JPH0366384B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] This invention is directed to sulfide stress corrosion cracking (hereinafter referred to as
Regarding high-strength steels with high resistance to SSCC), in particular structural members used in oil and gas wells, such as oil country tubular goods and line pipes, or equipment members used around oil and gas wells. The present invention relates to the above-mentioned high-strength steel suitable as a steel. [Background of the invention] In the development of oil fields and gas fields in recent years,
Rapidly increasing demand and advances in technology to meet the demand are driving the need for sulfide compounds, such as hydrogen sulfide (H ₂ S), that are buried deep underground and that were previously abandoned or difficult to exploit. Gradually, attention is being focused on the development of oil and gas that are in so-called sour environments, which are highly contaminated.
Therefore, in the field of oil and natural gas production, it is necessary to withstand external forces such as earth pressure (pressure of the formation), fluid pressure (pressure of oil and gas), and tensile loads due to the weight of the steel material, as well as to withstand external forces in sour environments. The advent of steel with high strength and strong resistance to SSCC, which can fully demonstrate the desired performance even when used,
In recent years, it has become more and more in demand. Sulfide stress corrosion cracking resistance of steel (hereinafter referred to as SSCC resistance)
Since 1950, various studies have been conducted on ways to improve
NACE Standard MR−01−75 (1977Revision)
The most effective way to prevent SSCC is to keep the strength of steel below the upper limit of hardness (strength) shown in
−80 [Lower limit of yield strength is 80000psi (56.2kgf/mm ² )]
has been added to the API standard. However, in the above-mentioned deep sour wells, if oil country tubing with reduced strength is used, the wall thickness must be increased, resulting in significant economic and operational disadvantages. From this point of view, conventionally, the above L
There is a strong demand for steel materials that are even stronger than -80 and have excellent SSCC resistance, and recently,
There is also an increasing demand for high-strength oil country tubular goods whose lower limit of yield strength exceeds 90,000 psi (63.3 Kgf/mm ² ). [Problems to be solved by the conventional technology and the invention] Against this background, the durability of high-strength steel has been improved.
In order to improve SSCC properties, various methods have been studied in the past, and as a result, the steel material has a structure obtained by completely quenching to obtain a uniform structure and then sufficiently tempering at high temperature. what is most suitable,
It is also known that the finer the structure and the fewer impurity elements, the better the SSCC resistance. Therefore, based on these findings, methods for improving hardenability and tempering softening resistance, various component systems and manufacturing methods for refining the structure, such as uniform microstructures formed by quenching and tempering treatments, are proposed. Use of AISI 4130 series steel with tempered martensitic structure, JP-A-59-222526, JP-A-56-33459
Although the manufacturing method disclosed in JP-A No. 59-123716 has been proposed, the above-mentioned AISI 4130 series steel still has a sufficiently satisfactory SSCC resistance.
Unable to obtain sex, the first Japanese Patent Application Publication No. 59-222526
The method described in the issue is exclusively for prevention of grain boundary segregation and Zr
We are simply pursuing control of sulfide morphology through addition, and the resulting steel has a low Si content.
Because no Al is added, the SSCC resistance in the sour environment mentioned above is insufficient, and the steel described in the following Japanese Patent Application Laid-Open No. 56-33459 also has low Mn content and has poor SSCC resistance. Although we are trying to improve,
Since no Zr is added, the SSCC resistance under the above environment is low, and the last
Although Zr is added to the steel obtained by the method described in No. 123716 for strength adjustment,
Since the contents of Mn and Ni are not kept low, this steel also cannot exhibit sufficient SSCC resistance under the above environment. On the other hand, if plastic deformation is applied to steel materials due to cold working, SSCC resistance will deteriorate, so steel materials used in sour environments generally require extreme care to avoid cold working after the final heat treatment process. However, even so, it is not possible to completely eliminate the possibility of plastic deformation such as hitting and gripping scratches occurring when the steel material is transported or handled at construction sites, so the above SSCC resistance required for the product is In order to guarantee this, it is necessary to evaluate the SSCC resistance of steel materials, taking into account the effects of such cold plastic deformation.
Even when the present inventors examined the prior art from this perspective, it was found that the prior art cannot provide sufficient SSCC resistance to steel materials that have undergone cold plastic deformation. By the way, as a method to check SSCC resistance,
The shell type test method, which is conducted in a corrosive liquid consisting of a 0.5% CH ₃ COOH solution saturated with H ₂ S, was developed through repeated research and comparison with numerous field results. The SSCC critical stress value (Sc value) is calculated using the formula: Sc (SMYS/0.75) x 10 ^-4 [However, SMYS stands for "standard minimum yield strength" and the unit is "psi". ] It is said that cracks will not occur if the above criteria are met, but none of the conventional low-cost, low-alloy, high-strength steels satisfies these strict criteria. In addition, the shell type test method is as shown in Fig. 1, in which a test piece 1 with two drilled holes in the center in the longitudinal direction is bent at three points as shown in Fig. 2. Stress is applied to the center part, the material is immersed in a corrosive liquid, and the apparent stress at which the cracking rate is 50% is measured and this is taken as the Sc value. In Figure 2,
2 is a test specimen support, 3 is a glass round rod with a diameter of 4 mm, and 4 is a bolt for applying load (stress). In addition to these improvements to the materials themselves, methods such as coating the steel material and injecting inhibitors into the corrosive environment are also being used, but none of these methods can be expected to be sufficiently effective. . [Purpose of the Invention] In view of the above-mentioned circumstances, the present inventors conducted research to obtain a high-strength steel that exhibits sufficient SSCC resistance in a wet sulfide environment even when subjected to cold plastic deformation. As a result, we obtained the findings shown in (a) to (f) below. [Findings] (a) Carbides in steel serve as a place for hydrogen to accumulate. Therefore, if these carbides exhibit a notch defect shape such as a needle or rod shape, or if they aggregate coarsely, it is likely that However, when Zr is added to steel, the carbides become spherical and become finely dispersed, which prevents the above phenomenon and significantly improves SSCC resistance. In that case, if the content of Mn added together with Zr is kept low, the Mn will further contribute to the uniform and fine dispersion of carbides, and the SSCC resistance will be improved even when the steel material undergoes cold plastic deformation. (b) Ni contained in steel promotes pitting corrosion in a wet sulfide environment, which causes SSCC to occur, but the Ni content can be suppressed to 0.03% or less. Therefore, the occurrence of pitting corrosion is suppressed and the resistance is improved.
(c) Increasing the s content increases sulfide inclusions, which are the starting point of SSCC, leading to deterioration of SSCC resistance, and also increases (d) When appropriate amounts of Cr, Mo, and B are added together, they are formed by the synergistic action of the Cr-Mo-B system. (e) Al added to steel improves durability by reducing non-metallic inclusions and improving the cleanliness of steel.
(f) Through the combined effects of (a) to (e) above, excellent SSCC resistance can be imparted to steel even when subjected to cold plastic deformation. [Means for solving the problem] This invention was invented based on the above findings, and includes: C: 0.20 to 0.40%, Si: 0.05 to 1.0%, Mn: 0.05 to less than 0.3%, Cr : 0.1-1.5%, Mo: 0.1-0.8%, Zr: 0.01-0.15%, B: 0.0003-0.0030%, Al: 0.01-0.10%, one or more of Nb, V and Ti: each 0.01 to 0.1%, but a total of 0.2%, and further contains Ca: 0.001 to 0.005% as necessary, and the remainder consists of Fe and unavoidable impurities, and P, S and
The composition has a Ni content of P: 0.020% or less, S: 0.005% or less, and Ni: 0.03% or less (all percentages mean weight% unless otherwise specified). and a structure mainly composed of martensite formed by tempering austenite with a fine grain size of AST number 8.5 or higher formed by quenching. This provides high-strength steel with excellent crackability. The heat treatment in this invention is carried out after the steel ingot melted to have the above composition is processed into, for example, a pipe material, and then heated to a temperature range that is generally above the Ac ₃ transformation point and does not cause coarsening of austenite grains. This is quenched to give a uniform martensitic structure with a fine austenite grain size of ASTM number 8.5 or higher, and then tempered at a temperature as high as possible, usually below the Ac ₁ transformation point, e.g. 640°C to 700°C. Generally, the steel material has a tempered martensitic structure of 90% or more. Next, in this invention, the reason why the component composition range, austenite crystal grain size, and structure are limited as described above will be explained. 1 Composition of steel (a) C C component is an essential component for improving the hardenability of steel, ensuring strength, and improving temper softening resistance, but its content is 0.02
If it is less than %, strength will decrease and hardenability will deteriorate.
Therefore, SSCC susceptibility increases because tempering at a low temperature is forced to achieve the desired strength, while if the content exceeds 0.40%, toughness deteriorates and quench cracking susceptibility increases during heat treatment. From, its content is 0.20~0.40%
It was determined that (b) Si In addition to having a deoxidizing effect, the Si component is an effective component for improving the strength and hardenability of steel, but if its content is less than 0.05%, the desired effect cannot be obtained; If it exceeds %%, it causes deterioration of toughness and deterioration of SSCC resistance due to grain aggregation, so the content was set at 0.05 to 1.0%. (c) Mn The Mn component is effective in improving strength and hardenability, and is widely used for such purposes. It is necessary for the content to be 0.05% or more; on the other hand, if it is 0.3% or more, carbide precipitation becomes uneven, and SSCC resistance tends to deteriorate, especially when the steel material undergoes cold plastic deformation. , the content was set at 0.05% to less than 0.3%. (d) Cr The Cr component has the effect of improving the strength, hardenability, and temper softening resistance of steel, and is extremely effective for increasing the strength. It also has the effect of improving SSCC resistance, but its content If the amount is less than 0.1%, the desired effect cannot be obtained;
If the content exceeds 1.5%, not only will the toughness deteriorate, but also the SSCC resistance will deteriorate due to the formation of coarse carbides, so the content was set at 0.1 to 1.5%. (e) Mo The Mo component also has the effect of improving the strength, hardenability, and resistance to temper softening of steel, and as mentioned above, it is a component that improves the hardenability especially in cooperation with Cr and B. If the content is less than 0.1%, the desired effect cannot be obtained;
Containing more than 0.8% not only causes embrittlement, but also
SSCC resistance due to inappropriate carbide precipitation form
The content should be reduced from 0.1 to 0.1 to avoid the deterioration of properties.
It was set at 0.8%. (f) Zr The Zr component has the effect of significantly improving SSCC resistance by dispersing carbides spherically, uniformly, and finely in steel, but if its content is less than 0.01%, the desired effect is not achieved. is not obtained, while
If the content exceeds 0.15%, toughness and
The content should be
It was set at 0.01-0.15%. In addition, Zr is originally C,
It is an element that has a high affinity with N, O, and S, and the content of N and O is not particularly specified in the present invention, but N, Regarding the O content, there is no problem with the above-mentioned amount of Zr added in order to obtain the desired SSCC resistance. (g) B Component B has the effect of improving hardenability, and in order to ensure the desired hardenability, it is necessary to add 0.0003
% or more, but if the content exceeds 0.0030, toughness and hot workability are impaired and coarse precipitates are formed at grain boundaries.
The content of SSCC is determined from the point where it promotes the occurrence of SSCC.
It was set at 0.0003 to 0.0030%. (h) Al The Al component acts as a deoxidizing agent to improve the cleanliness of steel, thereby contributing to improving the SSCC resistance, but if its content is less than 0.01%, the desired effect is not achieved. No effect was obtained, while 0.10
If the content exceeds 0.0%, inclusions will increase and toughness and SSCC resistance will deteriorate, so the content was set at 0.01 to 0.10%. (i) Nb, v, and Ti These components act effectively on improving strength and refining the structure, but if their content is less than 0.01%, the desired effects cannot be obtained; If the content exceeds 0.1%, the toughness and workability will deteriorate, so the respective contents should be adjusted accordingly.
The content was set at 0.01-0.1%, and the total content was set at 0.2% or less. (j) Ca Ca component has the effect of spheroidizing sulfide inclusions and improving SSCC resistance, so it is included as necessary, but its content is 0.001%.
If the content is less than 0.005%, the desired effect cannot be obtained, while if the content exceeds 0.005%, inclusions will increase and aggregate into clusters, resulting in poor resistance.
Since it deteriorates SSCC properties, its content should be reduced.
It was set at 0.001-0.005%. (k) P The P component is easy to segregate, and the resulting grain boundary segregation or segregation bands cause deterioration of SSCC resistance, and if its content exceeds 0.020%, the effect cannot be ignored. , the upper limit was set at 0.020%. (l) S The S component increases sulfide-based inclusions, which are the starting point of SSCC, leading to deterioration of SSCC resistance, and the effect becomes significant when the content exceeds 0.005%, so the upper limit is set to 0.005%. %. (m) Ni When stress is applied to the steel material, if the Ni content exceeds 0.03%, pitting corrosion will be promoted and the SSCC resistance will be significantly deteriorated, so the upper limit of Ni content should be determined. The value was set at 0.03%. 2 Austenite grain size and structure Sufficient resistance to high strength steel with the above composition
In order to impart SSCC properties, it is necessary to temper steel that has an austenitic structure with a fine grain size to create a structure mainly composed of martensite.
If the grain size is coarser than ASTM number 8.5 or the steel structure is not mainly composed of tempered martensite, the SSCC resistance will be extremely deteriorated, so the austenite grain size before tempering is
In addition to limiting the grain size to ASTM No. 8.5 or higher, the steel structure was defined to be mainly composed of tempered martensite. [Example] Next, the present invention will be explained using Examples and in comparison with Comparative Examples, but these Examples are merely to show examples that embody this invention,
It is not intended to limit the technical scope of this invention. First, steels A to O having the compositions shown in Table 1 were melted by a laboratory vacuum melting method.
Here, the molten steels A to H have compositions within the scope of this invention, and the molten steels I to O have compositions that are outside the scope of this invention at the points marked * in Table 1. , melted steel for comparison. Next, a plate material with a thickness of 15 mm and a width of 120 mm was manufactured by hot rolling these steels.
Completely austenitized by heating at 920℃ for 30 minutes, quenched in water, and then further heated to 700℃.
℃ for 30 minutes to produce plate-shaped inventive steels 1 to 8 and comparative steels 3 to 9, and to evaluate the influence of austenite grain size formed after quenching on SSCC resistance. In order to investigate some materials (molten steel A and B)
Regarding the above, the quenching heating temperature was set at 980°C to coarsen the austenite grain size, and comparative steels 1 and 2 in the same plate shape were manufactured. Next, in order to evaluate the SSCC resistance of the inventive steel and comparative steel obtained in this way, the cracking stress limit ratio was measured by the shell type test method described above, and the yield strength was also measured. Furthermore, during the production of these steels, the grain size of austenite in the steel that had not yet been tempered after quenching was measured by the picric acid etch method, and this was determined.
Expressed as ASTM No. The above measurement results are summarized in Table 2. Regarding SSCC resistance, each plate made of the above-mentioned inventive steel and comparative steel was divided into two parts, and one part was left as is, and the other part was cold rolled at a rolling reduction of 5%. After that, a test piece 1 as shown in FIG. 1 is cut out, and a test piece support shown in FIG. 2 is cut out.

【table】

【table】

【table】

〔Effect of the invention〕

From the results shown in Table 2, it is clear that the comparative steels whose composition and austenite grain size are outside the range of the present invention do not have sufficient SSCC resistance, whereas the steel of the present invention secures sufficient strength. However, it can be seen that the cracking stress limit ratio is extremely high, that is, the cracking stress limit ratio is extremely high not only when cold rolling is not performed, but also when cold rolling is performed. As mentioned above, according to the present invention, it is possible to provide a steel that has high strength and excellent SSCC resistance even when subjected to cold plastic deformation, so that it can be used particularly in cold environments in sour environments as described above. Industrially useful effects can be obtained, such as being able to provide a steel suitable for use in oil wells, for example, which requires a high-strength steel that exhibits excellent SSCC resistance even when subjected to inter-plastic deformation.

[Brief explanation of drawings]

Figure 1 shows an example of a test piece used in a shell type corrosion test. In Figure 1, a is its front view and b is its side view. FIG. 3 is a schematic front view showing a state in which the device is supported by a support. In the figure, 1... test piece, 2... support, 3
...Glass rod, 4...Stress adding bolt.

Claims (1)

[Claims] 1 C: 0.20 to 0.40%, Si: 0.05 to 1.0%, Mn: 0.05 to less than 0.3%, Cr: 0.1 to 1.5%, Mo: 0.1 to 0.8%, Zr: 0.01 to 0.15%, B: 0.0003 to 0.0030%, Al: 0.01 to 0.10%, one or more of Nb, V, and Ti: 0.01 to 0.1% each, but a total of 0.2%, and the rest is Fe and unavoidable impurities. P, S and in the inevitable impurities
The Ni content is P: 0.020% or less, S: 0.005% or less, and Ni: 0.03% or less. A high-strength steel with excellent sulfide stress corrosion cracking resistance, characterized by having a structure mainly composed of martensite formed by tempering austenite having a crystal grain size. 2 C: 0.20-0.40%, Si: 0.05-1.0%, Mn: 0.05-less than 0.3%, Cr: 0.1-1.5%, Mo: 0.1-0.8%, Zr: 0.01-0.15%, B: 0.0003-0.0030% , Al: 0.01 to 0.10%, one or more of Nb, V and Ti: 0.01 to 0.1% each, but a total of 0.2%, and further Ca: 0.001 to 0.005%, The remainder consists of Fe and unavoidable impurities, and the unavoidable impurities include P, S and
The Ni content is P: 0.020% or less, S: 0.005% or less, and Ni: 0.03% or less. A high-strength steel with excellent sulfide stress corrosion cracking resistance, characterized by having a structure mainly composed of martensite formed by tempering austenite having a crystal grain size.