JPH03120337A - Martensitic stainless steel and its manufacture - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

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

(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.

Traditionally, carbon steel or low-alloy steel was commonly used for oil country tubular goods, which are one of the common steel materials used in oil wells, but as the oil well environments in which they are used have become harsher, the amount of alloy has been increased. For example, in oil wells containing a large amount of carbon dioxide, it is known that the addition of Cr significantly improves corrosion resistance.
Cr-IMo steel or 503420 with 13% Cr added
Martensitic stainless steel has been widely used. However, Cr-added martensitic stainless steel has poor sulfide stress corrosion cracking resistance, and is extremely susceptible to stress corrosion cracking in environments that contain not only carbon dioxide (as mentioned above) but also hydrogen sulfide. The reality is that it is expensive and its use is restricted.

In such an oil well environment that contains carbon dioxide gas and hydrogen sulfide at the same time, 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, there is a significant increase in costs.

JP-A No. 60-174859 describes SUS4 (mentioned above)
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 containing hydrogen sulfide by reducing the amount of C to below %.

According to the steel type disclosed in this publication, in a corrosive environment containing only HlS, the corrosion resistance is certainly improved by adding Cr and Mo, but COt and Ht, which are particularly problematic today, improve corrosion resistance slightly.
The reality is that oil wells, which have extremely corrosive environments containing large amounts of corrosive impurities such as S and C2-, do not have sufficient corrosion resistance.

(Problem to be solved by the invention) In order to improve corrosion resistance, Cr, llo, Ni
It is well known that it is preferable to increase the alloying elements such as, but this increases the cost accordingly. Furthermore, Cr,
If Nt and Mo are increased carelessly, a martensitic single-phase steel cannot be obtained, and ferrite and austenite phases are mixed in, resulting in a decrease in strength and the problem that it cannot be applied to oil country tubular goods. As a result, costly strengthening such as cold working becomes necessary.

Therefore, an object of the present invention is to sufficiently improve stress corrosion cracking resistance in a corrosive environment containing hydrogen sulfide, carbon dioxide, and chloride ions while maintaining high strength without unnecessarily increasing alloying elements. steel, and steel products manufactured from it;
For example, an object of the present invention is to provide a method for manufacturing oil country tubular goods.

(Means for Solving the Problem) In order to achieve the above object, the present inventors first aimed to improve 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.

■The corrosion resistance of steel to which an appropriate amount of Nl has been added in the environment described above can be summarized by the amount (Cr+Mo) (wtχ). All you have to do is set an upper limit.

■In addition, due to the need to have high strength, a composition system that can stably obtain a martensitic single phase steel is required.

■The pitting resistance in environments containing carbon dioxide gas, hydrogen sulfide, and chloride ions is significantly improved when Mn is reduced to less than 0.5% in steel containing 1.5% or more of Mo.
The stress cracking characteristics that occur starting from pitting are also significantly improved.

■Reducing Mn reduces toughness and hot workability, so
To ensure both toughness and hot workability by tightening the upper limit of S content.

That is, according to the findings of the present inventors, Mo is 1.5%
In martensitic stainless steel with the above additions, especially Mn
When S is less than 0.5% and S is less than 0.0029A, steel with good corrosion resistance represented by sulfide stress corrosion sidewall resistance, good toughness and hot workability can be produced with a slight increase in cost. You can get it.

Here, the gist of the present invention is, in weight%, C: 0.05% or less, Si: 1.0% or less, Mn
: Less than 0.5%, P: 0.04% or less, S: 0.
002% or less, Cr: 8-15%, Mo: t,
5-7%, N[: 2-8%, Al: 0.0
01 to 0.1%, N: 0.1% or less, and Cr +
11.0% on Mo, 30CrQfe+36Mo5G+14Si (6Q 28
NH! IJ≦455)21Cr%+25Mo6Q+
This is a martensitic stainless steel for oil wells that simultaneously satisfies 175f% + 35NRI9≦731 and has a steel composition with the remainder consisting of Fe and unavoidable impurities, and has excellent sulfide stress corrosion cracking resistance.

According to the preferred B ladder of the present invention, Ti: 0.5%
Below, flb: 0.5% or less, V: 0.5% or less,
and Zr: 0.5% or less of one kind or 2N or more, and/or 0.001 to 0.05% of each of Ca and Mg.

It may contain one or more of La and Ce.

Another aspect of the present invention is that martensitic stainless steel having the above-mentioned steel composition is hot-formed into a steel material for a predetermined shape or purpose, such as a pipe material or a shape material, and then rapidly, cooled, or slowly cooled. This is a method for producing a martensitic stainless steel material for oil wells that has excellent sulfide stress corrosion cracking resistance.

According to another method, after hot forming the martensitic stainless steel material as described above, it is rapidly or slowly cooled to 70°C or lower, preferably to room temperature, and then
After heating again to the + point or higher, quenching or slow cooling may be performed.Furthermore, after heating to the 601 point or higher,
After cooling quickly or slowly to below A
After heating to the c1 point or lower, rapid cooling or slow cooling may be performed.

Here, the above-mentioned "hot forming" refers not only to hot rolling itself, but also to the entire pipe manufacturing process including hot rolling when the steel material is a pipe material. In addition, hot forming also includes hot processing of extrusion.

(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 0.305%, 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:3! A: Necessary as a deoxidizing agent in the regular steelmaking process.

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

Hn: An element having important significance for the present invention,
It is an element whose presence in steel reduces corrosion resistance, especially pinting resistance, in environments containing carbon dioxide gas, hydrogen sulfide, and chloride ions. If Mn is limited to less than 0.5%, the corrosion resistance will be good, and the less it is, the better the corrosion resistance will be.

Preferably it is 0.35% or less.

S: In steel with reduced Mn as in the present invention, S
Since the deterioration of hot workability and toughness becomes significant due to this, it is necessary to reduce as much as possible, and the upper limit is set at 0.002%. Preferably, it is less than or equal to 0.001. In addition, in the conventional technology, S is the martensitic stainless steel for oil wells.
In reality, it was contained in an amount of about 0.002 to 0.000s%.

P: When 0.04% is added, the sulfide stress cracking property is significantly reduced.

Cr: In order to form a corrosion-resistant film, it is necessary to add 8.0% or more. However, if it exceeds 15%, the cost increases more than the improvement in corrosion resistance, and the synergistic effect with Mo tends to produce ferrite, making it difficult to obtain strength (thus, the upper limit was set at 15%).

hoz is an important alloying element in the present invention and has a significant effect on corrosion resistance against hydrogen sulfide. If it is less than 1.5%, the effect is small, and if it exceeds 7%, ferrite is likely to be generated due to a synergistic effect with Cr, making it impossible to obtain strength. Therefore, in the present invention, the Mo content is set to 1.5%. ~7.0%
And so.

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. Ni: Cr + in the range of 2 to 8%
Corrosion resistance is significantly improved by adding Mo.

^Q: Used as a deoxidizing agent. If it is less than 0.0%, there is no effect, and if it exceeds 0.1%, inclusions will increase and corrosion resistance will be impaired.

N: Effective for improving strength, but if it exceeds 0.1%, strength increases too much and susceptibility to sulfide stress corrosion cracking increases. From the viewpoint of corrosion resistance, the lower the N content, the better, and preferably 0.
0.02% or less.

Cr + Mo: If this value is less than 11.0%, stress corrosion cracking resistance cannot be ensured sufficiently, preferably 1
It is 4% or more. The larger this value is, the better the stress corrosion cracking resistance is. However, it is not preferable to add too much of it because the formation of ferrite causes a decrease in strength and an increase in cost.

From the steel composition according to the invention described above, it does not exceed 22%.

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

30Cr61J +36Mo% + 14Si%-28
Ni69≦4556Q---Formula (1) 21Cr(!X2
+25Mo% +17Si eXa +35Ni
m≦731H...-Equation (2) In other words, 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 1100°C and transforms to martensitic steel when cooled. In order to form an austenite phase without producing hexaferrite at high temperatures, it is necessary to satisfy formula (1).

On the other hand, formula (2) must be satisfied in order to cool down to room temperature and become martensitic single steel without residual austenite. Of course, it is conceivable that other phases such as ferrite may be formed in some cases, but the martensitic stainless steel referred to in the present invention only needs to satisfy the above formulas (1) and (2).

According to the invention, several other alloying elements may be included, for example as described below.

↑L Nb, V, Zr: At least one or more of these alloying elements are added as necessary, but they form compounds with C and N during high-temperature hot working or solution treatment, and the free (C+ #) It has the effect of controlling the amount, and in actual production, it is possible to control the strength of as-rolled, as-solution-treated, or after tempering by adjusting the blending amount. When each exceeds 0.5%, the effect is saturated.

When at least one of these elements is blended, high strength can be obtained which tends to be stable and unaffected by variations in C1. Steels with such excellent strength properties can be easily used without being quenched or cooled.

Ca+Mg+La+Ce: At least one or more of these alloying elements may be added as desired, and both are intended to improve hot workability. If each content is less than 0.001%, there is no effect, while if it exceeds 0.05%, corrosion resistance decreases.

Steel having the above composition has appropriate strength and corrosion resistance even after it is formed into a tube by normal hot forming without any particular need for rapid cooling, but when it is further heat-treated, - The layer corrosion resistance is improved, and slow cooling is air cooling or cooling in a furnace.

According to the present invention, when heat treatment is performed after hot forming such as pipe making, one of the following methods is preferable.

(1) After hot forming, the product is rapidly cooled or slowly cooled to obtain Ac.

temper at will (n) After hot forming, the product is rapidly cooled or slowly cooled to obtain Ac.

It is heated on a dot plate to re-austenitize part or all, and then quenched by rapid or slow cooling.

(III) Furthermore, after tempering the material of (II) below the Acn 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 with a view to relieving residual stress during direct quenching.Therefore, tempering is preferably performed at 450°C or higher, where stress relaxation occurs.C1 Do it below the point.

In the case of (II), heat treatment is performed while quenching.

Ac, heated on a dot plate to partially or completely austenite, and then cooled. Since re-austenitization also has the meaning of homogenization, a temperature of Ac1 point or higher is desirable.

In the case of (III), the material quenched in (If) is tempered to relieve stress, so reheating is performed to Ac1 point or higher.

In the method for producing steel materials according to the present invention, the tube-making process, which is a typical example of hot forming, is not particularly limited, but examples include tube-making methods 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 as hot forming, but it will be understood by those skilled in the art that similar effects will be obtained even if the pipe manufacturing process as described above is performed. will be understood.

Examples A-Q steels shown in Table 1 were melted and hot rolled to 8.
After heat treatment shown in Table 2 was performed on A-1 steel, which is the steel of the present invention, J steel of the conventional example, and L-Q steel of the comparative example, which were made into thick plates, yield strength, corrosion rate, Tested for sulfide stress cracking.

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

The corrosion test was carried out using a 21mm thick x 10mm wide x 75mm length with 0.25R in the center as shown in Figure 1 (a), (b), and (c).
Two four-point bending test specimens l each having a notch 2 cut therein were prepared, and then, as shown in FIG. The bending stress was applied so that the stress applied was 1σy (σ,: 0.2% proof stress). The bent shape of the test piece 1 at this time was as shown in FIG. 2(b).

The test environment is 5% Nac2 + 0.03 atm fits
+30 atm Cot and after 336 hours of immersion test,
The test piece was taken out, and the corrosion weight 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 using an optical microscope. Note that the test temperature was 25°C.

The results of both these tests are summarized in Table 2.

In the table, in the column of sulfide stress cracking, "OO" indicates that no cracking occurred in both test pieces, and "XX" indicates that cracking occurred in both test pieces.

Conventional Examples 25 and 26 are made of conventional 13Cr steel and 9Cr-
I.M.

However, in this environment, the corrosion rate is high and stress cracking is also observed, which is not desirable.

Comparative Examples 27 and 28 each have a higher C and N content than the steels of the present invention, have significantly higher strength (but slightly lower corrosion rates, and suffer from sulfide stress cracking).

Comparative Example 29 has (Cr+Mo) l less than 11% and has poor corrosion resistance. In Comparative Examples 30 and 31, the components do not satisfy the respective formulas, and in particular, in Comparative Example 30, δ ferrite is produced, and in Comparative Example 31, residual austenite is produced, so that the strength is not suitable for oil country tubular goods.

Comparative Example 32 corresponds to the steel disclosed in JP-A-60-174859, in which Mn and S are too high, causing sulfide stress cracking.

However, as shown in Invention Examples 1 to 24, the steel of the present invention has both the necessary strength and corrosion resistance even when hot-rolled or subjected to various heat treatments, and can be used in the harsh environments described above. It can be seen that it can be suitably used as oil country tubular goods. All of these steels had a single martensitic phase.

(Leaving space below) Table 2 (Continued from Table 2) AC: E-cooling, Country: Oil-cooling, Country: Water-cooling (Effects of the invention) As is clear from the above examples, the present invention: The objective is to provide steel that has satisfactory corrosion resistance even in harsh environments where chloride ions, carbon dioxide gas, and hydrogen sulfide gas exist, and has sufficient strength for use as oil country tubular goods, etc., and the practical benefits are as follows: big.

[Brief explanation of the drawing]

Figures 1 (a), (b) and (c) are schematic diagrams showing the shape of the notched four-point bending test piece used in the examples; and Figures 2 (a) and (with) are FIG. 3 is an explanatory diagram showing the state of stress applied to a test piece using a bending jig. Figure 2

Claims (7)

[Claims] (1) In weight%, C: 0.05% or less, Si: 1.0% or less, Mn: 0.
Less than 5%, P: 0.04% or less, S: 0.002% or less, Cr: 8 to 15%, Mo: 1.5 to 7%, Ni: 2 to
8%, Al: 0.001 to 0.1%, N: 0.1% or less, and Cr+Mo≧11.0%, 30Cr(%)+36Mo(%)+14Si(%)-2
8Ni(%)≦455(%)21Cr(%)+25Mo
(%)+17Si(%)+35Ni(%)≦731(%
), and the remainder is Fe and unavoidable impurities. Martensitic stainless steel for oil wells with excellent sulfide stress corrosion cracking resistance. (2) Claim 1 further comprising one or more of Ti: 0.5% or less, Nb: 0.5% or less, V: 0.5% or less, and Zr: 0.5% or less. Martensitic stainless steel for oil wells as described. (3) Furthermore, 0.001 to 0.05% Ca
The martensitic stainless steel for oil wells according to claim 1 or 2, comprising one or more of , Mg, La, and Ce. (4) A martensitic stainless steel for oil wells having excellent sulfide stress corrosion cracking resistance, characterized in that the martensitic stainless steel according to any one of claims 1 to 3 is hot-formed and then rapidly cooled or slowly cooled. Method of manufacturing steel. (5) After hot forming the martensitic stainless steel according to any one of claims 1 to 3, the martensitic stainless steel is rapidly or gradually cooled to at least 70°C or lower, heated again to Ac_1 point or lower, and then rapidly or gradually cooled. A method for producing martensitic stainless steel for oil wells, which has excellent sulfide stress corrosion cracking resistance and is characterized by cooling. (6) After hot forming the martensitic stainless steel according to any one of claims 1 to 3, the martensitic stainless steel is quenched or slowly cooled to at least 70°C or lower, heated again to Ac_1 point or higher, and then quenched or slowly cooled. A method for producing martensitic stainless steel for oil wells, which has excellent sulfide stress corrosion cracking resistance and is characterized by cooling. (7) After hot forming the martensitic stainless steel according to any one of claims 1 to 3, the martensitic stainless steel is rapidly or gradually cooled to at least 70°C or lower, and then heated again to a temperature of Ac_1 point or higher, and then at least A martensitic stainless steel for oil wells with excellent sulfide stress corrosion cracking resistance, which is characterized by being rapidly or slowly cooled to 70°C or lower, then heated again to Ac_1 point or lower, and then rapidly or slowly cooled. Production method.