JP3033483B2 - Method for producing martensitic stainless steel welded pipe with excellent carbon dioxide gas corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a martensitic stainless steel welded pipe suitable for use as a line pipe or an oil country tubular good, particularly in a carbon dioxide gas environment.

[0002]

2. Description of the Related Art A martensitic stainless steel is usually manufactured by quenching and tempering so that the martensite volume ratio becomes 80% or more.

[0003] This martensitic stainless steel is made of C
Since the content of other expensive elements (for example, Ni and Mo) besides r is small, it is relatively cheaper than austenitic stainless steel and duplex stainless steel, and has good mechanical properties and corrosion resistance.

[0004] Also, since the required strength can be easily obtained by adjusting the heat treatment temperature, especially the tempering temperature, it has been widely used as a material for oil wells such as line pipes and oil well pipes.

Conventionally, a welded steel pipe made of martensitic stainless steel is formed by continuously passing a raw steel strip through a group of forming rolls to form a tubular shape, and butting both edges of the steel strip.
This butt portion is referred to as an electric resistance welding method (hereinafter, referred to as ERW method) or a gas tungsten arc welding method (hereinafter, referred to as G
It has been manufactured by welding and pipe forming by an arc welding method such as a TAW method or a submerged arc welding method (hereinafter, referred to as a SAW method). For example, Japanese Patent Application Laid-Open No. 4-191
Nos. 319 and 4-191320 disclose an ER using a steel strip made of low C martensitic stainless steel.
A method for producing line pipes and oil country tubular goods by W has been proposed.

However, when the martensitic stainless steel welded pipe is formed by arc welding such as the ERW method, the GTAW method or the SAW method, the cooling rate of the weld metal is low, so that the welding seam portion is not welded. A heat affected zone (hereinafter, HAZ) is formed adjacent to the metal. This HA
In Z, corrosion resistance deteriorates due to uneven distribution of elements. This is because the amount of Cr in the passive film decreases due to the precipitation of Cr at the grain boundaries, and the adhesion of the passive film and the like deteriorate. Such deterioration of the passive film immediately causes deterioration of carbon dioxide gas corrosion resistance. Therefore, if the as-welded steel pipe is used as an oil well steel pipe in a carbon dioxide gas environment without taking any measures, carbon dioxide corrosion resistance of the weld and the HAZ becomes a problem. For this reason, in the conventional welding method described above, HA
In order to improve the performance of the weld containing Z, a predetermined alloy component is added to the weld metal using a filler wire during pipe forming to improve the structure, or the entire pipe or HAZ after pipe forming is included. A predetermined post heat treatment was performed on the welded portion for the purpose of improving the structure.

However, since the HAZ adjacent to the weld metal is in a solid state even at the time of welding unlike the weld metal, the structure cannot be controlled by adding a predetermined alloy component using the filler wire. Performance cannot approach that of the base metal. When post-heat treatment is performed on the entire pipe or only the weld containing HAZ after pipe-forming welding, the performance of the HAZ can be made close to that of the base metal, but cannot be completely recovered. further,
Normally, in order to improve the structure of martensitic stainless steel, it is necessary to heat and hold at a quenching temperature of about 15 min and at a tempering temperature of about 30 min. It is necessary to increase the number of steps, and the product cost is undesirably increased.

In recent years, the application of laser welding to pipe welding has been promoted because of the improvement in productivity and the beauty of welds. Although not intended for martensitic stainless steel, for example, Japanese Unexamined Patent Publication (KOKAI) Showa No. 63-278688 discloses austenitic stainless steel.
No. 78689 is directed to ferritic stainless steel, and to JP-63-278690 is directed to Mo-containing alloy steel. By subjecting the entire pipe or a welded portion including HAZ to a predetermined post-heat treatment after laser welding pipe forming. HAZ
There has been proposed a method in which the performance of a welded portion including a base metal portion is made closer to that of a base metal portion. However, also in this case, as described above, the performance of the HAZ cannot be completely recovered only by performing the post-heat treatment, so that the carbon dioxide corrosion resistance of the HAZ is prevented from being deteriorated as compared with the base metal. Absent.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object the use of a martensitic stainless steel which is used as-welded and has excellent resistance to carbon dioxide gas corrosion in a weld containing HAZ. It is an object of the present invention to provide a method of manufacturing a steel welded pipe at low cost with good productivity by laser welding.

[0010]

The use of the laser welding method makes it possible to increase the cooling rate of the weld metal as compared with the conventional welding methods such as the ERW method, thereby preventing the HAZ from having a non-uniform element distribution.

The present inventors have proposed a method for preventing a non-uniform distribution of HAZ elements in the laser welding method and forming a passivation film exhibiting excellent corrosion resistance in HAZ when used in a carbon dioxide gas environment. Was experimentally examined, and the following was found.

(A) In order to secure the desired corrosion resistance and obtain a weld joint part which is completely penetrated without defects,
At least a Cr content of 10.0 to 15.0% by weight;
In addition, it is necessary to use a steel strip having a C content of 0.2% in impurities.

(B) Further, in order to perform full penetration welding by laser welding, the laser output is set to P (kW), the welding speed is set to V (m / min), the steel strip thickness is set to t (mm), and the laser Assuming that the preheating temperature of the steel strip edge before welding is T (° C.), laser welding must be performed under the conditions that satisfy the following equation.

0.4 ≦ P · exp (aT) / (Vt) where a is a constant obtained by experiment and 0.000
6 is assumed.

(C) Further, when used in a carbon dioxide gas environment, in order to form a passive film exhibiting excellent corrosion resistance in HAZ, the laser output is set to P (kW) in the same manner as described above. Assuming that the welding speed is V (m / min), the material strip thickness is t (mm), and the preheating temperature of the material strip edge before laser welding is T (° C.), the laser satisfies the following conditions. What needs to be welded.

5.0 ≧ P · exp (bT) / (Vt) where b is a constant obtained by experiment and 0.002
And

The gist of the present invention based on the above findings is the following method for producing a martensitic stainless steel welded pipe having excellent carbon dioxide gas corrosion resistance.

When a martensitic stainless steel welded pipe is welded by pipe-forming, it contains 10.0 to 15.0% by weight of Cr as a material and the C content in impurities is 0.2% by weight.
A martensitic stainless steel with excellent carbon dioxide gas corrosion resistance, characterized in that it is made from a steel strip made of the following martensitic stainless steel, and is laser welded under the following formula and the conditions shown in the formula to produce the product as it is. Manufacturing method of steel welded pipe.

0.4 ≦ P · exp (aT) / (Vt) ···· 5.0 ≧ P · exp (bT) / (Vt) ······ P: Laser output (kW) T: Preheating temperature of raw steel strip edge (° C) V: Welding speed (m / min) t: Raw steel strip wall thickness (mm) a: Constant (= 0.0006) b: Constant (= 0.002) In the above method of the present invention, “P · exp (aT)
/ (Vt) ”is 5.0 and“ P · exp (bT) /
(Vt) "is preferably set to 0.4.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting each condition in the present invention are as follows. In the following description, "%"
Both mean "% by weight".

<< Material Strip >> As a material strip used in the present invention, a martensitic stainless steel having a Cr content of 10.0 to 14.0% and a C content of impurities of 0.2% is used. Must be made of steel.

That is, when the Cr content is less than 10.0%, a passive film having sufficient corrosion resistance is not formed and formed on the surface of the steel including the base material portion, and when used in an environment containing carbon dioxide gas. Necessary carbon dioxide gas corrosion resistance cannot be secured. Conversely, when the Cr content exceeds 15.0%, a ferrite phase appears, causing element distribution between the martensite phase and the ferrite phase, and a passive film having sufficient corrosion resistance is formed on the steel surface as described above. It cannot be formed, and the necessary carbon dioxide gas corrosion resistance cannot be ensured when used in an environment containing carbon dioxide gas. Further, the Cr content is 15.0.
In order to obtain a desired martensite structure, a large amount of an alloying element such as Ni, which is an austenite-forming element, must be added and contained in a large amount, resulting in an increase in material cost and inexpensiveness. The economics of martensitic stainless steel are impaired. On the other hand, if the C content exceeds 0.2%, cracks occur in the welded seam after welding, and a sound penetration welded joint cannot be obtained. Therefore, the composition of the material strip used is limited as described above.

The lower limit of the C content need not be particularly defined, and may be as small as possible as long as the desired strength is obtained. However, it is preferably 0.002% or more from the viewpoint of economy.

The raw steel strip may be manufactured in any manner as long as it has sufficient corrosion resistance. However, from the viewpoint of reliably preventing the element distribution due to the precipitation of the ferrite phase or the retained austenite phase and the deterioration of the performance of the passive film due to the element distribution, it is desirable to use the one manufactured as follows. That is, it is preferable to use a material that has been subjected to quenching and tempering or annealing after hot rolling and adjusted so that the volume ratio of martensite is 80% or more.

The material strip may be any steel as long as it is composed of a martensitic stainless steel having a Cr and C content within the above range. For example, C
In addition to r and C, 1.0% or less by weight of Si, 5% or less of Mn, 0.005% or less of S, 0.04% or less of P,
Contains one or more of 7.0% or less Mo, 8.0% or less Ni, 0.1% or less Al, 0.75% or less Ti, and 2.0% or less Zr. It may be made to have been made.

Of the above elements, Mo and / or Ni
, The corrosion resistance can be further improved. When Ti or / and Zr is contained, abnormal increase in strength and deterioration of corrosion resistance due to precipitation of Cr carbide and / or V carbide can be suppressed. Further, Si and Al are usually added as a deoxidizing agent for steel, but if the content is not more than the above amount, the cleanliness of the steel can be ensured.

<< Laser Welding Conditions >> The above-mentioned material strip is continuously formed into an open pipe shape through a group of forming rolls, and both edges of the steel strip are butted with a squeeze roll. And weld pipes. At this time, in the present invention, it is necessary to perform laser welding under the following formula and the condition satisfying the formula.

That is, the laser output is P (kW), the steel strip thickness is t (mm), the welding speed is V (m / min),
When the preheating temperature of both edges of the material strip is T (° C),
It is necessary to satisfy the following expression and the expression.

0.4 ≦ P · exp (aT) / (Vt) 5.0 ≧ P · exp (bT) / (Vt) where a and b are constants and a = 0.0006,
b = 0.002, all of which are values obtained from many experimental results.

This is expressed as “P · exp (aT) / (V
If the value obtained in (t)) is less than 0.4, the heat input is insufficient, complete penetration welding becomes impossible, and a sound welded joint cannot be obtained. Also, “Pexp (b
T) / (Vt) ", the value H exceeds 5.0.
In AZ, the distribution of elements becomes non-uniform, and the corrosion resistance of the passive film formed and formed on the surface is deteriorated, and the carbon dioxide corrosion resistance of the welded portion is significantly deteriorated.

The above-mentioned "P.exp (aT) / (V
The value obtained in “t)” may be 0.4 or more, and it is not particularly necessary to set the upper limit. That is, if the value exceeds 5.0, the relationship between a and b is b> a, so that the above expression cannot be satisfied. From this expression, the upper limit (5.0) is inevitable. Because it is decided.

Similarly, the above-mentioned “P · exp (bT) / (V
t)) may be 5.0 or less, and there is no particular need to set the lower limit. That is, if the value is less than 0.4, the relationship between a and b is b> a, so that the above expression cannot be satisfied. From this expression, the lower limit (0.4) is inevitably obtained. Because it is decided.

In the present invention, as can be seen from the above and formulas, it is desirable to increase the welding speed V from the viewpoint of ensuring the carbon dioxide gas corrosion resistance of the welded portion and reducing the manufacturing cost. It is preferable to increase the laser output P.

In the present invention, it is not always necessary to preheat both edges of the steel strip.

However, when preheating is performed, a high-frequency heating means using a circular induction heating coil or a contact tip capable of locally heating used in the ERW method is arranged in front of the squeeze roll, and the input power is controlled. Then, it may be heated to a predetermined temperature. It is desirable that the preheating temperature for preheating be kept at 1300 ° C. or less. This is because when the preheating temperature exceeds 1300 ° C., δ ferrite precipitates, and a component distribution occurs between the matrix and the martensite phase, which causes deterioration of the corrosion resistance of the passive film.
Furthermore, in the present invention, the preheating temperature T of both edges of the raw steel strip when not preheating is set to 0 (zero) ° C.

[0036]

EXAMPLES Seven types having the chemical components shown in Table 1 (A to A)
G) a steel strip which had been subjected to a heat treatment of holding the steel at 900 ° C. for 15 minutes after hot rolling, cooling with water, then holding at 640 ° C. for 30 minutes, and air cooling.

[0037]

[Table 1]

These steel strips were formed into open pipes having the respective outer diameters shown in Tables 2 and 3, and a laser beam was applied from above to the butted portion of both edges of the steel strip. Laser welding was performed under the conditions shown in FIG. 3 to obtain a product as-welded.

From each of the obtained welded pipes, immersion test pieces (10 mm in width × 20 mm in length) were sampled so that the welded portion was located at the center in the width direction. At 150 ° C. with 30 atm CO ₂ -5% NaCl
Was immersed in an aqueous solution for 336 hours to investigate the corrosion rate.
The corrosion resistance of the weld containing AZ was evaluated. Further, for comparison, similar test pieces were taken from as-welded pipes formed by conventional GTAW method and ERW method, and the corrosion resistance of the HAZ-containing welds of these pipes was investigated by the same method. The results are shown in Tables 2 and 3.

[0040]

[Table 2]

[0041]

[Table 3]

As is clear from the results shown in Tables 2 and 3, the steel strips A to D whose chemical components are within the range specified in the present invention are used as the raw material strip steels, and the pipe forming welding is performed under the conditions specified in the present invention. In the present invention examples (No. 11 to No. 31), complete penetration welding was performed, and the corrosion rates including the welded portions under the above test conditions were all lower than the corrosion rate of the base material, and were equal to or higher than the base material. Good corrosion resistance to carbon dioxide.

On the other hand, a comparative example (N) in which a steel strip EF having a Cr content out of the range specified in the present invention was used as the material strip steel.
In o. 5 and No. 6), the corrosion resistance of the HAZ passive film deteriorates, and the HAZ is inferior in carbon dioxide gas corrosion resistance as compared with the base metal. In addition, a comparative example (No. 1) used for a steel strip composed of steel type G having a C content outside the range specified in the present invention.
In 7), cracks occurred in the welded seam after welding, and a sound complete penetration welded joint could not be obtained.

Further, although a steel strip made of steel type A whose chemical composition is within the range specified in the present invention is used, "P.
In the comparative examples (No. 1 to No. 4 and No. 10) in which the “exp (bT) / (Vt)” value exceeds 5.0, the non-uniform distribution of the elements of the weld metal occurs and the passive film of the HAZ is formed. The corrosion rate of the weld deteriorated, and the corrosion rate of the weld became higher than that of the base metal. In Comparative Examples (No. 8 and No. 9) in which the value of “P · exp (aT) / (Vt)” was less than 0.4, penetration welding was impossible because the heat input was too small. .

Further, in the conventional pipe welding by GTAW and ERW methods (No. 32 and No. 33), the corrosion resistance of the passive film of the HAZ deteriorates, and the corrosion rate of the welded portion is reduced by the base metal. Was higher than the corrosion rate.

[0046]

According to the method of the present invention, a martensitic stainless steel welded pipe excellent in carbon dioxide gas corrosion resistance at a welded portion can be manufactured at low cost by using it as it is.
Its industrial value is enormous.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-63-278688 (JP, A) JP-A-63-278689 (JP, A) JP-A-5-15986 (JP, A) JP-A 8- 257777 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B23K 26/00-26/18 B21C 37/08

Claims (1)

(57) [Claims] 1. A method for pipe-welding a martensitic stainless steel welded pipe, wherein the material contains Cr in an amount of 10.0 to 15.0% by weight, and a C content in impurities is 0.2% by weight.
A martensitic stainless steel with excellent carbon dioxide gas corrosion resistance, characterized in that it is made from a steel strip made of the following martensitic stainless steel, and is laser welded under the following formula and the conditions shown in the formula to produce the product as it is. Manufacturing method of steel welded pipe. 0.4 ≦ P · exp (aT) / (Vt) ···· 5.0 ≧ P · exp (bT) / (Vt) ····················· Laser Output (kW) T: Preheating temperature of material strip steel edge (° C) V: Welding speed (m / min) t: Material strip steel thickness (mm) a: Constant (= 0.0006) b: Constant (= 0.002)