181 nm and a groove angle θ of 30° at the position shown in the drawing.

The test results are listed in Table 2.

Example 2 The test method and results of testing the carbon dioxide corrosion performance of the base metal, the carbon dioxide gas local corrosion resistance of the weld, and the sulfide cracking resistance of the weld using the steel shown in Table 1 are described below. .

(1) Carbon dioxide corrosion performance of base material A test piece of 40mm x 2t was cut from the base material, polished with No. 320 emery, degreased, dried, and artificial seawater saturated with carbon dioxide using a closed loop testing device. flow velocity 10m/
s, after flowing for 500 hours at a liquid temperature of 80°C, the corrosion products were removed and the weight loss was measured. As an evaluation method, it is expressed as the corrosion rate when the amount of corrosion of steel 1 of comparison steel (conventional steel) is set as 100. The results are shown in Table 3.

As is clear from Table 3, compared to the comparative steel, this invention steel is superior in all of the corrosion resistance of the base metal, the corrosion resistance ratio of the welded part, and the sulfide cracking resistance.

It is clear that this invention steel has superior carbon dioxide corrosion resistance not only when compared with conventional line pipe steel but also when compared with comparative steels including Cr steel.

Furthermore, when the mechanical properties shown in Table 2 are taken into consideration, it can be seen that the steel of the present invention is extremely excellent as a steel for line pipes used under particularly severe conditions.

(2) Carbon dioxide local corrosion resistance of welded parts In order to examine the local corrosion of the weld heat-affected zone, a test piece measuring 70 mm wide x 50 mm long x 18 m 11 L thick was cut from a submerged arc weld by pickling. After descaling, a 500-hour test was conducted in CO□-saturated artificial seawater at a flow rate of 2.5 m/B and a liquid temperature of 80°C.

Table 3 shows the results of visually observing the presence or absence of corrosion and evaluating it in three stages.

In line pipes in which high-speed fluid flows, locally corroded areas serve as starting points for turbulent flow, further accelerating corrosion, so the corrosion resistance of this invention steel against localized corrosion is important.

(3) Resistance to sulfide cracking in the weld zone The base material has a resistance of rokg7'- or less, and sulfide cracking does not occur due to the presence of a trace amount of H2S coexisting with CO2.

However, welds are susceptible to sulfide cracking due to thermal effects and increased strength.

In order to investigate the sulfide cracking performance in the presence of a small amount of H2S in the coexistence of CO2 with a flow rate of 2.5 m/s, 40
°C, 50 ppm H2S in 5% NaCl, CO21
A stress corrosion cracking test was carried out in a fluid containing 80 ppm.The round test piece was a 4-point bending test piece with a notch with the welded part placed in the center of the test piece, and the applied stress was 1σY of the yield point of the base material.

[Brief explanation of drawings]

The drawing is an explanatory diagram of a test piece for weld bond toughness testing of the present invention. In the figure, 1... test piece, 2... test piece, θ
...Bevel angle, t...Plate thickness.

Claims (1)

[Claims] I C0.10% or less, Si0.10-1.00%,
Mn 0.10-1.20%, Cr 1.0-3.0%
, Nb0101~0.08%, AIo, 01~0.10
%, BO,0002~0.0050%, NO,0010
~0.015%, the balance consists of Fe and unavoidable impurities, B/N is 0.1 to 0.6, 80.010% of impurities
Hereinafter, a steel for line pipes having excellent wet carbon dioxide corrosion resistance and weldability, which has a CuO content of 0.04% or less and a weld crack susceptibility index PCM value of 0.23% or less. 2C0.10% or less, Si0.10-1.00%, Mn
0.10-1.20%, Cr 1.0-3.0%, N
b0101~0.08%, A10.01~0.10%,
BO, 0002~0.0050%, NO, 0010~0
．． 015%, further Mo 0.02-0.20%, Vo
, 01 to 0.10%, the remainder consists of Fe and unavoidable impurities, and the B/N is 0, 1 to 0.
．． 6, So in impurities, 010% or less, CuO004
% or less, and the following weld crack susceptibility index PCM value is 0.
Steel for line pipes with excellent wet carbon dioxide corrosion resistance of 23% or less and weldability. 3C0,10% or less, SiO,10-1,00%, M
n 0.10-120%, Cr 1.0-3.0%, 0.01-0.08%, AIo, 01-0.10%, B
o, 0002-0.0050%, NO, 0010-0.
015%, further ZrO, 005~0.005, CaO
,0005~0.01%, rare earth elements 0-0005~0
．． Contains one type of 05%, the remainder consists of Fe and unavoidable impurities, B/N is 0.1 to 0.6, 8 of the impurities
0.010% or less, Cu 0.04% or less, and lower = V
Steel for line pipes with excellent wet carbon dioxide corrosion resistance and weldability, with a joint cracking susceptibility index PCM value of 0.23% or less. 4C0.10% or less, Si0.10-1.00%, Mn
0.10-1.20%, Cr 1.0-3.0%, 0.01-0.08%, AIo, 01-0.10%, B
O,0Q02~0.0050%, NO,0O10~0.
015%, further Mo 0.02-0.20%, Vo, 0
One or two of 1 to 0.10% and Zr0.005
Contains one of ~0.10%, CaO,0005~0.01%, and rare earth elements 0.0005~0.05%,
The balance consists of Fe and unavoidable impurities, B/N is 0.1
~0.6, 80.010% or less in impurities, CuO,0
Below 4%, below potash = i-contact cracking susceptibility index PCM value is 0
．． Steel for line pipes with excellent wet carbon dioxide corrosion resistance of 23% or less and weldability.