JPH05263137A - Production of seamless tube of martensitic stainless steel excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To produce a seamless stainless steel tube excellent in corrosion resistance by subjecting a stainless steel containing specific amounts of C, Si, Mn, P, S, Cr, Ni, Al, N, Ti, Zr, and REM to heat treatment under specific conditions. CONSTITUTION:A steel which has a composition consisting of, by weight, <=0.05% C, <=0.5% Si, <=1% Mn, <=0.03% P, <=0.01% S, 11-17% Cr, 1.5-5% Ni, <=0.05% Al, 0.02-0.1% N, further one or >=2 elements among 0.003-0.4% REM, and the balance essentially Fe and satisfying C%+0.8N%>0.06 is hot- worked, subjected to natural air cooling down to room temp., reheated up to a temp. in the range between (Ac3 transformation point + 10 deg.C) and (Ac3 transformation point + 200 deg.C), cooled down to room temp. at rate not lower than air cooling rate, and successively subjected to tempering treatment at a temp. not higher than the Ac1 transformation point. By this method, the seamless tube of martensitic stainless steel excellent in CO2 corrosion resistance and having sulfide stress corrosion cracking resistance can be produced.

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 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, many mantensite stainless steel pipes excellent in CO ₂ corrosion resistance are used. In particular, martensite stainless steel of a type containing low to medium C and containing a few% of Ni has excellent corrosion resistance, and not only CO ₂ but also a trace amount of H ₂ as an oil country tubular good or a line pipe.
It is desired to be used in a severe corrosive environment containing S.

As this type of steel, AISI 414, 431 and the like specified in AISI are well known. However, since these steels are premised on being used as cast steel, their hot workability is extremely poor. Further, sulfide stress cracking easily occurs in a CO ₂ corrosive environment containing a trace amount of H ₂ S. Steels having improved hot workability and corrosion resistance of these steels are disclosed in JP-B-59-159.
77, JP-A-60-174859 and the like. However, these martensitic stainless steels have remarkably reduced the amounts of C and N added in order to improve the corrosion resistance, or have a low C content and a few% of Mo added, and therefore have a CO ₂ corrosion resistance. Although it is improved, the sulfide stress cracking resistance is still insufficient, and furthermore, when the steel is heated, a δ ferrite phase which deteriorates the hot workability is formed in the austenite matrix. Therefore, under severe processing conditions such as seamless rolling, cracks and flaws occur, cost increase due to yield reduction is unavoidable, and the production of seamless steel pipes with high corrosion resistance with such a composition system has been extremely difficult until now. It was difficult for me.

[0004]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems by adjusting the contained components and
An object of the present invention is to provide a method for producing a martensitic stainless steel seamless steel pipe having excellent corrosion resistance by forming a stable structure.

[0005]

From the results of many experiments, the present inventors have found that the CO ₂ corrosion resistance reduces C and reduces the required amount of C.
It has been found that if r is added, the sulfide stress cracking resistance can be maintained and that the sulfide stress cracking resistance can be improved by controlling the structure showing the cracking resistance. The hot workability is obtained by reducing P, S, etc. to suppress the formation of inclusions, and by controlling the addition amount of C and N, and further adding Ni, a different phase having different deformation resistance is obtained. It was found to be maintained by performing metallurgical operations such as controlling the fraction and shape. In particular, the present inventors paid attention to the effects of C and N and obtained the following findings.

That is, in FIG. 1, the base component is 1.5% N
₂ shows the CO ₂ corrosion resistance characteristics and the reduction value during hot working when the C and N contents were changed as i-12.5% Cr steel. In FIG. 1, C.I. R. Is the annual corrosion rate in 150 ° C. artificial seawater 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 the draw ratio when a sample heated to 1250 ° C. is uniaxially tensile deformed at 900 ° C. under a strain rate of 3 sec ⁻¹ , and if it is 70% or more, the hot deformability is good. In addition, in the CO ₂ corrosion test, after hot working, quenching and tempering treatment was performed, and the yield strength was 6
The one showing about 50 MPa was used. From Figure 1, CO ₂ resistance
In order to satisfy the corrosion property, C <0.05% is required, and in order to have sufficient hot workability, C is required.
% + 0.8 N%> 0.06 (unit of content of each element is wt%). Further, sulfide stress cracking of these steels occurs at the grain boundaries, which is a result of a decrease in the grain boundary strength due to P segregated at the grain boundaries. Therefore, it is useful to reduce the amount of P added, but especially for stainless steel, it is difficult to lower the P, and the cost is significantly increased. By the way, Ti, Zr, R forming a stable compound with P
It has been found that when EM is added, the solid solution P is reduced to substantially lower P, and the segregation P at grain boundaries is reduced.

The present invention is constructed on the basis of the above-mentioned findings, and the gist thereof is as follows. That is, as 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
%, Al ≦ 0.05%, N: 0.02 to 0.1%, and C% + 0.8N%> 0.06, and Ti:
0.005-0.1%, Zr: 0.01-0.2%, R
EM: 0.003 to 0.4% of 1 or 2 or more, or further Cu: 0.5 to 2% or Mo:
Steel containing 0.3 to 2.0% of 1 or 2 and the balance substantially consisting of Fe and unavoidable impurities is hot-worked and naturally cooled to room temperature, and then Ac ₃ transformation point + 10 ° C. ~ A
A martensitic stainless steel seamless steel pipe with excellent corrosion resistance, which is heated to a temperature of c ₃ transformation point + 200 ° C., cooled to room temperature at a rate of air cooling or more, and subsequently tempered at a temperature of Ac ₁ transformation point or less. It is a manufacturing method.

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 generation of the δ ferrite phase in the hot working temperature range of 900 to 1250 ° C. Added to. However, when an amount exceeding 0.05% is added, a large amount of carbides such as Cr carbides are deposited to form a Cr-deficient layer, and the CO ₂ corrosion resistance is deteriorated. Further, carbides are likely to precipitate at the grain boundaries, so that the sulfide stress cracking resistance is significantly reduced. Therefore, the C content is set to 0.05% or less.

Si is added as a deoxidizer for steelmaking,
When it is contained in the steel in an amount exceeding 0.50%, the toughness and the sulfide stress cracking resistance are deteriorated, so the content is set to 0.50% or less. 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 achieving austenite single phase. However, if added in excess of 1.0%, a large amount of inclusions are formed, so that the crack resistance and toughness in a corrosive environment deteriorate. Therefore, Mn
Content was 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%. Cr imparts the CO ₂ corrosion resistance which is the object of the present invention, and in order to have the corrosion resistance as stainless steel, 1
It is necessary to contain 1% or more. However, if more than 17% is added, the δ ferrite phase is likely to be generated,
The limited range was set to 11 to 17%.

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

Similar to Si, Al is added as a deoxidizing agent and remains. 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 δ ferrite phase in the hot working temperature range of 900 to 1250 ° C. .. As described above, when 1.5% Ni-12.5% Cr steel is used as a base component as described above, C% + 0.8N%> 0.06 (C and N are wt%).
It is effective in the range of the addition amount satisfying the above conditions. Therefore, C
In the case of <0.05%, it is necessary to add N by 0.02% or more in order not to generate the δ ferrite phase in the hot working temperature range and to obtain a good hot workability. In addition, since it is difficult to add 0.1% or more in the usual melting process,
The range of the addition amount was 0.02 to 0.1%.

Cu, like Ni, is a strong austenite forming element and also has an effect of not lowering the Ac ₁ transformation point. However, when added alone in excess of 2.0%, hot brittleness occurs, and the effect of providing corrosion resistance and phase stability is less than that of Ni, so addition alone does not show an effect. Therefore, when Cu is added, the addition amount should be 2.
It was set to 0% or less, and it was always added at the same time as Ni.
Mo has the effect of improving the CO ₂ corrosion resistance similarly to Cr, but the effect is not remarkable if the addition amount is 0.3% or less, while if added in excess of 2.0%, the δ ferrite phase is easily generated. Therefore, the upper limit is set to 2.0%.

Ti, Zr, and REM form a stable compound with P that is harmful to sulfide stress cracking resistance, and have the effect of reducing the solid solution P to substantially lower the P content. 0.0 each
If the amount is less than 0.05%, 0.01%, or 0.01%, the effect is not clear, and if added in a large amount, coarse oxides are formed and the toughness deteriorates, so the upper limits are 0.1% and 0.2%, respectively. ，
It was 0.4%. It should be noted that it is more effective to add these after deoxidizing with Al and Si. This is because when added at a stage where oxygen is large, the ability to form a large oxide and form a compound with P is reduced. Further, since the crystallizing / precipitating temperature of these P and the compound is extremely high, crystallizing / precipitating occurs during cooling from solidification.

Such a steel has a martensitic structure in a state where it is cooled to room temperature after rolling and may be immediately tempered, but it is usually re-austenite-tempered. The reasons for limiting the heat treatment conditions will be described. The upper limit of the heating temperature was the temperature at which the carbide was not completely dissolved in the γ loop of the Cr-containing stainless steel and coarsening of the crystal grains did not occur, and the lower limit was the lowest temperature at which the austenite phase was stable. That is, Ac ₃ transformation point +20
When heated to a temperature of 0 ° C. or higher, the carbide completely dissolves in solid solution, so that a large amount of Cr carbide and the like precipitates at the grain boundaries during cooling, significantly lowering the corrosion resistance, and further coarsening the crystal grains, resulting in toughness. descend. When heated to a low temperature of Ac ₃ transformation point + 10 ° C. or lower, the austenite phase is not stable and it is difficult to obtain stable strength. Therefore, the heat treatment temperature was set to Ac ₃ transformation point + 10 ° C to Ac ₃ transformation point + 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 remarkably deteriorates. 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, A
When heated to a temperature above the c ₁ transformation point, reverse transformation occurs and the toughness decreases significantly, so the tempering treatment is performed at a temperature below the Ac ₁ transformation point. 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.

[0018]

EXAMPLE 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 CO _{2 at} 40 atm. If the corrosion rate is 0.1 mm / year or less, it can be considered to have corrosion resistance. Resistance to sulfide stress cracking was measured at 25 ° C with a round bar tensile test piece.
99% CO ₂ + 1% H at 1 atm in 5% NaCl solution
_{2 In} a corrosive environment saturated with S gas, uniaxial tensile stress is applied,
The ratio (Rs value) of the maximum initial stress and the 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]

As described above, according to the method of the present invention, the amount of C is reduced and Cr and Mo are contained to improve the CO ₂ corrosion resistance, and the addition of Ti, Zr and REM improves the sulfide stress corrosion cracking resistance. And the hot workability is improved by reducing the balance of C and N and the generation of the δ ferrite phase due to the inclusion of Ni or Ni and C, and the martensite system having desired strength and excellent toughness. A stainless steel seamless steel pipe can be obtained.

[Brief description of drawings]

FIG. 1 shows the CO ₂ corrosion resistance (CR) of martensitic stainless steel and the reduction value (R.
A. ) A graph showing the relationship between C and N content.

Claims (2)

[Claims] 1. By 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%, Al ≦ 0.05%, N: 0.02 to 0.1%, and C% + 0.8N%>
0.06 is satisfied, and further, one or more of Ti: 0.005 to 0.1%, Zr: 0.01 to 0.2%, and REM: 0.003 to 0.4% is included, After hot-working steel having the balance substantially Fe and unavoidable impurities, it is naturally cooled to room temperature, reheated to Ac ₃ transformation point + 10 ° C to Ac ₃ transformation point + 200 ° C, and then air-cooled to room temperature or higher. Cooling at speed, followed by Ac
_A method for producing a seamless martensitic stainless steel pipe with excellent corrosion resistance, characterized by performing tempering at a temperature not higher than _one transformation point. 2. In% by 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%, Al ≦ 0.05%, N: 0.02 to 0.1%, and C% + 0.8N%>
0.06 is satisfied, and further, one or more of Ti: 0.005 to 0.1%, Zr: 0.01 to 0.2%, and REM: 0.003 to 0.4% is included, In addition, after hot working a steel containing one or two of Cu: 0.5 to 2% and Mo: 0.3 to 2.0%, the balance being substantially Fe and unavoidable impurities. After allowing to cool naturally to room temperature, Ac ₃ transformation point +1
After being reheated to 0 ° C to Ac ₃ transformation point + 200 ° C, it is cooled to room temperature at a rate of air cooling or more, and subsequently, tempered at a temperature of Ac ₁ transformation point or less, which is excellent in corrosion resistance. Martensitic stainless steel seamless steel pipe manufacturing method.