EP0177739A2 - Use of a steel for parts in refrigeration technology - Google Patents

Abstract

The invention relates to the use of a cold-tough ferritic steel with low C contents in the range 0.015-0.08%, which is alloyed with approximately 5.5% Ni, 0.2% V, 0.02-0.03 % N and optionally 0.5-1.5% Cu, which was melted, poured off, solidified, hot-rolled and cooled to room temperature in a conventional manner with this composition. It is then subjected to a single normal annealing with recrystallization, possibly with accelerated cooling. The combination of analysis and annealing treatment gives the steel its special properties, such as good weldability, improved cold toughness and high strength, and can then be used for the manufacture of refrigeration components in areas below - 100 ° C. This steel can be used particularly advantageously for containers for the transport and storage of liquefied natural gas (LNG) down to - 165 ° C and also up to temperatures of approx. - 200 ° C, a temperature range that was previously only possible with very complex steels is covered.

Description

The invention relates to the use of a weldable, cold-tough ferritic Ni steel

With increasing demand for natural gas for energy supply, tanker ships with containers are more and more needed, which are suitable for safely transporting liquefied natural gas (LNG = Liquid Natural Gas) to the consumer countries. In addition to LNG, there are other liquefied gases such as B. ethylene, nitrogen, hydrogen and certain noble gases for ship transport in question. Liquefied gas can be transported inexpensively since it takes up only a small part of its volume, which is gaseous at room temperature. Despite the complicated technology - you need liquefaction and re-evaporation systems, transport containers on special ships and land vehicles, storage containers etc. - the investment costs for such an LNG chain are only about a tenth of an underwater gas line.

Dieser Siedepunkt stellt die Betriebstemperatur der kryogenen Anlagen dar. Sie müssen bei dieser Temperatur eine ausreichende Sicherheit gegen Undichtigkeiten und Brüche aufweisen. Bei kryogenen Temperaturen verlieren übliche Stahllegierungen einen großen Teil ihrer Zähigkeit und werden sehr spröde. Für den Bau der genannten Anlagen werden dementsprechend "kaltzähe Stähle" benötigt. Kaltzähe Stähle sind ferritische oder austenitische Baustähle, die durch besonders gute Zähigkeitseigenschaften bis zu sehr tiefen Betriebstemperaturen gekennzeichnet sind. Solche Baustähle können den üblichen Verarbeitungsvorgängen, wie Kaltumformen, Warmumformen, thermisches Schneiden und Schweißen unterzogen werden. Für die Wahl der Stahlsorte im Druckbehälterbau ist in der Bundesrepublik Deutschland das A_D-Merk- blatt W 10 - Werkstoffe für tiefe Temperaturen, Eisenwerkstoffe - maßgebend. Die tiefste zulässige Anwendungstemperatur ist vom jeweiligen Beanspruchungsfall abhängig. Kaltzähe ferritische Baustähle kommen bis zu Betriebstemperaturen des flüssigen Stickstoffes bei - 196 °C zur Anwendung. Bei noch tieferen Betriebstemperaturen, wie sie z. B. bei flüssigem Wasserstoff oder Edelgasen auftreten, werden nur noch austenitische Stähle eingesetzt, die sowohl höher legiert als auch weniger fest sind.The liquefaction of gases under atmospheric pressure occurs at their boiling point. The boiling point of the gases mentioned above is shown in the following table:

This boiling point represents the operating temperature of the cryogenic plants. At this temperature, they must have adequate security against leaks and breaks. At cryogenic temperatures, common steel alloys lose a large part of their toughness and become very brittle. Accordingly, "low-temperature steels" are required for the construction of the systems mentioned. Low-temperature steels are ferritic or austenitic structural steels which are characterized by particularly good toughness properties down to very low operating temperatures. Such structural steels can be subjected to the usual processing operations such as cold forming, hot forming, thermal cutting and welding. In the Federal Republic of Germany, the A _D data sheet W 10 - materials for low temperatures, iron materials - is decisive for the choice of steel grade in pressure vessel construction. The lowest permissible application temperature depends on the particular application. Cold-tough ferritic structural steels are used up to operating temperatures of liquid nitrogen at - 196 ° C. At even lower operating temperatures, as z. B. occur with liquid hydrogen or noble gases, only austenitic steels are used, which are both higher alloyed and less solid.

The most important alloying element for achieving sufficient toughness of ferritic structural steels at low temperatures is known to be nickel. The γ region is expanded by nickel, the A ₃ conversion point and the critical cooling rate are markedly reduced. As the nickel content increases, the drop in toughness is shifted to lower temperatures. Up to a nickel content of around 5%, the addition of 1% Ni causes the transition temperatures to decrease by approx. 30 ° C, above that there are improvements of about 10 ° C per 1% Ni.

Accordingly, a steel with approx. 9% Ni must be used for an operating temperature down to - 196 ° C.

Other known important measures for achieving high toughness at low application temperatures are reducing the carbon content and increasing the manganese content. A reduction in the sulfur and phosphorus contents is also known to have a favorable effect on the toughness properties.

For the main application of cold-tough ferritic structural steels, the transport and storage of liquefied natural gas, the 9% nickel steel X 8 Ni 9 is used worldwide. In the area of the boiling point of methane (- 161.5 ° C), this material has considerable toughness reserves; it is used up to the temperature of liquid nitrogen (- 196 ° C).

• max. 0,10 % C
• max. 0,35 % Si
• 0,30 - 0,80 % Mn
• max. 0,025 % P
• max. 0,020 % S
• min. 0,015 % Al
• 8,5 - 10 % Ni entsprechend der Euronorm 129-76 entweder wasservergütet, d. h.
  
  
  oder luftvergütet werden, d. h.
  
  Erst durch eine der beiden alternativen Wärmebehandlungen, deren Auswahl der Hersteller treffen kann, wird ein Gefüge aus angelassenem Martensit mit einer gewissen Menge an fein verteiltem Austenit eingestellt.

To set a uniformly fine structure with correspondingly good mechanical properties and toughness, the steel X 8 Ni 9 with a composition of

• Max. 0.10% C
• Max. 0.35% Si
• 0.30 - 0.80% Mn
• Max. 0.025% P
• Max. 0.020% S
• min. 0.015% Al
• 8.5 - 10% Ni according to Euronorm 129-76 either water-treated, ie
  
  
  or air-conditioned, ie
  
  Only through one of the two alternative heat treatments, which the manufacturer can choose from, is a structure made from tempered martensite with a certain amount of finely divided austenite.

als optimal angesehen. In dieser Tabelle bedeuten R _e die Streckgrenze, R_m die Zugfestigkeit, A₅ die Bruchdehnung am kurzen Proportionalstab und A die Kerbschlagzähigkeit.The aforementioned temperature ranges are based on the state of the art to achieve the material properties required by Euronorm 129-76

regarded as optimal. In this table, R _{e means} the yield strength, R _m the tensile strength, A ₅ the elongation at break on the short proportional bar and A the notched impact strength.

The nickel content makes a significant contribution to good low-temperature properties. However, nickel is a relatively rare metal. As more recent publications show, efforts are therefore being made to save nickel through alloying measures and special heat treatments (1.) Transactions ISIJ, Vol. 11, 1971, pp. 402 - 411; 2.) Transactions ISIJ, Vol. 13, 1973, pp. 133-144).

From US Pat. No. 3,619,302, tough, ferritic steels with <0.20% C; 0.05-0.40% Si; 0.10 - 5.0% Mn; 1.50-10.0% Ni; 0.05-1.0% Mo; 0.1 - 2.0% Cu; 0.1-1.5% Cr; <1.0% Nb; <1.0% V, remainder iron and melting-related impurities known. The preferred nickel content is in the range 4-7.5%. Nitrogen is expressly referred to as the inevitable and harmful impurity that needs to be set, preferably by Al.

• - zunächst bei Temperaturen zwischen dem A_cl- und Ac3-Um- wandlungspunkt zu glühen. Sie müssen danach an Luft oder sogar schneller erkalten.
• - Dieser ersten Wärmebehandlung muß eine Anlaßglühung unterhalb des A_cl-Umwandlungspunktes mit Abkühlung an Luft oder schneller folgen.

These high-alloy steels are only suitable for low temperatures through extensive heat treatments; namely, they are at room temperature after hot rolling and cooling

• - First glow at temperatures between the A _cl - and Ac3 conversion point. You must then cool in air or even faster.
• - This first heat treatment must be followed by an annealing below the A _cl conversion point with cooling in air or faster.

This 2-stage glow cycle may need to be repeated several times.

The applicant of the US-PS has reported in detail on its development work leading to the above-mentioned US-PS in the above two Japanese publications. After that, the steels mostly had to be quenched with water from the two stages of the annealing cycle. According to this source, the steels contained nitrogen contents of 0.001 to 0.004%, which were bound exclusively and completely via Al.

Many attempts have been made to replace nickel with manganese. DE-OS 30 30 652 shows a cold-tough ferritic steel which essentially contains 0.02-0.06% carbon, 4-6% manganese, 0.1-0.4 molybdenum and 0-3% nickel and which also has to be subjected to a very complex heat treatment.

In the two cases of US Pat. No. 3,619,302 and DE-OS 30 30 652, “ultra-fine” microstructures and thus good low-temperature toughness are to be achieved by the aforementioned complicated heat treatments.

No information is given on the weldability of these known steels. It can therefore be assumed that welding of these materials will partially cancel out the "ultra-fine" microstructure that is achieved, and consequently the toughness properties in the welding area may deteriorate considerably.

Despite extensive worldwide investigations, the only cold-tough steel with a reduced Ni content is the steel X 7 Ni Mo 6 (steel-iron list: material no. 1.6349). This steel, which also requires complex heat treatment, does not replace the X 8 Ni 9 in terms of quality.

The invention has for its object to provide a weldable cold-tough ferritic steel that with the lowest possible nickel content for cryogenic applications, ie. H. is suitable for operating temperatures below - 100 ° C. For lower temperatures, such as the important field of application LNG at - 165 ° C and possibly also in lower temperature ranges down to approx. - 196 ° C, the steel of equivalent quality X 8Ni 9 should offer sufficient security against brittle failure. In addition to nickel, other alloy metals such as manganese and molybdenum should also be saved and the steel should also be easy to manufacture, i.e. its manufacturing process should do without complicated, time, control and space-requiring operations with multi-stage annealing cycles, accelerated cooling and the like.

To achieve this object, the invention proposes, according to claim 1, alloying a ferritic steel with specific analysis on the basis of closely selected C, Mn, and above all Ni contents with V and N, hot-rolling and allowing it to cool after melting . After normalizing under recrystallization, these steels are to be used as a weldable material with improved low-temperature toughness and high strength for the manufacture of refrigeration components in the range of - 100 ° C and below.

From the multitude of alloy compositions proposed for low-temperature steels used in apparatus and container construction, which have been recommended in various combinations and with various mass contents, a narrow, less complex alloy range has been selected. It results from the usual melting and hot rolling processes in cooperation with a significantly simplified heat treatment, namely only normal annealing, surprisingly such a low toughness, weldability and strength that these steels for the highest Low-temperature stress can be used, as they were previously reserved only for the 9% Ni steel X 8 Ni 9 according to the prior art.

The use claim 1 mentions various process steps important to the invention, in particular the basic composition; the decisive V and N alloy in the manner familiar to the person skilled in the art, e.g. B. by adding "nitrogen-manganese" or a vanadium-nitrogen alloy (z. B. trademark "Nitrovan"). Insofar as details of other process steps are not particularly specified, they correspond to the prior art.

The possible addition of 0.5 to 1.5% Cu mentioned in claim 1 leads to an increased yield strength and tensile strength (see Example 5).

Regarding the normalizing process step, it should be noted that the cooling rate of the normalization temperature can be varied over a wide range without sacrificing the excellent toughness.

The large strength range specified in the Euronorm 129-76 for the X 8 Ni 9 for adaptation to customer requirements can be achieved for the steels according to the invention solely by varying the cooling rate, it being known that increased strengths are set by faster cooling (see Examples 1, 2 and 3). In any case, the previously described "normalizing" represents an important simplification compared to the complex heat treatment of the X 8 Ni 9.

Since most refrigeration components are welded, the materials must be easy to weld. The steels according to the invention are e.g. B. with the known welding process for the X 8 Ni 9 crack-free and show a good low-temperature toughness of the weld (see Example 6).

Claim 2 preferably names Ni contents of 5-6%. Fig. 1 shows the influence of the V and N alloy according to the invention on the low-temperature toughness. The transition temperature Tü indicates the temperature up to which the notched impact strength with increasing cooling of the sample is still above the value of at least 42 J which is valid as the criterion of toughness for the longitudinal ISO sample.

Curve A (according to the invention) shows, in comparison to curve B (without V and N alloy,) that the greatest possible effect by V and N to increase the low-temperature toughness and at the same time the lowest transition temperature is achieved when the steels 5-6% Ni contain. The comparison of curve A with curve B further shows that the "VN effect" has an effect in the entire range of the Ni contents of 1-9%. It occurs preferably in the range of 4-7% Ni and preferably in a range restricted to 5-6% Ni.

An Al alloy should be avoided so that the "VN effect" is fully retained and is not affected by the setting of N to A1N (detectable by acid-soluble Al).

Claims 3 and 4 explain the hot rolling process step in a manner known per se by features which are known to the person skilled in the art under the term thermomechanical rolling. These measures lead to a structure refinement even in the rolled state (see Examples 1 - 6).

Claim 5 defines the use of the steels according to the invention in a temperature range which was previously reserved for more complex steels with regard to alloy and production.

Claim 6 describes the effect of the measure according to the invention on the structure of the structure and the low-temperature properties caused thereby, which are at least the same as the X 8 Ni 9 (see Examples 1, 3, 5) or far exceed it in terms of the transition temperature (see Example 2).

The attached FIGS. 1 to 6 represent the properties which are decisive for the use of the steels according to the invention.

• Fig. 1 shows the "VN effect". Ordinate (11): transition temperature Tu, this is the temperature up to which the notched impact strength with increasing cooling of the sample is still above the value of 42 J at ISO, which is the criterion of toughness -V longitudinal sample lies. Abscissa (12): Ni content in% by mass
• 2, 3 and 4 show the course of the notched impact strength (21, 31, 41) as a function of the temperature (22, 32, 42).
  Longitudinal (23) and transverse values (24) are shown separately in FIG. 2.

3 and 4 are only longitudinal values (33, 43).

• Fig. 5: ordinate (51): line limit R _e and tensile strength R _m abscissa (52): test temperature
• Fig. 6: has a diagram in the upper part
  • Ordinate (61): Notched impact strength (cross values)
  • Abscissa (62): position of the notch in a welded joint, which is shown schematically in the lower part of the figure
  • (63): Notch position in the base material
  • (64, 65): are notches in the heat affected zone
  • (66): Notch position in the weld metal

The invention is explained in more detail below on the basis of exemplary embodiments:

Example 1:

• 0,07 % C
• 0,27 % Si
• 0,58 % Mn
• 0,006 % P
• 0,005 % S
• 0,16 % V
• 0,024 % N
• 5,6 % Ni

A steel with the chemical composition of (mass%)

• 0.07% C
• 0.27% Si
• 0.58% Mn
• 0.006% P
• 0.005% S
• 0.16% V
• 0.024% N
• 5.6% Ni

Remainder of iron and usual impurities is pre-rolled with a usual pass reduction of 25%, cooled in a rolling break to approx. 850 ° C, then finish-rolled at a final roll temperature of approx. 780 ° C, cooled to room temperature and then annealed once (790 ° C, 30 min / ₀ C cooling 8 / min = _L uftabkühlung to 24 mm sheet).

As the A _v -T curves in FIG. 2 show, a notched bar impact work of 52 J on longitudinal ISO-V and 36 J on transverse samples is demonstrated on this steel at −196 ° C. The steel has a yield strength of 546 N / mm2, a tensile strength of 673 N / mm2 and an elongation of 29.7% at room temperature. The material properties required for the material X 8 Ni 9 according to Euronorm 129-76 are thus fully achieved.

Example 2:

• 0,04 % C
• 0,31 % Si
• 0,36 % Mn
• 0,006 % P
• 0,005 % S
• 0,25 % V
• 0,028 % N
• 5,2 % Ni

A steel with the chemical composition of

• 0.04% C
• 0.31% Si
• 0.36% Mn
• 0.006% P
• 0.005% S
• 0.25% V
• 0.028% N
• 5.2% Ni

was rolled out in the same way as in Example 1 and normalized.

From the A _v -T curve in Fig. 3 it can be seen that the steel has excellent low-temperature toughness.

The mechanical-technological test values are given in the table below.

Despite the lower tensile strength compared to example 1, the high yield strength of this steel allows a weight-saving construction. It can be derived from the A _v -T curve in FIG. 3 that the steel is still tough even at −230 ° C. and has considerable toughness reserves with a toughness of 200 J in the LNG application area.

Example 3:

• 0,037 % C
• 0,34 % Si
• 0,36 % Mn
• 0,005 % P
• 0,005 % S
• 0,26 % V
• 0,029 % N
• 5,8 % Ni

A steel with the chemical composition of

• 0.037% C
• 0.34% Si
• 0.36% Mn
• 0.005% P
• 0.005% S
• 0.26% V
• 0.029% N
• 5.8% Ni

was rolled out in the same way as in Example 1, then heated to 790 ° C. and then cooled in water. As the following test values show, this treatment results in a considerable increase in the yield strength and strength.

With a sufficient toughness of A = 70 J on ISO-V longitudinal samples at -196 ° C, the steel has a yield strength of 623 N / mm2, a tensile strength of 788 N / mm ² and an elongation of 22.5%.

Example 4:

Cold-tough steels are cold-formed to a greater or lesser extent, depending on the application. Since a stronger forming causes an excessive loss of toughness, these effects are eliminated, as is known to the person skilled in the art, by "stress relieving annealing" at a temperature of 530 ° C. to 580 ° C. To check its toughness after such annealing, the steel from Example 3 was annealed at 530 ° C.

As the following test values show, such stress relieving does not have a negative effect on the toughness properties of this steel.

Example 5:

Another way to increase the yield strength and tensile strength is to alloy the material with copper.

• 0,038 % C
• O, 27 % Si
• 0,57 % Mn
• 0,007 % P
• 0,005 % S
• 0,15 % V
• 0,024 % N
• 5,4 % Ni
• 1,05 % Cu

A steel with the chemical composition of

• 0.038% C
• O, 27% Si
• 0.57% Mn
• 0.007% P
• 0.005% S
• 0.15% V
• 0.024% N
• 5.4% Ni
• 1.05% Cu

was rolled out and annealed as in Example 1.

As the A _v -T curve in Fig. 4 shows, this steel has excellent toughness properties. It also has a yield strength of 591 N / mm2, a tensile strength of 666 N / mm2 and an elongation of 29.2%. The A _v value at - 196 ° C is 116 J (ISO-V longitudinal).

With this material, the material criteria required for an X 8 Ni 9 are also fully met. 5 shows the strength properties of this steel and the steel from Example 1 as a function of the test temperature. It should be emphasized that at - 196 ° C the yield strength values are 825 or 850 N / mm ² and the tensile strength values are 1045 N / mm2.

Example 6:

Steels for refrigeration components must be easy to weld and have sufficient toughness in the heat affected zone. As is known, the toughness property in this zone deteriorated with increasing C and Mn content. To test the weldability of a more critical composition, the steel with higher C and Mn contents from example 1 was therefore used.

An austenitic filler material was used for welding - as is common for steel X 8 Ni 9. No cracks were observed in the welded joints. The notched impact strength was tested on ISO-V specimens (transverse to the rolling direction) at - 160 ° C and - 196 ° C.

Special attention was paid to the heat affected zone, since there is always a drop in toughness. The notch of ISO-V samples was arranged at a defined distance from the melting line within the heat affected zone, as explained in the lower part of FIG. 6. The lowest toughness values were shown by the area Uo, about 0.5 mm from the melting line, Fig. 6. At - 160 ^o C test temperature, the toughness of this zone is still 46 J and at - 196 ° C 30 J (cross samples). It therefore fulfills the requirements placed on it.

With a further reduced C and Mn content (as in Example 2), even better toughness properties can be expected in the critical heat affected zone.

As explained and documented above, the weldable steels according to the invention can be used with sufficient security against brittle failure despite less complex alloy and significantly simplified production for high-strength components of refrigeration technology down to operating temperatures of -196 C and below.

Claims (6)

1. Use of a ferritic steel, its manufacturing process
the procedural steps - Melt with 0.015 - 0.08% C, (figures in mass%) 0.1 - 0.5% Si, 0.3 - 0.6% Mn, 0.015% P, 0.015% S, 4 - 7% Ni, optionally Cu in contents of 0.5 to 1.5% melt-related contents of N and other contaminating elements, remainder iron - Alloy vanadium and nitrogen to levels of 0.15 - 0.25% V 0.020 - 0.030% N - Hot rolling and cooling to room temperature and normalizing with recrystallization
includes
as a weldable material with improved cold toughness and high strength for the production of components in refrigeration technology in the range of - 100 ° C and below. 2. The method according to claim 1, characterized in that the steel to be used as cold tough contains Ni in contents of 5-6%. 3. The method according to claim 1 or 2, characterized in that the steel pre-rolled in a conventional manner with a conventional pass, cooled in a rolling break to 900 ° C to 840 ° C, then at a final roll temperature of 820 ° C to 770 ° C is finished rolled and cooled to room temperature. 4. The method according to claim 1, 2 or 3, characterized in that in the "hot rolling" step, the pass reduction during roughing is each about 25%. 5. Use of a steel according to claim 1, characterized in that the steel is used for the production of refrigeration components in the range of - 130 ° C and below. 6. The use of a weldable cold-tough ferritic steel according to one or more of the preceding claims, with a structure made of very fine-grained ferrite with embedded bainite and martensite islands, as a material for components of refrigeration technology, which at a temperature of - 196 ° C a notch impact ISO V longitudinal samples of more than 42 J required.

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