JPH0250942A - Ferritic stainless steel having superior resistance to stress corrosion cracking - Google Patents

Abstract

PURPOSE:To improve the resistance of ferritic stainless steel to stress corrosion cracking at a low cost while maintaining the superior resistance to corrosion by CO2 by specifying the compsn. of the steel. CONSTITUTION:The compsn. of ferritic stainless steel is composed of, by weight, <=0.15% C, 0.1-1.0% Si, 0.2-2.0% Mn, 9-16.0% Cr, <=0.02% P, <=0.02% S, 0.01-0.09% Al, 0.01-0.25% N, one or more among 0.001-0.06% Ca, 0.001-0.07% Zr and 0.001-0.07% Ba and the balance Fe with inevitable impurities. Steel for a line pipe or steel for an oil well pipe having satisfactory corrosion resistance and superior resistance to stress corrosion cracking is obtd. at a low cost.

Description

Detailed description of the invention [A field of application] The present invention has a yield strength of 135Ksi (94.5Kg/m
m2) oil country tubular goods and line pipes used in the energy field, which do not cause stress corrosion cracking and are
This invention relates to ferritic stainless steel that has excellent CO2 corrosion resistance even at temperatures around 0°C.

“Conventional technology” In applications such as oil country tubular goods and line pipes for natural gas development,
Carbon steel and low-alloy steel are generally used, but as development progresses in recent years, natural waste containing a large amount of CO2 gas has been mined, so this type of steel for natural waste has been used. 13XCr ferritic stainless steel, which has good corrosion resistance, has come to be used.

However, recent natural scum contains carbon dioxide and 11□S at the same time, causing hydrogen to enter the steel. It is becoming known that hydrogen and applied stress can cause sulfide stress corrosion cracking.

This is due to the manufacturing method or the quenching and tempering process.
The present inventors investigated the cause of grain boundary embrittlement and developed a heat treatment method for countermeasures.
It is disclosed in Publication No. 21. In addition, as a manufacturing method that does not cause grain boundary embrittlement, Japanese Patent Application No. 62-330445, 62-3
No. 30446 was filed. In all of the steels described in these documents, the metal structure is controlled from a quenched and tempered structure to a gold 1-like ferrite structure or a heinite structure, and the sulfide stress corrosion cracking resistance is improved. However, the above-mentioned steel must be manufactured under strict manufacturing conditions, and there was a need to reduce management costs and develop a lower-cost steel with excellent stress corrosion cracking resistance.

[Problems to be solved by the invention] Based on the above-mentioned circumstances, the present inventors conducted a rough and detailed study on the stress corrosion cracking mechanism9, and found that the conventional low alloy resistance is the starting point of grain boundary embrittlement. It was revealed that cracks occur due to stretched MnS clusters similar to those observed in sour line pipes, and cracks propagate secondarily through grain boundaries, resulting in sulfide stress corrosion cracking. As a result, it was found that in order to improve the resistance to sulfide stress corrosion cracking, spheroidization of the drawn MnS was most important. The spheroidization of drawn MnS has made it possible to provide a low-cost steel with excellent CO3 corrosion resistance and stress corrosion cracking resistance.

[Means for Solving the Problems] The present invention advantageously solves the above-mentioned problems, and its gist is as follows: C: 0.15% or less Si: 0.1-1. Oz Contains Mn: 02-2.0% Cr: 9-16.0% P: 0.02% or less S: 0.02X or less Al: 0.01-0.09% N: 0.01-025% At the same time, it further contains one or more of Ga: 0.001 to 0.06% Zr: O, OQl to 0.07% Ba: 0.001 to 0.07%, and further contains Ni as necessary.
: 02-2.5% Mo: 02-1.5% ■・0.02-1.5% Ti: 0.001-02% Nb: 0.02-1.5% One or more types It is a ferritic stainless steel with excellent stress corrosion cracking resistance, consisting of iron and unavoidable impurities.

Ferritic stainless steel with excellent stress corrosion cracking resistance can now be obtained at low cost for oil country tubular goods and line pipes used in the energy field with a yield strength of +35Ksi (94.5Kg#++m2) or less. Ta. The present invention has discovered that the most effective countermeasure for stress corrosion cracking resistance is to spheroidize the stretched MnS clusters.

Next, the reasons for limiting the steel components of the steel of the present invention will be described. The following percentages are all percentages by weight.

CC is effective for increasing the strength of steel. However, if the amount added exceeds 0.15%, the hardenability will deteriorate, the strength will increase, and the toughness will decrease. So C is 0, 1
．． 5% or less.

Si: Si is added for deoxidation. However, if the amount added is less than 1%, there is no effect, and if the amount added is 1.0%, the deoxidizing effect will not be sufficient or the toughness will deteriorate. Therefore, Si is set to 0.1 to 1.0y6.

Mn: Mn is added to improve toughness. However, if the added amount is less than 0.02%, it has no effect on the toughness ratio -2.0%.
When the strength exceeds 1, the strength increases, the strength increases, and the toughness decreases. Therefore, Mn is set to 02 to 20%.

Cr: Cr is an effective element for reducing CO3 corrosion. However, within the energy field targeted by the present invention, the conditions are quite severe: temperature 300°C, pressure 300°C.
In order to obtain sufficient corrosion resistance at atmospheric pressure, if the amount added is too small, the effect will not be obtained, and if it is less than Qya, corrosion resistance will not be obtained. 1
If the amount is 6.0%, corrosion resistance commensurate with the amount added cannot be obtained. Therefore, the amount of Cr added is 9 to 16%.

P and P embrittle steel. It is an element that condenses at the center of the thickness of the slab during casting, and together with expanded MnS, becomes the starting point of stress corrosion cracking. At 0.02%, stress corrosion cracking resistance decreases. Is it desirable that the content be as low as possible?
The upper limit of 0.02% has a slight effect on stress corrosion cracking resistance. Therefore, P should be 0.02% or less.

SS makes steel brittle. In normal cases, it becomes MnS and becomes the starting point of stress corrosion cracking. However, it is necessary to make the inclusions spheroidized by feeding an element that controls the inclusion morphology. If a large amount of spheroidal inclusions are present, they will reduce the cleanliness of the steel and become the starting point for stress corrosion cracking. Therefore, it is desirable that the S content and T content be as low as possible. The upper limit of 0.02% has only a slight effect on stress corrosion cracking resistance. Therefore, S should be 0.02% or less.

Al: Al is added for deoxidation. 0. OIX has no deoxidizing effect, and 0.09% OIX does not have a sufficient deoxidizing effect, but it lowers the cleanliness of the steel, reduces toughness, and becomes the starting point for stress corrosion cracking. Therefore, Al is 0.01-0.09
%.

N N has the effect of widening the γ loop in steel containing around 13% Cr, and can also provide strength due to the solid solution effect. This effect is not less than 0.01%.

On the other hand, addition of more than 0.025% cannot be easily added using normal processes. Therefore, the amount of N added is 0.01 to 025%.
shall be.

Further, one or more of Ca, 2r, and Ba are added to make the inclusions spheroidal.

Ca: Ca is added to make inclusions that become the starting point of stress corrosion cracking spherical. Lower limit t1. oo1% is the amount at which Ca addition starts to be effective in spheroidizing inclusions. If the upper limit of 0.06% is exceeded, Ca di, which is effective for the spheroidization effect, will be exceeded, and Ca oxides will be formed, reducing the cleanliness of the steel and becoming the starting point for stress corrosion cracking. It becomes an effect. Therefore, Ca should be 0.001 to 0.05%.

7. r: lr has the same effect as Ca. The lower limit is tl,
oo1%l Earth limit is 0.07%.

Ba: Ba has the same effect as Ca and 2r. The lower limit is 0
．． 001%, and the upper limit is 007%.

One or more of the above Cajr and Ra may be contained.

Ni, Mo, V, Ti, Nb These elements can be added arbitrarily singly or in combination. It is added in order to increase the strength by 1-1 by forming carbides. If the amount is less than the lower limit, the effect will be poor, and if it exceeds the lower limit, giant carbides will be formed and the toughness will be impaired. I want to do it
O,2=2.5%, Mo02~1.5%VO,02~Hot, 5$Ti 0.001~02%, Nb O,02~
The addition range is i, 5y6. It should be noted that since there is no difference between the case where these elements are added alone and the case where they are added in combination, one type or two or more types can be added depending on the required strength.

Examples of the present invention will be described below.

[Example] Table 1 shows the steel composition, mechanical properties, and various test results. After melting, the steel was hot rolled to a thickness of 15 mm. Hot rolling condition is 1200
After heating for 1 hour at a temperature of 950° C., the steel sheet was rolled at a finishing temperature of 950° C. After rolling, the steel plate was cooled in air. Mechanical properties of the steel plates were investigated using tensile test pieces according to JIS 82. We decided to conduct two types of corrosion tests. First, 1 targeted by the steel of the present invention
In order to see the cracks when 12S was crowded, a specimen W.
20 mm, fl, : 50 mm, t : 10 mm, was immersed in H2S saturated -5'8NaC9, -0.5%; acetic acid solution (N to C:E solution) for 96 hours. Since cracks appear parallel to the plate surface, UST performed flaw detection in the plate thickness direction, and expressed as a percentage by dividing the crack area by the area of the specimen. Next, the 002 corrosion test was performed using a specimen (W: 20 mm,
11: 50 mm, t: 5+nm) using an autoclave. Test conditions are temperature 300
℃, C02 pressure 300 atm, liquid used is 5! jiNa(4
It was immersed in liquid for 30 days, and its weight before and after corrosion was measured and evaluated. Finally, the stress corrosion cracking resistance test was conducted using the heavy constant load method. The specimen size is D; 6.35nut+,
1; 25.4mm. The test conditions were as follows: NACE liquid was used and the load stress was 0.9° and 0.8.07° below the yield point, and the test was determined by whether or not the product was broken after 720 hours. From now on
The steel of the present invention exhibits excellent properties in various L tests.

In addition, in Table 1, C (Note 1) Corrosion loss ◎ 0 ~ 49 mdd, 050 ~ 9! l mdd, △ 1
00~499 md(1,
・070σy or less [Effects of the Invention] The present invention provides a steel for line pipes that is lower in cost than conventional steels, has high corrosion resistance, and has excellent stress corrosion cracking resistance.
Steel for oil country tubular goods can be obtained, and its industrial effects are great.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of the specimen used in the corrosion test, and FIG. 2 is a front view of the dimensions of the specimen used in the stress corrosion cracking test.

Claims (1)

[Claims] 1. C: 0.15% or less Si: 0.1-1.0% Mn: 0.2-2.0% Cr: 9-16.0% P: 0. S: 0.02% or less Al: 0.01-0.09% N: 0.01-0.25% Ca: 0.001-0.06% Zr: 0.001 -0.07% Ba: 0.001-0.07% A ferritic stainless steel with excellent stress corrosion cracking resistance, containing one or more types of Ba:0.001-0.07%, with the remainder being iron and unavoidable impurities. 2. In weight%, C: 0.15% or less Si: 0.1-1.0% Mn: 0.2-2.0% Cr: 9-16.0% P: 0.02% or less S: 0 Contains .02% or less Al: 0.01-0.09% N: 0.01-0.25%, Ca: 0.001-0.06% Zr: 0.001-0.07% Ba: Contains one or more of 0.001 to 0.07%, and further includes Ni: 0.2 to 2.5% Mo: 0.2 to 1.5% V: 0.02 to 1.5% Ti: 0.001-0.2% Nb: 0.02-1.5% A ferritic stainless steel with excellent stress corrosion cracking resistance, consisting of one or more types of Nb, the balance being iron and unavoidable impurities.