JPS6254021A - Manufacture of high strength seamless steel pipe superior in sulfide stress corrosion cracking resistance - Google Patents

Abstract

PURPOSE:To obtain the titled steel pipe without contg. a large quantity of precious element and having a high yield strength, by specifying cooling rate at quenching after hot working of steel pipe contg. prescribed quantities of C, Cr, Mo and suppressed contents of P, S and raising tempering temp. CONSTITUTION:Hollow stock for seamless steel pipe contg. by weight <0.40-0.60% C, 0.20-0.35% Si, 0.4-1.2% Mn, 0.8-1.5% Cr, 0.6-1.0% Mo, and 0.005-0.1% Al, together with both P and S suppressed to <=0.02% and <=0.010% respectively is prepd. This is subjected to hot drawing, then heated to >=Ac3 transformation point temp. and quenched by cooling from the temp. to at least 300 deg.C by 1-50 deg.C/s average rate to obtain martensitic structure. Thereafter the material is tempered at >=680 deg.C - <=Ac1 transformation point range.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) This invention is applicable to oil country tubular goods, which are becoming increasingly deeper due to the trend toward sour oil, and no-line pipes for sour gas and sour oil.
Furthermore, regarding the manufacturing method of high-strength seamless steel pipes with excellent sulfide stress corrosion cracking resistance suitable for use in applications such as chemical plant piping, the risk of sulfide stress corrosion cracking is reduced to at least 75 kgf/m1T12. This paper discloses the development results regarding effective avoidance under high yield strength.

(Prior art) In response to the above-mentioned tendency for deeper wells to become sour,
In general, sulfide stress corrosion cracking resistance deteriorates as the strength increases, so Cr-Mo steel with a yield strength of 64 to 74 kgf/ml class 112 is currently considered to be the most excellent in terms of the balance between the two. ing.

By the way, recently, according to Japanese Patent Application Laid-Open No. 53-78917,
Compared to conventional 65kgf/H2 class [:r-Mo steel]
r, 75-9Qkgf/m by increasing Mo and adding a large amount of Katsu to improve sulfide stress corrosion cracking resistance.
M2 grade steel was developed, but it was expensive because it contained large amounts of expensive elements such as Mo and V, and because it contained a large amount of ■, it was prone to cracking during hot working by continuous casting.

Also, JP-A-57-19322 and JP-A-57-1932
In each publication No. 3, a La-added steel with excellent sulfide stress corrosion cracking resistance was proposed, but the yield strength of this steel is at most about 80 kgf / 11 m2, which is lower than conventional steel. There is virtually no improvement seen.

Furthermore, JP-A No. 57-35622 discloses a high-strength steel for oil wells with reduced P and S content, but although this steel has considerably improved strength, it is suitable for use in alkaline environments and H2S Resistance to stress corrosion cracking is guaranteed only when it is contained in only trace amounts.

Furthermore, JP-A-52-52114 and JP-A-5
4-119324, each publication states that when heating for quenching, AC, the average heating rate above the transformation point is set to 3 to 3.
A steel with improved sulfide stress corrosion cracking resistance has been proposed, which is made into a fine-grained steel through rapid heating of approximately b t/s (Japanese Unexamined Patent Publication No. 54-119324). These methods have limitations in achieving both high strength and resistance to sulfide stress corrosion cracking.

In fact, the yield strength of the steel shown in JP-A-54-119324 is about 55 kgf/mn++2, and the steel shown in JP-A-52-52114 has a yield strength of 75 kgf/mm2. The maximum load stress of steel that does not cause sulfide stress corrosion cracking is about 55 kgf/mm2 (73% of yield strength), and that of steel with yield strength e90 kgf/++un' is 40 kgf/mm' (44% of yield strength). It's only a matter of degree.

Under these current circumstances, the yield strength has recently been reduced to 75 kgf/m.
There is an increasing demand for materials that have a high strength of m' or higher and excellent resistance to sulfide stress corrosion cracking, and attempts have been made to use seamless pipes made of N1-based alloys such as Hastelloy and Inconel as oil country tubular goods. However, since these materials are too expensive, their use has to be limited, and there has been a strong demand for the development of inexpensive low-alloy steel materials.

(Problems to be Solved by the Invention) As mentioned above, all conventional steels have had some problems in terms of price or properties.

The present invention advantageously solves the above problems, and is an inexpensive composition system suitable for continuous casting that does not contain a large amount of the expensive elements listed above, and has excellent sulfide stress corrosion resistance. Breakability and yield strength are 75-120 kgf/mm
The purpose of the present invention is to provide an advantageous manufacturing method for seamless steel pipes having high strength.

(Means for Solving the Problems) The inventors have conducted extensive research in order to solve the above problems, and have obtained the knowledge described below.

(1) Sulfide stress corrosion cracking occurs more easily as the strength increases, but at the same level, the higher the tempering temperature, the better the sulfide stress corrosion cracking resistance.

[2) In order to obtain excellent sulfide stress corrosion cracking resistance with a steel having a yield strength of 75 kgf/mm2 or more, the tempering temperature must be at least 680°C or higher.

(3) When the tempering temperature is 680℃ or higher, 75k
In order to stably obtain a yield strength of gf/mm2 or higher,
In a Cr-Mo steel containing more than 0.4 Qwt% (hereinafter simply expressed as %) of C, it is necessary that Cr: 0.8% or more, 10: 0.6% or more.

(4) Even if the tempering temperature is 680°C or higher, if the steel contains 0.020% or more of P or 0.010% or more of S, good sulfide stress corrosion cracking resistance cannot be obtained. .

(5) As shown in item (3) above, Ti, V and N
Steels to which at least one of b. The ratio reaches over 90%. Here, the sour critical stress ratio is the ratio between the maximum stress at which sulfide stress corrosion cracking does not occur in a sour environment and the yield strength of the material, expressed as a percentage.

(6) However, 0.0, which is the target of this invention.
When quenching Cr-Mo steel containing a large amount of C exceeding 40% as a long, thick-walled pipe such as an oil country tubular goods, it is common practice to simply remove water from the heated state. If placed in a refrigerant such as oil, cracking will occur. Therefore, it is necessary to cool the steel relatively slowly, but if the cooling rate is too slow, hardening will not occur and the resistance to sulfide stress corrosion cracking after tempering will deteriorate significantly.

In order to prevent this, it is necessary to make the structure after quenching into 90% or more martensite, but for this purpose, the heating temperature must be set at 300°C, at which the martensitic transformation is almost completely completed.
It is necessary to set the average cooling rate to around 1°C to 1°C/S or more. On the other hand, in order to prevent quench cracking, the average cooling rate needs to be 50° C./S or less.

This invention is based on the above knowledge.

That is, this invention has C: more than 0.40 to 0.60%, S
l: 0.20-0.35%, Mn: 0.4-1.
2%, Cr: 0.8-1.5%, Mo: Q, 5-1.0
% and AI: 0.005-0.1%, 0.020%
P suppressed below and S suppressed below 0.010%
and sometimes further contains T in a range of 0.1% or less
A hollow material for a seamless steel pipe having a composition containing at least one of i, V, and Nb is subjected to hot working, and then heated to a temperature equal to or higher than the Ac3 transformation point. After forming a martensitic structure from this temperature to 300°C (average cooling rate: 1-b), AC at 680°C or higher.

This is a method for producing high-strength seamless steel pipes with excellent resistance to sulfide stress corrosion cracking, characterized by tempering at a temperature range below the transformation point.

Kuroshita: This invention will be explained in detail.

First, in this invention, the reason why the material components are limited to the above range will be explained.

C: more than 0.40 to 0.60% C is a useful element for obtaining high strength even in high temperature tempering, especially at 680°C as expected in this invention.
In order to stably obtain a yield strength of 75 kgf/mm2 or more through the above high temperature tempering treatment, at least 0.4
C is required in an amount exceeding 0%, but if it exceeds 0.60%, there is a risk of quench cracking, so it was decided to add in a range of over 0.40 to 0.60%.

Si: 0.20-0.35% Si is at least 0.20% to 0.35% for deoxidizing steel and improving strength.
Although 20% is required, if it exceeds 0.35%, the toughness deteriorates, so it is limited to a range of 0.20 to 0.35%.

! , In: 0.4-1.2% Mn is an element that is useful for improving hardenability and strength, and also for deoxidizing, but if the content is less than 0.4%, the effect of its addition is poor. , on the other hand, if it exceeds 1.2%, it is P.

Since it causes segregation of S and the like and deteriorates the resistance to sulfide stress corrosion cracking, it was decided to contain it in the range of 0.4 to 1.2%.

Cr: 0.8 to 1.5% Cr forms carbides during quenching and tempering treatment, and effectively contributes to increasing strength and tempering resistance. For that purpose, it is necessary to add 0.8% or more, but 145
%, the effect not only reaches saturation but also causes deterioration of sulfide stress corrosion cracking resistance.
The content was limited to a range of 0.068 to 1.5%.

Mo: 0.6-1.0%) 40 also increases strength and tempering resistance, similar to Cr,
Moreover, it effectively contributes to improving sulfide stress corrosion cracking resistance by preventing grain boundary segregation of P, but if the content is less than 0.6%, the addition effect is poor; If Mo is added in excess, the effect not only reaches saturation, but also tends to cause deterioration of toughness, which also increases costs, so the content of Mo was limited to a range of 0.6 to 1.0%. .

At: 0.005-0.1% AI is a useful element that not only contributes to deoxidation but also combines with N to refine crystal grains and improve toughness, strength, and sulfide stress corrosion cracking resistance. It is.

However, if the content is less than 0.005%, the effect of its addition is poor, while if it exceeds 0.1%, the effect not only reaches saturation, but also causes deterioration of toughness. .1% range.

P: 0.020% or less, S: 0,010% or less Both P and S are harmful elements that cause a significant deterioration of the sulfide stress corrosion cracking resistance of steel.According to this invention, Mo is added, When the tempering temperature is set to a high temperature of 680°C or higher, in order to ensure the initial strength and sulfide stress corrosion cracking resistance, P≦0.020% and S≦0.0, respectively.
It is necessary to suppress it to 10%.

After casting the molten metal prepared to have the above-mentioned preferred composition, a hollow material is obtained by drilling according to a conventional method.

The thus obtained hollow material for a seamless steel pipe is then subjected to a hot elongation process, and then subjected to a quenching and tempering process.

In this quenching treatment, the
It is necessary to make 0% or more of the martensitic structure, but for this purpose, after heating to a temperature above the C3 transformation point, cooling from this temperature to at least 300°C at an average cooling rate of 1 to b/S. There is a need to do. Here, the average cooling rate is 1 to b.If the uniform cooling rate is slower than 1℃/S, sufficient quenching will not occur, and satisfactory sulfide stress corrosion cracking resistance will not be obtained after tempering. This is because if it exceeds /S, there is a great possibility that quench cracking will occur.

Such cooling methods include, for example, methods in which refrigerant is sprayed onto steel pipes by spraying, showering, fogging, etc., in which the amount of refrigerant, spraying pressure, etc. are controlled, and methods in which the steel pipes are made in advance with high heat resistance and good heat retention before heating. There are various methods, such as wrapping the material in refractory material and cooling it with water or oil after heating, or preparing a refrigerant with a cooling capacity that suits the purpose. method may be adopted.

Further, the tempering treatment needs to be performed in a temperature range of 680° C. or higher, Ac, or lower than the transformation point. This is because, as initially developed in this invention, the yield strength is near 5 to 120 kgf/m [l
]2 and to obtain good sulfide stress corrosion cracking resistance,
This is because it is essential to perform tempering treatment at a high temperature of 680°C or higher.On the other hand, when the Ac transformation temperature is exceeded, austenite is formed, and when cooled to room temperature, this austenite becomes martensite that does not undergo tempering, making it resistant to sulfuration. This is because it causes a significant deterioration in the stress corrosion cracking properties of the material.

In this way, a high-strength seamless steel pipe with excellent sulfide stress corrosion cracking resistance is obtained. However, in this invention, in order to further improve the strength and sour critical stress ratio, T1 and N
b, V can be added.

TI, Nb and/or V: 0.1% or less Ti,
Like Cr and Mo, both Nb and (2) form carbides during quenching and tempering treatment, and effectively contribute to improving the hardenability and temper softening resistance of steel. However, if the amount of these added exceeds 0.1%, the precipitates become coarser, which not only deteriorates the sulfide stress corrosion cracking resistance (but also reduces the workability and toughness). and (2) must be added in an amount of 0.1% or less, whether added alone or in combination.

(Function) The reason why not only the strength but also the resistance to sulfide stress corrosion cracking is significantly improved by following the present invention has not yet been clearly elucidated, but it is thought to be as follows.

In other words, sulfide stress corrosion cracking occurs when hydrogen generated by corrosion of steel by an aqueous solution containing hydrogen sulfide penetrates into the steel.
It is considered to be a type of hydrogen embrittlement that accumulates in stress concentration areas such as inclusions, precipitates, and dislocations and embrittles steel. By reducing the dislocation density and making the precipitates spheroidal, and by performing high-temperature tempering in a complete quenching machine to make the precipitates more than 90% martensite, a structure in which the precipitates are finely and uniformly distributed can be obtained. This is thought to be due to the fact that stress concentration points where hydrogen accumulates are significantly reduced.

Normally, as mentioned above, when high-temperature tempering is performed to produce spherical precipitates with a low dislocation density, the strength of the steel material decreases, but with the combination of ingredients of this invention, precipitates with an appropriate shape and distribution can be produced. It is thought that this structure provides a structure with high strength and high strength without impairing sulfide stress corrosion cracking resistance.

(Example) After hot working, a hollow material for a seamless steel pipe having each component composition shown in Table 1 was heated to the Ac1 transformation point, and then cooled at the average cooling rate shown in Table 1. The products were subjected to tempering treatment at the temperatures shown in .

The yield strength of each seamless steel pipe thus obtained (Y, S,)
, tensile strength T, S, ) and sulfide stress corrosion cracking resistance are also listed in Table 1.

For sulfide stress corrosion cracking resistance, round bar tensile test pieces were tested with N
It was evaluated by a test in which a stress of 80% of the yield strength was applied in ACE liquid (0.5% acetic acid, 5% salt-added saturated hydrogen sulfide water), and those that did not break for 30 days in this test were marked with an ○. The broken pieces are also marked with an X.

As is clear from the table, all seamless steel pipes (symbols 1 to 10) obtained according to the present invention have high YS and TS values and excellent sulfide stress corrosion cracking resistance.

On the other hand, although the component composition satisfies the appropriate range,
Comparative steel whose tempering temperature is below the lower limit (symbol IL 12)
The comparative steel (symbol 25) whose cooling rate during quenching was less than the lower limit was inferior in sulfide stress corrosion cracking resistance. In addition, a comparative steel (symbol 24) whose cooling rate during quenching exceeded the upper limit had excellent sulfide stress corrosion cracking resistance, but suffered from quench cracking.

Further, among the components (symbols 13 to 23 and 26.27) in which any of the component compositions deviate from the appropriate range of the present invention, the C content is below the lower limit (symbol 18). , 26.27 is
Although the sulfide stress corrosion cracking resistance was good, only low YS and TS values were obtained. Other symbols 1
3-17 and 19-23 are all YS, TS
Although the value was good, the resistance to sulfide stress corrosion cracking was poor.

(Effects of the Invention) Thus, according to the present invention, an inexpensive component system that does not contain large amounts of expensive elements as in the past and is suitable for continuous casting.
A seam that has a high yield strength of near 5 to 120 kgf/++un2, but also has excellent sulfide stress corrosion cracking resistance that does not cause sulfide stress corrosion cracking even under a load stress of 80% of the yield strength. You can get steel-free pipes.

Claims (1)

[Claims] 1. C: more than 0.40 to 0.60 wt% Si: 0.20 to 0.35 wt% Mn: 0.4 to 1.2 wt% Cr: 0.8 to 1.5 wt% Mo : 0.6 to 1.0 wt% and Al: 0.005 to 0.1 wt%, together with P suppressed to 0.020 wt% or less and S suppressed to 0.010 wt% or less. The hollow material for use is subjected to hot working, then heated to a temperature above the Ac_3 transformation point, and then quenched from this temperature to at least 300°C at an average cooling rate of 1 to 50°C/s to form a martensitic structure. A method for producing a high-strength seamless steel pipe having excellent resistance to sulfide stress corrosion cracking, the method comprising: tempering at a temperature range of 680° C. or higher and Ac_1 transformation point or lower.