JPH09228001A - Martensitic stainless steel and martensitic stainless steel pipe with excellent pitting resistance - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To provide a martensitic stainless steel and a steel pipe having excellent pitting corrosion resistance even in an environment containing carbon dioxide. SOLUTION: Single amount, Cr: 10.0 to 14.0%, Ni: 0.2 to
Martensitic stainless steel and steel pipe containing 2.0%, Cu: 0.2 to 1.0%, C 0.02% or less, N 0.03
% Or less, P value = Cr + 3Cu-3C + Ti + V + Zr +
By setting Nb + Ta to 12.2 or more, weldability and pitting corrosion resistance are improved.

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 and a martensitic stainless steel pipe suitable for use in line pipes for transporting oil and natural gas.

[0002]

2. Description of the Related Art In recent years, oil and natural gas, which can be easily drilled, are exhausted, and there is no choice but to work on wells that have severe drilling environments such as severe corrosion, deep depth, cold regions and the seabed. It's gone. The oil and natural gas produced from such wells often contains a large amount of carbon dioxide gas, and in such an environment, carbon steel or low alloy steel is significantly corroded. Inhibitors have been added as an anticorrosion measure. However, since the use of inhibitors is high in cost and the effect is insufficient at high temperatures, there is a tendency in recent years to use corrosion-resistant materials that do not require the use of inhibitors. As such a corrosion resistant material, martensitic stainless steel containing 13% of Cr is widely used in oil country tubular goods.

[0003] On the other hand, for line pipes, 12% Cr martensitic stainless steel with a reduced C content is specified in the API standard, but this steel requires preheating and postheating for circumferential welding. Due to the high cost and the drawback of poor toughness of the weld, they are rarely generally used as line pipes. For this reason, duplex stainless steel containing higher Cr and containing Ni and Mo has been used as a corrosion-resistant line pipe material because of its excellent weldability and corrosion resistance. However, duplex stainless steel has the problem that it is of excessive quality or high cost in some wells.

As a technique for overcoming the above problems, for example, Japanese Patent Application Laid-Open No. 4-99128 proposes a method for producing a martensitic stainless steel line pipe. In the same publication, in 13% Cr-based stainless steel, C and N are reduced, Cu is added in an amount of 1.2 to 4.5%, and the quenching cooling rate after pipe making is cooled at a cooling rate higher than water cooling to obtain carbon dioxide. Shows excellent corrosion resistance even in corrosive environments containing gas,
A method for manufacturing a high-strength line pipe in which the hardness of the weld heat affected zone is low, there is no problem of fire cracking, and the productivity is excellent. However, this method still has a problem that the toughness of the heat affected zone (HAZ portion) is not sufficient.

[0005]

SUMMARY OF THE INVENTION The present invention solves the above problems, has high general corrosion resistance and pitting corrosion resistance even in a corrosive environment containing carbon dioxide, and has weld cracking resistance and welded portion. It is an object of the present invention to provide a martensitic stainless steel and a martensitic stainless steel pipe having excellent toughness.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies to achieve the above-mentioned objects, and as a result, Cr, Cu, and C significantly affect pitting corrosion resistance in a corrosive environment containing carbon dioxide gas.
The present invention was constructed by newly finding that the pitting corrosion resistance can be arranged by the formula of P = Cr% + 3Cu% -3C%. That is,
The present invention is, by weight%, C: 0.02% or less, Si: 0.5% or less, Mn: 0.8-3.0%, Cr: 10.0-14.0%, Ni: 0.2-.
2.0%, Cu: 0.2 to 1.0%, N: 0.03% or less, a P value represented by the following formula (1) of 12.2 or more, and the balance Fe and unavoidable impurities. It is a martensitic stainless steel with excellent pitting corrosion resistance.

P value = (Cr%) + 3 (Cu%)-3 (C%) (1) Further, in% by weight, C: 0.02% or less, Si: 0.5% or less, M
n: 0.8-3.0%, Cr: 10.0-14.0%, Ni: 0.2-2.0
%, Cu: 0.2 to 1.0%, N: 0.03% or less, and one or two selected from Ti, V, Zr, Nb, and Ta.
Contain 0.3% or less of the total of seeds and more, and the following (2)
It is a martensitic stainless steel excellent in pitting corrosion resistance, characterized in that the P value shown by the formula satisfies 12.2 or more, and the balance is Fe and inevitable impurities.

P value = (Cr%) + 3 (Cu%)-3 (C%) + (Ti%) + (V%) + (Zr%) + (Nb%) + (Ta%) .... (2) Further, in weight%, C: 0.02% or less, Si: 0.5% or less,
Mn: 0.8-3.0%, Cr: 10.0-14.0%, Ni: 0.2-2.0
%, Cu: 0.2 to 1.0%, N: 0.03% or less, the P value represented by the following formula (1) satisfies 12.2 or more, and the balance is Fe and inevitable impurities. It is a martensitic stainless steel pipe with excellent pitting corrosion resistance.

P value = (Cr%) + 3 (Cu%)-3 (C%) (1) Further, in% by weight, C: 0.02% or less, Si: 0.5% or less, M
n: 0.8-3.0%, Cr: 10.0-14.0%, Ni: 0.2-2.0
%, Cu: 0.2 to 1.0%, N: 0.03% or less, and one or two selected from Ti, V, Zr, Nb, and Ta.
Contain 0.3% or less of the total of seeds and more, and the following (2)
It is a martensitic stainless steel pipe excellent in pitting corrosion resistance, characterized in that the P value shown by the formula satisfies 12.2 or more and the balance is Fe and inevitable impurities.

P value = (Cr%) + 3 (Cu%)-3 (C%) + (Ti%) + (V%) + (Zr%) + (Nb%) + (Ta%) .... (2)

[0011]

First, the reasons for limiting the composition of the steel of the present invention will be described. C: 0.02% or less C should be reduced as much as possible from the viewpoints of hardness reduction in the weld heat affected zone, improvement of toughness and weld cracking resistance, and enhancement of corrosion resistance and pitting corrosion resistance in environments containing carbon dioxide. Is desirable. Especially, in order to be able to weld without preheating,
C needs to be 0.02% or less, and the upper limit of C amount is
0.02% or less. It is preferably 0.015% or less from the viewpoint of ensuring better weldability.

Si: 0.5% or less Si is added as a deoxidizing agent, but since it is a ferrite-forming element, if it is contained in a large amount, ferrite is likely to be generated and the toughness of the base material and the welded portion is deteriorated. Further, in the seamless steel pipe, the presence of ferrite may hinder the production. Therefore, the amount of Si is limited to 0.5% or less. It is preferably 0.3% or less.

Mn: 0.8 to 3.0% Mn is an element that acts as a deoxidizer and further increases strength. Furthermore, since it is an austenite forming element,
It also has a function of suppressing the formation of ferrite and improving the toughness of the base material and the welded portion. In order to obtain such an effect, 0.8% or more is necessary. However, even if added over 3.0%, the effect is saturated, so the Mn content is 0.8 to 3.0%, preferably 1.1 to 2.7%.

Cr: 10.0 to 14.0% Cr is a basic element necessary to secure a martensite structure and to enhance corrosion resistance and pitting corrosion resistance in a corrosive environment containing carbon dioxide gas. To get these effects 1
It is necessary to add 0.0% or more. Further, when the content exceeds 14.0%, ferrite is easily generated, and a large amount of austenite-forming element is required to be added in order to stably obtain the martensite structure, resulting in high cost. Therefore, the Cr amount is set to 10.0 to 14.0%.

Ni: 0.2-2.0% Ni is an austenite-forming element, which suppresses the formation of ferrite and improves the toughness of the base material and the weld. Further, it improves the corrosion resistance and pitting corrosion resistance in a corrosive environment containing carbon dioxide gas. 0.2% to obtain this effect
It is necessary to add the above, but if added over 2.0%, the transformation point will decrease, the tempering temperature will decrease, and the required strength
Long tempering is required to obtain toughness. Therefore, the Ni content is set to 0.2 to 2.0%. The content is preferably 0.5 to 1.7% from the viewpoint of securing more stable corrosion resistance and tempering characteristics.

Cu: 0.2 to 1.0% Cu is an austenite-forming element together with Ni and Mn, and suppresses the formation of ferrite, and is effective in improving the toughness of the heat-affected zone of welding and the general corrosion resistance. Further, Cu has the effect of stabilizing the passive film in an environment containing carbon dioxide and chloride, and contributes to the improvement of pitting corrosion resistance. To obtain these effects, it is necessary to add 0.2% or more. But 1.0
If it is contained in excess of%, a part thereof does not form a solid solution but precipitates, which adversely affects the toughness of the weld heat affected zone. Therefore, Cu
Was in the range of 0.2 to 1.0%. In addition, preferably 0.2 ~
0.7%.

N: 0.03% or less N, like C, is preferably as low as possible from the viewpoints of hardness reduction in the weld heat affected zone, improvement in toughness in the weld heat affected zone, and weld crack resistance. If it exceeds 0.03%, weld cracking occurs and the toughness of the weld heat affected zone deteriorates. Therefore, the N content is 0.03% or less, preferably 0.02% or less.

P value = Cr% + 3Cu% -3C% + Ti% + V%
+ Zr% + Nb% + Ti%: 12.2 or more P value is an index for evaluating pitting corrosion resistance in an environment containing carbon dioxide gas. When this P value is 12.2 or more, carbon dioxide gas partial pressure
No pitting corrosion occurs even in a harsh environment of 3.0 MPa and 20% NaCl solution. If the P value is less than 12.2, pitting corrosion occurs, so 12.2 was made the lower limit. In addition, preferably 12.2 ~
14.2. If the P value is too high, it may be difficult to secure the martensite structure.

One or two of Ti, V, Zr, Nb and Ta
Total of species or more: 0.3% or less Ti, V, Zr, Nb, and Ta have a strong affinity for C and a strong tendency to form carbides. One or two of Ti, V, Zr, Nb, Ta
At least one species is added to replace Cr carbide with Ti, V, Zr, Nb, and Ta carbides. As a result, the amount of Cr carbide is reduced, and the effective amount of Cr effective for corrosion resistance, especially pitting corrosion resistance, can be increased.

Ti, V, Zr, Nb and Ta are effective in improving the toughness of the base material and the weld heat affected zone, but if the total exceeds 0.3%, the weld crack susceptibility increases and the toughness deteriorates. , 0.3% was made the upper limit. 0.01 to 0.2% for Ti alone, V
It is preferably 0.01 to 0.1% by itself, 0.01 to 0.1% by Zr alone, 0.01 to 0.1% by Nb alone, and 0.01 to 0.1% by Ta alone. When compounded, the total content is preferably 0.03 to 0.2%.

Other elements are inevitably contained, but it is desirable to reduce them as much as possible. For example, P and S can be allowed up to 0.03% and 0.01%, respectively, but should be reduced as much as possible. O is acceptable up to 0.01%. The stainless steel having the above composition is melted in a converter or an electric furnace and solidified by a continuous casting method or an ingot-making method. Molten steel ladle refining, vacuum degassing, etc. may be carried out as necessary.

The solidified steel pipe material is made into a seamless steel pipe by hot rolling such as a plug mill system or a mandrel mill system, or a steel plate is formed by hot rolling and then welded to a UOE steel pipe, an electric resistance welded steel pipe, a spiral steel pipe, etc. To do. Both the method for producing a seamless steel pipe and the method for producing a welded steel pipe can be applied to the present invention. After pipe formation, heat treatment is performed. Heat treatment is
Hardening and tempering are the basics.

Quenching is performed after heating at A _{c 3} points or more and 1000 ° C. or less,
Cool to below 200 ℃ at a cooling rate of air cooling or higher. Tempering is preferably performed at a temperature of 600 ° C. or more and an _{Ac 1} point or less. After heating and holding at the above temperature, water cooling or air cooling is performed.

[0024]

【Example】

(Example 1) Steels having compositions shown in Table 1 were melted, continuously forged into slabs, and then hot rolled into steel plates having a thickness of 15 mm. Then, after heating to 900 ° C. and quenching by air cooling, it was _tempered at 680 ° C. below the _{Ac 1} point.

These steel sheets were subjected to a diagonal Y-shaped weld cracking test at 30 ° C. with preheating according to JIS Z 3158 to evaluate the weld cracking property. Those with cracks were marked with ◯, and those without cracks were marked with x. If no cross-sectional cracks were generated in this test, welded joints were prepared by welding at a heat input of 15 kJ / cm using the TIG welding method (preheating and post-heating were not performed). A Charpy impact test was performed. In the Charpy impact test, a JIS No. 4 test piece was sampled from the heat-affected zone (notch position: 1 mm from the bond) and tested at 0 ° C. to measure the absorbed energy.

Further, a carbon dioxide corrosion test of the base material was conducted on all the steel sheets to evaluate the pitting corrosion resistance and the general corrosion resistance.
In the carbon dioxide gas corrosion test, a specimen of 3.0 mm × 25 mm × 50 mm collected from a base material was immersed in a 20% aqueous NaCl solution saturated with 3.0 MPa of carbon dioxide in an autoclave, and kept at 80 ° C. for 7 days. . Regarding the pitting corrosion resistance, after the test, the test piece was washed with water and dried, and then the surface of the test piece was observed with the naked eye to examine the presence or absence of pitting corrosion. When one or more pits occurred, the evaluation was x, and the others were evaluated as ○.

The general corrosion resistance is determined by washing the test piece with water after the test.
After drying, the weight was measured, and the weight reduction rate was converted into the thickness reduction amount in one year for evaluation. Table 2 shows the test results.

[0028]

[Table 1]

[0029]

[Table 2]

In the examples of the present invention, no weld cracks were observed even at 30 ° C. preheating, and they have excellent weld crack resistance. Further, it can be seen that the HAZ portion also has an absorbed energy at 0 ° C. of 180 J or more and has a high toughness in the weld heat affected zone. Furthermore, no pitting corrosion was observed and the corrosion rate was 0.1.
mm / year or less, which is a practically usable level and has excellent pitting corrosion resistance and general corrosion resistance.

Steel plates outside the scope of the present invention are deteriorated in properties as compared with the examples of the present invention, such as weld cracking, low HAZ portion toughness, pitting corrosion, and high corrosion rate. (Example 2) Molten steel having the chemical composition shown in Table 3 was melted in a converter and continuously cast into a steel pipe material. This steel pipe material was formed into a 273 mmφ steel pipe by plug mill rolling. After pipe forming, it was heated to 900 ° C., water-quenched, and then heated and held at 680 ° C. below the _{Ac 1} point and air cooled.

Specimens were taken from the steel pipes thus treated and subjected to mechanical properties and corrosion tests. The corrosion test conditions were the same as in Example 1. TIG welding (voltage: 16V,
A steel pipe joint was produced at an electric current of 180 A and a welding speed of 6.0 cm / min, and a HAZ portion (1 mm from the bond) was subjected to a Charpy test. Table 3 shows the results. Both have pitting resistance,
It has excellent general corrosion resistance and toughness in the heat-affected zone of welding, and has sufficient properties for line pipes.

[0033]

[Table 3]

[0034]

As described above, according to the present invention, a martensitic stainless steel which exhibits excellent pitting corrosion resistance and general corrosion resistance in an environment containing carbon dioxide gas, and which has excellent toughness and weldability of a welded portion. In addition, steel pipes for line pipes for transporting oil, natural gas, etc. can be provided at low cost, which greatly contributes to industrial development.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Tomoya Koseki, Tomoya Mizushima Kawasaki-dori, Kurashiki City, Okayama Prefecture (no address) Inside the Mizushima Works, Kawasaki Steel Co., Ltd. (72) Takaaki Toyooka, Mizushima Kawasaki-dori, Kurashiki City, Okayama Prefecture Chome (No house number) Kawasaki Steel Co., Ltd. Mizushima Steel Works

Claims (4)

[Claims] 1. C: 0.02% or less, Si: 0.5% by weight
Hereinafter, Mn: 0.8 to 3.0%, Cr: 10.0 to 14.0%, Ni: 0.2
.About.2.0%, Cu: 0.2 to 1.0%, N: 0.03% or less, and the P value represented by the following formula (1) satisfies 12.2 or more, and the balance is Fe and inevitable impurities. A martensitic stainless steel with excellent pitting resistance. Note P value = (Cr%) + 3 (Cu%)-3 (C%) ......... (1) 2. In% by weight, C: 0.02% or less, Si: 0.5%
Hereinafter, Mn: 0.8 to 3.0%, Cr: 10.0 to 14.0%, Ni: 0.2
-2.0%, Cu: 0.2-1.0%, N: 0.03% or less, and further contains one or more selected from Ti, V, Zr, Nb, and Ta in a total of 0.3% or less. And the P value expressed by the following equation (2) satisfies 12.2 or more, and the balance Fe
And martensitic stainless steel with excellent pitting corrosion resistance, which is characterized by comprising unavoidable impurities. Note P value = (Cr%) + 3 (Cu%)-3 (C%) + (Ti%) + (V%) + (Zr%) + (Nb%) + (Ta%) ……… (2) 3. In% by weight, C: 0.02% or less, Si: 0.5%
Hereinafter, Mn: 0.8 to 3.0%, Cr: 10.0 to 14.0%, Ni: 0.2
.About.2.0%, Cu: 0.2 to 1.0%, N: 0.03% or less, and the P value represented by the following formula (1) satisfies 12.2 or more, and the balance is Fe and inevitable impurities. A martensitic stainless steel pipe with excellent pitting corrosion resistance. Note P value = (Cr%) + 3 (Cu%)-3 (C%) ......... (1) 4. In% by weight, C: 0.02% or less, Si: 0.5%
Hereinafter, Mn: 0.8 to 3.0%, Cr: 10.0 to 14.0%, Ni: 0.2
-2.0%, Cu: 0.2-1.0%, N: 0.03% or less, and further contains one or more selected from Ti, V, Zr, Nb, and Ta in a total of 0.3% or less. And the P value expressed by the following equation (2) satisfies 12.2 or more, and the balance Fe
And a martensitic stainless steel pipe excellent in pitting corrosion resistance, which is characterized by comprising unavoidable impurities. Note P value = (Cr%) + 3 (Cu%)-3 (C%) + (Ti%) + (V%) + (Zr%) + (Nb%) + (Ta%) ……… (2)