JPH101752A - Martensitic stainless steel and martensitic stainless steel pipe with excellent pitting resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain excellent pitting corrosion resistance and uniform corrosion resistance in the environment containing carbon dioxide gas and chloride by providing martensitic stainless steel and steel tube with specific composition. SOLUTION: This steel is a martensitic stainless steel having a composition consisting of, by weight, <=0.02% C, <=0.5% Si, 0.8-3.0% Mn, 10.0-14.0% Cr, 0.2-2.0% Ni, 0.2-1.0% Cu, 0.03-0.07% N, further <=0.3%, in total, of one or >=2 elements selected from the group consisting of Ti, V, Zr, Nb, and Ta, and the balance Fe with inevitable impurities.

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 are easily drilled, have been exhausted. Is gone. Oil and natural gas produced from such wells often contain a large amount of carbon dioxide gas. In such an environment, carbon steel or low alloy steel is significantly corroded. Inhibitors have been added as anticorrosion measures. However, since the use of inhibitors is expensive 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, a 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. This publication discloses that carbon and carbon are reduced by reducing C and N in 13% Cr stainless steel, adding 1.2 to 4.5% of Cu, and cooling the pipe after quenching at a cooling rate higher than water cooling. Shows excellent corrosion resistance even in corrosive environments containing gases,
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]

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

[0006]

Means for Solving the Problems The present inventors have conducted intensive studies to achieve the above object, and as a result, have reduced the amount of C to 0.02% or less, added Cu, and further increased the amount of N to 0.03 to 0.07%. The present inventors newly found that the pitting corrosion resistance of the martensitic stainless steel was significantly improved even in a corrosive environment containing carbon dioxide and chloride, and constituted the present invention.

That is, according to the present invention, C: 0.02
%, Si: 0.5% or less, Mn: 0.8-3.0%, Cr: 10.0
114.0%, Ni: 0.2-2.0%, Cu: 0.2-1.0%, N:
It is a martensitic stainless steel excellent in pitting corrosion resistance, characterized by containing 0.03 to 0.07%, the balance being Fe and unavoidable impurities. Also, the present invention provides, in weight%,
C: 0.02% or less, Si: 0.5% or less, Mn: 0.8 to 3.0%,
Cr: 10.0 to 14.0%, Ni: 0.2 to 2.0%, Cu: 0.2 to 1.0
%, N: 0.03-0.07%, Ti, V, Zr, Nb,
0.3 or more of one or more selected from Ta
% Martensitic stainless steel excellent in pitting corrosion resistance, characterized by containing not more than 0.1% and the balance being Fe and unavoidable impurities.

[0008] Further, the present invention relates to a method for producing C: 0.02% by weight.
Hereinafter, Si: 0.5% or less, Mn: 0.8 to 3.0%, Cr: 10.0 to
14.0%, Ni: 0.2-2.0%, Cu: 0.2-1.0%, N: 0.
It is a martensitic stainless steel tube excellent in pitting corrosion resistance, characterized in that it contains from 03 to 0.07% and the balance is Fe and unavoidable impurities. Also, the present invention provides, in weight%,
C: 0.02% or less, Si: 0.5% or less, Mn: 0.8 to 3.0%,
Cr: 10.0 to 14.0%, Ni: 0.2 to 2.0%, Cu: 0.2 to 1.0
%, N: 0.03-0.07%, Ti, V, Zr, Nb,
0.3 or more of one or more selected from Ta
% Martensitic stainless steel pipe excellent in pitting corrosion resistance, characterized by containing not more than 0.1% by weight and the balance being Fe and unavoidable impurities.

[0009]

First, the reasons for limiting the composition of the steel of the present invention will be described. C: 0.02% or less C is for reducing the hardness of the heat affected zone, improving the toughness, improving the resistance to weld cracking, and improving the overall corrosion resistance and pitting resistance in an environment containing carbon dioxide and chloride. It is desirable to reduce as much as possible from the point. In particular, in order to be able to perform welding without preheating, C must be set to 0.02% or less.
The upper limit of the amount was 0.02%. The content 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. However, since it is a ferrite-forming element, if it is contained in a large amount, ferrite is likely to be formed, thereby deteriorating the toughness of the base material and the weld heat affected zone. Further, if ferrite is present in a seamless steel pipe, hot workability is reduced, which may hinder production. Therefore, the amount of Si is limited to 0.5% or less. Preferably
0.3% or less.

Mn: 0.8-3.0% Mn is an element that acts as a deoxidizing agent and further increases the strength. Furthermore, because it is an austenite forming element,
It also has the function of suppressing ferrite formation and improving the toughness of the base metal and the heat affected zone. In order to obtain such an effect, 0.8% or more is necessary. However, even if added over 3.0%, the effect is saturated.
Preferably, it is set to 1.1 to 2.7%.

Cr: 10.0 to 14.0% Cr is a basic element necessary to secure a martensitic structure and to enhance corrosion resistance and pitting corrosion resistance in a corrosive environment containing carbon dioxide gas. To get these effects one
It is necessary to add 0.0% or more. In addition, when the content exceeds 14.0%, ferrite is easily formed, and in order to stably obtain a martensitic structure or to prevent a decrease in hot workability, it is necessary to add a large amount of austenite forming element. , Resulting in high costs. Therefore, the amount of Cr is 1
0.0-14.0%.

Ni: 0.2 to 2.0% Ni is an austenite forming element, suppresses the formation of ferrite, improves the toughness of the base material and the weld heat affected zone,
It has a function of suppressing a decrease in hot workability. Further, the corrosion resistance and pitting corrosion resistance in a corrosive environment containing carbon dioxide are improved. To achieve this effect, 0.2% or more must be added. However, if it exceeds 2.0%, the transformation point decreases, the tempering temperature decreases, and a long time is required to obtain the specified strength and toughness. Requires tempering. Therefore, the amount of Ni is set in the range of 0.2 to 2.0%. In addition, preferably, from the viewpoint of securing more stable corrosion resistance and tempering characteristics, 0.5 to
1.7%.

Cu: 0.2 to 1.0% Cu is an austenite-forming element together with Ni and Mn, suppresses the formation of ferrite, is effective in improving the toughness of the weld heat affected zone, and improving the overall corrosion resistance. There is also an effect of suppressing a decrease in workability. Further, Cu has an effect of stabilizing a passive film in an environment containing carbon dioxide and chloride, and contributes to improvement of pitting corrosion resistance. To obtain these effects, it is necessary to add 0.2% or more. However, if it is contained in excess of 1.0%, a portion will not be dissolved and will precipitate, adversely affecting the toughness of the heat affected zone. Therefore, Cu is 0.2
~ 1.0%. Preferably, 0.3-0.7%
It is.

N: 0.03 to 0.07% N has the effect of improving the pitting corrosion resistance in an environment containing carbon dioxide and chloride. Further, since it is an austenite forming element, it is effective in suppressing the formation of ferrite and improving the toughness of the welding heat affected blast. Further, it also has a function of suppressing a decrease in hot workability. To obtain these effects, it is necessary to add 0.03% or more. However, if it exceeds 0.07%, welding cracks occur and the toughness of the heat affected zone deteriorates. The range was 0.07%. In addition, it is preferably 0.04 to 0.06%.

One or more 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 decreases, and the effective Cr amount effective for corrosion resistance, particularly pitting corrosion resistance, can be increased.

Ti, V, Zr, Nb, and Ta are effective in improving the toughness of the base metal and the heat affected zone. However, if the total exceeds 0.3%, the weld cracking susceptibility increases and the toughness deteriorates. , And the upper limit was 0.3% in total. In addition, Ti alone is 0.01 ~
0.2%, V alone 0.01-0.1%, Zr alone 0.01-0.
1%, Nb alone 0.01-0.1%, Ta alone 0.01-0.1
% Is preferred. 0.03-0.2 in total when combined
% Is preferred.

Other elements are inevitably contained, but it is desirable to reduce them as much as possible. For example, P and S are acceptable up to 0.03% and 0.01%, respectively, but are 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. Ladle refining, vacuum degassing, etc. of molten steel may be performed as necessary.

The solidified steel pipe material is made into a seamless steel pipe by hot rolling such as a plug mill method or a mandrel mill method, or into a UOE steel pipe, an electric resistance welded steel pipe, a spiral steel pipe, etc. by forming a steel sheet by hot rolling and then welding. I 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.

The heat treatment is based on quenching and tempering. After quenching, the material is heated to a temperature from the _{Ac 3} point to 1000 ° C, and then cooled to 200 ° C or less at a cooling rate higher than air cooling. 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.

[0021]

【Example】

(Example 1) A steel having a composition shown in Table 1 was melted, ingot-formed, and then hot-rolled into a steel sheet having a thickness of 15 mm. Then, A
After quenching by heating air above _c3 point, tempering below A _c1 point, yield strength and (YS) and 550 ~600MPa.

These steel sheets were subjected to an oblique Y-shaped weld cracking test at 30 ° C. according to JIS Z 3158 to evaluate weld cracking properties. Those with cracks were rated as x, and those without cracks were rated as o. 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. The Charpy impact test was performed by collecting a JIS No. 4 test piece from the heat-affected zone (notch position: 1 mm from the bond portion). ○, -40 when the fracture surface transition temperature is -40 ° C or less
The case where the temperature exceeded ℃ was indicated by x.

Further, a carbon dioxide corrosion test of the base material was performed on all the steel sheets, and the pitting corrosion resistance and the general corrosion resistance were evaluated.
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. . 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 check for pitting corrosion. When one or more pits occurred, the evaluation was x, and the others were evaluated as ○.

After the test, the test piece was washed with water,
After drying, the weight was measured, and the rate of weight loss was evaluated by converting it to the amount of thickness reduction in one year. The case where the thickness reduction amount was less than 0.1 mm / year was indicated by "O", and the case where the thickness reduction was 0.1 mm / year or more was indicated by "X".
Table 2 shows the test results.

[0025]

[Table 1]

[0026]

[Table 2]

The example of the present invention shows no cross-sectional cracking even in the oblique Y-shaped welding crack test at a preheating temperature of 30 ° C., has excellent weld cracking resistance, and has a Charpy impact test failure of the weld heat affected zone. The plane transition temperatures are all -40 ° C or less, and have excellent weld heat affected zone toughness. Further, in the examples of the present invention, no pitting corrosion was observed, and the thickness reduction was less than 0.1 mm / year, which is a practically usable level, and has excellent pitting corrosion resistance and general corrosion resistance. .

Steel sheets outside the scope of the present invention may have weld cracks.
Causes low toughness in the heat affected zone and pitting
However, compared to the present invention, such as a large thickness reduction amount, the characteristics are
Has deteriorated. (Example 2) Molten steel having the composition shown in Table 2 was converted in a converter.
It was melted and made into a steel pipe material by continuous casting. This steel pipe material
Into a 273 mmφ steel pipe by plug mill rolling. Pipe making
Later, A _c3After heating above the point, water quenching, then A_c1
The temperature was kept below the temperature and air-cooled, and the YS was adjusted to 580 MPa.

Specimens were collected from the thus treated steel pipes and subjected to mechanical properties and corrosion tests. The corrosion test conditions were the same as in Example 1. TIG welding (voltage: 15V,
Current: 200 A, welding speed: 10 cm / min, to prepare a steel pipe joint in heat input 18 kJ / cm), the weld heat affected zone portion (notch position: a 1mm) Charpy test from the bond carried out at -40 ° C., the absorption energy _V E- ₄₀ (J) was determined.

Table 3 shows the results. In the present invention example, no pitting corrosion was observed, and the thickness reduction was less than 0.1 mm / year.
It has excellent resistance to pitting corrosion and general corrosion, as well as high Charpy absorbed energy at -40 ° C in the heat affected zone of welding, and excellent toughness in the heat affected zone of welding.

[0031]

[Table 3]

[0032]

As described above, according to the present invention, excellent pitting corrosion resistance and general corrosion resistance are exhibited in an environment containing carbon dioxide gas and chloride, and the toughness and weldability of the heat affected zone are excellent. Martensitic stainless steel can be provided, and furthermore, steel pipes for line pipes for transporting oil, natural gas, etc. can be provided at low cost, which greatly contributes to industrial development.

Continuing from the front page (72) Inventor Tomoya Koseki 1-1-1, Kawasaki-cho, Handa-shi, Aichi Kawasaki Steel Corporation Chita Works (72) Inventor Takaaki Toyooka 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Kawasaki Steel Co., Ltd. Chita Works

Claims (4)

[Claims] (1) In terms of% 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
A martensitic stainless steel having excellent pitting corrosion resistance, containing up to 2.0%, Cu: 0.2 to 1.0%, and N: 0.03 to 0.07%, the balance being Fe and unavoidable impurities. 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: 0.03-0.07%, and further contains one or more selected from Ti, V, Zr, Nb and Ta in a total of 0.3% or less. A martensitic stainless steel having excellent pitting corrosion resistance, comprising a balance of Fe and unavoidable impurities. 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
A martensitic stainless steel pipe excellent in pitting corrosion resistance, containing up to 2.0%, Cu: 0.2 to 1.0%, and N: 0.03 to 0.07%, the balance being Fe and unavoidable impurities. 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: 0.03-0.07%, and further contains one or more selected from Ti, V, Zr, Nb and Ta in a total of 0.3% or less. A martensitic stainless steel pipe excellent in pitting corrosion resistance, characterized by being composed of, the balance being Fe and unavoidable impurities.