JPH04268043A - High cr steel for high strength line pipe excellent in weldability - Google Patents

Abstract

PURPOSE:To provide a high Cr steel for high strength line pipe having high corrosion resistance to wet carbon dioxide and wet hydrogen sulfide and excellent in impact toughness in a weld heat-affected zone. CONSTITUTION:The steel has a composition consisting of 0.02-0.08% C, <=1% Si, <=2% Mn, 11-14% Cr, 1.1-4.0% Co, 0.005-0.2% Al, N by the amount reduced to <=0.015%, and the balance Fe with inevitable impurities, or further, respective contents of P and S are reduced or, if necessary, Ni, Cu, Mo, W, V, Ti, Nb, Ta, Zr, Hf, Ca, and REM are incorporated.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a high Cr steel for high strength line pipes with excellent weldability. The present invention relates to a high-strength line pipe steel that has corrosion resistance, excellent impact toughness in the welded heat affected zone, and reduced hardness in the welded heat affected zone.

0002

[Prior Art] Oil and natural gas produced in recent years include
Cases containing a large amount of wet carbon dioxide gas are increasing. It is well known that carbon steel and low alloy steel corrode significantly in such environments. For this reason, corrosion inhibitors have traditionally been added as a corrosion prevention measure for line pipes used for transportation. However, corrosion inhibitors often lose their effectiveness at high temperatures, and in addition to the fact that the cost of adding and recovering corrosion inhibitors is enormous, they are often not applicable to submarine pipelines. Therefore,
Recently, there has been an increasing need for corrosion-resistant materials that do not require the addition of corrosion inhibitors. In addition to corrosion resistance, materials used for line pipes are required to have high strength to withstand the pressure of the transport fluid flowing inside them, and to have excellent weldability. As a typical characteristic of weldability, it is necessary that the welded part has excellent impact toughness. Furthermore, when transporting a fluid containing hydrogen sulfide, it is also required that the hardness of the welded part be low. Of course, it is also necessary that the base material has excellent impact toughness.

[0003] As a corrosion-resistant material for petroleum and natural gas containing a large amount of carbon dioxide, the use of stainless steel, which has good corrosion resistance, was first considered. For example, L. J. Klein, Corrosion '84, Paper No. 2
11, oil country tubular goods without a welded structure contain 0.1 or 0.2% C, such as AISI (American Iron and Steel Institute) 410 steel or 420 steel, which are high-strength and relatively low-cost steels. , martensitic stainless steels containing 12-13% Cr are beginning to be widely used. However, since these steels have a high C content,
It is difficult to use it as a line pipe because the welded part becomes very hard and the impact toughness of the welded part is poor. Line pipes using AISI410 steel have recently received an A.
Although it has been standardized by PI (American Petroleum Institute), for example, Masataka Suga et al., NKK Technical Report, published in 1989, Vol.
As stated in No. 29, pages 15 to 22, the problem is that the impact toughness of the on-site welded part is poor. This is because, as reported by them, the weld heat affected zone becomes a coarse ferrite-based structure.

[0004] As steel for line pipes, JP-A-61-
In Japanese Patent No. 119654, C and N are reduced, and A
1 or Ca and V, and Ni and M
Steel containing o has been proposed. However, since this steel contains a large amount of Ni, which is an expensive alloying element, it is expensive and its properties are not satisfactory.

[0005]

SUMMARY OF THE INVENTION In view of the current situation, it is an object of the present invention to provide a high Cr steel for high strength line pipes that has sufficient corrosion resistance even in a carbon dioxide environment and has excellent weldability.

[0006]

[Means for Solving the Problems] The present inventor has studied various components of martensitic stainless steel in order to achieve the above object, and has finally found the following knowledge. First, the amount of C in steel containing 11 to 14% Cr is 0.
It has been found that when N is reduced to 0.08% or less and N is reduced to 0.015% or less, the hardness of the welded part can be reduced and the corrosion resistance in carbon dioxide-containing saline solution is improved. Co is added to the steel in an amount of 1.1 to 4.
It has been found that when added in an amount of 0%, the microstructure of the weld can be substantially made into a martensite single phase without increasing the hardness of the weld, and the impact toughness of the weld can be significantly improved.

Further, the inventor of the present invention further investigated, and found that the above-mentioned Cr content was reduced by 11 to 14%, and C and N were reduced.
Of the impurities in steel containing 1.1 to 4.0% Co, reducing P and S will further improve the toughness, and adding one or both of Ni or Cu will reduce the impact of the weld heat-affected zone. that it is effective in further improving toughness; that adding one or both of Mo or W is effective in improving corrosion resistance in a wet carbon dioxide environment; It was discovered that adding one or more of Zr and Hf is effective in further improving corrosion resistance, and that adding one or more of Ca and rare earth elements is effective in improving hot workability and corrosion resistance. Ta.

The present invention has been made mainly based on the above findings, and the gist of the first invention is that
So, C0.02~0.08%, Si1% or less, Mn2%
Below, Cr11-14%, Co1.1-4.0%, Al
High Cr for high-strength line pipes with excellent weldability, characterized by containing 0.005 to 0.2%, reducing N to 0.015% or less, and the remainder consisting of Fe and unavoidable impurities.
The gist of the second invention is that the weight percent of the inevitable impurities in the steel for line pipes of the first invention is
The gist of the third invention is a high Cr steel for high-strength line pipes characterized by reducing P to 0.025% or less and S to 0.010% or less. In the line pipe steel of the second invention, further, weight%
A high C for high-strength line pipe characterized by containing one or two of 4% or less Ni and 3% or less Cu.
r steel, and the gist of the fourth invention is that in each of the line pipe steels of the first, second, and third inventions, one of Mo2% or less, W4% or less, or The gist of the fifth invention resides in a high Cr steel for high strength line pipes characterized by containing two types:
In each of the line pipe steels of the first invention, second invention, third invention, and fourth invention, V0.5% or less, T
For high-strength line pipes containing one or more of i0.2% or less, Nb0.5% or less, Ta0.2% or less, Zr0.2% or less, and Hf0.2% or less The gist of the sixth invention is high Cr steel, and the gist of the sixth invention is the first invention, the second invention, the third invention, the fourth invention, and the fifth invention.
In each line pipe steel of the invention, Ca0
．． 0.008% or less, and one or two of rare earth elements and 0.02% or less.

[0009]

[Operation] The reasons for limiting the ingredients in the present invention will be described below. C: Since C is the most stable and low-cost element that increases the strength of martensitic stainless steel, it is necessary to ensure the necessary strength, and if it is reduced excessively, it will decrease the impact toughness of the weld heat affected zone. Therefore, add 0.02% or more. On the other hand, if added in excess of 0.08%, the impact toughness of the weld heat-affected zone decreases and the hardness of the weld heat-affected zone increases significantly, so the upper limit content is 0.08%.
It should be 0.8%.

[0010]Si: An element necessary for deoxidation,
If added in an amount exceeding 1%, the impact toughness is reduced, so the upper limit content is 1%. Mn: An effective element for deoxidizing and ensuring strength, but if added in an amount exceeding 2%, the effect is saturated, so the upper limit content is set to 2%. Cr: Cr is the most basic and essential element constituting martensitic stainless steel, and is necessary to provide corrosion resistance in a carbon dioxide environment, but if the content is less than 11%, the corrosion resistance will deteriorate. On the other hand, if it is added in excess of 14%, it will be difficult to make the microstructure of the weld heat affected zone into a single phase of martensite no matter how you adjust other alloying elements, so the upper limit content should be 14%. .

[0011] Co: Co improves the impact toughness of not only the base metal of steel with reduced C and N content but also the microstructure of the weld heat-affected zone by forming a martensitic structure, and also improves the corrosion resistance in a humid carbon dioxide environment. Although it is an extremely useful element for improving health, if the content is less than 1.1%, these effects are insufficient, and if it is added in excess of 4.0%, the effect will not only be saturated, but also be harmful. Since this only increases the cost, it is limited to a range of 1.1 to 4.0%.

[0012] Al: An element necessary for deoxidation, and if the content is less than 0.005%, the effect is not sufficient.
If added in excess of 0.2%, coarse oxide inclusions will remain in the steel and reduce toughness, so the content range should be 0.2%.
0.005 to 0.2%. N: If N exceeds 0.015%, it increases the hardness of the weld heat affected zone and reduces the impact toughness of the base metal and the weld heat affected zone, so the upper limit content is 0.015%.
Should be.

The above are the basic components of the present invention, but in the present invention, the following elements can be further added or reduced as necessary to further improve the characteristics. P: Since P is an element that reduces toughness, it is preferable to reduce it to a low level, but reducing it to an excessively low level will only unnecessarily increase costs and the effect of improving properties will be saturated, which is the purpose of the present invention. The content is as low as necessary and sufficient to ensure the desired toughness.
When the steel is reduced to 0.025% or less, the toughness is further improved.

S: Like P, S is an element that reduces toughness, so it is preferable to reduce it to a too low level, but reducing it to an excessively low level will only unnecessarily increase costs and the effect of improving properties will be saturated. because there is,
When the content is reduced to 0.010% or less, which is a sufficiently small amount to ensure the toughness targeted by the present invention, the toughness is further improved.

Ni: Ni coexists with 1.1 to 4.0% Co and is effective in further improving the impact toughness of the weld heat affected zone, but even if it is added in an amount exceeding 4%, the effect is The upper limit content is set at 4% because not only does it become saturated, but it also unnecessarily increases costs and increases the hardness of the weld heat-affected zone. Cu: Cu is also effective in coexisting with 1.1 to 4.0% Co to further improve the impact toughness of the weld heat affected zone.
Even if it is added in excess of 3%, the effect will not only be saturated, but also
Since it only reduces hot workability, the upper limit content is set at 3%.

Mo: Mo coexists with 1.1 to 4.0% Co and is effective in improving corrosion resistance in a wet carbon dioxide environment, but the effect is saturated even when added in excess of 2%. In addition, the upper limit content is set at 2% because it also reduces other properties such as the toughness of the base metal and the weld heat-affected zone. W: W also coexists with 1.1 to 4.0% Co and is effective in improving corrosion resistance in a wet carbon dioxide environment, but if it is added in excess of 4%, the effect will not only be saturated. , the upper limit content is set at 4% because other properties such as the toughness of the base metal and the weld heat-affected zone are deteriorated.

[0017] V, Ti, Nb, Ta, Zr, Hf:V,
Ti, Nb, Ta, Zr, and Hf are effective elements for further improving corrosion resistance;
If added in excess of 0.2% for Nb and 0.5% for Nb and V, coarse precipitates and inclusions will be generated and the SSC resistance will decrease in an environment containing hydrogen sulfide, so the upper limit content is 0.2% for Ti, Zr, Ta, and Hf;
For Nb and V, it was set to 0.5%.

Ca and rare earth elements: Ca and rare earth elements (REM) are elements that are effective in improving hot workability and corrosion resistance. If more than 0.02% of Ca is added, coarse nonmetallic inclusions will be generated and the hot workability and corrosion resistance will be deteriorated, so the upper limit content is 0.00% for Ca.
8%, and 0.02% for rare earth elements. In the present invention, the rare earth elements refer to elements having atomic numbers of 57 to 71 and 89 to 103, and Y.

[0019] In order to heat-treat stainless steel having the above components to give it a tempered martensitic structure and impart it with a predetermined strength, it is necessary to carry out appropriate heat treatment according to the desired properties such as strength, toughness, and corrosion resistance. good. It is not the original purpose of the present invention to describe the heat treatment conditions, but for reference, the austenitizing temperature is 920 to 1100°C, and the cooling rate after austenitizing is faster than water cooling. It is preferable that the tempering temperature be 580°C or higher and AC1 temperature or lower, and the cooling rate in cooling after tempering be higher than air cooling.

The steel of the present invention can be made into a steel plate by ordinary hot rolling, and then made into a line pipe by forming and welding, or it can be made directly into a steel pipe by ordinary hot extrusion or hot rolling. is also possible. In this case, the heat treatment is preferably performed after the final shape of the steel pipe is formed, that is, after welding, straightening, etc. are completed.

[0021]

[Examples] Examples of the present invention will be described below. Stainless steel with the ingredients shown in Tables 1 and 3 is melted and hot-rolled into a 14 mm thick steel plate, which is then quenched and tempered to have a 0.2% offset yield strength of 49 kg.
/mm2 or more of martensitic stainless steel. The cooling during quenching of the steel of the present invention was done by water cooling, and the cooling during tempering was done by air cooling. Next, these steels were welded by hand to create a joint, which corresponds to on-site circumferential welding when laying line pipes. Welding heat input was 17 kJ/cm. JIS No. 4 impact test piece (full size) from the base metal and the weld heat affected zone of the welded part
were sampled and subjected to an impact test. Further, the maximum hardness of the weld heat affected zone was determined by Vickers measurement with a load of 5 kg. In addition, test pieces were taken from the base metal and subjected to corrosion tests in a humid carbon dioxide environment. For the corrosion test in a humid carbon dioxide environment, a test piece with a thickness of 3 mm, a width of 15 mm, and a length of 50 mm was used, and the test piece was immersed in a 3% NaCl aqueous solution at a carbon dioxide partial pressure of 40 atm in an autoclave at a test temperature of 120°C. The corrosion rate was calculated from the weight change before and after the test after immersion for one day. The unit of corrosion rate is mm/y, but it is generally considered that if the corrosion rate of a material in a certain environment is less than 0.1 mm/y, the material is sufficiently corrosion resistant and can be used. .

The test results are shown in Tables 2 and 4. In the impact test results in both tables, ○ indicates that the fracture surface transition temperature is -30°C or lower;
× indicates that the fracture surface transition temperature exceeded -30℃ and below 0℃, XX indicates that the fracture surface transition temperature exceeded 0℃, and ○ indicates the maximum hardness of the weld heat affected zone. is 30
Less than 0;
Less than m/y, ○ indicates corrosion rate of 0.05 mm/y or more.
Less than 10 mm/y, × indicates corrosion rate of 0.1 mm/y or more and less than 0.5 mm/y, XX indicates corrosion rate of 0.5 mm/y
Each of these represents the above. In addition, Table 1
In the comparative steel No. 29 is AISI420 steel, No. No. 30 is a 9Cr-1Mo steel, and all of them are conventional steels that have been conventionally used in a humid carbon dioxide environment.

As is clear from Tables 1 to 4, steel No. 1, which is the steel of the present invention. Nos. 1 to 28 have extremely excellent impact toughness of the base metal and weld heat affected zone, and the hardness of the weld heat affected zone is sufficiently low, even at extremely high temperatures of 120°C for a line pipe in a humid carbon dioxide environment. It can be seen that the corrosion rate is lower than the practically usable corrosion rate of 0.1 mm/y, and it has excellent corrosion resistance and weldability. On the other hand, among the comparative steels, steel No. 29-33 already has a corrosion rate of 0.1 mm even at 120°C in a wet carbon dioxide environment.
/y, and comparative steel No. 29-3
No. 4 had poor impact toughness in the base metal and weld heat affected zone, and comparative steel No. 4 had poor impact toughness in the base metal and weld heat affected zone. Nos. 29 to 33 have high hardness of the weld heat affected zone.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

[0027]

[Table 4]

[0028]

As described above, the present invention makes it possible to provide a high Cr steel for high-strength line pipes that has excellent corrosion resistance and excellent weldability in a humid carbon dioxide environment, and is suitable for industrial use. The contribution it makes to the development of the world is extremely large.

Claims (6)

[Claims] Claim 1: Contains, in weight percent, 0.02 to 0.08% of C, 1% or less of Si, 2% or less of Mn, 11 to 14% of Cr, 1.1 to 4.0% of Co, and 0.005 to 0.2% of Al. A high Cr steel for high-strength line pipes with excellent weldability, characterized in that N is reduced to 0.015% or less, and the remainder consists of Fe and unavoidable impurities. 2. The high Cr for high-strength line pipe according to claim 1, characterized in that among the inevitable impurities, P is reduced to 0.025% or less and S to 0.010% or less. steel. [Claim 3] 4% Ni by weight as an additional component
The high Cr steel for high strength line pipes according to claim 1 or 2, characterized in that it contains one or two of the following: 3% or less of Cu. Claim 4: Mo2% by weight as an additional component
The high Cr for high strength line pipe according to claim 1, 2 or 3, characterized in that it contains one or two of the following W4% or less:
steel. Claim 5: As an additional component, V0.5 in weight%
% or less, Ti0.2% or less, Nb0.5% or less, Ta0.2% or less, Zr0.2% or less, and Hf0.2% or less. The high Cr steel for high strength line pipes according to 1, 2, 3 or 4. 6. As an additional component, Ca0.
The high Cr steel for high-strength line pipes according to claim 1, 2, 3, 4 or 5, characterized in that it contains one or two of the following: 0.008% or less, and 0.02% or less of rare earth elements.