JPS63230851A - Low-alloy steel for oil well pipe excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To develop a high-strength low-alloy steel for oil well excellent in corrosion resistance, particularly in resistance to sulfide stress corrosion cracking, by using, as a steel stock for oil well, a steel stock having a composition in which Ti and Zr for improving corrosion resistance are added and AlN content is minimal. CONSTITUTION:As a steel stock for oil well pipe and oil transport pipe used in the development of oil well and gas field, a low-alloy steel which has a composition containing, by weight, 0.15-0.45% C, 0.10-1.0% Si, 0.3-1.8% Mn, <0.010% sol. Al, N in an amount satisfying N<[0.0020+(Ti%+Zr%/8)]%, and <0.0050% AlN and further containing 0.005-0.10% Ti and/or 0.010-0.20% Zr or further containing one or more kinds among 0.05-2.00% Cr, 0.02-0.80% Mo, 0.005-0.20% Nb, 0.005-0.20% V, and 0.0001-0.0030% B is used. In this way, the low-alloy steel excellent in strength and corrosion resistance and having superior characteristics as a stock for oil well pipe, etc., can be obtained.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention provides a high-strength, low-alloy steel for oil wells that has superior corrosion resistance, particularly sulfide stress corrosion cracking resistance (hereinafter abbreviated as "5 SCC resistance"). It is related to.

<Background technology> In recent years, in light of future energy concerns, oil and gas field development efforts have focused on extracting corrosive substances (H,
s), etc.).

Therefore, in order to cope with the trend toward deeper wells and increased transport pressure, there is an even stronger demand for ``reducing unit weight through thinner walls'' for the oil country tubular goods and oil transmission pipes used in these products, and as a result, higher strength levels are required. A steel material (steel for oil country tubular goods) that exhibits excellent toughness and 5 SCC resistance has been desired.

However, as steel materials generally increase in strength, their corrosion resistance tends to decrease, and for this reason, steel for oil country tubular goods, where corrosion resistance is important, has no choice but to be subject to strength limitations.
A steel material with a 2% yield strength of 90 ksl (63 kgf/am") had the highest strength for practical use.

In other words, in conventional corrosion-resistant oil country tubular steels, "improvement of hardenability (increasing martensitic rate)" is based on the viewpoint that grain refinement and uniform dispersion of carbides are important factors for improving corrosion resistance (5SCC resistance). Emphasis was placed on ``improvement)'' and ``refining of crystal grains through heat treatment,'' while also taking into account toughness.
90ksi with excellent 5scc resistance based on f-killed steel
grade (0.2χ crab resistance 90~105ksi, i.e. 63~
Although steels with a strength of up to 73.5 kgf/mm") have been developed, it has not been possible to achieve a steel with a strength higher than this that satisfies the corrosion resistance.

By the way, various methods have been developed to evaluate the corrosion resistance of steel. Stress was applied under the test environment (room temperature, 0.5χCH
A method of determining the cracking limit by performing a test for 200 to 500 hours at z COOH, 1 atm H, S), and evaluating the 5 scc resistance using the cracking limit as the Sc value (corrosion resistance index) [Shell test] is well known, but other NACE tests (constant load method) and 5SRT tests (low strain rate tensile test) have the same tendency. Here, referring to the relationship between strength and 5scc resistance based on the Sc value, as the material strength increases, the 5scc sensitivity increases, and if you try to maintain the same level of 5scc resistance, you will need higher quality and a higher Sc value. It becomes necessary.

Figure 1 shows the 0.2z proof stress and required Sc of corrosion-resistant oil country tubular goods.
(Each strength grade requires strength and corrosion resistance indicated by diagonal lines in the figure)
, a steel with a strength level of 90ksi class could be satisfied as a corrosion-resistant oil country tubular steel if the Sc value was 12 or more, whereas 100ksi (minimum required crab resistance 70kgf/ms)
) class requires a Sc value of 13.3 or higher, which indicates that even though the steel is high in strength, it also requires higher corrosion resistance.

This requirement is extremely strict, and as the strength level increases, it has not yet been found that a practical material that satisfactorily satisfies this requirement in consideration of economic efficiency.

<Means for Solving the Problems> In view of the above-mentioned problems, the present inventors have improved the strength of oil well tubular steel materials to an even higher level by providing steel materials for oil country tubular goods with even better corrosion resistance. In order to expand the scope of practical application and further improve the performance of oil country tubular goods and oil transmission pipes, which have become increasingly suitable for harsh conditions, we have developed a variety of products that focus on the relationship between corrosion resistance and fine precipitates in steel. As a result of repeated research from the perspective of
N precipitates are also a significant factor, and in order to improve corrosion resistance, it is not enough to simply disperse carbides (mainly cementite) uniformly as in the past; They have discovered the fact that ``reducing ``is an indispensable condition for improving 5scc resistance in the high strength range.''

Therefore, the present inventors have found a concrete means to reduce A/N fine precipitates without adversely affecting strength, etc., and have stably and reliably produced steel that has both high strength and excellent corrosion resistance. As a result of repeated research, we found that (a) fine AA'N precipitates in steel can be reduced by limiting the N content and 5olAl content to below specific values and also adjusting other constituent components of the steel; It can be reduced to a sufficiently low level, and it can be achieved with high strength and excellent durability through normal heat treatment.
(b) Furthermore, when a specific amount of Ti or Zr is added as a component of the steel, the N in the steel is fixed as TiN or ZrN. It was discovered that the solid solution N is as low as possible, which is extremely advantageous for improving the Sc value of steel.

This invention was completed by adding the above-mentioned knowledge, and the steel for oil country tubular goods is made of C: 0.15 to 0.45% (hereinafter, % representing the component ratio).
is weight%), Si: 0.10 to 1.0%, Mn: 0.3
~1.8%.

5oi1. Al: 0.010% or less.

N: 0.0020+ (Ti(χ)+Zr(X)/
8 }% or less A I N : 0.0050% or less and Ti: 0.005 to 0.10%.

Zr: 0.010-0.20% or more, or in addition to this, Cr: 0.05-2.00%, Mo: 0.
02-0.80%.

Nb 70.005-0.20%, V: 0.
005-0.20%.

By including one or more of B: 0.0001 to 0.0030%, and configuring the composition as follows: Pa and other unavoidable impurities, it is possible to have both excellent corrosion resistance and high strength. It is characterized by the following points.

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

(Al C C is an important component to ensure strength and toughness during high-temperature tempering of steel to improve corrosion resistance.
In order to obtain a 0.2% yield strength of 0.15
% or more is necessary.

However, if the C content exceeds 0.45%, quench cracking is likely to occur during quenching, so the C content should be reduced to 0.
．． It was set at 15% to 0.45%.

(b) 5i Si does not have a particular effect on the corrosion resistance of steel, but AI is used to suppress AJN and improve 5scc resistance.
It is necessary to suppress its addition, and therefore it is an indispensable component from the standpoint of deoxidizing. If the content is less than 0.10%, a sufficient deoxidizing effect cannot be expected;
If the Si content exceeds 1.0%, the prior austenite grains after quenching become coarse, leading to a decrease in toughness, so the Si content was set at 0.10 to 1.0%.

(c) Mn The Mn component has the effect of improving the hardenability of steel and improving toughness by uniformly dispersing cementite after tempering, but if its content is less than 0.3%, the hardenability is reduced. If it is insufficient, corrosion resistance and toughness will not be sufficient, while if it is contained in excess of 1.8%, the effect will not only be saturated, but also increase micro-segregation, which may lead to deterioration of corrosion resistance. Therefore, the Mn content is 0.3 to 1.
8.

(dl Ti, and Zr) These components include N in the steel being replaced with TiN during cooling of the steel ingot.
Alternatively, it has a remarkable effect of reducing solid solution N by fixing it as ZrN, thereby improving the Sc value.
It can contain one species or two species. These N-fixing effects are very strong, so even if they are added, if they are added in appropriate amounts, the formation of fine precipitates of TiC and ZrC, which may deteriorate corrosion resistance, can be minimized. In particular, the N fixed effect of Zr is large, but Zr
is about twice the atomic weight of Ti, and therefore has the same effect on the Sc value as Ti in terms of weight ratio.

If the Ti content is less than 0.005%, and if the Zr content is less than 0.010%, the above N fixing effect will not be sufficient; on the other hand, if the Ti content exceeds 0.10%, Even if Zr is contained or contained in an amount exceeding 0.20%, the effect of N fixation will be saturated, and
Since the amount of fine precipitates increases and the Sc value decreases, the Ti content and Zr content were determined to be 0.005 to 0.10% and 0.010 to 0.20%, respectively. The amount of N fixed by Ti and Zr is (Tt(χ)+Zr
(χ))/8.

(d) sol, AI, N, and Aj! N Conventional corrosion-resistant steel pipes improve corrosion resistance by ensuring sufficient hardenability and then uniformly dispersing carbides (mainly cementite) during subsequent tempering, and grain refinement also contributes to this. was. However, as explained earlier, ``further improvement in corrosion resistance requires the use of finer precipitates (A) than cementite.
The inventors' research has revealed that "IN) holds an important key."

In other words, to improve the 5scc resistance, it is necessary to suppress the amount of AjlN in the steel, and the content is 0.0050% (5scc).
If it exceeds 0ppm), it will not be possible to secure the desired 5scc resistance. Therefore, the amount of AIN is 0.0050%
It was determined as follows.

Figure 2 shows Fe-0,25~0.29%G-0,3%S
i -0,5χMn-0.5%Cr-0.2%Mo-0
,03~0.08%Zr-0,005~0.060%s
oIt, AIt-0,0008~0.0084%
This is a graph showing the relationship between the Al1N content and the Sc value for steel with the composition N. From this figure, it can be seen that by suppressing the AIN content to 0.0050% or less, the 5scc resistance can be improved.
It is clear that the Sc value, which is an index of performance, can be sufficiently improved.

In order to reduce the amount of AIN to O, 0Q 50% or less, it is necessary to suppress the content of AI and N that constitute AIN to below a specific value, so the sol and Aβ content should be reduced to 0.0%.
10% or less, and the N content (0,0020+ (T
t(χ)+Zr(χ)/8)}% or less.

(e) Cr, Mo, Nb, V, and B These components are components that improve the strength and toughness of steel, so if further improvement in strength and toughness is required, one or more of these components may be included. However, the reason for limiting the content of each component will be explained in detail below.

1) Cr, M.

Cr and Mo are extremely effective components for improving hardenability, and are also effective for improving corrosion resistance because they do not cause micro-segregation as in the case of Mn.

However, if the content is less than 0.05% in the case of Cr and 0.02% in the case of Mo, the above effect cannot be obtained. ,
In the case of Mo, if it exceeds 0.80%, the hardenability will further improve, but the toughness will decrease, so the Cr content is 0.05 to 2.00%, and the Mo content is 0.02 to
Each was set at 0.80%.

1i) Nb Nb is a component that contributes to improving toughness, yield ratio, and corrosion resistance, especially through grain refinement, but if its content is less than 0.005%, the above effects cannot be obtained;
If the Nb content exceeds 20%, the toughness will decrease, the grain refining effect will be saturated, and furthermore, Nbc fine precipitates will increase and corrosion resistance will deteriorate, so the Nb content should be 0.005%.
It was set at ~0.20%.

1ii) V is an effective component for increasing strength during high-temperature tempering, but if its content is less than o, oos%, the above effect cannot be obtained,
On the other hand, if the content exceeds 0.20%, the toughness decreases, so the content (2) was determined to be 0.005 to 0.20%.

1v) B B is an element that is effective in improving hardenability, and from this point of view, is also effective in improving toughness and corrosion resistance, but when its content is 0.0
If it is less than 0.001%, the above effect will not be sufficient;
If B content exceeds 0.030%, the toughness after tempering will decrease, so the B content should be 0.0001 to 0.0030.
%.

The low-alloy steel for oil country tubular goods of the present invention has the above-mentioned composition, but the other unavoidable impurity elements P, S, O, Ni, and Cu each have P: 0.025. % or less, S: 0.005
% or less, O: 0.002% or less, Cu: 0.
It is desirable to suppress it to 0.5% or less. This is because the suppression of P and S mainly leads to the effect of preventing a decrease in toughness and the effect of improving corrosion resistance by preventing micro-segregation, the reduction of O is the effect of preventing a decrease in toughness, and the suppression of Ni and Cu is the effect of preventing deterioration of corrosion resistance such as pitting corrosion. Because it brings.

Note that in order to impart the required strength, toughness, and corrosion resistance to the steel of this invention, heat treatment is usually performed.

As this heat treatment, so-called "refining" of quenching and tempering is usually performed. That is, it is held at 880 to 940°C for 5 minutes to 2 hours, then cooled with water, and further heated to 600 to 72°C depending on the required strength.
Tempering is carried out by holding at 0°C for 10 minutes to 2 hours and then cooling in air. In this case, in order to refine the prior austenite grains and improve corrosion resistance and toughness,
~98 (held at °C for 5 minutes to 2 hours, cooled with water, 880~9
It is also preferable to carry out a two-time treatment of "maintaining at 4C for 5 minutes to 2 hours and then cooling with water."Of course, even if conventional steel is subjected to such a process, it will not have the high f-resistance as the steel according to the present invention. Needless to say, this cannot be obtained.This is because there is a limit to improving corrosion resistance by making Q grains finer, and it is essential to improve component Q.

In the case of the above heat treatment, if the quenching temperature is lower than the above-mentioned lower value, austenitization will be insufficient and carbides (mainly cementite) will not be sufficiently dissolved in solid solution, making it impossible to secure the desired quenching effect. On the other hand, if the quenching temperature exceeds the above-mentioned upper limit, coarsening of the grains will occur, which is not preferable. In the case of two-time quenching, the reason why the first quenching temperature is raised is that depending on the history before quenching, carbides may
It may be difficult to dissolve, so this is decided to prevent this.

In addition, if the holding time during heat treatment is shorter than the above lower limit, it will not be possible to secure the minimum temperature necessary for hardening to the center of the wall thickness, while on the other hand, if the holding time is longer than the above upper limit, the crystal grains will become coarse. This is not desirable because it causes oxidation.

1 Next, the present invention will be explained using examples while comparing it with a conventional example.

<Example> Xie First, steel having the composition shown in Table 1 was melted by a conventional method, and then hot rolled to a thickness of 15 fins.
The board was manufactured. The reason why ^βNfi could be controlled to below o, ooso% in the range where the N content was below 0.0100% is due to the N fixing action of Ti and Zr.

Next, this plate material was subjected to heat treatment under the conditions shown in Table 2, and was then subjected to a tensile test piece (rolling direction), a Charpy impact test piece (perpendicular to the rolling direction: full size with a notch), and a shell test piece ( 1 perpendicular to the rolling direction, at the center of the plate thickness). Then, the tensile properties and Charpy test fracture surface transition temperature (vTrs
) and Sc value were measured, and the results are also shown in Table 2.

As is clear from the results shown in Table 2, the steel according to the present invention has a 0.2% yield strength of 65 kgf/mow'' or more when tempered at 600°C or higher, and vTrs and Sc at -30°C or lower.
Value: 14.0 or higher, which shows high strength, high toughness, and high corrosion resistance, whereas conventional steels have a 0.2% lower proof stress (P steel, S steel) or vTrs exceeding 130°C. It can be seen that the Sc value is less than 13.3 (P steel, S steel, X14), and the performance as a 100 ksi class oil country tubular steel cannot be satisfied.

In addition, it was confirmed that the desired strength, toughness, and corrosion resistance (based on shell test and NACE test) were satisfied through investigation of actual steel pipes having a composition equivalent to C steel.

Furthermore, Fig. 3 is a drawing in which the Sc values shown in Table 2 are graphed in relation to strength, and from this Fig. 3, the 0.2% yield strength of the steel according to the present invention is 73.5. ~84kgf/ms
”, the Sc value satisfies 14.0 or more, and 100ks
While the conventional steel has a corrosion resistance of I class or 105 ksi class, the Sc value of the conventional steel is clearly lower than that of the steel of the present invention, and a clear difference can be recognized between the two.

<Summary of Effects> As explained above, according to the present invention, high-strength steel that not only has high strength but also stably exhibits excellent corrosion resistance (5 SCC resistance) can be realized at a relatively low cost. This will bring about industrially useful effects, such as making it possible to further improve the performance of oil country tubular goods, oil transmission pipes, etc., which have had to be used in harsh environments.

[Brief explanation of the drawing]

Figure 1 shows the strength and required corrosion resistance index (Sc
Figure 2 is a graph showing the relationship between the amount of AIN in steel and the Sc value, Figure 3 is the graph showing the relationship between the AIN content in the steel and the Sc value, and Figure 3 is the graph showing the relationship between the 0.2% proof stress and the Sc value for the steel shown in the example. It is a graph showing the relationship between

Claims (2)

[Claims] (1) C: 0.15-0.45%, Si: 0.10-1.0% by weight
, Mn: 0.3-1.8%, sol. Al: 0.010
% or less, N: 0.0020 + {Ti (%) + Zr (%)
/8}% or less AlN: 0.0050% or less, and further contains one or more of Ti: 0.005 to 0.10%, Zr: 0.010 to 0.20%, Fe and other A low-alloy steel for oil country tubular goods with excellent corrosion resistance, characterized by a composition consisting of unavoidable impurities: residue. (2) C: 0.15-0.45%, Si: 0.10-1.0% in weight percentage
, Mn: 0.3-1.8%, sol. Al: 0.010
% or less, N: 0.0020 + {Ti (%) + Zr (%)
/8}% or less AlN: 0.0050% or less, and one or more of Ti: 0.005-0.10%, Zr: 0.010-0.20%, and Cr: 0.005% or less. 05-2.00%, Mo: 0.02-0.8
0%, Nb: 0.005-0.20%, V: 0.005
~0.20%, B: 0.0001~0.0030%, and Fe and other unavoidable impurities: the remaining corrosion resistance. Low alloy steel for oil country tubular goods with excellent properties.