JPH06271939A - Production of high strength duplex stainless steel wire rod - Google Patents

Abstract

PURPOSE:To produce austenite-ferrite series duplex stainless steel wire rod in which the working amount in cold working exerting adverse influence on resisting performance to SCC and selective corrosion in an H2S coexisting environment can extremely be reduced. CONSTITUTION:Austenite-ferrite series duplex stainless steel contg. Cu is heated to >=1000 deg.C, subjected to hot working, thereafter rapidly cooled from >=800 deg.C, then subjected to warm working at 300 to 700 deg.C, is furthermore subjected to cold working or is moreover subjected to aging treatment at 450 to 700 deg.C after the cold working, or aging treatment is executed at 450 to 700 deg.C between the warm working and cold working or between direct solution treatment and warm working, by which the high strength wire rod can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention exhibits an excellent corrosion resistance in a carbon dioxide gas corrosive environment and a stress corrosive environment and also has a high strength. The present invention relates to a method for manufacturing a ferritic duplex stainless steel wire rod.

[0002]

2. Description of the Related Art In recent years, oil wells and natural gas wells have tended to deepen wells. With this trend, various measuring instruments have been introduced into wells during well drilling or oil or natural gas production.
It has become necessary to further improve the strength of the "oil well logging" for suspending jigs. In addition, carbon dioxide (CO ₂ ), hydrogen sulfide (H 2
₂ S), chlorine ions (Cl ^- From what has come to be directed eye development to the) oil wells and gas wells containing corrosive substances, such as, it has increased more importance also for the corrosion protection .

Conventionally, a method of introducing a corrosion inhibitor (inhibitor) into a well is known as the most general countermeasure for the corrosion of the well logging, but this method is expected to produce sufficient results. In many cases, it could not be done, and it could not be effectively used for offshore oil wells and gas wells.

In view of such circumstances, recently, there is a tendency to use higher-grade corrosion resistant materials, and attention is paid to the use of austenite / ferrite type duplex stainless steel having a high Cr content such as 22Cr steel and 25Cr steel. Are gathering.

However, this austenitic ferritic duplex stainless steel is able to obtain a tensile strength of about 80 kgf / mm ² at the solution treatment which is usually applied in the production of steel materials, and it is deep and deep. The strength as an oil well well logging line for wells could not be satisfied. Therefore, in applying the austenitic-ferritic duplex stainless steel, cold working is further performed after the solution treatment, and thereby the high strength required for the deep well oil well logging is provided. This is the current situation.

However, as a result of detailed experiments and studies conducted by the present inventors on the austenitic-ferritic duplex stainless steel, the following facts have become clear.
That is, an austenitic / ferritic duplex stainless steel showing excellent corrosion resistance in a high temperature humid CO ₂ environment of 150 to 250 ° C. also has a remarkable corrosion resistance if the environment is contaminated with H ₂ S or Cl ^−. It deteriorates. This CO ₂ −
H ₂ S-Cl ^- but mainly those of corrosion under oil well and gas well environments containing is stress corrosion cracking (referred to as hereinafter "SCC"), SCC in this case is usually completely different in behavior from that In addition to the presence of Cl ⁻ , the influence of H ₂ S is extremely large.

Then, further studies based on the above facts have revealed the following. A) In the austenite / ferrite type duplex stainless steel that has been subjected to the solution treatment, SCC will be generated when the H ₂ S partial pressure in the environment exceeds 10 atm. (B) Further, in the case of being strengthened by performing strong cold working, SCC occurs due to the development of the texture even if H ₂ S of about 1 atm is contained. C) Then, the presence of Cl ⁻ promotes the generation of this SCC. D) In addition, the austenite-ferrite duplex stainless steel in the above-mentioned CO ₂ —H ₂ S—Cl ⁻ environment selectively has a ferrite region selectively due to H ₂ S even if SCC is not generated. So-called "selective corrosion" that is dissolved
In some cases, cold working exerts a considerable adverse effect on this selective corrosion.

As described above, when the austenite-ferrite duplex stainless steel is subjected to the strong cold working in order to make up for the lack of strength, the corrosion resistance in the environment where H ₂ S is present is significantly deteriorated. Was recognized.

Therefore, the object of the present invention is to "H ₂
Resistance performance against SCC and selective corrosion under S coexisting environment "
Has established a strengthening means for austenite / ferrite type duplex stainless steel that can reduce the amount of cold working which adversely affects the heat treatment. For example, a trace amount of H ₂ S with a partial pressure of about 1 atm or less.
CO ₂ -H ₂ S-Cl containing ^- deep oil well environment, to provide a high corrosion resistance and high strength wire which can be sufficiently satisfactory as an oil well logging lines and the like for gas wells.

[0010]

Means for Solving the Problems The present inventor has obtained the following new findings as a result of further research by repeating trial and error in order to achieve the above object. a) Among the austenitic / ferritic duplex stainless steels, especially those containing "Cu" are directly hot-worked and then rapidly quenched (direct solution treatment), followed by Doing warm working, on frozen hot working strain and warm working strain, the strength by further superposing effect of the precipitation of Cu at the time of processing warm greatly improved, CO ₂ -
The SCC resistance of H ₂ S—Cl ⁻ under oil well and gas well environments remains well maintained. Therefore, even if cold working is performed for further strengthening, a large amount of strength can be obtained with a small working amount, and the corrosion resistance after this cold working is also subject to strong cold working after the conventional reheating solution treatment. It is better than the ones that were made.

B) In order to improve SCC resistance, C in the material
Keep the content as low as possible (less than 0.1% by weight, especially 0.03
However, even if such a material has a low C content, which is one of the strength securing components of steel, if it contains Cu as a constituent component, it is possible to directly form the solution. A combination of treatment and subsequent warm working exerts a sufficiently large strengthening effect. In addition, the effect of strengthening the material is further enhanced by performing a combination treatment of performing a warm working and then an aging treatment or a aging treatment and then a warm working after directly performing the solution treatment. c) Further, when the direct solution treatment temperature is 800 ° C. or higher, the SCC resistance becomes particularly good.

D) Moreover, when direct solution treatment, followed by warm working and cold working, followed by aging treatment, further Cu precipitation further improves the material strength. . Therefore, in this case, the cold working amount for obtaining the required strength becomes smaller, so that the SCC resistance becomes extremely good as compared with the conventional method.

E) On the other hand, even when the aging treatment is performed after the direct solution treatment and the subsequent warm working, or when the aging treatment is performed between the direct solution treatment and the subsequent warm working. , Frozen hot working strain and warm working strain,
Further, the material strength is significantly improved by the superposition effect of Cu deposited during warm working and aging, and the SCC resistance is not deteriorated. Therefore, also in this case, it is possible to reduce the cold working amount to be applied thereafter for further strengthening, and the SCC resistance becomes extremely good as compared with the conventional method.

The present invention has been made on the basis of the above-mentioned findings. "The austenitic-ferritic duplex stainless steel containing Cu is heated to 1000 ° C. or higher to perform hot working, and then, as it is. Quenching (direct solution treatment) from a temperature of 800 ° C or higher, then performing warm working at 300 to 700 ° C and further cold working, or aging treatment at 450 to 700 ° C after this cold working. Or by performing an aging treatment at 450 to 700 ° C. between the warm working and the cold working or between the direct solution treatment and the warm working, CO ₂ corrosion resistance and SCC resistance And the like, which is capable of stably obtaining an austenitic / ferrite-based duplex stainless steel wire rod having excellent corrosion resistance and sufficiently high strength ”.

More specifically, the present invention has been completed by comprehensively and organically combining the following technical means and the actions and effects brought by them. That is, 1) Among the austenite / ferrite type duplex stainless steels, the one containing Cu in particular is selected, and this is heated to a high temperature to decompose and solidify the carbide and sigma phase, and then hot work. And then quenching (direct solution treatment)
However, if appropriate warm working is performed so that precipitation of coarse carbide and sigma phase is not observed, frozen hot working strain and warm working strain, and further Cu precipitation during warm working The strength is significantly improved by the overlapping action with.

2) When the material thus obtained is cold-worked in order to impart even greater strength, the working amount is based on the usual reheat-solution-annealed material. It is possible to significantly reduce the corrosion resistance, and thus it is possible to prevent the corrosion resistance from deteriorating under an environment containing a minute amount of H ₂ S at a partial pressure of about 1 atm or less.

3) Further, if an aging treatment is carried out after the cold working, the frozen hot working strain and warm working strain and the action of Cu precipitated during warm working, and further the cold working strain. And the effect of Cu that precipitates by aging superimpose, and the material strength increases significantly. Therefore, in this case, the cold working amount for obtaining the required strength can be significantly reduced as compared with the case where the normal reheat solution treatment material is used as the base, and deterioration of the corrosion resistance hardly occurs. Absent.

4) Even if the aging treatment described above is performed between the warm working and the cold working or between the direct solution treatment and the warm working, in the final cold working, ,
Strengthening by cold working is superposed on the frozen hot working strain and warm working strain and the strength increase due to precipitation of Cu during warm working and aging, and the material strength increases significantly. Therefore, also in this case, the working amount in the final cold working can be significantly reduced as compared with the case where the normal reheat solution treatment material is used as the base, and deterioration of the corrosion resistance hardly occurs. Absent.

5) Further, in the conventional solution heat treatment, after hot working, the material which was once cooled to room temperature in the air was reheated to a high temperature and rapidly cooled, but in the direct solution heat treatment according to the present invention, This brings about a secondary effect of reheating to the solution heat temperature and saving heat energy for maintaining the solution temperature.

As a means for thermomechanical treatment of steel, which is similar in appearance to the above-mentioned direct solution treatment, there are known treatments such as "direct quenching" and "ausforming". However, they are completely different from the above direct solution treatment in the following points. (i) Direct quenching is a process in which steel is hot-worked in a stable austenite region and then immediately quenched to cause martensitic transformation, which is often followed by tempering. However, since the quenching is performed immediately after hot working, the austenite grains are larger than in the case of reheating and quenching, so that quenching becomes easier, the hardening ability of the steel is remarkably increased, and the strength is improved. However, the strengthening by the treatment of the present invention does not utilize the strengthening by this transformation.

(Ii) Ausforming is suitable for an austenitic steel having a structure as austenite obtained by quenching it to the temperature of the inlet of the isothermal transformation diagram at that temperature. A process of giving a plastic deformation, then quenching to cause martensitic transformation, and then tempering,
It is fundamentally different from the strengthening treatment of the austenitic ferritic duplex stainless steel according to the present invention in that it causes working and transformation at a constant temperature. Moreover, a C content of approximately 0.1% by weight or more is required to cause remarkable strengthening by osforming, but in the case of the strengthening treatment of the duplex stainless steel according to the present invention, the examples described later are used. As is clear from the results of the above, a large strengthening effect can be obtained even with a low C material of less than 0.1% by weight.

By the way, the "austenite-ferrite duplex stainless steel" to which the method of the present invention is applied is Cu
The type is not particularly limited as long as it contains Cu, but the Cu content is preferably 0.2% by weight or more in order to improve the strength and corrosion resistance, while the hot workability is deteriorated. Considering the above, it can be said that an appropriate upper limit is 3.0% by weight.

Further, in order to obtain excellent SCC resistance in a CO ₂ -H ₂ S-Cl ^- environment, 20% by weight or more of Cr and Mo are used.
1.5% by weight or more, Ni 6% by weight or more, and N 0.1%
It is preferable that the content of Mn is not less than 1.5% by weight, and conversely, the amount of Mn is preferably not more than 1.5% by weight.

Further, when the austenitic-ferritic duplex stainless steel contains W, it exhibits a better SCC resistance, but the addition of a large amount of W deteriorates the hot workability, so its upper limit is 4. It is preferable to set the content to about wt%. Regarding the amount of C, the desired SCC resistance can be obtained by directly performing solution treatment to decompose and solidify carbides and the like.
It is desirable that the amount be less than 0.1% by weight (preferably 0.03% by weight or less) so that the properties are obtained. It is desirable that the Si content be suppressed to a necessary minimum amount of 1% by weight or less, which is sufficient for deoxidation, yet does not impair ductility and can secure corrosion resistance.

Next, the reason why the manufacturing conditions of the steel material are limited as described above in the present invention will be described in detail. First, the heating lower limit temperature for hot working the austenitic / ferritic duplex stainless steel was set to 1000 ° C., because the deformation resistance of the material becomes large in the heating in the low temperature region below this temperature, In addition to the difficulty in processing, the decomposition and solid solution of carbides and sigma phases are inadequate, which deteriorates hot workability. In addition, the desired microstructure cannot be obtained when direct solution treatment is applied. This is because the SCC resistance is deteriorated.

The upper limit temperature of this heating is not specified, and may be a temperature that does not cause embrittlement at high temperature during material processing. That is, it suffices to select a temperature (for example, 1200 to 1250 ° C.) at which the drawing value in the high temperature tensile test using the gleeble tester becomes 50% or more.

On the other hand, the reason why the rapid cooling start temperature after hot working, that is, the lower limit temperature of the direct solution treatment is set to 800 ° C. is that the SCC resistance deteriorates when gradually cooled to a temperature range below this temperature. This is to prevent

The warm working is performed in the temperature range of 300 to 700 ° C. This is because the SCC resistance deteriorates in the warm working in the temperature range lower than 300 ° C as in the case of the cold working, while in the warm working in the temperature range higher than 700 ° C, the heat treatment in the preceding stage occurs. This is because the hot working strain and the warm working strain are released, and Cu precipitated during the warm working becomes coarse and is not effective for strengthening.

Further, the aging treatment is performed in the temperature range of 450 to 700 ° C. This is because the precipitation of Cu is not sufficient in the aging treatment at a temperature lower than 450 ° C, while the precipitation Cu is coarsened in the aging treatment at a temperature higher than 700 ° C, and the hot working strain is also caused. This is because the warm working strain and the cold working strain are released, so that it is not effective for strengthening. Moreover, if the aging treatment is performed for a long time at a temperature exceeding 700 ° C,
Coarse carbide and precipitation of sigma phase occur, and the SCC resistance also deteriorates.

By the way, the amount of processing in the "cold working after warm working or after aging treatment" in the method of the present invention is the same as that obtained by subjecting the conventional reheated solution treatment to cold working to obtain the same strength level. Since it can be made significantly smaller than the case, it is extremely effective in preventing deterioration of corrosion resistance.

Next, the present invention will be described with reference to Examples in comparison with Comparative Examples.

【Example】

[Test Example 1] Austenite / ferrite type duplex stainless steels (3 types of A to C) having the component compositions shown in Table 1 were melted by a usual method.

[0032]

[Table 1]

Next, these steel pieces were soaked at 1220 ° C. and then hot-rolled to obtain a wire having an outer diameter of 5.5 mm. Then, subsequent to this, the wire rod was directly subjected to solution treatment or ordinary reheat solution treatment.

Subsequently, the directly solution treated material is subjected to warm working, cold working and further aging treatment, while
The reheated solution treatment was cold-worked and the tensile strength was measured. Table 2 shows the measurement results of the tensile strength together with the conditions of various treatments after hot rolling.

[0035]

[Table 2]

From the results shown in Table 2, the treatment according to the present invention significantly strengthens the austenitic-ferrite duplex stainless steel, and the "normal reheat solution treatment material" is used with a small cold working amount. It can be confirmed that a wire rod with a high strength comparable to that obtained by applying a large amount of cold working can be obtained.

[Test Example 2] Steel A shown in Table 1 was heated to 1240 ° C.
After soaking, the wire rod was hot-rolled to obtain a wire rod with an outer diameter of 8.0 mm. Then, the solution treatment was carried out directly or by reheating under the conditions shown in Table 3. Then
Further, each treatment under the conditions shown in Table 3 was performed, and the tensile strength of the obtained wire rod was measured. The results are also shown in Table 3.

[0038]

[Table 3]

From the results shown in Table 3, it is clear that the treatment according to the present invention stably and significantly strengthens the austenitic-ferrite duplex stainless steel, and a wire having high strength can be easily obtained. is there.

[Test Example 3] The C steel in Table 1 was 1250.
The plate thickness is 5 to 12 mm after soaking to ℃ and hot strip rolling.
It was used as a plate material. Then, the solution treatment was carried out directly under the conditions shown in Table 4 or by reheating. Next, each treatment under the conditions shown in Table 4 was further performed, and from the obtained plate material, "thickness 2 mm x width 10 mm x length 7" was applied at right angles to the rolling direction.
A 5 mm "test piece was sampled and a corrosion resistance test was performed.

[0041]

[Table 4]

In the corrosion resistance test, as shown in FIG. 1, a T-shaped test piece 1 was used by using a 4-point support beam jig 3 for supporting the test piece 1 at four support points 2, 2 ,. screw 4 in tensile strength in a state where the corresponding stress was loaded 65% "H ₂ S partial pressure was changed to various H ₂ S-3 atm CO ₂ -3% NaCl (solution temperature 2
100 ° C) "for 100 hours and examined for the presence or absence of selective corrosion in the SCC and / or ferrite areas.

The results of this corrosion resistance test are summarized in Table 4 together with the tensile strength. The results shown in Table 4 also show that those treated in accordance with the method of the present invention have high strength, and are superior in corrosion resistance to those treated by direct solution treatment from a low temperature range below 800 ° C. It can be confirmed that the corrosion resistance is better than that of the conventional "reheat solution treatment + strong cold work treatment material" of the same strength level.

[0044]

[Summary of Effects] As described above, according to the present invention, a deep oil well containing corrosive substances such as moist CO ₂ , H ₂ S, and Cl ⁻ , an oil well logging line for gas wells, etc. is sufficient. Austenite with excellent corrosion resistance and high strength
Industrially useful effects are brought about, such as the stable production of ferritic duplex stainless steel wire rods.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a 4-point bending corrosion test.

[Explanation of symbols]

 1 Test piece 2 Support point 3 4 point support beam jig 4 T type screw

Claims (4)

[Claims] 1. An austenitic / ferritic duplex stainless steel containing Cu is heated to 1000 ° C. or higher to perform hot working, then rapidly cooled from a temperature of 800 ° C. or higher, and then 300 to 700 ° C. A method for producing a high-strength duplex stainless steel wire rod, which comprises performing warm working in step 1 and then cold working. 2. An austenitic / ferritic duplex stainless steel containing Cu is heated to 1000 ° C. or higher to perform hot working, and then rapidly cooled from a temperature of 800 ° C. or higher, and then 300 to 700 ° C. A method for producing a high-strength duplex stainless steel wire rod, which comprises performing warm working at 1, then cold working, and then performing aging treatment at 450 to 700 ° C. 3. An austenitic / ferritic duplex stainless steel containing Cu is heated to 1000 ° C. or higher to perform hot working, then rapidly cooled from a temperature of 800 ° C. or higher, and then 300 to 700 ° C. After warm working with 4
A method for producing a high-strength duplex stainless steel wire rod, which comprises aging treatment at 50 to 700 ° C. and then performing cold working. 4. An austenitic / ferritic duplex stainless steel containing Cu is heated to 1000 ° C. or higher to perform hot working, and then rapidly cooled from a temperature of 800 ° C. or higher, and then 450 to 700 ° C. After aging treatment with 3
A method for producing a high-strength duplex stainless steel wire rod, which comprises performing warm working at 00 to 700 ° C. and further cold working.