JPS634047A - High-tensile steel for oil well excellent in sulfide cracking resistance - Google Patents

Abstract

PURPOSE:To obtain a high-tensile steel for oil well excellent in sulfide cracking resistance by specifying a composition consisting of C, Si, Mn, Cr, Mo, Ti, Al, B, Zr, Hf, Nb, V, and Fe and by properly limiting the amounts of inevitable- impurity elements. CONSTITUTION:The high-tensile steel for oil well excellent in sulfide cracking resistance has a composition which consists of, by weight, 0.15-0.45% C, 0.1-0.8% Si, 0.2-0.8% Mn, 0.2-<1.0% Cr, 0.1-0.8% Mo, 0.001-<0.010% Ti, 0.005-0.060% Al, 0.0001-0.0030% B, 0.01-0.15% Zr and/or 0.001-0.150% Hf, 0.01-0.15% Nb and/or 0.01-0.15% V, and the balance essentially Fe and further containing, if necessary, one or more kinds among 0.001-0.010% Ca and 0.001-0.030% rare earth elements and in which amounts of P, S, Cu, Ni, N, and O among inevitable impurities are limited to <=0.01%, <=0.005%, <=0.15%, <=0.05%, <=0.0150%, and <=0.0050%, respectively and, moreover, it is free from deterioration due to plastic deformation.

Description

[Detailed Description of the Invention] Industrial Application Fields The present invention provides high resistance to sulfide cracking (hereinafter abbreviated as SSC) caused in a so-called "sour environment" containing moist hydrogen sulfide. The present invention relates to high-strength steel suitable as oil well steel particularly used in sour environments.

SSC is a type of environmental embrittlement phenomenon that occurs when stress is applied to steel materials in a sour environment. For example, when this happens to oil country tubular goods, it can lead to large economic losses that may even lead to the abandonment of oil wells. Moreover, there is a great risk of serious accidents due to leakage of toxic gas, etc. Therefore, first of all, steel materials used in sour oil wells and sour gas wells are required to have excellent SSC resistance.

On the other hand, when designing a well, it is necessary to consider the tensile stress due to the steel's own weight and the capacity of the rig used, and for this purpose, it is necessary to reduce the weight of the steel, that is, to increase its true tension. The recent development of deep wells has made this demand even more urgent.

However, since the strength and SSC resistance of steel materials are generally incompatible with each other, the development of sour oil wells and sour gas wells in deep geological formations has been severely constrained.

<Prior art and its problems> By the way, great efforts have been made to elucidate the metallurgical factors that affect the SSC resistance of steel materials, and the following points are the most important. was known to. That is, ta) The SSC resistance of steel deteriorates as the material strength increases, and conversely improves as the material strength decreases;
~79 kgf/mi”) are generally SSC
is unlikely to occur.

(bl) When compared at the same strength level, materials with a "tempered martensitic structure" as the steel structure have the best SSC resistance, and this is due to the high homogeneity of the m-weave. If it has such a structure, it has the advantage that high strength can be obtained with the addition of a relatively small amount of alloying elements.

In order to homogenize the steel structure through quenching and tempering, it is first necessary to completely quench the structure to make it martensitic.
s Addition of Mo and B is effective.

(C) A high temperature tempering temperature within a range that does not exceed the ACl transformation point is advantageous for SSC resistance, and this is thought to be due to the reduction of internal strains such as dislocations and the spheroidization of carbides.

(d) The finer the crystal grains to -1'ffl, the better the SSC resistance of the steel material, so adding Nb or the like or making the grains finer by heat treatment such as rapid heating quenching is effective in improving the SSC resistance.

(e)! Regarding impurity elements in the Pal material, SSC resistance is often deteriorated due to increase in inclusions and grain boundary segregation, and in particular, the lower the content of P and S, the better.

As described above, the relationship between SSC resistance and metallurgical factors has become qualitatively clear, although it cannot be said to be complete, and the API (American Petroleum Institute) has established a sour environment The standardization of oil country tubular goods for commercial use is “C-90 grade” (yield strength: 63.3 to 73.8 kgf/mm2).
)” strength level has even been developed.

However, even though the relationship between the metallurgical factors of copper materials and SSC has been elucidated to some extent, sufficient targeted knowledge has not been obtained to perfect SSC countermeasures. Research has shown that (a') SSC occurs even in low-strength steels when corrosive environments and stress conditions become severe; - It is only a guideline for general material selection and does not provide guidelines for improving SSC resistance. (b゛) Even though steel materials with a tempered martensitic structure exhibit good SSC resistance, In the case of high-strength steel, simply forming a tempered martensitic structure does not necessarily give a practically sufficient 1tssc property. In addition, improving the hardenability is an effective means to obtain a homogeneous tempered martensitic 'lJi texture and achieve good SSC resistance, and the addition of C%Sl, in, Crs Mo and B improves the hardenability. Although this is a method that has been used in conventional oil well steels, the above elements often deteriorate SSC resistance depending on the amount added, and it is difficult to improve hardenability and SSC resistance. (C') High tempering temperature (Although it improves SSC resistance, tempering at high temperature reduces material strength; How to secure the necessary strength is an extremely difficult problem in practice.For this reason, it is possible to add various solid solution elements and carbide-forming elements, but the elements necessary to ensure SSC resistance (d') Is grain refinement certainly effective in improving SSC resistance?Although this is a necessary condition for improving the SSC resistance of steel materials, there is no clear knowledge regarding the types and amounts added. It was clarified that (C') reduction of pJ?)s, which is an impurity element, is a necessary condition for improving the SSC resistance of steel materials, but it is not a sufficient condition, and strength grades were standardized. If the SSC resistance becomes even higher than that of the standard, it suggests that sufficient SSC resistance cannot be stably achieved based on conventional knowledge, and further research is needed to improve the local stability even with already standardized grades. They also found that plastic deformation significantly deteriorates SSC resistance and may not be suitable for practical use.

For example, even if oil country tubular goods are manufactured and handled with the utmost care to avoid cold deformation, some degree of local plastic deformation is unavoidable when connecting them with joints at oil well sites. It is. Therefore, taking these into consideration, a steel with low deterioration in SSC resistance due to plastic deformation is required for sour applications, but with conventional steel, this deterioration is large, and the higher the strength, the less affected by plastic deformation. I couldn't avoid the easy problem.

<Means for solving the problem> From the above-mentioned viewpoint, the present inventors have developed an excellent SSC-resistant
It has both high strength and high strength, and also has SS resistance due to plastic deformation.
As a result of further research aimed at producing high-strength oil well steel without deterioration of carbon properties, the following findings were obtained.

i) In oil well steel, it is essential to add a certain amount of C, Si, Mn, Cr and 4O to ensure desired strength and hardenability, and highly homogeneous tempered martensite is required to improve l5sc properties. However, the deterioration of SSC resistance due to plastic deformation is largely influenced by the carbide distribution in the tempered structure, and the strength-ensuring components Si, Mn, Cr, and Mo are Since carbide distribution tends to be uneven, it is necessary to comprehensively and sufficiently adjust their content.

ii) In other words, in order to reduce the internal strain of the steel and make carbides spheroidal to improve toughness, oil well steel must be tempered at as high a temperature as possible, but in conventional oil well steel, high temperature tempering reduces prior austenite grains. The carbides in the area tend to become coarse, and when subjected to plastic deformation, strain concentrates in the vicinity, causing a significant deterioration of SSC resistance.

Moreover, the coarsening tendency of the carbides is caused by Si, M in n.
The amounts of n, Cr, and Mo have a large influence, and as their contents increase, the tendency to form coarse carbides becomes stronger.

iii) Therefore, even if high-temperature tempering is performed, if the coarsening of carbides at prior austenite grain boundaries is suppressed, the deterioration of SSC resistance due to plastic deformation can be suppressed as much as possible.
For this purpose, it is effective to limit the contents of Si, Mn, Cr and Mo; iv) By the way, S
If the content of i, Mn, Cr, and chromium is increased by 1 jill, there is a risk that the desired strength and hardenability of the steel material cannot be secured, but by adding an appropriate amount of B and a very small amount of Ti, Zr or Hf When B is contained, the above-mentioned concerns are alleviated by taking into account the hardenability improving effect of B, and also the effect of a small amount of Ti to prevent grain boundary carbide coarsening, and the addition of Zr or Hf.
Si has a synergistic effect with the action of forming fine nitrides and preventing coarsening of grain boundary carbides.
The restrictions on the content of Mn, Cr and Mo are also somewhat relaxed,
Practical disadvantages regarding strength and hardenability are eliminated.

■) Furthermore, not only S and P in the inevitable impurities, but also NIX
Since N, O, and Cu also have a negative effect on the SSC resistance of steel materials, in addition to the measures mentioned above, if the content of the above impurity elements is also comprehensively controlled, the SSC resistance of steel materials will be further improved regardless of the presence or absence of local plastic deformation. An object of the present invention is to obtain a steel material for oil wells having improved strength and SSC resistance that are sufficiently satisfactory in practice.

vi) Furthermore, when a predetermined amount of Ca or rare earth element (REM) is added to the oil well mouth material that has taken the above measures, the shape of the sulfide-based inclusions in the steel becomes spheroidal, resulting in improved SSC resistance.
It is possible to obtain a steel material for oil wells with even better SSC resistance, taking into account the effect of improving the properties of steel.

This invention was made based on the above knowledge, and oil well steel is made of: C: 0.15 to 0.45% (hereinafter, % representing the component ratio is a weight ratio), Si: 0.1 ~0.8%, Mn: 0.2~0.
8%, Cr: 0.2% or more and less than 1.0%, Mo:
0.1 to 0.8%, Ti: 0.001% or more and less than 0.010%, Al
: 0.005-0.060%, B: 0.000
1 to 0.0030%, and further one or more of Zr: 0.01 to 0.15%, Hf: 0.001 to 0.150%, Nb: 0.01 to 0.15%, V: 0.01 to 0.15%, or further contains Ca:
0.001-0.010%, rare earth elements: 0.00
1 to 0.030% of one or more types, the balance is substantially composed of Fe, and the content of Cu, Ni-N and O, which is regulated as an unavoidable impurity, is P: 0.01% or less, S: 0
．． 0.005% or less, Cu: less than 0.15%, Ni
: 0.05% or less, N: 0.0150% or less, O
: By configuring it so that it satisfies 0.0050% or less, it is possible to exhibit excellent SSC resistance and high strength without being particularly affected by tempering temperature or local plastic deformation. It has the following characteristics.

Next, the reason why the component ratio of steel is numerically limited as described above in this invention will be explained.

a) C The C component has the effect of imparting the necessary strength to steel as oil well steel and improving hardenability, but if its content is less than 0.15%, the desired effect cannot be obtained. On the other hand, if the C content exceeds 0.45%, the toughness will be adversely affected (not only that, but it will also be more likely to cause quench cracking during quenching. Therefore, the C content is set at 0.15 to 0.45%. However, from the viewpoint of ensuring more stable performance and ease of manufacture, it is preferable to adjust the C content to 0.25 to 0.35%.

b) Si The Si component is necessary as both a deoxidizing agent and a hardenability improving element for steel, but if its content is less than 0.1%, it is difficult to obtain the desired deoxidizing effect and hardenability improving effect. On the other hand, if it is contained in excess of 0.8%, the crystal grains will become coarser and the SSC resistance and toughness will deteriorate.
The i content was determined to be 0.1 to 0.8%. However, preferably, the Si content is adjusted to 0.15% or more.

c) Mn The Mn component is added mainly to improve strength and hardenability, and if its content is less than 0.2%, the desired effect cannot be ensured. - On the other hand, if Mn is contained in an amount exceeding 0.8%, it will segregate in the steel and cause a locally hardened structure, deteriorating the SSC resistance.
It was set at 0.8%.

d) Cr The Cr component also has the effect of improving strength and hardenability, but if its content is less than 0.2%, the desired effect cannot be obtained; If Cr is contained, coarse carbides are formed at grain boundaries when high-temperature tempering is performed, resulting in deterioration of SSC resistance. Therefore, the Cr content was set at 0.2% or more and less than 1.0%.

e) M.

Mo is also added to improve strength and hardenability, and is indispensable to ensure the necessary strength especially during high-temperature tempering, but if its content is less than 0.1%, the desired effect will not be achieved. On the other hand, if the content exceeds 0.8%, coarse carbides are formed and the SSC resistance deteriorates, so the io content was set at 0.1 to 0.8%.

f) Ti The Ti component exhibits the effect of preventing coarsening of grain boundary carbides and improving SSC resistance only when added in an extremely small amount, but if the content is less than 0.001%, the desired effect is not achieved. Cannot obtain the effect of -, 0.010%
If the Ti content exceeds this amount, coarse and stable TiN will be formed and the SSC resistance will deteriorate, so the Ti content was set at 0.001% or more and less than 0.010%.

g) To AI! The component is added to deoxidize the steel and make the grain finer, but if the content is less than 0.00%, the desired effect cannot be obtained; A! Content is 0.005-0.0
It was set at 60%.

h) B The B component has the effect of significantly improving the hardenability of steel when added in small amounts, and in the case of the steel of this invention, the amounts of Si, Mns, Cr, and Mo added, which are effective in improving the hardenability, improve SSC resistance. Therefore, it is essential to secure the desired hardenability instead of these. However, if the content is less than 0.001%, the desired effect cannot be obtained in the above action, and on the other hand, if the content exceeds 0.0030%, coarse carbides are formed at grain boundaries, resulting in poor SCC resistance. The S content is 0.001 to 0.00 because the toughness deteriorates.
It was set at 30%.

i) Zr and Hf These components form fine nitrides and prevent coarsening of grain boundary carbides, thereby improving SSC resistance. However, if the content is less than 0.01% in the case of Zr, and in the case of If, o,
If the amount is less than 0.00%, the desired effect cannot be obtained, and even if both Zr and Hf are contained in amounts exceeding 0.15%, not only will no further SSC resistance improvement effect be achieved, but the opposite effect will occur. leads to toughness deterioration (therefore, Zr content is 0.
01-0.15%, and ) If content is 0.001-0.001%.
Each was set at 150%.

j) Nb, and ■ These components all have the effect of refining the steel structure and improving strength, so they are added either singly or in combination of two types, but none of these components is 0. If it is less than 0.01%, the desired effect cannot be obtained;
．． If the content exceeds 15%, the toughness deteriorates, so the Nb and ■ contents are each 0.01 to 0.15%.
It was determined that

k) P P is an impurity element that tends to segregate at the grain boundaries of steel and deteriorate SSC resistance, but if its content is 0.015
% or less, no particular problem will arise in practice, so
The P content was determined to be 0.015% or less.

1) S S is an impurity element that increases sulfide-based inclusions and deteriorates the SSC resistance of steel, but if the content is 0.015% or less, the above disadvantages can be tolerated, so the S content was set at 0.015% or less.

m) (:u Cu is an impurity element that embrittles the grain boundaries during rolling heating of steel, but if its content is less than 0.15%, liquid problems can actually be tolerated, so Cu content is 0. .15%
It is set as less than

n) Ni Ni is an impurity element that promotes pitting corrosion in sour environments and deteriorates SSC resistance, but if its content is 0.0
Since practical inconveniences can be tolerated if it is 5% or less, the Ni content is limited to 0.05% or less.

0) N If a large amount of N, an impurity element, is contained in steel, coarse nitrides will increase and the SSC resistance will deteriorate, but if the content is up to 0.0150%, problems exceeding the allowable limit will occur. The N content was set at 0.0150% or less since this would not cause any damage.

p) 0 If a large amount of O, an impurity element, is contained in steel, oxide inclusions will increase and the SSC resistance will deteriorate, but if the content is up to 0.0050%, the inconvenience exceeds the permissible limit. The O content was set at 0.0050% or less to avoid causing the above.

q) Ca and rare earth elements (REM) Each of these components has the effect of spheroidizing the shape of sulfide inclusions and improving the SSC resistance of steel, so one or more of these components may be used as necessary. Although it can be contained,
In either case, if the content is less than o or ooi%, the desired effect cannot be obtained. In either case, inclusions in the steel increase and conversely deteriorate SSC resistance and toughness.
The a content was determined to be 0.001 to 0.010%, and the rare earth element was determined to be 0.001 to 0.030%.

By configuring the steel as described above, it is possible to create a high-tensile oil well with very little deterioration in SSC resistance even when subjected to local compositional deformation, which means that it has very good durability in sour environments and practical handling. Although side rails can be obtained, such an effect is limited by the addition of less than 0.5% of Ta, Sn, and S in the cough steel.
b, AS% Te-, Bi, 5esZns Pbs M
Even if one or more of g, Y, and Co are included, no damage will occur. Furthermore, if the grain size of the prior austenite grains, which is the predecessor of tempered martensite, is large, the grain boundary carbides in the steel material after tempering tend to become coarse and cause deterioration of SSC resistance. 1
It is preferable to adjust the thickness to 6 μm.

Next, the present invention will be explained using examples and comparing with comparative examples.

<Example> First, a sheet material having a composition shown in Table 1 was melted in a 150 kg high frequency furnace, and then a plate material having a thickness of 121 mm was produced by hot rolling. Next, heat this to 880℃ or 930℃
After heating for 1 hour and quenching in water,
A steel material having a "structure mainly composed of tempered martensite" was obtained by subjecting it to a tempering treatment at 0 to 720°C for 30 minutes.

Regarding the steel materials thus obtained, the crystal grain size, strength, and SSC resistance of prior austenite were investigated, and the results are shown in Table 2.

The SSC resistance was evaluated by performing a type of three-point bending stress adding test called a "shell tie test" as shown in FIG. 1 to determine the critical added stress at which SSC occurs. In this test method, a drill hole is provided in the center in the longitudinal direction as shown in FIG.

The test specimen 1 used here is one in which the small pores in the center are plastically deformed due to stress concentration, so that practical SSC resistance can be evaluated. The test solution used was a 0.5% acetic acid solution (20°C) saturated with hydrogen sulfide.
The immersion time was 500 hours.

As is clear from the results shown in Table 2, all steels that satisfy the conditions of the present invention have excellent strength and SSC resistance.
However, it can be seen that steels in which the composition and the particle size of prior austenite grains deviate from the conditions specified in the present invention do not exhibit sufficient SSC resistance.

As described above, according to the present invention, it is possible to produce high-strength steel that not only has high strength but also stably exhibits excellent SSC resistance at a relatively low cost, and is suitable for use in sour environments. This brings about extremely useful effects industrially, such as making it possible to further improve the performance of oil well components.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing the supporting state of the test piece in the shell-type test, and Figure 2 shows the shape of the test piece in the shell-type test. 'b) is a side view. In the drawings, 1... test piece, 2... glass round bar, 3...
...Stressed bolt.

Claims (2)

[Claims] (1) Weight percentage: C: 0.15-0.45%, Si: 0.1-0.8%, Mn: 0.2-0.8%, Cr: 0.2% or more1. 0%
Mo: 0.1 to 0.8%, Ti: 0.001 or more and less than 0.010%, Al: 0.005 to 0.060%, B: 0.0001 to 0.0030%, and further One or more of Zr: 0.01-0.15%, Hf: 0.001-0.150%, and Nb: 0.01-0.15%, V: 0.01-0.15%. P, which also contains one or more types of P, the remainder of which is substantially composed of Fe, and which is regulated as an unavoidable impurity;
The contents of S, Cu, Ni, N and O are P: 0.01% or less, S: 0.005% or less, Cu: less than 0.15%, Ni: 0.05% or less, N: 0. A high tensile oil well steel having excellent sulfide cracking resistance, which satisfies O: 0.0150% or less and O: 0.0050% or less. (2) Weight percentage: C: 0.15-0.45%, Si: 0.1-0.8%, Mn: 0.2-0.8%, Cr: 0.2% or more1. 0%
Mo: 0.1 to 0.8%, Ti: 0.001% to less than 0.010%, Al: 0.005 to 0.060%, B: 0.0001 to 0.0030%. , and further one or more of Zr: 0.01-0.15%, Hr: 0.001-0.150%, Nb: 0.01-0.15%, V: 0.01-0.15% and one or more of Ca: 0.001 to 0.010%, rare earth elements: 0.001 to 0.030%, and the remainder substantially consists of Fe, and unavoidable P, S, and C regulated as impurities
The contents of u, Ni, N and O are respectively P: 0.01% or less, S: 0.005% or less, Cu: less than 0.15%, Ni: 0.05% or less, N: 0.0150% Hereinafter, a high tensile oil well steel having excellent sulfide cracking resistance is characterized by satisfying an O content of 0.0050% or less.