JPH0641638A - Production of martensitic stainless steel seamless pipe excellent in toughness and stress corrosion cracking resistance - Google Patents

Abstract

PURPOSE:To provide the method for manufacturing a martensitic stainless steel seamless pipe excellent in toughness and stress corrosion cracking resistance. CONSTITUTION:Steel contg. components satisfying 0.02 to 0.05% C, 0.50% or lower Si, <= 0.03% or lower P, 0.01% or lower S, 11 to 17% Cr, 1.5 to 5.0% Ni, 0.5 to 2.0% Mo, 0.05% or lower Al and 0.02 to 0.1% N or, furthermore, 0.5 to 2.0% Cu as well as C+0.8N>0.06, and the balance substantial Fe with inevitable impurities is subjected to hot working and is naturally air-cooled to a room temp. Next, it is heated to the Ac3 point +10 deg.C or above to the Ac3 point + 200 deg.C or below and is cooled from this heating temp.-800 deg.C to 600-350 deg.C at 2 deg.C/s or a higher rate. Successively, the steel is cooled to a room temp. at a rate of air cooling or above and is thereafter tempered at the Ac1 point or below. By this control of the cooling at the time of the normalizing, the precipitation of intergranular carbides is controlled to improve its toughness and stress corrosion cracking resistance.

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 seamless martensitic stainless steel pipe having excellent toughness 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. Particularly, as a martensitic stainless steel excellent in corrosion resistance and hot workability, JP-B-59-15977 and the like can be mentioned. However, in this martensitic stainless steel, the amounts of C and N added are remarkably reduced in order to improve the corrosion resistance, and a δ ferrite phase that deteriorates the hot workability is formed in the austenite matrix during heating of the ingot. There is. 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.

Further, in the production of such a martensitic stainless steel, Japanese Patent Publication No. 63-60808 discloses "a low C martensitic stainless steel having 900-
Heat treatment method of heating and holding in a temperature range of 1000 ° C. and then gradually cooling, or further heating and holding in a temperature range of 350 ° C. or lower and gradually cooling ”, and Japanese Patent Publication No. 1-258810, column 6,“ Generally adopted. The heat treatment is a normal normalization / tempering process. Forged and rolled molten steel is standardized at 950 ° C or higher and then tempered at 700 ° C or higher and Ac ₁ or lower ”. In addition, cracking is likely to occur when cooling from the heating temperature after rolling is rapidly cooled, such as water cooling, so that it is manufactured by performing slow cooling such as air cooling. However, although the martensitic stainless steel heat-treated by such a method can be obtained as a product having excellent corrosion resistance without residual stress or cracks, on the other hand, there is a problem that the toughness and the stress corrosion cracking resistance are not sufficient. .

[0004]

SUMMARY OF THE INVENTION In view of the above situation, an object of the present invention is to provide a method for producing a martensitic stainless steel seamless steel pipe excellent in toughness and stress corrosion cracking resistance.

[0005]

The present inventors have found from a number of experimental results that CO ₂ corrosion resistance can be maintained by reducing C and adding necessary amounts of Cr and Mo. It was found that the cracking property is improved by controlling the structure showing the cracking resistance.

Further, the hot workability is different in the deformation resistance by reducing the contents of P and S to suppress the formation of inclusions, and by controlling the addition amount of C and N and further adding Ni. It has been found that it is maintained by performing metallurgical operations such as controlling the phase fraction and shape of different phases. In particular, the present inventors have paid attention to the effects of C and N and obtained the following findings. In Fig. 1, the base component is 1.5% Ni-12.5%
Resistance to CO ₂ when changing C and N contents as Cr steel
The corrosion characteristics and the reduction value during hot working are shown. In FIG. 1, C.I. R. Is 150 ° C equilibrated with 40 atm CO _2.
The annual corrosion rate in artificial seawater of C.I. R.
If it is <0.1 mm / y, it can be evaluated as having sufficient corrosion resistance. In addition, R. A is a sample heated to 1250 ° C.
It is a reduction ratio when uniaxially tensile deformed at 0 ° C. and 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, in the CO ₂ corrosion test, after hot working, quenching and tempering are performed, and the yield strength is 650MP.
The one showing the degree a was used. From FIG. 1, it is necessary to set C <0.05% in order to satisfy the CO ₂ corrosion resistance characteristic, and in order to have sufficient hot workability, C +
It can be seen that it is necessary to make 0.8N> 0.06 (the unit of the content of each element symbol is wt.%). Also,
When gradually cooled or air-cooled during normalization, coarse and plate-like thin Cr-based carbides are precipitated along the former austenite grain boundaries, and a Cr-deficient layer is formed around them to substantially reduce the Cr content. Since it is selectively corroded, the resistance to stress corrosion cracking of steel decreases. Further, since the coarse plate-shaped thin carbide serves as a starting point of cracking, the toughness of steel deteriorates. When the temperature at which this coarse carbide precipitates was investigated, it was 800 to 600.
It was found to be ° C. Therefore, in order to improve the toughness and stress corrosion cracking resistance of the above martensitic stainless steel, it is necessary to employ rapid cooling as a method for suppressing the formation of such coarse Cr-based carbides.

The present invention is constructed by combining the above-mentioned findings, and the gist thereof is C ≤
0.05%, Si ≦ 0.50%, Mn ≦ 1.0
%, P ≦ 0.03%, S ≦ 0.01%,
Cr: 11 to 17%, Ni: 1.5 to 5%, M
o: 0.5 to 2%, Al ≦ 0.05%, N:
0.02-0.1%, or even Cu: 0.5
.About.2% and a component (% by weight) satisfying C + 0.8N> 0.06, with the balance being substantially Fe.
And, the steel consisting of unavoidable impurities is hot-worked and naturally cooled to room temperature, and then heated to a temperature of Ac ₃ transformation point + 10 ° C. or higher and Ac ₃ transformation point + 200 ° C. or lower.
After cooling from 0 ° C. cooling start temperature to 600 ° C. to 350 ° C. cooling stop temperature at a rate of 2 ° C./sec or more, and then cooling to room temperature at an air cooling rate or more, the Ac ₁ transformation point This is a method for producing a martensitic stainless steel seamless steel pipe excellent in toughness and stress corrosion cracking resistance which is tempered at the following temperature.

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 is 9 in the hot working temperature range.
It has an effect of suppressing the generation of the δ ferrite phase at 00 to 1250 ° C. However, when an amount exceeding 0.05% is added, a large amount of carbides such as Cr carbides precipitate to form a Cr-deficient layer, which lowers 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, the C content is (0.02
% Or more) 0.05% or less.

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

Mo is added because it has the effect of enhancing pitting corrosion resistance and improves stress corrosion cracking resistance, but it is a strong ferrite-forming element, and if added in excess of 2%, it causes the formation of δ phase. Therefore, the upper limit was set to 2%.

Similar to Si, Al is added as a deoxidizer, and 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 a base component, it is effective in the range of the additive amount 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%.

Cu, like Ni, is a strong austenite forming element and has an advantage that it does not lower 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 is set to 2.0% or less, and it is always added at the same time as Ni.

Next, the reasons for limiting the heat treatment conditions will be described. The upper limit of the 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 is stable. . That is, when heated to a temperature of Ac ₃ transformation point + 200 ° C. or more, the carbide completely dissolves in solid solution, so that a large amount of Cr carbide and the like precipitates on the grain boundaries during cooling, the corrosion resistance is significantly reduced, and the crystal grain becomes coarser. As a result, the toughness decreases. Also, Ac ₃ transformation point +10
When heated to a low temperature of ℃ or less, 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.
˜Ac ₃ transformation point + 200 ° C.

The martensitic stainless steel thus heated is heated at a temperature of from 800 ° C. to a cooling start temperature of 600 to 350 ° C. to a cooling stop temperature of 2 ° C./sec.
Cool at the above speed. The reason for setting the controlled cooling conditions is to pass the temperature range of 800 to 600 ° C. where the plate-shaped Cr-based carbide precipitates in a short time to suppress the precipitation of carbides. However, rapid cooling down to 350 ° C. or lower is likely to cause cracking, so the rapid cooling must be stopped at 350 ° C. or higher. On the other hand, at 600 to 800 ° C., nucleation / growth of carbides is fast, and at a cooling rate slower than 2 ° C./sec, plate-like carbides are precipitated at grain boundaries.

When the steel cooled to the temperature of 600 to 350 ° C. is further cooled to room temperature at a rate higher than the air cooling rate, martensite transformation occurs and becomes a martensite single phase structure. The residual stress in this 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. The steel pipe manufactured by the method of the present invention as described above is excellent in toughness and stress corrosion cracking resistance.

[0021]

EXAMPLES Steels having the chemical composition shown in Table 1 were cast in a usual melting process, and then a steel pipe was manufactured by hot rolling, which was subjected to heat treatment and tempering treatment. The toughness, CO ₂ corrosion resistance, and sulfide stress cracking resistance were investigated.
The heat treatment temperature at that time, the cooling rate between 800 and 600 ° C., and materials such as strength are shown in Table 2. 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 / year or less, it can be considered to have corrosion resistance. Resistance to sulfide stress cracking was measured by applying a round bar tensile test piece to 1% in a 5% NaCl solution at 25 ° C.
A uniaxial tensile stress was applied in a corrosive environment saturated with 99% CO ₂ + 1% H ₂ S gas at atmospheric pressure, and the ratio (Rs value) of the maximum initial stress and the yield stress at which breakage did not occur at 720 hours was determined. R
If s ≧ 0.8, it can be said that the characteristics are excellent.

[0022]

[Table 1]

[0023]

[Table 2]

From the results shown in Table 2, the steel pipe produced by the method of the present invention exhibits 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. For steel pipes cracked by hot working, test pieces were cut out from the part without cracks.

[0025]

INDUSTRIAL APPLICABILITY As described above, according to the present invention, the martensitic stainless steel seam having good hot workability and excellent toughness and stress corrosion cracking resistance is specified by specifying the steel composition and thermo-mechanical treatment conditions. Can manufacture steelless pipes.

[Brief description of drawings]

FIG. 1 shows the CO ₂ corrosion resistance characteristics and the reduction value during hot working when the C and N contents were changed with 1.5% Ni-12.5% Cr steel as the base component.

Claims (2)

[Claims] 1. C by weight <0.05%, Si <0.50%, Mn <1.0%, P <0.03%, S <0.01%, Cr: 11-17%, Ni : 1.5 to 5%, Mo: 0.5 to 2%, Al ≦ 0.05%, N: 0.02 to 0.1%, and C + 0.8N> 0.
Steel containing a component satisfying 06 and the balance substantially consisting of Fe and unavoidable impurities is hot-worked and allowed to cool naturally to room temperature, and then Ac ₃ transformation point + 10 ° C or more Ac ₃ transformation point +20
It is heated to a temperature of 0 ° C or lower, and is cooled at a rate of 2 ° C / sec or higher from the heating start temperature of 800 ° C to the cooling stop temperature of 600 ° C to 350 ° C, and then to the room temperature by air cooling. A method for producing a seamless martensitic stainless steel pipe excellent in toughness and stress corrosion cracking resistance, characterized by performing tempering at a temperature not higher than an Ac ₁ transformation point after cooling at the above rate. 2. 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%, Mo: 0.5 to 2%, Al ≦ 0.05%, N: 0.02 to 0.1%, and C + 0.8N> 0.
Steel satisfying No. 06 and containing Cu: 0.5 to 2% and the balance substantially consisting of Fe and unavoidable impurities is hot-worked and naturally cooled to room temperature, and then Ac ₃
2 ° C / sec from the heating start temperature to 800 ° C to the cooling start temperature to the cooling stop temperature of 600 ° C to 350 ° C by heating to a temperature of not less than the transformation point + 10 ° C and not more than Ac ₃ transformation point + 200 ° C.
Excellent toughness and stress corrosion cracking resistance, characterized by cooling at the above rate, then cooling to room temperature at the rate of air cooling or more, and then tempering at a temperature below the Ac ₁ transformation point. Martensitic stainless steel seamless steel pipe manufacturing method.