DE1924487A1 - Rust-resistant ferrite-austenitic steel - Google Patents

Description

AKTIEBOLAGET BOFORS, 690 20 Bofors / Sohweden

Rust-resistant ferrite-austenitic steel

The present invention relates to a method for producing a ferrite-austenitic Steel with high resistance to intergranular corrosion.

In those cases where you used to have good corrosion resistance, high strength, high toughness and wished for good forgeability, so far ferrite-austenitic steels have often been used, the

$ 0.1 carbon
$ 27 chrome
5 i · Niokel and
1.5 i * molybdenum

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which corresponds to a bar according to SXS 2324. (SXS a Swedish Industry Standard). It has However, it has been shown that steel of the type mentioned in

many cases, for example whale oil separators, where a relatively aggressive medium is combined with different operating temperatures, is subjected to extremely obvious intergranular corrosion and is therefore unsuitable. This intergranular corrosion has been demonstrated on a laboratory scale by boiling with sodium chloride solution saturated with 3% silver chloride. By such laboratory tests it has been found that the aforementioned steel according to SIS 2324 is extremely sensitive to intergranular corrosion in heat treatments to 65O ⁰ G, even with relatively short treatment times of the order of 1 min. From this behavior results in that, even when using Water cooling, after a heat treatment at about 1000 C, practically cannot prevent such a sensitivity to intergranular corrosion from occurring in the steel type mentioned, especially when it comes to coarser parts, such as spherical bodies for separators.

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The present invention made it completely surprising proved to be possible, this sensitivity to intergranular corrosion in rust-resistant Eliminate ferrite-austenitic steels and at the same time the good forgeability that maintain high general corrosion resistance and good strength properties.

The method according to the invention for the production of rust-resistant ferrite-austenitic steel with high resistance to intergranular corrosion is characterized in that such a large amount of niobium is added to a ferrite-austenitic steel that the quotient Nb _R goes up to at least 8, with Hb-. denotes the total niobium content reduced by the amount of niobium that is required to bind the nitrogen in the steel to form miobium nitride, and C refers to the total carbon content in the steel. The ferrite-austenitic steel to which niobium is added can expediently have the following composition

C max. 0.12 Jl

Si max. 20th I. ο * Mb max k 2.0 N max. 1 O, 1 Cr - 3O * Ni - 8th < Mon -25

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The remainder is iron as well as the usual impurities for the steel type and accompanying substances. Said ferritaustenite also Steel can contain with advantage!

C $ 0.02 - $ 0.08>

Si max. 0.5 #

Mn max * 1.0%

N max. 0.06 #

Cr 24 - 27 #

Ni 5 - 7 % ■ '

Mon 1.3-1.8 #.

The invention also relates to steel made according to any of the aforementioned types.

It has been suggested in the past to add niobium in order to avoid intergranular corrosion, but these were just conditions related to welding and you has assumed that an addition of niobium, which corresponds to eight times the carbon content, is sufficient would be to eliminate susceptibility to intergranular corrosion. Both indications However, in the specialist literature as well as our own tests have shown that such an addition of niobium does not the sensitivity to intergranular corrosion could eliminate, and it must therefore be described as extremely surprising that such, obtained significantly improved resistance to intergranular corrosion in the present invention has been.

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The invention will now be further elucidated with reference to the study of a number of different steels are described, the chemical composition of which is given in Table 1. Steel No. 1 in this one Table corresponds to the aforementioned steel type naoh SIS 2324. And the other steels are close to the increasing Value for the quotient Nb_, arranged

In addition to the chemical composition listed in Table 1, the third to last column also shows the calculated amount of carbon that is not bound as niobium carbide, taking into account the behavior that the niobium is primarily due to the nitrogen occurring in the steel Table 1 also shows the values for the quotients of the total niobium content and carbon (Nb,.) and the quotient Nb _R wor- - ™ - -

You steels 1 and 9 in Table 1 have been Silberchlorld saturated 3 ^ Lger sodium chloride solution examined with the use of cooking "old in Beaug corrosion at various isothermisohen treatments · The result of these investigations from Figure" 1 out ₉ in the curve A the TeM -peratur @ n (ΐ) in ⁰ G and treatment areas (t) in

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Hinuten indicating where a weight loss of 5 Ί "has arisen for steel Nos. 1, while the curve B represents the weight loss of .10 £ for steel 1 * The curve C corresponds to a weight loss of 5 i» in the steel Nos. 9 and curve D finally shows a weight loss of 10 ή » for steel 9 · Within the areas to the left of curves A and B, the corrosion took place mainly in the form of point attacks. Within the areas between curves A and B or * C and D, the attacks on the respective steel consisted of both point attacks and intergranular corrosion. On the right of curves B and D, the corrosion attacks were predominantly of an intergranular nature. From Fig. 1 it is apparent that in 2324 the intergranular corrosion has its maximum value at about 65O ⁰ C for steel according to SIS and that, as has been mentioned earlier, even with such short treatment times, such as about a minute, an apparent intergranular corrosion receives 1 can also be seen that a steel (No. * 9) according to the present invention has significantly less sensitivity to intergranular corrosion and that, in contrast to steel according to SIS 2324, there are no practical difficulties with steel

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* ^% ORlGHiALi ^ iFEGTi

according to the present invention with sieve, brings to achieve a so son eiles cooling after heating to about 1000 ⁰ C, that runs even for large dimensions is no risk of this range, the sensitivity to intergranular corrosion arises, to pass.

The steels listed in Table 1 have been subjected to various heat treatments with regard to corrosion attacks have been investigated, and in Table 2 are the types of corrosion attack for some of the examined steels and at the same time the quotients Nb. . and Nb ", indicated. From Table 2, German

C C

revealed that vigorous intergranular corrosion attacks as well as point attacks even at such high levels

Nb. . Quotients such as 12 (steel iir _e h) occur, where to * c

however, the quotient Nb _n . is only 1. For

kind

Steel no. 6 with the quotient Nb. 'equal to 22 and

Nb _n . equal to 7 »can still be certain if nest

relatively moderate intergranular corrosion attacks can be found in addition to point attacks. With regard to steels 7, 8 and 9, which all have the quotient Nb _n,. Values above 8, the corrosion

sion mainly in the form of point attacks. If-

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For example, steel no.7 has a lower vert for the quotient Nb. . As steel No. 6 has, however, the intergranular corrosion of steel No. 7 is much smaller than that of steel No. 6. In other words, Table 2 clearly shows that the quotient Nb ₁₃ , the corresponding

cT
de factor for the type of corrosion, whereas the quotient Nb,, in this context of

C.
is of ordered importance. Similar investigations with corrosion conditions during boiling for 2k h in 3 ig * r sodium chloride solution saturated with silver chloride are shown in Fig. 2. As can be seen from Fig To emphasize this, the difference "wipe" the weight loss ( ^v _D 4f "f ·) in the quenched state and the weight loss in the quenched state mentioned in connection with isothermal treatment is plotted on the vertical y axis in FIG. 2 . This isothermisohe treatment was carried out for 2 hours at 600 ° C (θ) on one side and the other sum at 70o ⁰ C for 2 h (ca). The proportions between niobium and carbon have been illustrated on the horizontal ζ-axis.

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point (coordinate start) has chosen the point where the quotient Nb _n . = 7.8, ie where on

ΛΘ8Ό

C.
1 mole of Nb, the _{remainder of} 1 mole of carbon, is omitted. At this point there is neither free carbon nor free niobium, but these two elements are completely bound to one another in the form of niobium strbide (NbC). When calculating the amount

Nb _n . is, as has been pointed out earlier, bextesu

It has been taken into account that the nitrogen occurring in steel combines with niobium before carbon to form niobium nitride (NbN) In fact, a certain amount of niobium can be bound in the form of niobium oxide or other niobium-containing slag5 but, as is normally the case with steels of this type If aluminum occurs in an amount of a few hundredths $, such bonding of niobium to oxygen is of no practical importance. For values of the quotient Nb ". Which are below 7» S, the niobium will of course not be sufficient to bind all the carbon, but rather a certain amount of free carbon will be present in the steel, and this amount along d * a right part of the horizontal x-aohae alc % freer

-, 10 -

9098 48/0728 oi »^ ^L » * ^SFECTED

Carbon (C ") has been drawn. For values of the quotient Nb _n ,, which are above 7 »8,

KcSl

C.
Niobium is in excess and the steel contains a certain amount of free niobium and this amount has been plotted on the left part of the horizontal x-axis as i> free niobium (Nb-.) "

From Fig. 2 it is clear that for the Steels for which the quotient Nb _n . considerably

i £ 2JLi

_i £ 2JLi is below 8 (steel nos. 1 to 5) 1 ^the weight loss caused by intergranular corrosion is considerably greater than for steels (nos. ₉ 7-11), in which the quotient Nb_ is greater than

the vert 8 lies.

The addition of niobium should not, however, be made too large, since the undesirable tendency towards ε ~ phase formation, which has already been determined in steel according to SIS 2324, increases with the addition of niobium. This tendency to increased 6 * -Phasenbildung with larger niobium additive is shown in Table 3, where the 6 "-Phasenbildung for most of recorded in Table 1 Steels verschiedenett at SWEi Wärmebehandlungearten was examined on the one hand heating at 975 ⁰ C for 1 h and cooling in water as well as

- 11 -

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ORIGINAL INSPECTED

h following sensitization at 7O0 ° C for 2 and h for other equal Absehrecken by heating 975 ° C for 1 and then sensitize for 2 hours, but this time seen at 800 ⁰ C, Vie from Table 3, could not ό "phase can be detected at the lower sensitization temperature, while, on the other hand, at the higher sensitization temperature the amount of detectable & phase increases with increasing Vert for the

Ratio Nb _R increased. This (5 ^ phase formation <j

however, one can counteract that the

Quotient Nb_. xwisohen 8 and 20 is held and

C.
at the same time the silicon content is low, two-fold

wise at about 0.5 fL,

Table 3 also shows the austenite content in steels after quenching from heating for 1 hour at 975 ° C. This shows that steel No. 2 had a significantly higher austenite content than the other steels, and this high austenite content was also found to be less suitable, since the moldability obviously deteriorated, with the yield point being lowered at the same time. The austenite content should be between 5 and 30 pounds and preferably between 15 and 25 i * , lower austenite

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tenite content primarily has an unfavorable effect on the notch toughness and in order to obtain a suitable austenite content in steel, the quantities of nickel, manganese and chromium must be coordinated in an appropriate manner. Chromium is absolutely necessary in order to give the steel the desired good corrosion properties, but its content should not exceed approx. 30 fd , as otherwise the tendency towards Cf phase formation increases. The presence of molybdenum in the steel is also useful for increased corrosion resistance and the molybdenum content can preferably be around 1.5 ^. The carbon content should be for two reasons kept low becoming one side, the consequent low carbide favorable effect on the Sctimiedbarkeit and Kerbsähigkeit and sum other is a reduced carbon content reduce the amount Zusatzniob that required 1st, so that the ratio Nb _n. up to at least 8. To Koh-

. Kef t

To keep fuel contents below 0.04 · , however, special metallurgical processes are required, which obviously make the steel more expensive, and in the present case therefore around 0.05 Ί » carbon seems to be

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• - 13 -

to be appropriate. Ow the nitrogen content should be kept low to limit the amount of niobium addition required.

Finally, table k shows some strength properties for the steels examined. «These strength properties are in accordance with ISO Recommendations No. R 82 (1959) and R ik8 (1960). As can be seen from Table k , the strength properties have largely been retained in spite of the addition of niobium, and the present invention has therefore made it possible to produce a rust-resistant, ferrite-austenitic steel which has practically equivalent strength properties as the Ferrite made from tenitia ο he steel according to SIS 2324 previously used, but has a much lower sensitivity to intergranular corrosion.

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Claims (1)

PATENT CLAIMS 1 · Process for producing rust-resistant (stainless) ferrite-austenite steel with high resistance to intergranular corrosion, characterized in that such a large amount of niobium is added to a stainless steel that the quotient ^Nb Re, t
C. is at least 8, where Nb _n . Total niobium content Me s χ less by the amount of niobium means that are used to bind from nitrogen found in steel is consumed to niobium nitride, and C is total carbon in steel means, 2, method according to claim 1, characterized in that the ferrite-austenitic steel, to which niobium is added, has the following composition C. Max. 20th 0 , 12 % Si Max. H J .0 £ Mn Max. 1 2 , 0 * N Max. 0 ,1 * Cr - 30 Ni - 8th i, Mon - 2nd , 5 *, The remainder of sisen as well as the usual impurities for the steel type and accompanying substances. - 15 909848/0728 3 · Method according to claim 1, thereby characterized in that the ferrite-austenitic steel to which niobium is added has the following composition C 0.02 - 0.08 i » Si max. 0.5 i » Mn max. 1.0% N max. 0.06 <ft Cr 24-27% Ni 5 - 7% Mon 1.3 - 1.8 £, The remainder is iron as well as the usual impurities for the steel type and accompanying substances k. Rust-resistant ferrite-austenitic steel, characterized in that it has been manufactured according to one of the preceding claims. 909848/0728 Table 1 - Chemical composition of investigated steel reading systems stole C. Si Percentage of Cr Ni Mon Nb N Calculated amount
Carbon the ^ dead 0 ^Nb rest 0 I. No. 0.088 0.37 Mn 27.0 5.3 1.42 - 0.030 not available as NbC
is bound C. 0
6th C. 0
0 m
jt
\
I. CD
O
CO 1 0.017
0.024 0.57
0.40 0.71 25.8
24.8 8.0
5.4 1.57
1.43 0.15 0.047
0.051 0.088 12th 1 XO
* · » 2
3 0.021 0.38 0.84
0.80 26.1 5.5 1.48 0.25 0.034 0.017
0.024 7th 5 OS
> ». 4th 0.11 0.53 0.83 25.7 5.1 1.48 0.71 0.029 0.018 22nd 7th O
- «3
N) 5 0.025 0.37 0.77 25.8 . 5.6 1.51 0.56 0.059 0.044 17th 11th cn 6th 0.066 0.36 0.79 25.7 5.2 1.40 1.14 0.060 0.004 39 19th 7th 0.022 0.40 0.74 25.9 5.6 1.45 0.86 0.066 0 36 27 8th 0.025 0.40 0.81 27.0 6.1 1.41 0.90 0.030 0 50 41 9 0.024 0.38 0.65 25.5 6.0 1.48 1.19 0.032 0 60 45 10 0.025 0.34 0.82 25.4 5.9 1.41 1.50 0.059 0 11th 0.77 0 ■■ Es ·· ' Table 2 - Type of corrosion attacks after heat treatment 975 ⁰ C 1 h - 6OQ ⁰ G, 2 h. V / ater stole σ 0
6th Nb _rest 0
0 Corrosion attack No. . 12th σ 1 1
3 22nd 7th Strong intergranular corrosion
+ Point attacks 4th 17th 11th 6th 39 19th Massive intergranular attacks
+ Point attacks; 7th 36 ' 27 Mainly point attacks 8th - "- 9 Table 3 - Structure according to different types of Y / heat treatment stole Austenite content (#) £ "phase salary ($) 975 ° C, 1 h
800 ⁰ C for 2 h No. .975 G, 1 h, water 975 ⁰ G, 1 h-700 ° G, 2 h traces 1
2 25th
45 None (5 "phase after
assignable traces 3 24 No Ö phase after
assignable - »- 4th 18th • ^w - - "- 6th 15th - 0.5 7th 6th - '· -. -0.5 .8th 7th -.si- ; -1.0 9 24 - ·· - j -1.0 ίο Ι 10 ¹ -1.5
i 11th 6th "9 03 8 4 870738 foy Tabel * ί - Mechanical properties of investigated steel lofts 33 ' .Stole
No. Stretch limit
kp / mia Breaking point
kp / mm Dehnuni !;
ψ Constriction
* Notched impact strength
KCV
kpm 1 57 72 21 _ 9 2
ι 47 70 34 71 20th 3 48 67 31 69 VJl 5 52 68 31 63 16, ■ 52 66 25th 65 8 OQ.'l ~ τ7 '■ " 59 69 24 56 6th 8th 56 · 67 22nd 59 7th 9 58 68 23 59 . '. ^6th 10 54 67 * 26th 61 6th 11th 55 66 23 54 6th IC S- OO Empty soap