JPH0375332A - Martensitic stainless steel having high strength and excellent corrosion resistance and its manufacture - Google Patents

Abstract

PURPOSE:To obtain the martensitic stainless steel having excellent corrosion resistance in a wet carbon dioxide environment and having high resistance to cracking caused by wet hydrogen sulfide by forming it from the compsn. contg. each prescribed amt. of C, Si, Mn, Cr, Cu, Al and N and the balance Fe with inevitable impurities. CONSTITUTION:The martensitic stainless steel contg., by weight, 0.03 to 0.12% C, <=1% Si, <=2% Mn, >14 to 18% Cr, 1.2 to 5% Cu, 0.005 to 0.2% Al and 0.005 to 0.15% N is treated as follows. Namely, the stainless steel is austenitized at 900 to 1100 deg.C, is thereafter cooled at a cooling rate more than that in air cooling and is then subjected to tempering treatment at >=580 deg.C to the Ac1 temp. or below. Next, the above steel after subjected to the tempering treatment is cooled at a cooling rate more than that in air cooling, by which the objective martensitic stainless steel having high strength and excellent corrosion resistance can be obtd.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a martensitic stainless steel with high strength, excellent corrosion resistance, and stress corrosion cracking resistance, and a method for producing the same.
More specifically, the present invention relates to a high-strength steel that has high corrosion resistance and cracking resistance in environments containing wet carbon dioxide gas and wet hydrogen sulfide, for example, in the drilling, transportation, and storage of oil and natural gas, and a method for producing the same.

(Prior Art) Oil and natural gas produced in recent years increasingly contain a large amount of wet carbon dioxide. It is well known that carbon steel and low alloy steel corrode significantly in such environments. For this reason, corrosion inhibitors have traditionally been added to prevent corrosion of oil country tubular goods used for drilling, line pipes used for transportation, and the like. However, corrosion inhibitors often lose their effectiveness at high temperatures, and in offshore oil wells and submarine pipelines, corrosion inhibitors are
The costs required for recovery processing are enormous and are often not applicable. Therefore, the need for corrosion-resistant materials that do not require the addition of corrosion inhibitors has recently increased.

As a corrosion-resistant material for petroleum and natural gas containing a large amount of carbon dioxide gas, the application of stainless steel with good corrosion resistance was first considered.
As described in Paper No. 211, martensitic stainless steel containing 12 to 13% Cr, such as Al5I410 or Al5I420, has begun to be widely used as a high-strength, relatively inexpensive steel. However, these steels are sensitive to high temperatures even in a humid carbon dioxide environment.
For example, in an environment of 120° C. or higher or an environment with a high concentration of CI- ions, the corrosion resistance is insufficient and the corrosion rate is high. Furthermore, these steels have the disadvantage that when hydrogen sulfide is contained in oil or natural gas, their corrosion resistance is significantly degraded, causing general corrosion, localized corrosion, and even stress corrosion cracking.

For this reason, the use of the above-mentioned martensitic stainless steel has been limited to cases where the HzS partial pressure contains a very small amount of HzS, such as 0.001 atm, or where all<His is not included.

On the other hand, as martensitic stainless steel with increased resistance to cracking due to hydrogen sulfide, for example,
The ropes shown in Japanese Patent Application Laid-open No. 0-174859 and Japanese Patent Application Laid-Open No. 62-54063 have been proposed. However, these steels do not necessarily have sufficient corrosion resistance in a CO2 environment,
Furthermore, since a large amount of nickel, which is an expensive alloying element, is used, the cost is high.

(Problems to be Solved by the Invention) In view of the current situation, the present invention has sufficient corrosion resistance even in a carbon dioxide environment with high temperature and high CI-ion concentration, and has SCC (Stress Corro) even when containing hydrogen sulfide.
The purpose of the present invention is to provide an inexpensive martensitic stainless steel that has high cracking resistance against ion cracking and a method for manufacturing the same.

(Means for Solving the Problem) The present inventors have studied various components of martensitic stainless steel in order to achieve the above object, and as a result, they have finally found the following knowledge.

First, it was discovered that when more than 14% Cr was added to steel, the corrosion rate in a wet carbon dioxide environment was significantly reduced, and when Cu was added to such steel, the corrosion rate was further reduced. It has been found that the effect of adding Cu is significant when the amount added is 1.2% or more, and it can be used practically at temperatures of 180° C. or higher in a humid carbon dioxide environment. It has also been found that when 1.2% or more of Cu is added, a martensitic structure can be obtained after quenching even if the amount of C is reduced to 0.12% or less. Cu is Ni
Since it is a much cheaper element than , the increase in material cost is small even if 1.2% or more is added. On the other hand, it has been found that when steel containing 1.2% or more of Cu and 0.12% or less of C contains 0.005% or more of N, even higher strength is obtained and corrosion resistance is also improved. At this time, new findings were also obtained that steel with such components has high cracking resistance even in environments containing hydrogen sulfide.

Furthermore, the present inventors further investigated and added 1.2% or more of Cu, reduced C to 0.12% or less, and added 0.005% of N.
The above added P in steel is reduced to 0.025% or less, and S
It was revealed that cracking resistance in an environment containing hydrogen sulfide is further improved by reducing O to 0.010% or less or reducing O to 0.004% or less. On the other hand, Ni and Mo are added to these steels.

It has also been found that the addition of W can further reduce the corrosion rate in a humid carbon dioxide environment at high temperature or high CI-ion concentration.

The present invention has been made based on the above findings, and the gist of the first invention is that, in weight%, C0.03
~0.12%, Si1% or less, Mn2% or less, Cr1
More than 4% but not more than 18%, Cu1.2-5%, Ar1.005
-0.2%, N0.005-0.15%, and the remainder consists of Fe and inevitable impurities, and is characterized by high strength and excellent corrosion resistance.Summary of the second invention The steel of the first invention has high strength and excellent corrosion resistance, and is characterized by reducing P to 0.025% or less and S to 0.025% or less and OIO% or less among the inevitable impurities in the steel of the first invention. The gist of the third invention lies in the martensitic stainless steel.
The fourth aspect of the invention resides in a martensitic stainless steel with high strength and excellent corrosion resistance, characterized in that among the inevitable impurities in the steel of the invention, 0 is reduced to 0.004% or less by weight. However, each of the steels of the first invention, the second invention, or the third invention is characterized in that it contains one or more of Ni 4% or less, MO 2% or less, and W 4% or less in weight percent. The fifth invention is a martensitic stainless steel with high strength and excellent corrosion resistance.
In the keynote of the third invention or the fourth invention, in weight%,
V 0.5% or less, Ti 0.2% or less, NbO 35%
Below, Ta 0.2% or less, Zr 0.2% or less, Hf0
The gist of the sixth invention is a martensitic stainless steel with high strength and excellent corrosion resistance, which is characterized by containing one or more of two or more of the following: invention,
In the keynote of the third invention, fourth invention, or fifth invention,
In weight%, Ca 0.008% or less, rare earth elements 0.0
The gist of the seventh invention is a martensitic stainless steel with high strength and excellent corrosion resistance, which is characterized by containing one or two of the following: 2% or less. , in the third invention, the fourth invention, the fifth invention, or the sixth invention, after austenitizing at 900 to 1100°C, cooling at a cooling rate higher than air cooling, and then cooling at a temperature of 580″C or more A c and a temperature below The present invention provides a method for producing martensitic stainless steel having high strength and excellent corrosion resistance, which is characterized by performing tempering treatment at a high temperature and then cooling at a cooling rate higher than that of air cooling.

(Function) The reasons for limiting the components and heat treatment conditions in the present invention will be described below.

C: Since C is the most stable and low-cost element that increases the strength of martensitic stainless steel, it is necessary to add 0.03% or more to ensure the necessary strength, but 0.12 If it is added in an amount exceeding 0.1%, the corrosion resistance will be significantly reduced, so the upper limit content should be 0.12%.

St: St is an element necessary for deoxidation, but 1
If added in excess of 1%, the corrosion resistance will be significantly reduced, so the upper limit content should be 1%.

Mn: Mn is an effective element for deoxidizing and ensuring strength, but its effect becomes saturated when added in excess of 2%, so the upper limit content is set to 2%.

Cr: Cr is the most basic and essential element constituting martensitic stainless steel, and is necessary for imparting corrosion resistance. However, if the content is less than 14%, the corrosion resistance will not be sufficient; If it is added in excess of 18%, it will be difficult to obtain a martensitic structure and ensure strength no matter how the other alloying elements are adjusted, so the upper limit content should be 18%.

Cu: Cu is an extremely useful element that significantly reduces the corrosion rate of martensitic stainless steel in a humid carbon dioxide environment and significantly reduces the cracking susceptibility in an environment containing hydrogen sulfide by adjusting the C and N content. However, if the content is less than 1.2%, these effects are insufficient, and if it is added in excess of 5%, the effects not only become saturated but also significantly reduce hot workability. , limited to a range of 1.2 to 5%.

AI:N is an element necessary for deoxidation, and the content is o.
, below oos%, the effect is not sufficient, and 0.2%
If it is added in excess of 2%, coarse oxide-based inclusions will remain in the steel and reduce the cracking resistance in hydrogen sulfide, so the content range was set to 0,00% to 0.2%.

N: Like C, N is effective as an element for increasing the strength of martensitic stainless steel, but at 0.005%
If it is less than 0.15%, the effect will not be sufficient, and if it exceeds 0.15%, Cr nitrides will be produced, reducing corrosion resistance and cracking resistance. Therefore, the content range is from 0.005 to 0.
．． It was set at 15%.

The above are the basic components in the present invention, but in the present invention, the following elements can be further added as necessary to further improve the characteristics.

P: Since P is an element that increases stress corrosion cracking susceptibility, it is preferable to have less P, but reducing it to too low a level will only unnecessarily increase costs and the effect of improving properties will be saturated. Corrosion resistance as an objective of the present invention.

When the content is reduced to 0.025% or less, which is sufficiently small to ensure stress corrosion cracking resistance, stress corrosion cracking resistance is further improved.

Like P, SO3 is an element that increases stress corrosion cracking susceptibility, so it is better to have less SO3, but reducing it to too low a level will only unnecessarily increase costs and the effect of improving properties will be saturated. , the content is 0.010 as low as necessary and sufficient to ensure the corrosion resistance and stress corrosion cracking resistance targeted by the present invention.
% or less, stress corrosion cracking resistance is further improved.

O: If present in a large amount, coarse oxide-based nonmetallic inclusion clusters will be generated and stress corrosion cracking susceptibility will increase, so it is preferable to have a small amount, but reducing it to a too small level will unnecessarily increase costs. Since the property improvement effect is saturated with only 0.004%, the content is as low as necessary and sufficient to further improve the corrosion resistance and stress corrosion cracking resistance that are the objectives of the present invention.
When the stress corrosion cracking resistance is reduced to below, the stress corrosion cracking resistance is further improved.

Ni: Ni coexists with 1.2% or more of Cu and is effective in further improving corrosion resistance in a humid carbon dioxide environment, but if it is added in an amount exceeding 4%, the effect not only becomes saturated, but also becomes undesirable. Therefore, the upper limit content is set at 4%.

Mo: Mo coexists with 1.2% or more of Cu and is effective in improving corrosion resistance in a wet carbon dioxide environment, but 2%
If it is added in excess of 2%, the effect will not only be saturated, but also the toughness and other properties will be degraded, so the upper limit content is set at 2%.

WOW is also effective in coexisting with 1.2% or more Cu to improve corrosion resistance in a humid carbon dioxide environment, but adding more than 4% not only saturates the effect, but also improves toughness and other properties. The upper limit content is set at 4% because the properties of

V, Ti, Nb, Ta, Zr+ Hf: V,
Ti, Nb, Ta, Zr.

Hf is an effective element to further improve corrosion resistance, but 0.2% for Ti, Zr, Ta, and Hf, and 0.2% for Nb.
, V, if added in excess of 0.5%, will produce coarse precipitates and inclusions and reduce cracking resistance in an environment containing hydrogen sulfide, so the upper limit content is Ti,
It is 0.2% for Zr, Ta, and Hf, and 0.5% for Nb and V.

Ca, rare earth elements: Ca and rare earth elements (REM) are elements that are effective in improving hot workability and corrosion resistance, but Ca exceeds 0.008%, and rare earth elements exceed 0.02%. If added in excess, coarse non-metallic inclusions will be formed and the hot workability and corrosion resistance will deteriorate, so the upper limit content is set at 0.008% for Ca and 0.02% for rare earth elements. In addition, in the present invention, rare earth elements include those having atomic numbers of 57 to 71 and 89 to 103.
Refers to the number element and Y.

When heat-treating stainless steel having the above components to form a martensitic structure and imparting a predetermined strength, the austenitizing temperature was set to 900 to 1100°C.
This is because at temperatures lower than 1100°C, austenitization is not sufficient and it is therefore difficult to obtain the necessary strength, and when the austenitization temperature exceeds 1100°C, crystal grains become significantly coarsened and cracking resistance in an environment containing hydrogen sulfide decreases. Therefore, the austenitizing temperature is 900-1
The temperature shall be 100°C.

The reason why the cooling rate in cooling after austenitization was set to be higher than air cooling is because martensite is not sufficiently generated at a cooling rate slower than air cooling, making it difficult to secure a predetermined strength.

The reason why the tempering temperature was set to 580°C or higher and lower than Ac+ temperature is because
If the tempering temperature is less than 580°C, sufficient tempering will not be performed, and if the tempering temperature exceeds A c 1 temperature, a part of the steel will become austenite, and fresh martensite will be produced during subsequent cooling, and neither will be sufficiently tempered. This is because no martensite remains to increase cracking susceptibility in hydrogen sulfide-containing environments.

The reason why the cooling rate in cooling after tempering was set to be higher than air cooling is because toughness decreases at a cooling rate slower than air cooling.

The steel of the present invention can be used as a steel plate by ordinary hot rolling, can be used as a steel pipe by hot extrusion or hot rolling, and of course can be used as a bar or wire. It is possible. The steel of the present invention has many uses, such as as oil country tubular goods or line pipes, as well as parts for valves and pumps.

(Example) Examples of the present invention will be described below.

Stainless steel with the composition shown in Table 1 is melted and hot-rolled into a steel plate with a thickness of 12 mw*, and then quenched and tempered under the conditions shown in Table 1 to achieve a content of 0.2%.
It is made of high-strength stainless steel with an offset yield strength of 63 kg/- or more. Note that the tempering temperatures in Table 1 are all below the base ACl temperature. Next, test pieces were taken from these steel materials and subjected to corrosion tests in a humid carbon dioxide environment.
and an SCC test in an environment containing hydrogen sulfide. For corrosion testing in a humid carbon dioxide environment, thickness 3
ttrm, width 15 mm, and length 50 m, in an autoclave at test temperatures of 150°C and 180°C, and at a partial pressure of carbon dioxide gas of 40 atm, in a 3% NaCl aqueous solution.
After immersion for 0 days, the corrosion rate was calculated from the weight change before and after the test. The unit of corrosion rate was expressed in cm/y.
y or less, the material is considered to be sufficiently corrosion resistant and usable.

For cracking tests in environments containing hydrogen sulfide, NAcH
The test was conducted in accordance with NAcE standard TM0177, which is a standard test method established by the American Society of Corrosion Engineers (American Society of Corrosion Engineers), and the hydrogen sulfide partial pressure was 0.1 atm and the test temperature was 120'C. A constant uniaxial tensile stress was applied to a test piece set in a 5% NaCl + 0.5% acetic acid aqueous solution under the above test conditions, and it was examined whether it would break within 720 hours. The test stress was set to a value of 60% of the 0.2% offset proof stress of each steel material.

The test results are also shown in Table 1. In Table 1, in the corrosion test results, ◎ indicates that the corrosion rate is less than 0.05 m/y.
○ means the corrosion rate is 0.05 m/y or more, and 0.10 m/y
Less than y, × indicates corrosion rate of 0.1 mm/7 or more, 0.5
less than tug/y, XX indicates corrosion rate of 0.5 am
/ y is 2, respectively, and S
In the CC test results, ◎ indicates that the sample did not break, and × indicates that it did. In addition, in Table 1, the comparative steel 29 is Al5I 420 steel, -
030 is a 9Cr-IMo steel, and both are conventional steels that have been conventionally used in a humid carbon dioxide environment.

As is clear from Table 1, the steel of the present invention kl=2
8 is a corrosion rate that can be practically used even at a high temperature of 180°C in a humid carbon dioxide environment, which is unthinkable for conventional martensitic stainless steel.
, 1 ms/y, and did not break even in a cracking test in a hydrogen sulfide-containing environment, indicating that it has excellent corrosion resistance and stress corrosion cracking resistance. On the other hand, 29 = -3 for the comparison steel, rope.
In case of No. 4, the corrosion rate was already much higher than 0.1 run/y even at 150°C in a humid carbon dioxide environment, and it fractured in every cracking test in an environment containing hydrogen sulfide.

(Effects of the Invention) As described above, the present invention makes it possible to provide a steel having excellent corrosion resistance in a wet carbon dioxide environment and high cracking resistance against cracking caused by wet hydrogen sulfide, and a method for manufacturing the same. Therefore, it is extremely important to contribute to the development of industry.

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Claims (7)

[Claims] (1) In weight%, C 0.03 to 0.12%, Si 1% or less, Mn 2% or less, Cr more than 14% and 18% or less, Cu 1.2 to 5%, Al 0.005 to 0.2%, N 0.005 A martensitic stainless steel with high strength and excellent corrosion resistance, characterized by containing ~0.15% and the remainder consisting of Fe and unavoidable impurities. (2) A martensitic system with high strength and excellent corrosion resistance according to claim 1, characterized in that among unavoidable impurities, P is reduced to 0.025% or less and S to 0.010% or less. stainless steel. (3) The martensitic stainless steel steel with high strength and excellent corrosion resistance according to claim 1 or 2, characterized in that among the inevitable impurities, O is reduced to 0.004% or less by weight. (4) The high-strength steel according to claim 1, 2 or 3, characterized in that it contains one or more of Ni 4% or less, Mo 2% or less, and W 4% or less as an additional component. Martensitic stainless steel with excellent corrosion resistance. (5) As an additional component, one of the following in weight%: V 0.5% or less, Ti 0.2% or less, Nb 0.5% or less, Ta 0.2% or less, Zr 0.2% or less, Hf 0.2% or less The martensitic stainless steel with high strength and excellent corrosion resistance according to claim 1, 2, 3, or 4, characterized in that it contains two or more of the following. (6) Claim 1, 2, 3, 4, or 5, characterized in that the additional component contains one or two of Ca0.008% or less and rare earth element 0.02% or less in weight percent. Martensitic stainless steel with high strength and excellent corrosion resistance. (7) The martensitic stainless steel according to claim 1, 2, 3, 4, 5, or 6 is austenitized at 900 to 1100°C, then cooled at a cooling rate higher than air cooling, and then at Ac_1 temperature of 580°C or higher. A method for producing martensitic stainless steel with high strength and excellent corrosion resistance, which comprises tempering at a temperature below and then cooling at a cooling rate higher than air cooling.