STEEL FOR MAKING ACID-GAS RESISTANT PART

Abstract

A steel for making acid-gas resistant parts for power generation, pipes, tanks, pipelines for gases or other fluids, for parts in contact with hydrogen, bipolar plates of fuel cells and heat exchangers. The high-manganese austenitic steel. The steel has in weight percent 0.02 to 0.12% carbon, 0.05 to 0.5% nitrogen, 10 to 20% manganese, 10 to 20% chromium, and remainder iron with usual impurities.

Description

FIELD OF THE INVENTION

The present invention relates to use of a steel for making acid-gas resistant parts. More particularly this invention concerns the actual steel for making acid-gas resistant parts.

BACKGROUND OF THE INVENTION

The invention is based on an austenitic stainless steel St 1.4404 according to German material data sheet Edelstahlwerke from 8 Jun. 2016. Such stainless steel is useful for many areas of application.

The main disadvantage of this steel is that it is alloyed with a high proportion of nickel, from 10 to 13 percent by weight. This leads due to the high cost of nickel to a considerable increase in the price of the product, so that due to the high price the application of this material is out of the question for many applications.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide an improved steel for making acid-gas resistant parts.

Another object is the provision of such an improved steel for making acid-gas resistant part that overcomes the above-given disadvantages, in particular that can be produced at low cost, making it suitable for many applications.

SUMMARY OF THE INVENTION

A steel for making acid-gas resistant parts for power generation, pipes, tanks, pipelines for gases or other fluids, for parts in contact with hydrogen, bipolar plates of fuel cells and heat exchangers. The high-manganese austenitic steel. The steel has according to the invention in weight percent

0.02 to 0.12% carbon,

0.05 to 0.5% nitrogen,

10 to 20% manganese,

10 to 20% chromium, and

remainder iron with usual impurities.

According to the invention, a nickel-free high-manganese austenitic steel is employed for the specified use, is extremely useful, and thus also makes it usable for new areas of application.

Such a so-called high manganese austenite is sufficiently corrosion-resistant and easily shapable, in particular cold shapable, the steel being inexpensive because of its low cost so it can be applied to new areas of application.

The steel material can in addition to the specified components have in weight percent at least one other ingredient, namely

up to 0.8% silicon,

up to 4% copper,

up to 4% nickel,

up to 2% molybdenum,

at most 0.05% phosphorus,

at most 0.05% sulfur,

up to 0.5% titanium, up to 0.5% niobium,

up to 0.5% vanadium.

The addition of nickel is only exceptional and in small amounts to optimize ductility and to improve corrosion resistance. If improved corrosion resistance is unnecessary, the proportion of nickel will remain at zero.

The components of alloy elements are preferred as follows in weight percent:

0.05 to 0.08% carbon,

0.08 to 0.15% nitrogen,

13 to 17% manganese,

13 to 17% chromium,

1 to 2% nickel, as well as preferably

0.1 to 0.2 silicon,

1 to 2% copper,

maximum 0.2% molybdenum, and as permissible impurities

a maximum of 0.005% sulfur,

no more than 0.02% phosphorus, no more than 0.5% titanium,

a maximum of 0.5% niobium,

at most 0.5% vanadium,

the remainder iron, possibly with the usual impurities.

The specified steel material preferably has the following mechanical technological properties:

tensile strength R_m=500 up to 800 mpa,

yield strength R_p 0.2=200 to 500 mpa,

stretch-to-break A80=at least 42%.

It is particularly preferably provided that the material is cold shaped using predominantly a TWIP mechanism containing less than 5% strain-indexed martensite.

A special feature is also that the technological properties of the steel material are determined by rolling of the material with a suitable degree of rolling and/or annealing of the material at a suitable annealing temperature.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing that is a Schaeffler—DeLong diagram illustrating the invention.

SPECIFIC DESCRIPTION OF THE INVENTION

In the diagram of the drawing the austenite formers are opposite the ferrite formers.

Influence of the Elements:

A fundamental distinction is to be made between the alloying elements: whether they are carbide, austenite or ferrite formers or which the purpose for which they are alloyed with the steel.

Each individual element gives the steel depending on its proportion certain specific properties. If several are present elements, the effect can be increased. There are, however alloy variants in which the individual elements related to a certain property do not have the same positive influence direction, but negatively affect one another.

The presence of the alloying elements in the steel ensures only that prerequisite for the desired properties; only the processing and heat treatment allow this to be achieved.

The extremely fine grain of the inventive steel makes it possible to use extremely thin plates when making fuel cells. Such thin plates can be deformed with out fracturing, which is a big advantage in the production of a fuel cell.

Carbon (C) Generally:

Carbon is the most important and most influential alloy element in steel. Besides carbon, all unalloyed steel contains silicon, manganese, phosphorus and sulfur, which accidentally added during manufacture. The addition further alloy elements to achieve special effects as well as the deliberate increase in the manganese and silicon content leads to alloy steel. The higher the C content, the higher the strength and toughness of steel, whereas its elongation, forgeability, weldability and machinability (by cutting tools) can be reduced. The corrosion resistance to water, acids and hot gases is practically not influenced by the carbon.

According to the invention, the proportion of carbon is held relatively low, so as to optimally maintain the relationship between strength and elongation. The goal is to achieve maximum shapability. Furthermore carbon forms austenite.

Manganese (Mn) Generally:

• • Mn very strongly decreases the critical cooling rate and thus the hardenability.
  • The yield strength is increased as the Mn content increases.
  • When the Mn content is above 4% causes slow cooling of brittle martensitic structure.
  • Steel with an Mn content above 12% and a high C content are austenitic because Mn increases the γ region.
  • Mn is an austenite former and is according to the invention mainly used as a cheaper replacement for Ni. Therefore a content of 10-20% is also used in this concept.

Chromium (Cr) Generally:

• • Reduces the critical value required for martensite formation cooling speed
  • Increases hardenability and improves temperability
  • Cr increases resistance to scaling
  • Cr cuts off the γ-area and thereby expands the ferrite area; however, stabilizes the austenite in austenitic Cr—Mn or Cr—Ni steels
  • Chromium forms starting at 10.5% mass fraction a chromium-oxide layer that prevents further oxidation. If this oxide layer is damaged, bare metal is exposed to the atmosphere, and forms automatically a new passivating layer, that is the slayer is self-healing.

According to the invention, the main task of Cr is to protect against corrosion. Because in this concept the product in used as part of a corrosive medium a Cr content of 14-16% is used to provide an increased corrosion protection.

Nickel (Ni) Generally:

Nickel is one of the alloying elements that enhances formation of a stable iron-carbon system. By lowering the critical cooling rate, nickel increases hardening and tempering. Further, nickel increases especially shapability, especially at low temperatures, has an effect on the grain, and reduces the sensitivity to overheating. De 18/10 chrome-nickel steel (1.4301) is one of the main types of corrosion-resistant austenitic steels.

As mentioned earlier, nickel is a relatively expensive alloy element and, according to the invention, the proportion is here replaced by manganese. However, a certain content level increases shapability increases and also positively influences stability of the austenite. Furthermore, nickel has a positive effect on corrosion resistance.

Nitrogen (n) Generally:

Nitrogen is as an austenite former, like nitrogen. The lower maximum solubility of iron for nitrogen however is significantly larger than for carbon. By alloying of other elements or pressurization the nitrogen content in the steel can be increased significantly, with a part of the nickel in austenitic steels replacing the effect of carbon so the carbon content can be reduced.

According to the invention, the nitrogen serves as a substitute for the carbon and improves corrosion resistance. Of furthermore, nitrogen acts as a substitute for the austenite former nickel.

Austenite Formers:

Alloying elements expand the austenite area and stabilize austenite. Ni, Co, Mn, N and C are the most important agents. With the help of the Schaeffler Delong diagram the resulting structure of high-alloy steels based on the chemical composition can be determined.

Claims

1. A method of making steel for acid-gas resistant parts for power generation, pipes, tanks, pipelines for gases or other fluids, for parts in contact with hydrogen, bipolar plates of fuel cells and heat exchangers, the high-manganese austenitic steel of the parts comprising in weight percent:

0.02 to 0.12% carbon,

0.05 to 0.5% nitrogen,

10 to 20% manganese,

10 to 20% chromium, and

remainder iron with usual impurities.

2. The method of making steel according to claim 1, further comprising:

up to 0.8% silicon,

up to 4% nickel,

up to 2% molybdenum,

a maximum of 0.05% phosphorus,

not more than 0.05% sulfur,

up to 0.5% titanium,

up to 0.5% niobium, and

up to 0.5% vanadium.

3. The method of making steel according to claim 1, wherein the steel has:

a tensile strength Rm=500 to 800 mpa,

a yield strength Rp0.2=200 to 500 mpa, and

a stretch to break A80=at least 42%.

4. The method of making steel according to claim 1, wherein the steel is cold shaped solely by using a TWIP mechanism alone and contains less than 5% deformation-indexed martensite.

5. The method of making steel according to claim 1, wherein technological properties of the steel material are determined by rolling the material with a suitable degree of rolling and/or annealing the material at a suitable annealing temperature.

6. A steel for making acid-gas resistant parts for power generation, pipes, tanks, pipelines for gases or other fluids, for parts in contact with hydrogen, bipolar plates of fuel cells and heat exchangers, the high-manganese austenitic steel, the steel comprising in weight percent:

0.02 to 0.12% carbon,

0.05 to 0.5% nitrogen,

10 to 20% manganese,

10 to 20% chromium, and

remainder iron with usual impurities.