DE2550643B2 - LOW-ALLOY HEAT-RESISTANT FERRITIC STEEL - Google Patents

Description

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3. A method for the heat treatment of low-alloy, heat-resistant ferritic steel according to claim 2, characterized by normalizing at 1000 to 1200 ° C and tempering at 600 to 800 ° C.

4. Use of a steel according to one of claims 1 to 3 as a material for the production of Components for operating temperatures up to 575 ° C.

The invention relates to a low alloy ferritic heat-resistant steel to have a high creep strength and high creep resistance for operating temperatures up to 575 ⁰ C, a good Kriechverformbarkeit and good toughness at ambient temperature.

For objects made of heat-resistant steel such as chemical reactors, boilers, boiler tubes, steam pipes etc., its cold formability and its service life at operating temperature play a significant role Role. Ordinary steels with good cold formability properties also have considerable properties Creep resistance problems due to the life span of the steels made from such steels Workpiece is shortened considerably in this way. On the other hand, there are steels with good creep resistance certain problems of cold deformation.

Because steel is the only relatively inexpensive material that is used at temperatures of up to 800-1000 ° C Many types of steel have been developed for use at such temperatures.

The high-temperature steels can be divided into three broad classes:

(1) Austenitic steels with a high nickel and chromium content, which can reach temperatures in the Range of 900-1000 ° C can be used;

(2) Ferritic steels without nickel, which contain chromium up to 12% for temperatures up to about 800 ° C exhibit;

(3) Low alloy ferritic steels, which are almost are nickel-free and contain up to approx. 5% chromium for temperatures up to approx. 600 ° C.

According to the invention, a steel of this last group is now to be proposed, a group to which include several low alloy steels for use at temperatures up to about 600 ° C.

For example, Japanese Patent 72 47 488 relates to a steel with the following percentage Composition: 1.15-032 C; 0.05-0.35 Si;

less than 0.5% silicon,
less than 1% manganese,
0.01 to 0.013% nitrogen,

Remainder iron and manufacturing-related impurities.

2. Low-alloy, creep-resistant ferritic steel according to claim 1, characterized in that the sum of the contents of vanadium and niobium is between 0.35 and 0.4% and

'"^ ^b ) <1.2 <""C-""N)

0 "> - 08Mn; 0.5-3.0Cr; 0.3-1.5Mo; 0.1-0.35 V; at least an element of Ta or Nb 0.05-0.5; 0.02-0.1 N; and the rest iron.

The advantage of this steel is that its Creep resistance is about twice that recorded for common Cr-Mo-V steels.

Soviet patent 3 38 551 relates to a steel with the following percentage composition zung x 038-0.44 C; 0.3-0.6 Mn; 0.17-0.37 Si; 2-5Cr; 1.2-1.6Ni; 1.4-1.8Mo; 0.25-0.4 V; <0.03Si; <0.03 P; and the rest iron. Such a composition gives the steel a special one Conferred toughness; it is suitable for hot pressing of Al alloy profiles.

Japanese patent specification 73 13 803 relates to a steel with high creep resistance or high fatigue strength at temperatures below 500 ° C, which is weldable and has good toughness. His percentage composition is given as: 0.08-0.20C; 0.20-0.40Si; 1.0-1.6Mn; 0.05-0.35Cr; 0.10-0.60 Mo; 0.005-0.10 V; 0.0004-0.02 N; and the remainder essentially iron. B (0.001 to 0.01%) and Nb (0.01-0.05%) can also be added.

British patent specification 12 18 927 relates to a steel with the following percentage composition: 0.21-0.35C; 0.35 Si; 0.39-1Mn; 0.4-1Ni; 0.7-14Cr; 0.5-1.5 Mo; 0.2-0.6 V; 0.03-0 _: 15T '+ Nb and / or Ta; 0.002-0.01 B; 0.5-3 Co; and the remainder essentially iron. This steel is particularly recommended for turbine shafts for aircraft engines or parts of steam or gas turbines.

Japanese Patent 71 27 656 describes a steel with the following percentage composition:

0.3 C; 0.1-0.5Si; 0.2-1.5Mn; 0.01 Al; 0.004-0.02 N; 0.002-0.1 Se and / or Te and optionally 0.2-1.0Cr and 0.1-1.2 Mo; and the rest iron. This type of steel is special

suitable for high temperature and pressure vessels.
Surprisingly, according to the invention, a low-alloy, heat-resistant ferritic steel has now been found which is excellent for welding. This consists of:

0.04 to 0.08% carbon;

0.9 to 1.2% chromium,

0.9 to 1.1% molybdenum,

0.2 to 0.25% vanadium,

0.14 to 9.18% niobium,

0 to 0.006% boron,

less than 0.5% silicon,

less than 1% manganese,

0.01 to 0.013% nitrogen,

The remainder is iron and contaminants from the air.

The steel according to the invention is particularly advantageous for seamless tubes that are designed for a maximum operating temperature of 575 ° C are determined. Due to its good weldability, it is also suitable for welded Parts such as boiler components, large steam lines, etc. are suitable.

The low content of alloying elements makes production easier and more economical, without sacrificing properties such as creep resistance, impact resistance and processability would be. The elongation at break under creep shows a constant value for fatigue strengths up to 10,000 hours.

The exact relationships between the steel components are also used for special purposes Elements important. It has been found that there is a precise stoichiometric ratio between Carbon, nitrogen, vanadium and niobium are fundamental to the above Properties to be able to achieve. Thus, the steel according to the invention is characterized by its generality Composition as well as the fact that the sum of the percentages of vanadium and Niobium must preferably be between 0.35 and 0.40% and the sum of the percentages of niobium and Vanadium in relation to the sum of the percentages of carbon and nitrogen according to dei must be the following formula:

(C + N |

Nb

1.2 (C + N)

The purpose of the invention is to propose a low-alloy ferritic bainitic steel, the properties of which are obtained after hardening by precipitation or precipitation. That is why the carbon content was kept as low as possible, compatible with the usual oxygen blowing techniques, in order to obtain a steel with good weldability properties. Both niobium and vanadium are hardly added in sufficient proportions to completely bind the carbon and nitrogen present in the bath with the formation of carbides and carbonitrides. This ensures the hardening of the ferritic poultry (matrix) when these are excreted in appropriate quantities and distributions, namely on the basis of a suitable heat treatment to be discussed later. Molybdenum is also added so that it provides hardening by solids solution. Chromium is added to promote resistance to oxidation in the heat; however, its content should not exceed 1% in order to avoid its negative influence, if present in higher concentrations, on the creep resistance. It is recommended to add boron because of its positive effect on the hot workability and increasing the hardenability of the steel. It has been found that the most favorable Ausscheidungsbehanalung for steel according to the invention in the normalization at 1000 to 1200 ° C, followed by tempering to 600 to 800 ⁰ C, can be seen.

The achievable characteristics of the steel according to the invention, which was normalized at 1050 ⁰ C and tempered at 700 to 72o ⁰ C, are shown below, for example, those of another bezüelich up for

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Maximah temperatures of 575 ° C used compared. This reference steel, which is already commercially available, has the following composition in Weight percent: 0.11 C; 0.40Cr; 0.60 Mo; 0.25 V and the remainder essentially iron, apart from the usual percentages of Si, Mp, S and P.

The first table shows a comparison between the impact strength of both steels; the steel according to the invention was normalized at 1050 ° C, while the reference number has been normalized at 960 ⁰ C, which was the temperature that was applied for the industrial production of the reference steel and through which the steel the best properties were imparted. The impact resistance properties are given below as a function of tempering or temperature.

Table 1

stole Charpy-v-kgm 750 ⁰ C 80O = C Tempering temperature 700 ⁰ C Steel after 24.1-25.0 22.0-24.4 the invention 24.3-25.2 21.8-24.0 A. 23.9-24.2 24.5-26.0 21.9-23.8 B. 24.0-24.2 24.3-24.9 21.2-23.3 C. 23.6-24.3 24.2-24.8 21.5-24.0 D. 23.9-24.1 24.0-24.5 22.0-23.7 E. 24.1-24.3 F. 23.8-24.2 8.4-10.6 5.0-5.5 Reference steel 7.3- 9.9 4.4-5.3 1 5.3- 8.4 2 4.2- 8.0

The table clearly shows that the steel according to the invention is considerable compared to the reference steel higher and more regular values in the field of the investigated or applied tempering temperatures shows, so that the precise attainment of the tempering temperature is not critical.

The steel exhibits higher toughness properties than those found in those now commercially available Meets steel. The properties in terms of weldability are also far better than those of the Reference steel.

4s Tables 2 and 3 each show comparisons between the steels listed in Table 1 compared with the breaking stresses after 10 ⁵ hours (Table 2) and the percentage elongations after breakage due to creep. In this case too the steels were at

normalized to 1050 ° C and tempered at 700 to 720 ° C.

Table 2

stole Breaking stress after 10 ⁵ hours (kg / mm ² ) 550 ⁰ C 600 ⁰ C Test temperature 500 ° C Steel after 15.5-18 10-11 the invention 15-16.5 9.5-11 A. 24-26 16-17 10.5-12 B. 25-26.5 16-16.5 11th C. 24.5-25.5 16 17 11-12.5 D. 23.5-24.5 15-18 10-11.5 E. 15-26 F. 25-26.5 8.5-10 3.1-3.8 Reference steel 8.2- 9.3 3.1-4.2 1 14.8-18.5 2 15.3-19.8

Plate 3

stole strain muh break up t't'utid by KrifctiL-r bc-i 550 ( Hruch / L'it in hours 100 K) OO 10,000 Steel after the invention A. 19-22 18-20 17-19 B. 20-21 18-19 15-Ib C. 21-22 19-20 lb-18 D. 18-20 19-2b 17-19 E. 20-21 17-19 15-lc F. 20-21 18-19 14-lb Reference steel! 1 18-20 1 7-20 15-18 2 18-21 18-20 lb-18

As shown in Table 2, the steel according to the invention has higher breaking stresses than the reference steel: < with similar creep strain (Table 3). The steel after the invention shows a considerable advance, compared to the reference steel, in terms of it Cold formability and high temperature strength. There are other advantages of this steel over other steels in its economic production and its general properties.

Claims (1)

Patent claims: 1. Low-alloy, heat-resistant ferritic steel, characterized by a composition. Existing the end 0.04 to 0.08% carbon, 0.9 to 1.2% chromium, 0.9 to 1.1% molybdenum, 0.2 to 0.25% vanadium, 0.14 to 0.18% niobium, 0 to 0.006% boron,