JPS5945753B2 - Steel for line pipes with excellent hydrogen sulfide cracking resistance and its manufacturing method - Google Patents

Description

Detailed Description of the Invention The present invention is a hydrogen sulfide resistant (H2
S) This relates to line pipe steel with excellent crackability and its manufacturing method.

In recent years, due to the tight energy situation, large amounts of H2S,
As so-called sour gas fields and sour oil fields are being actively developed, the demand for line pipes for transporting the produced gas and crude oil is also increasing.

However, in such line pipes for transporting sour gas and sour crude oil, cracks may occur due to wet H2S, and there is a risk of destruction.

Hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSCC) are known as cracks that occur in steel materials due to wet H2S.

Since HIC occurs regardless of material strength and even in the absence of external stress, it is a major problem even in relatively low-strength materials such as line pipes.

HIC is a type of hydrogen embrittlement in which hydrogen generated by corrosion in an environment containing H2S enters the steel, accumulates at the interface of non-metallic inclusions, and is caused by the internal pressure. It is known that adding Cu to steel to suppress hydrogen intrusion and controlling the shape of nonmetallic inclusions by treating with Ca and rare earth metals are effective.

On the other hand, 5scc is a crack that occurs when stress is applied to a relatively high-strength material, and even in line pipes, it becomes a problem in welded parts where hardened structures are likely to occur.It is generally known that reducing material strength is effective in preventing 5scc. It is being

By the way, recent trends in line pipes include an increasingly strong demand for thick walls and high strength to increase operating pressure and transport efficiency, and for high toughness for use in cold regions. It is often incompatible with breakability.

Generally, the higher the strength, the greater the susceptibility to H2S cracking, so it is difficult to prevent H2S cracking in high-strength materials by simply combining conventional techniques.

In order to increase the strength, various alloying elements are usually used, but some of these elements are harmful from the viewpoint of H2S cracking resistance, and the hardness of the weld increases, making it difficult to prevent 5scc of the weld.

Furthermore, line pipes are manufactured and installed by welding, and high-strength materials are prone to low-temperature cracking during welding, and prevention of such cracking is an important issue.

The present invention provides a steel material that maintains the strength, toughness, and weldability necessary for line pipe materials and has excellent cracking resistance in a humid H2S environment, and a method for manufacturing the same.
Targeting line pipe materials with high strength in the ≦H2S Provides a steel material that has crackability.

Conventionally, it has been known that adding a small amount of Cu or suppressing the shape of inclusions through Ca treatment is effective in improving H2S cracking resistance in a sour environment. In this case, it becomes difficult to completely prevent cracking no matter what countermeasures are taken.

This means that the higher the material strength, the higher the H2S cracking susceptibility (
This is due to the general tendency to increase the strength, but furthermore, increasing the amount of alloying elements added in order to achieve high strength and high toughness has a large influence.

For example, Mo and Ni are elements that are often used to achieve high strength and high toughness, but these do not impair the effect of suppressing hydrogen penetration by adding a small amount of Cu, but they degrade H2S cracking resistance.

Furthermore, an increase in alloying elements increases the segregation when they are segregated in the steel, which tends to lead to the formation of low-temperature transformation products that are highly susceptible to cracking, thereby degrading the H2S cracking resistance.

Furthermore, since it causes an increase in the hardness of the welded part, it becomes extremely difficult to prevent 5SCC.

In view of the above situation, the present inventors have developed a method for achieving high strength and toughness and H resistance.
As a result of repeated basic studies in order to achieve both 2S crackability, we have optimized the combination of alloy components in the steel and the range of those components, and have also improved rolling and heat treatment to reduce segregation in the slab. It was discovered that the purpose could be achieved by combining the two.

Specifically, C<0.05%, Si 0.50%, Mn
1.0-2-0%, P<;: 0.030%, S 0.
002%, CuO, 20~0.80%, Cr<1.0
%, N i <, 1.0%, Mo <0.05%, A
Io, 01-0.1%, N 0.01%, CaO900
1 to 0008%, and further contains Nb O,01 to 0,1
5%, Vo, 01-0.15%, Ti 0.01-0.
10%, Zr 0.01-0.10%, B 0.000
Contains 5 to 0.0050% of one or two kinds, the remainder substantially consists of iron and unavoidable impurities, and C, Si, M
The contents of n, Cu, Cr, Ni, Mo, V, and B are as follows (
1) It is characterized by satisfying the formula, and furthermore, a slab of steel having the above composition is heated at 1100 to 1350°C for 5 to 7
After 0% rolling, it is held at 1100 to 1350°C for 1 to 100 hours, and then final rolled to a predetermined thickness.

Next, the reason for limiting each component of the steel of the present invention will be described.

C is generally a major strengthening element for steel, but in the present invention, the upper limit is set at 0.05% from the viewpoint of H2S cracking resistance.
It is characterized by the fact that its strength is ensured by other alloying elements.

When C exceeds 0.05%, APIX, which is the object of the present invention,
Since sufficient H2S cracking resistance cannot be obtained with -60 to X-80 class high strength steels, the upper limit is set at 0.05%.

Note that lowering the amount of C is also effective in improving weldability.

Si is added as a deoxidizing agent and is also an element useful for adjusting strength, but if it exceeds 0.50%, the toughness of the steel deteriorates, so the upper limit is set at 0.50%.

Mn is an important element both as a deoxidizing agent and as a strength adjusting component.

In particular, in the present invention, it is necessary to add 1.0% or more of C to compensate for the decrease in strength caused by keeping the amount of C low.

However, if added in excess of 2.0%, toughness and weldability deteriorate, so the upper limit is set at 2.0%.

If P exceeds 0.03%, it is not favorable for toughness and weldability, and it is an element that tends to segregate, increasing the H2S cracking susceptibility of the segregated parts, so the upper limit is set at 0.03%.

Since S forms sulfide-based inclusions that become the starting point of HIC, it is necessary to reduce it as much as possible.

In the present invention, the shape of inclusions is controlled by Ca treatment in order to improve H2S cracking resistance, but the upper limit of S needs to be 0.002% in order to fully exhibit the effect.

Cu improves H2S resistance by suppressing hydrogen intrusion into steel.
It is an element that is effective in improving crackability, and if it is less than 0.20%, the effect is not sufficient, so it is necessary to add more.

On the other hand, if the content exceeds 0.80%, weldability and hot workability deteriorate, so the upper limit is set at 0.80%.

Cr is used for strength adjustment and is useful as a reinforcing element because it does not adversely affect H2S cracking resistance.

However, if it exceeds 1.0%, the toughness deteriorates, so 1.0%
The upper limit is %.

Mo is effective as a reinforcing element, but in terms of H8 cracking resistance, even if it is reduced in a very small amount, it will not harm the hydrogen intrusion suppressing effect of Cu, but since it degrades H2S cracking resistance, it should be reduced to 0.05%.
It is necessary to do the following.

Ni is an undesirable element from the viewpoint of H2S cracking resistance, but in the combination of ingredients of the present invention, there is no particular problem as long as it is 1.0% or less, so Ni is added in an amount of 1.0% or less for the purpose of improving toughness. Do the following.

AI is necessary as a deoxidizing agent and effective in refining the structure, so it is added at 0.01% or more, but if it exceeds 0.10%, it impairs the cleanliness of the steel, so the upper limit is 0.10%. .

From the viewpoint of 5SCC resistance of the welded part, N content is set to 0.01% or less because if it exceeds 0.01%, cracking tends to occur.

Ca is effective in controlling the shape of inclusions and improving resistance to H2S cracking, and it is necessary to add 0.001% or more to obtain this effect.

On the other hand, if it exceeds o, oos%, it causes cluster-like accumulation of inclusions and deteriorates the H2S cracking resistance, so this is set as the upper limit.

Nb, ■, Ti, and Zr are all effective in adjusting strength, but their upper limits are determined from the viewpoints of toughness and economical efficiency.

B is added in an amount of 0.0005% or more as necessary to improve hardenability, but if it exceeds 0.0050%, the toughness will deteriorate.

Note that Ti is preferably added in combination to further enhance the effect of B.

Finally, C, Si, Mn, Cu, Cr,
Mo.

The amount of Ni, V, and B is limited by the PCM value of equation (1) because if the PCM value exceeds 0.160, the 5 SCC resistance of the welded part deteriorates.

Sufficient H resistance by setting the PCM value to 0.160 or less
In addition to achieving 2S crackability, low-temperature cracking during welding is also prevented, and welding workability can be significantly improved.

The steel of the present invention is characterized by the above-mentioned combination of ingredients and optimization of the amount of ingredients, and the synergistic effect thereof provides a material with predetermined high strength and toughness as well as excellent H2S cracking resistance. The slab should be heated to 1100-1350℃.
After primary rolling of 5 to 70% at 1100 to 135
It is characterized by a treatment of holding at 0°C for 1 to 100 hours.

The purpose of this treatment is to homogenize the segregation of steel in the slab, and a sufficient effect cannot be obtained at temperatures below 1,100°C, while the upper limit is set at 1,350°C for economic efficiency and to avoid operational troubles. do.

Regarding the rolling reduction ratio in the primary rolling, if it is less than 5%, a sufficient effect cannot be obtained, while if it exceeds 70%, the rolling reduction ratio in the final rolling becomes too small, which is not practical and makes it difficult to obtain sufficient mechanical properties. It becomes difficult.

Regarding retention time, the effect is small if it is less than 1 hour, and 10
The reason for setting the upper limit to 0 hours is for economic reasons.

The present invention is applicable not only to line pipe materials for transporting sour gas and sour crude oil, but also to various materials such as crude oil storage tanks and refining equipment used in a humid H2S environment.

Examples of the present invention will be described below.

41 types of steel shown in Table 1 were melted and hot rolled into a 25 mm thick steel plate.

A part of the material was rolled as N, and a part was subjected to quenching and tempering treatment of heating at 920° C. for 30 minutes, water cooling, and heating and air cooling at 620° C. for 60 minutes.

Thereafter, mechanical properties, H2S cracking tests, and weld cracking tests were conducted.

H2S cracking test is 20w x 1oozx for the base metal part.
A test piece of 23 t (with one dragon cut on each surface) was cut in parallel and the HIC performance was examined.

Also, regarding the welded part after submerged welding, 15W x 115
A 1 x 5 t test piece was cut out, a notch was made in the weld bond part so that the degree of stress concentration was 2.5, and a stress equivalent to the yield stress of the steel plate was applied by four-point bending, and then the 5SCC performance was examined.

The test conditions were 2 conditions: artificial seawater saturated with H2S and 0.5% acetic acid-5% saline + H2S saturated phase, and immersion was carried out for 96 hours.

Welding cracks were investigated using a cellulose welding rod, assuming circumferential welding of pipes.

As is clear from the test results listed in Table 1, all of the steels of the present invention satisfy the strength level of APIX-60 to X-80, have good low-temperature toughness, and have excellent H2S resistance.
It shows crackability and weld cracking resistance.

On the other hand, in Takami-Sengite Steel A and B, which have a C content, both HIC and 5SCC occur under severe test conditions with low pH (acidity), and the H2S cracking resistance is not sufficient.

Furthermore, Comparative Steel B has a high PCM value, which causes cold cracking during welding.

Comparative steel C has a high amount of S (although its H2S cracking resistance is still insufficient).

Comparative steel E contains Mo in a larger amount than the range of the present invention, and the H2S cracking resistance is poor because the effect of suppressing hydrogen penetration by the addition of Cu is impaired.

F has a high N content, so the 5scc resistance of the welded part is poor.

Comparative steels G and H have a high PCM value, which causes cold cracking during welding, and shows that the 5scc resistance of the welded part is poor.

The above results are for materials that have been subjected to 30% primary rolling at 1300°C in the slab stage and then subjected to homogenization heat treatment at 1300°C for 16 hours, and are then rolled into a 25mm thick steel plate as described above. Hot rolled.

This treatment is carried out because if this treatment is not carried out, the HIC resistance performance under low pH test conditions may deteriorate slightly and minute cracks may occur.

Claims (1)

[Claims] IC: 0.05%, Si: 0.50%, Mn: 1. O
~2.0%, P 0.030%, S 0.002%, C
uO, 20-0.80%, Cr<1.0%, Ni<
1.0%, Mo <0.05%, Al O, 01-0.
10%, N 0.01%, Ca O, 001~0.00
8%, and further contains NbO, 01-0.15%, Vo,
01~0.15%, TiO, 01~0.10%, Zr
0.01-0.10%, Bo, 0005-0.005
0% of one or more kinds, the remainder substantially consists of iron and unavoidable impurities, and C, Si, Mn1Cu
,,Cr,Mo. A line pipe steel having high strength and toughness and excellent resistance to hydrogen sulfide cracking, characterized in that the contents of Ni, V, and B satisfy the following relational expression. C (%) + Si (%) / 30 + (Mn (%) + Cu (
%)+Cr(%))/20+Ni(%)/60+Mo
(%)/15+V(%)/10+5XB(%)<0.
160゜2 C: 0.05%, Si: 0.50%, Mn: 1. O
~2.0%, P 0.030%, S 0.002%, C
uO, 20-0.80%, Cr<1.0%, Ni<1.
0%, Mo <0.05%, AI 0.01-0.10
%, N 0.01%, Ca O, 001-0.008%
Contains Nb 0.01-0.15%, Vo, 0
1-0.15%, TiO, 01-0.10%, Zr
0.01-0.10%, Bo, 0005-0.005
0% of one or more kinds, the remainder substantially consists of iron and unavoidable impurities, and C, Si, Mn, Cu
, Cr, Mo, Ni, V, B content satisfies the following relational expression, continuous casting slab or blooming slab is 11
A high-strength, high-strength steel sheet that is characterized by performing primary rolling at 00-1350°C with a reduction rate of 5-70%, then heat treatment at 1100-1350°C for 1-100 hours, and then final rolling to a predetermined thickness. A method for manufacturing line pipe steel that is tough and has excellent hydrogen sulfide cracking resistance. C (%) + Si (%) / 30 + (Mn (%) 10 Cu
(%)10Cr(%))/20+Ni(%)/60+M
o (%)/15+V(%)/10+5XB(%)<:
0.160°