JPH08260038A - Production of martensitic stainless steel excellent in co2 corrosion resistance and sulfide stress cracking resistance - Google Patents

Abstract

PURPOSE: To provide heat treatment conditions for refining a martensitic stainless steel into low strength. CONSTITUTION: A martensitic stainless steel, having a composition consisting of 0.005-0.05% C, 1.5-6% Ni, 11-16% Cr, 0.5-3% Mo, and the balance essentially Fe with inevitable impurities or further containing 0.2-4% Cu, is used. This steel is hot-worked, naturally air-cooled down to room temp., heated up to a temp. at which the fraction of austenitic phase becomes 3-60% by area ratio in a two-phase temp. region between the Ac1 point and the Ac3 point, and naturally air-cooled down to room temp., and further, if necessary, this steel is heated up to a temp. between (Ac3 +30 deg.C) and (Ac3 +200 deg.C) and cooled down to room temp. prior to the heating treatment in the two-phase region and then tempered at a temp. between Ac1 and (Ac1 -150 deg.C) after the heating treatment in the two-phase region. The resultant martensitic stainless steel whose strength is regulated to a low value has excellent toughness and sulfide stress cracking resistance.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to CO ₂ corrosion resistance and sulfide stress crackin resistance (SSC), which are extremely important characteristics especially when used as oil country tubular goods.
g) The present invention relates to a method for producing martensitic stainless steel having excellent properties.

[0002]

2. Description of the Related Art In recent years, CO ₂ for the development and secondary recovery of oil wells and gas wells that produce gas containing large amounts of CO _2.
Injection has become widespread. In such an environment, the corrosion of the steel pipe is severe, so that martensitic stainless steel pipes excellent in CO ₂ corrosion resistance are often used. So far, the most commonly used steel is AISI type 410, 420 series steel.

However, in recent years, with the deepening of wells in oil wells, there is an increasing need for steels having CO ₂ corrosion resistance at higher temperatures and crack resistance under H ₂ S-containing environment. As a martensitic stainless steel having improved CO ₂ corrosion resistance and SSC resistance, Japanese Patent Publication No. 6-43626 discloses Ni- and Mo-added steels and JP-A-2-217444. Japanese Patent Laid-Open Publication proposes a steel containing Cu and Ni, Mo added as selective elements.

[0004]

However, when a few% of Ni is added to such a steel, the Ac ₁ point becomes so low that it becomes 600 ° C. or less, so that the tempering process is limited to a low temperature range and tempering is possible. The strength range is API (American Petroleum Institute) standard, P110 grade (YS (yield stress) is 758-86.
It has the drawback of being limited to very high strengths of 1 MPa)) or more. As the strength of the material increases, the crack resistance (SSC resistance) of the material in an H ₂ S-containing environment decreases, see “Corrosion Damage of Metals and Anticorrosion Technology,” published by Agne Inc. (19
83) p197 and the like.

Therefore, in order to obtain good sour resistance in these steels, it is necessary to find manufacturing conditions capable of adjusting to L80 grade (YS; 551 to 655 MPa) or C95 grade (YS; 655 to 758 MPa). It is necessary to clarify the effect of additional elements.

[0006] In response to such a need, if the two-phase region heat treatment is performed before the tempering treatment, the tempering can be performed to a relatively low strength,
As a result, the toughness is improved.
No. 18 publication. However, the effect on SSC resistance has not been clarified. Furthermore, as a result of detailed research after that, not only the heat treatment may not be performed to low strength depending on the condition even if the two-phase heat treatment is applied, the stress-strain curve of the tensile test shows an extremely low elastic limit and is stable. It has become clear that it may be difficult to obtain such strength.

[0007]

DISCLOSURE OF THE INVENTION The present inventors have found by many studies that the SSC resistance is first improved by adding Mo and lowering the material strength. 1% Mo-containing steel (◆, ◇ in the figure), Mo-free steel (●,
The influence of H ₂ S partial pressure and pH on SSC sensitivity when (○) is used is shown. ◇ and ○ indicate that no cracks occurred, and ◆ and ● indicate that cracks occurred. 1% Mo
Addition of SSC decreases the critical pH at which SSC is generated from 5 to 4, and it is clearly recognized that the sour resistance is improved. Further, in such a martensitic stainless steel, in order to further improve the SSC resistance, it is better to lower the material strength as described above.

Therefore, the present inventors have investigated in detail the heat treatment characteristics of the martensitic stainless steel containing Mo or Cu which has a strong ability to precipitate during tempering and increase the material strength by precipitation strengthening. As a result, Ac
Ferrite / austenite with ₁ or more and Ac ₃ or less 2
It was found that YS can be reduced by heating in the phase region.

When heated to the temperature of the two-phase region, a structure in which the reverse-transformed austenite phase is mixed in the matrix mainly composed of the tempered martensite phase is formed. At this time, C in the mother phase
Although carbides such as r and Mo are precipitated, the increase in strength due to precipitation strengthening is small because they are overaged. Further, in the austenite phase generated by the reverse transformation, C, Cu, Ni
The austenite-stabilizing elements such as and the like are concentrated, and the concentration of these elements is decreased in the matrix. With the above mechanism, the strength can be stably reduced by heating to the two-phase region temperature.

It has been clarified that this strength lowering effect depends on the austenite phase fraction. 0.03% C in Figure 2
-1.1% Cu-4.7% Ni-12% Cr-1.7%
The heat treatment conditions that affect YS (0.2% offset yield strength) of Mo-0.05% N steel and the relationship between the austenite phase fraction at the two-phase region temperature are shown. This steel has an Ac ₁ transformation point of 615 ° C. and an Ac ₃ transformation point of 820 ° C. If the austenite phase fraction is less than 3%, the target L8
It is difficult to adjust the strength to 0 (YS; 551 to 654 MPa) and C95 (YS; 654 to 758 MPa). Further, if the austenite phase fraction exceeds 60%, Ac ₁ after the two-phase region heat treatment
It is clear that when tempered just below the transformation point, it cannot be tempered to a strength of C95 or lower. Therefore, the optimum tempering condition is the temperature at which the austenite phase fraction is 3 to 60% in the two-phase region temperature range. Furthermore, in order to stabilize the tensile strength at a low level and to reduce the substantial strength, it has been found that it is better to carry out a normalizing treatment before the two-phase region heat treatment or a tempering treatment after the two-phase region heat treatment. Obtained.

The present invention is constructed on the basis of the above findings, and its gist is as follows. Ie% by weight
And C: 0.005 to 0.05%, Ni: 1.
5-6%, Cr: 11-16%, M
o: 0.5 to 3% or further Cu: 0.2 to 4%, the balance being substantially Fe and unavoidable impurities after martensitic stainless steel is hot worked and naturally cooled to room temperature. , Ferrite + austenite 2 phase temperature range, austenite phase fraction is 3% or more in area ratio 6
Heating to a temperature of 0% or less and cooling to room temperature, and if necessary, heating to a temperature of Ac ₃ + 30 ° C. or more and Ac ₃ + 200 ° C. or less and cooling to room temperature before the two-phase region heat treatment, In addition, if necessary, after the two-phase region heat treatment, Ac
_A method for producing martensitic stainless steel having excellent CO ₂ corrosion resistance and sulfide stress cracking resistance, which is characterized by performing a tempering treatment at ₁ point or less and Ac ₁ −150 ° C. or more.

The contents of the present invention will be described in more detail below. First, the reasons for limiting the components of the martensitic stainless steel in the present invention will be described below. C: C is the most powerful austenite stabilizing element,
It has the effect of suppressing the formation of the δ-ferrite phase that causes scratches and cracks during hot working. Therefore, it is desirable to add it. However, if the addition amount is 0.005% or less, the above effect is small, and therefore it is necessary to add more than that. On the other hand, not only does the amount of solid solution Cr, which is effective in corrosion resistance, decrease due to the formation of carbides by combining with Cr during heat treatment, and the precipitated carbides become cathode reaction sites, which promotes the corrosion reaction and thus decreases corrosion resistance. In particular, when the C content exceeds 0.05%, the corrosion resistance remarkably decreases, so the upper limit of the addition amount was made. In order to achieve both austenite phase stability during hot working and more stable high corrosion resistance, 0.01 to 0.03 is required.
It is desirable to adjust to the range of%.

Cr: Cr is an element necessary for improving general corrosion resistance. Since the effect is small when the added amount is a small amount less than 11%, the lower limit of the added amount is set to 11%. Further, Cr is a strong ferrite stabilizing element, and if added in excess of 16%, a δ phase is generated during hot working, resulting in deterioration of hot workability. Therefore, the range of the added amount is set to be 11 to 16%. Desirably, 12 to 15% is the optimum addition range.

Ni: Ni is an austenite stabilizing element and has the effect of suppressing the formation of the δ phase in the hot working temperature range. Further, even if the δ phase is present, the morphology is controlled, and the hot workability is harmed. The effect is 1.
Since it is brought about by the addition of 5% or more, the minimum addition amount is set to 1.5%. However, the addition of a large amount increases the cost and destabilizes martensite, so the upper limit was made 6%. Desirably 3 ~
Addition of 5% is optimal.

Mo: As described with reference to FIG. 1, Mo has the effect of improving the local corrosion resistance in an environment containing H ₂ S, suppressing microcracks, and improving the SSC resistance.
The effect is not sufficient if less than 0.5% is added, so the minimum value of the added amount was set to 0.5%. On the other hand, it is a strong ferrite stabilizing element, and if it is added in excess of 3%, a δ phase is generated in the hot working temperature range, resulting in deterioration of hot workability. From such knowledge, the range of addition amount is 0.5-
It was 3%. However, in order to impart more stable SSC resistance, addition of 1% or more is desirable.

Cu: Cu has the function of improving the corrosion resistance in an environment containing CO ₂ . In particular, since it is possible to raise the corrosion resistance limit temperature to about 200 ° C. by adding Ni in combination, it is desirable to add Ni when corrosion resistance in a CO ₂ -containing environment is required. However, if less than 0.2% is added, the effect is insufficient. Further, if added excessively, it segregates at the austenite grain boundaries in the hot working temperature range, lowering the grain boundary strength and degrading the hot workability. In particular, this decrease in hot workability becomes remarkable when the Cu content exceeds 4%. Therefore, the Cu addition range is set to 0.2 to 4%. It is desirable to add 0.5% or more.

Steel having such a chemical composition shows a structure in which most of it is composed of martensite when hot-worked and allowed to cool to room temperature. Ac ₁ or more points Ac
_When heated to two-phase temperature below ₃ points, supersaturated C atoms combine with Fe, Cr, Mo, etc. and precipitate in the form of carbides, so-called tempered martensite structure, and old austenite grain boundaries or C, Ni, etc. In the region in which the austenite stabilizing element is concentrated, a two-phase coexisting structure having a majority of reverse transformed austenite formed by aggregation of austenite stabilizing elements such as C and Ni is exhibited. At this time, in the tempered martensite structure, the precipitated carbides agglomerate and coarsen to re-dissolve C and newly generate a reverse transformation austenite phase, or solid-phase diffusion of C into a nearby austenite phase. Occurs repeatedly.

In this way, the C and Ni concentrations in the tempered martensite, which is the matrix phase, decrease, and the effects of precipitation strengthening and solid solution strengthening decrease, so the overall strength decreases. At this time, as shown in FIG. 2, when the austenite phase fraction is 3% or more and 60% or less, it is clear that YS is stable in the L80, C95 class. On the other hand, when the austenite phase fraction is 3% or less, the strength extremely decreases to L80 class or less. It is considered that this is because the austenite phase is stabilized by air cooling after heating in the two-phase region and a part of retained austenite is included without undergoing martensite transformation. When heated to a temperature at which the austenite fraction exceeds 60%, C, Ni, etc. cannot be sufficiently diffused because the austenite phase is excessively present and the difference in the C, Ni, etc. concentrations is less likely to occur. Along with this, it is considered that the strength of the matrix is hardly reduced and the strength does not change as compared with the usual quenching (or normalizing) / tempering treatment. Therefore, the heat treatment temperature of the austenite + ferrite two-phase region must be between 3% and 60% in terms of austenite phase fraction. In particular, since the strength-reducing effect is large in the austenite phase fraction between 3% and 40%, it is desirable to heat to a temperature range that brings about this.

Further, the more excellent sour resistance and toughness can be obtained by normalizing before the two-phase region heat treatment, or further,
It was also clarified that it is possible to stably impart the material by performing the tempering treatment after the two-phase region heat treatment.

First, the normalizing treatment has the effect of adjusting the austenite grain size during heat treatment in the two-phase region to an equivalent size and shape, thereby making the size and shape of the reverse-transformed austenite during heating in the two-phase region substantially equal. On the basis of the above, it is performed as necessary for the purpose of stably providing the strength lowering effect. However, when the heating temperature at this time is Ac ₃ + 30 ° C. or lower, precipitation of Cr, Mo carbides or carbonitrides occurs in the austenite phase, and even if the two-phase region heat treatment is performed after that, it is sufficiently redissolved. However, the strength is not sufficiently reduced because the precipitation strengthening effect is brought about. Also, the heating temperature is Ac ₃
When the temperature is + 200 ° C. or higher, austenite grains rapidly grow due to the complete solid solution of carbides, and so-called coarsening of crystal grains occurs. As described above, when the initial austenite grain size is coarse, the reverse transformation austenite during heating in the two-phase region is nonuniformly generated, so that not only stable strength reduction cannot be obtained but also toughness decreases. Therefore, the heating temperature of the normalizing treatment is Ac ₃ + 30 ° C. to Ac ₃
The range of + 200 ° C is preferred. In particular, in order to stabilize the effect of heat treatment in the two-phase region, Ac ₃ + 50 ° C to A
It is desirable to heat in the range of c ₃ + 150 ° C. After the austenitizing treatment is performed at such a temperature, the temperature may be cooled to room temperature at a rate of air cooling or more, and subsequently the two-phase region heat treatment may be performed.

If necessary, a tempering treatment is applied after the two-phase region heat treatment. This treatment causes precipitation of solid solution C and the like remaining in the martensite structure after the heating in the two-phase region and recovery of dislocations, and stabilizes the strength and improves the toughness. Although it increases, the tensile strength decreases and it becomes possible to obtain more stable strength and toughness. However, the heating temperature is Ac ₁ point-
When the temperature is 150 ° C. or less, Cr carbide and the like are finely precipitated to bring about a precipitation strengthening action, so that the strength unnecessarily increases, so the lower limit of the heating temperature was set. Also, Ac
_{Since the} heat treatment exceeding ₁ point is meaningless because it repeats the two-phase region heat treatment, the upper limit of the heating temperature of the tempering treatment was set. Desirably, Ac ₁ point −100 ° C. to Ac ₁
Heating between the points has a greater effect on strength reduction and strength stabilization.

As described above, even in the martensitic stainless steel having a low Ac ₁ point and a high tempering temperature that cannot be set, the two-phase heat treatment is adopted after the hot working, or further, if necessary, By performing either one or both of the normalizing treatment after the hot working and before the heat treatment in the two-phase region and the tempering treatment after the heat treatment in the two-phase region, it is possible to perform heat treatment to L80 class or C95 class, sour resistance and It is possible to improve toughness.

[0023]

EXAMPLES The contents of the present invention will be described in more detail with reference to the following examples. Using the steels having the components shown in Table 1, the strength / toughness, CO ₂ corrosion resistance and SSC resistance were examined. Transformation points (Ac ₁ , Ac ₃ , M _s , M _f ) measured for each steel
Is also shown in Table 1.

Table 2 shows the results of heat treatment conditions and properties applied to each steel. YS is 0.2% offset proof stress.
The toughness was evaluated by the impact absorption energy value (vE- ₄₀ ; J) when a Charpy impact test was performed at -40 ° C using a test piece with a 2 mm V notch. CO ₂ corrosion resistance is 16
The corrosion rate (CR; mm / y) in an environment where 4 MPa of CO ₂ was saturated in 0 ° C. artificial seawater was shown. If the corrosion rate is 0.1 mm / y or less, it can be said that it has corrosion resistance in this environment. Further, the SSC resistance is Rs value (= Rs value when a constant load tensile test is performed in an environment in which a mixed gas of 10% H ₂ S + N ₂ is saturated in a 5% NaCl solution having a pH of 3.5.
It was evaluated by σ _th / YS). σ _th is a threshold stress value at which breakage does not occur at 720 hours. If Rs is 0.8 or more, it can be evaluated as having SSC resistance.

[0025]

[Table 1]

[0026]

[Table 2]

The steels of the present invention are all L8 grade oil well pipes.
Class 0 (YS; 551 MPa to 655 MPa) or Class C95 (YS; 65
It can be seen that it has excellent toughness, CO ₂ corrosion resistance and SSC resistance at the same time as having 5 MPa to 758 MPa. On the other hand, it is clear that each of the comparative steels has a strength deviated from the L80 grade and the C95 grade, or one of the above characteristics is inferior to the invention steel.

[0028]

As described above, the martensitic stainless steel whose strength is adjusted to be low by the method of the present invention is
It has good toughness, excellent CO ₂ corrosion resistance, and sulfide stress cracking resistance.

[Brief description of drawings]

FIG. 1 is a diagram showing the effect of the amount of Mo on cracking susceptibility in a hydrogen-containing flow-containing environment.

FIG. 2 is a diagram showing heat treatment conditions affecting strength (YS = 0.2% offset proof stress) and austenite phase fraction during heating in a two-phase region.

Claims (8)

[Claims] 1. By weight%, C: 0.005 to 0.05%, Ni: 1.5 to 6%, Cr: 11 to 16%, Mo: 0.5 to 3%, the balance being substantially After martensitic stainless steel consisting of Fe and unavoidable impurities is hot worked and naturally cooled to room temperature, Ac ₁ point or more A
c Austenite phase fraction in the area of 2 phases of ferrite + austenite of ₃ points or less is 3% to 60% in area ratio
A method for producing martensitic stainless steel excellent in CO ₂ corrosion resistance and sulfide stress cracking resistance, characterized by heating to the temperature below and cooling to room temperature. 2. By weight%, C: 0.005 to 0.05%, Ni: 1.5 to 6%, Cr: 11 to 16%, Mo: 0.5 to 3%, and the balance substantially. After martensitic stainless steel consisting of Fe and unavoidable impurities is hot worked and naturally cooled to room temperature, Ac ₁ point or more A
c Austenite phase fraction in the area of 2 phases of ferrite + austenite of ₃ points or less is 3% to 60% in area ratio
And heated to a temperature equal to or less than the cooling to room temperature, excellent resistance to CO ₂ corrosion properties as well as sulfide stress cracking resistance, characterized in that the tempering treatment at Ac ₁ point below Ac ₁ -150 ° C. or more temperature Martensite Site-based stainless steel manufacturing method. 3. By weight%, C: 0.005 to 0.05%, Ni: 1.5 to 6%, Cr: 11 to 16%, Mo: 0.5 to 3%, and the balance substantially. Of martensitic stainless steel consisting of Fe and unavoidable impurities is hot-worked and naturally cooled to room temperature, then Ac ₃ + 30 ° C
Above, it is heated to a temperature of Ac ₃ + 200 ° C or less and cooled to room temperature, and then ferrite of Ac ₁ point or more and Ac ₃ points or less +
Excellent in CO ₂ corrosion resistance and sulfide stress cracking resistance characterized by heating to a temperature at which the austenite phase fraction is 3% or more and 60% or less in area ratio in the austenite two-phase temperature range and cooling to room temperature Martensitic stainless steel manufacturing method. 4. By weight%, C: 0.005 to 0.05%, Ni: 1.5 to 6%, Cr: 11 to 16%, Mo: 0.5 to 3%, and the balance substantially. Of martensitic stainless steel consisting of Fe and unavoidable impurities is hot-worked and naturally cooled to room temperature, then Ac ₃ + 30 ° C
Above, it is heated to a temperature of Ac ₃ + 200 ° C or less and cooled to room temperature, and then ferrite of Ac ₁ point or more and Ac ₃ points or less +
In the austenite two-phase temperature range, heating to a temperature at which the austenite phase fraction is 3% or more and 60% or less in area ratio, cooling to room temperature, and tempering at a temperature of Ac ₁ point or less and Ac ₁ -150 ° C or more, A method for producing martensitic stainless steel having excellent CO ₂ corrosion resistance and sulfide stress cracking resistance. 5. By weight%, C: 0.005 to 0.05%, Cu: 0.2 to 4%, Ni: 1.5 to 6%, Cr: 11 to 16%, Mo: 0.5. After martensite stainless steel containing ~ 3% and the balance consisting essentially of Fe and unavoidable impurities is hot-worked and naturally cooled to room temperature, Ac ₁ point or more A
c Austenite phase fraction in the area of 2 phases of ferrite + austenite of ₃ points or less is 3% to 60% in area ratio
A method for producing martensitic stainless steel excellent in CO ₂ corrosion resistance and sulfide stress cracking resistance, characterized by heating to the temperature below and cooling to room temperature. 6. By weight%, C: 0.005 to 0.05%, Cu: 0.2 to 4%, Ni: 1.5 to 6%, Cr: 11 to 16%, Mo: 0.5. After martensite stainless steel containing ~ 3% and the balance consisting essentially of Fe and unavoidable impurities is hot-worked and naturally cooled to room temperature, Ac ₁ point or more A
c Austenite phase fraction in the area of 2 phases of ferrite + austenite of ₃ points or less is 3% to 60% in area ratio
And heated to a temperature equal to or less than the cooling to room temperature, excellent resistance to CO ₂ corrosion properties as well as sulfide stress cracking resistance, characterized in that the tempering treatment at Ac ₁ point below Ac ₁ -150 ° C. or more temperature Martensite Site-based stainless steel manufacturing method. 7. By weight%, C: 0.005 to 0.05%, Cu: 0.2 to 4%, Ni: 1.5 to 6%, Cr: 11 to 16%, Mo: 0.5. ~ 3%, the balance consisting essentially of Fe and unavoidable impurities martensitic stainless steel is hot worked and allowed to cool naturally to room temperature, then Ac ₃ + 30 ° C.
Above, it is heated to a temperature of Ac ₃ + 200 ° C or less and cooled to room temperature, and then ferrite of Ac ₁ point or more and Ac ₃ points or less +
Excellent in CO ₂ corrosion resistance and sulfide stress cracking resistance characterized by heating to a temperature at which the austenite phase fraction is 3% or more and 60% or less in area ratio in the austenite two-phase temperature range and cooling to room temperature Martensitic stainless steel manufacturing method. 8. In% by weight, C: 0.005 to 0.05%, Cu: 0.2 to 4%, Ni: 1.5 to 6%, Cr: 11 to 16%, Mo: 0.5. ~ 3%, the balance consisting essentially of Fe and unavoidable impurities martensitic stainless steel is hot worked and allowed to cool naturally to room temperature, then Ac ₃ + 30 ° C.
Above, it is heated to a temperature of Ac ₃ + 200 ° C or less and cooled to room temperature, and then ferrite of Ac ₁ point or more and Ac ₃ points or less +
In the austenite two-phase temperature range, heating to a temperature at which the austenite phase fraction is 3% or more and 60% or less in area ratio, cooling to room temperature, and tempering at a temperature of Ac ₁ point or less and Ac ₁ -150 ° C or more, A method for producing martensitic stainless steel having excellent CO ₂ corrosion resistance and sulfide stress cracking resistance.