JPH07207337A - Method for producing high strength duplex stainless steel - Google Patents

Abstract

(57) Abstract: OBJECTIVE CO ₂ -H ₂ S-Cl ^- also provides a sufficiently high corrosion resistance and high strength duplex stainless steel capable of satisfying a deep oil well environment, oil well logging lines and the like for gas wells . [Composition] Austenite / ferrite duplex stainless steel solution-containing material containing Cu, with a cross-section reduction rate of 35%
After performing the above cold working, it was once heated to a temperature range of 800 to 1150 ° C. at a heating rate of 50 ° C./sec or more, then rapidly cooled, and then warm worked at 300 to 700 ° C. After that, cold working is performed again, or after this cold working, aging treatment is performed at 450 to 700 ° C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention exhibits an excellent corrosion resistance even 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 material.

[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
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, 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) In addition, in the case of strengthening by subjecting to strong cold working, even if H ₂ S of about 1 atm is contained due to the development of texture, S
Produce a CC. C) Then, the presence of Cl ⁻ promotes the generation of this SCC. D) Moreover, the above-mentioned austenite-ferrite duplex stainless steel in the CO ₂ —H ₂ S—Cl ⁻ environment is
Even if SCC is not generated, so-called "selective corrosion" may occur in which the ferrite region is selectively dissolved due to the effect of H ₂ S, and cold working also has a considerable adverse effect on this selective corrosion. ing.

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

Therefore, the object of the present invention is "H ₂ S
The amount of cold working that adversely affects the resistance to SCC and selective corrosion under coexisting environment can be reduced as much as possible.
Establishing a reinforcing means of austenite-ferrite duplex stainless steel, for example, the partial pressure in CO ₂ -H ₂ S-Cl containing trace H ₂ S of more than about 1 atm ^- deep oil well environment, oil wells for gas wells It is an object of the present invention to provide a high-corrosion-resisting and high-strength steel material that is sufficiently satisfactory as a logging wire and the like.

[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, the solution-treated material of "containing Cu" is ultra-fine recrystallized by subjecting it to strong cold working followed by rapid heating / quenching. If it becomes a grain structure material and if further warm working is performed after this treatment, the effect of microstructure refinement is superposed by the warm working strain, and the effect of this warm working strain and the precipitation of Cu during warm working. The strength is greatly improved. Moreover, even by these treatments, the SCC resistance and the selective corrosion resistance in an oil well or gas well environment containing CO ₂ —H ₂ S—Cl ₂ ^— are maintained well. Therefore, even if cold working is performed for further strengthening, a sufficiently large strength level can be achieved with a small amount of cold working, so the corrosion resistance after cold working is also strong after cold remelting. It is better than that obtained by the method of performing hot working (hereinafter referred to as "conventional method").

B) In this case, the reduction rate of the cold working performed before the rapid heating / quenching treatment is 35% or more in terms of cross-section reduction rate,
Further, when the heating rate during rapid heating is adjusted to 50 ° C./sec or higher and the ultimate heating temperature is adjusted to 800 to 1150 ° C., respectively, the SCC resistance and the selective corrosion resistance become particularly good.

C) Further, if the duplex stainless steel that has been subjected to strong cold working followed by rapid heating / quenching, warm working and cold working is then aged, further Cu precipitation causes the material to precipitate. Strength is further improved. Therefore, in this case, the amount of "cold working after warm working" performed to obtain the required strength can be further reduced, and the SCC resistance and selective corrosion resistance are better than those obtained by the conventional method. It will be extremely good.

The present invention has been made on the basis of the above findings and the like. "A solution-treated material of austenite-ferrite duplex stainless steel containing Cu is used as a solution treatment material having a cross-section reduction rate of 35% or more. After hot working, it is heated once to a temperature range of 800 to 1150 ° C at a heating rate of 50 ° C / sec or more, then rapidly cooled, and then warm worked at 300 to 700 ° C and then cooled again. By performing hot working or aging treatment at 450 to 700 ° C. after this cold working, excellent corrosion resistance such as CO ₂ corrosion resistance, SCC resistance, and selective corrosion resistance and sufficiently high strength are obtained. -Stainite / ferrite type duplex stainless steel materials (wires, rods, plates, pipes, etc.) can be stably obtained. "

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, a so-called “solution treatment material” is selected which contains Cu in particular and precipitates such as carbides and σ phase in the matrix structure. If this is subjected to the specified strong cold working, then rapidly heated to an appropriate temperature range and rapidly cooled, ultrafine recrystallized grains in which carbides and σ phase are formed as a solid solution are obtained. When proper warm working is performed so that precipitation of carbides and σ phase is not observed, strength is significantly improved by superposition of fine structure and warm working strain, and further Cu precipitation during warm working. To do.

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 heat-treated material. It is possible to significantly reduce the corrosion resistance, and thus it is possible to prevent the corrosion resistance from deteriorating in 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 performed after the cold working, the microstructure, the warm working strain that has been frozen, the action of Cu precipitated during the warm working, and the action of the cold working strain are performed. And superimpose with the action of Cu that precipitates by aging, 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.

By the way, the "austenitic-ferrite duplex stainless steel" to which the method of the present invention, which has been completed under the above-described concept, is applicable is of a type that is special if it contains Cu. The Cu content is not limited to 0.2% by weight in order to improve strength and corrosion resistance.
It is preferable that the above amount be satisfied. On the other hand, considering the deterioration of the hot workability, the upper limit of 3.0% by weight is appropriate.

Further, in order to obtain excellent SCC resistance and selective corrosion resistance in a CO ₂ -H ₂ S-Cl ^- environment, Cr is 20 wt% or more, Mo is 1.5 wt% or more, and Ni is 1.5 wt% or more. It is preferable that the content of Nn is 6 wt% or more and the content of N is 0.1 wt% or more, and conversely, the amount of Mn is preferably 1.5 wt% or less.

Further, when the austenitic-ferritic duplex stainless steel contains W, it exhibits better SCC resistance and selective corrosion resistance, but the addition of a large amount of W deteriorates the hot workability. Therefore, the upper limit is preferably about 4% by weight. The amount of C is preferably less than 0.1% by weight (preferably 0.03% by weight or less) so that the desired SCC resistance can be obtained. The Si content is preferably controlled to a necessary minimum amount of 1% by weight or less, which is sufficient for deoxidation, does not impair the ductility, and ensures 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, prior to the rapid heating / quenching treatment, the austenite / ferrite type 2
The reduction amount of the cross-section reduction rate applied to the duplex stainless steel solution treated material was set to 35% or more because the cold-working amount lower than this amount was the recrystallization of fine grains in the next rapid heating / quenching treatment. This is because the structure cannot be obtained and the SCC resistance and the selective corrosion resistance are deteriorated. The upper limit of the cold working amount does not have to be specified in particular, and is determined from the aspect of equipment.

By the way, the solution heat treated material may be not only a normal reheat solution heat treated material but also a direct solution heat treated material immediately quenched after hot working. Further, the “area reduction rate” referred to in the present invention is defined by the following equation. Area reduction rate (%) = {(S ₀ −S ₁ ) / S ₀ } × 100, where S _{1 is} the area of the cross section perpendicular to the main processing direction of the processed steel, S _{0 is} the main processing direction of the material. The area of the cross section perpendicular to.

Next, the heating rate in the rapid heat treatment after cold working of the solution heat treated material is set to 50 ° C./sec or more, because the recrystallized grains grow and coarsen at a heating rate lower than this. In addition to not being effective for strengthening, SCC resistance
This is because the deterioration of corrosion resistance and selective corrosion resistance occurs. The upper limit of the heating rate does not have to be set in particular, either, and is also determined from the aspect of equipment.

The reason why the heating temperature in the above rapid heat treatment is limited to 800 to 1150 ° C. is that the cold worked structure remains as it is or the recrystallization becomes insufficient at a temperature below 800 ° C. Or σ phase may be precipitated, resulting in deterioration of SCC resistance and selective corrosion resistance. On the other hand, at a temperature higher than 1150 ° C., recrystallized grains are coarsened and are no longer effective for high strength. Further, the SCC resistance and the selective corrosion resistance are deteriorated. It is desirable that the holding at 800 to 1150 ° C. be within 20 seconds in order to obtain a fine structure.

The rapid cooling after the rapid heat treatment is intended to maintain the fine recrystallized structure to increase the strength and prevent the precipitation of carbides and σ phase during cooling to prevent the deterioration of SCC resistance and selective corrosion resistance. This is to prevent it. The quenching means may be water cooling, oil cooling or the like as long as the above object can be achieved.

The warm working is carried out in the temperature range of 300 to 700 ° C. In the warm working in the temperature range of less than 300 ° C., the texture develops and the SC resistance is improved as in the case of the cold working.
This is because the C property and the selective corrosion resistance are deteriorated.
This is because the warm working strain is released in the warm working in the temperature range exceeding 0 ° C., and Cu precipitated during the warm working becomes coarse and is not effective for increasing the strength.

Furthermore, the aging treatment carried out as required is 4
It is performed in a temperature range of 50 to 700 ° C. This is 450
Precipitation of Cu is not sufficient in the aging treatment at a temperature lower than ℃, while in the aging treatment at a temperature higher than 700 ℃, in addition to coarsening of the precipitated Cu, warm working strain and cold working This is because the strain is released and it is no longer effective for increasing the strength. Moreover, if the aging treatment is carried out at a temperature higher than 700 ° C. for a long time, coarse carbides and σ phase are precipitated and the S resistance is increased.
CC property and selective corrosion resistance also deteriorate.

By the way, the processing amount in the "cold working after warm working" in the method of the present invention is higher than that in the case where the conventional reheated solution treatment is subjected to cold working to obtain the same strength level. Since it can be made extremely small, 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] First, an austenitic / ferritic duplex stainless steel (A to C 3) having the composition shown in Table 1 was prepared.
A reheated solution-finished wire rod having an outer diameter of 8.5 mm was prepared.

[0029]

[Table 1]

Next, the processing and heat treatment shown in Table 2 were applied to these wire rods, and the tensile strengths were measured. Table 2
The results of tensile strength measurements are shown in Fig. 1 along with the conditions of processing and heat treatment.

[0031]

[Table 2]

The results shown in Table 2 indicate that the treatment according to the present invention significantly strengthens the austenite-ferrite duplex stainless steel, and that the amount of cold work is "conventional". It can be confirmed that it is possible to obtain a wire rod having a high strength comparable to that obtained by subjecting a heat-solution treated material to a large cold working amount.

[Test Example 2] A reheated solution-annealed finish wire (outer diameter: 7.5 mm) made of the steel A in Table 1 was prepared, and after the working and heat treatment shown in Table 3 were performed, the tensile strength of each wire was measured. It was measured. Table 3 shows the measurement results of the tensile strength together with the processing and heat treatment conditions.

[0034]

[Table 3]

From the results shown in Table 3, it can be seen that the treatment according to the present invention can stably significantly increase the strength of the austenite-ferrite duplex stainless steel, and easily obtain a high-strength wire. it is obvious.

[Test Example 3] A reheated solution-finished sheet material (sheet thickness 6 to 30 mm) made of the C steel in Table 1 above was prepared and subjected to the processing and heat treatment shown in Table 4 and then the tensile strengths thereof, respectively. Was measured. Further, a test piece of "2 mm thickness x 10 mm width x 75 mm length" was taken from the plate material after processing / heat treatment at right angles to the rolling direction, and a corrosion resistance test was carried out.

[0037]

[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, .... "H ₂ S-5 atmospheric pressure CO ₂ -5% NaCl (liquid temperature 210 ° C) with various H ₂ S partial pressures" with a stress equivalent to 65% of the tensile strength being applied with the screw 4.
It was immersed in the solution for 120 hours, and the presence or absence of selective corrosion in the SCC and / or ferrite region was investigated.

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 the material subjected to the treatment according to the method of the present invention has a high strength, and has the same strength level as the conventional "reheat solution treatment + strong cold work treatment material". It can be confirmed that the corrosion resistance is better than that of ".

[0040]

As described above [Summary of effects] According to the present invention, wet _{CO 2, H 2 S, Cl} - deep oil wells containing corrosive substances such as, as oil well logging lines and the like for gas well enough It is possible to stably produce an austenitic / ferrite-based duplex stainless steel material having excellent corrosion resistance and high strength, and industrially useful effects are brought about.

[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 (2)

[Claims] 1. A cross-section reduction rate of 3 is applied to a solution-treated material of austenite / ferrite duplex stainless steel containing Cu.
After performing cold working of 5% or more, once heat at a heating rate of 50 ° C / sec or more to a temperature range of 800 to 1150 ° C, quench it, and then warm work at 300 to 700 ° C. A method for producing a high-strength duplex stainless steel material, characterized by performing cold working again after the hot working. 2. A cross-section reduction ratio of 3 is applied to a solution-treated material of austenite / ferrite duplex stainless steel containing Cu.
After performing cold working of 5% or more, once heat at a heating rate of 50 ° C / sec or more to a temperature range of 800 to 1150 ° C, quench it, and then warm work at 300 to 700 ° C. After that, cold working is performed again, and then 450 to 70
A method for producing a high-strength duplex stainless steel material, characterized by performing an aging treatment at 0 ° C.