JP4010017B2 - Method for producing martensitic stainless steel pipe with excellent SSC resistance - Google Patents

Description

【００２１】
【表２】

【００２２】
【表３】

【００２３】
【発明の効果】
以上のように、本発明によれば、耐ＳＳＣ性に優れたマルテンサイト系ステンレス鋼油井管を得ることが可能となる。
【図面の簡単な説明】
【図１】昇温速度とＡｃ₁ 変態点の関係を示す。
【図２】焼戻の昇温速度と耐ＳＳＣ性の関係を示す。なお、プロット横数値は表３におけるNo．を意味する。
【図３】本発明の対象となる焼戻前の熱履歴の概要を示す。
【符号の説明】
Ｒ ：圧延まま
Ｒ−Ｎ ：圧延まま材をＡｃ₃ 点以上に再加熱して常温まで冷却
Ｒ−Ｌ ：圧延まま材をＡｃ₁ 〜Ａｃ₃ 点の温度域に再加熱して常温まで冷却
Ｒ−Ｎ−Ｌ：Ｒ−Ｎ材をＡｃ₁ 〜Ａｃ₃ 点の温度域に再加熱して常温まで冷却[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a martensitic stainless steel pipe excellent in SSC resistance.
[0002]
[Prior art]
Martensitic stainless steel has been applied as an oil well pipe since about 1980 because it is excellent in strength and CO ₂ corrosion resistance, as represented by AISI 420 steel. Recently, the demand for CO ₂ corrosion resistance superior to that of 420 steel and further the market demand for H ₂ S resistance has increased, and as shown in JP-A-3-120337, low C and Ni—Mo added steel, or As described in JP-A-2-247360, steel types such as low C and Ni—Cu—Mo added steels (so-called modified 13Cr steel) have been developed. Such steel is, as seen in Japanese Patent Application Laid-Open No. 3-120337, etc., (1) A quenching treatment (or quenching) in which after hot working, it is heated and held again at a temperature of Ac ₃ point or higher and then cooled to Ms point or lower. Or (2) Ac ₁ point to Ac ₃ point temperature range, heat and hold again, then cool to Ms point or less, or (3) Ac ₁ point temperature or less. Then, it is heated and held again and then subjected to a tempering process for cooling to the Ms point or lower, or a heat treatment is performed by appropriately combining (1), (2), and (3).
[0003]
[Problems to be solved by the invention]
However, such a heat treatment method does not give sufficient conditions for maximizing the functions inherent in steel, and as a result, even though it is excellent as steel, it has not always been satisfactory as an oil well pipe. . For example, in the method (1), since it is a hardened martensite structure, the yield strength (YS) is abnormally low and the tensile strength (TS) is abnormally high, which is not practical for forming a structure. In addition, in the above (2) or (1) + (2), since it becomes a mixed structure of quenched martensite and tempered martensite, YS is greatly influenced by slight fluctuations in heat treatment conditions and is stably within a predetermined range. It is difficult to build in YS. If the method (3) is used, the steel becomes tempered martensite, so that the balance between the yield strength and the tensile strength is moderate, and the necessary YS can be stably obtained. From this, the heat treatment of (1) + (3), (2) + (3), (1) + (2) + (3) is practical. However, if the conditions are not sufficiently defined even after the heat treatment with these tempering treatments (3) as essential, characteristics that should be satisfied as corrosion-resistant oil well pipes, that is, sufficient sulfide stress cracking (SSC) characteristics. There is a problem that cannot be obtained.
[0004]
This invention solves this problem, and provides the manufacturing method of the martensitic stainless steel oil well pipe which ensures the SSC resistance required as a corrosion-resistant oil well pipe by regulating the conditions in a tempering process. With the goal.
[0005]
[Means for Solving the Problems]
The present inventors applied Mannesmann seamless steel pipe rolling to various component materials, allowed to cool to room temperature, and then tempered by changing the temperature and temperature rise rate for materials having various previous histories. The treatment was carried out and the properties of the resulting material (YS / TS balance and SSC resistance) were evaluated. As a result, it was found that the SSC resistance depends on the YS / TS ratio (YR), and the higher the YR, the better the SSC resistance even in the same YS. Furthermore, the conditions (tempering temperature, temperature increase rate) of the tempering process for obtaining high YR were clarified.
[0006]
The present invention is configured based on this finding, the gist of which is (1) wt%,
C: ≦ 0.050, Si: ≦ 0.5,
Mn: ≦ 1.5, P: ≦ 0.03,
S: ≦ 0.005, Cr: 11.0-14.0,
Ni: 4.0-7.0, Mo: 1.0-2.5,
Cu: 1.0 to 2.5, Al: ≦ 0.05,
N: 0.01-0.10
The martensitic stainless steel having a composition consisting of Fe and unavoidable impurities is cooled to a temperature below the Ms point after hot working, and then heated to a temperature T of 550 ° C. or higher and Ac ₁ or lower, 500 to T It is a method for producing a martensitic stainless steel seamless pipe characterized by performing a tempering treatment by raising the temperature so that the average heating rate at ℃ is 1.0 ° C / sec or more,
(2) By weight%
C: ≦ 0.050, Si: ≦ 0.5,
Mn: ≦ 0.5, P: ≦ 0.03,
S: ≦ 0.005, Cr: 11.0-14.0,
Ni: 4.0-7.0, Mo: 1.0-2.5,
Cu: 1.0 to 2.5, Al: ≦ 0.05,
N: 0.01-0.10, Ca: 0.001-0.020,
The martensitic stainless steel having a composition consisting of Fe and unavoidable impurities is cooled to a temperature below the Ms point after hot working, and then heated to a temperature T of 550 ° C. or higher and Ac ₁ or lower, 500 to T It is a method for producing a martensitic stainless steel seamless pipe characterized by performing a tempering treatment by raising the temperature so that the average heating rate at ℃ is 1.0 ° C / sec or more,
(3) After hot working, it is allowed to cool to a temperature of Ms point or lower, and then reheated to a temperature of Ac ₃ point or higher and subjected to quenching treatment or normalizing treatment, and further a temperature T of 550 ° C. or higher and Ac ₁ lower than The martensite as described in (1) or (2) above, wherein the tempering treatment is performed by raising the temperature so that the average heating rate of 500 to T ° C is 1.0 ° C / sec or more. a method for producing a system stainless seamless steel pipe, (4) the the ratio of the yield strength and tensile strength of the heat treated alms steel tube material, characterized in that at 0.77 or more (1) to (3) section A method for producing a martensitic stainless steel seamless pipe according to any one of the above.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
The reasons for limiting the components of the martensitic stainless steel in the present invention are as follows.
C: Since C is an element that forms Cr carbide and the like and deteriorates corrosion resistance, the lowest possible level is preferable. The maximum value is set to 0.05% in consideration of the current level of industrial refining technology.
Si: Si is added and remains as a deoxidizer in the steelmaking process, and is not an element to be deliberately added. If it exceeds 0.5%, the toughness and SSC resistance deteriorate, so the upper limit is 0. 0.5%.
[0008]
Mn: Mn is an austenite stabilizing element and effective in preventing rolling wrinkles by suppressing the precipitation of δ phase during hot working. However, if added over 1.5%, the grain boundary strength is lowered and the SSC resistance is reduced. Deteriorates, so 1.5% was made the upper limit.
P: The upper limit was made 0.03% in order to segregate at the grain boundary to lower the grain boundary strength and deteriorate the SSC resistance.
[0009]
S: In order to deteriorate hot workability and reduce CO ₂ corrosion resistance and SSC resistance, the upper limit was made 0.005%.
Cr: A basic element for improving corrosion resistance. In order to obtain sufficient CO ₂ corrosion resistance, addition of 11% or more is necessary, but it is also a ferrite stabilizing element, and if it is too much, a δ phase precipitates during hot working. As a result, the hot workability deteriorates and the product does not become a martensite single phase, and the SSC resistance is deteriorated. Therefore, the upper limit is set to 14%.
[0010]
Ni: Effective for improving CO ₂ corrosion resistance and toughness. Moreover, it is an austenite stabilizing element and suppresses the formation of a δ phase that leads to rolling wrinkles. These effects are insufficient if the addition amount is less than 4.0%, and the effect is saturated even if the addition amount exceeds 7.0%. Therefore, the optimum addition range is set to 4.0 to 7.0%.
Al: It is added for the purpose of deoxidation, but a sufficient effect can be obtained with addition of 0.05% or less.
[0011]
N: Since it is a strong austenite stabilizing element, it is added as an alternative element for expensive Ni. However, if added too much, the hardness in the martensite state increases and delayed fracture susceptibility increases, so the optimum addition amount was made 0.01 to 0.10%.
Similar to Mo: Cr, it is an element that improves corrosion resistance and is a ferrite-forming element. From the viewpoint of CO ₂ corrosion resistance, SSC resistance, and hot workability, the optimum addition range is set to 1.0 to 2.5%.
[0012]
Cu: Similar to Ni, it is an element effective for improving CO ₂ corrosion resistance and an austenite stabilizing element and effective for preventing rolling flaws. However, if it is less than 1.0%, these effects cannot be obtained sufficiently. When adding over 2.5%, the hot workability is remarkably deteriorated, so the optimum range was made 1.0 to 2.5%.
It suppresses hot workability deterioration due to Ca: S, and is added as necessary. However, if it is less than 0.001%, the effect is not manifested, and the effect is saturated even if added over 0.02%. Therefore, the optimum addition amount is set to 0.001 to 0.02%.
[0013]
Next, a heat treatment method for steel having such a composition will be described.
The steel having such a composition was reduced in C mainly for improving the corrosion resistance for the purpose of use, and added with Ni, Cu or the like in order to prevent the precipitation of δ ferrite harmful to the SSC resistance caused thereby. However, the addition of such a γ-forming element greatly reduces the Ac ₁ transformation point. Therefore, the steel having such a composition is inevitably subjected to low-temperature tempering. However, if low-temperature tempering is performed, the strength increases, and as a result, sufficient SSC resistance cannot be obtained.
[0014]
However, as a result of intensive studies by the present inventors, it has been clarified that the Ac ₁ transformation point of such a composition steel depends on the heating rate. That is, as shown in FIG. 1, the Ac ₁ transformation point becomes higher as the heating rate is higher. The reason for this is considered to be that the α → γ transformation is delayed due to the progress of precipitation, aggregation, and solid solution being delayed as the temperature rise rate of the precipitate increases. The heating rate here is an average heating rate in the temperature range of tempering temperature T from 550 ° C. to Ac ₁ point or less, which is related to the state change of the precipitate.
[0015]
The above behavior is the same even if the previous history is different, and is the same as rolled (cooled to room temperature (sign: R)), then reheated to Ac ₃ point or higher and then cooled to room temperature (sign: RN) ), Reheated to Ac ₁ point or more and Ac ₃ point or less following the process of symbol R (cooled to room temperature) (symbol: RL), ac ₁ point or more and Ac ₃ points or less following the process of symbol RN Similar results were obtained with four types of specimens (FIG. 3) that were reheated to room temperature and then cooled to room temperature (sign: R-N-L). In a normal furnace heating method, only a heating rate of about 0.3 ° C./sec can be obtained, so the Ac ₁ point is about 630 ° C., but at a high temperature heating of 1.0 ° C./sec or more, the Ac ₁ point is 650. ℃ or more. For this reason, if the temperature is raised at a high speed, the Ac ₁ transformation point is increased, and as a result, high-temperature tempering is possible, and it is possible to prevent an increase in strength harmful to SSC resistance. Examples of the heat treatment method that enables such rapid temperature increase include an induction heating method and a salt bath heating method. The tempering holding time is preferably within 1 minute because the Ac ₁ transformation point decreases when held for a long time.
[0016]
Next, the SSC characteristics of the steel material obtained by the above heat treatment method will be described. An SSC test was performed using the steel material obtained by the above heat treatment method, and the results are organized in relation to the rate of temperature increase and are shown in FIG. In the SSC test, H ₂ S was saturated at a partial pressure of 0.3 atm in a state in which a bending stress that is 100% of YS was applied to a strip-shaped test piece of t3.0 × 10 × 65 mm size by a four-point support method. The presence or absence of cracks was evaluated by dipping in a 5% acetic acid solution at room temperature. As is clear from FIG. 2, regardless of the history before tempering (signs: R, RN, RL, RNNL, see FIG. 3), the temperature rise rate is excellent at 1 ° C./sec or more. SSC resistance can be obtained. The reason for this is that YR increases by tempering at a high temperature increase rate, and therefore TS increases as the material has a high temperature increase rate even in the same YS.
[0017]
Next, the reason why it is desirable to define YS / TS ratio (YR) ≧ 0.77 as the mechanical characteristics to be satisfied by the steel material that has been subjected to the tempering process by high-speed temperature rise will be described.
An SSC test was conducted using a material that had been tempered to 69.1-75.1 kg / mm ² (corresponding to 95 ksi class as an oil well pipe) by the above heat treatment method, and the results were organized in relation to YR. did. In the SSC test, H ₂ S was saturated at a partial pressure of 0.3 atm in a state in which a bending stress that is 100% of YS was applied to a strip-shaped test piece of t3.0 × 10 × 65 mm size by a four-point support method. The presence or absence of cracks was evaluated by dipping in a 5% acetic acid solution at room temperature. As a result, regardless of the history before tempering (signs; R, RN, R-L, R-N-L, see FIG. 3), if YR is less than 0.77, the SSC resistance is slightly reduced. . Therefore, if the condition of YR ≧ 0.77 is added to the steel material obtained by the above heat treatment method, more excellent SSC resistance can be obtained.
[0018]
【Example】
Examples of the present invention will be described below.
The test was performed using the steel shown in Table 1. Press roll piercing-elonator rolling method seamless steel pipe rolling using a rectangular cross-section bloom as a raw material, and giving a history before tempering treatment under the conditions shown in Table 2 using a steel pipe cooled to room temperature as shown in Table 3. Tempering treatment was performed under conditions. The tempering holding time was within 1 minute. A test piece was collected from the specimen and subjected to a tensile test and an SSC test. SSC characteristics are obtained by saturating H ₂ S at a partial pressure of 0.3 atm with a strip test piece of t3.0 × 10 × 65 mm size subjected to a bending stress of 100% of yield strength by the four-point support method. It was evaluated by immersing it in a 0.5% acetic acid solution at room temperature and determining the presence or absence of cracks.
[0019]
The results of both tests are shown together in Table 3. In Table 3, the present invention No. 1-4 and 8-11 show a high YR and show excellent SSC resistance. On the other hand, as for the comparative example, No. Nos. 22 and 23 have insufficient SSC resistance because the chemical components of the specimens are outside the scope of the present invention. 15, 17, 18, 19, 20, 21, and 22 cannot obtain sufficient SSC resistance because the rate of temperature rise is outside the range of the present invention. No. No. 14 has an insufficient SSC resistance because the tempering temperature exceeds Ac1. Nos. 16 and 23 fall outside the scope of the present invention in terms of both the rate of temperature rise and the tempering temperature, and the SSC resistance deteriorates.
[0020]
[Table 1]

[0021]
[Table 2]

[0022]
[Table 3]

[0023]
【The invention's effect】
As described above, according to the present invention, it is possible to obtain a martensitic stainless steel oil well pipe having excellent SSC resistance.
[Brief description of the drawings]
FIG. 1 shows the relationship between the rate of temperature rise and the Ac ₁ transformation point.
FIG. 2 shows the relationship between the tempering rate and the SSC resistance. Note that the horizontal values of the plots are No. in Table 3. Means.
FIG. 3 shows an outline of a heat history before tempering which is an object of the present invention.
[Explanation of symbols]
R: As-rolled RN: Re-heated as-rolled material to Ac ₃ point or higher and cooled to room temperature RL: Re-rolled as-rolled material to temperature range of Ac ₁ to Ac ₃ point and cooled to room temperature R -N-L: RN material is reheated to a temperature range of Ac ₁ to Ac ₃ points and cooled to room temperature.

Claims (4)

% By weight
C: ≦ 0.050
Si: ≦ 0.5,
Mn: ≦ 1.5,
P: ≦ 0.03
S: ≦ 0.005,
Cr: 11.0-14.0,
Ni: 4.0-7.0,
Mo: 1.0 to 2.5,
Cu: 1.0 to 2.5,
Al: ≦ 0.05,
N: 0.01-0.10
The martensitic stainless steel having a composition consisting of Fe and unavoidable impurities is cooled to a temperature below the Ms point after hot working, and then heated to a temperature T of 550 ° C. or higher and Ac ₁ or lower, 500 to T A method for producing a martensitic stainless steel seamless pipe, wherein the temperature is raised so that the average heating rate at 1 ° C. is 1.0 ° C./sec or more and tempering is performed. % By weight
C: ≦ 0.050
Si: ≦ 0.5,
Mn: ≦ 1.5,
P: ≦ 0.03
S: ≦ 0.005,
Cr: 11.0-14.0,
Ni: 4.0-7.0,
Mo: 1.0 to 2.5,
Cu: 1.0 to 2.5,
Al: ≦ 0.05,
N: 0.01-0.10
Ca: 0.001 to 0.020,
The martensitic stainless steel having a composition consisting of Fe and unavoidable impurities is cooled to a temperature below the Ms point after hot working, and then heated to a temperature T of 550 ° C. or higher and Ac ₁ or lower, 500 to T A method for producing a martensitic stainless steel seamless pipe, wherein the temperature is raised so that the average heating rate at 1 ° C. is 1.0 ° C./sec or more and tempering is performed. After hot working, it is allowed to cool to a temperature of Ms point or lower, then reheated to a temperature of Ac ₃ point or higher and subjected to quenching or normalizing treatment, and further to a temperature T of 550 ° C. or higher and Ac ₁ lower than 500. The martensitic stainless seamless steel pipe according to claim 1 or 2, wherein the tempering treatment is performed by raising the temperature so that the average heating rate at ~ T ° C is 1.0 ° C / sec or more. Manufacturing method. The method for producing a martensitic stainless steel seamless pipe according to any one of claims 1 to 3 , wherein the ratio of the yield strength and the tensile strength of the heat-treated steel pipe material is 0.77 or more.

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