JPH0128815B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical Field) The present invention relates to an austenitic material containing Cu, Ni, Cr, and Mo, particularly a method for manufacturing an austenitic oil country tubular material, and a method for producing an austenitic material for oil country tubular goods that has excellent corrosion resistance, especially stress corrosion cracking resistance. This relates to a manufacturing method. (Background technology) In recent years, there has been a marked trend toward deeper oil and natural gas wells, which requires high-strength oil country tubular goods, and in addition, produced oil and gas contain wet hydrogen sulfide (H ₂ S).
including carbon dioxide gas (CO ₂ ) and chloride ions (Cl ^- ).
Increasingly, corrosive substances such as Along with this trend, the operating conditions for oil country tubular goods have become harsher, and countermeasures against corrosion have become even more important for stable operation. The most common method to prevent corrosion of oil country tubular goods is to add corrosion inhibitors, but this method is often not effective in offshore oil and gas wells. Furthermore, it is often not possible to expect sufficient results. Other measures such as protective coatings on the pipes have also been used, but these measures are not expected to yield sufficient results. In view of these circumstances, there has been a recent trend toward the use of higher-grade corrosion-resistant materials, and high-alloy materials such as austenitic stainless steel and Incoloy and Hastelloy (all trade names) are being adopted. However, since these materials are austenitic stainless steels or alloys, their strength, especially yield strength (0.2% yield strength), is low when subjected to solution treatment, which is the usual manufacturing method, and they are not suitable for use as oil country tubular goods for deep wells. The strength cannot be satisfied. Therefore, by adding large amounts of precipitation-strengthening elements such as Nb and Ti to these materials, adding N in particular for solid solution strengthening, or subjecting them to cold working, these materials meet the requirements for OCTG for deep wells. Currently, efforts are being made to provide high strength. However, according to the experiments and research of the present inventors, H ₂ S−
The main type of corrosion in CO ₂ −Cl ⁻ oil and gas well environments is stress corrosion cracking (SCC), but the behavior of SCC in this case is completely different from that of general austenitic stainless steel. It is. In other words, while general SCC is deeply related to the presence of Cl ^- , in the oil and gas well environments mentioned above, the influence of H ₂ S is greater than that of Cl ^- . It became clear. On the other hand, for steel pipes used for practical use as oil country tubular goods, Nb, a precipitation-strengthening element, and
When a large amount of Ti is added, these elements may impair hot workability. In addition, when applying cold working to strengthen, a large amount of processing (reduction amount) is required, which not only may cause restrictions on equipment, but also the strong cold working described above
In some cases, resistance to SCC is also significantly reduced. Furthermore, when a large amount of N is added for solid solution strengthening, there is a problem that melting and ingot making become difficult. In view of this current situation, the present inventors have developed Nb and
The highly corrosive H ₂ S-
As a result of our research, we aimed to provide high-strength OCTG that is suitable for deep wells and exhibits excellent durability even in CO ₂ -Cl ^- oil and gas well environments.
They obtained the following knowledge. (a) Austenitic stainless steels and high alloys containing Cu, Ni, Cr, and Mo will freeze if they are rapidly cooled (direct solution treatment) immediately after hot working and then subjected to aging treatment. The strength is greatly improved by the superposition of hot working strain and Cu precipitation, and the SCC resistance is good in H ₂ S−CO ₂ −Cl ⁻ oil and gas well environments. (b) In order to improve SCC resistance, it is preferable to reduce the C content of the material to less than 0.1% by weight, but even such low C materials can be treated by a combination of the above direct solution treatment and subsequent aging treatment. The reinforcing effect must be sufficiently large. (c) When the direct solution treatment temperature is 800℃ or higher,
Particularly good SCC resistance can be obtained. (d) Furthermore, if cold working is performed between the above direct solution treatment and the subsequent aging treatment, greater strength and durability can be obtained with a small amount of processing (reduction amount).
The SCC properties are also better than those using conventional methods. (e) Alternatively, even if cold working is performed after the above direct solution treatment and subsequent aging treatment, high strength and durability can be obtained with a small amount of work.
The SCC properties are also better than those using conventional methods. Now, among the processing heat treatments for steel, direct quenching treatment and ausforming are known as similar to the above-mentioned solution treatment. However,
They are completely different from the solution treatment described above in the following points. In other words, (1) direct quenching is a process in which steel is hot-worked to a stable austenite range and then quenched immediately;
This is a treatment that causes martensitic transformation, and is often used after tempering. However, by quenching immediately after hot working, the austenite grains are larger than when reheating and quenching, so the steel is hardened. The strength is significantly increased, that is, it becomes easier to burn, and therefore the strength increases. However, the reinforcement by the treatment of the present invention does not utilize the reinforcement by this transformation. (2) Ausforming involves rapidly cooling the austenitized steel to the temperature at the inlet of the isothermal transformation diagram, applying appropriate plastic deformation to the austenitic structure at that temperature, and then quenching it to martensite. It is a process of causing transformation and then tempering, and is very different from the austenitic material strengthening process of the present invention in that it involves processing and transformation at a constant temperature. Furthermore, in order to cause significant strengthening by ausforming, a C content of approximately 0.1% by weight or more is required, but in the case of the direct solution treatment of the austenitic material of the present invention, the amount of C is less than 0.1% by weight. Even with a low C material, a large strengthening effect can be obtained as shown in the examples described later. (Summary of the Invention) The present invention has been made based on the above findings, and uses austenitic materials containing Cu, Ni, Cr, and Mo, such as austenitic stainless steels such as SUS316J1, Incoloy, and Hastelloy (both A direct method in which high alloys such as Ni-based alloys (product name) are heated to a temperature of 1000°C or higher, hot worked, and immediately quenched after the hot working is finished at a temperature range of 800°C or higher. Strengthen the austenitic material for high-strength oil country tubular goods with excellent SCC resistance by applying solution treatment and further aging treatment, or by performing further cold working between or after the above quenching treatment and aging treatment. It is characterized by the fact that it does. (Detailed Description) The present inventors heated an austenitic material containing Cu, Ni, Cr, and Mo to a high temperature to dissolve carbides and precipitates into the austenite, and then hot-processed the material. If direct solution treatment is performed to rapidly cool the material immediately after hot working, and then an appropriate aging treatment is performed to prevent the precipitation of coarse carbides and sigma phase, frozen hot working strain and Cu precipitation can be eliminated. The strength increases significantly due to the superimposed action of It was discovered that even a mill with low power can be strengthened.
If the above-mentioned cold working is performed directly after the direct solution treatment, the frozen hot working strain and cold working strain will overlap with the Cu precipitation during the final aging treatment, and a large strength will be obtained. Furthermore, if cold working is performed after aging treatment, the strength increases due to hot working strain and Cu precipitation, and the strengthening due to cold working is superimposed, resulting in a significant improvement in strength. In addition, in conventional solution treatment, after hot working, the product is cooled in the air to room temperature, then reheated to a high temperature and then rapidly cooled, but the direct solution treatment according to the present invention involves heating to this solution temperature. This also has the secondary effect of saving thermal energy for holding. Next, in the present invention, the lower limit heating temperature for hot working an austenitic material is set at 1000°C because heating in a low temperature range below this temperature increases the deformation resistance of the material and makes hot working impossible. In addition to this, hot workability deteriorates due to insufficient solid solution of carbides and precipitates into austenite, and in addition, the desired microstructure cannot be obtained with direct solution treatment, resulting in poor SCC resistance. This is because it will lead to sexual deterioration. The upper limit temperature for this heating is not specified, and it may be set to a temperature that does not cause brittleness at high temperatures during material processing, and a temperature at which the reduction of area is 50% or more in a high-temperature tensile test using a Greeble tester. All you have to do is choose the temperature (for example, 1200 to 1250°C). On the other hand, the lower limit temperature of the process of rapid cooling immediately after hot working, that is, the direct solution treatment, was set at 800°C in order to prevent SCC resistance from deteriorating if the material was slowly cooled below this temperature. Further, the aging treatment is preferably carried out at a temperature range of 500 to 700°C. This is because aging treatment at temperatures below 500°C does not sufficiently precipitate Cu, and aging treatment at temperatures over 700°C causes the precipitated Cu to become coarse, as well as hot work distortion and cold stress. This is because the processing strain is released, making it no longer effective for strengthening, and furthermore, if aging treatment is performed at temperatures exceeding 700°C for a long time, coarse carbides and sigma phase will precipitate, which will reduce the resistance.
Since the SCC property deteriorates, to prevent this, the
Aging treatment at 700°C is preferable. 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 the same as that of conventional reheating solution treatment. The intensity level can be significantly reduced compared to the case where the intensity level is obtained. On the other hand, as described above, intense cold working deteriorates the SCC resistance, so the amount of cold working is preferably 30% or less in terms of cross-sectional compressibility. Here, the sectional compressibility (RA) is defined by the following equation. RA (%) = S ₀ - S ₁ /S ₀ ×100 where RA: Sectional compressibility S ₀ : Area before cold working of the cross section perpendicular to the main processing direction Si: Perpendicular to the main processing direction The area after cold working of the cross-section of Preferably, it is applied to materials of 0.03% by weight or less. On the other hand, Cu is used to improve strength and corrosion resistance.
It is preferable to contain 0.2% or more, and on the other hand, it is preferable to contain 3.0% or less in order not to deteriorate hot workability. (Example) Next, an example of the present invention will be described. Example 1 An austenitic material containing Cu, Cr, Ni, and Mo having the composition shown in Table 1 was produced by a conventional method. Next, these steel pieces (alloy pieces) are soaked at 1180℃, hot rolled, and then subjected to direct solution treatment or ordinary reheating solution treatment, and then subjected to aging treatment and cooling. The yield strength (0.2% yield strength) was measured after performing a temporary processing treatment. Table 2 shows the conditions of various treatments after hot rolling as well as the measurement results of yield strength.

【table】

[Table] From the results in Table 2, it is clear that the treatment according to the present invention can significantly strengthen the material, and that even with a small amount of cold working, it can achieve a high strength comparable to that of a conventional reheated solution-treated material subjected to a large amount of cold working. It is clear that strength is obtained. Example 2 Alloy 3 in Table 1 above was soaked at 1150°C, then hot rolled, and then subjected to direct solution treatment or reheating solution treatment under the conditions shown in Table 3, and then each The yield strength (0.2% yield strength) was measured after cold working and the results are shown in Table 3.

[Table] (Note) * Temperature of direct solution treatment ** Temperature of reheating for solution treatment (all quenching methods are water-cooled)
It is clear from this Table 3 that reinforcement can be easily achieved by the treatment of the present invention. Example 3 Alloy 1 in Table 1 above was soaked at 1080°C or 1200°C, then hot rolled, then subjected to direct solution treatment or ordinary reheating solution treatment, and then aged for each. 2 mm perpendicular to the rolling direction from the obtained plate material after treatment and cold working treatment.
Collect a specimen of thickness x 10mm width x 75mm length and perform SCC
A test was conducted. In this SCC test, a stress equivalent to the yield strength (0.2% yield strength) is applied to the above test piece 1 using a three-point support beam jig 2 that is supported at three points as shown in the attached drawing. 8 atm
_H2S , 10% saturated with _H2S , _{CO2 at} 10 atm CO2
A method was used in which the samples were immersed in a NaCl solution (temperature 175°C) for 150 hours and the presence or absence of cracks was observed. Table 4 shows the conditions of various treatments after hot rolling, as well as yield strength,
The SCC test results are summarized below.

【table】

[Table] According to Table 4, the products treated with the present invention have greater strength and are more resistant to SCC than those treated with direct solution treatment in the low temperature range below 800°C, followed by aging treatment and cold working. It is also clear that the SCC resistance is excellent compared to conventional reheat solution treatment + intense cold working materials at the same strength level. As explained above, the effects of the present invention are extremely large, and its industrial value is high.

[Brief explanation of drawings]

The attached drawing shows the SCC test jig for plate-shaped specimens. In the figure, 1...test piece, 2...jig, 3...screw holder, 4...support point.

Claims (1)

[Claims] 1. Hot working is performed after heating an austenitic material containing Cu, Ni, Cr, and Mo to a temperature of 1000°C or higher, and the hot working is finished in a temperature range of 800°C or higher. A method for producing a high-strength austenitic material for oil country tubular goods having excellent corrosion resistance, which comprises performing a direct solution treatment by immediately quenching the material, followed by an aging treatment. 2. Direct heating in which an austenitic material containing Cu, Ni, Cr, and Mo is heated to a temperature of 1000°C or higher, then hot worked, and immediately quenched after the hot working is completed at a temperature of 800°C or higher. A method for producing a high-strength austenitic material for oil country tubular goods having excellent corrosion resistance, which comprises performing solution treatment, followed by cold working, and then aging treatment. 3. Direct heating in which an austenitic material containing Cu, Ni, Cr, and Mo is heated to a temperature of 1000°C or higher, then hot worked, and immediately quenched after the hot working is completed at a temperature of 800°C or higher. A method for producing a high-strength austenitic material for oil country tubular goods having excellent corrosion resistance, which comprises performing solution treatment, followed by aging treatment, and then cold working.