EP0010755B1 - Use of manganese and nickel-containing fine-grained structural steel - Google Patents

Abstract

1. Use of a manganese-nickel fine-grain structural steel, containing 0.04 to 0.09% carbon, 1.2 to 1.8% manganese, 0.1 to 0.4% silicon, 0.03 to 0.08% niobium, 0.5 to 1.5% nickel, up to 0.25% aluminium, up to 0.015% sulphur and optionally 0.2 to 0.4% copper, remainder iron including smelt dependent impurities, which is normalized, as material for components which, like pipes and vessels, come into contact with liquefied gas at temperatures down to - 120 degrees C.

Description

The invention relates to the use of a manganese-nickel fine-grained structural steel with 0.04 to 0.09% carbon, 1.2 to 1.8% manganese, 0.1 to 0.4% silicon, 0.03 to 0, 08% niobium, 0.5 to 1.5% nickel, up to 0.25% aluminum, up to 0.015% sulfur and optionally 0.2 to 0.4% copper, remainder iron including impurities due to melting.

An alloy steel of the aforementioned type is known from German Offenlegungsschrift 2 407 338; it contains 0.01 to 0.10% carbon, 0.5 to 2% manganese, 0.1 to 0.9% silicon, 0.001 to 0.10% niobium, 0.01 to 0.3% aluminum and 1, 4 to 3.5% nickel. This steel has a certain cold strength if it has been hot rolled in a controlled manner depending on the nickel content. However, hot rolling controlled as a function of the respective nickel content proves to be difficult and, in particular, complex in practice. In addition, the cold toughness of this steel is not sufficient to use the steel at temperatures such as that of liquid methane and, in particular, liquid ethylene.

Furthermore, it is known from German Offenlegungsschrift 2,461,087 that copper is used in a steel with 0.05 to 0.20% carbon, 0.01 to 0.8% silicon, 0.5 to 1.6% manganese 0.03% phosphorus, 0.002 to 0.02% sulfur and 0.2 to 0.8% copper, remainder iron and iron companion increases the strength, but affects the hot formability. In addition, the copper should increase the resistance to hydrogen or suppress the formation of hydrogen cracks, provided the steel contains no or at most 0.6% nickel; however, this is only at the expense of hot workability. This steel is suitable as a material for oil or gas pipes.

From German published patent application 2 157 305 is a low-alloy steel with 0.04 to 0.09% carbon, 0.1 to 0.9% silicon, 1.7 to 2.2% manganese, each with 1.0% copper or nickel, 0.3 to 1.5% copper and nickel and 0.01 to 0.10% niobium, which has limited cold toughness with an additional vanadium content of 0.05% and a zirconium content of 0.06%. Nothing is known about the resistance of the steel to hydrogen cracks.

The same applies to a steel known from German Offenlegungsschrift 2323738 with up to 0.2% carbon, at least 0.75% manganese and at least 0.015% niobium, the mechanical properties of which, however, depend on a certain grain size. With a nickel content of 0.29% and a chrome content of 0.54% at -51.1 ° C, this steel achieves a notched impact strength of 51.9 J / cm ² .

For the transport and storage of liquid gases, however, materials are required that have sufficient strength and toughness at temperatures down to -196 ° C. In addition, these materials must be weldable to enable the economical production of pipes and containers.

It is known that stainless steels have grown to operating temperatures below -270 ° C. The carrier of the cold toughness is in particular the nickel. However, the high proportion of expensive alloy components places limits on the use of stainless steels, which led to the search for cheaper alloy steels. This has led to the development of a number of steels having about 9% nickel, 0.1% carbon, 0.80% manganese and 0.020% of phosphorus, characterized by a in comparison with the stainless steels and a higher tensile strength to about -200 ⁰ C mark sufficient toughness. However, a prerequisite for the high cold toughness is a two-stage normalizing and tempering, which aims to set a sufficient proportion of austenite in a ferritic structure. This is based on the finding that toughness increases with an increasing proportion of austenite.

In this context, tests have shown that the cold toughness increases with decreasing contents of carbon, phosphorus and manganese. Furthermore, it was shown that a gradual reduction in the nickel content to 2.1% leads to an increasing impairment of the cold toughness. For example, the notched impact strengths of normalized and tempered steels containing 8.5 to 9.5% nickel decreased from 42.5 J / cm ² at -196 ° C to 3.25 to 3.75% nickel to 25 J / cm ² at -100 ° C and with steel containing 2.1 to 2.5% nickel to 22.5 J / cm ² at -68 ° C. Steels with a nickel content of less than 9% are therefore not suitable for extremely low temperatures.

The invention is based on the object of proposing an alloy steel for use at temperatures down to -120 ° C. which can be welded, has a high yield strength at room temperature and cold toughness and resistance to hydrogen cracks and is accordingly particularly suitable as a material for welded parts, which, like pipes and containers, serve for the transport and storage of liquid gases even in the presence of hydrogen sulfide and water. In particular, the steel is said to be resistant to liquid ethylene and to withstand temperatures up to 120 ° C.

The solution to this problem is to use as a material for parts such as pipes and containers with liquefied gas at temperatures down to -120 ⁰ C to come into contact, a steel of the type mentioned in the normalized condition. After such a heat treatment, the steel has a room temperature yield point of at least 420 N / mm ² and a transition temperature of the impact strength of 51 J / cm ² transverse to the rolling direction of at least -120 ⁰ C and a notched impact strength of at least 280 J / cm ² at room temperature .

If the steel contains 0.2 to 0.4% copper, its crack resistance is particularly high in the presence of traces of hydrogen sulfide. This is of considerable importance insofar as liquefied gases often contain traces of hydrogen sulfide which, when water is present at the same time, has a corrosive effect and in particular leads to hydrogen-induced cracks.

The low carbon content of the steel on the one hand ensures good welding behavior and on the other hand promotes notched impact strength. All in all, the excellent properties of the proposed steel are explained by the synergistic interaction of nikkel, niobium and manganese.

The steel is preferably annealed normally until the core temperature is 30 to 50 ° C above the Ae ₃ point and then annealed for 2 to 4 minutes at 550 to 650 ° C, in particular at 630 ° C, for 2 millimeters of material thickness in order to improve the cold toughness adjust.

• Fig. 1 die Abhängigkeit der Raumtemperatur-Kerbschlagzähigkeit vom Nickelgehalt und der Art der Wärmebehandlung.
• Fig. 2 die Abhängigkeit der Übergangstemperatur vom Nickelgehalt und der Wärmebehandlung,
• Fig. 3 die Abhängigkeit der Kerbschlagzähigkeit und des Verformungsbruchs eines unter die Erfindung fallenden Stahls im Vergleich zu bekannten Stählen von der Prüftemperatur,
• Fig. den Gehalt an gelöstem Wasserstoff in Abhängigkeit vom Kupfergehalt nach einem 96stündigen Tauchen in ein mit Schwefelwasserstoff gesättigtes Seewasser und
• Fig. 5 die Länge der wasserstoffinduzierten Risse in Abhängigkeit vom Wasserstoffgehalt.

The invention is explained below on the basis of diagrams shown in the figures and of exemplary embodiments. The drawing shows:

• Fig. 1 shows the dependence of the room temperature notched impact strength on the nickel content and the type of heat treatment.
• 2 shows the dependence of the transition temperature on the nickel content and the heat treatment,
• 3 shows the dependence of the notched impact strength and the deformation fracture of a steel covered by the invention in comparison to known steels on the test temperature,
• Fig. The content of dissolved hydrogen depending on the copper content after a 96-hour immersion in sea water saturated with hydrogen sulfide and
• 5 shows the length of the hydrogen-induced cracks as a function of the hydrogen content.

The tests on which the diagrams in FIGS. 1 and 2 are based were carried out on steels 1 to 5 with the composition shown in the table below. Of the specified steel 2 and 3 fall under the invention.

Samples of the test steels were subjected to the heat treatments shown in the diagrams and examined with regard to their notched impact strength and cold toughness. The results can be seen from the diagrams of FIGS. 1 and 2 and show that both the notched impact strength at room temperature and the transition temperature in the range from 0.5 to 1.5% nickel go through an optimum regardless of the respective heat treatment, without it this requires special measures. This is surprising in that, according to conventional wisdom, a decreasing nickel content is accompanied by a reduction in the cold and notched impact strength, unless special measures such as controlled hot rolling are used to adjust the cold toughness.

• 1. Stahl-Eisen-Werkstoffblatt 680-70
• 2. Hoesch-Berichte 1/72, Seite 5-20
• 3. Jasper H.K. Achtelik, «Kaltzähe Stähle», Nikkelinformationsbüro, 1964.

The figures in Figure 3 show the superiority of the steel to be used according to the invention (Erf.St.) with 0.61% nickel compared to conventional standard steels, whereby it should be noted that the steel to be used according to the invention is cross-tests, in the other cases, with one exception, are longitudinal samples. The comparative steels, each with an index number, are described in the following publications:

• 1. Steel-iron material sheet 680-70
• 2. Hoesch reports 1/72, page 5-20
• 3. Jasper HK Achtelik, “Cold-tough steels”, Nikkel information office, 1964.

The steels examined also each had a yield strength of at least 420 N / mm ² and a notched impact strength of at least 280 J / cm ² at room temperature.

Furthermore, the diagrams in FIGS. 5 and 6 show that the crack sensitivity in the presence of hydrogen sulfide is particularly low at copper contents above about 0.02%, so that the proposed steel is particularly suitable for the transport and storage of contaminated liquid gas. The high resistance to cracking is due to the fact that a weak acid develops during operation under the influence of hydrogen sulfide and water. The resulting hydrogen ions migrate into the material and are molecularly separated at the grain boundaries. This results in pressures that lead to cracking in conventional steels. In the steel to be used according to the invention, on the other hand, part of the copper dissolves in the acid. The resulting ions migrate to the material surface through ion exchange and form a molecular protective layer made of copper. This copper layer acts as a barrier layer against further penetration of the hydrogen and explains the high hydrogen resistance of the steel to be used according to the invention which can be seen from FIG. 4.

Claims (4)

1. Use of a manganese-nickel fine-grain structural steel, containing 0.04 to 0.09% carbon, 1.2 to 1.8% manganese, 0.1 to 0.4% silicon, 0.03 to 0.08% niobium, 0.5 to 1.5% nickel, up to 0.25% aluminium, up to 0.015% sulphur and optionally 0.2 to 0.4% copper, remainder iron including smelt dependent impurities, which is normalized, as material for components which, like pipes and vessels, come into contact with liquefied gas at temperatures down to ―120°C.

2. Use according to Claim 1, characterized in that a steel is used which has also been annealed.

3. Use according to Claim 1 or 2, characterized in that a steel is used, which has been normalized for 2 to 4 minutes at a core temperature of 30 to 50°C above the AC₃-point.

4. Use according to Claim 2 or 4, characterized in that a steel is used, which has been annealed at 550 to 650°C for 2 to 4 minutes per mm of material thickness.

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