CA1149647A

CA1149647A - Application of a fine grain manganese-nickel structural steel

Info

Publication number: CA1149647A
Application number: CA000339072A
Authority: CA
Inventors: Constantin M. Vlad; Klaus Hulka
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-11-02
Filing date: 1979-11-02
Publication date: 1983-07-12
Also published as: NO151506B; ATE4228T1; EP0010755B2; DE2847506C2; DE2847506A1; NO151506C; EP0010755B1; NO793516L; EP0010755A1

Abstract

ABSTRACT OF THE DISCLOSURE

The invention relates to the use of a fine grain manganese-nickel structural steel which contains 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 0.025% aluminum, up to 0.015% sulfur and, optionally, 0.2 to 0.4% copper, the residue being iron including the impurities resulting from smelting as a material for workpieces, e.g., pipes and vessels, which may get in contact with liquid gases at temperatures of at least -120°C.

Description

The present invention relates to the use of fine grain manganese structural steel which contains 0.04 to 0.09%
carbon 1.2 to 1.8% manganese; 0.1 to 0.4 silicon 0.03 to 0.08% niobium up to 0.025% aluminum: up to 0.015% sulfur 0.5 to 1.5% nickel and, optionally 0.2 to 0.4% copper the residue being iron including the impurities resulting frorn smelting.
An alloy steel of the above type has already been known from German Patent no. 24 07 338. This steel 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. If this steel undergoes a controlled hot rolling which is dependent on its nickel content, it will acquire a certain strength at low temperatures. However, in practice, a hot rolling, which is dependent on the nickel-content of a particular material, has been found to be difficult and expensive. In addition, the ductility in the cold state of such steel was found to be insufficient to enable it to come into contact with liquid methane and, in particular, liquid ethylene.
~ or transporting and storing liquid gases, there must be used structural materials which possess sufficient strength and ductility at temperatures of at least -196C. In addition, these materials should be weldable in order to ensure economical production of pipes and reservoirs.
It is known that stainless steel can be used at working temperatures of at least -270C. In this case, it is nickel that may be considered responsible of cold ductility.
The large amount of expensive components9 however, limits the use of stainless steel and to solve the problem the use of less expensive alloy steels should be considered. As a result, a 114969~7 series of steels has been developed, which contain about 9%
nickel 0.1% carbon, 0.80% manganese and 0.020% phosphorus, and which as compared with stainless steels are characterized by higher strength and cold ductility at temperatures of at least -200C. The precondition for high cold ductility, however, is a two-stage normalizing and air-cooling intended to bring a suitable portion of austenite into the ferretic matrix. This process is based on the recognition that ductility increases with an increase in the portion of austenite.
Tests have shown that cold ductility increases with a decrease in the contents of carbon, phosphorus, and manganese. Furthermore, it has also been established that a gradual reduction of the nickel content to values as low as 2.1% gradually increases cold ductility. For example, the notch impact strength of normalized and air-cooled steel containing 8.5% to 9.5% nickel is 34 J at -196C it decreases to 20 J at -100C in the case of steels containing 3.25 to 3.75% nickel, ancl to 18 J at -68C in the case of steels containing 2.1 to 2.5% nickel. Therefore, steels with a nickel content less than 9% cannot be regarded as suitable for use at low temperatures.
The aim of the present invention is to provide an alloy steel which can be welded, has a high yield point at room temperature, cold ductility, resistance to hydrogen-cracks and which, as a result, is particularly suitable to be used as a material for welded pieces, such as pipes and reservoirs, useful for transporting and storing liquid yases even in the presence of hydrogen sulfide or water. In particular, this steel should be resistant to ethylene and should be suitable to be used at temperatures of at least ~96~7 It is recommended that the solution to this problem consists of providing a steel having the composition mentioned in the introduction, thereby achieving the above aim.
Despite its very low nickel content, even in the hard-rolled and air-cooled state, this steel has such a high notch impact strength and transition t~mperature that it can be used at temperatures of at least -70C. However, the proper-ties of this material will be fully developed only after the steel which is proposed has been normalized and, if necessary, again tempered. With a heat-treatment of this kind, this steel has a yield point of at least 420 N/mm at room temperature and a transition temperature of at least -120C with a notch impact strength of 51 J/cm2 across the rolling direction, and a notch impact strength of at least 280 J/cm at room temperature.
If this steel contains 0.2 to 0.4% copper, its resistan~e to crack-formation in the presence of traces of hydrogen sulfide will be particularly high. This fact is of considerable importance since liquid gases often contain traces of hydrogen sulfide which, in the case of a simultaneous presence of water, has a corroding effect and results in cracks due to the presence of hydrogen.
The low carbon content ensures a good behavior in welding on the one hand, and it promotes the notch impact resistance, on the other. As a whole, the reason for the excellent properties of the recommended steel is the synergistic action of nickel, niobium, and manganese.
Advantageously, the steel is normalized until the core temperature is about 30 to 50C higher than the AC3- point after which it is tempered at 550-650C, preferably at 630C, for two to four minutes per 2 mm thickness of the material, in ~96~7 order to adjus-t cold ductility.
The invention is better explained with reference to the drawings which describe diagrams and explain preferred embodiments. In the drawings:
Figure 1 illustrates the notch impact strength at room temperature as a ~unction of the nickel content and the type of heat-treatment.
Figure 2 illustxates the transition temperature as a function of the nickel content and of the heat-treatment.
Figure 3 illustrates the notch impact strength and the deformation at break of a steel in the invention as compared with known steels at the testing temperature.
Figure 4 illustrates the amount of dissolved hydrogen as a function of the copper content after a 96 hour immersion into sea water saturated with hydrogen sulfide.
Figure 5 illustrates the length of hydrogen induced cracks as a function of the hydrogen content.
The tests which served aQ a basis for the diagrams of Figures 1 and 2 wer~ conducted on steels 1 to 5 having 20 composition given in the Table below. Out o~ these steels, steels 2 and 3 are steels in the invention.

TABLE
C Si Mn P S N Al Nb Ni Steel % (%) (%) (%) (%) (%) (%) (%) (%) 1 o.og 0.28 I.37 0.015 0.013 0.0061 0.050 0.07 0.05

2 o.og 0.31 1.52 0.014 0.013 0.0077 0.051 0.08 0.61

3 0.09 0.31 1.46 0.013 0.013 0.0077 0.030 0.0~ 1.40

4 0.09 0.33 1.46 0.015 0.013 0.0079 0.046 0.08 2.23 0.0~ 0,32 1.43 0.015 0.014 0.0078 0.038 0.08 3.10 Samples taken ~rom the experimental steels underwent the heat-treatment given in the diagrams and tests were made to determine the notch impact strength and the cold ~l~g6~7 ductility. The results illustrated in the diagrams of Figures 1 and 2 show that, within the 0.5-1.5% nickel-content range, both the notch impact strength at room temperature as well as the transition temperature, without taking any special measure and independently of the type of heat-treatment are optimum. This is a surprising fact since, in accordance with traditional considerations, a reduced nickel content goes together with a reduction of cold ductility and notch irnpact strength, unless of course special measures, such as controlled hot rolling, are taken to adjust the cold ductility.
As compared with traditional normal steels, the superiority of the steel of the invention is evidenced from the diagrams of Figure 3, whereby it should be remembered that in the case of the steel of the invention the samples are in transverse direction while in the other samples (with one exception), are in the longitudinal direction.
In all cases, the steels under study have a yield point at room temperature of at least 420 n/mm and a notch impact strength of at least 280 j/cm2.
Furthermore, the diagrams of Figures S and 6 show that the sensitivity to crack formation in the presence of hydrogen sulfide is particularly low when a copper content is above about 0.02%. In this manner, the suggested steel is also particularly suitable for transport and storage of contaminated liquid gases. The high resistance to crack-formation is explained by the fact that, under working conditions, a weak acid is formed under the action of hydrogen sulfide and water. The hydrogen ions generated in this process travel along the material and a molecular precipitation takes place on the grain boundaries. In the case of traditional steel, it is a pressure that results from this phenomenon which ~1~9647 leads to the formation of cracks. On the other hand, in the case of the steel of the invention, a portion of copper is dissolved in the acid. The ions formed in this process travel, by way of ion exchange, along the surface of the material and form thereon a molecular protecting copper layer.
This copper layer acts as an inhibiting layer against further intrusion of hydrogen and explains the high resistance against hydrogen of the steel of the invention, as it may be seen in Figure 4.

Claims

The embodiments of the invention in which an exclusive property or pxivilege is claimed are defined as follows:

1. A fine grain manganese-nickel structural steel which contains 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 0.025% aluminum, up to 0.015% sulfur and, optionally, 0.2% to 0.4% copper, the residue being iron including impurities.

2. A steel according to claim 1 which has been normalized.

3. A steel according to claim 2 which has been normalized until the core temperature is between 30 and 50°C
above AC3 point.

4. A steel according to claim 1 which has been tempered.

5. A steel according to claim 4 which has been tempered at between 550 and 650°C between two and four minutes for each two mm of thickness of material.