JPH09170019A - Production of seamless martensitic stailess steel pipe excellent in ssc resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a martensitic stainless steel oil well pipe in which SSC resistance required of a corrosion resisting oil well pipe is secured by controlling the conditions in tempering treatment. SOLUTION: A martensitic stainless steel, which has a composition consisting of, by weight, <=0.050% C, <=0.5% Si, <=1.5% Mn, <=0.03% P, <=0.005% S, 11.0-14.0% Cr, 4.0-7.0% Ni, 1.0-2.5% Mo, 1.0-2.5% Cu, <=0.05% Al, 0.01-0.10% N, and the balance Fe with inevitable impurities and further containing, if necessary, 0.001-0.020% Ca, is hot-worked and then cooled down to a temp. not higher than the Ms point. Subsequently, heat treatment, consisting of temp. raise up to a temp. T in the range between 550 deg.C and the Ac1 point at >=1.0 deg.C/sec average heating rate between 50 and T deg.C and cooling down to a temp. no higher than the Ms point, is carried out.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a martensitic stainless seamless steel pipe having excellent SSC resistance.

[0002]

2. Description of the Related Art Martensitic stainless steel has been developed using AI.
As represented by SI 420 steel, it has been used as an oil country tubular good since around 1980 due to its excellent strength and CO ₂ corrosion resistance. Recently, it has better C resistance than 420 steel.
Market demand for O ₂ corrosion characteristics, and further H ₂ S resistance has increased, and low C and Ni—Mo added steels such as those found in JP-A-3-120337 or JP-A-2-247360. Low C as seen in
And steel types such as Ni-Cu-Mo added steel (so-called
A steel type called Modified 13Cr steel) has been developed. Such steel is, as seen in Japanese Patent Laid-Open No. 3-120337, a quenching treatment (or normalizing treatment) 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 Ac ₁ point to Ac
A two-phase region treatment is performed in which the material is heated and held again at a temperature of ₃ points and then cooled to the Ms point or lower, or a tempering treatment is performed where it is heated and held again to a temperature of Ac ₁ point or lower and then cooled to the Ms point or lower. It is manufactured by performing heat treatment by appropriately combining or.

[0003]

However, such a heat treatment method does not give sufficient conditions for maximizing the functions of the steel itself, and as a result, even if it is excellent as a steel, it is not always suitable for an oil country tubular good. It was not satisfactory. For example, the above method is a material having an extremely low yield strength (YS) and an abnormally high tensile strength (TS) because it has a quenched martensite structure, and is not practical at all for forming a structure. Further, in the above or +, since a mixed structure of quenched martensite and tempered martensite is obtained, YS is greatly affected by slight changes in heat treatment conditions, and it is difficult to stably build YS within a predetermined range. . When the above method is used, the steel becomes tempered martensite, so that the yield strength and tensile strength are well balanced, and necessary YS can be stably obtained. From this fact, the heat treatment of +, +, ++ is practical. However, even if these heat treatments that require tempering treatment are performed, if the conditions are not sufficiently specified, sufficient properties as a corrosion resistant oil country tubular good, that is, sufficient sulfide stress cracking (SSC) properties can be obtained. There is a problem that there is no.

The present invention solves the above problem and provides a method for producing a martensitic stainless steel oil country tubular goods which secures the SSC resistance required as a corrosion resistant oil country tubular goods by regulating the conditions in tempering treatment. The purpose is to provide.

[0005]

[Means for Solving the Problems] The inventors of the present invention have performed a Mannesmann system seamless steel pipe rolling on materials of various components and allowed to cool to room temperature. The characteristics of the obtained material (YS / TS balance and SSC resistance
Sex) was evaluated. As a result, it was found that the SSC resistance depends on the YS / TS ratio (YR), and the higher the YR is, the better the SSC resistance is even in the same YS. Furthermore, the conditions of the tempering treatment (tempering temperature, temperature rising rate) for obtaining high YR were clarified.

The present invention is constructed on the basis of this finding, and the gist thereof is (1)% by weight, C: ≤0.050, Si: ≤0.5, Mn: ≤1.5, P : <0.03, S: <0.005, Cr: 11.0 to 14.0, Ni: 4.0 to 7.0, Mo: 1.0 to 2.5, Cu: 1.0 to 2 .5, Al: ≤0.05, N: 0.01 to 0.10, and the balance of Fe and inevitable impurities having a composition of martensitic stainless steel having a composition of Ms point or less after hot working. Cool to temperature, then 550 ° C or higher A
Martensite, characterized in that a temperature T of c ₁ or lower is subjected to a heat treatment in which the temperature is raised to an average heating rate of 500 to T ° C. of 1.0 ° C./sec or higher and then cooled to a temperature of the Ms point or lower. Is a method for producing stainless steel seamless steel pipe,
(2) In% 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: 1.0-2.5, Cu: 1.0-2.5, Al: ≤0.05, N: 0.01-0.10, Ca : 0.001 to 0.020, with the balance being Fe and inevitable impurities, the martensitic stainless steel is hot-worked and then cooled to a temperature not higher than the Ms point, and then 550 ° C. or higher A
Martensite, characterized in that a temperature T of c ₁ or lower is subjected to a heat treatment in which the temperature is raised to an average heating rate of 500 to T ° C. of 1.0 ° C./sec or higher and then cooled to a temperature of the Ms point or lower. Is a method for producing stainless steel seamless steel pipe,
(3) After hot working, it is cooled to a temperature of Ms point or lower, then reheated to a temperature of Ac ₃ point or higher and then cooled to a temperature of Ms point or lower, and a temperature T of 550 ° C. or higher and Ac ₁ or lower.
(1) or (2) above, which is subjected to a heat treatment in which the temperature is raised to an average heating rate of 500 to T ° C. of 1.0 ° C./sec or more and then cooled to a temperature of the Ms point or less.
The method for producing a martensitic stainless seamless steel pipe according to item (4), after cooling to a temperature of Ms point or less after hot working, and then reheating to a temperature of Ac ₁ point or more and Ac ₃ point or less Cool to a temperature below the Ms point, and then cool to 55
After heating to a temperature T of 0 ° C. or more and Ac ₁ or less so that the average heating rate of 500 to T ° C. is 1.0 ° C./sec or more, M
The method for producing a martensitic stainless seamless steel pipe according to the above (1) or (2), characterized by performing a heat treatment of cooling to a temperature of s point or lower, and (5) Ms after hot working. Cool to below the point, then Ac ₃
After being reheated to a temperature not lower than the point, it is cooled to a temperature not higher than the Ms point, subsequently reheated to a temperature not lower than Ac ₁ point and not higher than Ac ₃ point, and then cooled to a temperature lower than the Ms point, and further not lower than 550 ° C and not higher than Ac _{1 point.} The temperature T of 500 to T ° C. is subjected to a heat treatment of raising the temperature to an average heating rate of 1.0 ° C./sec or more and then cooling to a temperature below the Ms point (1) or The method for producing a martensitic stainless seamless steel pipe according to the item (2), wherein the ratio of the yield strength to the tensile strength of the heat-treated steel pipe material is 0.77 or more. A method for producing a martensitic stainless seamless steel pipe according to any one of items (1) to (5).

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. The reasons for limiting the components of the martensitic stainless steel in the present invention are as follows. C: C is an element that forms Cr carbide or the like and deteriorates corrosion resistance, so that the lowest possible level is preferable. The maximum value is 0.05 in consideration of the achievement level of the current industrial refining technology.
%. Si: Si is added and remains as a deoxidizer in the steelmaking process, and is not an element to be intentionally added, but 0.5
%, The toughness and SSC resistance deteriorate, so the upper limit was made 0.5%.

Mn: Mn is an austenite stabilizing element and is effective in preventing rolling flaws by suppressing precipitation of the δ phase during hot working, but if added in excess of 1.5%, the grain boundary strength decreases. Since SSC resistance deteriorates, 1.5
% As the upper limit. P: The upper limit was set to 0.03% because it segregates at the grain boundaries to lower the grain boundary strength and deteriorate the SSC resistance.

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, and 11% or more is required to obtain sufficient CO ₂ corrosion resistance, but it is also a ferrite stabilizing element, and if too much, δ during hot working.
The upper limit was set to 14% because the phases precipitate and the hot workability deteriorates, and the product does not become a martensite single phase and the SSC resistance deteriorates.

Ni: Effective for improving CO ₂ corrosion resistance and toughness. Further, it is an austenite stabilizing element and suppresses the formation of a δ phase which leads to rolling defects. These effects are insufficient if the addition amount is less than 4.0%, and the effects are 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.

N: Since it is a strong austenite stabilizing element, it is added as an alternative element to expensive Ni. However, if added too much, the hardness in the martensitic state becomes high and the delayed fracture susceptibility increases, so the optimum addition amount was made 0.01 to 0.10%. Similar to Mo: Cr, it is an element for improving corrosion resistance and an element for forming ferrite. From the viewpoint of CO ₂ corrosion resistance, SSC resistance, and hot workability, the optimum addition range was 1.0 to 2.5%.

Similar to Cu: Ni, it is an element effective in improving the CO ₂ corrosion resistance and an austenite stabilizing element, and is also effective in preventing rolling flaws. However, if it is less than 1.0%, these effects are sufficient. If not added and added in excess of 2.5%, the hot workability deteriorates significantly, so the optimum range is 1.0-
It was set to 2.5%. It suppresses the deterioration of hot workability due to Ca: S, and is added as necessary, but if it is less than 0.001%, the effect is not exhibited, and if it exceeds 0.02%, the effect is saturated. Therefore, the optimum addition amount is set to 0.001 to 0.02%.

Next, a heat treatment method for steel having such a composition will be described. The steel having such a composition is mainly added with Ni and Cu for the purpose of use in order to reduce C mainly for improving the corrosion resistance and to prevent the precipitation of δ ferrite which is harmful to the SSC resistance. However, the addition of such a γ-forming element significantly lowers the Ac ₁ transformation point. Therefore, although steel having such a composition is inevitably subjected to low temperature tempering, low temperature tempering increases strength, resulting in insufficient SSC resistance.

However, as a result of diligent research conducted by the present inventors, it became clear 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 increases. It is considered that the reason for this is that the higher the heating rate of the precipitate, the later the progress of precipitation, aggregation, and solid solution, which delays the α → γ transformation. The heating rate here is the average heating rate in the temperature range of 550 ° C. to the tempering temperature T of Ac ₁ point or less related to the change in the state of the precipitate.

The above behavior is the same even if the previous history is different, that is, as-rolled and cooled to room temperature (reference: R),
After that, it was reheated to Ac ₃ points or more and then cooled to room temperature (code: RN), and subsequently, after the treatment of code R, it was reheated to Ac ₁ point or more and Ac ₃ points or less and then cooled to room temperature (code: RN). R-L), subsequently Ac ₁ to the processing of the code R-N
Similar results were obtained with the four types of test materials (FIG. 3) that were reheated to a temperature not lower than the Ac _{3 and} lower than the Ac _{3 and} then cooled to room temperature (symbol: RN-L). 0.3 ° C / sec with normal furnace heating method
Ac ₁ point is 630 ° C because only a moderate heating rate can be obtained
Although it is about the level, Ac ₁
The point is 650 ° C or higher. From this, it is possible to raise the Ac ₁ transformation point by performing high-speed temperature rise, and as a result, it becomes possible to perform high temperature tempering and prevent the strengthening which is detrimental to the SSC resistance. Examples of the heat treatment method that enables the rapid temperature rise include an induction heating method and a salt bath heating method.
It is desirable that the tempering retention time be within 1 minute because the Ac ₁ transformation point decreases when retained for a long time.

Next, the SSC characteristics of the steel material obtained by the above heat treatment method will be described. An SSC test was conducted using the steel material obtained by the heat treatment method described above, and the results are summarized in relation to the temperature rising rate and shown in FIG. The SSC test was performed by saturating H ₂ S at a partial pressure of 0.3 atm while applying a bending stress of 100% of YS to a strip-shaped test piece of size t3.0 × 10 × 65 mm by a four-point supporting method. It was immersed in a 5% acetic acid solution at room temperature and evaluated for cracks. FIG.
As is apparent from the history before tempering (code: R, R-
It is possible to obtain excellent SSC resistance at a temperature rising rate of 1 ° C./sec or more regardless of N, RL, RN, and FIG. 3). The reason for this is that, since tempering at a high temperature rising rate increases the YR, the higher the temperature rising rate, the lower the TS in the same YS.

Next, the reason why it is desirable to define YS / TS ratio (YR) ≧ 0.77 as the mechanical properties to be satisfied by the steel material subjected to the tempering treatment by the high-speed temperature rise will be described. The YS of the steel material was changed to 69.
An SSC test was performed using a material that had a quality of 1 to 75.1 kg / mm ² (corresponding to 95 ksi class as an oil country tubular good), and the results were summarized in relation to YR. SSC test is t3.0 ×
H ₂ S was applied to a 10 × 65 mm strip test piece under a bending stress of 100% of YS by a four-point support method.
Was immersed in a solution of 5% acetic acid saturated with a partial pressure of 0.3 atm at room temperature, and the presence or absence of cracks was evaluated. As a result, regardless of the history before tempering (signs: R, RN, RL, RN, 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]

Embodiments of the present invention will be described below. The test was conducted using the steels shown in Table 1. A seamless steel tube is rolled using a press roll piercing-elongator rolling method using a rectangular cross-section bloom as a raw material, and a steel pipe cooled to room temperature is used as a raw material to give a history before tempering under the conditions shown in Table 2, and further shown in Table 3. A tempering process was performed under the conditions. Temporary retention time is 1
Within minutes A test piece was sampled from this test material and subjected to a tensile test and an SSC test. SSC characteristic is t3.0
In a state in which a bending stress of 100% of the yield strength is applied to a strip-shaped test piece of a size of × 10 × 65 mm by a four-point supporting method,
It was evaluated by immersing H ₂ S in a 0.5% acetic acid solution saturated with a partial pressure of 0.3 atm at room temperature and determining the presence or absence of cracks.

The results of both tests are also shown in Table 3. In Table 3, the present invention No. 1 to 13 have high YR and show excellent SSC resistance. On the other hand, regarding comparative examples, No.
Nos. 22 and 23 have insufficient SSC resistance because the chemical composition of the sample material is out of the range of the present invention. 15, 17, 1
No. 8,19,20,21,22 cannot obtain sufficient SSC resistance because the heating rate is out of the range of the present invention. Also, N
o. No. 14 has an insufficient SSC resistance because the tempering temperature exceeds Ac ₁ , and No. 14 In Nos. 16 and 23, both the heating rate and the tempering temperature are out of the range of the present invention, and the SSC resistance deteriorates.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

As described above, according to the present invention, SS resistance
It is possible to obtain a martensitic stainless steel oil country tubular good having excellent C property.

[Brief description of the drawings]

FIG. 1 shows the relationship between the temperature rising rate and the Ac ₁ transformation point.

FIG. 2 shows the relationship between the temperature rising rate of tempering and SSC resistance. The plot lateral values are shown in Table 3. Means

FIG. 3 shows an outline of a thermal history before tempering which is a target of the present invention.

[Explanation of symbols]

R: As-rolled RN: As-rolled material is reheated to Ac ₃ points or higher and cooled to room temperature RL: As-rolled material is reheated to Ac ₁ to Ac ₃ points temperature range and cooled to room temperature R -N-L: cooling the R-N material to room temperature and re-heated to a temperature range of Ac ₁ to Ac ₃ point

Claims (6)

[Claims] 1. By weight%, C: ≦ 0.050, Si: ≦ 0.5, Mn: ≦ 1.5, P: ≦ 0.03, S: ≦ 0.005, Cr: 11.0 to 14.0, Ni: 4.0 to 7.0, Mo: 1.0 to 2.5, Cu: 1.0 to 2.5, Al: ≤0.05, N: 0.01 to 0.10. , And the balance is Fe and unavoidable impurities, and a martensitic stainless steel having a composition consisting of inevitable impurities is cooled to a temperature not higher than the Ms point after hot working, and then 550 ° C. or higher A
Martensite, characterized in that a temperature T of c ₁ or lower is subjected to a heat treatment in which the temperature is raised to an average heating rate of 500 to T ° C. of 1.0 ° C./sec or higher and then cooled to a temperature of the Ms point or lower. Of stainless steel seamless steel pipe. 2. In% by weight, C: ≦ 0.050, Si: ≦ 0.5, Mn: ≦ 1.5, P: ≦ 0.03, S: ≦ 0.005, Cr: 11.0 to 14.0, Ni: 4.0 to 7.0, Mo: 1.0 to 2.5, Cu: 1.0 to 2.5, Al: ≤0.05, N: 0.01 to 0.10. , Ca: 0.001 to 0.020, with the balance being Fe and inevitable impurities, the martensitic stainless steel is cooled to a temperature not higher than the Ms point after hot working, and then 550 ° C. or higher. A
Martensite, characterized in that a temperature T of c ₁ or lower is subjected to a heat treatment in which the temperature is raised to an average heating rate of 500 to T ° C. of 1.0 ° C./sec or higher and then cooled to a temperature of the Ms point or lower. Of stainless steel seamless steel pipe. 3. After hot working, the material is cooled to a temperature below the Ms point, then reheated to a temperature above the Ac ₃ point and then cooled to a temperature below the Ms point, and further to a temperature T above 550 ° C. and below Ac _1. In addition, the average heating rate from 500 to T ° C is 1.0 ° C / s.
The method for producing a martensitic stainless seamless steel pipe according to claim 1 or 2, wherein the heat treatment is performed such that the temperature is raised to ec or higher and then cooled to a temperature equal to or lower than the Ms point. 4. After hot working, it is cooled to a temperature below the Ms point, then reheated to a temperature below the Ac ₁ point and below the Ac ₃ point, then cooled to a temperature below the Ms point, and further above 550 ° C. to Ac ₁ The temperature T below is subjected to a heat treatment in which the temperature is raised to an average heating rate of 500 to T ° C. of 1.0 ° C./sec or more and then cooled to a temperature below the Ms point. The method for producing a martensitic stainless seamless steel pipe according to claim 2. 5. After hot working, it is cooled to a temperature of Ms point or lower, then reheated to a temperature of Ac ₃ point or higher and then cooled to a temperature of Ms point or lower, and subsequently to a temperature of Ac ₁ point or higher and Ac ₃ point or lower. After being reheated to a temperature, it is cooled to a temperature not higher than the Ms point, and further heated to a temperature T of 550 ° C. or higher and Ac ₁ or lower, 500
3. The martense according to claim 1 or 2, wherein the heat treatment is performed such that the temperature is raised to an average heating rate of 1.0 to T ° C or more to 1.0 ° C / sec or more and then cooled to a temperature equal to or lower than the Ms point. Site-based stainless steel seamless steel pipe manufacturing method. 6. The martensite system according to claim 1, wherein the yield strength and the tensile strength of the heat-treated steel pipe material are 0.77 or more. Manufacturing method of stainless steel seamless steel pipe.