JPS5831068A - Ferritic-austenitic duplex stainless steel - Google Patents

Abstract

PURPOSE:To obtain a ferrite-austenite 2-phase stainless steel with enhanced resistance to nitric acid by increasing the Cr and Ni contents of a steel and specifying the Ni balance. CONSTITUTION:This ferrite-austenite 2-phase stainless steel consists of <=0.03% C, <=2.0% Si, <=2.0% Mn, 25-35% Cr, 6-15% Ni, <=0.35% N and the balance Fe with inevitable impurities or further contains 0.001-0.030% B and satisfies -13<Ni balance <-9. The Ni balance is represented by Nieq-1.1Creq+8.2 in the equation (I). The amounts of P and S among the inevitable impurities are regulated to <=0.010% P and/or <=0.005% S. By said steel composition the high Cr 2-phase stainless steel having superior corrosion resistance in an environment contg. nitric acid and superior weldability is obtd. at a low cost.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a ferrite-austenitic duplex stainless steel having excellent corrosion resistance in a nitric acid environment.

In a nitric acid environment, stainless steel with a high Cr content has strong dovetail resistance, and depending on the degree of nitric acid immersion, grain boundary corrosion is extremely high. For example, 310LC13/θLCNb or the like is used. However, since the solid solubility limit of carbon (Q) is small in such austenitic stainless copper with a high Nl content, chromium carbides precipitate at the grain boundaries due to heating at 300 to 900 C or under the influence of welding heat, making it difficult to resist. It has weaknesses such as a tendency for intergranular corrosion to deteriorate, a high sensitivity to solidification cracking during welding, and a lack of reliability in welded parts.

On the other hand, ferrite-austenite duplex stainless steel has a high Cr' content and high resistance to solidification cracking during welding, but on the other hand, it is susceptible to selective corrosion between structures due to the effects of welding heat. This tendency is strong, and conventional duplex stainless steel has problems with reliability as a nitric acid-resistant material with a welded structure.

The present inventors conducted research on the influence of the structure and trace elements on the nitric acid resistance of stainless steel, and as a result, they solved the above-mentioned drawbacks of austenitic stainless steel and duplex stainless steel, and achieved excellent nitric acid resistance and weldability. However, it is obtained from 452-phase stainless steel at a low price. That is, the present invention sets the C and N1 contents higher than the conventional 7-erite-austenite bi-phase pots, which are Cr -3 to 3% and Nl 4I to A%, while at the same time achieving a specific N1 balance. , and also 31θLC and 31
Although the N1 component, which is expensive compared to 0LCNb, is lower, the nitric acid resistance is superior to those of these materials.We have found a structure with the best nitric acid resistance. In the present invention, IF 0.00/~0.03% of 8 is further added to improve the nitric acid resistance, and P%S contained as an unavoidable impurity is reduced to P O,0/0% or less, Sθ ,0
The nitric acid resistance is further improved by reducing the nitric acid resistance to 0.3% or less, and has the composition shown below.

(11% by weight Teco, 03% or less, sl 2.0% or less, Mn 2.0% or less, p O, 04/o% or less Sθθ3°% or less, Cr23-33%, NlA ~/
J%, NO, 33% or less, the balance V and unavoidable impurities, and satisfies the following formula: (2) 0.00/~0.0.7% B in the above steel. (31 above* in (1) and (2), P and S are added singly or simultaneously P O,0/ 0% or less, fJ O,00j %
The following is one thing that will be destroyed.

Next, the reason for limiting the range of the chemical composition of the present invention will be explained.

C: C is an effective austenite-forming element, but since it forms carbides and increases susceptibility to intergranular corrosion, it is better to reduce the amount as much as possible. However, the upper limit is set at 0.03% within a range that can be easily manufactured industrially.

S1 and 'QMn; Sl and Mn are elements used as deoxidizers during the steelmaking process, and for easy industrial production, it is usually necessary to add 81 and Mn in an amount of 2.0% or less. , each below 2.0%.

Cr; Cr is a ferrite-forming element, and is an important element for forming a two-phase structure of austenite and hepteryte. At the same time, it is extremely important for increasing corrosion resistance, especially resistance to nitric acid, and has good resistance. One thing to get nitric acidity! It is necessary to add 5% or more.

Nitric acid resistance improves as the Cr content increases under an appropriate microstructure balance, but if it exceeds 33%, workability deteriorates, making it difficult to manufacture steel materials and similar products, and it is impractical. The upper limit is set at 33%.

Nl: Nl is an austenite-forming element and is an important element for the formation of a two-phase structure together with Cr, but it is also an extremely important element for reducing the active dissolution rate such as general corrosion, and Cr is the main ferrite-forming element.
In order to obtain a preferable ferrite-austenite structure balance corresponding to the amount, it is necessary to add k in a range of 6% to 73%.

N: Like 0%N1, N is a strong austenite-forming element and is an effective element for improving corrosion resistance such as pitting corrosion resistance, but if N exceeds 0.3.5%, the steel manufacturing process In this case, blowholes may occur in the steel ingot and hot workability will deteriorate, so the content is limited to 0.33% or less.

In the present invention, there is no meaning in specifying each of these elements individually, and excellent effects are achieved only in an optimal combination.The range of each component is limited so as to satisfy the following formula. It has been found that this is necessary, and that this is also one of the features of the present invention.

One layer 3 < Nl balance <-9 Tohede rigid balance = rigid Q -/, / X CrB +
8. KoNIeq=N1%+O-! 5XMn%-+-3
0x(C+N)%Creq=:Cr%-)/,,4XS
If the Nl-4 lance is 1% or less, the selective corrosion tendency between the composite fibers increases, and under these conditions, even if C is increased, the nitric acid resistance cannot be improved (,Nl). Shifting the balance to a direction that is less favorable in terms of corrosion resistance may actually accelerate corrosion.On the other hand, increasing the N1 balance to greater than -9 increases the amount of expensive Nl added, which is economically disadvantageous. Not only this, but also the hot workability is inhibited and the corrosion resistance is also deteriorated, so the N1 balance is limited to -73 or more and -9 or less.

The improvement effect of nitric acid resistance by BIB addition is 0.00/%
It becomes noticeable when more than 0.03% is added, and if it exceeds 0.03%, the deterioration of weldability and workability is significant, so 0.00/
~0. Limited to O7%.

P and S; impurity elements P and S reduce this 4e
t2 is preferable, but as is clear from JIS and the like, po, o, 0% or less, and SO, 030% or less are normally permissible ranges. However, P 0010% or less, so, o
When O is limited to 3% or less, the effect of improving nitric acid resistance becomes even greater.

It has been found that adding a small amount of rare earth elements (REM) such as Lm and CI has the same effect in reducing P and S.

Next, the features of the duplex stainless steel according to the present invention will be explained in detail using examples.

The table below shows the chemical composition of the sample materials and their performance.

The corrosion degree values shown in Figure 1 and above show the results of the actual test of 3776 in a nitric acid plug.As is clear from Figure 1, Fe-Cr
The nitric acid resistance of -Nl-based duplex stainless steel is improved as the Cr content increases in the range of N1 balance (-73 to -9) in the present invention, and these types are better than conventional steels (310LC, 310LCNb). It shows excellent corrosion resistance. 0 as a criterion for corrosion resistance. /1 back・hr(
Approximately 0. / tightening part), C' must be 233 or more within the N1 balance range of the present invention.N1
Although the balance is outside the above range, the nitric acid resistance will eventually change.
The corrosion resistance is particularly bad, especially in steels with -/3 or less. One of these types is a welded part (welded metal part, limp part,
Corrosion in the heat-affected area is particularly severe, and selective corrosion in the austenite phase is becoming a problem. On the other hand, in the case of the present invention, there was no selective corrosion at the welded portion and sound corrosion resistance was confirmed by microscopic observation. Figure 1 shows the cross-sectional inspection results of a sample of welded material subjected to a practical corrosion test in a nitric acid plant. In the figure, (a) (・') shows a 3x magnification and 100x micrograph of perfume 1 (comparative steel), and (b) (b') shows perfume 2 (comparative steel).
Comparative steel), (cXc') is Ichiban 15 (comparative steel), (d)
(d') is Perfume 17 (comparative steel), (e>(a') is Perfume 6 (invention steel), (z) (f') is Ichiban 7 (invention! 1li), and ( g) (g') is a similar photograph of Steel No. 11 (invention).As is clear from the photograph, the invention steel is sound including the welded parts, but the composition of the invention- At a ratio of 111- which is outside the range, selective corrosion is observed in the welded portion.

In addition, even if the Nl balance is met, steel containing Mo is highly corroded, and Mo is an ingredient that should be avoided in this application (see 6.3.4). The nitric acid test results in the table above indicate 63% HNOs + Cr. "H
The results are shown as the average value of the amount of corrosion of the three tests. The nitrile resistance of the steel of the present invention is clearly superior to that of the comparative steel.
) and the PUS-reduced steel (m 13.14) have a particularly low amount of corrosion among the Honsokumei steels, and their nitric acid resistance improvement effect is remarkable.

The above table also shows the Fisco test (J
IS713/A! The results of evaluating the weld cohesive cracking susceptibility in terms of cracking rate (%) are shown below.

As is clear from this, duplex stainless steel is the steel of the present invention,
None of the comparative steels shows any cracking susceptibility, whereas 310LC and 310LCNb#, which are famous as nitric acid resistant materials, have a cracking rate of -0% and -co%, and are extremely sensitive to cracking.

As described above, the present invention is not a scale that has better nitric acid resistance than the austenitic high Cr -Nl- that has been conventionally used as a nitric acid-resistant material, but also has a reliability of welded parts, which is often a problem with conventional steels. Significant improvements were also observed in the areas of weld cracking and intergranular corrosion in the heat-affected zone of welds, so it is of great interest in industrial applications.

[Brief explanation of the drawing]

Figure 1 shows the degree of corrosion and C'' in actual tests at a nitric acid plant.
This shows the relationship with the content, and the diagrams (a) to
@, (a') to (g') are 11ll mirror (3x, 100x) photographs of welded materials of the present invention and comparative steel. 211 (l) (l') Figure 2 C non (pu〈') 2nd v! j (Cλ 21st!1 Cat) 2nd WI □ -11″Ce) (C'2 Figure 2 (fλ rf') Figure 2 (Piλ (f) Procedure Amendment (Method) 1. Display Showa J4 patent application No. 130≠412
No. 2, Title of the invention: Phaethoaustenitic two-phase stainless steel 3. Person making the amendment Relationship to the case: Applicant 4, Agent 5, Date of amendment order: 1326/1981 6゜8, Contents of the amendment/ Specification No. 1/Page 1 Line 1 2 Figure 1 “2nd to
Figure 11” is corrected. U Correct ``(a)(a')'' in line 20 of page 1 of the same book to read ``Figures 2 and 3''. 3. Change '(b)(b')' in page 1, line λ of the same book to 'Hirozu, Figure 5' K or '(c)(c') @, to '
Figures 6 and 7 have been corrected. Hiroshi Ibid., page 7.2, line 3' (d) (d') '
t-nff diagram, Figure 2”. '(@)(・')1 in page 12 of the same book is changed to 'Figure 70, Figure 1/', and '(f)(f')'' is changed to '(f)(f')''.
Figures 12 and 73 have been corrected. O, correct "(g)(g')-" in line j of page 12 of the same book as ``1st Hirozu, 1jmJ.Z. a)～(
g), (Fist ~(g')'' is corrected as 'λth~/jWJJ. L Correct Figure 2 (a) ~ (g), (a') ~ (g') as shown in the attached sheet. Figure 2Figure 3Figure 4Figure 5'Figure 6Figure 7Figure 8Figure 9Figure 10Figure 11Figure 12Figure 13Figure 14Figure 15

Claims (1)

[Claims] / CO, 03% or less, 81 J, 0% or less, Mn 2
．． 0% or less, Cr:lk ~33%, N16~73%,
A ferrite-austenite two-phase stainless steel having excellent nitric acid resistance, which is characterized by comprising No. 33% or less, the balance being Fe and unavoidable impurities, and satisfying the following formula (%). (c) P and S, which are unavoidable impurities, can be removed singly or simultaneously.
The ferrite-austenite dual-phase stainless steel according to claim 1, wherein the ferrite-austenite dual-phase stainless steel has a content of O, 0/0% or less and SO, 0,003% or less. 3. C0.03% or less, 81.1. Q% or less, Mn
2aQ% or less, Cr2A ~3!3%, N14~7
3%, No, 33% and B O,00/-0,030
%, the remainder consists of eaves d and unavoidable impurities, and the following formula %
A ferrite-austenite dual-phase stainless steel with excellent nitric acid resistance that satisfies the formula (%).
The ferrite-austenite dual-phase stainless steel copper according to claim 3, which has a content of O, 0/0% or less and so, o s%% or less.