JP2001140040A - Low carbon ferrite-martensite duplex stainless welded steel pipe excellent in sulfide stress cracking resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a ferrite-martensite duplex stainless welded steel pipe exhibiting excellent SSC resistance and toughness in in oil well environment even without executing quenching and tempering heat treatment and annealing heat treatment for a long time to a hot rolled steel sheet as the stock and a steel pipe after pipe making by welding. SOLUTION: This ferrite-martensite duplex stainless welded steel pipe has a composition containing, by mass, <=0.02% C, <=0.04% P, <=0.01% S, 2 to 8% Ni, 11.5 to 15% Cr, 1.5 to 4% Mo, 0 to 1% Si, 0 to 1 % Mn, 0 to 0.1% sol.Al, 0 to 1.2% Cu, 0 to 0.2% Ti, <=0.02% N, 0.1% V, and the balance Fe with impurities, and in which the content of ferrite in the metallic structure is 15 to 40 volume %.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low carbon ferrite-martensite duplex stainless steel welded steel pipe excellent in sulfide stress cracking resistance and suitable for pipes of line pipes, oil country tubular goods or chemical plants.

[0002]

2. Description of the Related Art Low carbon martensitic stainless steel is a type of steel that has been recently developed as a material for oil wells. This type of steel is inexpensive because it contains less expensive elements such as Cr than duplex stainless steel, and has good corrosion resistance in a humid environment containing only carbon dioxide or a mixture of carbon dioxide and a trace amount of hydrogen sulfide. Is shown.

[0003] Steel pipes made of low-carbon martensitic stainless steel are generally often manufactured as seamless steel pipes. Seamless steel pipes are highly valued for reliability, but have some problems. One is that it is difficult to manufacture a thin-walled pipe having a thickness of 10 mm or less due to the principle of the pipe-making method. When welding the line pipe, it is needless to say that it is desirable that the wall thickness be as thin as possible within the range of the strength, from the viewpoint of reducing the number of laminations during welding and reducing the construction cost. Further, when a ferrite phase is precipitated in a metal structure, hot workability is significantly reduced, and scratches such as rash are generated. Therefore, the structure must be a martensite single phase as much as possible.

[0004] For these reasons, in recent years, a method for producing a high corrosion resistant stainless steel pipe by welding has been developed. Low carbon martensitic stainless steel has good weldability due to its low carbon content, and a girth weld joint by gas tungsten arc welding (hereinafter referred to as GTAW) or gas metal arc welding (hereinafter referred to as GMAW). It is suitable for a prerequisite line pipe.

For example, Japanese Patent Application Laid-Open Nos. 4-191319 and 4-191320 disclose that a low carbon martensitic stainless steel material strip is formed into a tubular shape, and a butt portion is formed by an electric resistance welding method (hereinafter, referred to as "welding"). A method for pipe welding by the ERW method is disclosed. For small diameter pipes, GTAW method or plasma welding method (hereinafter, PAW method)
Butt-pipe welding by the method).

In recent years, a butt pipe welding method using a high-power laser welding machine has also been developed.
No. 25 discloses a method for improving the corrosion resistance by producing a pipe by butt laser welding and then performing an appropriate post heat treatment in the vicinity of the welded portion.

[0007] Also, demand for pipes having a larger diameter than seamless steel pipes is increasing. For large-diameter pipes, a steel plate is used as a material and submerged arc welding (hereinafter referred to as SAW).
Pipe-forming welding).

[0008] Since the welded steel pipe made of low carbon martensitic stainless steel has a single-phase structure of martensite, it has a high-strength and coarse-grained structure as rolled, and has toughness,
Corrosion resistance such as resistance to sulfide stress cracking (hereinafter referred to as SSC) is reduced. For this reason, in the case of martensitic single-phase steels, it is generally necessary to secure the toughness and corrosion resistance by performing quenching and tempering heat treatment for the purpose of grain refinement after hot rolling and long-time annealing heat treatment for the purpose of softening.

[0009] Further, according to API standard (American Petroleum Institute standard) 5LC, which is required strength as a line pipe when hot rolled as it is, the strength becomes higher than X56 class to X80 class (yield stress is 386 to 655 MPa). There are many. For this reason, a heat treatment for softening is required before or after welding.

For example, Japanese Patent Application Laid-Open No. 4-191319 discloses that the winding temperature after hot rolling is set to 600 ° C. or higher,
It shows that it is necessary to perform quenching and tempering heat treatment after ERW pipe production. However, adding such a heat treatment step increases the production cost.

[0011]

SUMMARY OF THE INVENTION An object of the present invention is to provide a hot rolled steel sheet and a steel pipe after welding and pipe forming which are excellent in SSC resistance in an oil well environment without being subjected to quenching, tempering heat treatment or long-time annealing heat treatment. It is an object of the present invention to provide a welded steel pipe exhibiting the properties and toughness.

[0012]

Means for Solving the Problems The inventors of the present invention have conducted intensive experiments and studies in order to solve the above-mentioned problems, and as a result, if it is assumed that a product is obtained as it is during hot rolling, the martensite phase which is a parent phase If the ferrite phase and the martensite phase have a two-phase metal structure in which a ferrite phase is precipitated at a predetermined ratio, high strength can be suppressed even during hot rolling, and good SSC resistance can be obtained. I got the knowledge. The present invention has been made based on such knowledge, and the gist is as follows.

(1) In mass%, C: 0.02% or less,
P: 0.04% or less, S: 0.01% or less, Ni: 2 to 2
8%, Cr: 11.5 to 15%, Mo: 1.5 to 4%,
Si: 0 to 1%, Mn: 0 to 1%, sol.
1%, Cu: 0 to 1.2%, Ti: 0 to 0.2%, N:
0.02% or less, V: 0.1% or less, the balance being F
A low carbon ferrite-martensitic duplex stainless steel welded pipe comprising e and impurities and having an amount of ferrite in the metal structure of 15 to 40% by volume by volume and excellent in sulfide stress cracking resistance.

It is generally said that the precipitation of a ferrite phase in martensitic stainless steel deteriorates performance such as toughness and corrosion resistance. For example, with respect to toughness, ferritic stainless steel has poor toughness, and it is easily presumed that toughness will deteriorate if a ferrite phase precipitates in martensitic stainless steel. Regarding the corrosion resistance, the precipitation of the ferrite phase causes the elements Cr and Mo, which are effective in protecting the corrosion-resistant coating, to be absorbed from the martensite of the mother phase, so that the corrosion resistance of the mother phase cannot be sufficiently secured. That is to be expected. Therefore, there has been no conventional example of utilizing the precipitation of a ferrite phase in a steel pipe used in an oil well environment.However, the present inventors focused on the fact that the ferrite phase is a softened phase as compared with the martensite phase, and Such a test was performed.

In mass%, C: 0.012%, Si: 0.1%.
43%, Mn: 0.51%, Mo: 2.53%, sol.A
l: 0.033%, Ti: 0.034%, Ni: 3.5
5%, V: 0.02% as a basic component and a Cr content of 1%
0-17%, NI content is 0-10%, and Mo content is variously changed in the range of 0-5%. Hot rolling was performed at various temperatures of 1100 to 1250 ° C. Using this hot-rolled steel sheet, a tensile test,
A Charpy impact test and an SSC resistance test were performed.

FIG. 1 is a diagram showing the relationship between the amount of ferrite and the yield stress obtained from the results of a tensile test. The strength decreases as the amount of ferrite in martensite increases.

FIG. 2 is a diagram showing the relationship between the ferrite content (% by volume) obtained from the Charpy impact test results and the fracture surface transition temperature. When the amount of ferrite is 15% or more, the transition temperature is -20 ° C or less. As a result of examining the metal structure,
Even when rolled, the structure was extremely fine when the amount of ferrite was 15 to 80%.

FIG. 3 is a graph showing the relationship between the amount of ferrite (% by volume) obtained in the SSC resistance test and the partial pressure of hydrogen sulfide. As in the case of the toughness, a fine structure is obtained when the amount of ferrite is 15% or more, so that good SSC resistance is exhibited. However, from the viewpoint of protection of the corrosion-resistant coating, the ferrite phase is Cr or M
Since o is absorbed from the martensite phase of the parent phase and indirectly lowers the corrosion resistance of the martensite phase, it can be seen that if the ferrite content exceeds 40%, the SSC resistance is rather lowered. Was.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the chemical composition and metal structure specified in the present invention will be described in detail. All percentages of the content of the chemical components are mass%, and the indication of the amount of the metal structure is volume%.

C: 0.02% or less C is preferably as low as possible from the viewpoint of securing weldability. However, since excessive reduction of the C amount involves a cost increase, it is desirable to set the content to 0.002% or more from the viewpoint of economy. If it exceeds 0.02%, the strength of the martensite phase becomes too high, and the heat-affected zone during welding (hereinafter, referred to as
HAZ) causes significant hardening and SSC resistance
Therefore, the upper limit was set to 0.02%.

P: 0.04% or less P is inevitably present in steel as an impurity and segregates at grain boundaries to deteriorate the SSC resistance. In particular, if the content exceeds 0.04%, the SSC resistance deteriorates significantly, so the content needs to be 0.04% or less. In addition,
It is desirable that the P content be as low as possible in order to increase the SSC resistance.

S: 0.01% or less S is inevitably present in steel as an impurity like P, but it is segregated at grain boundaries and forms a large amount of sulfide-based inclusions. Decreases SSC resistance. If the content exceeds 0.01%, the SSC resistance is significantly reduced, so the content needs to be 0.01% or less.
It is desirable that the content of S be as low as possible in order to increase the SSC resistance.

Ni: 2 to 8% or less Ni has the effect of increasing the amount of martensite similarly to Mn. From this viewpoint, it is necessary to contain 2% or more. On the other hand, if it is contained excessively, it becomes expensive steel and the economic efficiency is impaired, and the solid solution strengthening causes an increase in the strength of the martensite phase, thereby deteriorating the SSC resistance. From this viewpoint, the upper limit is set to 8%.

Cr: 11.5 to 15% Cr is an element that protects a corrosion-resistant film and enhances SSC resistance. In order to obtain this effect, the content needs to be 11.5% or more. On the other hand, since Cr is a ferrite stabilizing element, if it is contained in excess of more than 15%, it becomes necessary to increase the amount of expensive alloying elements such as Ni, which is a martensite stabilizing element, and economic efficiency is impaired. From this viewpoint, the upper limit is set to 15%.

Mo: 1.5 to 4% or less Mo is an element that protects a corrosion-resistant film and enhances SSC resistance. In order to obtain this effect, it is preferable to set it to 1.55 or more. Further, since it is a ferrite stabilizing element like Cr, if it is contained excessively, it becomes necessary to increase the amount of an expensive alloying element such as Ni, which is a martensite stabilizing element, which impairs economic efficiency. From this viewpoint, the upper limit is set to 4%.

Si: 0 to 1% or less Si need not be particularly added, but if added, it is effective for deoxidizing molten steel. In order to obtain the effect, the content is preferably set to 0.2% or more. However, if the content exceeds 1%, the grain boundary strength is reduced and the SSC resistance is reduced, so the upper limit is 1%.

Mn: 0 to 1% or less Mn may not be added, but if added, has the effect of increasing the proportion of martensite. When it is added, it is preferable to contain 0.2% or more. However, when the content exceeds 1%, the grain boundary strength is weakened, or active dissolution in hydrogen sulfide is performed, thereby lowering the SSC resistance.
Therefore, the upper limit is set to 1%. Desirable Mn content is 0.1.
Not more than 05%.

Sol. Al: 0.1% or less Al need not be added, but if added, it is effective in deoxidizing molten steel. In order to obtain the effect, the content is preferably set to 0.02% or more. However, when the content exceeds 0.1%, coarse Al-based inclusions increase and the SSC resistance decreases. Therefore, the upper limit is set to 0.1%.

Ti: 0 to 0.2% Ti is contained as necessary, but if contained, it has an effect of fixing N, which is an impurity in steel, as TiN. In addition, excess Ti than required for fixing N becomes carbide and traps C, thereby suppressing hardening in the HAZ of the circumferential weld. However, when the content exceeds 0.2%, the workability is lowered and the carbonitride itself becomes the starting point of SSC. Therefore, the upper limit is set to 0.2%. A preferred lower limit is 0.1%.

V: 0.1% or less V is an element which is inevitably mixed as an impurity due to a loss in dissolution. In particular, if it exceeds 0.1%, fine VC is precipitated, so that the strength becomes too high, and the SSC resistance is lowered. Therefore, the upper limit is made 0.1%. Desirable V amount is 0.05% or less.

N: 0.02% or less N is present in steel as an impurity, and its content is 0.0% or less.
If it exceeds 2%, the hot workability is impaired, the production becomes difficult, and the strength of the martensite phase is increased, resulting in an SSC resistance.
Is reduced. Desirable N content is 0.01% or less.

Cu: 0 to 1.2% Cu is contained as necessary, but if it is contained, it is resistant to SSC.
It has the effect of enhancing the nature. 0.1
% Or more is preferable. On the other hand, if it exceeds 1.2%, the effect on corrosion resistance is saturated, and the strength of the martensite phase is increased, thereby deteriorating the SSC resistance. From this viewpoint, the upper limit is set to 1.2%.

Metal structure: If the metal structure is made into a martensite single phase, a heat treatment for strength adjustment is required after hot rolling or welding and pipe making, so that it has a two phase structure of a ferrite phase and a martensite phase. .

With the two-phase structure, the grain growth of each phase is suppressed, the structure becomes extremely fine even as rolled, and the toughness and SSC resistance are improved.

In order to obtain a yield strength of 655 MPa or less as-rolled and obtain good toughness, it is necessary to precipitate 15% or more of a ferrite phase. On the other hand, from the viewpoint of SSC resistance, when the ferrite phase exceeds 40%, Cr and Mo are absorbed from the martensite phase of the mother phase and the SS resistance is indirectly increased.
Decreases C property. From this viewpoint, the volume fraction of the ferrite phase is set to 15 to 40%.

The amount of ferrite can be adjusted by a combination of the contents of Cr and Mo and the heating temperature for hot rolling. For example, when increasing the residual amount of ferrite, the ferrite former Cr,
It is preferable to increase the Mo content and increase the heating temperature.

Next, a method for manufacturing a welded steel pipe will be described below.

As the material steel plate of the welded steel pipe, a hot rolled steel plate or a thick steel plate manufactured by ordinary slab rolling and hot rolling is used. As for the hot rolling method, after heating to a normal heating temperature, for example, in a range of 1100 ° C. or more and 1250 ° C. or less, rolling and finishing may be performed by a normal method. However, in order to adjust the amount of ferrite as described above, it is necessary to determine the heating temperature in consideration of the chemical composition. In addition, tempering for a short time after rolling may be performed for fine adjustment of strength.
Quenching, tempering heat treatment and long-time annealing heat treatment are not desirable from the viewpoint of economy and do not meet the purpose of the present invention.

A hot-rolled steel plate or a thick steel plate is cut into a width substantially equal to the outer peripheral length of a target steel pipe, formed into a cylindrical shape, and abutted portions are welded to form a welded steel pipe. There is no particular limitation on the welding method, and any method may be used as long as the performance of the welded portion is guaranteed. GT for thin tube
An arc welding method such as an AW method, a GMAW method, or a plasma welding method may be used, or an ERW method may be used from the viewpoint of reducing pipe production costs. In addition, from the viewpoint of ensuring the quality of the weld,
An electron beam welding method or a laser welding method may be used.

In pipe forming welding, a hot-rolled steel sheet is formed into an open pipe shape by a processing device such as a forming roll group, and both edges of the steel strip are butted together by means such as a squeeze roll. A method of pipe-forming welding may be employed.
In order to improve the pipe-making speed, pipe-forming welding may be performed after preheating by a tubular induction heating coil capable of locally heating used in the electric resistance welding method or a high-frequency heating means using a contact tip. Further, after the pipe forming by welding, a local heat treatment for recovering the structure of the welded portion may be performed by using a high frequency heating means.

For the production of a thick steel pipe, pipe production by SAW is preferable. Thick steel plate can be used for normal C press, U press and O
It is formed into a tubular shape step by step with a press,
After welding and pipe-forming by AW, a method such as making a product as welded may be used. The welding conditions and the components of the weld metal are not particularly limited as long as the desired performance can be obtained.

[0042]

EXAMPLES Steels of 16 chemical compositions shown in Table 1 were melted,
The ingot was forged into a slab. Symbols A to H in the same table are within the range defined by the present invention for the chemical composition, and 1 to 8 are outside the specified range. Each slab was heated in various temperature ranges from 1100 ° C. to 1250 ° C., and then hot-rolled to obtain a hot-rolled steel sheet. The heating temperature was changed to adjust the amount of ferrite.

[0043]

[Table 1]

Using each hot-rolled steel sheet as a raw material, welded tubes were produced by laser welding, SAW, ERW, PAW and GTAW. For laser welding, ERW, and PAW, pipe-forming welding was performed without using a filler metal. GTAW, SAW
Was welded using a 22Cr or 25Cr ferritic austenitic duplex stainless steel as a filler metal. In all cases, no post heat treatment was performed after welding.

The volume fraction of the ferrite phase in the metal structure is
The resin-filled material of the cross section of the steel sheet as hot rolled was corroded with a virella reagent, the structure was observed, and the measurement was performed by the point calculation method. For each steel plate, three cross sections were measured, and the average value was calculated as the volume fraction.

From the hot-rolled steel sheet, various types of round bar tensile test pieces corresponding to the thickness of the material steel sheet were sampled in the width direction, and a tensile test was performed at room temperature to measure the yield strength (YS).

Further, Charpy impact test specimens corresponding to the thickness of the base steel sheet were sampled in the width direction of the hot-rolled steel sheet, and subjected to an impact test at various temperatures. vTs) was measured.

Further, the SSC test showed that the thickness was 2 mm and the width was 1
A stress corrosion test specimen having a length of 0 mm and a length of 75 mm is sampled in the width direction from the base material and the welded portion of the welded pipe, and a 100% stress of YS of the material steel is applied by a four-point bending constant strain method. It was immersed for 336 hours and examined for the occurrence of SSC. The test solution contains 0.001 to 0.01 MPa, H ₂ S (CO ₂
A 5% aqueous solution of NaCl, which was adjusted to a pH of 3.5 by adding a predetermined amount of acetic acid-sodium acetate saturated with (balance), was used.

Tables 2 and 3 show the test results.

[0050]

[Table 2]

[0051]

[Table 3]

The evaluation criteria in the SSC resistance column in Tables 2 and 3 are as follows:
If no occurrence of SSC was observed, it was evaluated as “Good”, S
A sample in which SC occurred was evaluated as "bad".

As is evident from Table 2, in steel having a chemical composition and a metal structure within the range specified in the present invention, Y
S is 648 MPa or less, and has excellent toughness and corrosion resistance.

On the other hand, as shown in Table 3, even if the chemical compositions of Test Nos. 21 to 32 are within the range specified in the present invention, if the ferrite content is out of the specified range, the toughness, strength and resistance to resistance are reduced. One or more of the SSC properties are poorly practical. Test numbers 33 to 40 whose chemical composition is out of the range specified in the present invention are inferior in SSC resistance, toughness,
Poor yield stress characteristics.

[0055]

According to the present invention, a hot-rolled steel sheet of a material or a steel pipe after welding and pipe-forming is used.
A welded steel pipe exhibiting excellent SSC resistance and toughness in an oil well environment can be provided at low cost without performing quenching, tempering heat treatment or long-time annealing heat treatment, and its industrial value is great.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the amount of ferrite and the yield stress.

FIG. 2 is a diagram showing the relationship between the amount of ferrite and the fracture surface transition temperature.

FIG. 3 is a diagram showing the relationship between the amount of ferrite and the partial pressure of hydrogen sulfide.

Claims (1)

[Claims] (1) C: 0.02% or less, P: 0.
04% or less, S: 0.01% or less, Ni: 2 to 8%, C
r: 11.5 to 15%, Mo: 1.5 to 4%, Si: 0
-1%, Mn: 0-1%, sol. Al: 0-0.1%, C
u: 0 to 1.2%, Ti: 0 to 0.2%, N: 0.02
%, V: 0.1% or less, with the balance being Fe and impurities, the ferrite content in the metal structure being 15 to 40% by volume in sulfide stress cracking resistance. Excellent low carbon ferrite-martensite duplex stainless steel welded pipe.