JPH07110970B2 - Method for producing acicular ferritic stainless steel with excellent resistance to stress corrosion cracking - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a steel used in the energy field having a yield strength of about 135 ksi (94.5 kg / mm ² ) or less, particularly, it does not cause stress corrosion cracking and is resistant to CO. _{2 The} present invention relates to a method for manufacturing a steel material having excellent corrosiveness.

(Prior Art) Generally, carbon steel or low alloy steel is used as a line pipe for natural gas development. However, with the progress of development in recent years, the above steel has a corrosion resistance against natural gas containing a large amount of carbon dioxide gas. It is not enough. 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. This martensitic stainless steel is usually manufactured by quenching and tempering (QT), but it can be said that it is 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 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 a countermeasure against these problems, the present inventors have proposed, for example, Japanese Patent Laid-Open No.
Japanese Unexamined Patent Publication No. 60-197821 discloses a heat treatment method for Cr-based stainless steel oil country tubular goods having excellent resistance to stress corrosion cracking. It is described therein that it is possible to improve the stress corrosion cracking resistance by controlling the structure by an optimal combination of quenching cooling rate control and 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 is obtained as a result of repeated studies based on the above-mentioned actual conditions. The above-mentioned ferritic stainless steel is used as a basic component, and Cr, C, N The mutual addition amount of the three elements is regulated, the rolling reduction is processed within a certain range, and the γ
By tempering steel with fine grained particles once at a low cost and with good workability, by obtaining an acicular ferrite structure,
An object of the present invention is to provide a steel that can inherit the excellent corrosion resistance of this steel as it is and can impart excellent stress corrosion cracking resistance.

(Means for Solving Problems) The present invention advantageously solves the above problems, and the gist of the present invention is C: 0.15% or less by weight% Si: 0.1-0.5% Mn: 0.2- 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 to 0.25%, or together with these 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% One or more types, balance iron and unavoidable impurities, Cr% ≧ 10 × C (%) + 30 × N (% ) After heating steel with a composition satisfying +8 to a heating temperature of 950 ° C to 1250 ° C, hot working is performed at a finishing temperature of 750 ° C or higher at a processing rate of 60% to 95%, and after processing it is cooled to room temperature by air cooling or higher. And then tempered at a temperature of 400 ° C to less than 750 ° C for 15 minutes to 60 minutes to form an acicular ferrite structure with an area ratio of 80% or more, which is excellent in stress corrosion cracking resistance. Acicular It is in the method of manufacturing ferritic stainless steel.

The present invention will be described in detail below.

(Working) In order to obtain high strength and sulfide stress corrosion cracking resistance, it is effective to make the structure extremely fine and uniform. This is because the addition of appropriate chemical components and the control of hot working conditions and subsequent It can be achieved by controlling the tempering conditions to obtain a needle-like ferrite structure of 80% or more.

The present inventors have studied the relationship between the structure and the sulfide stress corrosion cracking property obtained by hot working and tempering treatment, the acicular ferrite structure is more resistant to sulfide stress corrosion cracking than the conventional tempered martensitic structure. I found it to be excellent.

That is, the present invention is a high-strength stress corrosion cracking-resistant steel, which has as its essence a combination of the alloy addition necessary for obtaining an acicular ferrite structure effective for sulfide stress corrosion cracking resistance, hot working conditions and tempering conditions. .

Next, the reasons for limiting the steel composition of the steel produced by the present invention will be described. All the following percentages are weight percentages.

C: C is effective for increasing the strength of steel. However, if the addition amount exceeds 0.15%, the hardenability is increased, the structure becomes martensite easily, and the acicular ferrite (AF) structure becomes difficult to appear. Therefore, C is 0.15% or less.

Si: Si is added for deoxidation. However, if the addition amount is less than 0.1%, there is no effect, and if the addition amount 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 makes steel brittle. The lower the cost to obtain the toughness, the better the cost. Therefore, the upper limit of the content that does not substantially pose a 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, but the cleanliness of the steel is reduced and toughness is reduced. 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, if the addition amount is less than 0.01%, there is no effect of expanding the γ loop, 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, No, Nb, V, Ti: One or more of these elements 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, huge carbides are formed, so Ni0.2 to 2.5%, Mo0.2 to 1.5%, V0.02 to 1.5%, T
i 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 kind or two or more kinds 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 ferrite austenite state without heating. Result of experiment Cr% ≧ 10 × C (%) + 30 ×
It is necessary to satisfy N (%) + 8.

Next, the hot rolling process of the present invention will be described.

The reason for limiting the heating temperature of the slab consisting of the steel components of the present invention is that hot working in the complete austenite region is necessary to obtain an acicular ferrite structure, and therefore it is 950 ° C. or higher, and austenite coarsening is prevented. Therefore, the temperature shall be 1250 ° C or lower.

The hot working rate is required to be 60% or more to form a fine structure that does not deteriorate the sulfide stress cracking resistance.

On the other hand, in terms of the finished plate thickness, the normal working rate is 95% or less, so the upper limit was made 95% or less.

The processing temperature is required to be 750 ° C or higher because it needs to be processed in the complete austenite region in order to obtain the acicular ferrite structure. After processing, it is necessary to start cooling as soon as possible after processing in order to prevent coarsening due to recrystallization and growth of austenite particles after processing. Regarding the cooling rate, the cooling rate was set to be air cooling or higher because it takes a high cost to control cooling if it is less than air cooling, and there is no structural problem even if cooling is made to be air cooling or higher.

The reason for limiting the tempering temperature is as follows.

Tempering is essential for removing internal stress that is harmful to sulfide stress corrosion cracking resistance, and its effective tempering range is 400 ° C to 750 ° C.
Up to a temperature below ℃. The lower limit of 400 ° C. or higher is because there is no effect on stress relief below the lower limit and ferrite precipitation occurs at the upper limit of 750 ° C. or above. Further, it is desirable that the tempering time is 15 to 60 minutes from the viewpoint of work efficiency.

As described above in detail, if the component steel of the present invention is manufactured under the processing conditions of the present invention and the steel structure is made to have the acicular ferrite structure of 80% or more, the sulfide stress corrosion cracking resistance can be remarkably improved. . The original excellent corrosion resistance of this steel can be directly accepted.

(Example) Steels having the compositions shown in Table 1 were processed under the manufacturing conditions shown in Table 1 to produce steels, and the results of testing each steel are shown in FIG. Steels A to T are according to the method of the present invention, and U to Z are comparative examples according to the conventional method.

The test was carried out by using a four-point bending jig in NACE liquid as the critical stress (σth), which is the minimum stress that does not fracture even after 336 hours have been applied with various stresses. The horizontal axis of Fig. 1 shows the proof stress (YS) of the sample steel, and the vertical axis shows the critical stress.
Indicates steel according to the method of the present invention, and ● indicates comparative steel.

As is clear from the results shown in FIG. 1, the steel according to the present invention has a significantly improved resistance to sulfide stress corrosion cracking.

Regarding the carbon dioxide corrosion resistance of the steel according to the present invention, it goes without saying that it has the same excellent corrosion resistance as the conventional 13% Cr steel.

(Effects of the Invention) According to the present invention, the sulfide stress corrosion cracking resistance that cannot be obtained by the conventional reheated QT steel is achieved, and the reheat quenching step is not required. Since it is possible to produce a low-cost steel that can be advantageously used for the above, its industrial effect is great.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the critical stress and the proof stress of the steels manufactured by the method of the present invention and the comparative example.

Claims (2)

[Claims] 1. C .: 0.15% 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%, balance iron and inevitable impurities, and Cr% ≥
After steel with a composition satisfying 10 x C (%) + 30 x N (%) + 8 is heated to a heating temperature of 950 ° C to 1250 ° C, hot working is performed at a finishing temperature of 750 ° C or higher and a working rate of 60% to 95%. After processing, cool to room temperature at a cooling rate of air cooling or higher, and then temper at a temperature of 400 ° C to less than 750 ° C for 15 minutes to 60 minutes to make the acicular ferrite structure have an area ratio of 80% or more. A method for producing an acicular ferritic stainless steel having excellent stress corrosion cracking resistance. 2. Weight% C: 0.15% or less 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%, 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%, 1 type or 2 types or more, and balance iron and Steel consisting of unavoidable impurities and having a composition satisfying Cr% ≥ 10 x C (%) + 30 x N (%) + 8 is heated to a heating temperature of 950 ° C to 1250 ° C, and a working rate of 60% at a finishing temperature of 750 ° C or higher. ~ 95% hot working, after processing cool to room temperature at a cooling rate of air cooling or higher, and then temper for 15 to 60 minutes at a temperature of 400 ° C to less than 750 ° C to obtain an area of acicular ferrite structure Of 80% or more in terms of percentage, a method for producing acicular ferritic stainless steel with excellent stress corrosion cracking resistance.