JP3077576B2 - Method for producing low carbon martensitic stainless steel welded pipe - 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 low carbon martensitic stainless steel pipe excellent in sulfide stress cracking resistance and the like suitable for line pipes, oil well pipes, pipes for chemical machinery and the like.

[0002]

2. Description of the Related Art Low-carbon martensitic stainless steel has a low content of expensive elements such as Ni and Mo.
It is relatively cheaper than austenitic stainless steel and duplex stainless steel, and has good mechanical properties and corrosion resistance. Furthermore, low carbon reduces the amount of precipitation of Cr carbides and also reduces the increase in hardness when welding, so that corrosion resistance and weldability are improved as compared with general martensitic stainless steel. Therefore, low carbon martensitic stainless steel is
Widely used for oil well pipes and materials for chemical machinery.

Conventionally, in a welded pipe made of the above-mentioned low-carbon martensitic stainless steel, a steel strip, which is a material, is continuously formed through a group of forming rolls to form a tubular shape, and abutted at the edge of the steel strip. It was manufactured by welding.
Welding methods include ERW (hereinafter referred to as ERW) and gas tungsten arc welding (hereinafter GT).
AW method) or submerged arc welding method (hereinafter, referred to as SAW method) is used. For example, JP
JP-A-191319 and JP-A-4-191320 disclose a method for producing a line pipe and an oil country tubular good by the ERW method using a steel strip made of a low carbon martensitic stainless steel.

When a low carbon martensitic stainless steel welded pipe is welded by an arc welding method such as the ERW method, the GTAW method, and the SAW method, a heat-affected zone (hereinafter, referred to as a weld seam portion) is formed adjacent to the weld metal. HAZ). In the HAZ, although varying in degree, even if the carbon content is low, hardening due to solid solution of carbon and deterioration of corrosion resistance due to uneven distribution of elements occur. This is because the adhesion of the passive film containing Cr deteriorates. Such deterioration and increase in hardness of the passivation film immediately cause deterioration of sulfide stress cracking resistance. Therefore, if an as-welded steel pipe is used in an environment containing hydrogen sulfide without taking any measures, sulfide stress cracking of the HAZ occurs. Therefore, in the above-mentioned welding method, in order to improve the performance of the welded portion including HAZ, a predetermined alloy component is added to the weld metal at the time of welding by using a filler wire to improve the structure, or the hardness is reduced after pipe welding. Post-heat treatment was applied to the entire pipe or the welded part for the purpose.

[0005] However, even if an attempt is made to improve the post-heat treatment, the corrosion resistance of HAZ is not completely restored, and the sulfide stress cracking resistance of the weld containing HAZ is inferior to that of the base metal. For example, the corrosion resistance of the HAZ remains deteriorated only by performing post-heat treatment by high-frequency heating using an annular induction heating coil or a contact tip that can be locally heated.

If the post-heat treatment of the entire tube is performed in an electric furnace, the hardness and corrosion resistance of the HAZ approach those of the base material.
It is not desirable in terms of manufacturing cost to cut a long pipe that has been continuously manufactured and individually perform post-heat treatment.

In recent years, the application of the laser welding method to pipe making has been increasing because of the improvement in productivity and the beauty of the welded portion. Austenitic stainless steel (Japanese
No. 278688), a ferritic stainless steel (Japanese Unexamined Patent Publication No. 63-278689) and a Mo-containing alloy (Japanese Unexamined Patent Publication No. 63-278690) were used to determine the mechanical properties of the welded portion of a laser-welded tube. Ways to improve are presented. According to the document, it is described that it is necessary to perform post-heat treatment on the weld metal constituting the weld seam portion to recover the mechanical properties of the weld metal.

However, after a short post-heat treatment, H
AZ could not be completely softened, and the sulfide stress cracking resistance of HAZ was inevitably degraded compared to the base metal.

[0009]

SUMMARY OF THE INVENTION The present invention relates to a method for producing a low carbon martensitic stainless steel welded pipe having a beautiful welded portion having excellent resistance to sulfide stress cracking with good productivity by laser welding. The purpose is to provide.

[0010]

When a laser welding method is used to join both butted edges, a heating rate and a cooling rate of the weld metal can be increased as compared with a conventional welding method such as ERW, and therefore, the hardening width of the HAZ can be increased. Can be narrowed, and uneven distribution of elements can be prevented. But HAZ
The hardness increase is greater than in conventional welding methods. The cause of this is that in the HAZ by the conventional welding method, although there is hardening due to solid solution of carbon, the HAZ is annealed at the time of cooling due to the heat input during welding, so that the dislocation density in martensite of the HAZ decreases. In the case of the laser welding method, in HAZ, in addition to hardening of martensite due to solid solution of carbon, annealing is insufficient due to a small amount of heat input during welding, and dislocations remain in martensite at high density. .

The present investigators have studied by experiment the method of reducing the hardness of the HAZ in the laser welding described above, and have confirmed that hardening of the HAZ can be suppressed by increasing the laser output. In particular, it was confirmed that the HAZ hardly hardened if laser post-heating was performed under an output of 15 kW or more and further post-heat treatment was performed. Also, if the hardening problem is solved in this way, the high cooling rate of the weld metal specific to the laser welding method itself has the advantage of reducing the deterioration of the corrosion resistance of the coating.

[0012] In addition to these methods, if both edges are heated (preheated) before welding, the hardness of the HAZ in the product can be more effectively reduced.

In the method of the present invention, the laser output P
(KW), in addition to the welding speed V (m / m
in), the thickness of the steel strip t (mm) of the material, and the temperature T (° C.) of both edge portions before welding must satisfy the following conditions.

P ≧ 15 kW 0.4 ≦ P · {exp (a · T)} / (V · t) ≦
2 where a: constant (= 0.0006) Further, since the weld metal itself has a coarse quenched martensite structure as it is, the sulfide stress cracking resistance may be insufficient. Alternatively, it is necessary to perform one of the post heat treatments in (b).

(A) 850 to 1000 ° C. (850 ° C. or higher and 1
000 ° C or less, hereinafter the same meaning), cooled to 300 ° C or less at a cooling rate of 20 ° C / s or more, and then heated to a temperature range of 600 to 700 ° C and 20 ° C.
Cool to room temperature at a cooling rate of not more than / s.

(B) After heating to a temperature range of 700 to 900 ° C.,
Cool to room temperature at a cooling rate of 20 ° C./s or less.

The steel strip to be subjected to the laser welding is a low carbon martensitic stainless steel. C and C of the steel
The content of r must be within a predetermined range. However, the types and contents of the other alloy components may be arbitrarily selected so as not to fall outside the category of martensitic stainless steel. The following chemical composition is desirable as a steel strip that is inexpensive and has excellent corrosion resistance.

In weight%, C: 0.05% or less, Cr: 1
0 to 14%, Si: 1% or less, Mn: 0.5% or less,
P: 0.04% or less, S: 0.005% or less, Ni: 8
%, Mo: 7% or less, Al: 0.1% or less, Ti:
It contains 0.75% or less and V: 2% or less, the balance being Fe and unavoidable impurities.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting each condition in the present invention are as follows. In the following description, "%"
In each case, “% by weight” is indicated.

1. Chemical composition of strip steel The strip steel used as the raw material in the present invention is a stainless steel strip having a carbon content of 0.05% by weight or less and containing 10 to 14% by weight of Cr. In the following description, other chemical composition limitations other than the C content and the Cr content are within a desirable range.

C: The C content is preferably low. However,
Unnecessarily reducing the amount of C is accompanied by a remarkable increase in cost. If it exceeds 0.05%, Cr carbides precipitate, leading to deterioration of corrosion resistance and remarkable hardening of HAZ.
5% is the allowable upper limit.

Cr: If Cr is less than 10%, a film having corrosion resistance is not formed on the surface of the base metal and the welded portion. Therefore, when Cr is used as an oil well steel pipe in an environment containing carbon dioxide gas or hydrogen sulfide, Sulfide stress cracking easily occurs. On the other hand, if it exceeds 14%, the ferrite phase exceeds a certain limit,
Element distribution between the martensite phase and the ferrite phase occurs strongly, and the corrosion resistance of the coating deteriorates. In order to suppress the ferrite phase below a certain limit even when the Cr content exceeds 14%, it is necessary to increase the amount of expensive Ni, which is an austenite-forming element, which impairs the economics of martensitic stainless steel. Therefore, the Cr content is set to 10 to 14%.

Si: Si need not be added. However, Si is an element effective as a deoxidizing agent. Therefore, Si is added when increasing the yield of Al, which is a strong deoxidizing element, or when avoiding deterioration of cleanliness of steel due to Al oxide.
However, if it exceeds 1%, the cleanliness of the steel is degraded by the Si oxide itself, and sulfide stress cracking is induced.

Mn: Mn may not be added. However, Mn has an effect of suppressing the formation of a ferrite phase, and is added when further strengthening is desired. But,
If it exceeds 0.5%, sulfide stress cracking is promoted.

P: Since P promotes sulfide stress cracking,
It is better to be as low as possible. However, since the reduction of the amount of P necessarily involves an increase in cost, the P amount is set to be below the allowable range in performance.

If it exceeds 0.04%, the susceptibility to sulfide stress cracking becomes extremely high, so it must be limited to 0.04% or less.

S: Since S promotes sulfide stress cracking,
0.005% or less. When used in a severe environment including hydrogen sulfide, etc.
% Is desirable.

Ni: Ni may not be added. However, if the Cr content is increased beyond 14% in order to further increase the corrosion resistance, the ferrite phase exceeds a certain limit, and the corrosion resistance of the film deteriorates. Ni is added. But 8
%, The hot workability is deteriorated and the economic efficiency is reduced.

Mo: Mo may not be added. However, Mo has an effect of improving the corrosion resistance even when used alone or in the presence of Ni. Therefore, Mo is added in order to further obtain this effect. However, if it exceeds 7%, sulfide stress cracking is promoted.

Al: Al may not be added. However, since Al is extremely effective as a deoxidizing agent, it is added when strongly deoxidizing and reducing the amount of oxygen in the molten steel after deoxidation to a low level. However, if the content exceeds 0.1%, the cleanliness of the steel deteriorates, and the sulfide stress cracking resistance deteriorates.

Ti: Ti may not be added. However, Ti has an effect of preventing an abnormal increase in hardness and deterioration of corrosion resistance due to precipitation of Cr carbide. Therefore, Ti is added when these effects are to be obtained. However, if the content exceeds 0.75%, the workability is deteriorated.
5% or less.

V: V may not be added. But V
Like Ti, Ti has an effect of preventing an abnormal increase in hardness and deterioration of corrosion resistance due to precipitation of Cr carbide. Therefore, it is added when this effect is to be obtained. However, since it is an expensive element, if it exceeds 2%, economic efficiency is impaired. Therefore, even if it is added, the content is set to 2% or less.

N (nitrogen) does not need to be particularly added or reduced, but a martensitic stainless steel with a specially increased N content for improving strength or a reduced N content in consideration of workability and the like. It is of course possible to apply the method of the present invention to martensitic stainless steel.

O (oxygen) does not need to be particularly added or reduced, but it is necessary to increase the amount of O in order to disperse the oxide or to take into account mechanical properties such as fatigue.
It is perfectly acceptable to apply the method of the present invention to a reduced amount.

It is desirable that the steel strip having the above-mentioned chemical composition be subjected to ordinary quenching and tempering treatment so as to be adjusted to the following martensite phase volume ratio.

Martensitic phase: The martensitic stainless steel before laser welding, after quenching and tempering after hot rolling, preferably has a martensite phase volume ratio of about 80% or more. When the martensite phase is less than 80%, that is, when the total volume ratio of the remaining ferrite phase and the retained austenite phase and the like exceeds 20%, element distribution occurs between these phases, and the corrosion resistance of the film becomes poor. to degrade.

2. Laser Welding Conditions FIG. 1 is a drawing showing an outline of manufacturing a product from a steel strip. In the figure, a steel strip 1 is formed by a group of forming rolls 11.
To form an open pipe 2, and the edges of the steel strip are butted against each other by a squeeze roll 13 to form a butt pipe 3. Here, the “open pipe” refers to a pipe-shaped intermediate product in which both edges are not yet abutted and are kept apart. The term "butt pipe" refers to a transitional state of an intermediate workpiece immediately after passing through a squeeze roll, where both edges are butted but not joined by welding.

The temperature of both edge portions immediately before welding is from room temperature to
Must be 1000 ° C.

When welding at a temperature exceeding normal temperature, a high-frequency heating device 12 using a ring-shaped induction heating coil or a contact tip capable of locally heating used in the ERW method is arranged in front of the squeeze roll 13. And
What is necessary is just to control the input electric power and make it into a predetermined temperature range. The maximum temperature (preheating temperature) for this heating is 1000 ° C.
The following is assumed. If the preheating temperature exceeds 1000 ° C., the preheated portion is quenched and hardened after cooling, and the sulfide stress cracking resistance is deteriorated. Since the difference between the preheating temperature of the edge portion and the temperature immediately before welding is small, in this manual, the two are referred to as “preheating temperature” or “temperature before welding” without distinguishing the two.

The butted portion is irradiated with a laser beam from above by a laser beam welding machine 14 and welded. At this time, the laser output P is set to 15 kW or more (condition), and laser welding is performed under the following conditions.

0.4 ≦ P · {exp (a · T)} /
(V · t) ≦ 2 Here, t (mm) is the thickness of the strip, V (m / min)
Represents the welding speed, T (° C.) represents the temperature of both edges of the steel strip before welding, and a represents a constant (= 0.0006).

In the following, P · ｛ex in the condition
The value of p (a · T)｝ / (V · t) may be referred to as “heating index”.

When the laser output P is less than 15 kW, H
The hardness of AZ increases and the sulfide stress cracking resistance deteriorates.
On the other hand, if the heating index exceeds 2, H
The corrosion resistance of the film is deteriorated due to the hardening of the AZ and the decrease in the cooling rate, and the sulfide stress cracking resistance of the weld is significantly deteriorated. On the other hand, when the heating index is less than 0.4, the amount of heat input per unit length of welding is insufficient, so that complete penetration welding becomes impossible. Welds that are not full penetration welds are poor fusion welds and must be avoided.

The upper limit of the laser output P does not need to be particularly determined. Expression of the above heating index (= P ｛exp
As is clear from (a · T)｝ / (V · t), when the heating index is kept constant within the above range, the welding speed V can be increased as the laser output P increases, so that the productivity can be improved. To achieve this, it is desirable to increase the laser output P.

3. Post heat treatment conditions FIG. 2 is a drawing schematically showing the following post heat treatment conditions (a) and (b).

After the laser welding, the post-heat treatment of the following conditions (a) or (b) is performed on the welded portion by using the same equipment 15 as the above-mentioned preheating to obtain a product 5.

(A) After heating to a temperature range of 850 to 1000 ° C., cooling to a temperature of 300 ° C. or less at a cooling rate of 20 ° C./s or more, and then heating to a temperature range of 600 to 700 ° C.
Cool to room temperature at a cooling rate of not more than / s.

(B) After heating to a temperature range of 700 to 900 ° C.,
Cool to room temperature at a rate of 20 ° C./s or less.

If the post-heating conditions are out of these ranges, the HAZ cannot be softened, or the post-heated portions harden and harden, deteriorating the sulfide stress cracking resistance.

Under the post-heating condition (a), if the initial heating temperature is lower than 850 ° C., the austenitization is insufficient, and the volume ratio of the martensite phase after cooling (quenching) is insufficient. On the other hand, when the temperature exceeds 1000 ° C., a coarse martensite phase is formed, and sulfide stress cracking resistance is deteriorated. The cooling rate after this is set to 20 ° C./s or more because if it is less than 20 ° C./s, the corrosion resistance of the coating deteriorates. The cooling rate is preferably as high as possible, and there is no particular upper limit. In order to secure a required volume fraction of the martensite phase, cooling is performed to 300 ° C. or less. Then 600-7
The reason for heating to 00 ° C. is to soften the HAZ by tempering. If the temperature is lower than 600 ° C., the softening is insufficient and the sulfide stress cracking resistance is deteriorated. On the other hand, if the temperature exceeds 700 ° C., the corrosion resistance of the film is deteriorated. The following cooling rate of 20 ° C./s or less
If the temperature exceeds 0 ° C./s, a large residual stress is generated due to the uneven cooling. Cooling at this cooling rate is performed to room temperature to reduce the residual stress.

Although the lower limit of the cooling rate is not particularly set here, it is desirable to set the cooling rate to 3 ° C./s or more from the viewpoint of work efficiency.

Under the post-heating condition (b), if the initial heating temperature is lower than 700 ° C., the softening of the HAZ is insufficient and the sulfide stress cracking resistance is deteriorated. On the other hand, when the temperature exceeds 900 ° C., austenitization occurs halfway in the post heat treatment (b), element distribution occurs, and it does not disappear later. The subsequent cooling rate of 20 ° C./s or less is for suppressing excessive residual stress. Cooling in this cooling rate range is performed to room temperature to prevent generation of residual stress. The cooling rate here is desirably 3 ° C./s or more from the viewpoint of work efficiency.

The post-heating conditions (a) and (b) generally have no difference in performance, and are easy to apply depending on the material of stainless steel to which the present invention is applied or the available equipment specifications. One may be used.

The post-heat treatment may be performed in a batch-type heating furnace, or may be performed by a high-frequency heating device or the like.

There is no particular limitation on the holding time when heating to each of the temperature ranges (a) and (b), but it is preferable that the holding time is as short as several seconds to several tens seconds in order to improve productivity.

[0056]

EXAMPLES Table 1 is a list showing the chemical composition of the strip steel used in the experiment. The steel strip having the chemical composition of A to G shown in the same table was kept at 900 ° C. for 15 minutes, and then water-cooled, and then 640 ° C.
And then air-cooled to obtain a material for laser welding.

Tables 2 and 3 are lists summarizing welding conditions, post-heat treatment conditions, and weld characteristics obtained as a result of applying the conditions. Table 2 shows a comparative example, and Table 3 shows an example of the present invention. Each steel strip was formed into an open pipe having an outer diameter shown in these tables, the butted portion was laser-welded under the conditions shown in the table, and then subjected to a post-heat treatment to obtain a product.

[0058]

[Table 1]

[0059]

[Table 2]

[0060]

[Table 3]

The characteristics of the welds were evaluated by visually observing whether or not a sulfide stress crack test and complete penetration welding were performed.

FIG. 3 is a drawing showing a cross section in a state where stress due to four-point bending is applied in a sulfide stress cracking test. A sulfide stress cracking test piece (thickness 2 mm x width 10 mm x length 75 mm-no notch) was collected from each welded tube such that the welded portion was located at the center in the width direction. The actual yield strength (YS) of the strip by four-point bending at the center of these specimens
0.01 atm with 100% stress applied
_{(1atm = 0.1013MPa) H 2 S} -30atm
It was immersed in a 25 ° C. aqueous solution of CO ₂ -5% NaCl. After elapse of 336 hours after immersion, the presence or absence of cracks was investigated and HA
The weld properties including Z were evaluated. In addition, conventional GTAW
After welding, the entire pipe was kept at 900 ° C. for 15 minutes and then water-cooled, and then kept at 640 ° C. for 30 minutes and air-cooled. further,
900W by high frequency heating after pipe making by ERW method
A welded tube cooled to 300 ° C. or lower at a rate of 30 ° C./s after holding at 30 ° C. for 30 seconds, and then cooled at a rate of 6 ° C./s after holding at 640 ° C. for 30 seconds. These GTs
Table 2 also shows the results of the welded pipe by the AW method and the ERW method as the conventional method.

As is clear from the results shown in Tables 2 and 3, pipe steels A, B, C and D whose chemical compositions are within the scope of the present invention were used for pipe welding under the conditions within the scope of the present invention. In the present invention examples (Nos. 16 to 36), no cracks were generated under the test conditions, and good characteristics equivalent to those of the base metal were exhibited.

On the other hand, comparative examples using strip steels E, F and G having compositions outside the range of the present invention (No. 10 to No. 1)
In 2), hardening of the HAZ or deterioration of the corrosion resistance of the film occurred, and sulfide stress cracking occurred.

In Comparative Examples (No. 1 to No. 5) in which the laser output P was less than 15 kW, the HAZ was significantly hardened and sulfide stress cracking occurred. Similarly, when the laser output P is 1
In a comparative example (No. 6) having a heating index of 5 kW or more and a heating index exceeding 2 at the same time, curing of the HAZ and deterioration of the corrosion resistance of the film occurred, and cracks occurred. Moreover, although the laser output P was 15 kW or more, in the comparative examples (No. 7 and No. 8) in which the heating index was less than 0.4, the heat input was too small, so that the penetration welding was not performed (see “Table 2”). No "
Display). Further, although the laser power P and the heating index are within the range of the present invention, in the comparative example (No. 9) in which the preheating temperature is 1200 ° C. (outside the limit of the present invention “normal temperature to 1000 ° C.”), the preheating temperature is Was too high and the preheated site was significantly hardened and cracked. Comparative examples (Nos. 14 and 1) in which post-heat treatment conditions were out of the range of the present invention (post-heat conditions (c) and (d) in Table 2).
In the case of 5), the portion subjected to the post-heat treatment was hardened and cracks occurred.

These examples clearly show that the method of the present invention is very effective in preventing sulfide stress cracking in welds.

[0067]

According to the method of the present invention, a low carbon martensitic stainless steel welded pipe having a beautiful welded portion having excellent sulfide cracking resistance and the like can be produced at low cost with high productivity. it can.

[Brief description of the drawings]

FIG. 1 is a drawing showing an outline of manufacturing a product from a steel strip.

FIG. 2 (a) shows the post-heating condition (a), and FIG.
(B) is a drawing schematically showing the post-heating condition (b).

FIG. 3 is a drawing showing a cross section in a state where a stress due to four-point bending is applied in a sulfide stress cracking test.

[Explanation of symbols]

1 ... strip steel (material), 2 ... open pipe (intermediate processed product), 3 ... butt pipe (transitional state of the intermediate processed product where both edges have been butted immediately after leaving the squeeze roll), 4 ... welded As-pipe (intermediate product), 5: post-heat treated pipe (product), 11: forming roll group, 12: high frequency heating device, 13: squeeze roll, 14: laser welding machine, 15: high frequency heating device or batch type Heating furnace, 21: sulfide stress cracking test piece, 22: outer support rod, 23: inner support rod

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ identification symbol FI C22C 38/00 302 C22C 38/00 302Z 38/46 38/46 (58) Fields investigated (Int.Cl. ⁷ , DB name) B21C 37/08 B23K 26/00

Claims (2)

(57) [Claims] 1. The method according to claim 1, wherein the carbon content is 0.05% by weight or less,
Of the stainless steel strip containing 10 to 14% by weight of the aluminum alloy was formed into a tube, and the butted edges in a temperature range from room temperature to 1000 ° C. were laser-welded under the following conditions and then 850 to 1000 ° C. Heated to a temperature range, cooled to 300 ° C or less at a cooling rate of 20 ° C / s or more, and then heated to a temperature range of 600 to 700 ° C,
A method for producing a low carbon martensitic stainless steel welded pipe characterized by cooling to room temperature at a cooling rate of not more than ° C / s. P ≧ 15kW 0.4 ≦ P · {exp (a · T)} / (V · t) ≦
2 However, P: Laser output at the time of pipe welding (kW) a: Constant (= 0.0006) T: Temperature of both butted edges before welding (° C) V: Welding speed (m / min) t: Strip steel Thickness (mm) 2. The method according to claim 1, wherein the carbon content is 0.05% by weight or less,
Of a stainless steel strip containing 10 to 14% by weight of a stainless steel, and the two butt portions in a temperature range from room temperature to 1000 ° C. are laser-welded under the following conditions and then at a temperature of 700 to 900 ° C. A method for producing a low carbon martensitic stainless steel welded pipe characterized by heating to a temperature range and cooling to room temperature at a cooling rate of 20 ° C / s or less. P ≧ 15kW 0.4 ≦ P · {exp (a · T)} / (V · t) ≦
2 However, P: Laser output at the time of pipe welding (kW) a: Constant (= 0.0006) T: Temperature of both butted edges before welding (° C) V: Welding speed (m / min) t: Strip steel Thickness (mm)