JPH08929B2 - Method for producing ferritic stainless steel excellent in stress corrosion cracking resistance and carbon dioxide corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a method for producing steel used in the energy field having a yield strength of about 135 ksi (94.5 kg / mm ² ) or less, and particularly stress corrosion. This is related to the production of materials that do not crack and have excellent CO ₂ corrosion resistance.

(Prior Art) Generally carbon steel as a line pipe for natural gas development,
Although low-alloy steel is used, as the development progresses in recent years, the corrosion resistance of the above-mentioned steel is becoming insufficient with respect to natural gas containing a large amount of carbon gas. For this reason, the use of stainless steel, which has good corrosion resistance, as a steel for this type of natural gas has been studied. For example, martensitic stainless steel containing 13% Cr represented by AISI 410 steel and 420 steel has good corrosion resistance to carbon dioxide. Although it is usually manufactured by quenching and tempering (QT), it can be said to be a relatively excellent steel type whose strength can be freely changed depending on the QT conditions.

However, the greatest weakness of this stainless steel is that it causes stress corrosion cracking. It is said that the normal stress corrosion cracking strength range is at a yield point of 75 ksi (52.5 kg / mm ² ) or more. It is considered that this is because the manufacturing method is QT, which is caused by the structure of tempered martensite and the susceptibility of tempered martensite to stress corrosion cracking increases in proportion to the yield point. As measures against these problems, the present inventors have disclosed, for example, in Japanese Patent Application Laid-Open No. 60-197821, a heat treatment method for a Cr-based stainless steel oil country tubular good having excellent resistance to stress corrosion cracking. Therefore, it is stated that it is possible to improve the stress corrosion cracking resistance by controlling the structure by optimally controlling the quenching cooling rate and the tempering temperature. However, in this method, the heat treatment has to be performed in two steps of quenching and tempering, and it is necessary to reduce the cost.

(Problems to be Solved by the Invention) The present invention was obtained from the results of repeated studies based on the above-mentioned actual conditions, and the components of the above ferritic stainless steel were used as basic components, and Cr, C, A steel that can inherit the excellent corrosion resistance of this steel as it is and also give excellent stress corrosion cracking resistance by controlling the mutual addition amount of N three elements and obtaining a uniform acicular ferrite structure in one heat treatment. It aims at providing the manufacturing method of.

(Means for Solving Problems) The present inventors first need to obtain an austenite single phase in order to obtain a uniform structure.
Forms a γ loop, so what is important in heat treatment is this γ
We paid attention to the fact that it is important to use an ingredient system that is included in the loop.

Therefore, we investigated the effects of changes in Cr and C and N additions, Ni addition, and Mo addition on the range of the γ loop.

Table 1 shows the results of the microstructure investigation when the components of the test material, the heating temperature, and the cooling rate were different. As the test material, a 25 kg melted material was used after being normally rolled. Judgment of metal structure
After heating at 930 ° C., three types of cooling, that is, accelerated cooling, air cooling, and furnace cooling were performed, and the cross-sectional optical microscope observation was performed. Relation between Cr addition amount and C, N on γ loop Formula Cr% ≧ 10 × C
When (%) + 30 × N (%) + 8 is satisfied, the γ single phase can be heated and the structure is martensite single phase due to accelerated cooling, especially the cooling rate below air cooling that does not forcefully cool, It was found that an acicular-ferrite structure (AF) was obtained with a structure composed of ferrite and carbide. It has been found that in the case of these component systems, the ferrite-pearlite structure does not form even if the cooling is slowed down. In the case of a component system that does not satisfy the above formula, since the α + γ heating region, the ferrite in the α portion remains as it is after cooling, and the γ portion becomes a martensite or AF mixed structure depending on the cooling rate. It was also found that the effect of Ni and Mo addition on the γ loop was almost nonexistent due to the small addition amount.

 Next, the relationship between the structure and the stress corrosion cracking property was examined.

Table 2 shows the results of stress corrosion cracking test for the steels used in Table 1. It was revealed that the structure and the stress corrosion cracking property are closely related. That is, it was found that stress corrosion cracking occurs as expected when the structure becomes martensite, and the stress corrosion cracking resistance is large only in the case of the AF structure. It was also found that in the case of the AF structure, grades of steel with different yield points can be obtained depending on the cooling rate.

The test conditions were a JIS A2 tensile test for the yield point and a 4-point bending test for the stress corrosion cracking evaluation. Bending load stress is 1.25 times, 1.0 times, and 0.85 times the yield point, and the stress is 14% in corrosive solution H ₂ S saturated 5% NaCl + 0.5% acetic acid solution.
The usual method of judging cracking after taking out after soaking for a day was used.

The test pieces and jigs are shown in FIGS. 1 and 2. First
In the figure, the thickness t = 3 mm in the plate thickness direction, the sample width W = 15 mm,
The sample length l = 115 mm. In FIG. 2, 1 is a 3t test piece, the jig body 2 has a gauge length L, and the test piece 1 is supported by a ceramic rod 3 at two central points and both ends, and a lock nut is provided at the central part. A bending stress σ is given by being pressed by a threaded rod that is freely movable back and forth. The ceramic rods are used at each fulcrum to prevent the potential generated by the difference in the electrochemical characteristics of the jig and the specimen generated in the immersion liquid. Bending stress σ and indentation amount δ
The relational expression of is shown by the following expression.

The present invention has been made based on the above new findings, and regulates the mutual addition amounts of the three elements of the components Cr, C and N, realizes austenite uniform heating, and provides an AF structure that does not form martensite. By doing so, the tempering process can be omitted and the cost can be reduced, and the ferritic stainless steel excellent in stress corrosion cracking resistance and carbon dioxide gas corrosion resistance can be manufactured.

That is, the gist of the present invention is, in wt% C: 0.25% or less Si: 0.1 ~ 0.5% Mn: 0.2 ~ 1.0% Cr: 9 ~ 16.0% P: 0.02% or less S: 0.02% or less Al: 0.01 ~ 0.05% N: 0.01 to 0.25%, the balance of iron and unavoidable impurities, and the composition of steel satisfying Cr% ≧ 10 × C (%) + 30 × N (%) + 8, or Ni: 0.2 to 2.5% Mo: 0.2 to 1.5% V: 0.02 to 1.5% Ti: 0.001 to 0.2% Nb: 0.02 to 1.5% Steel containing at least one kind in the austenite region 91
A method for producing a ferritic stainless steel excellent in stress corrosion cracking resistance and carbon dioxide gas corrosion resistance, which is characterized in that it is heated at 0 to 1020 ° C. and then cooled at a cooling rate not higher than air cooling.

(Operation) Next, the reason for limiting the components in the steel targeted by the present invention will be described.

C: C is effective for increasing the strength of steel. However, if the addition amount exceeds 0.25%, the hardenability is increased, the structure is easily martensitic, and the AF structure is difficult to appear. Therefore, C is 0.25% or less.

Si: Si is added for deoxidation. However, the addition amount is 0.1%
If it is less than 0.5%, there is no effect. If the amount added exceeds 0.5%, the deoxidizing effect is sufficient, but the toughness deteriorates. Therefore Si is 0.1 ~
0.5%

Mn: Mn is added to improve toughness. However, if the added amount is less than 0.2%, it has no effect on improving the toughness, and if it exceeds 1%, it is an element that improves hardenability, so that the structure easily becomes martensite and AF is difficult to occur. Therefore, Mn
Is 0.2 to 1.0%.

Cr: Cr is an element effective in reducing CO ₂ corrosion. However, in the case of use in the energy field, which is the object of the present invention, under extremely severe conditions, if the addition amount is small, the effect is not obtained. The lower limit is determined by the addition amount at which the effect of reducing corrosion begins to appear. The upper limit of the amount added does not have the meaning of being added even if it exceeds the effective range. Therefore, the Cr addition range is 9 to 16.0%.

P: P embrittles steel. However, in the case of the steel of the present invention, unlike the conventional one in which the structure is tempered martensite, since it has an AF structure, the degree of P embrittlement of the steel is low. Therefore, if it is set to 0.02% or less of the normal level, there is no concern about embrittlement. Therefore, P is 0.02% or less.

S: S also embrittles steel. The lower the content, the better in order to obtain toughness, but since it costs more, the upper limit of the content that does not pose a practical problem is about 0.02%. Therefore, S is 0.02% or less.

Al: Al is added for deoxidation. If it is less than 0.01%, there is no deoxidizing effect, and if it exceeds 0.05%, the deoxidizing effect is sufficient.
It lowers the cleanliness of steel and reduces toughness. Therefore, the amount of Al added is 0.01 to 0.05%.

N: N has the effect of expanding the γ loop in steel with around 13% Cr, and plays an important role in controlling the microstructure. However, there is no effect to widen the γ loop whose addition amount is less than 0.01%, and it is necessary to add 0.01% or more. On the other hand, the higher the upper limit value, the better, but the addition amount that can be easily added in a normal process is about 0.25%. Therefore, the amount of N added is
0.01 to 0.25%

Ni, Mo, Nb, V, Ti: These elements are elements to which one or more kinds can be arbitrarily added. It is added in order to increase the strength by the formation of carbide when the tissue is made into AF. If the addition amount is less than the lower limit, the effect is poor, and if it exceeds the upper limit, large carbides are formed, so Ni0.2 to 2.5%, Mo0.2 to 1.5%, V0.02 to 1.5
%, Ti 0.001 to 0.2%, Nb 0.02 to 1.5%. Since there is no difference between the case where these elements are added in combination and the case where they are added alone, one or more of these elements can be added if necessary.

Relational expression of Cr, C, N addition amount: In order to obtain a uniform AF structure, it is necessary to make the austenite state not containing ferrite during heating. Result of experiment Cr% ≧ 10 × C (%) + 30
It was found that a uniform AF structure can be obtained by cooling below the air cooling satisfying × N (%) + 8.

Heat treatment condition: In the present invention, it is important to make the structure different from the conventional heat treatment into an AF structure. Therefore, it is necessary that the cooling rate be equal to or lower than the air cooling rate and that the martensite structure with quenching not be formed. The cooling rate equal to or less than the empty age in the present invention is a cooling rate while standing still in a furnace or in air. When AF is selected, tempering can be omitted,
Moreover, it is possible to obtain a steel having high strength and free from stress corrosion cracking. The heating temperature is set to a temperature of 910 to 1020 ° C. at which γ becomes uniform, but the lower limit is determined as a temperature at which α is not mixed, and the upper limit is set as a range in which coarsening of γ grains does not easily occur.

As described above, according to the present invention, it is possible to obtain a ferritic stainless steel, which has a yield strength of about 135 ksi or less and is used in the energy-related fields, which does not cause stress corrosion cracking and carbon dioxide corrosion by one heat treatment. It will be possible.

 Next, the present invention will be described with reference to examples.

(Examples) Steels having chemical compositions shown in Table 3 were melted and then subjected to normal rolling to obtain steel plates, which were held at each heating temperature for 30 minutes and then cooled by air cooling at a cooling rate.

Although there are some differences in the cooling conditions depending on the plate thickness, in the case of the present invention, it does not matter at all if the cooling speed is equal to or lower than the air cooling, even if it is not strictly regulated. Details of the test method, such as the light microscope, the tensile test, and the atmospheric pressure test in the stress corrosion cracking test, are the same as those described above, and therefore will be omitted.

As a supplement to the high-pressure CO ₂ + H ₂ S test, not to mention the purpose of this test, it is carried out in order to investigate cracks under conditions that simulate the actual service conditions of line pipes. That is, at normal pressure, the pH of the solution is made acidic with a test solution using acetic acid to facilitate corrosion, and therefore the actual high-pressure environment may not be reproduced depending on the combination of acetic acid and steel. The test piece size, the stress loading method, the salt concentration of the immersion liquid, and the immersion period are completely the same as the normal pressure method. In the test, each jig was put in an autoclave after stress loading, and after sealing, H ₂ S was first introduced at 10 atm and then CO ₂ was introduced at 10 atm to obtain a corrosive environment. Since the gas is absorbed in the immersion liquid after the gas is added and the pressure drops, in that case, the gas was added again at an appropriate time and the test was continued.

Table 4 shows the results of the stress corrosion cracking test. It can be seen that the steel produced according to the present invention does not crack at all and has a small corrosion weight loss.

(Effects of the Invention) As described above, according to the present invention, the yield strength is 135 ksi (9
Among steels used in the energy field of about 4.5 kg / mm ² ) or less, it is possible to manufacture steel with excellent stress corrosion cracking resistance and CO ₂ corrosion resistance at low cost, which is an effective industrial effect. Is brought about.

[Brief description of drawings]

FIG. 1 is a dimensional drawing of a 4-point bending test piece, and FIG. 2 is a sectional view showing a bent state of the test piece shown in FIG. 1 ... Test piece, 2 ... Jig, 3 ... Ceramic rod, 4 ...
… Supporting table, 5… Pushing rod.

Claims (2)

[Claims] 1. C: 0.25% or less in weight% Si: 0.1 to 0.5% Mn: 0.2 to 1.0% Cr: 9 to 16.0% P: 0.02% or less S: 0.02% or less Al: 0.01 to 0.05% N: 0.01 〜0.25%, balance iron and unavoidable impurities, and Cr% ≧ 10
Stress corrosion cracking resistance and carbonation resistance, characterized in that a steel having a composition satisfying × C (%) + 30 × N (%) + 8 is heated to an austenite range of 910 to 1020 ° C. and then cooled at a cooling rate not higher than air cooling. A method for producing ferritic stainless steel having excellent gas corrosiveness. 2. C: 0.25% or less by weight% Si: 0.1 to 0.5% Mn: 0.2 to 1.0% Cr: 9 to 16.0% P: 0.02% or less S: 0.02% or less Al: 0.01 to 0.05% N: 0.01 〜0.25% and Ni: 0.2〜2.5% Mo: 0.2〜1.5% V: 0.02〜1.5% Ti: 0.001〜0.2% Nb: 0.02〜1.5% and the balance iron and inevitable impurities. And Cr% ≧ 10 × C (%) + 30 × N
(%) + 8 steel with composition satisfying austenite region 91
A method for producing a ferritic stainless steel having excellent stress corrosion cracking resistance and carbon dioxide gas corrosion resistance, which comprises heating at 0 to 1020 ° C. and then cooling at a cooling rate not higher than air cooling.