JPH07179943A - Production of high toughness martensitic strainless steel pipe excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To provide a method for producing a martensitic stainless steel pipe excellent in corrosion resistance. CONSTITUTION:A steel contg. components constituted of, by weight, <=0.05% C, <=0.50% Si, <=1.0% Mn, <=0.03% P, <=0.01% S, 11 to 17% Cr, 0.2 to 4.0% Cu, 0.5 to 2% Mo, <=0.05% Al and 0.02 to 0.l% N and satisfysing C+0.8N>0.06 is subjected to hot working, is naturally air-cooled to a room temp., is thereafter heated to a temp. from the Ac3+10 deg.C to the Ac3200 deg.C is cooled to a room temp. and is subsequently subjected to tempering treatment at the Ac1 point or below. Thus, its CO2 corrosion resistance is made good in the case the content of C is regulated to <=0.05% and Cr, Cu and Ni are added in the proper combination, and its crack sensitibity in an environment contg. H2S improves by the addition of the same elements + Mo.

Description

Detailed Description of the Invention

[0001]

The present invention has excellent CO ₂ corrosion resistance and sulfide stress cracking resistance (hereinafter referred to as SSC resistance. S
SC: abbreviation for Sulfide Stress Cracking) and a method for producing a high toughness martensitic stainless steel seamless steel pipe.

[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, as a martensitic stainless steel excellent in corrosion resistance, C: less than 0.015%, Si: 0.1 to 0. 0, as disclosed in Japanese Patent Publication No. 59-15977.
8%, Mn: 0.1 to 2.0%, Cr: 11 to 17%,
Ni: 0.1-3.0%, Nb: 0.01-0.05
%, N: 0.015% or less, Al: 0.01 to 0.10.
%, And if necessary, a seamless steel pipe having a martensite structure containing a small amount of Co or Cu. These steels are conventional steels, SUS410, SUS420J1, J
Since the C content is limited to be lower than that of No. 2 and the like, the precipitation amount of Cr carbide is small, and the amount of solid solution Cr effective for corrosion resistance can be secured, so that the corrosion resistance is improved. Also,
To improve the corrosion resistance in CO ₂ environment, Ni, C
It has also been clarified that the combined addition of u is effective.

[0003]

However, when the addition amounts of C and N are remarkably reduced in the martensitic stainless steel as described above, the δ ferrite phase which deteriorates the hot workability in the austenite matrix during the heating of the steel ingot is formed. It has the drawback of being easy to generate. Therefore, under severe processing conditions such as seamless rolling, cracks and flaws are generated, and it is unavoidable that the yield is reduced and the cost is increased. Further, although the corrosion resistance is improved by adding Ni and Cu in combination, there is a problem that minute cracks are generated due to uneven corrosion in an environment containing H ₂ S. Therefore, it has heretofore been difficult to manufacture a martensitic stainless steel seamless steel pipe having excellent CO ₂ corrosion resistance and SSC resistance.

[0004]

The present inventors have found from many experimental results that the CO ₂ corrosion resistance reduces C and reduces the required amount of Cr,
It can be maintained by adding Ni and Cu, SSC resistance
It was found that the property is improved by controlling the structure showing crack resistance and adding Mo. The hot workability is
To reduce the formation of inclusions by reducing P, S, etc.,
It has been found that a metallurgical operation is performed to control the phase fraction and shape of different phases having different deformation resistances by controlling the addition amounts of C and N and further adding Ni.

In particular, the present inventors first focused on the effects of C and N and obtained the following findings. In FIG. 1, C and N are used as base components with 2% Cu-4% Ni-15% Cr steel.
The CO ₂ corrosion resistance characteristics when the content is changed and the drawing value during hot working are shown. In FIG. 1, C.I. R. Is 40 at
is the annual corrosion rate in 200 ° C. artificial seawater equilibrated with m ₂ CO ₂ . R. If it is <0.1 mm / y, it can be evaluated as having sufficient corrosion resistance. In addition, R. A. Is 1
It is the reduction ratio when a sample heated to 250 ° C. is uniaxially tensile deformed at a strain rate of 3 sec ⁻¹ at 900 ° C.
A. If it is> 70%, it can be said that the hot deformability is good. In the CO ₂ 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 is necessary to set C <0.05% in order to satisfy the CO ₂ corrosion resistance characteristic, and to set C + 0.8N> 0.06 in order to have good hot workability. It can be read that it is necessary (the unit of the content of each element symbol is wt.%).

Next, in FIG. 2, 2% Cu-4% Ni-when artificial seawater was used in a CO ₂ environment of 25 ° C. and 4 MPa.
Partial pressure of H ₂ S, Mo on crack susceptibility of 15% Cr steel
The effect of quantity is shown. If the Mo content is 0.5% or less, SS
It can be read that C or minute cracks occur, but cracks do not occur when the content exceeds 0.5%. This is Ni,
This is because the combined addition of Cu reduces the amount of hydrogen intrusion accompanying the suppression of general corrosion, and the addition of Mo further reduces the local corrosion.

The present invention is constructed by combining the findings described above, and the gist thereof is as follows.
C ≦ 0.05%, Si ≦ 0.50%, Mn
≦ 1.0%, P ≦ 0.03%, S ≦ 0.0
1%, Cr: 11 to 17%, Ni: 1.5
~ 5%, Cu: 0.2-4%, Mo: 0.5-
2%, Al ≦ 0.05%, N: 0.02 to 0.
Component satisfying 1% and C + 0.8N> 0.06 (w
t. %), With the balance consisting essentially of Fe and unavoidable impurities, after hot working and spontaneous cooling to room temperature,
It is heated to a temperature of Ac ₃ transformation point + 10 ° C to Ac ₃ transformation point + 200 ° C and then cooled to room temperature at a cooling rate of air cooling or higher, and subsequently tempered at a temperature of Ac ₁ transformation point or lower, or After the steel having the above components is hot worked and naturally cooled to room temperature, the Ac ₃ transformation point + 10 ° C to the Ac ₃ transformation point +
Heating to a temperature of 200 ° C. and cooling to room temperature at a cooling rate of air cooling or higher, heating to a temperature of Ac ₁ transformation point to Ac ₃ transformation point and cooling to room temperature at a rate of air cooling or higher, and subsequently Ac ₁ transformation This is a method for producing a seamless martensitic stainless steel pipe, which is excellent in corrosion resistance and is tempered at a temperature below the point.

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. Added to. 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 set to 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. 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 a single phase of austenite. 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, 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%. 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 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 is CO only when combined with Ni.
₂ Effective to reduce the corrosion rate in the environment. Therefore, Ni and Cu are always added at the same time. It is also an austenite stabilizing element and has the advantage of not lowering the Ac ₁ transformation point. However, if the content is 0.2% or less, the effect of improving the corrosion resistance is not sufficient, and if it exceeds 4%, it is excessively segregated at the grain boundaries during heating to reduce the grain boundary strength, resulting in a decrease in hot workability. Therefore, add 0.2
The range was limited to 4%.

Similar to Si, Al is added as a deoxidizer, and if added in excess of 0.05%, Al is added.
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 C + 0.8N>
It is effective in the range of the added amount satisfying 0.06 (C and N are wt.%). Therefore, when C <0.05%, it is necessary to add 0.02% or more of N in order to obtain a good hot workability without generating a ferrite phase in the hot working temperature range. Also, 0.1% in the usual melting process
Since the above addition is difficult, the range of addition amount is set to 0.
It was set to 02 to 0.1%.

Mo is an element effective for improving pitting corrosion resistance and is essential as an additional element because it has an effect of reducing cracking susceptibility in an environment containing H ₂ S.
However, the effect is small with the addition of 0.5% or less. Also, since Mo alone does not improve the general corrosion characteristics, it is necessary to add Cu and Ni in combination. Furthermore, 2
%, The above effect is not improved, and since it is a strong ferrite stabilizing element and the addition of more than 2% tends to generate the δ phase, the limited range is 0.5.
~ 2%.

Next, the reasons for limiting the heat treatment conditions will be described. First, the austenitizing heating temperature is the upper limit of the temperature at which carbides do not completely form a solid solution and coarsening of crystal grains does not occur in the γ loop of Cr-containing stainless steel, and the minimum temperature at which the austenite layer becomes stable. Was set as the lower limit.
That is, when heated to a temperature of Ac ₃ transformation point + 200 ° 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, the corrosion resistance is significantly reduced, and the crystal grains are coarsened. As a result, the toughness decreases.
When heated to a low temperature of Ac ₃ transformation point + 10 ° C. or lower, the austenite phase is not stabilized 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 at the grain boundaries 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 this martensitic structure is eliminated by recovery, and supersaturated carbon atoms are precipitated as carbides to enhance toughness and ductility, and tempering treatment is performed to obtain desired strength. In particular, since Mo and Cu are added to this component system and precipitation of Mo carbide and Cu clusters is caused by the tempering treatment, the grade of oil country tubular goods can be tempered to a high strength of C95 or higher, while Ni is contained. Very high toughness is obtained. In addition, during this tempering treatment, Ac ₁
When heated to a temperature above the 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.

Further, for the purpose of obtaining excellent toughness and SSC resistance, if necessary, before the tempering treatment after the austenitizing treatment, the temperature range from the Ac ₁ transformation point to the Ac ₃ transformation point may be satisfied. A two-phase region heat treatment is performed by heating to the above. This aims to temper the steel to a relatively low strength which cannot be obtained by a single tempering treatment. By heating to this temperature range, Cu and Mo
Since the size of the carbides is increased, the precipitation hardenability is reduced, and it is possible to temper even lower strength. C9 using this process
By tempering to a grade 5 or lower strength, it becomes possible to impart more excellent toughness and SSC resistance to the steel. The steel pipe manufactured by the method of the present invention as described above is excellent in CO ₂ corrosion resistance, SSC resistance and toughness.

[0019]

EXAMPLE Steels having the chemical composition shown in Table 1 were cast in a usual melting process and then hot rolled to produce steel pipes. First, the surface of the as-rolled steel pipe was observed to investigate the occurrence of flaws. Next, the heat treatment and the tempering treatment were performed, and the strength, toughness, CO ₂ corrosion resistance and SSC resistance were investigated. Table 2 shows materials such as heat treatment temperature and strength at that time.

The resistance to CO ₂ corrosion was evaluated by the corrosion rate (mm / y) in 200 ° C. artificial seawater equilibrated with CO _{2 at} 40 atm. If the corrosion rate is 0.1 mm / y or less, it can be considered to have corrosion resistance. The SSC resistance was measured by applying a round bar tensile test piece to a 5% NaCl solution at 25 ° C. under 1 atmosphere of 90% CO ₂ + 10% H.
_{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.

[0021]

[Table 1]

[0022]

[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, SSC resistance and high toughness, whereas the comparative pipes out of the scope of the present invention show It is clear that these properties are inferior.

[0024]

As described above, according to the present invention, the martensitic stainless steel seamless steel having excellent CO ₂ corrosion resistance, SCC resistance and high toughness can be obtained by specifying the constituent elements and the treatment conditions. A steel pipe can be easily obtained.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between C and N contents, CO ₂ corrosion resistance, and drawing value during hot working.

Fig. 2 H ₂ influence on cracking susceptibility using artificial seawater
The figure which shows the relationship between S partial pressure and Mo content.

Claims (2)

[Claims] 1. C ≦ 0.05%, Si ≦ 0.50%, Mn ≦ 1.0%, P ≦ 0.03%, S ≦ 0.01%, Cr: 11-17%, Ni: 1 0.5 to 5%, Cu: 0.2 to 4%, Mo: 0.5 to 2%, Al ≦ 0.05%, N: 0.02 to 0.1%, and C + 0.8N> 0.06 Content (wt.%) Is satisfied, and the balance is substantially Fe.
And, steel consisting of unavoidable impurities is hot-worked and naturally cooled to room temperature, then heated to a temperature of Ac ₃ transformation point + 10 ° C to Ac ₃ transformation point + 200 ° C, and then cooled to room temperature at a cooling rate of air cooling or higher. Then, a method of producing a high toughness martensitic stainless steel seamless steel pipe having excellent corrosion resistance, which is characterized by performing a tempering treatment at a temperature not higher than the Ac ₁ transformation point. 2. C ≦ 0.05%, Si ≦ 0.50%, Mn ≦ 1.0%, P ≦ 0.03%, S ≦ 0.01%, Cr: 11-17%, Ni: 1 0.5 to 5%, Cu: 0.2 to 4%, Mo: 0.5 to 2%, Al ≦ 0.05%, N: 0.02 to 0.1%, and C + 0.8N> 0.06 Content (wt.%) Is satisfied, and the balance is substantially Fe.
And, steel consisting of unavoidable impurities is hot-worked and naturally cooled to room temperature, then heated to a temperature of Ac ₃ transformation point + 10 ° C to Ac ₃ transformation point + 200 ° C, and subsequently cooled to room temperature at a rate of air cooling or higher. , Further heating to a temperature from the Ac ₁ transformation point to the Ac ₃ transformation point, and then cooling to room temperature at a rate of air cooling or higher,
Thereafter, a method of producing a high toughness martensitic stainless steel seamless steel pipe having excellent corrosion resistance, which is characterized by performing a tempering treatment at a temperature not higher than an Ac ₁ transformation point.