JP2007186796A - High chromium steel with high toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide high chromium steel having adequate strength and sufficient toughness, further suitable for use under a high CO<SB>2</SB>environment of ≥2 MPa, and excellent in economical efficiency. <P>SOLUTION: The high chromium steel with high toughness has a composition containing, by weight, 0.035 to 0.05% C, ≤0.5% Si, ≤2.0% Mn, ≤0.05% P, ≤0.005% S, 10 to 12.5% Cr, 1.5 to 3.0% Ni, ≤0.02% N and 0.01 to 0.1% Al, further containing one or more elements among ≤0.5% Mo, ≤0.02% Nb, ≤0.03% Ti, ≤0.005% B, ≤0.5% Cu, ≤0.1% V and ≤0.005% Ca, and having the balance iron with inevitable impurities. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to an inexpensive high chromium steel which is an oil well material used for oil wells or gas wells, is suitable for use in a high CO ₂ environment, has high yield strength of 80 ksi (552 to 655 MPa) grade, and has high toughness. .

In recent years, the development of oil and gas wells in various environments, such as deep wells, high-temperature and high-pressure gas fields, and cold regions, has been progressing. Accordingly, corrosion by high CO ₂ partial pressure, since the corrosion cracking (SSC) is also a big problem due to the H _{2 S} in the case of oil wells that contain more H _{2 S,} withstand these severe corrosive environment In addition, the demand for steel pipes having both strength and high toughness of 80 ksi (552 MPa) or more necessary for oil well pipes for deep layers has increased.

  Conventionally, AISI (American Iron and Steel Institute) steel 410 or 420 has been used as oil well materials, which are relatively inexpensive and yield strength of 80 ksi or higher by heat treatment, but have sufficient corrosion resistance. It cannot be said that it shows. In addition, water cooling cannot be performed in the manufacturing process, which hinders manufacturing efficiency. To date, several proposals have been made for martensitic stainless steel having the above-described strength, high toughness, and high corrosion resistance, and a method for producing the same.

For example, Patent Document 1 proposes a steel containing C: 0.005 to 0.04%, Cr: 12 to 17%, and Ni: 1.5 to 6.0%, and a manufacturing method thereof. The strength range is 80 to 110 kgf / mm ² (784 to 1078 MPa), which is higher than the general-purpose 80 ksi (552 to 655 MPa) grade, and in such a high strength, there is a problem with the corrosion resistance in a high CO ₂ environment. is there. Patent Document 2 proposes a steel containing C: 0.15% or less, Cr: 9.0 to 16.0%, Ni: 0.2 to 2.5%, and a manufacturing method thereof. Since rolling is required, there is a problem in the efficiency of the manufacturing process, and there are restrictions on manufacturing equipment. Patent Document 3 proposes a steel containing C: 0.03% or less, Cr: 11-17%, Ni: 3.5-7.0%, and a manufacturing method thereof, but 3.5% or more. Therefore, there is a problem in economic efficiency because of the need for addition of Ni. Thus, in the prior art, an oil well pipe suitable for use in a high CO ₂ environment of ₂ MPa or more, having a yield strength of 80 ksi (552 to 655 MPa) grade, high toughness, and excellent in economic efficiency is obtained. Not.
Japanese Patent No. 2665209 Japanese Patent No. 2091532 Japanese Patent No. 2995524

As a result of various studies to solve the problems of these prior arts, the present inventors have suitable strength and sufficient toughness, and further suitable for use in a high CO ₂ environment of ₂ MPa or more, We have found high-chromium steel with excellent economic efficiency. Accordingly, an object of the present invention is to provide a high chromium steel having high toughness with a yield strength of 80 ksi (552 to 655 MPa) grade and a method for producing the same based on such knowledge.

  The present inventors have restricted the amount of C within an economical range to suppress carbide precipitation, thereby improving the corrosion resistance and enabling water cooling during production. As the amount of C is limited, it is found that even if the amount of Cr is lower than the added amount of 13% of the conventional material such as 420 steel, it has the same corrosion resistance. Reduced costs. After hot forming the melted ingot, the temperature range of quenching and tempering after heating was changed, and an appropriate strength-toughness correlation was examined. The present invention is configured based on such knowledge, and the gist thereof is as follows.

  The high chromium steel having high toughness according to the first invention is, by weight, C: 0.035 to 0.05%, Si: 0.5% or less, Mn: 2.0% or less, P: 0.00. 05% or less, S: 0.005% or less, Cr: 10 to 12.5%, Ni: 1.5 to 3.0%, N: 0.02% or less, Al: 0.01 to 0.1% , With the balance being iron and inevitable impurities.

  The high chromium steel having high toughness according to the second invention is the same as the high chromium steel according to the first invention, in terms of weight percent, Mo: 0.5% or less, Nb: 0.02% or less, Ti: 0.00. It further contains at least one element selected from the group consisting of 03% or less, B: 0.005% or less, Cu: 0.5% or less, V: 0.1% or less, and Ca: 0.005% or less.

  The high chromium steel having high toughness according to the third invention is the same as the high chromium steel according to the first invention except that Mo: 0.1 to 0.5% and Ti: 0.08% or less. It contains.

  The high chromium steel having high toughness according to the fourth aspect of the invention is the same as the high chromium steel according to the third aspect of the invention, in terms of% by weight, Nb: 0.02% or less, B: 0.005% or less, Cu: 0.00. It further contains at least one element consisting of 5% or less, V: 0.1% or less, and Ca: 0.005% or less.

In the development process of the present invention, the chemical composition and heat treatment conditions were changed, and the strength, toughness, corrosion resistance under high CO ₂ environment, SSC resistance under H ₂ S environment, and hot workability were investigated. As a result, it has been found that a high chromium steel having a yield strength of 552 MPa or more suitable for use under high CO ₂ conditions is obtained by controlling the chemical composition and heat treatment conditions of the present invention. .

  Hereinafter, the reason why the chemical component is limited to the above range in the present invention will be described.

C: 0.035 to 0.05%
C increases the strength by solid solution strengthening and precipitation strengthening. If the content is less than 0.035%, the effect of increasing the strength is small, and if it exceeds 0.05%, the toughness and the corrosion resistance deteriorate. Therefore, from the viewpoint of high chromium steel excellent in economy, the C content is set to 0.035 to 0.05%.

Si: 0.5% or less Si has a deoxidizing effect. If the content exceeds 0.5%, the toughness deteriorates. Therefore, the Si content is limited to 0.5% or less.

Mn: 2.0% or less Mn has a deoxidizing effect in the same manner as Si. If the content exceeds 2.0%, the toughness deteriorates. Therefore, the amount of Mn is limited to 2.0% or less.

P: 0.05% or less P is an impurity element, which causes toughness deterioration, and is preferably as low as possible. If the content exceeds 0.05%, the toughness is remarkably deteriorated, so the P content is limited to 0.05% or less.

S: 0.005% or less S is an impurity element like P, and causes deterioration of toughness and lowers hot workability. Therefore, S is preferably as low as possible. Therefore, the amount of S is limited to 0.005% or less.

Cr: 10 to 12.5%
Cr has the effect of improving the corrosion resistance. If the addition amount is less than 10%, sufficient corrosion resistance cannot be obtained. The effect is saturated even if added over 12.5%. Therefore, the Cr content is limited to a range of 10 to 12.5%.

Ni: 1.5-3.0%
Ni has an effect of improving corrosion resistance and toughness. If it is less than 1.5%, δ ferrite tends to be generated, and hot workability is lowered. The effect is saturated even if added over 3.0%. Therefore, the amount of Ni is limited to a range of 1.5 to 3.0%.

N: 0.02% or less N increases strength by solid solution strengthening and precipitation strengthening, but combines with V, Nb, Ti, etc. to form coarse precipitates, deteriorates toughness, and hot workability Reduce. Therefore, the N content is limited to 0.02% or less.

Al: 0.01 to 0.1%
Al has a deoxidizing effect. If the addition amount is less than 0.01%, a sufficient effect cannot be obtained. If it exceeds 0.1%, AlN is formed by nitriding, and the grain boundary strength is lowered to deteriorate toughness. Therefore, the addition amount of Al is limited to 0.0 l to 0.1%.
The balance is iron and inevitable impurities.

  In the present invention, the following chemical component is further added to the chemical component of the first invention to improve the strength, toughness, corrosion resistance, SSC resistance and hot workability. Hereinafter, the reasons for limiting the chemical components will be described.

Cu: 0.5% or less Cu has an effect of improving corrosion resistance. If it exceeds 0.5%, hot workability deteriorates. Therefore, the amount of Cu is limited to 0.5% or less.

V: 0.1% or less V has the effect of forming a nitride with N and increasing the strength. Even if added over 0.1%, the effect is saturated, and the toughness deteriorates due to coarsening of precipitates. Therefore, the V amount is limited to 0.1% or less.

Ca: 0.005% or less Ca has an effect of controlling the form of sulfide and improving toughness and corrosion resistance. When added over 0.005%, the Ca inclusions increase, leading to deterioration of toughness and corrosion resistance. Therefore, the Ca content is limited to 0.005% or less.

Mo: 0.5% or less Mo has an effect of improving SSC resistance. If it exceeds 0.5%, hot workability is lowered. Therefore, the addition amount of Mo is limited to 0.5% or less.

Nb: 0.02% or less Nb increases strength by fine Nb carbides precipitated during tempering, and refines austenite grains and improves toughness by Nb carbides precipitated during quenching. If added over 0.02%, the strength becomes high and cannot be controlled within an appropriate strength range. Therefore, the amount of Nb added is limited to 0.02% or less.

Ti: 0.03% or less, 0.08% or less (when Mo is added)
Ti precipitates as carbides and nitrides, thereby reducing the solid solution amount of C and N in the steel and lowering the strength. Even if added over 0.03%, the effect is saturated, and coarse precipitates and inclusions are formed to lower the SSC resistance. Therefore, the amount of Ti added is 0.03% or less.
However, when added simultaneously with 0.1 to 0.5% Mo, TiMoC having a slow growth rate is formed, and coarsening of the grains is suppressed. From the relationship with the amount of addition of Mo, addition up to 0.08% is effective, but even if added over 0.08%, the effect is saturated and SSC resistance is reduced. Therefore, the addition amount of Ti is set to 0.08% or less.

B: 0.005% or less B has an effect of strengthening grain boundaries. When the content exceeds 0.005%, a compound having a low melting point tends to be generated at the grain boundary, and the hot workability is deteriorated. Therefore, the addition amount of B is 0.005% or less.

  Next, manufacturing conditions in the present invention will be described.

Quenching heating temperature: 780-960 ° C
When the heating temperature at the time of quenching is less than 780 ° C., a complete austenite single phase structure is not formed at the time of heating, and therefore a martensite single phase structure cannot be obtained even after cooling, resulting in an unstable structure. On the other hand, when heated above 960 ° C., austenite grains become coarse and toughness deteriorates. Accordingly, the quenching heating temperature is in the range of 780 to 960 ° C.

Tempering temperature: 600-750 ° C
The steel that is the subject of the present invention has high strength and is not sufficiently tough when it is quenched, so that proper tempering is required. If the tempering temperature is less than 600 ° C., the desired strength cannot be obtained. Moreover, when it exceeds 750 degreeC, toughness will fall. Accordingly, the tempering temperature is in the range of 600 to 750 ° C.

As described above, according to the present invention, a high chromium steel having a high toughness with a yield strength of 80 ksi (552 to 655 MPa) grade can be obtained. And the high chromium steel is excellent in corrosion resistance under high CO ₂ environment, SSC resistance under H ₂ S environment, and hot workability by performing an appropriate heat treatment, and is manufactured at low cost. be able to. Therefore, a seamless steel pipe for a line pipe suitable for use in a high CO ₂ environment can be provided at a low cost.

  For the melting of the steel of the present invention, any method may be used as long as it is a converter, an electric furnace, or any other manufacturing method that can control chemical components within the scope of the invention. Since the molten steel is mainly used as an oil well steel pipe, it is cast or rolled into a shape such as a billet. Thereafter, a seamless steel pipe is formed through a process such as piercing by an extrusion type piercing machine or an inclined roll type piercing and rolling machine, a rolling process, and the like, and a predetermined heat treatment is performed.

  Tables 1 and 2 show the steels of the present invention (Nos. 1 to 29), and Tables 3 to 4 show the chemical components, heat treatment temperatures, and characteristics of the comparative steels (Nos. 30 to 55).

Each steel was vacuum-melted in an experimental furnace, and the resulting steel ingot was made into a steel plate (plate thickness 12 mm) by hot working. These were subjected to heat treatment, and the strength, toughness, corrosion resistance, SSC resistance and hot workability were examined. For strength, a JIS 14B round bar test piece (6 mm) was sampled from the center of the plate thickness, a tensile test was performed, and the yield strength was evaluated. Toughness was evaluated by cutting out a full-size V-notch shear-peel specimen from the center of the plate thickness and performing an impact test at a test temperature of −40 ° C. Corrosion resistance was evaluated by conducting a corrosion test at 100 ° C. × 336 hr in artificial seawater saturated with carbon dioxide (10% NaCl + CO ₂ solution, pH = 4.0, PCO ₂ = 30 atm). The SSC resistance was applied for 30 days by applying 90% of the yield stress of each steel as a load in a 10% NaCl and 0.5% CH ₃ COOH solution (pH = 4.0, PH ₂ S = 0.035 atm). The presence or absence of breakage after standing was examined. The hot workability was evaluated by collecting a torsion test piece from an as-cast ingot and performing a hot torsion test at 1200 ° C.
As target values, the strength is 80 ksi (552 to 655 MPa) yield strength, the toughness is absorption energy (vE-40) at −40 ° C. of 100 J or more, the corrosion resistance is corrosion rate of 0.2 mm / year or less, and the SSC resistance is There was no break during the 30-day test period, and the hot workability was good when the number of rotations until the break in the hot torsion test was 10 or more.

In Tables 1 to 4, the present invention steel No. 1 satisfying the scope of the invention for both chemical components and production conditions. It was confirmed that 1-29 showed sufficient strength, toughness, corrosion resistance, SSC resistance, and hot workability.
FIG. 1 shows the steel No. 1 of the present invention when tempered from a predetermined quenching temperature. 1 and No. 6 shows the relationship between the yield strength of 6 and vE-40. From this figure, it can be seen that the steel of the present invention has an extremely high toughness with a yield strength of 80 ksi (552 to 655 MPa) grade.
On the other hand, although the manufacturing conditions are within the scope of the invention, the comparative steel No. 1 whose chemical composition is outside the scope of the invention. In 30 to 51, any of strength, toughness, corrosion resistance, SSC resistance, and hot workability does not reach the target value. In addition, although the chemical composition is within the scope of the invention, the comparative steel no. As for 52-55, either intensity | strength or toughness has not reached | attained the target value.

It is a figure which shows the correlation of the intensity | strength and toughness of the high chromium steel of this invention.

Claims (4)

  C: 0.035 to 0.05%, Si: 0.5% or less, Mn: 2.0% or less, P: 0.05% or less, S: 0.005% or less, Cr: 10% by weight -12.5%, Ni: 1.5-3.0%, N: 0.02% or less, Al: 0.01-0.1%, the balance is made of iron and inevitable impurities High chromium steel with high toughness characterized by   Mo: 0.5% or less, Nb: 0.02% or less, Ti: 0.03% or less, B: 0.005% or less, Cu: 0.5% or less, V: 0.1% The high chromium steel having high toughness according to claim 1, further comprising at least one element composed of Ca: 0.005% or less.   The high chromium steel having high toughness according to claim 1, further comprising, by weight%, Mo: 0.1 to 0.5% and Ti: 0.08% or less.   1% or more of elements consisting of Nb: 0.02% or less, B: 0.005% or less, Cu: 0.5% or less, V: 0.1% or less, Ca: 0.005% or less by weight% The high chromium steel having high toughness according to claim 3, further comprising:

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