JPH0375339A - 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, Al and N. CONSTITUTION:The above martensitic stainless steel is formed from the compsn. contg., by weight, 0.03 to 0.12% C, <=1% Si, >2 to 7% Mn, >14 to 18% Cr, 0.005 to 0.2% Al, 0.005 to 0.15% N and the balance Fe with inevitable impurities. For obtaining the stainless steel, the steel having the above componental compsn. is austenitized at 900 to 1100 deg.C and is thereafter cooled at a cooling rate more than that in air cooling to satisfactorily form martensite. Next, the above steel after cooled is subjected to tempering treatment at 580 deg.C to the Ac1 temp. or below and is thereafter 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 and excellent corrosion resistance and a method for producing the same. The present invention also relates to a high-strength steel that has high corrosion resistance and cracking resistance in an environment containing wet carbon dioxide gas and wet hydrogen sulfide during storage, 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, in addition to the fact that corrosion inhibitors often lose their effectiveness at high temperatures, they are often not applicable to offshore oil wells and submarine pipelines because the cost of adding and recovering them is enormous. . 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, such as J, Klein, Corrosion °84. As described in Paper No. 211, martensitic stainless steel containing 12 to 13% Cr, such as Al5r410 or Al5r420, has begun to be widely used as a high-strength, relatively inexpensive steel. However, these steels have the disadvantage that even in a humid carbon dioxide environment, their corrosion resistance is insufficient and their corrosion rate is high in a high temperature environment, for example, 120° C. or higher, or in an environment with a high concentration of CI- ions. Furthermore, these steels have the disadvantage that when hydrogen sulfide is contained in oil or natural gas, their corrosion resistance is significantly deteriorated, 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 H2S partial pressure contains a very small amount of H2S, such as 0.001 atm, or where the H2S partial pressure is less than all H2S.

On the other hand, as martensitic stainless steel with increased resistance to cracking due to hydrogen sulfide, for example,
Steels disclosed 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.

(Problem B to be Solved by the Invention) In view of the current situation, the present invention has been developed to provide an inexpensive method that has sufficient corrosion resistance even at high temperatures and carbon dioxide environments with high CI-ion concentration, and has high cracking resistance even when containing hydrogen sulfide. The purpose of this research is to provide a martensitic stainless steel and a method for producing 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 Mn was added to such steel, the corrosion rate was further reduced. The effect of this old addition is remarkable when it is added in excess of 2%, and the effect is significant when added in excess of 2%.
It has been found that the above method can be used practically. It has also been found that when Mn is added in excess of 2%, a martensitic structure can be obtained after quenching even if Cl is reduced to 0.12% or less. Since Mn is a much cheaper element than Ni, the material cost will not increase much even if it is added in an amount exceeding 2%.

On the other hand, N is added to steel containing more than 2% of Mn and 0.12% or less of C. It has been found that when OO is contained in an amount of 5% or more, significantly higher strength is obtained and corrosion resistance is also improved. At this time, new findings were also obtained that cavities containing such components have high cracking resistance even in environments containing hydrogen sulfide.

Furthermore, the present inventors conducted studies and added Mn in excess of 2%, reduced C to 0.12% or less, and reduced N to 0.005%.
% or more of added P in steel to 0.025% or less,
Reduce S to 0.010% or less or O to 0.004%
It has been revealed that cracking resistance in environments containing hydrogen sulfide can be further improved by applying any of the following reductions. On the other hand, these steels contain Ni, Cu, Mo
It has also been found that the addition of W can further reduce the corrosion rate in a humid carbon dioxide environment with 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%, Si 1% or less, Mn over 2% and 7% or less, C
r more than 14% and 18% or less, AZo, 005 to 0.2%.

N0. The second invention resides in a martensitic stainless steel with high strength and excellent corrosion resistance, which is characterized by containing 5 to 0.15% OO, with the remainder consisting of Fe and unavoidable impurities. In the steel of. Among the inevitable impurities, P is 0.025% by weight.
Hereinafter, the gist of the third invention is a martensitic stainless steel with high strength and excellent corrosion resistance characterized by reducing S to 0.010% or less.
Among the inevitable impurities in the steel of the invention, O is 0.0% by weight. The gist of the fourth invention is a martensitic stainless steel with high strength and excellent corrosion resistance characterized by reduced OO to 4% or less. , 1% of Ni 4% or less, Cu 1% or less, Mo 2% or less, W 4% or less
The gist of the fifth invention resides in a martensitic stainless steel with high strength and excellent corrosion resistance characterized by containing one or more of the following: the first invention, the second invention,
In the keynote of the third invention or the fourth invention, in weight%,
■0.5% or less, Ti0.2% or less, Nb0.5% or less, Ta0.2% or less, Zr0.2% 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 2% or less. invention,
In the keynote of the third invention, fourth invention, or fifth invention,
In weight%, Ca 0.008% or less, rare earth elements 0.02
% or less, the martensitic stainless steel has high strength and excellent corrosion resistance, and the gist of the seventh invention is that the first invention, the second invention, In the keynote of the third invention, fourth invention, fifth invention, or sixth invention, after austenitizing at 900 to 1100°C, cooling at a cooling rate higher than air cooling, and then tempering at a temperature of 580°C or higher and lower than AcI temperature. The present invention provides a method for producing martensitic stainless steel having high strength and excellent corrosion resistance, which comprises 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: Si 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, ensuring strength, and improving corrosion resistance in a carbon dioxide environment, but its effect is insufficient if the content is less than 2%, and if the content is less than 7%.
If more than 2% is added, the effect will not only be saturated, but also austenite may be formed after quenching and tempering heat treatment, so the content should be more than 2% and 7% or less.

Cr: Cr is the most basic and essential element constituting martensitic stainless steel and is an element necessary to impart corrosion resistance, but if the content is less than 14%, corrosion resistance is insufficient; 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%.

/V: A7 is an element necessary for deoxidation, and if the content is less than 0.005%, its effect will not be sufficient;
If added in excess of 2%, coarse oxide inclusions remain in the steel and reduce cracking resistance in hydrogen sulfide, so the content range should be 0. OO5 to 0.2%.

Like C, NUN is effective as an element for increasing the strength of martensitic stainless steel, but 0, 0 O5
If the content is less than 0.15%, the effect will not be sufficient, and if it exceeds 0.15%, Cr nitrides will be formed, reducing corrosion resistance and cracking resistance, so the content range is 0.15%. OO5~
It was set to 0.15%.

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.

Since FDP is an element that increases stress corrosion cracking susceptibility, it is preferable to reduce it to a low level, but reducing it to too low a level will only unnecessarily increase costs and the effect of improving properties will be saturated. Stress corrosion cracking resistance is further improved when the content is reduced to 0.025% or less, which is a sufficiently small amount to ensure the desired corrosion resistance and stress corrosion cracking resistance.

S: Like P, S is an element that increases stress corrosion cracking susceptibility, so it is preferable to have less S, but reducing it to too low a level will only unnecessarily increase costs and the effect of improving properties will be saturated. Therefore, the content is 0.01 as low as necessary and sufficient to ensure the corrosion resistance and stress corrosion cracking resistance that are the objectives of the present invention.
When the content is reduced to 0% or less, stress corrosion cracking resistance is further improved.

O: If O is present in a large amount, it will generate coarse oxide-based nonmetallic inclusion clusters and increase stress corrosion cracking susceptibility, so it is preferable to have a small amount, but reducing it to too low a 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 more than 2% Mn and is effective in further improving corrosion resistance in a wet carbon dioxide environment, but adding more than 4% not only saturates the effect but also unnecessarily increases costs. The upper limit content is 4% as it will cause an increase in
shall be.

Cu: Cu is also effective in further improving corrosion resistance in a wet carbon dioxide environment, but the effect is saturated even if it is added in an amount exceeding 1%, so the upper limit content is set at 1%.

Mo: Mo coexists with more than 2% Mn and is effective in improving corrosion resistance in a wet carbon dioxide environment, but adding more than 2% not only saturates the effect but also improves other properties such as toughness. The upper limit of the content is set at 2% since it degrades the properties.

WOW is also effective in coexisting with more than 2% Mn to improve corrosion resistance in a wet carbon dioxide environment, but adding more than 4% not only saturates the effect, but also improves toughness and other properties. Therefore, the upper limit content is set at 4%.

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

) If is an effective element to further improve corrosion resistance, but in Ti, Zr, Ta, If, 0.2%, N
If B and V are added in excess of 0.5%, coarse precipitates and inclusions will be generated and the cracking resistance in an environment containing hydrogen sulfide will be reduced, so the upper limit content is T.
0.2% for i, Zr, Ta, Hf, Nb, V
In this case, it is set to 0.5%.

Ca and rare earth elements: Ca and rare earth elements (REM) are elements that are effective in improving hot workability and corrosion resistance, but for Ca it exceeds o, o o s%, and for rare earth elements it exceeds 0.02. If it is added in excess of %, coarse nonmetallic inclusions will be formed and the hot workability and corrosion resistance will deteriorate, so the upper limit content is o, o o for Ca.
s%, and 0.02% for rare earth elements.

In addition, in the present invention, rare earth elements have an atomic number of 57 to
Refers to elements 71 and 89 to 103 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 Ac or lower was because
If the tempering temperature is less than 580°C, sufficient tempering will not take place, and if the tempering temperature exceeds A c 1 temperature, part of the material will become austenite and produce fresh martensite during subsequent cooling, and neither will be sufficiently tempered. This is because free martensite remains and increases cracking susceptibility in an environment containing hydrogen sulfide.

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.

After melting stainless steel with the ingredients shown in Table 1 and hot rolling it into a steel plate with a thickness of 12 mm, it is quenched and tempered under the conditions shown in Table 1 with an offset of 0.2%. It is made of high-strength stainless steel with a yield strength of 63 kg/- or more. In addition, all tempering temperatures in Table 1 are based on A.
c The temperature is below 1 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
mn+, using a test piece with a width of 15 mm and a length of 50 mm, in an autoclave with a test temperature of 150°C and 180°C, and a carbon dioxide gas partial pressure 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 is kaku/y, but - the corrosion rate of a certain material in the environment of boat fishing is 0.1 m/y
y or less, the material is considered to be sufficiently corrosion resistant and usable.

For cracking tests in environments containing hydrogen sulfide, NAcE
The test was conducted in accordance with NAcE standard TM 0177, which is a standard test method established by the American Society of Corrosion Engineers (American Society of Corrosion Engineers), with a hydrogen sulfide partial pressure of 0.1 atm and a test temperature of 120°C. Under the above test conditions, a constant uniaxial tensile stress was applied to a test piece set in a 5% NaCj + 0.5% acetic acid aqueous solution, and it was examined whether it would break within 720 hours. The value was set to 60% of the .2% offset proof stress.

The test results are also shown in Table 1. In Table 1, in the corrosion test results, ◎ indicates a corrosion rate of 0.05flIl/
Less than y, ○ indicates corrosion rate of 0.05 m/y or more 0, 1
Less than 0 am/y;
mm/7 or more, respectively.
In the SCC test results, ◎ indicates that the sample did not break, and × indicates that it did. In Table 1, the comparison steel Na29 is Al5I 420 steel, and Sou30 is 9Cr-IMo steel, both of which are conventional steels that have been conventionally used in a humid carbon dioxide environment.

As is clear from Table 1, steel bars 1 to 28, which are steels of the present invention,
is a corrosion rate that is practically usable even at a high temperature of 180"C in a humid carbon dioxide environment, which is unthinkable for conventional martensitic stainless steel.
, 1 mm/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, comparative steels 29 to 34
Even at 150°C in a humid carbon dioxide environment, the corrosion rate was already much higher than 0.1 m/y, and it fractured in a 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 more than 2% to 7%, Cr more than 14% to 18%, Al 0.005 to 0.2%, N 0.005 to 0. A martensitic stainless steel with high strength and excellent corrosion resistance, characterized by containing .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 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) As an additional component, it contains one or more of Cu 1% or less, Ni 4% or less, Mo 2% or less, and W 4% or less in weight percent. Martensitic stainless steel with high strength and 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.