JPH11158551A - Production of martensitic stainless steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a martensitic stainless steel pipe excellent in carbon dioxide corrosion resistance, cathode corrosion resistance and weldability capable of producing a seamless steel pipe suitably usable for the arrangement in a seawater atmosphere of a line pipe transporting petroleum contg. carbon dioxide, natural gas or the like at a low cost. SOLUTION: A billet contg. 0.005 to 0.02%, C, <=1.0% Si, 0.2 to 2.0% Mn, <=0.03% P, <=0.005% S, 7.0 to 14.0% Cr, 1.0 to 7.0% Ni, 3.5(C+N) to 0.15% Ti, <=0.016% N, and the balance Fe with inevitable impurities is subjected to hot pipe making working so as to regulate the cross-sectional compressibility in finish rolling to >=40% and also to regulate the finishing temp. to >=900 deg.C, after the completion of this pipe making working, it is successively reheated at 900 to 1000 deg.C and is subjected to water quenching, and, after the completion of this water quenching, it is tempered at 500 to 750 deg.C.

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 pipe, for example, which is excellent in carbon dioxide gas corrosion resistance and cathodic corrosion resistance and can be used as welded without requiring heat treatment on site. Therefore, it is possible to manufacture a seamless steel pipe which can be suitably used for arranging a line pipe for transporting petroleum or natural gas containing carbon dioxide gas in a seawater atmosphere at a low cost. The present invention relates to a method for producing a martensitic stainless steel tube having excellent resistance, cathodic protection and weldability.

[0002]

2. Description of the Related Art Conventionally, the environment of wells for collecting oil wells and gas wells has become increasingly harsh. For example, pipes laid in seawater to transport carbon dioxide-containing oil or natural gas, etc. Corrosion is a major problem. Conventionally, this corrosion has been dealt with by using an inhibitor for general carbon steel or by using duplex stainless steel.

[0003] Such a pipe is excellent in both general corrosion resistance and corrosion performance against carbon dioxide gas (hereinafter referred to as "carbon dioxide corrosion resistance"), and has both high strength and high toughness. Further, it is required that on-site welding can be performed, and in particular, martensitic stainless steel pipes have been attracting attention in recent years due to its excellent economic efficiency.

[0004] When a seamless steel pipe made of 13Cr SUS420 steel, which is a typical martensitic stainless steel, is used for the above-mentioned pipe, both the weld toughness and the delayed fracture resistance are inferior. , It was necessary to perform heat treatment after welding. For this reason, an increase in local construction costs is inevitable, and improvements have been required.

On the other hand, regarding the production of steel products that require a huge production equipment, generally, the process and energy saving by continuously performing the processing and heat treatment on-line are actively studied and promoted. ing. However, with regard to martensitic stainless seamless steel pipes, in order to ensure high quality that satisfies the reliability required when used as the above-mentioned pipes, it is still necessary to cool the pipes to room temperature once after pipe forming, and then offline. Due to the heat treatment, it had to be manufactured.

That is, a martensitic stainless steel seamless pipe is generally formed by piercing a billet with a piercer by an inclined rolling mill, stretching (rolling) with a plug mill or a mandrel mill, and then finish rolling with a sizer or a reducer. Is performed by performing quenching and tempering after the pipe forming process, but it is necessary to perform air cooling quenching in order to prevent quenching cracks due to a high C content. For this reason, quenching requires a long time, and it is impossible to perform quenching online after the pipe forming process from the manufacturing process. Therefore, after cooling to room temperature after the pipe making process, it is charged into a heat treatment furnace installed independently of the pipe making line, heated again, and air-cooled quenching and tempering are performed offline.

[0007] In Japanese Patent Publication No. 6-104868, C:
0.02 to 0.08% (hereinafter, "%" means "% by weight" unless otherwise specified), N: 0.015% or less of a billet made of martensitic stainless steel having 0.015% or less. Is cooled to normal temperature once after forming the pipe under normal conditions, reheated to 920-1100 ° C to form austenite, then cooled at a cooling rate of water cooling or higher, and then at a temperature of 600 ° C or higher and Ac ₁ temperature or lower. There has been proposed an invention in which a high-strength martensitic stainless steel pipe is manufactured by cooling at a cooling rate equal to or higher than air cooling after tempering in step (1).

[0008]

As described above, all of the conventional methods for producing a martensitic stainless steel pipe require a large amount of heat treatment after the pipe-forming process. Therefore, there is a problem that the total energy amount for performing the heat treatment by any of the methods is large, and the martensitic stainless steel pipe cannot be economically manufactured.

Here, an object of the present invention is to provide, for example, carbon dioxide gas which is excellent in corrosion resistance to carbon dioxide gas and corrosive in seawater and which can be used as welded without requiring heat treatment on site. Prerequisite for mass production at low cost by reducing the total energy required for heat treatment of seamless steel pipes that can be suitably used for the installation of line pipes for transporting contained oil and natural gas in seawater In fact, it is an object of the present invention to provide a method for producing a martensitic stainless steel pipe which can be produced and is excellent in carbon dioxide gas corrosion resistance, cathodic protection and weldability.

[0010]

SUMMARY OF THE INVENTION The inventor of the present invention uses the calorie of the workpiece after the pipe making process. Means for ensuring high quality that satisfies the required reliability has been earnestly studied, and the following new findings (a) to (d) have been obtained, and the present invention has been completed.

(A) In finish rolling when a billet made of martensitic stainless steel having a specific composition is formed into a seamless steel pipe, it is converted to a cross-sectional compression ratio of 40% or more at a finishing temperature of 900 ° C. or more. A large working strain is applied by applying a high pressure, and after finishing rolling, the steel sheet is successively reheated to 900 to 1000 ° C. without cooling after cooling, and quenching is performed online.

(B) After the quenching, tempering is performed while maintaining the temperature at 500 to 750 ° C. (c) After finish rolling, by reheating and maintaining at 900 to 1000 ° C., uniform recrystallized grains can be obtained. As a result, a martensitic stainless steel seamless pipe having various performances equivalent to that of a martensitic stainless steel pipe that was heat-treated off-line was actually used for mass production.
Can be manufactured.

(D) In order to continuously reheat to 900 to 1000 ° C. without cooling after finish rolling, for example, when reheating is performed between rough rolling and finish rolling during pipe making, or in a conventional method, As compared with the case where the heat treatment is performed off-line after the pipe making process, both the heat treatment equipment and the running cost required for the heat treatment equipment can be significantly reduced.

Here, the gist of the present invention is as follows.
C: 0.005 to 0.02%, Si: 1.0% or less, Mn: 0.2 to 2.0
%, P: 0.03% or less, S: 0.005% or less, Cr: 7.0 to 1
4.0%, Ni: 1.0 to 7.0%, Ti: 3.5 (C + N) to 0.15
%, N: 0.015% or less, and Mo: 0.2 to 3.
A billet consisting of 0%, the balance of Fe and unavoidable impurities was hot-pipe-processed so that the cross-sectional compression ratio in finish rolling was 40% or more and the finishing temperature was 900 ° C or more. After finishing the pipe processing, re-heat to a temperature of 900 to 1000 ° C and perform water quenching,
After the completion of this water quenching, tempering is performed at a temperature of 500 to 750 ° C. to produce a seamless steel pipe. A martensitic stainless steel excellent in carbon dioxide gas corrosion resistance, cathode corrosion resistance and weldability. This is a method for manufacturing a steel pipe.

The “cross-sectional compression ratio” in the present invention is calculated by (cross-sectional area before finish rolling−cross-sectional area after finish rolling) ÷ cross-sectional area before finish rolling × 100. The martensitic stainless steel pipe produced according to the present invention can be suitably used for steel pipes for transporting petroleum containing carbon dioxide, natural gas, etc. because of its excellent carbon dioxide corrosion resistance, and contains carbon dioxide. In other industrial fields, for example, it can be used as a material for forming pipes for decarbonation gas facilities, pipes for geothermal power generation, and tanks for liquids containing carbon dioxide gas. It can be used even in a carbon dioxide gas environment where the amount of mixed hydrogen sulfide is very small.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a method for producing a martensitic stainless steel pipe excellent in carbon dioxide gas corrosion resistance, cathodic protection and weldability according to the present invention will be described in detail with reference to the accompanying drawings. I do. First, the reason for limiting the composition of the billet used in the present invention will be described.

(C: 0.005 to 0.02%) If the C content exceeds 0.02%, the hardness as-quenched increases, and the susceptibility to quenching cracks increases. It also deteriorates the weldability of the product and adversely affects the corrosion resistance of the weld. From this point of view, the lower the C content, the better. However, to reduce the content to less than 0.005%, various treatments are required, and the melting cost is significantly increased. Therefore, in the present invention, the C content is limited to 0.005% or more and 0.02% or less. From the same viewpoint, it is desirable that the upper limit of the C content is 0.015% and the lower limit is 0.008%.

(Si: 1.0% or less) Si is an effective element for deoxidizing steel, but if it exceeds 1.0%, toughness is reduced. Therefore, in the present invention, in order to secure good toughness, the Si content is limited to 1.0% or less. From the same viewpoint, the Si content is desirably 0.50% or less.

(Mn: 0.2 to 2.0%) Mn is an element effective for deoxidizing and desulfurizing steel, and also effective for improving hot workability and obtaining an appropriate structure. However, if the Mn content is less than 0.2%, such an effect is insufficient. On the other hand, if it exceeds 2.0%, toughness and weldability deteriorate. Therefore, in the present invention, the Mn content is limited to 0.2% or more and 2.0% or less. From a similar viewpoint, the upper limit of the Mn content is 1.20.
%, And the lower limit is desirably 0.30%.

(P: 0.03% or less) P is an impurity in the present invention, and if it exceeds 0.03%, toughness is deteriorated. Therefore, in the present invention, the P content is limited to 0.03% or less.

(S: 0.005% or less) When S exceeds 0.005%, the hot workability is deteriorated and flaws are generated during pipe production. Therefore, in the present invention, the S content is limited to 0.005% or less.

(Cr: 7.0-14.0%) When Cr is added in an amount of 7.0% or more, the corrosion resistance to carbon dioxide gas is improved. But 14.0
%, The hot workability is reduced. Therefore, in the present invention, the Cr content is limited to 7.0% or more and 14.0% or less. From the same viewpoint, the upper limit of the Cr content is 13.0% and the lower limit is
Desirably 10.0%.

(Ni: 1.0 to 7.0%) Ni is a high Cr martensitic stainless steel which is the object of the present invention, since delta ferrite is generated and flaws are likely to be generated during hot pipe forming. Add 1.0% or more to adjust the component balance. However, since it is an expensive element and there is a limit in suppressing the generation of hot flaws, if added over 7.0%, the cost will only increase significantly, and no further improvement in quality will be obtained. Therefore, in the present invention, the Ni content is 1.0% or more and 7.0% or more.
% Or less. From the same viewpoint, the upper limit of the Ni content is
Desirably, the lower limit is 6.0% and the lower limit is 1.5%.

(Ti: 3.5 (C + N) -0.15%) Ti is a carbonitride forming element, and if it exceeds 0.15%, the toughness is remarkably reduced, and large inclusions of TiN are formed to form a large amount of TiN. Causes flaws. Further, by containing 3.5 (C + N)% or more of Ti, when performing the on-line heat treatment in the present invention, it fixes free C and N which cause a variation in strength and also deteriorate the toughness and corrosion resistance of the welded portion. . Therefore, in the present invention, the Ti content is 3.5 (C +
N) The content is limited to not less than 0.15%. From a similar perspective, Ti
The upper limit of the content is desirably 0.10%.

(N: 0.015% or less) N exists as an impurity in steel. When the N content is 0.015%, the toughness is reduced, and the hardness as-quenched is increased, so that the susceptibility to quenching cracks is remarkably increased, and the hot workability is also deteriorated. Therefore, in the present invention, the N content is limited to 0.015% or less.

(Mo: 0.2 to 3.0%) Mo is an optional additive element in the present invention, and prevents local corrosion in a carbon dioxide gas environment in the presence of Cr. Also, when used in an environment where trace amounts of hydrogen sulfide coexist, it exhibits particularly high corrosion resistance. Such an effect can be obtained by containing 0.2% or more, but the effect is saturated even if the content exceeds 3.0%, resulting only in an increase in cost.
Therefore, in the present invention, the content when Mo is added is 0.
2% or more and 3.0% or less.

Other compositions are Fe and inevitable impurities. Next, a method for manufacturing a martensitic stainless steel pipe according to the present invention will be described in the order of steps.

FIG. 1 is a graph showing an example of a temperature curve given to a workpiece according to the present invention. In FIG. 1, for reference, the temperature curve of the invention proposed in Japanese Patent Publication No. 6-104868 is also shown by a dashed line.

(Piping) In the present invention, the billet having the above-mentioned composition is subjected to heat treatment so that the cross-sectional compression ratio in finish rolling at 1100 ° C. or less is 40% or more and the finishing temperature is 900 ° C. or more. Pipe forming is performed between them. That is, in carrying out the present invention, a mandrel mill and a sizer capable of performing finish rolling with a cross-sectional compression ratio of 40% or more at a finishing temperature of 900 ° C. or more are used.

When the cross-sectional compression rate is a processing rate of less than 40%,
At the time of subsequent reheating, recrystallization does not proceed smoothly, and not only an effect of miniaturization cannot be obtained, but also crystal grains may grow abnormally in some cases.

On the other hand, if the finishing temperature is lower than 900 ° C., the deformation resistance of the rolled material as a hollow shell becomes large, making it difficult to perform a strong reduction with a cross-sectional compression ratio of 40% or more. The amount of energy required for reheating to obtain a desired microstructure increases, and the effect of reducing the total amount of energy required for heat treatment decreases.

Therefore, in the present invention, the cross-sectional compression ratio in the finish rolling of hot pipe forming is limited to 40% or more, and the finishing temperature is limited to 900 ° C. or more. In the present invention,
Other tube-forming conditions may be any known conditions, and are not particularly limited.

(Reheating) In the present invention, after finishing the pipe forming process, the workpiece is reheated to a temperature of 900 to 1000 ° C., that is, without cooling the workpiece to room temperature.

In the present invention, reheating is performed after the end of the pipe forming process, instead of reheating between the rough processing and the finishing process in the pipe forming process as in the related art. Therefore, it is not necessary to set the reheating temperature in consideration of processing after reheating, and the reheating temperature can be set to the lowest temperature at which recrystallization proceeds. Thereby, regular recrystallized grains can be obtained.

If the reheating temperature is lower than 900 ° C., it takes a long time to recrystallize the martensitic stainless steel having the composition specified in the present invention, and the pipe production efficiency is extremely reduced. . On the other hand, if the reheating temperature exceeds 1000 ° C., the crystal flow grows greatly and coarsening becomes severe,
A decrease in toughness, cracking during secondary processing, and a decrease in HAZ toughness during welding occur. Therefore, in the present invention, the reheating temperature is 90
Limit to 0 ° C or more and 1000 ° C or less.

According to the present invention, the recrystallization is induced by appropriately combining the pipe-making condition and the reheating condition, and the crystal grains can be sized. Also,
According to the present invention, for example, a workpiece which has been subjected to pipe forming at a high temperature of about 800 to 900 ° C. is continuously cooled without cooling to room temperature, and then charged into a reheating furnace and ignited to 900 ° C. or more and 1000 ° C. or less. Therefore, the residence time in the heating furnace can be shortened as compared with the conventional method of once cooling to room temperature after the end of the pipe making process, so that the energy required for the heat treatment can be reduced and the energy can be saved.

(Quenching and tempering) In the present invention, 900 ° C. or more
After reheating to 00 ° C. or lower, water quenching is performed, and after completion of the water quenching, tempering is performed at a temperature of 500 to 750 ° C. to produce a seamless steel pipe. In the present invention, the quenching is limited to the water quenching in order to shorten the quenching time and eliminate the time constraint on a continuous manufacturing process line.

The tempering temperature depends on the desired proof stress (strength).
To 500 ° C or more and 750 ° C or less to impart
Also, the holding time may be any normal holding time,
For example, 30 minutes or more. In the present invention, other quenching and tempering conditions may be publicly known conditions, and are not particularly limited.

The seamless steel pipe made of martensitic stainless steel obtained according to the present invention is excellent in carbon dioxide gas corrosion resistance and cathode corrosion resistance and can be used as welded without requiring heat treatment in the field. It can be suitably used for line pipes for transporting carbon dioxide-containing oil, natural gas, and the like, and can also be suitably used for disposing this line pipe in a seawater atmosphere.

Further, according to the present invention, it is possible to manufacture such a seamless steel pipe made of martensitic stainless steel at low cost and on the premise of mass production.

[0041]

EXAMPLES A method for producing a martensitic stainless steel pipe according to the present invention will be described more specifically with reference to examples. Thirteen kinds of billets having a diameter of 150 mm were produced by subjecting steels A to G having the compositions shown in Table 1 to melt slab-rolling by a usual method, and these were used as rolled materials.

[0042]

[Table 1]

Each of these billets was subjected to pipe forming, reheating, water quenching and tempering so as to have a temperature curve as shown in FIG. 1 to obtain an outer diameter of 168 mm and a wall thickness of 15 mm. The seamless steel pipes of Sample No. 1 to Sample No. 13 shown in FIG.

[0044]

[Table 2]

In this embodiment, the heating temperature before perforation in FIG. 1 is 1250 ° C., and the finish rolling conditions (cross-sectional compression ratio Rd, finishing temperature) and the reheating temperature are both as shown in Table 2. Further, the tempering temperature was as shown in Table 2.

Samples No. 1 to No. 8 in Table 2 are examples of the present invention manufactured under all the conditions specified in the present invention. Sample Nos. 9 to No. 13 are steel grades. This is a comparative example in which at least one of the finish rolling conditions and the reheating temperature does not satisfy the conditions specified in the present invention.

Sample No. 1 to Sample Manufactured in this Way
After butt-welding the No. 13 seamless steel pipe under the following welding conditions, a test piece was cut out from the weld and subjected to a tensile test, a toughness test, a carbon dioxide gas corrosion resistance test, and a cathodic protection test under the following conditions. Was.

[Welding conditions] Full-layer TIG welding, ASTM-1G, welding rod: 25Cr duplex stainless steel welding material [tensile test] Test piece: 25.4 mm wide arc, parallel portion 50.8 mm, test temperature: normal temperature [toughness] Test] Test piece: 10 × 10 mm, 2 mm V notch, Test temperature: Performed at various temperatures. Transition temperature (vTrs): Temperature at which the brittle fracture rate becomes 50% Test piece cutting position: Base material L direction (BM), welding heat affected zone (HA)
Z).

[Test for Corrosion Resistance to Carbon Dioxide Gas] Test piece: 22 mm wide, 3 mm thick, 75 mm long (four-point bending) Test condition: Saturate 30 atm CO ₂ gas in a 5% NaCl aqueous solution,
Immersion at 125 ° C for 720 hours [Cathode corrosion test] Specimen: width 10mm, thickness 2mm, length 75mm (4-point bending) Test conditions: artificial seawater, potential 1300mV vs. SCE 25 ° C, additional stress: 100% of proof stress.

The test results of these tests are summarized in Table 2. In Table 2, the transition temperature vTrs was −30.
The case where the temperature was not higher than -30 ° C was evaluated as ○, and the case where the temperature was higher than -30 ° C was evaluated as ×. The carbon dioxide corrosion resistance was evaluated as ○ when there was no local corrosion on the surface after the test, and evaluated as x when there was some corrosion.
Further, in the cathodic protection test, the case where there was no crack on the surface after the test was evaluated as ○, and the case where there was no crack was evaluated as ×.

As is clear from Table 2, Sample Nos. 1 to 8 obtained according to the present invention exhibited good test results in all the tests. Thereby, the present invention example is excellent in carbon dioxide gas corrosion resistance, and also excellent in the toughness of the base material and the weld heat affected zone, so that it can be used as welded without requiring heat treatment on site, and It can be seen that since the outer surface is excellent in cathodic protection when exposed to a seawater atmosphere, it can be suitably used as a seamless steel pipe for line pipes for transporting petroleum, natural gas, and the like containing carbon dioxide.

In addition, since Sample No. 1 to Sample No. 8 are subjected to online quenching and tempering treatment without cooling to room temperature after pipe forming, they are low cost and are truly supposed to be mass-produced. In fact, you can see that it was manufactured.

On the other hand, in Sample No. 9, since the cross-sectional compression ratio in the finish rolling was lower than the lower limit of the range of the present invention, the recrystallization did not proceed smoothly during reheating, and an effect of miniaturization was obtained. As a result, weld toughness deteriorated.

In sample No. 10, since the reheating temperature was higher than the upper limit of the range of the present invention, remarkable coarsening occurred, the toughness of the weld was deteriorated, and a crack occurred in the cathodic protection test.

In sample No. 11, the cross-sectional compression ratio in the finish rolling was lower than the lower limit of the range of the present invention. Therefore, similarly to sample No. 9, the weld toughness was deteriorated and a crack was generated in the cathodic protection test. In Sample No. 12, since the reheating temperature was higher than the upper limit of the range of the present invention, remarkable coarsening occurred, and the weld toughness was deteriorated.

Further, in Sample No. 13, the C content of the billet used exceeded the upper limit of the range of the present invention, so that the susceptibility to sintering cracks increased and cracks occurred. As a result, the free C and N in the steel could not be fixed, and the toughness of the weld deteriorated and cracks occurred in the cathodic protection test.

[0057]

As described in detail above, according to the present invention, carbon dioxide gas is excellent in carbon dioxide gas corrosion resistance and cathode corrosion resistance and can be used as welded without requiring heat treatment on site. To manufacture seamless steel pipes that can be suitably used for line pipes that transport oil, natural gas, etc., at low cost by reducing the total energy required for heat treatment, and on the premise of mass production. Can be done. The significance of the present invention having such an effect is extremely remarkable.

[Brief description of the drawings]

FIG. 1 is a graph showing an example of a temperature curve given to a workpiece according to the present invention.

Claims (2)

[Claims] C .: 0.005 to 0.02% by weight, Si: 1.% by weight.
0% or less, Mn: 0.2 to 2.0%, P: 0.03% or less, S: 0.0
05% or less, Cr: 7.0 to 14.0%, Ni: 1.0 to 7.0%, T
i: 3.5 (C + N)-0.15%, N: 0.015% or less, balance Fe
In addition, the billet consisting of unavoidable impurities has a cross-sectional compression ratio of 40% or more in finish rolling and a finishing temperature
At 900 ° C or higher, hot pipe processing is performed,
After completion of the pipe making process, subsequently 900-1000 ° C
And then tempering at a temperature of 500 to 750 ° C. after the completion of the water quenching to produce a seamless steel pipe. A method for producing a martensitic stainless steel pipe having excellent corrosion resistance and weldability. 2. The composition according to claim 1, wherein the billet is M
o: 0.2 to 3.0%.
A method for producing a martensitic stainless steel pipe having excellent carbon dioxide corrosion resistance, cathodic corrosion resistance and weldability as described.