EP0123054B1 - Stainless chromium steel and process for the manufacture thereof - Google Patents

Abstract

The corrosion resistant chromium steel which includes 3 to 45% chromium, 0.2 to 5% nitrogen and other elements. Its structure contains at least 50% ferromagnetic structure components. It is directly magnetizable and, at 400 DEG C., has a yield strength of Rp0.2>400 N/mm2 and, at 600 DEG C., a yield strength of Rp0.2>250 N/mm2. To produce this chromium steel, a prealloy containing at least 50% ferromagnetic structure components is nitrogen enriched under pressure, is then hot worked, then annealed at 800 DEG to 1250 DEG C. and finally cooled to room temperature. A tempering treatment at 450 DEG to 750 DEG C. may follow.

Description

The invention relates to a corrosion-resistant chromium steel which consists of 3 to 45% chromium, 0.001 to 0.5% carbon, 0 to 10% nickel, 0 to 10% manganese, 0 to 10% molybdenum, 0 to 5% vanadium, 0 up to 2% silicon, 0 to 2% titanium, niobium and / or tantalum, 0 to 1% cerium, 0 to 0.3% aluminum and the rest iron and the structure of which contains at least 50% ferromagnetic structure components. The invention further relates to a method for producing this steel.

AT-PS 277 301 discloses a nitrogen-containing steel with a high yield strength and good toughness properties, which contains up to 0.6% carbon, 5 to 40% chromium, up to 30% manganese, up to 5% molybdenum, up to 20% Contains nickel, 1.5 to 5% nitrogen and the rest iron and has an austenitic structure. The nitrogen content is introduced into the steel by first adding nitrogen-containing iron-chromium or iron-manganese alloys to the melt and then introducing gaseous nitrogen into the melt or into the slag. The teaching of AT-PS 277 301 is based on the long-known knowledge that austenitic chromium-nickel and chromium-manganese alloys increase austenite stability by nitrogen and that in semi-ferritic and ferritic chromium steels with more than 18% chromium nitrogen occur of austenite or 4ur increases the part of the structure that can be converted, whereby 0.1% nitrogen can replace 2% nickel for austenite stabilization (see E. Houdremont, Handbuch der Sonderstahlkunde, 1956, pages 1327 to 1331).

Compared to the austenitic chromium-nickel steels, the chromium steels with a ferromagnetic structure are characterized by higher strength properties and very good resistance to stress corrosion cracking. Even in the temperature range up to 400 ° C, the strength properties of ferritic chromium steels, which have a ferromagnetic structure, are far above the values of austenitic chromium-nickel steels, while the deformation parameters are clearly below the values of austenitic steels. However, the heat resistance of the ferritic chrome steels drops considerably by 450 ° C as a result of the embrittlement phenomena that begin in this temperature range. The use of these steels for continuous operation is therefore restricted to temperatures below 300 ° C (see Materials Science of Common Steels, Part 2, Verlag Stahleisen mbH, Düsseldorf, 1977, page 165).

For example, the corrosion-resistant chromium steels with a ferromagnetic structure have 12 to 18% chromium, 0.5 to 1% manganese, 0.05 to 1.2% carbon, 0 to 1% silicon, 0 to 2.5% nickel, 0 up to 1.3% molybdenum, 0 to 2% vanadium, 0 to 0.3% aluminum and the rest iron, in the annealed or tempered state the following material properties:

The ferromagnetic structure of these corrosion-resistant steels consists of ferrite or of ferrite and pearlite in the annealed state and of ferrite and transformation structure or transformation structure or martensite in the tempered state.

DD-PS 115 508 discloses a corrosion and heat-resistant chrome-nickel steel which contains 0.005 to 0.065% carbon, 0.1 to 1.0% silicon, 0.5 to 4.0% manganese, 22 , 5 to 28.0% chromium, 3.5 to 8.0% nickel, 0.08 to 0.40% nitrogen and the rest iron, which at a ferrite content of 30 to 70% by primary deformation at temperatures above 1 155 ° C and a further deformation at temperatures below 1,000 to 800 ° C to a 0.2% proof stress of at least 75 kp / mm ² with good notch toughness and for the manufacture of objects in the chemical industry, especially in fermentation technology as well as food and paper industry is used. Although the steels known from DD-PS 115 508 have a very high yield strength R _P0.2 at room temperature (the maximum value is stated at 865 N / mm ² ), they have only low strength values at higher temperatures. This also applies to the chrome-nickel steels described in DD-PS 142 894 and up to 0.1% carbon, up to 1% silicon, 4 to 6% manganese, 22 to 28% chromium, 3, Contain 5 to 5.5% nickel, 1 to 3% molybdenum, 0.35 to 0.6% nitrogen and the rest iron and 30 to 70% austenite. These steels also have a minimum yield strength of more than 600 N / mm ² at room temperature; at higher temperatures, their strength is too low for many applications.

So far, it has not been possible to produce a heat-resistant, corrosion-resistant, nitrogen-containing chromium steel with a predominantly ferromagnetic structure from a chromium steel with a predominantly ferromagnetic structure, although it would be of technical interest to be able to use the chromium steels with a ferromagnetic structure due to their good properties even at higher temperatures .

The invention is therefore based on the object of providing a corrosion-resistant chromium steel which, even at temperatures above 400 ° C., has the favorable strength properties Chromium steels with ferromagnetic structure has, without the appearance of embrittlement. Furthermore, the invention is intended to provide a method for producing this steel.

The object on which the invention is based is achieved in that the chromium steel of the type mentioned at the outset has a nitrogen content which is between 0.2 and 5% and is at least 10% greater than the nitrogen solubility limit at 1 bar and 20 ° C., which is at 400 ° C has a yield strength R _p0.2 > 400 N / mm ² and at 600 ° C a yield strength R _p0.2 > 250 N / mm ² and which can be magnetized. Although it was not to be expected that a corrosion-resistant chrome steel with predominantly ferromagnetic microstructure components would have a high heat resistance at temperatures of more than 400 ° C., it was surprisingly found that the corrosion-resistant chrome steel according to the invention also has a high heat resistance at temperatures above 400 ° C. without that brittle phases occur. This advantageously means that the components made from the steel according to the invention can be dimensioned smaller because of the favorable relationship between tribochemical resistance and high heat resistance. The good heat resistance of the chrome steel according to the invention is attributed to the high nitrogen content, which must be considerably greater than the nitrogen solubility limit at 1 bar and 20 ° C. Since the nitrogen content of the steels known from the two DD patents 115 508 and 142 894 is far below the nitrogen solubility limit at 1 bar and 20 ° C., the person skilled in the art was not encouraged by this prior art to exceed the nitrogen solubility limit and he was able to also do not expect this measure to result in a significant improvement in properties.

The object is also achieved by the creation of a method for producing a corrosion-resistant chromium steel, in which a master alloy consisting of 3 to 45% chromium, 0.001 to 0.5% carbon, 0 to 10% nickel, 0 to 10% manganese, 0 to 10% molybdenum, 0 to 5% vanadium, 0 to 2% silicon, 0 to 2% titanium, niobium and / or tantalum, 0 to 1% cerium, 0 to 0.3% aluminum and the rest iron and a structure with at least 50% ferromagnetic microstructure, nitrogen is introduced by nitrogen pressure, which is between 0.2 and 5% and must be at least 10% greater than the nitrogen solubility limit of the master alloy at 1 bar and 20 ° C, at which the embroidered Alloy is thermoformed, in which the embroidered thermoformed alloy is annealed at 800 to 1 250 ° C and then cooled to room temperature. The master alloy is embroidered under pressure and can be carried out, in particular, by electroslag remelting. The glow time can be, for example, 0.5 to 10 hours. According to the method of the invention, a corrosion-resistant chromium steel with predominantly ferromagnetic structure components is produced, which can also be used at temperatures above 400 ° C, since it contains no brittle phases.

In a further embodiment of the invention it is provided that the steel, after it has been cooled at 450 to 750 ° C., is subjected to a tempering treatment and then cooled to room temperature. The duration of the tempering treatment is, for example, 1 to 10 hours. The tempering treatment advantageously achieves an additional improvement in the strength properties, in particular the deformation parameters.

Finally, according to the invention, it is particularly advantageous if the corrosion-resistant chrome steel is used for the production of parts for steam and gas turbines, since particularly high demands must be made on these parts with regard to their heat resistance.

The subject matter of the invention is explained in more detail below on the basis of exemplary embodiments. The ferritic chromium steel 1.400 2, which consists of 0.06% carbon, 0.5% silicon, 1% manganese, 13 _% chromium, 0.01% nitrogen, 0.1% aluminum, the rest iron and has a ferromagnetic structure after annealing at 800 ° C the following mechanical properties:

The structure of the chrome steel consists of ferrite. At a test temperature of 400 ° C, the yield strength of the steel is approximately 200 N / mm ^2.

After annealing at 950 to 1,000 ° C and cooling in oil or air and after tempering at 700 to 750 ° C and cooling in air, the steel has the following mechanical characteristics:

At a test temperature of 400 ° C this steel had a yield point Rp _o.2 = 280 N / mm ^2. The structure of the steel consists of ferrite and transformation structure.

A nitrogen content of 0.51% was introduced into a master alloy with a composition that corresponds to the composition of the material 1,400 2 by means of electroslag remelting under pressure. The embroidered master alloy was hot worked by forging at 1,180 ° C and then subjected to various heat treatments. It was found that three significantly different strength levels can be set by slightly changing the heat treatment, especially at room temperature. It was also found that at a test temperature of over 400 ° C there is no sudden drop in the heat resistance properties. The results of these tests are summarized in Table 1. The materials characterized in Table 1 have an extremely fine-grained structure. Annealing at temperatures above 800 ° C with subsequent cooling in air without tempering treatment (see Table 1, transverse column 3) cause the formation of a nitrogen-induced martensitic structure, which, in contrast to carbon martensite, has a higher ductility with significantly higher strength properties. Tempering treatments following the annealing (see Table 1, transverse columns 1 and 2) in turn cause a regression to a ferritic structure with simultaneous formation of the finest precipitates, primarily chromium nitride. The fact that the heat resistance of the embroidered steels at 400 ° C is far above the values of the known stainless ferritic chromium steels with a ferromagnetic structure and does not suffer a drop above this temperature is probably due to the restriction of the atomic mobility in the lattice typical of high-alloy chromium steels when the temperature rises.

The composition of the material 1.4002 was changed by adding 2.9% nickel and 3.5 _% molybdenum and by reducing the carbon content to 0.03%. The structure of this starting alloy was largely ferritic. A nitrogen content of 0.51% was introduced into this predominantly ferritic master alloy by electroslag remelting under pressure. The embroidered alloy was hot worked by forging at 1 180 ° C and then subjected to different heat treatments. The properties of the materials produced in this way are summarized in Table 2. Table 2 shows that the materials characterized there have strength properties that are far above those of conventional corrosion-resistant ferritic chromium steels. The different heat treatments result, among other things, in a change in the R _p0.2 / R _m ratio. If the homogenization annealing is carried out below 1000 ° C, this ratio is approx. 0.7. In the case of annealing above 1,000 ° C, this ratio is approximately 0.5. At the higher test temperatures, the strength level of the steels according to the invention characterized in Table 2 is far above the strength level that the austenitic chromium-nickel steels have. Metallographic investigations have shown that the materials characterized in Table 2 are mainly composed of ferrite, transformation structure and chromium nitride precipitates.

The possibility of using the materials characterized in Tables 1 and 2 at temperatures above 400 ° C was confirmed by examining the creep rupture strength at 400 to 750 ° C over a period of more than 1,000 hours.

. All percentages relating to the composition of the materials and alloys are% by weight. The percentages that relate to the individual structural components are vol%. The structural components can be determined by electron microscopy or by X-ray diffraction. The term room temperature means a temperature of 20 ° C.
(See tables 1 and 2 page 5 f.)

Claims (4)

1. Corrosion-resistant chromium steel which consists of 3 to 45 % chromium, 0.001 to 0.5 % carbon, 0 to 10 % nickel, 0 to 10 % manganese, 0 to 10 % molybdenum, 0 to 5 % vanadium, 0 to 2 % silicon, 0 to 2 % titanium, niobium and/or tantalum, 0 to 1 % cerium, 0 to 0.3 % aluminium, an increased nitrogen content and remainder iron, and the structure of which contains at least 50 % ferromagnetic structure constituents, characterised in that the nitrogen content lies between 0.2 and 5 % and is at least 10 % greater than the nitrogen solubility limit at 1 bar and 20 °C, in that the steel has at 400 °C a yield point Rp_o.₂ > 400 N/sq - mm², and at 600 °C a yield point R_p0,2 > 250 N/sq - mm² and in that it is magnetisable.

2. Process for the production of the corrosion-resistant chromium steel according to Claim 1, characterised in that a nitrogen content which lies between 0.2 and 5 % and must be at least 10 % greater than the nitrogen solubility limit of the prealloy at 1 bar and 20 °C is introduced by nitrogenation under pressure into a prealloy which consists of 3 to 45 % chromium, 0.001 to 0.5 % carbon, 0 to 10 % nickel, 0 to 10 % manganese, 0 to 10 % molybdenum, 0 to 5 % vanadium, 0 to 2 % silicon, 0 to 2 % titanium, niobium and/or tantalum, 0 to 0.3 % aluminium and remainder iron and possesses a structure with at least 50 % ferromagnetic structure constituents, in that the nitrogenated alloy is hot-deformed, in that the nitrogenated hot-deformed alloy is annealed at 800 to 1,250 °C, and then cooled to room temperature.

3. Process according to Claim 2, characterised in that the steel after cooling is subjected to a tempering treatment at 450 to 750 °C and then cooled to room temperature.

4. Use of the corrosion-resistant chromium steel according to Claim 1 for the production of parts for steam and gas turbines.