JPH0688130A - Production of martensitic stainless steel seamless steel pipe excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To obtain a high toughness martensitic stainless steel pipe excellent in CO2 corrosion resistance and having sulphide stress cracking resistance with high productivity. CONSTITUTION:Steel constituted of, by weight, <=0.05% C, <=0.50% Si, <=1.0% Mn, <=0.03% P, <=0.01% S, 11 to 17 Cr, 1 to 4% Cu, 1.5 to 5% Ni, <=0.05% Al and 0.02 to 0.1% N as well as C+0.8N>0.06, and the balance substantial Fe with inevitable impurities is subjected to hot working allowed to naturally cool to a room temp., heated thereafter to the Ac3 transformation point + 10 deg.C to the Ac3 transformation point + 200 deg.C and then cooled to a room temp. at a rate of air cooling or higher. Next, it is heated to the Ac1 transformation point to the Ac3 transformation point, cooled to a room temp. at a rate of air cooling or higher and successively subjected to tempering treatment at the Ac1 transformation point or below.

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 high toughness martensitic stainless steel seamless steel pipe having excellent CO ₂ corrosion resistance and sulfide stress cracking resistance.

[0002]

2. Description of the Related Art In recent years, development of gas wells for producing gas containing a large amount of CO ₂ and CO ₂ injection for secondary recovery have been widely performed. In such an environment, since the steel pipe is severely corroded, a martensitic stainless steel pipe excellent in CO ₂ corrosion resistance is often used. In particular, Japanese Patent Publication No. 59-15977 is cited as a martensitic stainless steel having excellent corrosion resistance and hot workability. However, in this martensitic stainless steel, the contents of C and N are remarkably reduced in order to improve the corrosion resistance, and a δ ferrite phase which deteriorates the hot workability is formed in the austenite matrix during heating of the ingot. It has drawbacks. Therefore, under severe processing conditions such as seamless rolling, cracks and flaws occur, cost increase due to yield reduction is inevitable, and the production of seamless steel pipes with high corrosion resistance with such a composition system has been extremely difficult until now. It was very difficult.

[0003]

SUMMARY OF THE INVENTION The present invention solves the above problems and provides a high toughness martensitic stainless steel seamless steel pipe having excellent CO ₂ corrosion resistance and sulfide stress cracking resistance. The present invention provides a method for manufacturing with high productivity.

[0004]

From the results of many experiments, the inventors of the present invention have found that CO ₂ corrosion resistance reduces C and reduces the required amount of Cr.
It was found that the addition of Mo, Cu, and Cu can maintain the sulfide stress cracking resistance, and that the sulfide stress cracking resistance can be improved by controlling the structure showing the cracking resistance. Further, the hot workability is obtained by reducing P, S, etc. to suppress the formation of inclusions, and by controlling the content of C, N, and further adding Ni, the phases of different phases having different deformation resistances can be obtained. It was found to be maintained by performing metallurgical operations such as controlling the fraction and shape.

In particular, the present inventors have paid attention to the effects of C and N and have obtained the following findings. FIG. 1 shows the CO ₂ corrosion resistance characteristics and the drawing value during hot working when the C and N contents were changed with 1.5% Ni-12.5% Cr steel as the base component. In FIG. 1, C.I. R. Is the annual corrosion rate in artificial seawater at 180 ° C. equilibrated with 40 atm CO ₂ . R. If it is <0.1 mm / y, it can be evaluated as having sufficient corrosion resistance. In addition, R. A. Is 125
It is the reduction ratio when the sample heated to 0 ° C. is uniaxially tensile deformed at 900 ° C. under a strain rate of 3 sec ^{−1. If} it is 70% or more, it can be said that the hot deformability is good. In addition, CO
_{(2) For the} corrosion test, the one having a yield strength of about 720 MPa was used after quenching and tempering after hot working.
From FIG. 1, it was found that in order to satisfy the CO ₂ corrosion resistance, C <
It can be read that it is necessary to set the content to 0.05%, and it is necessary to set C + 0.8N> 0.06 in order to have sufficient hot workability (the unit of the content of each element symbol is wt.%). The present invention is constructed on the basis of the above-mentioned findings, and its gist is as follows.
That is, in weight%, C ≦ 0.05, Si ≦ 0.50, Mn ≦ 1.0, P ≦ 0.03, S ≦ 0.01, Cr: 11 to 17, Ni: 1.5 to 5, Cu: 1-4, Al ≦ 0.05, N: 0.02-0.1, and C
Steel satisfying + 0.8N> 0.06 or further containing Mo: 0.5 to 2 and the balance being substantially Fe and unavoidable impurities is hot worked and naturally cooled to room temperature. ₃ transformation points +10
℃ ~ Ac ₃ transformation point + 200 ℃ temperature is cooled to room temperature at a rate of air cooling or more, followed by tempering treatment at a temperature of Ac ₁ transformation point or less, or hot working of the steel of the above components After allowing to cool naturally to room temperature, Ac ₃ transformation point +1
It is heated to a temperature of 0 ° C to Ac ₃ transformation point + 200 ° C and cooled to room temperature at a rate of air cooling or higher, and then Ac ₁ transformation point to Ac
₃ Martensitic stainless steel seamless steel pipe with excellent corrosion resistance, which is heated to the temperature of ₃ transformation points, cooled to room temperature at a rate of air cooling or higher, and then tempered at a temperature of the Ac ₁ transformation point or lower Is.

The present invention will be described in detail below. First, the reasons for limiting the steel components will be described. C is an element that forms Cr carbide and deteriorates the corrosion resistance, but is a typical austenite forming element and has an effect of suppressing the occurrence of the δ ferrite phase in the hot working temperature range of 900 to 1250 ° C. Contained in. However, if the content exceeds 0.05%, a large amount of carbides such as Cr carbides precipitate to form a Cr-deficient layer, which deteriorates the CO ₂ corrosion resistance and also causes carbides to easily precipitate at grain boundaries. Therefore, the sulfide stress cracking resistance is significantly reduced. Therefore C
The content was 0.05% or less.

Si is added as a deoxidizing agent in steelmaking and remains. When it is contained in the steel in an amount of more than 0.50%, toughness and sulfide stress cracking resistance are deteriorated. ．
It was set to 50% or less.

[0008] Mn is an element that forms inclusions and impairs crack resistance in a corrosive environment, but is added because it is a useful component for forming an austenite single phase. However, 1.
If added in excess of 0%, a large amount of inclusions are formed, so that the crack resistance and toughness in a corrosive environment deteriorate. Therefore, the Mn content is set to 1.0% or less.

P segregates at the grain boundaries, weakens the grain boundary strength, and reduces hot workability and sulfide stress cracking resistance.
It was set to 03% or less. Since S forms inclusions as sulfides and deteriorates hot workability, the upper limit was made 0.01%.

In order to impart the CO ₂ corrosion resistance, which is the object of the present invention, and to have the corrosion resistance as stainless steel, Cr must be contained in an amount of 11% or more. However, if it is added in excess of 17%, a ferrite phase is likely to be formed, so the limited range is set to 11 to 17%.

Ni has the effect of improving the corrosion resistance of Cr-containing steel. Moreover, it is a strong austenite forming element, and it has the effect of suppressing the formation of the δ ferrite phase during high temperature heating, and also of suppressing the growth of cracks formed inside the δ ferrite phase during hot working by making the shape thin and short. Therefore, it also has an effect of improving hot workability.
However, when N: 0.02%, addition of Ni: 1.5% or less does not show these effects, and when it exceeds 5%, the Ac ₁ point becomes extremely low and tempering becomes difficult. In addition, since the retained austenite phase is formed and the strength and toughness are impaired, the limiting range is set to 1.5 to 5%.

Cu has the effect of improving the CO ₂ corrosion resistance. Further, it is an austenite stabilizing element, and Ac
_{1 It} also has the advantage of not lowering the transformation point. However,
When the content is 1% or less, the effect of improving the corrosion resistance is not sufficient, and when the content is more than 4%, it is sensitive to hot cracking and the hot workability is deteriorated. Therefore, the content is limited to the range of 1 to 4%. Further, since the above effect is small when Cu alone is added, it is always added in combination with Ni.

Al, like Si, was added as a deoxidizer and remained. If added in excess of 0.05%, Al
A large amount of N is formed and the toughness is significantly reduced. Therefore, the upper limit of the amount added is set to 0.05%.

N is harmless to the corrosion resistance and is a typical austenite forming element like C, and has the effect of suppressing the formation of the ferrite phase in the hot working temperature range of 900 to 1250 ° C. The effect is 1.
When 5% Ni-12.5% Cr steel is used as the base component, it is effective in the range of the content satisfying C + 0.8N <0.06 (C and N are wt.%). Therefore, C <
In the case of 0.05%, in order to obtain a good hot workability without generating a ferrite phase in the hot working temperature range, N is set to 0.
It is necessary to add 02% or more. In addition, since it is difficult to add 0.1% or more in the usual melting process, the range of the addition amount is set to 0.02 to 0.1%.

Mo is an element effective for improving the pitting corrosion resistance, and is added if necessary. However, 0.
The effect is small with the addition of 5% or less. It is also a strong ferrite stabilizing element, and if added in excess of 2%, δ
Since the phase is easily generated, the limited range is 0.5.
~ 2%.

Next, the reasons for limiting the heat treatment conditions will be described. The upper limit of the austenitizing heating temperature is the temperature at which the carbide is not completely dissolved in the γ loop of the Cr-containing stainless steel and coarsening of the crystal grains does not occur, and the lower limit is the lowest temperature at which the austenite phase becomes stable. And That is, a steel pipe that has been hot worked and cooled has an Ac ₃ transformation point of +2.
When heated to a temperature of 00 ° C. or higher, the carbide is completely dissolved, so that a large amount of Cr carbide or the like is precipitated at the grain boundary during cooling, the corrosion resistance is significantly lowered, and the coarseness of the crystal grain occurs, so that the toughness is improved. descend. Also, Ac ₃ transformation point + 10 ° C
When heated to the low temperature below, the austenite phase is not stabilized and it is difficult to obtain stable strength.
Therefore, the heat treatment temperature is Ac ₃ transformation point + 10 ° C to A
The c ₃ transformation point was set to + 200 ° C. If the cooling rate after this heating is slower than that of air cooling, carbide precipitates in the grain boundary in the form of a plate and the toughness is significantly reduced, so the cooling rate was limited to air cooling or higher.

When cooled to room temperature in this way, martensitic transformation occurs to form a martensitic single-phase structure. The residual stress in the martensite structure is eliminated by recovery, and supersaturated carbon atoms are precipitated as carbides to enhance toughness and ductility, and a tempering treatment is performed to obtain desired strength. At this time, if the material is heated to a temperature above the Ac ₁ transformation point, reverse transformation occurs and the toughness is significantly reduced, so the tempering treatment is performed at a temperature below the Ac ₁ transformation point.

Before the tempering treatment after the austenitizing treatment, if necessary, the Ac ₁ transformation point to Ac ₃
Two-phase heat treatment is performed by heating in the temperature range of the transformation point. This is aimed at tempering the steel to a low strength that cannot be obtained by a single tempering treatment. By tempering the steel to a low strength using this treatment, sufficient sulfide stress crack resistance of the steel can be obtained. It is possible to impart the sex. The steel pipe manufactured by the method of the present invention as described above is excellent not only in CO ₂ corrosion resistance and sulfide stress cracking resistance but also in toughness.

[0019]

EXAMPLES First, after casting a steel having the chemical composition shown in Table 1 in a usual melting process, a steel pipe was manufactured by hot rolling and subjected to heat treatment and tempering treatment.
The strength, toughness, CO ₂ corrosion resistance, and sulfide stress cracking resistance were investigated. Table 2 shows materials such as heat treatment temperature and strength at that time. The CO ₂ corrosion resistance was evaluated by the corrosion rate in artificial seawater at 150 ° C. equilibrated with 40 atm of CO ₂ . If the corrosion rate is 0.1 mm / y or less, it can be considered to have corrosion resistance. The resistance to sulfide stress cracking is 2 for round bar tensile test pieces.
99% CO ₂ +1 at 1 atm in 5% NaCl solution at 5 ° C
A uniaxial tensile stress was applied in a corrosive environment saturated with% H ₂ S gas, and the ratio (Rs value) of the maximum initial stress and yield stress at which breakage did not occur at 720 hours was determined. If Rs ≧ 0.8, it can be said that the characteristics are excellent.

[0020]

[Table 1]

[0021]

[Table 2]

From the results shown in Table 2, the steel pipes produced by the method of the present invention show good CO ₂ corrosion resistance, sulfide stress cracking resistance and high toughness, while the comparative method out of the range of the present invention. It is clear that one of the characteristics is inferior.

[0023]

INDUSTRIAL APPLICABILITY As described above, the present invention provides a martensitic stainless steel seamless steel pipe excellent in CO ₂ corrosion resistance and sulfide stress cracking resistance by specifying the contained components and controlling the structure. Can be provided.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of C and N on the corrosion rate in a CO ₂ corrosive environment and the drawing rate during hot deformation.

Claims (4)

[Claims] 1. By weight%, C ≦ 0.05, Si ≦ 0.50, Mn ≦ 1.0, P ≦ 0.03, S ≦ 0.01, Cr: 11 to 17, Cu: 1 to 4 , Ni: 1.5 to 5, Al ≦ 0.05, N: 0.02 to 0.1, and C + 0.8N> 0.06, with the balance being substantially Fe and unavoidable. After hot working the steel made of impurities and allowing it to cool naturally to room temperature, the Ac ₃ transformation point + 10 ° C to the Ac ₃ transformation point + 200 ° C
Of the martensitic stainless steel with excellent corrosion resistance, which is characterized by heating to a temperature of room temperature, cooling to room temperature at a rate of air cooling or more, and subsequently tempering at a temperature of Ac ₁ transformation point or less. . 2. In% by weight, C ≦ 0.05, Si ≦ 0.50, Mn ≦ 1.0, P ≦ 0.03, S ≦ 0.01, Cr: 11-17, Cu: 1-4 , Ni: 1.5 to 5, Mo: 0.5 to 2, Al ≦ 0.05, N: 0.02 to 0.1, and C + 0.8N> 0.06. Of steel substantially consisting of Fe and unavoidable impurities is hot-worked and naturally cooled to room temperature, and then Ac ₃ transformation point + 10 ° C to Ac ₃ transformation point + 200 ° C.
Of the martensitic stainless steel with excellent corrosion resistance, which is characterized by heating to a temperature of room temperature, cooling to room temperature at a rate of air cooling or more, and subsequently tempering at a temperature of Ac ₁ transformation point or less. . 3. By weight%, C ≦ 0.05, Si ≦ 0.50, Mn ≦ 1.0, P ≦ 0.03, S ≦ 0.01, Cr: 11 to 17, Cu: 1 to 4 , Ni: 1.5 to 5, Al ≦ 0.05, N: 0.02 to 0.1, and C + 0.8N> 0.06, with the balance being substantially Fe and unavoidable. After hot working the steel made of impurities and allowing it to cool naturally to room temperature, the Ac ₃ transformation point + 10 ° C to the Ac ₃ transformation point + 200 ° C
Temperature was heated to and cooled by air cooling or faster to room temperature, then, Ac ₁ and heated to a temperature of transformation point to Ac ₃ transformation point is cooled at a rate of more than air cooling to room temperature, followed by less Ac ₁ transformation point A method for producing a seamless martensitic stainless steel pipe with excellent corrosion resistance, characterized by performing tempering treatment at 4. By weight%, C ≦ 0.05, Si ≦ 0.50, Mn ≦ 1.0, P ≦ 0.03, S ≦ 0.01, Cr: 11-17, Cu: 1-4 , Ni: 1.5 to 5, Mo: 0.5 to 2, Al ≦ 0.05, N: 0.02 to 0.1, and C + 0.8N> 0.06. Of steel substantially consisting of Fe and unavoidable impurities is hot-worked and naturally cooled to room temperature, and then Ac ₃ transformation point + 10 ° C to Ac ₃ transformation point + 200 ° C.
Temperature was heated to and cooled by air cooling or faster to room temperature, then, Ac ₁ and heated to a temperature of transformation point to Ac ₃ transformation point is cooled at a rate of more than air cooling to room temperature, followed by less Ac ₁ transformation point A method for producing a seamless martensitic stainless steel pipe with excellent corrosion resistance, characterized by performing tempering treatment at