JP2000144337A - Low-C high-Cr alloy steel having high corrosion resistance and high strength, and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow the subject steel to exhibit high stress corrosion cracking resistance even in environment contg. gaseous moist carbon dioxide and moist hydrogen sulfude in particular by alloying it to have a specified compsn. contg. C, Cr, Si, Mn, Ni, Mo, V and N, and in which each content of Ni, Cr and Mo satisfies specified relation. SOLUTION: This alloy steel has a compsn. contg. 0.005 to 0.05% C, 12 to 16% Cr, <=1% Si, 0.05 to 0.3% Mn, 3.5 to 6% Ni, 1.5 to 2.5% Mo, 0.01 to 0.05% V, <=0.02% Ni, and the balance Fe with inevitable impurities and satisfying the following conditions: 705-25×Ni%+5×Cr%+25×Mo%>=680. In the production, steel having this compositional range is melted into a slab, which is subjected to hot working, is thereafter heated at Ac3 point to 980 deg.C, is austenitized and is thereafter cooled. Next, it is tempered at 550 deg.C to the Ac1 point.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength martensitic stainless steel having excellent resistance to stress corrosion cracking and a method for producing the same.
The present invention relates to a high-strength low-C high-Cr alloy steel (high-strength martensitic stainless steel) having high stress corrosion cracking resistance in an environment containing wet carbon dioxide gas and wet hydrogen sulfide in excavation and transportation of natural gas, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, petroleum and natural gas produced in recent years often include a large amount of wet carbon dioxide gas and wet hydrogen sulfide. In excavation and transportation, 13Cr stainless steel or the like is used in place of conventional carbon steel. Martensitic stainless steel has been used. However, conventional martensitic stainless steels have excellent corrosion resistance to wet carbon dioxide gas (hereinafter simply referred to as corrosion resistance), but have insufficient stress corrosion cracking resistance to wet hydrogen sulfide (hereinafter simply referred to as stress corrosion cracking resistance). There has been a demand for a martensitic stainless steel having improved stress corrosion cracking resistance while maintaining strength, toughness, and corrosion resistance.

As those satisfying the requirements of stress corrosion cracking resistance in addition to strength, toughness and corrosion resistance, Japanese Patent Publication Nos. 61-3391, 58-199850 and 61-2.
No. 07550 is disclosed.

On the other hand, a martensitic stainless steel having improved stress corrosion cracking resistance in an environment where the partial pressure of hydrogen sulfide exceeds 0.01 atm has been proposed.
JP-A-174859 and JP-A-62-54063 are disclosed.

[0005]

However, the steels described in JP-B-61-3391, JP-A-58-199850, and JP-A-61-207550 contain only a trace amount of hydrogen sulfide. Although it exhibits stress corrosion cracking resistance in an environment, stress corrosion cracking occurs in an environment where the partial pressure of hydrogen sulfide exceeds 0.01 atm, and there has been a problem that it cannot be used in an environment containing a large amount of hydrogen sulfide.

Further, Japanese Patent Application Laid-Open No. Sho 60-174859,
The steels described in JP-A-62-54063 cannot completely prevent stress corrosion cracking due to hydrogen sulfide.

Further, from the viewpoint of strength, in any of the above-mentioned martensitic stainless steels, when an attempt is made to increase the strength, the toughness and the stress corrosion cracking resistance are significantly deteriorated. There was also a problem that one had to sacrifice one sex. for that reason,
For example, there is a drawback that it cannot be applied to a deep oil well where high strength, stress corrosion cracking resistance, corrosion resistance and toughness are simultaneously required.

An object of the present invention is to improve the strength, stress corrosion cracking resistance and toughness of a conventional martensitic stainless steel at the same time in order to solve the above-mentioned problems in the prior art, thereby maintaining corrosion resistance. Another object of the present invention is to provide a high-strength low-C high-Cr alloy steel (martensitic stainless steel) that can be used without causing stress corrosion cracking even in an environment containing a large amount of hydrogen sulfide, and a method for producing the same. Here, the target performance was set as follows in view of the performance required for excavating and transporting oil and natural gas containing carbon dioxide and hydrogen sulfide.

(1) Strength: 758 MPa at 0.2% proof stress
(2) Toughness: Absorbed energy value (referred to as Charpy impact value) of Charpy full-size test piece at −20 ° C. is 100 J or more. (3) Corrosion resistance: 5% NaCl solution, 180 ° C., 30 atm CO Under the environment of ₂ , the corrosion rate is 0.5 mm / y or less, (4) stress corrosion cracking resistance: 0.1
The test piece was subjected to a stress of 80% with a 0.2% proof stress in a 5% NaCl solution saturated with a hydrogen sulfide gas at a pressure of 720 atm.
Hold without breaking for more than an hour.

[0010]

In order to solve the above problems and achieve the object, the present invention uses the following means.

(1) The alloy steel of the present invention contains C:
0.005 to 0.05%, Cr: 12 to 16%, S
i: 1% or less, Mn: 0.05 to 0.3%, Ni:
3.5-6%, Mo: 1.5-2.5%, V: 0.
It has high corrosion resistance and high strength, characterized in that it contains 0.1 to 0.05% and N: 0.02% or less, and satisfies the following formula (1) and is composed of the balance of Fe and inevitable impurities. Low C high Cr alloy steel.

705-25 × Ni% + 5 × Cr% + 25 × Mo% ≧ 680 (1) (2) The alloy steel of the present invention further contains Nb: 0.01 to 0.1% by weight as an alloy component. 1%, Ti: 0.01 to
Low C and high C having high corrosion resistance and high strength as described in (1) above, wherein at least one of 0.1% is contained.
r alloy steel.

(3) In the production method of the present invention, after the alloy steel having the composition described in the above (1) or (2) is hot-worked, it is heated to a temperature of Ac ₃ point to 980 ° C. to austenitize. And then tempered at a temperature of 550 ° C. to _one point of Ac, wherein the carbide after tempering precipitates uniformly in the grains and does not preferentially precipitate at grain boundaries, and has high corrosion resistance and high strength. This is a method for producing a low C high Cr alloy steel having:

[0014] The term "carbide precipitates uniformly in grains" as used herein means a state in which carbides precipitate discontinuously at grain boundaries, and fine particles are dispersed and precipitated in grains.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of intensive studies to solve the above-mentioned problems, the present inventors have obtained the following findings.

It is effective to increase Cr to improve the corrosion resistance of martensitic stainless steel. However, an increase in Cr, on the other hand, forms a δ-ferrite phase and degrades strength and toughness. Therefore, the austenite forming element N
Although there is a method of suppressing the formation of the δ-ferrite phase by increasing i, there is a limitation in terms of the tempering temperature in increasing Ni. An increase in C is also effective in suppressing the formation of the δ-ferrite phase, but its content should be rather limited because carbides precipitate during tempering and deteriorate corrosion resistance and stress corrosion cracking resistance.

On the other hand, in general, when the strength of steel is increased, toughness and stress corrosion cracking resistance are deteriorated. However, V is contained in an appropriate amount, and carbide is dispersed as fine precipitates in the stainless steel matrix by heat treatment. Thereby, the strength can be increased without deteriorating these.

However, in order to uniformly disperse and precipitate fine carbides of V, it is particularly necessary to control the tempering conditions.

Based on the above findings, the present inventors considered that the low-C high-Cr alloy steel contains a certain amount of V, and Adjusted within a certain range, carbides are uniformly dispersed and precipitated in the grains, and new toughness, high strength, and excellent stress corrosion cracking resistance that were not realized with conventional martensitic stainless steel. The present inventors have found a martensitic stainless steel (low-C high-Cr alloy steel) and a method for producing the same, and have completed the present invention.

That is, according to the present invention, the corrosion resistance is maintained by simultaneously improving the strength, stress corrosion cracking resistance and toughness of a conventional martensitic stainless steel by limiting the alloy steel composition and production conditions to the following ranges. In addition, it is possible to provide a high-strength, low-C, high-Cr alloy steel that can be used without causing stress corrosion cracking even in an environment containing a large amount of hydrogen sulfide.

The reasons for adding the components of the present invention, the reasons for limiting the components, and the reasons for limiting the manufacturing conditions will be described below.

(1) Component composition range C: 0.005 to 0.05% C is a strong austenite-forming element and an element indispensable for obtaining high strength. However, during tempering, it combines with Cr to form carbides and precipitates,
Deterioration of stress corrosion cracking resistance and toughness. If the C content is less than 0.005%, sufficient strength cannot be obtained,
If it exceeds 5%, the deterioration becomes remarkable, so 0.005 to 0.5%.
The content is 05%.

Cr: 12 to 16% Cr is a basic element constituting martensitic stainless steel, and is an important element exhibiting corrosion resistance. If the content is less than 12%, sufficient corrosion resistance can be obtained. Without
If it exceeds 16%, δ-
The content is 12 to 16% because the amount of ferrite phase formed increases and the strength and toughness deteriorate.

Si: not more than 1% Si is an element necessary as a deoxidizing material, but is also a strong ferrite-forming element.
The content is 1% or less to promote the formation of a ferrite phase.

Mn: 0.05 to 0.3% Mn is an austenite-forming element which is effective as a deoxidizing and desulfurizing agent and suppresses the appearance of a δ-ferrite phase. However, Mn is detrimental to stress corrosion cracking resistance, with an upper limit of 0.3%. If it is less than 0.05%, deoxidation becomes insufficient and inclusions increase, so that the Mn content is 0.05 to 0.3%.

Ni: 3.5 to 6% Ni is an element which is extremely effective in improving corrosion resistance and forming austenite, but its effect is small at less than 3.5%, while transformation increases when its content increases. Point (A
6% to lower the tempering temperature by lowering c ₁ point)
Is the upper limit. Mo: 1.5 to 2.5% Mo is an element particularly effective for stress corrosion cracking resistance and corrosion resistance, but its effect does not appear at a content of less than 1.5%, and it exceeds 2.5%. And an excessive δ-ferrite phase, the upper limit is 2.5%.

V: 0.01-0.05% V is a strong carbide-forming element, and makes fine crystal grains finer by precipitating fine carbides, thereby improving stress corrosion cracking resistance. Further, precipitation of fine carbides also contributes to improvement in strength. However, it is also a ferrite forming element and increases the δ-ferrite phase. If the content is less than 0.01%, the effect of improving stress corrosion cracking resistance does not appear, and if it exceeds 0.05%, the effect is saturated and the δ-ferrite phase increases. 01-0.05%.

N: 0.02% or less N is an element harmful to the improvement of corrosion resistance, but is also an austenite-forming element. If the content exceeds 0.02%, it becomes a nitride during tempering and precipitates, and the corrosion resistance, stress corrosion cracking resistance and toughness deteriorate, so the content is 0.02% or less.

705-25 × Ni% + 5 × Cr% + 25
× Mo% ≧ 680 This is an equation that gives the relationship between the Ac ₁ point and the main additive element (Ni, Cr, Mo). When the Ac ₁ point decreases, it becomes difficult to obtain a sufficient tempered martensite structure,
Stress corrosion cracking resistance deteriorates. Therefore, 705-25
× Ni% + 5 × Cr% + 25 × Mo% ≧ 680.

In the present invention, one or more of the following selected components (Nb, Ti) may be contained in addition to the above basic components.

(Selective components) Nb: 0.01-0.1%, Ti: 0.01-0.1% Nb, Ti is a strong carbide-forming element, and forms crystal grains by precipitating fine carbides. Refines and improves stress corrosion cracking resistance. However, it is also a ferrite forming element and increases the δ-ferrite phase. Content 0.01
%, The effect of improving stress corrosion cracking resistance does not appear,
If it exceeds 1%, the effect is saturated and the δ-ferrite phase increases, so that the content of both Nb and Ti is 0.01%.
~ 0.1%.

Inevitable impurities include P and S, both of which are elements that degrade the hot workability and the stress corrosion cracking resistance of steel, and are preferably as small as possible.
However, if the content of P is 0.04% or less and the content of S is 0.01% or less, the stress corrosion cracking resistance, which is the object of the present invention, can be secured, and the production of a hot-rolled steel sheet or a seamless steel pipe is not hindered. It does not appear.

By adjusting to the above-mentioned component composition range, the strength, stress corrosion cracking resistance and toughness of the conventional martensitic stainless steel are simultaneously improved, and the corrosion resistance is maintained while maintaining the corrosion resistance even in an environment containing a large amount of hydrogen sulfide. It is possible to obtain a high-strength low-C high-Cr alloy steel (martensitic stainless steel) that can be used without causing stress corrosion cracking.

The steel having such characteristics can be manufactured by the following manufacturing method.

(2) Steel Manufacturing Process (Manufacturing Method) Steel adjusted to the above-mentioned composition range is melted in a converter or an electric furnace, and cast into slabs by a normal ingot-making method or a continuous casting method. After producing it into a seamless steel pipe or steel plate by hot working, it is heated to a temperature of Ac ₃ point to 980 ° C., austenitized, cooled, and then 550 ° C.
ＡAc Tempering is performed at a temperature of _one point.

A. Heating temperature: Ac ₃ points to 980 ° C. If the heating temperature is less than Ac ₃ points, the lower limit is Ac ₃ points because austenitizing is not performed and the effect of quenching cannot be obtained. On the other hand, if the heating temperature exceeds 980 ° C., the crystal grains become coarse, not only cannot obtain sufficient strength, but also the toughness is deteriorated, so the upper limit is 980 ° C.

B. Tempering temperature: 550 ° C. to Ac ₁ point As described above, the tempering treatment is to disperse and precipitate fine carbides of V uniformly and to increase the strength without deteriorating toughness and stress corrosion cracking resistance. Required for However, when the temperature exceeds the Ac ₁ point, a part is transformed into an austenite phase and becomes a quenched martensite phase during cooling, and the components and strength between the remaining tempered martensite phase and the non-uniformity occur. The upper limit is Ac ₁ point. When the temperature is lower than 550 ° C, the strength is increased without obtaining a sufficient tempering effect, and the toughness and the stress corrosion cracking resistance are deteriorated. Therefore, the lower limit is 550 ° C.

Hereinafter, the effects of the present invention will be proved by giving examples of the present invention.

[0039]

EXAMPLES The present inventors made the steel No. 1 of the present invention. Samples 1 to 6 and comparative steels a to f were melted as test steels, and subjected to heat treatment after hot rolling to obtain a steel sheet having a thickness of 12 mm, and the tests specifically described below were performed.

Table 1 shows the steel No. of the present invention. The main components, selected components, and other components of 1 to 6 and comparative steels a to f are shown. These steels were austenitized at 950 ° C., air-cooled,
As a result of tempering at 0 ° C. for 30 minutes to test the mechanical properties, corrosion resistance and stress corrosion cracking resistance, Ac ₁ , A
Table 2 shows the c ₃ transformation temperature and the carbide precipitation state.

As shown in Table 2, the steel No. of the present invention. 1-6
In each case, the 0.2% proof stress and the Charpy impact value all exceeded the target values. In addition, the corrosion resistance and stress corrosion cracking resistance also cleared the targets.

On the other hand, in the comparative steels a to f, any one of the components is out of the range of the present invention, so that the test results do not achieve the targets of the corrosion resistance and the stress corrosion cracking resistance.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

According to the present invention, by specifying the alloy steel composition and the production conditions, not only the corrosion resistance against carbon dioxide gas corrosion but also the high strength and low strength corrosion resistant cracking resistance in an environment containing a large amount of hydrogen sulfide can be obtained. It has become possible to provide C high Cr alloy steel.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tatsuo Maeda 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan Inside Nihon Kokan Co., Ltd. (72) Inventor Yusuke Minami 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan Honko Tube Co., Ltd.

Claims (3)

[Claims] C: 0.005 to 0.05% by weight
, Cr: 12 to 16%, Si: 1% or less, Mn:
0.05-0.3%, Ni: 3.5-6%, Mo:
1.5-2.5%, V: 0.01-0.05%,
N: 0.02% or less, and satisfying the following formula (1), the balance being Fe and unavoidable impurities, and characterized by high corrosion resistance and high strength, a low C high Cr alloy steel. 705-25 × Ni% + 5 × Cr% + 25 × Mo% ≧ 680 (1) 2. The alloy steel component further comprises N in weight%.
The low C height having high corrosion resistance and high strength according to claim 1, characterized in that it contains at least one of b: 0.01 to 0.1% and Ti: 0.01 to 0.1%. Cr alloy steel. 3. After hot-working the alloy steel having the composition according to claim 1 or 2, the steel is heated to a temperature of Ac _{3 to} 980 ° C., austenitized, cooled, and then cooled to 550.
Tempering at a temperature of ₁ ° C. to Ac, and low carbide and high Cr having high corrosion resistance and high strength, characterized in that carbide after tempering precipitates uniformly in grains and does not preferentially precipitate at grain boundaries.
Manufacturing method of alloy steel.