JPS63230847A - Low-alloy steel for oil well pipe excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To develop a low-alloy steel excellent in strength and corrosion resistance, by minimizing the precipitation of AlN by reducing respective contents of N and Al in a low-alloy steel for oil well pipe to traces. CONSTITUTION:A steel tube for use in oil fields and gas fields, such as oil well pipe, oil transport pipe, etc., is manufactured by using a low-alloy steel stock 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, <0.0020% N, and <0.0050% AlN 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. By the above composition, the quantity of the fine precipitates of AlN is minimized, and the low-alloy steel as an oil well pipe stock excellent in resistance to sulfide stress corrosion cracking in a high-strength region can be manufactured.

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 been focused on the development of deep underground and corrosive substances (Hz).
This has even spread to resources contaminated with S.S., etc.). Therefore, the oil country tubular goods and oil transmission pipes used for these are
In order to cope with the trend toward deeper wells and increased transport pressure, the demand for "reducing unit weight through thinner walls" has become even stronger.
steel for oil country tubular goods) was 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 ksi (63 kgf/am) had the highest strength for practical use.

In other words, in conventional steels for corrosion-resistant oil country tubular goods, emphasis has been placed on "improving hardenability" from the viewpoint that uniform dispersion of carbides is an important factor in improving corrosion resistance (5scc resistance), and also on improving toughness. In addition, the basic method is to use killed steel as a base and then quench and temper it, so the 0.2z yield strength is inevitably 90 ksi (63 ksi).
It was decided that it would be limited to up to kgf/nvr class.

By the way, various methods have been developed to evaluate the corrosion resistance of steel. Stress was applied under the test environment (room temperature, 0.5χCH3C
This is a method to determine the cracking limit by performing a test for 200 to 500 hours at 1 atm (H, S).
Method for evaluating 5scc resistance as a value (corrosion resistance index) [
shell test] is well known, but other NAC
The tendency is the same for the E test [constant load method] and the 5SRT test [low strain rate tensile test]. Here, referring to the relationship between strength and 5scc resistance based on the Sc value, the 5scc sensitivity increases as the material strength increases, and the same level of 5scc resistance increases.
In order to maintain C properties, higher quality and higher Sc value are required.

Figure 1 shows the required 0.2X proof stress and 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/s+I
It can be seen that when the steel is in the +") grade, a Sc value of 13.3 or more is required, which means that even though the steel is high in strength, it is also required to have 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 are increasingly being applied to harsh conditions, we are developing various technologies that also focus on the relationship between corrosion resistance and fine precipitates in steel. As a result of repeated research from this perspective, we found that ``fine A/
N precipitates are also a significant factor, and in order to improve corrosion resistance, it is not enough to uniformly disperse carbides (mainly cementite) as in the past; "Reducing the 5scc resistance is an essential condition for improving 5scc resistance in the high strength range."
We have come to discover this fact.

This invention has been achieved through further research based on the above knowledge. The amount of precipitates can be sufficiently reduced, and through normal heat treatment, high strength and excellent durability can be achieved.
It was completed by adding the knowledge that "it will be possible to stably produce steel materials that have both SCC properties". Hereinafter, % represents the ingredient ratio
is weight%), Si: 0.10 to 1.0%, Mn: 0.3
~1.8%.

so 1, A j! : 0.010% or less, N
: 0.0020% or less.

8/N: 0.0050% or less, or further Cr: 0.05-2.00%, Mo
sQ, 02-0.80%.

Nb: 0.005-0.20%, V: 0
．． 005-0.20%.

B: By containing one or more of 0.0001 to 0.0030%, and configuring the composition to consist of Fe and other unavoidable impurities: the remainder, 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, and 63 kgf/
In order to obtain 0.2% yield strength of 0.1 mm” or more, 0.1
It is necessary to contain 5% or more.

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%.

Cbl 5i Si does not have a particular effect on the corrosion resistance of steel, but AI is used to suppress AIN 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 St content exceeds 1.0%, the prior austenite grains after quenching will become coarse, leading to a decrease in toughness, so the St 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 the content [1] is less than 0.3%, the hardenability On the other hand, if the content exceeds 1.8%, the effect not only becomes saturated, but also increases micro-segregation, which may lead to deterioration of corrosion resistance. Therefore, the Mn content is between 0.3 and 1.
8.

(d) soiAA, N, and AIN Conventional corrosion-resistant steel pipes have improved corrosion resistance by ensuring sufficient hardenability and then uniformly dispersing carbides (mainly cementite) through subsequent tempering. Granulation also contributed to this. However, as explained earlier, research by the present inventors has revealed that ``precipitates (AIN), which are even finer than cementite, hold the key to further improving corrosion resistance.'' It is.

In other words, to improve the 5scc resistance, it is necessary to suppress the amount of Al1N in the steel as much as possible, 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 AlN is 0.0050%
It was determined as follows.

Figure 2 shows Fe-0,25~0.29%C-0,16~
0.21%St-1,39~1.46%Mn-0,
005~0.060% soj, Al-0.0008~0
．． This is a graph showing the relationship between the amount of AlN and the Sc value for steel with a composition of 0084%N, and from this figure 2,
It is clear that by suppressing the amount of AlN to 0.0050% or less, the Sc value, which is an index of 5scc resistance, can be sufficiently increased.

In order to make ANNil 0.0050% or less, it is necessary to suppress the content of 82 and N that constitute AAN to below a specific value, so the sol, Al content should be reduced to o,
oio% or less, and the N content was determined to be 0.0020% or less.

Note that suppressing the amount of AlN is also effective in preventing cracks that occur during cooling of the steel ingot after it solidifies, and defects on the surface of the steel pipe during pipe production.

(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.

i) 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 MoC content is 0.02 to
Each was set at 0.80%.

1i) Nb Nb is a component that contributes to improving toughness and yield ratio especially through grain refinement, but if its content is less than 0.005%, the above effects cannot be obtained; on the other hand, if its content exceeds 0.20% Even if Nb is contained, the effect not only becomes saturated, but also causes a decrease in toughness.
It was set at 20%.

1ii) V (2) is an effective component for increasing strength during high-temperature tempering, but if its content is less than 0.005%, 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 effective element for improving hardenability, but if its content is less than 0.0001%, the above effect is not sufficient, while if it is contained in more than 0.0030%, toughness decreases after tempering. The B content is 0.0001 to 0.0.
It was set at 0.030%.

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: 0.05
It is desirable to keep it below %. 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 0 leads to the effect of preventing a decrease in toughness, and the suppression of Ni and Cu leads to 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 the temperature at 0°C for 10 minutes to 2 hours and then cooling it in air.
After holding at 980°C for 5 minutes to 2 hours, cooled with water and heated to 880-940°C.
It is also preferable to carry out a two-time treatment of ``holding at ℃ for 5 minutes to 2 hours followed by water cooling''.Of course, even if conventional steel is subjected to such treatment, it will not be possible to obtain the high corrosion resistance as the steel according to the present invention. Needless to say, this is because there is a limit to improving corrosion resistance through grain refinement, and it is essential to improve the ingredients.

In the case of the above heat treatment, if the quenching temperature is below the lower limit value described above, austenitization will be insufficient and carbides (mainly cementite) will not be sufficiently dissolved, 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 crystal grains will occur, which is not preferable. The reason why the temperature of the first quenching is made higher in the case of quenching twice is to prevent carbides from forming a solid solution in some cases depending on the history before quenching.

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.

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

<Example> First, steel having the composition shown in Table 1 was melted by a conventional method, and a plate material having a thickness of 15 days was produced by hot rolling.

Next, this plate material was subjected to heat treatment under the conditions shown in Table 2, and was then subjected to tensile test pieces (in the rolling direction), Charpy impact test pieces (perpendicular to the rolling direction: full size with ■ notches), and shell test pieces (in the rolling direction). At right angles to the center of the plate thickness). Then, the tensile properties, Charpy test fracture surface transition temperature (vTrs), and Sc value were measured using the sampled test pieces, 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 high strength and toughness, with a 0.2% yield strength of 60 kgf/n+m" or more and vTrs of -30°C or less when tempered at 600°C or higher. In contrast, it can be seen that the 0.2% proof stress of conventional steel is low or vTrs exceeds 130°C.

Furthermore, by investigating actual steel pipes having a composition equivalent to D@, it was confirmed that the desired strength, toughness, and corrosion resistance (based on shell test and NACE test) were satisfied.

Furthermore, Fig. 3 is a graph showing the Sc values shown in Table 2 in relation to strength, and from this Fig. 3, the steel according to the present invention clearly has a higher Sc value than the conventional steel. It can be seen that it has excellent 5scc resistance. Furthermore, depending on the composition and tempering conditions, the 0.2% proof stress is 70 to 80.5k.
It is clear that all the specimens with gf/nun2 have an Sc value of 14.5 or more, and satisfy 100 ksi class corrosion resistance. On the other hand, looking at the case of conventional steel, conventional steel has 9
It can be confirmed that there are some that satisfy the 0 ksi class, but none that satisfy the 100 ksi class.

<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 drawings]

Figure 1 shows the strength and required corrosion resistance index (Sc
Figure 2 is a graph showing the relationship between Alx content in steel and Sc value, Figure 3 is a graph showing the relationship between 0.2% proof stress and 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% or less, AlN: 0.005
A low alloy steel for oil country tubular goods having excellent corrosion resistance, characterized by having a composition consisting of 0% or less Fe and other unavoidable impurities: the remainder. (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% or less, AlN: 0.005
In addition to the components of 0% or less, Cr: 0.05 to 2.00%, Mo: 0.02 to 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.