AT395018B - METHOD FOR PRODUCING A CORROSION-RESISTANT FERRITIC-AUSTENITIC STEEL - Google Patents

Description

AT 395 018 B

The present invention relates to the precipitation hardening of fenitic-austenitic corrosion-free steel with very high strength, good ductility and notched impact strength and good resistance to general corrosion and grain boundary corrosion.

One of the special properties of high-chromium ferritic-austenitic steels is that they have very good corrosion resistance even in aggressive environments. Steel of grade SIS 2324, which contains approx. 0.1% C, 26% Cr, 5% Ni and 1.5% Mo, is an example of this type of steel. This steel is used for many purposes where high demands are placed on corrosion resistance, and steel of this grade also has high strength compared to conventional austenitic corrosion-free steel. Often, however, there is a great need for a steel with even higher strength than steel of grade SIS 2324 in Combination with good ductility and impact strength as well as good corrosion resistance

Precipitation hardening is a frequently used method for obtaining such increased strength. In order to achieve the finely dispersed precipitation which is necessary for the desired increase in strength, a prerequisite is that the steel has a suitable alloy composition and a suitable aging treatment is carried out. which is conventionally carried out at a relatively low temperature usually follows solution heat treatment at a high temperature.

As far as the aforementioned high-chromium ferritic-austenitic steels of grade SIS 2324 are concerned, it has proven to be difficult to carry out precipitation hardening with satisfactory results. If the canting is carried out at a temperature between about 400 ° C and 525 ° C, the well-known 475 ° C embrittlement occurs, and if the canting is carried out between 700 ° C and 850 ° C, embrittlement embrittlement also occurs the σ phase.

It is therefore difficult to carry out the aging in practice within the above-mentioned temperature ranges without having to accept an unacceptable reduction in the impact strength. Also, aging treatments between 525 ° C and 700 ° C are unfavorable because aging treatments between 500 ° C and 750 ° C very quickly lead to an unacceptably high susceptibility to grain boundary corrosion.

In the past fifteen years, however, various precipitation hardening processes for ferritic corrosion-free steels have been proposed in the technical literature and in patent applications. For example, it has proven possible to use the 475 ° C embrittlement to a certain extent for precipitation hardening if the austenite content is kept sufficiently high. Precipitation hardening with Cu and Al is just as possible as with Be. For goods subject to wear, it is even possible to use the σ phase for the purpose of precipitation hardening. Precipitation hardening with austenite or ferrite has also been proposed (e.g. in U.S. Patent 4,353,755). However, no industrially manufactured steel has yet been obtained as a result of this work. The main reason for this is an unfavorable combination of properties of the steel or the sensitivity to cracking during forging, or the steels require a solution annealing temperature which is practically unrealistic (> 1200 ° C).

When looking for a high-strength ferritic-austenitic steel with good corrosion properties, it has now surprisingly proven to be possible to achieve these properties with ferritic-austenitic steels of the following type: approx. 20% Cr, approx. 6% Ni, approx. 2% Mo etc. if the Si content is increased to approx. 2% and if Nb or Ti is added to the steel to bind C and N, and if the steel is solution annealed at 1050-1150 ° C and then at 500 -700 ° C is outsourced. Furthermore, favorable hot forming properties can be obtained if the steel composition is otherwise properly balanced and if the proportion of impurities is kept relatively low. The present steel is therefore new and satisfies a high demand in the market for cast and forged products.

The present invention can therefore be defined as a method for producing a corrosion-resistant ferritic-austenitic steel with high strength and toughness and good manufacturing properties, which method is characterized in that the steel has the following composition in percent by weight: C as low as possible, but at most 0.10, preferably at most 0.06, suitably at most 0.04;

Si 1.0-3.0, preferably between 1.5 and 2.5;

Mn at most 5.0, preferably at most 2.5, but expediently at most 2.0;

Cr 18.0-26.0, preferably 18.0-24.0, but expediently 18.0-22.0;

Ni 4.0-10.0, preferably 4.0-8.0;

Mo 1.0-4.0, preferably 1.5-3.5, but expediently 1.5-3.0;

Nb at most 2.0, preferably at most 1.5, but expediently at most 1.0;

Ti at most 1.5, preferably at most 1.0, but expediently at most 0.5;

Remainder iron and impurities as well as additional components usual for this type of steel, -2-

AT 395 018 B where (atomic% Nb + atomic% Ti) > Atomic% C, preferably 2 (atomic% C + atomic% N) and that the steel is solution annealed at 1000-1200 ° C and aged at 500-700 ° C. The aging is advantageously subjected to pre-aging at 400-500 ° C and a final aging at 500-700 ° C.

The following examples serve to illustrate the invention.

example 1

Tables I and Π relate to investigations of a »number of steels 1-4 which were investigated in the as-cast state. The tables show examples of high Si content (> 1%) defined by the composition limits of the claims as defined above, and the examples further clarify the precondition that the steel should be stabilized by Nb or Ti. Steels 3 and 4 thus fall within the ranges specified for the invention. lamp

composition

Stole -

No. C Si Mn Cr Ni Mo Ti Nb N 1 0.02 0.8 0.6 22.0 6.4 2.4 0.35 _ 0.05 2 0.03 1.9 0.7 18.8 5.4 2.4 - - 0.06 3 0.03 1.9 0.7 18.8 6.0 2.4 - 0.72 0.06 4 0.03 2.0 0.6 193 6, 3 23 0.39 - 0.04

Following solution annealing at 1150 ° C, followed by aging at 600 ° C, the following thickness values were measured.

Table!!

Binell hardness (HB) after solution annealing after aging Increase hardness 1 235 245 10 2 245 260 15 3 255 315 60 4 250 310 60

Solution heat treatment at 1200 ° C gave a higher hardness by about 5-15 units HB, while solution heat treatment at 1000 ° C or lower gave a relatively insignificant precipitation hardening effect in all steels. A pre-aging at approx. 475 ° C, followed by a final aging at 600 ° C, has proven to be very favorable from the point of view of precipitation hardening. With steels 3 and 4, hardnesses of up to 350 HB could be achieved through this double outsourcing.

BeigpieU

In order to be able to investigate the properties of a steel according to the invention more thoroughly, a steel of the following composition was produced:

C Si Mn Cr Ni Mo Ti N 0.03 2.0 0.6 19.5 6.0 2.0 037 0.04 -3-

Claims (10)

AT 395 018 B This steel had excellent hot forming properties and could be easily forged to the desired dimensions without the risk of cracking. The steel was examined in a series of different conditions of heat treatment and sample thickness. The following properties were obtained with a thickness of 60 mm: heat treatment ^ p02 ^ 5 KU Mpa% J 1125 ° C, water + 550 ° C 750 19 49 tt + 575 ° C 720 20 32 tt + 600 ° C 690 20 35 1050 ° C, water + 550 ° C 720 22 45 tt + 600 ° C 660 28 41 RpQ ^ = yield strength 0.2 A ^ = elongation (elongation at break) in the tensile test at a measuring length L = 5d for circular cross-section or a general measuring length of L = 5.65 A. Both the diameter d and the cross-sectional area A relate to the original dimensions. KU = impact strength (J) using U-notched samples according to Charpy. By pre-outsourcing at z. B. 475 ° C before final aging at 550-600 ° C, the precipitation hardening effect could be increased with approx. 50 MPa. With a thickness of 25 mm of the samples, a yield strength that was 20-50 MPa higher than stated above was measured, while the yield strength at a thickness of 200 mm of the sample was estimated to be 20-50 MPa below the aforementioned values, based on simulated heat treatment tests Reference is made. Under the aforementioned heat treatment conditions, the steel has shown good resistance to grain boundary corrosion in tests in boiling NaCl solution which was saturated with AgCl and Ca (OH) 2. Other properties related to corrosion were also good. The present invention therefore specifies a steel which has very interesting features for future use. The steel has a comparatively simple composition, is easy to manufacture and can be heat-treated in conventional heat treatment furnaces. PATENT CLAIMS 1. Process for producing a corrosion-resistant ferritic-austenitic steel with high strength and toughness and good manufacturing properties, characterized in that the steel has the following composition in% by weight: C max. 0.10 Si 1.0 to 3.0 Mn max. 5.0 Cr 18.0 to 26.0 Ni 4.0 to 10.0 Mo 1.0 to 4.0 Nb max. 2.0 Ti max. 1.5 -4- AT395 018 B remainder iron and impurities as well as normal additional components for this type of steel, whereby (atom% Nb + + atom% Ti) > Atomic% C, preferably > (Atomic% C + atomic% N), and that the steel is solution-annealed at 1000 to 1200 ° C and aged at 500 to 700 ° C. 2. The method according to claim 1, characterized in that the steel solution annealed at 1050 to 1150 ° C and then aged at 500 to 700 ° C. 3. The method according to claim 1 or 2, characterized in that the steel is pre-outsourced at 400 to 500 ° C before it is finally outsourced at 500 to 700 ° C. 4. The method according to claim 1, characterized in that the composition of the steel in wt .-% is as follows: C max. 0.06 Si 1.5 to 2.5 Mn max. 2.5 Cr 18.0 to 24.0 Ni 4.0 to 8.0 Mo 14 to 3.5 Nb max. 1.5 Ti max. 1.0 balance iron and impurities as well as normal additional components for this type of steel, where (atom% Nb + + atom% Ti) £ atom% C, preferably > (Atomic% C + atomic% N). 5. Process according to claims 1 to 4, characterized in that the composition of the steel in wt .-% is as follows: C max. 0.04 Si 14 to 2.5 Mn max. 2.0 Cr 18.0 to 22.0 Ni 4.0 to 8.0 Mo 1.5 to 3.0 Nb max. 1.0 Ti max. 0.5 remainder iron and impurities as well as normal additional components for this type of rod, where (atom% Nb + + atom% Ti) > Atomic% C, preferably £ (atomic% C + atomic% N). 6. Corrosion-free ferritic-austenitic steel with very high strength, good ductility and impact strength as well as good resistance to general corrosion and grain boundary corrosion, characterized in that it has the following composition in% by weight: C max. 0.04 Si 14 to 2.5 Mn max. 2.0 Cr 18.0 to 22.0 Ni 4.0 to 8.0 Mo 1.5 to 3.0 Nb max. 1.0 Ti max. 0.5 balance iron and impurities as well as normal additional components for this type of steel, where (atom% Nb + + atom% Ti) > Atomic% C, preferably > (Atomic% C + atomic% N) and that the steel has a structure achieved by solution heat treatment at 1000 to 1200 ° C and aging at 500 to 700 ° C. -5- AT 395 018 B 7. Steel according to claim 6, characterized in that it contains a maximum of 0.1 N. 8. Steel according to claim 7, characterized in that it has a structure which is obtained by precipitation hardening by means of solution annealing at 1050 to 1150 ° C and subsequent aging) at 500 to 700 ° C 5 9. Steel according to claim 7 or 8, characterized in that it is pre-outsourced at 400 to 500 eC at 400 to 500 ° C before final aging 10. Steel according to claim 8 or 9, characterized in that it has the following nominal composition: 10 c 0.01 to 0.03 Si 2.0 Mn max. 2.0 Cr 20 Ni 6 Mo 2 Nb (Ti + -) 0.3 to 0.5 2 N max. 0.1 balance iron, impurities and additional components in normal proportions. 25 30 35 40 45 50 -6- 55