JPH0953157A - High corrosion resistant martensitic stainless steel low in hardness in weld zone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a martensitic stainless steel having the hardness in the heat affected zone equal to that of the base metal and having excellent CO2 corrosion resistance. SOLUTION: This high corrosion resistant martensitic stainless steel is the one having a compsn. contg., by weight, <=0.005% C, 0.1 to 1.0% Mn, <=0.03% P, <=0.005% S, 1.5 to 5% Ni, 1 to 3% Cu and 9 to 13% Cr, also satisfying the relation of 40C+34N+Ni+0.3Cu-1.1Cr>=-10.5, moreover contg., at need, 0.5 to 2.0% Mo and one or >= two kinds among Ti, Zr, Ca and rare earth metals, and the balance substantially Fe and having a martensitic structure, and in which the hardness of the heat affected zone is low comparably to that of the base metal, or is the one obtd. by subjecting the same to tempering or two phase region tempering. Thus, the martensitic stainless steel excellent in the characteristics in the heat affected zone which is nearly impossible in the conventional steel and good in CO2 corrosion resistance can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a martensitic stainless steel having excellent CO ₂ corrosion resistance and low weld hardness.

[0002]

2. Description of the Related Art In recent years, a gas well for producing a gas containing a large amount of CO ₂ has been developed, and CO ₂ injection has been widely performed. Since corrosion is severe in such an environment, an oil country tubular good made of 13% Cr martensitic stainless steel excellent in CO ₂ corrosion resistance as represented by AISI 420 steel is used. Since line pipes that have been exposed to the surface are welded together, materials such as those used for oil country tubular goods that do not take weldability into consideration cannot be used. Therefore, higher grade duplex stainless steel line pipes are used unavoidably. However, from the viewpoint of economy, it is desired to apply a material of about 13% Cr steel to the line pipe.

In order to improve weldability, it is generally necessary to reduce C, and a martensitic stainless steel material having reduced C and improved weldability is disclosed in, for example, Japanese Patent Laid-Open No. 4-99128. Japanese Patent Laid-Open No. 4-99127 and the like are mentioned. However, these steels also have hard welds and have many problems in terms of properties. Above all, the weld heat-affected zone does not have sufficient resistance to sulfide stress cracking due to its hardness, and has not reached the level at which it can be used in place of duplex stainless steel.
Therefore, there is a need for steel that can obtain CO ₂ corrosion resistance at the operating temperature of the line pipe and a weld hardness comparable to that of the base metal.

[0004]

DISCLOSURE OF THE INVENTION The present invention is intended to solve the above-mentioned conventional problems, and by adjusting to specific components, good CO ₂ corrosion resistance and equivalent base material can be obtained. It is an object of the present invention to provide a martensitic stainless steel having a weld heat affected zone having the hardness and properties of.

[0005]

DISCLOSURE OF THE INVENTION From the results of many experiments, the present inventor has found that the CO ₂ corrosion resistance is remarkably improved by the combined addition of Cu and Ni in a specific range to high Cr steel. . Fig. 1 shows the effect of elements on CO ₂ corrosion resistance, which is based on 2 to 4% Ni for Cr,
This is a summary of the corrosion rates of steel when the amounts of Mo and Cu change. ● indicates steel containing Cu; 1 to 3%, and ○ indicates steel not containing Cu. Clearly, the steel containing Cu and Ni has higher corrosion resistance than the steel containing only Ni, and the same corrosion resistance as when adding 4% more Cr is obtained. As described below, the material must be martensite single phase. However, although the corrosion resistance is improved by increasing the amount of Cr added, it becomes difficult to form a martensite single phase. If the corrosion resistance is improved by the combined addition of Cu and Ni, the addition amount of Cr, which is a ferrite forming element, can be reduced,
Moreover, since Cu and Ni are austenite-forming elements, addition of these elements is advantageous from the viewpoint of martensite single-phase formation.

Considering the hot workability of the material, the resistance to weld cracking, the toughness of the weld heat affected zone, etc., the structure must be the martensite phase containing no δ ferrite (the austenite phase at high temperatures). Requires the addition of an austenite forming element such as Ni. The weld heat-affected zone of steel having such a structure undergoes martensite transformation irrespective of welding heat input, and therefore the hardness of that portion becomes higher than that of martensite. Since the hardness of martensite is determined by the C content, it is necessary to reduce the content of such elements in order to reduce the hardness. FIG. 2 shows the effect of the amount of C on the hardness of the weld heat affected zone obtained by the simulated heat cycle test and the hardness of the base metal after heat treatment. It was found that when the C content is reduced to a certain level, the hardnesses of the two become almost the same. Since the weld heat affected zone is heated to a high temperature, it is generally not effective to reduce the solid solution C by forming carbide. Further, it is not easy to find a condition where δ ferrite does not appear at such an extremely low C. Therefore, we have systematically conducted experiments to find such chemical composition conditions that the ultra low C content and δ ferrite do not appear. It was found that by combining these findings, it is possible to obtain a martensitic stainless steel having a low weld heat-affected zone hardness and good corrosion resistance.

The present invention is constructed based on the above findings, and the gist thereof is as follows. That is, C in weight%; 0.005% or less,
Si; 0.50% or less, Mn; 0.1 to 1.0
%, P; 0.03% or less, S; 0.00
5% or less, Cr; 8.2 to 13%, Ni;
1.5-5%, Cu; 1.0-3.0
%, Al; 0.06% or less, N; 0.0
08% or less, and Mo as required; 0.5 to 2.0
%, And the formula Cr + 1.6Mo ≧ 9 between these components.
Which satisfies the formula 40C + 34N + Ni + 0.3Cu-
1.1Cr-1.8Mo ≧ -10.5, and Ti: 0.005-0.1 if necessary.
%, Zr; one or two of 0.01 to 0.2%, and / or Ca; 0.001 to 0.02
%, REM; 0.003 to 0.4% of one or two or more of them may be contained, and the weld zone hardness is characterized by exhibiting a martensitic structure in which the balance is substantially Fe. It is a high corrosion resistant martensitic stainless steel with a low corrosion resistance. Also, heating this steel to a temperature above the Ac ₃ transformation point,
If necessary, tempering or two-phase region tempering may be performed.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. C: A strong austenite forming element which is effective for strengthening and has an effect of suppressing the formation of the δ ferrite phase. However, in order to increase the toughness of the weld heat affected zone by lowering C and further reduce the hardness of the weld heat affected zone to the level of the base metal as shown in FIG. 2, it is necessary to limit it to 0.005% or less. is there. The lower limit is not always clear, but 0.001
% Is still valid.

Si: It is added as a deoxidizing agent on steelmaking and remains. If it is contained in the steel in an amount of more than 0.50%, toughness and sulfide stress cracking resistance are deteriorated.
0.50% or less. Mn: An element that lowers the grain boundary strength and impairs crack resistance in a corrosive environment. However, MnS is formed to promote detoxification of S, and is an effective element for austenite single-phase formation, so it is added, so it is added. However, if the content is 0.
If it is less than 1%, there is no effect, and if it exceeds 1.0%, the grain boundary strength is significantly reduced, so the Mn content is 0.1 to 1.
0%.

P: Segregated at the grain boundaries to weaken the grain boundary strength and reduce the sulfide stress cracking resistance and low temperature toughness.
03% or less. S: The upper limit was made 0.005% in order to form sulfide-based inclusions and reduce hot workability and ductility.

Ni: A strong austenite-forming element, which is useful for realizing a martensitic structure and improving hot workability. Further, it has a role of increasing the toughness of the weld heat affected zone which is a low C martensitic structure. Content is 1.
If it is less than 5%, the effect is not sufficient, and if it exceeds 5%, the Ac ₁ transformation point becomes too low and the tempering becomes difficult, so the limiting range was made 1.5 to 5%. Also, C
There is an important role to improve the corrosion resistance by the combined addition with u, and for this purpose too, addition of 1.5% or more is necessary.

Similar to Al: Si, it is added as a deoxidizing agent and remains. If the content exceeds 0.06%, A
A large amount of 1N is formed and the toughness decreases. Therefore, the upper limit of the content is set to 0.06%. N: An element that is unavoidably contained in steel, but increases the hardness of the weld heat affected zone and deteriorates the toughness, so a maximum of 0.00
8%.

Mo: Similar to Cr in the next section, it has the effect of improving the CO ₂ corrosion resistance and further improving the SSC property, so it is added if necessary. If the content is less than 0.5%, there is no effect, so the content was determined to be 0.5% or more. On the other hand, even if added in a large amount, the effect saturates, and the hot deformation resistance increases and the hot workability decreases, so the upper limit is 2
%.

Cr: The most important element for improving the CO ₂ corrosion resistance. If the content of Cr alone is less than 9%, the corrosion resistance is not sufficient, while if it exceeds 13%, the δ ferrite phase is likely to be formed. Therefore, 9 ~ 13
%. Also, as mentioned above, Mo also works in the same way,
The contribution rate is 1.6 times that of Cr as a result of experimental determination. Therefore, when Mo is contained instead of Cr alone, Cr + 1.6Mo is set to 9% or more, and the lower limit of Cr is 8.2.
%.

Coexisting with Cu: Ni has the effect of enhancing corrosion resistance. If the content is less than 1.0%, the effect is not sufficient, while if it exceeds 3.0%, the hot workability is impaired.
The addition range was 1.0 to 3.0%.

The steel having the above-mentioned compositional range exhibits good CO ₂ corrosion resistance. However, with a component containing a large amount of ferrite forming elements such as Cr and Mo, a ferrite phase is formed in the weld heat affected zone and the toughness deteriorates. Therefore, it is necessary to limit the content of the ferrite forming element. From the conventional knowledge, C, N and Ni suppress the generation of ferrite phase,
r and Mo accelerate. Steel with different element concentrations was melted and the contribution rate of each was determined experimentally. As a result, I _ps = 40C + 34N + Ni-1.1Cr-1.8Mo of the following formula is obtained.
If ≧ -10.5 is satisfied, it was found that the ferrite phase was not generated and the single phase was martensite. Therefore, C, N, Ni, Cr,
Mo needs to satisfy this relationship.

Ti: dispersed as TiN or Ti oxide,
Suppresses grain growth in the weld heat affected zone and suppresses deterioration of toughness. If it is too small, it has no effect.
Precipitates and rather deteriorates the toughness. Therefore, the addition amount of Ti is set to 0.005 to 0.1%. Zr: A stable compound is formed with P, which is harmful to sulfide stress cracking resistance, and solid solution P is reduced to substantially reduce P.
Has the effect of promoting A small amount has no effect, and an excessive amount causes formation of a coarse oxide to rather reduce toughness and sulfide stress cracking resistance, so the content was made 0.01 to 0.2%.

Ca, REM: An element effective for making the inclusions spherical and harmless. If it is too small, the effect will not be obtained, and if it is too large, inclusions will increase and the resistance to sulfide stress cracking will be reduced.
2% and 0.003-0.4%.

The steel having the above components has a martensite structure after being hot-worked or after being reheated to the Ac ₃ transformation point or higher. However, since the deformability may not be sufficient with martensite as it is, tempering or two-phase region tempering is performed as necessary. In the low C steel of the present invention, the strength change is small even if tempered, but the residual stress is reduced, the elongation is improved, and the deformability is increased. When two-phase region tempering is performed, a mixed structure containing tempered martensite and martensite, and in some cases further austenite is formed, and the structure becomes finer to further improve toughness and elongation.

[0020]

Example After melting and casting the steel having the chemical composition shown in Table 1, a seamless steel pipe was manufactured by a model rolling machine and heat-treated as required. CO ₂ corrosion resistance is 40 atm CO
_The test piece was immersed in artificial seawater at 100 ° C in equilibrium with the _two gases, and the corrosion rate was measured from the corrosion weight loss. Resistance to sulfide stress cracking is adjusted by adjusting the pH to 4.5 by mixing 1N acetic acid and 1mol / l sodium acetate, and adding 1% hydrogen sulfide + 9
In a liquid saturated with a mixed gas of 9% nitrogen gas, a tensile stress equivalent to 80% of the yield strength was applied to a smooth round bar test piece (parallel part diameter 6.4 mm, parallel part length 25 mm) to give a breaking time. It was measured. At that time, the test was conducted up to 720 hours, and it can be considered that those which did not break had excellent resistance to sulfide stress cracking. In addition, a reproduction heat cycle test corresponding to a heat input of 2 kJ / mm was performed to measure hardness and transition temperature (vTrs) using a JIS No. 4 Charpy test piece. Table 2 shows the test results.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

As described above, according to the present invention, the martensitic stainless steel in which the steel composition is specified and the steel structure is specified has the same weld hardness as the base metal and excellent CO ₂ corrosion resistance. It has characteristics.

[Brief description of drawings]

[Fig. 1] C on corrosion rate in wet carbon dioxide environment
The figure which shows the influence of u and Ni compound addition.

FIG. 2 is a diagram showing the effect of the amount of C on the hardness of the heat-affected zone of welding and the hardness after tempering.

Claims (5)

[Claims] 1. C by weight%; 0.005% or less; S
i; 0.50% or less, Mn; 0.1 to 1.0%, P;
0.03% or less, S; 0.005% or less, Cr; 9 to
13%, Ni; 1.5 to 5%, Cu; 1.0 to 3.0
%, Al: 0.06% or less, N: 0.008% or less, and the formula 40C + 34N + Ni + 0.3Cu-1.1Cr ≧ -1 between the components.
A highly corrosion-resistant martensitic stainless steel with a low weld hardness, which has a relationship of satisfying 0.5 and has a martensitic structure in which the balance is substantially Fe. 2. C by weight%; 0.005% or less; S
i; 0.50% or less, Mn; 0.1 to 1.0%, P;
0.03% or less, S 2; 0.005% or less, Cr;
2 to 13%, Mo; 0.5 to 2.0%, Ni; 1.5 to
5%, Cu: 1.0 to 3.0%, Al: 0.06% or less, N: 0.008% or less, satisfy the formula Cr + 1.6Mo ≧ 9 between these components, and further formula: 40C + 34N + Ni + 0.3Cu-1.1Cr-1.
A highly corrosion-resistant martensitic stainless steel having a low weld hardness, which has a relationship of satisfying 8Mo ≧ -10.5 and has a martensitic structure in which the balance is substantially Fe. 3. The steel according to claim 1 or 2, further comprising one or two of Ti; 0.005 to 0.1% and Zr; 0.01 to 0.2% by weight. High corrosion resistance martensitic stainless steel with low weld hardness. 4. The steel according to claim 1, further comprising Ca in a weight percentage of 0.001 to 0.02.
%, REM; high corrosion-resistant martensitic stainless steel with low weld hardness, containing one or more of 0.003 to 0.4%. 5. A welded part hardened by heating the steel according to any one of claims 1, 2, 3 or 4 to a temperature not lower than the Ac ₃ transformation point and tempering or two-phase region tempering. Highly corrosion resistant martensitic stainless steel with low hardness.