JPS6123713A - Production of high-strength two phase stainless steel - Google Patents

Abstract

PURPOSE:To obtain a two phase stainless steel which has excellent corrosion resistance and strength and is adequate as a material for an oil well pipe by subjecting a Cu-contg. austenitic-ferrite two phase stainless steel to hot working then cooling quickly said steel from a specific temp. and subjecting the steel to an aging treatment or to cold working before or after said treatment. CONSTITUTION:The austenitic-ferrite two phases stainless steel ingot such as 22Cr steel or 25Cr steel contg. 0.2-3.0wt% Cu is heated to >=1,000 deg.C and is subjected to hot working such as hot rolling. The steel ingot is quickly cooled straightly from >=800 deg.C and is subjected to a solutionizing treatment. The steel is subjected to the aging treatment in succession thereto or is subjected to the cold working such as cold rolling either before or after the aging treatment. The Cu- contg. austenitic-ferrite two phase stainless steel which has excellent corrosion resistance in a corrosive atmosphere of moist gaseous CO2, H2S or Cl<-> ion or in a stress corrosion environment, has excellent strength and is adequate as the material for the oil well pipe is thus produced.

Description

[Detailed Description of the Invention] <Field of Industrial Application> The present invention provides a method for producing high-strength duplex stainless steel with excellent corrosion resistance. 2. Cu-containing austenite, which also has high strength and is suitable for oil country tubular goods.
The present invention relates to a method for producing ferritic duplex stainless steel.

<Conventional technology> In recent years, oil wells and natural gas wells have become increasingly deep, and therefore there is a strong demand for improved strength of oil country tubular goods. carbon dioxide gas〔
CO2), hydrogen sulfide (H2S) and chlorine ions (
As development efforts have begun to focus on oil and gas wells that contain corrosive substances such as CA), countermeasures against corrosion have become even more important.

Conventionally, the most common method to prevent corrosion of oil country tubular goods has been to inject corrosion inhibitors into wells, but this method is often not expected to produce sufficient results. Another problem was that it could not be used effectively for offshore oil wells, gas wells, etc.

In addition, attempts have been made to prevent corrosion by applying protective coatings to oil country tubular goods, but these efforts have not yielded satisfactory results.

In view of these circumstances, there has recently been a trend to use higher-grade corrosion-resistant materials for oil country tubular goods than in the past. In order to cope with the severe corrosion phenomenon called A1.5I410 steel and 420 steel, 13Cr
Attempts have been made to use steel, but the current situation is that the l/groove structure cannot maintain resistance to CO2 for a long period of time in a high temperature environment exceeding 150°C.

Therefore, in a high temperature environment exceeding 150℃, 2
The use of austenitic-ferritic duplex stainless steels with a higher Cr content, such as 2Cr steel and 25Cr steel, is attracting attention.

However, this austenitic-ferritic duplex stainless steel has a yield strength of 0.2% (yield strength) of 265 to 80K when left in solution embedding, which is the usual method for manufacturing steel materials.
si (45.7 to 56.2 Kpf/nrA), and the strength as oil country tubular goods for deep wells was not sufficiently satisfied.

Therefore, when applying austenitic-ferritic duplex stainless steel, the current practice is to further cold-work the steel after solution treatment to provide the high strength required for oil country tubular goods for deep wells.

By the way, detailed experiments and research by the present inventors have revealed that wet carbonic acid i
Even though duplex stainless steel has excellent corrosion resistance under a gas ring, its corrosion resistance deteriorates significantly if the environment is contaminated with H2S or C4-. This CO2-H2S-CA-
The main type of corrosion in oil and gas well environments, including stress corrosion cracking (rSCCJ), is
The behavior of SCC in this case is completely different from that of normal ones, and it has become clear that not only is the presence of C7- present, but the influence of H2S is even more significant. Austenitic/ferritic type 2 as-treated
In phase stainless steel, SCC will occur if the partial pressure of H2S in the environment exceeds 10 atm, and in steel that has been strengthened by cold working, SCC will occur even if H2S of about 1 atm is contained. . The presence of Cl- promotes the occurrence of SCC.

Moreover, the above CO2,-H2S-CA! - Even if duplex stainless steel does not generate SCC in the environment, so-called "selective corrosion" may occur in which the ferrite region is selectively dissolved due to the influence of H2S. It was discovered that "cold working has considerable negative effects."

As described above, duplex stainless steel has the problem that if it is subjected to severe cold working to compensate for its lack of strength, its corrosion resistance will deteriorate significantly in an environment where H2S exists. Since this requires a large amount of processing (reduction amount), it has also been subject to restrictions in terms of equipment.

<Objective of the Invention> The main object of the present invention is to solve the various problems mentioned above, and to eliminate the need for cold working, which has a negative effect on resistance to SCC and selective corrosion C2 in an H2S coexisting environment. , discovered a means of strengthening austenitic-ferritic duplex stainless steel, and created an oil country tubing material that exhibits excellent durability even in oil and gas well environments where the H2S partial pressure is 10 atm or less, or at least 5 atm or less. In addition, we also provide a trace amount of H2S with a partial pressure of about 1 atm in the afternoon.
For duplex stainless steel materials used in environments including It is also an object of the present invention to provide a means for achieving the desired reinforcement while minimizing the amount of damage.

<Structure of the Invention> From the above-mentioned viewpoint, the inventors of the present invention have repeatedly conducted research through trial and error in order to achieve the above-mentioned objective, and have obtained the findings shown in fa) to (dl) below. be.

That is, (a) among austenitic-ferritic duplex stainless steels, those containing Cu, after hot working,
If a direct quenching treatment (direct solution treatment) is performed in that state, followed by an aging treatment, the strength will be significantly improved due to the combined effect of frozen hot working strain and Cu precipitation. (b) In order to improve SCC resistance, it is preferable to keep the C content in the material as low as possible, particularly to 0.03% by weight or less; Even if such a material has a low C content, it still contains Cu as a constituent component.
is included, a sufficiently large strength improvement effect can be obtained by performing the above-mentioned "direct solution treatment" in combination with the subsequent "aging treatment"; The corrosion resistance of the resulting material is particularly good when the temperature is 800°C or higher, fdl Furthermore, between the direct solution treatment and the subsequent aging treatment or between the direct solution treatment and the aging treatment, If mild cold working is performed after this process, greater strength can be imparted to the material with a small amount of processing (reduction amount), and the corrosion resistance is also better compared to conventional methods.

This invention was made based on the above knowledge, and involves hot working a Cu-containing austenitic-ferritic duplex stainless steel by heating it to a temperature of 100°C or higher, and then hot working it as it is to a temperature of 800°C or higher. Carbon dioxide corrosion resistance can be achieved by rapid cooling from a temperature of This method is characterized in that it produces a duplex stainless steel that is excellent in corrosion resistance such as corrosion resistance and SCC resistance, and has sufficiently high C2 strength.

More specifically, this invention has been achieved by comprehensively and organically combining the following technical means and the effects brought about by them. In other words, ■ Among austenitic and ferritic duplex stainless steels, those containing Cu are particularly selected, and heated to high temperatures to decompose carbides, σ phase, etc.
After forming a solid solution, it is processed hot, then rapidly cooled,
Furthermore, if proper aging treatment is performed to avoid precipitation of coarse carbides and σ phase, frozen hot working strain and Cu
The strength increases significantly due to the combination of precipitation and precipitation.

■ Since the material obtained in this way has not been subjected to cold working, it has corrosion resistance comparable to that of ordinary reheated and solution-treated materials, as will be described in the examples below. , H2S partial pressure: 10 atmospheres or less, especially H2S
The strength grade of the duplex stainless steel material, which is suitable as an oil country tubular material and which exhibits excellent corrosion resistance under Co2-H2S-C1- oil and gas well environments with a partial pressure of 5 atm or less, can be increased.

■ In addition, in order to provide even greater strength (2. Even when an additional cold working process is incorporated, the amount of processing is lower than that of materials based on ordinary reheated solution treatment materials. Therefore, it is possible to significantly reduce the partial pressure by 1
It is now possible to improve the strength of duplex stainless steel oil country tubular goods, etc. used in environments containing trace amounts of H2S at atmospheric pressure or below, even by using a mill with low power, and with a reduced amount of cold processing. Since only a small amount is required, there is almost no deterioration in corrosion resistance. In addition, if the above-mentioned cold working is performed directly after the solution treatment, the frozen hot working strain and cold working strain will overlap with the precipitation of Cu during the final aging treatment, resulting in a large strength improvement. On the other hand, when cold working is performed after aging treatment, the strengthening effect due to cold working is superimposed on the strength increasing effect due to hot working strain and Cu precipitation, and the strength is significantly improved.

[Co., Ltd.] Furthermore, in conventional solution treatment, after hot working, the material is cooled to room temperature in the atmosphere, then reheated to a high temperature and then rapidly cooled, but the ``direct solution treatment'' in this invention is This also has the secondary effect of saving thermal energy for reheating to the solution temperature and maintaining it at that temperature.

By the way, treatments such as "direct quenching" and "ausforming" are known as processing heat treatment means for steel similar to the "direct solution treatment" described above.

However, they are completely different from the above-mentioned "direct solution treatment" in the following points.

In other words, direct quenching is a process in which steel is hot-worked in the stable austenite region and then immediately quenched to cause martensitic transformation. Since quenching is performed immediately after processing, the austenite grains are larger than in the case of reheating and quenching, and therefore quenching occurs more easily, significantly increasing the hardenability of the steel and improving its strength.

However, (2) Strengthening through processing according to the present invention does not utilize the above-mentioned "strengthening through metamorphosis."

(b) Ausforming involves rapidly cooling austenitized steel to the temperature at the inlet in the isothermal transformation diagram, giving the austenitic structure an appropriate amount of plastic deformation at that temperature, and then quenching it. This treatment involves causing martensitic transformation and then tempering, and is fundamentally different from the duplex stainless steel strengthening treatment of the present invention in that processing and transformation are performed at a constant temperature.

Furthermore, in order to achieve significant strengthening by ausforming, the C content must be approximately 0.1% by weight or more, whereas in this invention, direct solution treatment of duplex stainless steel is required. ” (If it depends on two, 0.
Even with a low carbon content of less than 1% by weight, a large reinforcing effect can be obtained.

Now, the type of "austenitic-ferritic duplex stainless steel" to which the method of the present invention is applied is not particularly limited as long as it contains Cu, but the Cu content does not affect the strength. In addition, in order to improve corrosion resistance, it is preferable to set the content to 0.2% by weight or more.On the other hand, in consideration of deterioration in hot workability, it is recommended to set the upper limit to 3.0% by weight.

In addition, in order to sufficiently decompose and dissolve carbides etc. through direct solution treatment and obtain high SCC resistance, this treatment should be applied to materials with a C content of 0.03% by weight or less. It's good.

Next, in the method of the present invention, the lower limit heating temperature for hot working the austenitic-ferritic duplex stainless steel and the quenching start temperature after hot working (lower limit temperature for direct solution) are determined using the above-mentioned values. Explain the reason for the limitation.

A) Lower heating limit temperature The lower limit heating temperature for hot working austenitic-ferritic duplex stainless steel was set at 1000°C.
Heating in a low temperature range below this temperature increases the deformation resistance of the material, making hot working difficult, and decomposition and solid solution of carbides and σ phases become insufficient, resulting in poor hot workability. In addition, if direct solution treatment is used, a desired microstructure cannot be obtained, resulting in deterioration of SCC resistance. In addition, the upper limit temperature for this heating is not specified, and it may be a temperature that does not cause brittleness at high temperatures during material processing.
Temperature C such that the aperture value in a high temperature tensile test using a Grieple tester is 5096 or more, e.g. 1200 to 12
50℃).

B) Rapid cooling start temperature The reason why the rapid cooling start temperature after hot working, that is, the lower limit temperature for direct solution, is set at 800°C is to prevent SCC resistance from deteriorating if it is slowly cooled to a temperature below this temperature. This is to do so.

In addition, the "aging treatment" performed after direct solution treatment is
It is preferable to carry out the reaction in a temperature range of 500 to 700°C if possible. This is because aging treatment at temperatures below 500°C tends to result in insufficient Cu precipitation.
On the other hand, aging treatment at temperatures exceeding 700°C tends to result in insufficient strengthening due to the coarsening of precipitated Cu and the release of hot and cold working strains. . Furthermore, if the steel is aged for a long time at a temperature exceeding 700° C., carbides and σ phase may precipitate, leading to deterioration of SCC resistance.

In addition, when it is necessary to impart greater strength, cold working is performed before or after aging treatment, but the amount of cold working at this time is different from the conventional method of ``reheating and solution treatment.'' This can be significantly smaller than in the case of "cold working to obtain the same strength level."

Next, the present invention will be explained by examples and in comparison with comparative examples.

<Examples> Example 1 First, austenitic-ferritic duplex stainless steel having the composition shown in Table 1 was produced by a conventional method.

Next, these steel pieces were soaked at 1200° C. and then hot rolled, and the hot rolling was completed at finishing temperatures as shown in Table 2.

Subsequently, direct solution treatment or normal reheating solution treatment 9 aging treatment under the conditions also shown in Table 2.

and cold working treatment.

A test piece was taken from each treated material obtained in this way, the yield strength (0.296 yield strength) was measured, and the results were
It is also shown in the table.

From the results shown in Table 2, it is clear that the treatment of the present invention can significantly improve the strength of duplex stainless steel with excellent corrosion resistance. It is clear that high strength comparable to conventional materials obtained by subjecting the treated materials to cold working with a large amount of processing can be obtained.

Example 2 Regarding the austenitic-ferritic duplex stainless steel having the composition shown in Table 3, the steel piece was soaked at 1220°C, then hot rolled, and the hot rolling was carried out at the finishing temperature shown in Table 4. Finished rolling.

Subsequently, direct solution treatment or normal reheating solution treatment, aging treatment, and cold working treatment were performed under the same conditions shown in Table 4.

A test piece was taken from each treated material and the yield strength was 0.
296 proof stress) was measured, and the results are also shown in Table 4.

From the results shown in Table 4, it is clear that a duplex stainless steel material with high strength can be obtained by the method of the present invention.

Example 3 Regarding austenitic-ferritic duplex stainless steel having the composition shown in Table 5, the steel piece was soaked at 1080°C or 1250°C, then hot rolled, and the finishing temperature was as shown in Table 6. The hot rolling process was completed.

Subsequently, direct solution treatment or normal reheating solution treatment 1 aging treatment under the conditions also shown in Table 6.

and cold working treatment.

From each obtained treated material, thickness = 2 in the direction perpendicular to the rolling direction.
A test piece with dimensions of 10 m x width: 10 m x length = 75 mm and a notch in the center was taken and a corrosion resistance test was conducted.

In addition, the corrosion resistance test was carried out using simple test piece 1 as shown in the attached drawing.
is supported by a four-point support beam jig 3 supported by four fulcrums 2, and furthermore, a T-shaped screw holder 4 is used to increase the yield strength (0.2% yield strength).
H2S - 10 atmospheres Co2. -5% NaCl solution (liquid temperature 210°C) for 120 hours to investigate the presence or absence of selective corrosion in the SCC and/or ferrite regions.

The corrosion resistance test results obtained in this way were also used in the 6th test.
The yield strength is summarized in the table 5. (0.2% yield strength).

From the results shown in Table 6, it can be seen that the samples directly solution treated under the conditions of the present invention (test numbers 27, 28 and 31) had a
Direct solution treatment from low temperature range below 00℃ (
Test No. 36) has superior corrosion resistance, and it has the same strength level as that obtained by cold working after conventional reheating solution treatment (Test No. 33), but has superior corrosion resistance. it is obvious.

Furthermore, these test numbers 27, 28 and 31 have the same corrosion resistance level as the conventional reheated solution treatment (test number 32), but have greater strength and are therefore suitable for deeper wells. It is also clear that it can be done.

On the other hand, among those treated under the conditions of the present invention, those that were cold worked after direct solution treatment and before or after aging treatment (test numbers 29 and 30) showed that the conventional reheating solution treatment with a small amount of cold working After treatment, strong cold working was performed (test number 34)
and 35), and a corrosion resistance level superior to that of 35). Therefore, it can be seen that the method of the present invention is extremely effective for increasing the strength of duplex stainless steel oil country tubular goods with excellent corrosion resistance.

<Overall Effects> As explained above, according to the present invention, ■
It is possible to obtain duplex stainless steel that exhibits excellent durability in the 2-H2S-Cl- oil well or gas well environment, making it possible to increase the strength grade of oil country tubing materials, etc., and promoting the trend toward deeper wells. ■ Can strengthen duplex stainless steel with low cold working amount,
Therefore, it is easy to manufacture high-strength oil country tubular goods made of duplex stainless steel that can be used in environments containing trace amounts of H2s at a partial pressure of about 1 atm or less, and also ensures sufficient corrosion resistance. It brings about excellent effects such as being able to save the thermal energy required for reheating to the heating temperature and maintaining it at that temperature, and its industrial value is extremely high.

[Brief explanation of drawings]

The attached drawing shows a four-point bending corrosion test jig for plate-shaped test pieces with notches. In the drawings: 1. Test piece, 2. Support point, 3. Jig, 4. T-type screw holder.

Claims (3)

[Claims] (1) Austenite and ferrite containing Cu 2
The phase stainless steel is heated to 1000°C or higher, hot worked, and then quenched from a temperature of 800°C or higher,
A method for producing high-strength duplex stainless steel with excellent corrosion resistance, which is characterized by subjecting it to an aging treatment thereafter. (2) Austenite/ferritic system containing Cu 2
A stainless steel with excellent corrosion resistance, which is characterized by heating phase stainless steel to 1000℃ or higher and hot working it, then quenching it from a temperature of 800℃ or higher, cold working it, and then aging it. A method for producing high-strength duplex stainless steel. (3) Austenite/ferritic system containing Cu 2
A stainless steel with excellent corrosion resistance, which is characterized by heating phase stainless steel to 1000℃ or higher and hot working it, then quenching it from a temperature of 800℃ or higher, aging treatment, and then cold working it. A method for producing high-strength duplex stainless steel.