JPH10121202A - High-strength steel used in an environment requiring sulfide stress cracking resistance and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel having an austenite single phase structure and excellent in sulphide stress cracking resistance by specifying the contents of Ni, Cr and Mn in a steel and executing solution heat treatment and cold working under specified conditions after hot working. SOLUTION: This steel is composed of, by weight, <=1.2% C, 0.05 to 1% Di, 5 to 45% Mn, <=0.03% P, <=0.03% S, <=10% Cr+Ni+Mo, <=3% Cu, <=0.6% N, and the balance Fe with inevitable impurities. The steel is subjected to hot working and is thereafter subjected to solution heat treatment in which it is held at 1,000 to 1,200 deg.C for >=10min, and is thereafter rapidly cooled to allow carbides or the like to perfectly enter into solid dilution. Next, cold working is executed at a working degree of >=20%. In this way, the high strength steel having a metallic structure substantially composed of austenite and εmartensite and >=77.3kgf/mm<2> proof stress, and in which cracking generating limit stress (δth) in an NACE-TM-0177 bath is regulated to >=80% to the stress proof can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a steel material used in an environment requiring sulfide stress cracking resistance, for example, an oil well environment containing hydrogen sulfide, and a method for producing the same.
Since the steel material of the present invention has excellent sulfide stress cracking resistance together with high strength, it is suitable for use as a pipe used for extracting crude oil containing hydrogen sulfide, natural gas, etc., transporting, and other processes. Things.

[0002]

2. Description of the Related Art A steel material used in an oil well environment containing hydrogen sulfide is required to have resistance to sulfide stress cracking (hereinafter referred to as "sulfide stress cracking resistance") as its main property.

[0003] Sulfide stress cracking is a type of hydrogen embrittlement in which hydrogen generated on the surface of a steel material in a corrosive environment diffuses into the steel and breaks due to a synergistic effect with the stress applied to the steel material.

[0004] In general, the higher the strength of a steel material, the higher the susceptibility to sulfide stress cracking, and sulfide stress cracking easily occurs with a load stress lower than the proof stress of the steel material.

[0005] The sulfide stress cracking resistance is greatly affected by the metal structure of the steel material. Therefore, many studies have been made on a method for improving sulfide stress cracking resistance from the metal structure side. Generally, it is said that a tempered martensite structure (hereinafter, simply referred to as “α martensite”) is the most effective for improving sulfide stress cracking resistance, and a fine grain structure is desirable.

For example, JP-A-61-9519 discloses a method in which rapid heating means such as induction heating is applied to heating steel, and JP-A-59-232220 discloses that steel is hardened twice. The invention of making crystal grains fine by a method is disclosed. Other than that, for example,
Japanese Patent Application Laid-Open No. 3-93822 discloses a method for improving performance by using a steel material structure of bainite.

[0007] The steels targeted in many of the prior arts as described above are all carbon steels (low alloy steels) mainly composed of α-martensite or ferrite (partially bainite).

[0008] All of these steels are body-centered cubic (hereinafter, referred to as “body-centered cubic”).
"BCC"). However, BCC is inherently highly susceptible to hydrogen embrittlement. Therefore, there is no method for completely preventing sulfide stress cracking in steel having α-martensite or ferrite as a main structure. In particular, the sulfide stress cracking susceptibility increases as the strength increases. Therefore, it is practically impossible to obtain a steel material having high strength and excellent sulfide stress cracking resistance.

On the other hand, if a highly corrosion-resistant alloy such as a stainless steel or a high Ni alloy having a face-centered cubic (hereinafter referred to as “FCC”) austenitic structure which is essentially low in hydrogen embrittlement susceptibility is used, sulfides can be reduced. Stress cracking can be prevented. However, austenitic steels generally have low strength. Further, in order to obtain a stable austenite structure, it is necessary to add a large amount of expensive component elements such as Ni, and the production cost of steel materials is significantly increased.

However, steel materials used for applications such as oil country tubular goods are strongly required to have strength and economy. For this reason, conventional stainless steel containing a large amount of Ni or a high Ni alloy cannot satisfy the demand.

[0011]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a steel material having an austenitic single-phase structure, excellent sulfide stress cracking resistance and high strength, at a low cost. Specifically, the yield strength (YS) is 77.3.
１０９109.0 kgf / mm ² (110 to 155 ksi)
At an intensity level of NACE-TM-0177 bath (0.5
% Acetic acid + 5% salt, 1 atm hydrogen sulfide, 25 ° C.), a high-strength steel material having a sulfide stress cracking resistance of 80% or more of the above proof stress, and the steel material at low cost It is intended to provide a method of manufacturing.

[0012] The strength level is defined as 77.
The reason for setting the pressure to 3 kgf / mm ² or more is as follows. In other words, the main use of the steel material of the present invention is a pipe used for extracting and transporting crude oil and natural gas. Ie, 77.3-10
This is because it is desired to have a strength of 9.0 kgf / mm ² class or more.

On the other hand, regarding the sulfide stress cracking resistance,
The evaluation was based on the critical stress (σth) for crack initiation in the NACE-TM-0177 bath recommended by the “National Association of Corrosion Engineers”.
If the crack generation limit stress (σth) is 80% or more of the proof stress, it can sufficiently withstand the use in a recent severe corrosive environment.

[0014]

It is said that a material having an austenite structure generally does not cause hydrogen embrittlement unlike a ferrite structure and an α-martensite structure.
However, stainless steels and high-Ni alloys having an austenitic structure have a strong effect of improving the corrosion resistance of the film by mainly containing Cr and Ni in an environment containing hydrogen sulfide. It is not yet clear whether or not does not cause hydrogen embrittlement.

As a result of experimental studies using a large number of test materials, the present inventors have found that an austenitic single-phase steel material containing Mn, which is a much cheaper element than Ni and Cr, as a main alloy component is excellent. Sulfide stress cracking resistance.

However, simply adding Mn to form an austenitic single phase does not provide the necessary strength as a steel material for the above-mentioned applications. Therefore, as a result of various experimental studies on the method of imparting a predetermined strength to this steel material, a part of austenite was found to have a densest hexagonal crystal (hereinafter referred to as “HC”).
P)), but it is most economical to perform cold working that does not cause hydrogen embrittlement and does not deteriorate sulfide stress cracking resistance. And the manufacturing method was also developed.

The gist of the present invention is as follows: (1) a high-strength steel material; and (2) a manufacturing method thereof.

(1) By weight%, C: 1.2% or less, S
i: 0.05-1%, Mn: 5-45%, P: 0.03
%, S: 0.03% or less, Cr + Ni + Mo: 10
%, Cu: 3% or less, N: 0.6% or less, the balance being Fe and unavoidable impurities, and a metal structure substantially consisting of austenite and ε martensite;
7.3kgf / mm ² or more proof stress and NAC
Critical stress for crack initiation in E-TM-0177 bath (σt
h) is 80% or more of the proof stress, wherein the high-strength steel material is used in an environment requiring sulfide stress cracking resistance.

(2) By weight%, C: 1.2% or less, S
i: 0.05-1%, Mn: 5-45%, P: 0.03
%, S: 0.03% or less, Cr + Ni + Mo: 10
%, Cu: 3% or less, N: 0.6% or less, the balance being steel which is hot-worked from Fe and unavoidable impurities, and then quenched after holding at 1000 to 1200 ° C. for 10 minutes or more. A method for producing a high-strength steel material used in an environment requiring sulfide stress cracking resistance, which comprises subjecting a sulfide heat treatment to cold working at a working ratio of 20% or more.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION

<< High strength steel of the present invention >> The steel of the present invention is characterized by the following.

Containing Mn of 5 to 45% (hereinafter, “%” of the component content means “% by weight”), the metal structure substantially consisting of austenite and ε martensite, 77 0.3 kgf / mm ² or more, a crack initiation critical stress (σth) in NACE-TM-0177 bath is 80% or more of the above proof stress, and sulfide stress cracking resistance is required. It must be used in the environment.

The above-mentioned "substantially composed of austenite and ε martensite" means that the total amount of austenite and ε martensite is not necessarily 100%.
It does not need to be. Of course, it is most desirable that the sum of austenite and ε martensite is 100%, but it is permissible that a small amount of other structures (for example, up to about 5% in area ratio), such as ferrite, coexist.

As described above, it is generally said that steel having an austenitic structure of FCC hardly causes hydrogen embrittlement. However, it was unclear whether such characteristics could be obtained even with an austenitic steel not containing a large amount of Ni or Cr.

As described above, the steel material of the present invention has a low cost
n as the main austenite stabilizing element,
It has excellent sulfide stress cracking resistance. Moreover, for example, by the manufacturing method of the above (2),
And high strength as described above.

First, the reason why the composition of steel is determined as described above will be described.

Mn: 5 to 45% Mn is an element necessary for changing the metal structure of the steel material to an austenitic single phase structure at normal temperature. However, when the content is less than 5%, a substantial mixed structure of austenite and ε-martensite cannot be obtained, and the sulfide stress cracking resistance is significantly deteriorated due to excessive precipitation of a ferrite phase. On the other hand, when the content exceeds 45%, Mn dissolves in the hydrogen sulfide environment, increasing the corrosion rate and deteriorating sulfide stress cracking resistance. Therefore, the Mn content is set to 5 to 45%.

Even if the Mn content is less than 5%, a mixed structure of austenite and ε martensite can be obtained by adding a large amount of an austenite stabilizing element such as Ni or N, but an expensive element such as Ni is used. Use of a large amount of is contrary to the object of the invention to provide an inexpensive steel material.

In the above range, in an area where Mn is relatively low, an austenite stabilizing element such as Ni or N may be used in combination. However, excessive addition of these components may cause It is not preferable because the corrosion rate is lowered due to the excessive improvement of the corrosion resistance and the sulfide stress cracking resistance is lowered. Therefore, in the steel material of the present invention, it is basically desirable to obtain a stable mixed structure of austenite and ε martensite only with Mn, and the preferable range of the Mn content is 15 to 45%.

Si: 0.05-1% Si is an element necessary for deoxidizing steel, and if its content is less than 0.05%, deoxidation becomes insufficient and nonmetallic inclusions Remains, and the desired sulfide stress cracking resistance cannot be obtained. On the other hand, when the content exceeds 1%, the grain boundary strength is weakened, and the sulfide stress cracking resistance is reduced. Therefore, Si
The content was 0.05-1%. The preferred range is 0.1 to 0.9%, and the more preferred range is 0.3 to 0.
8%.

C: not more than 1.2% C may not be added, but when added, stabilizes austenite, suppresses ferrite precipitation, improves sulfide stress cracking resistance, and improves strength. It is an element that contributes to improvement. Therefore, if these effects are desired, they can be added. However, if the content is less than 0.05%, the above effect is small. On the other hand, if the content exceeds 1.2%, corrosion proceeds at the interface between the carbide and the base material with an increase in the carbide, and the corrosion rate increases to reduce the wall thickness and deteriorate the sulfide stress cracking resistance. Therefore, the C content when added is desirably 0.05% or more, but its upper limit must be 1.2% or less. In addition, when adding, the preferable range is 0.1 to 1%, and the more preferable range is 0.4 to 0.6%.

P: 0.03% or less P is inevitably present in steel as an impurity.
If it exceeds 03%, it segregates at the grain boundaries and deteriorates sulfide stress cracking resistance. Therefore, the P content is set to 0.03% or less. The lower the P content, the better.
However, an excessive decrease causes an increase in the production cost of the steel material. Therefore, the lower limit is desirably set to about 0.001%, more preferably about 0.005%.

S: 0.03% or less S is inevitably present in steel as an impurity like P, but if it exceeds 0.03%, it segregates at grain boundaries,
Generates sulfide-based inclusions and reduces sulfide stress cracking resistance. Therefore, the S content is set to 0.03% or less.
The lower the content of S, the better. However, an excessive decrease leads to an increase in steel production costs,
The lower limit is about 0.002%, more preferably 0.02%.
It is desirable to keep it at about 05%.

Cu: 3% or less Cu need not be added, but when added, is an element that strengthens the coating to improve corrosion resistance and contributes to improvement in strength. Therefore, if these effects are desired, they can be added. However, if the content is less than 0.5%, the above effect is small. On the other hand, if the content exceeds 3%, the hot workability of steel is significantly deteriorated,
Production of steel materials becomes difficult. Therefore, when adding Cu
The content is desirably 0.5% or more, but the upper limit must be 3% or less. In addition, when adding, a preferable range is 1 to 2.5%, and a more preferable range is 1.%.
5 to 2.5%.

N: 0.6% or less N may not be added, but when added, austenite is stabilized to suppress precipitation of ferrite, improve sulfide stress cracking resistance, and improve strength. It is an element that contributes to improvement. Therefore, if these effects are desired, they can be added. However, if the content is less than 0.05%, the above effect is small. On the other hand, if the content exceeds 0.6%, the hot workability of steel is significantly deteriorated, and it becomes difficult to manufacture steel materials. Therefore, it is desirable that the N content when added is 0.05% or more, but the upper limit must be 0.6% or less. In addition, when adding, the preferable range is 0.1-0.5%,
A more preferred range is 0.1 to 0.3%.

Cr + Ni + Mo: 10% or less Cr, Ni and Mo do not need to be added, but when added, all elements are elements that improve the film corrosion resistance and contribute to the improvement in strength. Ni also has the effect of stabilizing austenite, suppressing the formation of ferrite, and improving sulfide stress cracking resistance. Therefore, if you want to get these effects,
One, two or more of Cr, Ni and Mo can be added. However, if the content of each element is less than 1%, the above effect is small. On the other hand, if the total content exceeds 10%, the effect of improving the corrosion resistance of the film becomes excessively large, so that stress corrosion cracking accompanying the occurrence of pitting corrosion occurs. Therefore, the content of these elements when they are added is desirably 1% or more for each element, but the total content must be 10% or less.
The preferred upper limit of the total content when added is 9
%, More preferably 7%.

<< Method of Manufacturing Steel Material of the Present Invention >> As described above, the steel material of the present invention has a thickness of 77.3 kgf / mm ² (11
It is a high strength steel material having a proof stress of 0 ksi) or more. One of the manufacturing methods for providing such strength is the method (2). Hereinafter, the method will be described in the order of steps.

1. Melting and casting Melting and casting may be performed by a general method for manufacturing an austenitic steel material. The casting may be ingot casting or continuous casting.

2. Hot working (forging, piercing, rolling) After casting, hot working such as forging, piercing, or rolling is performed. In the production of seamless steel pipes, a method of directly punching a billet obtained by continuous casting has been put to practical use, and in this case, a forging step is not required. When the steel material is a seamless steel pipe, rolling is performed using a mandrel mill or a plug mill after the above-described drilling step. When the steel material is a plate material, the process involves rough rolling of the slab, followed by finish rolling. Desirable conditions for hot working such as forging, piercing, and rolling are as follows.

<In the case of a seamless steel pipe>
It may be performed to the extent that hot piercing in a piercing mill is possible,
Desirable is between 1000 ° C and 1250 ° C. There is no special restriction on piercing rolling and rolling by other rolling mills such as mandrel mills and plug mills, but from the viewpoint of hot workability, specifically, to prevent surface flaws,
The finishing temperature is desirably 900 ° C. or higher. There is no particular upper limit on the finishing temperature, but it is preferable to keep the finishing temperature up to 1100 ° C.

<In the case of steel plate>
Temperature at which hot rolling is possible, for example, from 1000 ° C. to 1250
It is enough to be between ° C. The pass schedule of the hot rolling is optional, but it is desirable to set the finishing temperature to 900 ° C. or higher in consideration of hot workability for reducing the occurrence of surface flaws, edge cracks, and the like of the product. The finishing temperature is preferably up to 1100 ° C. as in the case of the seamless steel pipe.

<Solution Heat Treatment> After hot working, the steel is heated to a temperature sufficient to completely dissolve carbides and the like, and then rapidly cooled. In this case, it is necessary to keep the temperature in the range of 1000 to 1200 ° C. for 10 minutes or more and then rapidly cool it. That is, if the heating temperature is less than 1000 ° C., carbides,
In particular, when Cr or Mo is added, Cr-Mo-based carbides cannot be completely dissolved, and a Cr- and Mo-depleted layer is formed around the Cr-Mo-based carbides, resulting in stress corrosion caused by pitting. Cracking occurs, and the desired sulfide stress cracking resistance cannot be obtained. On the other hand, when the heating temperature exceeds 1200 ° C., a different phase such as ferrite is precipitated, and the desired sulfide stress cracking resistance cannot be obtained. In addition, the holding time is 10
If the heating time is less than 1000 minutes, the effect of solution treatment becomes insufficient and carbides cannot be completely dissolved.
For the same reason as in the case where the temperature is lower than 0 ° C, the desired sulfide stress cracking resistance cannot be obtained.

The upper limit of the holding time also depends on the size and shape of the steel material, and cannot be generally determined. In any case, the time required for the entire steel material to be soaked is necessary, but from the viewpoint of reducing manufacturing costs, too long time is not desirable,
Usually, it is appropriate to set the time within one hour. In addition, cooling
In order to prevent precipitation of carbides (mainly Cr-Mo-based carbides) and other intermetallic compounds during cooling, it is necessary to cool at a cooling rate higher than oil cooling.

The lower limit of the holding time is a holding time when the steel material after hot working is once cooled to a temperature of less than 1000 ° C. and then reheated to the above temperature range of 1000 to 1200 ° C. However, when the end temperature (finish temperature) of hot working is set in the range of 1000 to 1200 ° C., if the supplementary heat is performed at that temperature for about 5 minutes or more, the same effect as the solution heat treatment under the above conditions is obtained. And can be rapidly cooled without reheating. Therefore,
The lower limit value of the holding time in the present invention includes the case where the end temperature (finish temperature) of hot working is in the range of 1000 to 1200 ° C. and the supplementary heat is performed at that temperature for about 5 minutes or more.

<Cold work> The steel material after the solution heat treatment has a target high strength, that is, 77.3 kgf /
Cold working is performed to obtain a strength of mm ² (110 ksi) or more. In this case, it is necessary to perform cold working with a working ratio (cross-section reduction rate) of 20% or more. That is, if the degree of work to be imparted is less than 20%, a target high strength, that is, a strength of 77.3 kgf / mm ² (110 ksi) or more cannot be obtained.

The cold working method is not particularly limited as long as it can uniformly work the steel material.
However, when the steel material is a steel pipe, it is industrially advantageous to use a so-called cold drawing machine using a perforated die and a plug, or a cold rolling mill called a cold pilger mill. When the steel material is a plate material, it is industrially advantageous to use a rolling mill used for manufacturing a normal cold-rolled steel sheet.

The upper limit of the degree of processing to be applied is not particularly limited. However, excessive processing is not desirable from the viewpoint of suppressing the production cost, and its upper limit is desirably set to about 50%. Further, the working ratio for obtaining the desired strength can be freely selected within a range of 20% or more. For example,
The degree of working to obtain the desired strength may be determined according to the cold strengthening curve shown in FIG.

Here, as described above, part of the austenite is transformed into the ε-martensite phase of HCP by the cold working, but this ε-martensite does not cause hydrogen embrittlement and has resistance to sulfide stress cracking. Does not deteriorate.

[0048]

【Example】

<< Test 1 >> Twenty kinds of steels having the chemical components shown in Table 1 were melted in a 150 kg vacuum furnace, and the cross-sectional dimension was 80 mm.
It was cast into an ingot of × 75 mm and length of 130 mm.

[0049]

[Table 1]

In Table 1, steel Nos. A to Q are austenitic steels of the present invention and comparative steels, and steel Nos. RT are conventionally considered to have excellent sulfide stress cracking resistance. Ferritic (α-ferritic) low alloy steel.

After heating each of the above ingots at 1250 ° C. for 1 hour, it was hot-rolled at a finishing temperature of 1000 ° C. to form a plate having a thickness of 12 mm, a width of 90 mm and a length of 700 mm. Alternatively, only heat treatment was performed.

<Steel Subjected to the Present Invention and Comparative Steel> Solution treatment: After holding at 1000 ° C. for 1 hour, oil cooling. Cold working: Cold rolling at a working degree of 25 to 40%. As shown in Tables 2 and 3, for some plate materials (steel Nos. A, C, and D), the solution treatment conditions and the working ratio during cold working were variously changed.

<Ferritic low alloy steel> Quenching treatment: After holding at 900 ° C. for 15 minutes, water cooling. Tempering treatment: treatment at the temperature and time shown in Table 4, followed by cooling. Cold working: Not performed.

Using the obtained test materials, mechanical properties and sulfide stress cracking resistance were investigated. The sulfide stress cracking resistance was evaluated using two round bar-type tensile test pieces (parallel part 6.35φ × 25.4 mm) taken from the L direction (rolling progress direction) of the test material. The applied stress was set to 80% of the proof stress of the base material.

The test solution was NACE-TM-0177-
90 (Test Method by National Association of Corro
NACE-TM-01 specified in the “sion engineers”
77 baths (0.5% acetic acid + 5% saline, 1 atm hydrogen sulfide saturation, 25 ° C.). It is determined whether the sample is kept in this solution for 720 hours or not to determine whether or not it is broken. It was evaluated as poor "x".

Tables 2 to 4 show heat treatment conditions, cold working conditions, strength and sulfide stress cracking test results of the inventive examples, comparative examples and conventional examples. Tables 2 to 4 also show the amount of austenite and the amount of ε-martensite (all of which are area ratios) measured by the point calculation method.

[0057]

[Table 2]

[0058]

[Table 3]

[0059]

[Table 4]

As is clear from the results shown in Tables 2 to 4, the test steels No. A to E of the present invention (steel Nos. A to E in Table 1) were manufactured under the conditions specified in the present invention. 1 to
The metal structure of the steel material of No. 15 of the present invention is substantially a mixed structure of austenite and ε martensite. And
Its proof strength is all 77.3 kgf / mm ² or more,
Some of them have high strength exceeding 100 kgf / mm ² . That is, the steel material of the present invention conforms to the API standard C11.
It corresponds to class 0 or higher. And despite having such high strength, it did not break in the test in NACE-TM-0177 bath. The stress applied in this test is 80% of the proof stress of the base material. That is, the steel material of the present invention had extremely excellent sulfide stress cracking resistance, in which the critical stress for crack initiation was 80% or more of the proof stress.

However, even if the steel subject to the present invention is used, if the production conditions are out of the range specified in the present invention, the desired strength cannot be obtained, or even if the strength is obtained, the desired sulfuration resistance can be obtained. No material stress cracking property was obtained. That is, Test No. 1
6-18, 20-22, 24-26,
Even when the cold working conditions were within the range of the present invention, when the solution heat treatment conditions were out of the range of the present invention, the target strength was obtained, but the sulfide stress cracking resistance was poor. Conversely, test N
o. As is clear from FIGS. 19, 23 and 27, even if the solution heat treatment is within the scope of the present invention, the sulfide stress cracking resistance is not good when the cold working conditions are outside the scope of the present invention. However, the target strength was not obtained.

On the other hand, the production conditions were within the range of the present invention, but when comparative steels having a component composition outside the range specified in the present invention were used, the sulfide cracking resistance was poor in all cases.

That is, when a comparative steel having a too low Mn content (steel I in Table 1) was used (Test Nos. 31 and 4).
3, 55) showed a ferrite structure exceeding 5% and poor sulfide stress cracking resistance. When a comparative steel with excessive Mn content (steel J in Table 1) was used (Test No. 3)
2, 44, 56) had a substantially austenitic single phase structure, but had poor sulfide stress cracking resistance. This is a result of an increase in the corrosion rate due to elution of Mn in the steel as described above.

Further, comparative steels having too low a Si content (Table 1)
(G steel in the middle) (Test Nos. 29, 41 and 53), the deoxidation was insufficient and the inclusions remained, resulting in poor sulfide stress cracking resistance. When a comparative steel (H steel in Table 1) having an excessive Si content was used (Test Nos. 30, 4)
In Nos. 2 and 54), the sulfide stress cracking susceptibility of the portion was increased due to the low strength of the grain boundary, and the sulfide stress cracking resistance was poor.

Further, when a comparative steel having an excessive C content (steel F in Table 1) was used (Test Nos. 28, 40 and 52).
Means that carbides, especially Cr-Mo-based carbides, remain and Cr
And Mo deficient layers were formed, resulting in poor sulfide stress cracking resistance. Comparative steel with excessive P content (K steel in Table 2)
When (No. 33, 45, 57) was used, excessive P segregated at the grain boundaries, and the sulfide stress cracking resistance was poor. When a comparative steel (Steel L in Table 1) having an excessive S content was used (Test Nos. 34, 46 and 58), excess S was segregated at the grain boundaries while formation of sulfide-based inclusions was performed. As a result, the sulfide stress cracking resistance was poor.

Furthermore, when comparative steels (M, N and O steels in Table 1) containing excessive amounts of Cr, Ni and Mo were used (Test Nos. 35 to 37, 47 to 49, 59 to 59)
In No. 61), the effect of improving the corrosion resistance of the film was excessive, and cracking occurred due to the occurrence of pitting corrosion, and the sulfide stress cracking resistance was poor. When a comparative steel with excessive Cu content (P steel in Table 1) and a comparative steel with excessive N content (Q steel in Table 1) were used (Test Nos. 38, 39, 50, 51, 62). , 63)
All of them had poor hot workability and could not be hot rolled.

The conventional ferritic low-alloy steels (R, S and T steels in Table 1) shown for reference and subjected to quenching and tempering (Test Nos. 64 to 69) have high strength. However, in each case, the sulfide stress cracking resistance was poor. This is because sulfide stress cracking susceptibility is increased at such a high strength because the metal structure is a structure mainly composed of α-martensite.

<< Test 2 >> 0.3-0.5% C-0.3- in which the total content of Cr, Ni and Mo was variously changed
0.5% Si-17-22% Mn-0.001-0.0
The effect of the total content of Cr, Ni, and Mo on the corrosion rate and the relationship between the occurrence of sulfide stress cracking and the use of several types of steel consisting of 03% P-0.005% to 0.02% S and the balance of Fe. Examined.

In the test, a thickness of 5 mm and a width of 1 mm were taken from the L direction (rolling progress direction) of the sheet obtained by rolling in Test 1.
0 mm, a plate-shaped test piece having a length of 30 mm was immersed in the same NACE-TM-0177 bath as used in Test 1 for 720 hours, and then the test piece was taken out of the bath and its corrosion weight loss was measured. The corrosion rate was calculated from the corrosion weight loss. The test results are shown in FIG.

As is apparent from FIG. 1, the corrosion rate decreases as the total content of Cr, Ni and Mo increases. However, if the total content exceeds 10%,
It can be seen that sulfide stress cracking has occurred. this is,
As described above, Cr, Ni, and Mo are elements that improve the film corrosion resistance, and the film corrosion resistance is improved. However, in a severe corrosive environment containing hydrogen sulfide, if local corrosion such as pitting occurs, a corrosion cell is formed between the film and the inside of the pit, and the corrosion inside the pit remarkably progresses. This is because sulfide stress cracking occurs.

<< Test 3 >> Using steels A and B shown in Table 1, the effects on the proof stress were examined by variously changing the working ratio during cold working. The other test conditions were the same as Test No. 1 of Test 1. The test results are shown in FIG.

As shown in FIG. 2, if the working ratio is not 20% or more, the target 77.3 kgf / mm ² (110 k
It can be seen that a proof stress higher than si) cannot be obtained.

[0073]

The steel material of the present invention has a mixed structure of austenite and ε martensite which does not cause hydrogen embrittlement, has excellent sulfide stress cracking resistance, and has high strength. These properties are virtually impossible to achieve with conventional carbon steels and low alloy steels.

The steel material of the present invention is much cheaper than an austenitic stainless steel or a Ni-based alloy containing a large amount of Ni, and can be manufactured only by performing cold working after solution heat treatment. Since the cost can be reduced, it is economically excellent. Therefore, it is extremely useful as a material for oil country tubular goods and the like used in a severe corrosive environment containing hydrogen sulfide.

[Brief description of the drawings]

FIG. 1 is a graph showing the results of examining the effect of the total content of Cr, Ni and Mo on corrosion rate and sulfide stress cracking resistance.

FIG. 2 is a diagram showing the results of examining the effect of the working degree during cold working on the proof stress for an example of the steel material of the present invention.

Claims (2)

[Claims] (1) C: 1.2% or less and Si: 0.
05-1%, Mn: 5-45%, P: 0.03% or less,
S: 0.03% or less, Cr + Ni + Mo: 10% or less,
Cu: 3% or less, N: 0.6% or less, the balance being Fe
A metal structure substantially consisting of austenite and ε martensite, and 77.3k
With a proof stress of gf / mm ² or more, and NACE-TM
A high-strength steel material used in an environment requiring sulfide stress cracking resistance, wherein a critical stress (σth) at which cracking occurs in a 0177 bath is 80% or more of the proof stress. 2. C .: 1.2% or less by weight, Si: 0.
05-1%, Mn: 5-45%, P: 0.03% or less,
S: 0.03% or less, Cr + Ni + Mo: 10% or less,
Cu: 3% or less, N: 0.6% or less, the balance being Fe
And after hot working steel consisting of unavoidable impurities,
2. The method for producing a high-strength steel material according to claim 1, wherein a solution heat treatment for quenching after holding at 000 to 1200 [deg.] C. for 10 minutes or more is performed, and thereafter, cold working is performed at a working ratio of 20% or more.