JPH02243740A - Martensitic stainless steel material for oil well and its manufacture - Google Patents

Abstract

PURPOSE:To easily obtain the steel material having sufficient corrosion resistance, good strength and excellent sulphide stress corrosion cracking properties even in an oil well environment with industrial stability by subjecting a stainless steel having limited compsn. to hot working and thereafter to rapid cooling or gradual cooling. CONSTITUTION:A steel constituted of, by weight, <=0.05% C, <=1.0% Si, 0.5 to 3.0% Mn, <=0.04% P, <=0.005% S, 9.0 to 15% Cr, 0.1 to 7.0% Mo, 2 to 8% Ni, 0.001 to 0.1% Al and <=0.1% N, furthermore constituted of one or more kinds among <=0.5% Ti, <=0.5% Nb, <=0.5% V and <=0.5% Zr and the balance iron with inevitable impurities is prepd. (where Cr + Mo is regulated to >=10.5% and inequalities I and II are satisfied). The above steel is subjected to hot forming and thereafter to rapid cooling or gradual cooling. If required, the steel is furthermore successively heated to the Ac1 point or above, is thereafter subjected to rapid cooling or gradual cooling, is then reheated to the Ac1 point or below and is thereafter subjected to rapid cooling or gradual cooling.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to oil well steel materials used in oil wells or gas wells (hereinafter simply referred to as "oil wells") and a method for manufacturing the same. The present invention relates to a steel material having corrosion resistance and strength suitable for use in oil wells (gas wells), which contain corrosive impurities such as hydrogen sulfide and chloride ions and are subject to extremely corrosive environments, and a method for manufacturing the same.

(Conventional technology) In recent years, the environment in wells used to extract oil or natural gas has become increasingly harsh.In addition to increasing depth, the number of oil wells that contain carbon dioxide gas and hydrogen sulfide has also increased. While strength of materials is required, material embrittlement due to corrosion has become a major problem.

Conventionally, oil country tubular goods, one of the common steel materials for oil wells, have typically been made of carbon steel or low-alloy steel, but as the oil well environments in which they are used have become harsher, steel with increased alloy content has been used. is starting to be used. For example, in oil wells containing a large amount of carbon dioxide gas, it is known that the addition of Cr significantly improves corrosion resistance.
r-IMo steel and 5US420 martensitic stainless steel with 13% Cr added have been widely used. However, Cr-added martensitic steel has poor sulfide stress corrosion cracking resistance, and is extremely susceptible to stress corrosion cracking in environments that contain not only carbon dioxide gas but also hydrogen sulfide as mentioned above. The reality is that its use is restricted.

In such an oil well environment containing both carbon dioxide gas and hydrogen sulfide, currently we have no choice but to use duplex stainless steel or austenitic stainless steel with even higher alloying elements, but the addition of alloying elements is increasing. Because of this, the cost rises significantly.

JP-A-60-174859 has the above-mentioned 5US4
Based on 20 steel, with the addition of Ni and Mo and 0.02
An attempt has been made to ensure hydrogen sulfide corrosion resistance in a highly corrosive oil well environment by reducing the amount of C to below %.

According to the steel type disclosed in this publication, it is true that Cr, Mo
Although corrosion resistance is improved by the addition of C, it is costly to industrially produce steel with a low C content of 0.02% or less. on the other hand,
The strength of martensitic stainless steel strongly depends on the C content, and variations in the C content will lead to variations in strength, so strict control of the C content is required to maintain a constant strength level. However, cost increases cannot be avoided.

Moreover, it is hardened and tempered to ensure strength levels of L80 and C90 of API standards.

(Problems to be Solved by the Invention) The general purpose of the present invention is to solve the problems of the prior art, and to provide a steel material for oil wells that has the high strength required and is resistant to carbon dioxide gas. It is an object of the present invention to provide a steel material that has good corrosion resistance even in an environment where hydrogen sulfide coexists, does not unnecessarily increase alloying elements, and satisfies economic efficiency, and a method for manufacturing the same.

Conventional 9Cr-IMo steel and 5US420 steel had good corrosion resistance against carbon dioxide gas, but insufficient corrosion resistance against hydrogen sulfide. In particular, in recent years there have been reports of the so-called Hatateria corrosion problem, in which the only corrosive impurity in oil wells is carbon dioxide, but as oil extraction continues, hydrogen sulfide is generated by Hatatelia. There is a need for steel materials that are resistant to sulfide stress corrosion cracking. Currently, we have no choice but to use duplex stainless steel and high alloys, which significantly increase costs, but these steels do not have sufficient strength for, for example, oil country tubular goods, and cannot be strengthened by cold working. There is also a weakness in that it is not possible to manufacture anoposesoto products in which the tube end is thickened in advance by upsetting forging in order to satisfy the requirements.

In the case of oil country tubular goods, such upcent processing is a necessary step in order to ensure a predetermined strength, since threads for connecting the pipes to each other are formed at the ends of the pipes, resulting in thinner walls.

Therefore, a more specific object of the present invention is to sufficiently improve the stress corrosion cracking resistance against hydrogen sulfide without unnecessarily increasing the alloy content, and to have high strength suitable for oil country tubular goods, and to provide an amplifier set product. It is an object of the present invention to provide a steel that has workability that allows it to be manufactured, and steel materials using the same, such as oil country tubular goods, and a method for manufacturing the same.

(Means for Solving the Problems) In order to achieve the above object, the present inventors first investigated corrosion resistance, mainly stress corrosion cracking resistance in an environment containing carbon dioxide gas, hydrogen sulfide, and chloride ions. As a result of various experiments and studies to investigate the effects of alloying elements, the following findings were obtained.

■In steel with an appropriate amount of Ni added, the corrosion resistance in the above environment can be summarized by the amount of (Cr+Mo) wt%, and C and N are added to increase the amount of effective Cr and effective Mo that has not become carbides or nitrides. All you have to do is set an upper limit.

③ Due to the need for higher strength, a composition system that stably obtains martensitic stainless steel is required.

■Since the strength of such low C martensitic steels changes significantly with Cl during quenching, it is difficult to manufacture products with industrially stable strength.
When r is added, the strength does not change even if the amount of C changes.

That is, according to the findings of the present inventors, Ti, Nb, V
By adding Zr and Zr, there is no need to unnecessarily lower C, and sufficient corrosion resistance is ensured even with C: 0.05% or less, and the amount of C does not vary to some extent.
ζ also makes it possible to manufacture products with stable strength. Furthermore, since the addition of carbide stabilizing elements such as Ti, Nb, V, and Zr reduces the strength of the as-quenched material,
This makes it possible to obtain an innovative steel that has appropriate strength and corrosion resistance even as it is quenched, without going through the quenching and tempering treatments that have been common knowledge for martensitic stainless steels up until now.

Therefore, the gist of the present invention is as follows in weight%: C: 0.05% or less, Si: 1.0% or less, M
n: 0.5-3.0%, P: 0.04% or less, S
: 0.005% or less, Cr: 9.0-15%, M
o: 0.1-7.0%, Ni: 2-8%, 8Q
70.001 to 0.1%, N: 0.1% or less, Ti: 0.5% or less, Nb: 0.5% or less, ■20. 5% or less and Zr: 0.5% or less, Cr+Mo: 10.5% or more, the remainder consisting of Fe and unavoidable impurities, and 30CrCX)+36MoeFJ+14SiN 28
Ni (N≦455)21CrQQ + 25Mof'
N + 17Si (%) + 35Ni C'X>≦73
This is a martensitic stainless steel material for oil wells that has a steel composition of 1 (g) and has excellent sulfide stress corrosion cracking resistance.

The above steel composition may further contain Ca: 0.001~
0.05%, Mg: 0.001-0.05%, La:
0.001 to 0.05% and Ce: 0.001 to
It may contain one or more of 0.05%.

Therefore, according to the present invention, the strength variation is small (,
A steel material having high strength and excellent corrosion resistance can be obtained. Furthermore, since the strength variation during quenching is small, strength control after tempering is also easy.

As described above, according to the present invention, the steel material according to the present invention can withstand use even when hot worked, as quenched, quenched and tempered, and even as cast or welded. , a previously unknown martensitic stainless steel material.

Note that "steel materials" here include not only plate materials and frame materials, but also pipe materials.

(Function) Next, the reason why the steel composition is limited as described above in the present invention will be explained in detail. In this specification, "1%" means "wt%" unless otherwise specified.

C: If the content exceeds 0.05%, the strength will increase too much and the susceptibility to sulfide stress cracking will increase, so the upper limit should be set to 0.0.
It was set at 5%. In addition, from the viewpoint of corrosion resistance, the lower the C content, the better, and it is preferably 0.02% or less.

Si: Necessary as a deoxidizer in normal steelmaking processes.

If the content exceeds 1.0%, the toughness decreases, so the upper limit was set at 1.0%.

Mn: 0.5% or more of Mn is required to improve hot workability. If it is added in excess of 3.0%, the effect will be saturated and the toughness will decrease.

If Mnl is too large, retained austenite is likely to be formed, so it is preferably 0.5 to 1.0%.

S: The smaller the hot workability, the better.

If the upper limit is set to 0.005% in consideration of desulfurization cost, normal hot working is possible.

P: When it exceeds 0.04%, sulfide stress cracking resistance is significantly reduced.

Cr: 9.0% or more is required to form a corrosion-resistant film. If it exceeds 15%, the cost increases more than the improvement in corrosion resistance, and ferrite tends to be generated due to the synergistic effect with Mo, making it impossible to obtain strength, so the upper limit was set to 15% or less.

Mo: Effective in corrosion resistance against hydrogen sulfide. 0.1
If it is less than 7%, the effect is small, and if it exceeds 7%, ferrite tends to be generated due to the synergistic effect with Cr, making it impossible to obtain strength, so the upper limit was set to 7.0% or less.

Cr+Mo: If this value is less than 10.5%, stress corrosion cracking resistance is not sufficiently ensured. Preferably it is 12% or more. The larger this value is, the better the stress corrosion cracking resistance is.

Ni: It is added to ensure the necessary strength and corrosion resistance, and if it is less than 2%, the effect will not be sufficient, while if it is 8%
If it exceeds this amount, the amount of retained austenite increases and strength cannot be ensured. Especially in the range of Ni=2 to 8%, Cr+Mo
Corrosion resistance is significantly improved by addition.

Al: Used as a deoxidizing agent. If it is less than 0.001%, there is no effect, and if it exceeds 0.1%, inclusions will increase and corrosion resistance will be impaired.

If N: exceeds 0.1%, the strength increases too much and the susceptibility to sulfide stress corrosion cracking increases. From the viewpoint of corrosion resistance, the lower the N content, the better, and preferably 0.02% or less.

Ti, Nb, V, Zr: These alloying elements form compounds with C and N during high-temperature hot working or solution treatment.
It has the effect of controlling the amount of free (C+N) in steel, and in actual production, it is possible to control the strength of as-rolled, as-solution-treated, or after tempering by adjusting its blending amount. When each exceeds 0.5%, the effect is saturated.

By blending at least one of these elements, stable and high strength can be obtained without being affected by variations in the amount of C. Steel with such excellent strength properties can be used without hardening or cooling, which is of great significance.

Ca, Mg, La, Ce: These alloying elements are added as desired and used to improve hot workability. If each content is less than 0.001%, there is no effect, and if it exceeds 0.05%, corrosion resistance decreases.

Furthermore, in the present invention, the steel composition must satisfy the following formula.

30Cr('# +36Mo(%> +1451m-2
8Nifi≦455 CIIQ-...Formula (1) 21Cr
QQ + 25Mo (%) + 17Sil'X +35
Ni ('i4≦731e%)...Equation (2), that is,
Since the target steel type of the present invention is for oil wells, martensitic single-phase steel is preferable in order to ensure excellent strength and corrosion resistance.
It is necessary that it becomes an austenitic single phase steel at ℃ and transforms to martensitic steel when cooled. Equation (1) is used to form an austenite phase without generating δ ferrite at high temperatures.
need to be satisfied.

On the other hand, formula (2) must be satisfied in order to cool down to room temperature and become martensitic single steel.

Steel with the above composition has appropriate strength and corrosion resistance even after it is formed into a tube body through normal hot working, without the need for rapid cooling, but when it is further heat-treated, - Improves layer corrosion resistance. Note that there is no problem even if the material is rapidly cooled after hot working.

According to the present invention, when heat treatment is performed while pipe production is being performed, one of the following methods is desirable.

(?) After hot working, the product is rapidly cooled or slowly cooled, but not tempered below the Ac+ point. (II) After hot working, the product is rapidly cooled or slowly cooled.

It is heated on a dot plate, partially or completely re-austenitized, and then rapidly or slowly cooled and quenched.

(I[[) Furthermore, after tempering the material of (11) below the Act point, it is rapidly or slowly cooled.

In the case of (1), it is a direct quenching-tempering process, and the heating temperature range is determined in order to relieve the residual stress during direct quenching. Therefore, it is preferably carried out at a temperature of 450° C. or above and below the Acn point, where stress relaxation occurs.

In the case of (11), the heat treatment is performed as-quenched.

It is heated to Ac1 point or higher to partially or completely austenite, and then cooled. Since re-austenitization also has the meaning of homogenization, it is preferable to use Ac at a temperature above the dot plate.

In the case of (Iff), the material quenched in (II) is tempered to relieve stress, so reheating is performed under Ac and dotted.

Note that the pipe manufacturing method of the present invention is not particularly limited, but examples include pipe manufacturing methods that involve steps such as the Mannesmann mandrel mill method.

Next, the present invention will be explained in more detail with reference to Examples.

In addition, in the following examples, only hot rolling is performed, but those skilled in the art will understand that the same effect will be exhibited even if a pipe manufacturing process is performed.

Examples A-U steels having the compositions shown in Table 1 were each melted,
A plate with a thickness of 12° C.m was obtained by hot rolling. Among these, F, G, H, and I steels, which are within the scope of the present invention and have Ti, Nb, V, and Zr added in a similar composition system, and Ti, Nb
, (2) Q, R, S, and T to which Zr was not added were subjected to the heat treatment shown in Table 2, and tensile test pieces with a diameter of 4B and a parallel portion of 34 mm were taken to measure the tensile strength. RQJ then A for those that have been quenched (quenched) by water cooling, oil cooling, etc., including those that have been air cooled (slowly cooled) after being heated to Ac3 points or higher.
A material that has been heated and tempered to a temperature below the c+ point is referred to as "OT".

The results are summarized in Table 2, and FIG. 1 is a graphical representation of the correlation between C content and tensile strength for each steel that was heat treated and then tempered.

As is clear from the illustrated graph, in both the as-quenched material (0) and the after-tempered material (OT), as the C content increases, the tensile strength of the comparative steel increases significantly. but,
In the steel of the present invention, the strength is kept constant without being affected by variations in the amount of C. Therefore, the amount of C in the steel is m! In industrial processes that are extremely difficult to control, it can be seen that the addition of Ti, Nb, ■, and Zr according to the present invention is extremely useful as an effect of stabilizing strength.

In addition, in the case of the steel of the present invention, a single set of martensite was present during cooling.

Furthermore, steels A to ■ according to the present invention, J and K steels of conventional examples,
Comparative Examples LP and U@ were subjected to the heat treatments shown in Table 3, and then tested for strength, corrosion rate, and sulfide stress corrosion cracking resistance.

The tensile test was conducted by taking a tensile test piece with a diameter of 4 mm and a parallel portion of 34 mm.

Corrosion tests were conducted using 21 mm thick x 10 mm wide x 80 mm.
Two long U-bend bending test pieces were prepared, and as shown in Figure 2, test piece 1 was subjected to bending stress using bending jig 2 so that the radius of curvature R was 7.5B. Ta. Test environment: 5% NaCQ + 0.01 atm H2S + 3
After a 336-hour immersion test at 0 atmosphere CO□, the sample was taken out, and the corrosion loss was measured, and the presence or absence of cracks was investigated by visually observing the appearance and observing the cross section of the test piece with an optical microscope. Note that the test temperature was 25°C.

The results of both these tests are summarized in Table 3.

Conventional examples 25 and 26 are conventional 13Cr steel and 9Cr-IMo
fi results, the corrosion rate is high and cracks are observed in this environment, which is not preferable.

Comparative Examples 27 to 30 each have a higher C and N content than the steels of the present invention, and although their strength is significantly high and their corrosion rate is good, stress cracking occurs.

In Comparative Example 31, the amount of (Cr + Mo) was less than 10.5%, and the corrosion resistance was poor. Comparative Examples 32 and 33 did not satisfy the calculated values of the formulas, and their strength was not suitable for use as oil country tubular goods.

Comparative Example 34 corresponds to the steel disclosed in JP-A-60-174859, and since Ti, Nb, etc. are not added, the as-quenched strength is too high, causing sulfide stress cracking.

However, as shown in Examples 1 to 24 of the present invention, the steel of the present invention has both the tensile strength and corrosion resistance necessary for oil country tubular goods even when subjected to various heat treatment conditions or hot rolled. , it can be seen that it can be suitably used as oil country tubular goods used in the above-mentioned harsh environments. All of these steels were martensitic single pairs.

(The following is a blank space) Table (Effects of the Invention) As is clear from the examples above, the present invention has satisfactory corrosion resistance even in a harsh environment where chloride ions, carbon dioxide gas, and trace amounts of hydrogen sulfide gas exist, and It provides a truly useful steel that has suitable strength for oil country tubular goods and can be easily obtained industrially as a homogeneous steel material with small variations in strength, and its practical benefits are great.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the tensile strength after heat treatment organized by the amount of C in the steel; and FIG. 2 is a diagram showing the state of stress applied to the U-Huff 1 bending test piece used in the example.

Claims (6)

[Claims] (1) In weight%, C: 0.05% or less, Si: 1.0% or less, Mn: 0.
5-3.0%, P: 0.04% or less, S: 0.005%
Below, Cr: 9.0-15%, Mo: 0.1-7.0%
, Ni: 2-8%, Al: 0.001-0.1%, N:
0.1% or less, further Ti: 0.5% or less, Nb: 0.5% or less, V: 0.5
% or less and Zr: one or two of 0.5% or less
Species or more, however, Cr + Mo: 10.5% or more, the remainder consists of Fe and unavoidable impurities, and 30Cr (%) + 36Mo (%) + 14Si (%) -2
8Ni(%)≦455(%)21Cr(%)+25Mo
(%)+17Si(%)+35Ni(%)≦731(%
) A martensitic stainless steel material for oil wells with excellent sulfide stress corrosion cracking resistance. (2) In weight%, further Ca: 0.001 to 0.05%, Mg: 0.00
1-0.05%, La: 0.001-0.05%, and Ce: 0.001-0.05%. Stainless steel material. (3) A method for producing a steel material for oil wells having excellent sulfide stress corrosion cracking resistance, which comprises hot forming the martensitic stainless steel according to claim 1 or 2 and then rapidly cooling or slow cooling. (4) After hot forming using the martensitic stainless steel according to claim 1 or 2, the A
A method for producing oil well steel materials with excellent sulfide stress corrosion cracking resistance, the method comprising heating to below c_1 point and then rapidly cooling or slow cooling. (5) After hot forming using the martensitic stainless steel according to claim 1 or 2, the A
c_A method for producing oil well steel materials with excellent sulfide stress corrosion cracking resistance, which comprises heating to a temperature of 1 or more and then rapidly or slowly cooling. (6) After hot forming using the martensitic stainless steel according to claim 1 or 2, the A
c_After heating to a temperature of 1 point or more, quenching or slow cooling,
A method for producing oil well steel materials with excellent sulfide stress corrosion cracking resistance, which comprises then reheating the steel material to a temperature of Ac_1 point or lower, and then rapidly or slowly cooling it.