JPS61223166A - High strength steel having superior resistance to sulfide stress corrosion cracking - Google Patents

Abstract

PURPOSE:To improve the strength of a steel and the resistance to sulfide stress corrosion cracking by hardening the steel contg. prescribed percentages of C, Si, Mn, Cr, Mo, Zr, etc. and restricted percentages of P, S and Ni. CONSTITUTION:A steel contg., by weight, 0.2-0.4% C, 0.05-1% Si, 0.05-0.3% Mn, 0.1-1.5% Cr, 0.1-0.8% Mo, 0.01-0.15% Zr, 0.0003-0.003% B, 0.01-0.1% Al and <=0.2% in total of one or more among 0.01-0.1% each of Nb, V and Ti is produced. The amounts of P, S and Ni as impurities in the steel are restricted to <=0.02% P, <=0.005% S and <=0.03% Ni. The steel is hardened to form austenite having a grain size represented by ASTM grain size No. >=8.5. The austenite is then tempered to form a martensite-base structure.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention is directed to sulfide stress corrosion cracking (hereinafter referred to as 5SCC) caused by a humid environment containing hydrogen sulfide.
Regarding high-strength steels having high resistance to It concerns high strength steel.

[Background of the invention]

In the development of oil and gas fields in recent years, the rapidly increasing demand and the advancement of technology to meet this demand have led to the development of oil and gas fields that were previously neglected or difficult to develop.
Increasingly, attention is being focused on developing oil and gas in so-called sour environments, which are buried deep underground and are heavily contaminated with sulfides such as hydrogen sulfide (HtS). In the field of natural gas production, it can withstand external forces such as tonosho (pressure of the formation), fluid pressure (pressure of oil and gas), and tensile loads due to the weight of the steel material, and is also suitable for use in sour environments. In recent years, there has been an increasing demand for a steel with high strength and strong resistance to 813CG, which can exhibit the desired performance.

Since 1950, various studies have been conducted on ways to improve the sulfide stress corrosion cracking resistance (hereinafter referred to as 5SCC resistance) of steel, and currently, for example, N A C B S
t, andard MR-(H-75 (1g) 7Rev
It is said that keeping the strength of steel below the upper limit of hardness (strength) shown in
o [The lower limit of yield strength is 80,000 pa1 (56,2kr
f/isl) :l has been added to the AP engineering standard.

However, in the sour deep well as mentioned above,
If OCTG with reduced strength is used, the wall thickness must be increased, which brings about economic and operational disadvantages.From this point of view, conventional , there is a strong desire for a steel material that is even higher in strength than the above-mentioned L-80 and also has excellent 5scc resistance.
Demand for high-strength oil country tubular goods exceeding 3.3 kPf/5 nJ) is also increasing.

[Problems to be solved by conventional technology and invention]

Against this background, various methods have been studied to improve the 5BCC resistance of high-strength steel, and as a result, the steel material has been completely hardened to have a uniform structure. It has been found that a structure obtained by sufficient tempering at a high temperature is most suitable, and that the finer the structure and the fewer impurity elements, the better the 5 SCC resistance.

Therefore, based on these findings, methods for improving hardenability and temper softening resistance, various component systems and manufacturing methods for refining the structure, such as uniform tempered martensitic structures formed by quenching and tempering treatments, are proposed. The use of A/S 4130 series steel and JP-A-59
Although the manufacturing methods disclosed in JP-A-222526, JP-A-56-33459 and JP-A-59-123716 have been proposed, the AIS'I 4130 series steel still has a sufficiently satisfactory 5scc resistance. However, the method first described in JP-A-59-222526 only sought to prevent grain boundary segregation and control the sulfide morphology by adding Zr, and the steel obtained by this method only sought to prevent grain boundary segregation and control the sulfide morphology by adding Zr. Because the S1 content is low and M is not added, the 5SCC resistance in the sour environment described above is insufficient, and the steel described in the following JP-A-56-33459 also has a reduced Mn content. However, since Zr is not added, the 5SCC resistance under the above environment is low, and the last Japanese Patent Application Laid-Open No. 59-1
Although Zr is added to the steel obtained by the method described in No. 23716 for strength adjustment,
! Since the contents of n and Ni are not kept low, this steel cannot exhibit sufficient 5scc resistance under the above environment.

On the other hand, when plastic deformation is applied to steel materials due to cold working, it can withstand up to 5 seconds.
Due to the deterioration of CC properties, great care is generally taken to ensure that steel materials used in sour environments are not subjected to cold working after the final heat treatment process. Since it is impossible to completely eliminate the possibility of plastic deformation such as hitting or gripping scratches occurring during handling in It is necessary to evaluate the 5scc resistance of steel materials, taking into account the influence. Even if the present inventors examine the prior art from this perspective,
It has been found that the above-described prior art cannot sufficiently impart 8SCC resistance to steel materials that have undergone cold plastic deformation.

By the way, as a method to confirm 5SCC resistance, H,S
The shell type test method, which is carried out in a corrosive liquid consisting of a 0.5% CH3CO0H solution saturated with
5scc critical stress value (sc
Although it is said that cracking does not occur if the value (value) satisfies the formula,
None of the conventional low-cost, low-alloy, high-strength steels satisfied such strict standards.

In addition, the shell type test method is a test piece 1 with two drilled holes in the center of the length, as shown in Figure 1.
As shown in Fig. 2, stress is applied to the central part by three-point support bending, and the specimen 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. It is.

In Figure 2, the reference numeral 2 is a test specimen support, the reference numeral 3 is a glass round rod with a diameter of 4H, and the reference numeral 4 is a bolt for applying load (stress). .

In addition to these improvements to the materials themselves, methods such as coating steel and injecting inhibitors into the corrosive environment have been used, but none of these methods have been expected to be sufficiently effective.

[Purpose of the invention]

In view of the above-mentioned circumstances, the present inventors have conducted research in order to obtain a high-strength steel that exhibits sufficient resistance to 5 SCC in a wet sulfide environment even when subjected to cold plastic deformation. As a result, the findings shown in (a) to (f) below were obtained.

[Knowledge]

(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 coarsely agglomerate, this becomes the starting point and the 5scc However, Zr in steel
When Zr is added, the carbides become spherical and are finely dispersed, thereby preventing the above phenomenon and significantly improving the 5SCC resistance. However, in this case, if the content of Mn added together with Zr is kept low , the Mn further contributes to uniform and fine dispersion of carbides, further improving the 5scc resistance even when the steel material undergoes cold plastic deformation; (1)
) Ni contained in steel promotes pitting corrosion in a wet sulfide environment and causes BBCC to occur from there, but
If the Ni content is kept below 0.034 Jg, the occurrence of pitting corrosion will be suppressed, and the 5SCC resistance will also be improved. (d) Adding an appropriate amount of Cr%Mo, B (e) M added to steel reduces non-metallic inclusions and improves the cleanliness of the steel. (f) Through the combined effect of (a) to (e) above, the steel has excellent 5SCG resistance even when subjected to cold plastic deformation. What you can do.

[Means for solving problems]

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 0.3%, Cr: 0.1-1.5%, Mo: Q, 1-0.8%, Zr: 0.01-0.15%, B:
0.0003-0.0030%, A11: 0.01-
0.10%, one or more of Nb, V and Ti: 0.01 to 0.1% each, with a total of <0.2 Ti, and if necessary, ca: o , o Ol ~ 0.005%, the remainder consists of Fe and unavoidable impurities, and the contents of p, s and Ni in the unavoidable impurities are P: 0.020- or less, El: 0 0.005% or less, Ni: 0.03% or less, composition (more than % by weight, hereinafter, unless otherwise specified, all % means weight%) and ASTM number 8.5 formed by quenching. characterized by having a structure mainly composed of martensite formed by tempering austenite having a fine grain size of
The present invention provides a high-strength steel with excellent resistance to sulfide stress corrosion cracking.

The heat treatment in the present invention involves processing a steel ingot melted to have the above composition into, for example, a pipe material, heating it to a temperature range that is generally above the AC3 transformation point and not causing coarsening of austenite grains, and then quenching. and AST this
A uniform martensitic structure having a fine austenite crystal grain size with an M number of 8.5 or more is formed, and then this is usually heated under Ac, at a temperature as high as possible below the transformation point, e.g. 640°C ~
It is tempered under conditions of 00°C to obtain a steel material having a tempered martensitic structure of usually 90 cm 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 The C component is an indispensable component for improving the hardenability of steel, ensuring strength, and improving temper softening resistance. If the content is less than 20%, the strength will decrease and the hardenability will deteriorate, and the desired strength will be forced to be tempered at a low temperature, resulting in a high 5scc sensitivity.On the other hand, if the content exceeds 0.40%,
The content was determined to be 0.20 to 0.40% because it not only deteriorates toughness but also increases susceptibility to quench cracking during heat treatment.

(b) The 131 Si component has a deoxidizing effect and is an effective component for improving the strength and hardenability of steel, but when its content is 0.05
If it is less than 1%, the desired effect cannot be obtained;
．． If it exceeds 0%, toughness will deteriorate and the resistance to 5scc will decrease due to coarse graining.
The content should be set at 0.05 to 1.
It was set as 0chi.

(c) Mn The Mn component is effective in improving strength and hardenability, and is widely used for such purposes. However, in order to prevent grain boundary embrittlement due to S, its content must be increased. 0.0
It needs to be 5% or more; on the other hand, if it exceeds 0.3 inches, carbide precipitation becomes uneven and the 5scc resistance tends to deteriorate, especially when the steel material undergoes cold plastic deformation. Therefore, the content was determined to be 0.05 to 0.3 chi.

(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 5SCGSC properties, but its content If the content is less than 0.14, the desired effect cannot be obtained, while if the content exceeds 1.5, the toughness is deteriorated and the 5SCC resistance is also deteriorated due to the formation of coarse carbides. The content was determined to be 0.1 to 1.5%.

(e) M.

Mo acid component has the effect of improving the strength, hardenability, and temper softening resistance of steel, and as mentioned above, it is a component that improves hardenability especially in cooperation with Cr and B. If the content is less than 111, the desired effect cannot be obtained; on the other hand, if the content exceeds 0.8 h, not only will it cause embrittlement, but the precipitation form of carbides will not be appropriate, resulting in poor BBC resistance.
Since the C property deteriorates, the content should be increased from 0.1 to 0.
It was set at 8%.

(f) Zr The Zr component has the effect of significantly improving the ICC resistance by dispersing carbides in spherical and uniform fine particles in the steel, but if its content is less than 0.01%, the desired effect may not be achieved. However, if the content exceeds 0.15%, the toughness and 5SCC resistance deteriorate, so the content was set at 0.01 to 0.15%. In addition, Zr
is an element that originally has a high affinity with C, N, and O1S, and in the present invention, the content of N and 0 is not particularly specified, but it is With respect to the N and O contents, there is no problem with the above-mentioned amount of Zr added in order to obtain the desired 5EICC resistance.

(b)) B The B component has the effect of improving hardenability, and in order to secure the desired hardenability, it is necessary to add 3% or more of O,0O0. The content was determined to be 0.003 to 0.0030 Chi, since if it exceeds 0.000%, toughness and hot workability will be impaired, and coarse precipitates will be formed at the grain boundaries, promoting the generation of BSCG.

Color) M The M component acts as a deoxidizing agent and improves the cleanliness of steel, thereby contributing to improving the BBCC resistance, but if its content is less than O, Ol 4, the desired effect is not achieved. However, the effect of 0. If the content exceeds 10%, inclusions will increase and the toughness and 5scc resistance will deteriorate, so the content was set at 0.01 to 0.10%.

(i) Nb, V-o! These components have an effective effect on improving strength and refining the structure, but if the content is less than O.014, the desired effects cannot be obtained; If the content exceeds that amount, the toughness and workability will deteriorate, so each content was set at 0.01 to 0.1%, and the total content was set at 0.2% or less.

(j) P The P component is easy to segregate, and the resulting grain boundary segregation or segregation bands cause deterioration of 5SCC resistance, and if its content exceeds 0.020%, the effect cannot be ignored. Therefore, the upper limit was set at 0.020%.

佽）i) The content of O,
As the effect becomes significant when OO exceeds 5%,
The upper limit was set at 5%.

2. Austenite grain size and structure In order to impart sufficient 5SCC resistance to high-strength steel having the above composition, the steel with an austenitic structure having a fine grain size is tempered, and this is made into a structure mainly composed of martensite. If the grain size of the austenite is coarser than ASTM number 8.5, or if the steel structure is not mainly composed of tempered martensite, its 5SCC resistance will be extremely low. Therefore, the austenite grain size before tempering was limited to a fine grain size of ASTM 48.5 or higher, and the structure of the steel was defined as a structure mainly composed of tempered martensite.

〔Example〕

Next, this invention will be explained by comparing it with Examples and Comparative Examples, but these Examples are merely to show examples embodying this invention, and do not limit the technical scope of this invention. It is not intended to.

First, steels A and O having the compositions shown in Table 1 were melted using a laboratory vacuum melting method.

Here, the molten steels A and H have compositions within the scope of this invention, and the molten steel weight ~0 has a composition that is outside the scope of this invention in the points marked with * in Table 1. This is a melted steel for comparison.

Next, a plate material with a thickness of 15 fl and a width of 120 m manufactured by hot rolling these steels was heated to 920°C.
After being completely austenitized by heating for 30 minutes at
A tempering treatment is carried out by holding the steel for 5 minutes, thereby producing plate-shaped steels of the present invention 1 to 8 and comparative steels 3 to 9, and the austenite grain size formed after quenching is
In order to investigate the effect on the properties of some materials (molten steel A
As for and B), the quenching heating temperature was set at 980° C. to coarsen the austenite grain size, and thereby comparative steels 1 and 2 in the same plate shape were manufactured.

Next, in order to evaluate the 5scc resistance of the inventive steel and comparative steel obtained in this way, the cracking stress limit ratio was measured by the above-mentioned shell tie test method, and the yield strength was also measured. During the production of the steel, the grain size of austenite in the steel which had not yet been tempered after quenching was measured by the picric acid etching method, and this was expressed in ASTMA.

The above measurement results are summarized in Table 2.

Regarding BBCC 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, test pieces 1 as shown in FIG. 0.59g saturated with hydrogen sulfide
The cracking stress limit ratio was measured by immersing the test piece 1 in an acetic acid aqueous solution for 200 hours, and the value is shown.

Note that the bottle mark in Table 2 indicates that the conditions marked with it are outside the scope of the present invention.

〔Effect of the invention〕

On the other hand, the steel of the present invention has a high cracking stress limit ratio while ensuring sufficient strength. It can be seen that it exhibits a high cracking stress limit ratio.

As mentioned above, according to this invention, along with high strength,
Excellent resistance to 5scc, especially when subjected to cold plastic deformation
As we can provide steel with excellent properties, it has excellent resistance to cold plastic deformation under the above-mentioned sour environment.
Industrially useful effects can be obtained, such as being able to provide a steel suitable for use in oil wells, which requires high-strength steel exhibiting BCC properties, for example.

[Brief explanation of the drawing]

Figure 1 shows an example of a test piece used in a shell-type corrosion test. In Figure 1, (a) is its front view;
ψ) is a side view thereof, and FIG. 2 is a schematic front view showing the test piece supported by a test piece support. In the figure, 1... test piece, 2... support, 3...
Glass rod, 4... Stress adding bolt.

Claims (2)

[Claims] (1) C: 0.20-0.40%, Si: 0.05-1.0%, Mn: 0.05-0.3%, Cr: 0.1-1.5%, 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 of Nb, V and Ti Species or two or more species: 0.01 to 0.1% each, however, total ≦0.2%, with the remainder consisting of Fe and unavoidable impurities, and the inclusion of P, S, and Ni in the unavoidable impurities. The composition (in weight% or more) has the following amounts: P: 0.020% or less, S: 0.005% or less, Ni: 0.03% or less, and the ASTM number 8.5 or more formed by quenching. A high-strength steel with excellent resistance to sulfide stress corrosion cracking, characterized by having a structure mainly composed of martensite formed by tempering austenite having a fine grain size of . (2) C: 0.20-0.40%, Si: 0.05-1.0%, Mn: 0.05-0.3%, Cr: 0.1-1.5%, 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 of Nb, V and Ti Species or two or more species: 0.01 to 0.1% each, but total ≦0.2%, further containing Ca: 0.001 to 0.005%, and the remainder being Fe and unavoidable impurities and the contents of P, S and Ni in the inevitable impurities are respectively P: 0.020% or less, S: 0.005% or less, Ni: 0.03% or less, % by weight) and a structure mainly composed of martensite formed by tempering austenite having a fine grain size of ASTM number 8.5 or higher formed by quenching. High-strength steel with excellent stress corrosion cracking resistance.